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The role of prefrontal cortical dopamine (DA) in the modulation of working memory functions is well documented, but substantial evidence indicates that the locus ceruleus noradrenergic system also modulates working memory via actions within the prefrontal cortex (PFC). This study shows that PFC noradrenaline (NA) and DA dialysate levels phasically increase when rats perform correctly in a delayed alternation task in a T-maze, a test of spatial working memory. However, NA levels were markedly enhanced in animals trained to alternate compared with rats that acquired the spatial information about the location of food in the maze but were untrained to make a choice to obtain the reward. In contrast, PFC DA elevations occurred independently of whether the animal had acquired the trial-specific information for correct task execution. The contribution of anticipatory responses to catecholamine efflux was also evaluated by exposing rats to an environment signaling the presence of the reward in the successive alternation task. No conditioned NA efflux was observed in either group. In contrast, in both groups, DA efflux increased in the anticipatory phase of the test to the same levels of those reached during the task. These data provide the first direct evidence for a selective activation of PFC NA transmission during a spatial working memory task. We propose that, in the working memory task, DA is primarily associated with reward expectancy, whereas NA is involved in the active maintenance of the information about a goal and the rules to achieve it.

Objective: Previous studies indicate that the nitric oxide (NO(.)) pathway is involved in the acute or chronic effects of ethanol on the central nervous system. However, direct evidence for the effect of ethanol on NO(.) production in vivo is lacking, and it is not clear whether it is inhibition or stimulation of the NO(.) pathway that contributes to the behavioral effects of ethanol. Herein the release of NO(.) in the rat striatum in vivo in response to NMDA receptor activation--the dominant mechanism controlling NO(.) formation-has been investigated after systemic or local injections of ethanol., Methods: NMDA-induced release of authentic NO(.) was measured directly in the striatum of urethane-anesthetized (1.2 g/kg intraperitoneally) male Sprague-Dawley rats by using a direct-current amperometric method coupled to an electrically modified carbon microelectrode. An injector cannula was implanted in the proximity of the electrode (250 microm apart) for focal drugs application., Results: Local application of NMDA (1 microl, 100 microM) produced a sharp and transient NO(.) signal. Systemic ethanol, 1 or 2.5 g/kg intraperitoneally, caused a long-lasting, dose-dependent inhibition of NMDA-induced NO(.) release to 12.2 +/- 5.9 and 6.4 +/- 3.7% of control, respectively, 60 min after ethanol administration. Dizocilpine (0.5 mg/kg intraperitoneally) mimicked the ethanol effect, inhibiting NO release to 1.6 +/- 0.66% of control. Local application of ethanol (1 microl, 2.5% v/v) in the striatum reduced the NMDA-induced response to 28.6 +/- 3.8% of control. Focal application of the competitive NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (100 microM) or the nonselective NO synthase inhibitor L-N(G)-nitro-arginine methyl esther (100 microM) also caused inhibition of NMDA-induced NO(.) release to 2.4 +/- 0.7 and 4.3 +/- 0.9% of control, respectively., Conclusions: Ethanol, at pharmacologically significant doses, strongly inhibits striatal NO(.) production and release apparently through inhibition of NMDA receptor function. Inhibition of NMDA receptor-mediated activation of the NO(.) pathway could be a primary neurobiological mechanism contributing to the effects of ethanol.

Increased activity of glutamate N-methyl-d-aspartate (NMDA) receptors is the dominant mechanism by which nitric oxide (NO.) is generated. By using a selective direct-current amperometry method, we characterized real time NO* release in vivo in response to chemical stimulation of NMDA receptors in the rat striatum. The application of NMDA caused the appearance of a sharp and transient oxidation signal. Concentration-response studies (10-500 microM) indicated an EC(50) of 48 microM. The NMDA-induced amperometric signal was suppressed by focal application of the nitric-oxide synthase inhibitor L-nitro-arginine methyl ester (L-NAME, 100 microM) or D-(-)-2-amino-5-phosphonopentanoic acid (AP-5, 100 microM) or by systemic injection of dizocilpine (1 mg/kg i.p.), drugs that, when given alone, had no effect on baseline oxidation current. Repeated injections of NMDA at short intervals (approximately 80 s) resulted in a progressive reduction of the amperometric signal with a decay half-life of 1.36 min. Sixty min after the last NMDA application the amperometric response was restored to initial levels. Finally, the coapplication of glycine (50 or 100 microM), which, when given alone had no effect, potentiated the NMDA-induced response. Thus, NMDA receptor-mediated activation of striatal NO* system shuts off quickly and undergoes rapid desensitization, suggesting a feedback inhibition of NMDA receptor function. To the extent of NO* release can represent a correlate of NMDA receptor activity, its amperometric detection could be useful to assess in vivo the state of excitatory transmission under physiological, pharmacological, or pathological conditions.

Low doses of ethanol (0.5 g/kg i.p.) increased, while higher doses (1 g/kg i.p.) reduced acetylcholine (ACh) release in the rat prefrontal cortex (PFC). Ethanol (50-300 mM) applied in the nucleus basalis through a second dialysis probe caused concentration-dependent biphasic changes in prefrontocortical ACh release. Ethanol apparently acts on cholinergic fibers to modulate ACh transmission in the PFC. These results could be of relevance for the bidirectional modulation of working memory by ethanol.

Background: It is common knowledge that ethanol causes cognitive and memory impairments. Although these deficits are attributed to its central depressant properties, ethanol has biphasic effects and at low doses can produce excitatory actions., Methods: Here we examined whether ethanol could have biphasic effects on performance in a delayed alternation task in a T-maze, a behavioral test of working memory., Results: A dose-response study showed that intermediate doses of ethanol (1 g/kg) were associated with impairments of working memory in rats, as assessed at short intertrial intervals (10 sec). In contrast, at longer delays (120 sec), when the delayed alternation performance was reduced markedly in controls, a lower dose of ethanol (0.5 g/kg) significantly improved working memory., Conclusions: These results demonstrate a dose-dependent, bidirectional effect of ethanol on working memory and implicate the prefrontal cortex, the site of working memory function, as a target of ethanol action. The cognitive improvements caused by low, excitatory doses of ethanol may be perceived as rewarding and could have relevance for chronic ethanol consumption in humans.

Extracellular glutamate levels were measured by microdialysis in the prefrontal cortex (PFC) of anaesthetised rats in response to a short, experimenter-provoked mechanical movement of the animal head. Movement caused significant, nerve impulse-independent elevations of glutamate levels (maximum increase, 300+/-30% of baseline). This study reveals a possible artifact in the measurement of extracellular glutamate concentrations by microdialysis and suggests that, in awake animals, treatments associated with stimulation of motor activity can cause non-specific efflux of glutamate in the PFC.

It has been hypothesised that rat lines genetically selected for their alcohol preference consume large amounts of ethanol because they have a low 5-HT content. Since brain tryptophan (TRP) availability controls the rate at which neurons synthesise and release serotonin (5-HT), we assessed whether the administration of a TRP-supplemented or TRP-free diet for 3 consecutive days influenced alcohol intake in alcohol-preferring and non-preferring sP and sNP rats, respectively. In the same animals extracellular 5-HT concentration was monitored by microdialysis in the frontal cortex. A TRP-free diet progressively and markedly decreased cortical extracellular 5-HT in sP and sNP rats during the treatment period with respect to a balanced diet. However, the TRP-free diet failed to modify alcohol consumption and preference in sP and sNP rats. The TRP-supplemented diet also failed to alter the intake of alcohol in either group of rats. Therefore, these results do not support a specific role of 5-HT transmission in ethanol intake and preference in sP and sNP rats.

The effect of NMDA on the motility of the rat portal vein was studied in an isolated preparation. NMDA induced a concentration-dependent (10(-7) - 10(-4) M) increase of the contraction frequency (maximum increase, 148+/-6% of control at NMDA 10(-4) M). The NMDA-induced excitatory response was prevented by the competitive NMDA receptor antagonists (+/-)-2-Amino-5-phosphonopentanoic acid (AP-5, 5x10(-4) M) or (RS)-3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid (CPP, 10(-4) M). Tetrodotoxin (TTX, 10(-6) M) or atropine (10(-4) M) abolished the NMDA-induced increase of the portal vein motility and reversed the excitatory effect to a concentration-dependent inhibition (maximum inhibition, 52+/-8 and 29+/-7% of controls, respectively, at NMDA 10(-3) M). Removal of the endothelium abolished the NMDA-induced inhibitory response. Sodium nitroprusside concentration-dependently (10(-7) - 10(-5) M) inhibited the portal vein motility, while L-N(G)-nitro-arginine methyl ester (L-NAME, 10(-4) M) reversed the inhibitory effect of NMDA (in the presence of TTX), restoring the portal vein spontaneous activity to control values. These results show that NMDA modulates the portal vein motility in a biphasic manner: via indirect activation, through prejunctional NMDA receptors presumably located on intrinsic excitatory neuronal afferences, or via direct inhibition, through endothelial NMDA receptors activating the nitric oxide pathway. Overall these findings support the hypothesis of the existence of a peripheral glutamatergic innervation modulating the contractile activity of the rat portal vein. British Journal of Pharmacology (2000) 129, 156 - 162

Withdrawal from chronic ethanol intoxication is associated with a reduction of dopamine neurotransmission. However, the mechanisms of dopamine depletion, a putative neurochemical correlate of the dysphoric symptomatology, are not yet understood. To assess the role of L-type calcium channels in the inhibition of the dopaminergic system in the withdrawal state, the effects of the dihydropyridine calcium channel antagonist nimodipine on the extracellular levels of dopamine were studied in the nucleus accumbens shell of awake rats 10 h after withdrawal from chronic ethanol intoxication. In control, chronic sucrose-withdrawn rats, nimodipine did not change extracellular dopamine levels. However, in ethanol-withdrawn rats nimodipine (5 or 10 mg/kg s.c.) increased extracellular dopamine to 136 +/- 16 and 305 +/- 19% of pre-administration values, respectively, the latter dose elevating levels above those of controls. The elevations of extracellular DA by nimodipine (10 mg/kg) were associated with a significant reduction (-17%) of the overall behavioural score of the withdrawal symptomatology, as evaluated for 11 behavioural items. Significant reductions of the score for convulsions (-47%) and, to a lesser extent, for catatonia (-30%) and tremors (-15%) contributed to the overall effect. It is suggested that overactivity of L-type calcium channels is involved in the mechanisms of dopamine depletion as well as in certain behavioural/neurological signs associated with ethanol withdrawal. By restoring depleted dopamine levels, dihydropyridines might ameliorate the dysphoric symptoms of ethanol abstinence.

Long-term ethanol consumption in humans and laboratory animals is associated with morphological and functional alterations of brain structures involved in cognitive processes. In the present experiments, we assessed whether voluntary long-term consumption of ethanol by alcohol-preferring (sP) rats under free choice condition with water (also) caused alterations in memory performance and hippocampal acetylcholine (ACh) release in vivo. A group of sP rats were offered a 10% v/v ethanol solution in a free choice with water for 36 weeks; controls had only tap water available. After withdrawal of ethanol, rats were tested in one trial passive avoidance test and thereafter were trained in a food-reinforced radial arm maze task for 12 days. One day after the last session in the radial-arm maze, rats were implanted with a microdialysis probe in the dorsal hippocampus and dialysate concentrations of ACh were measured. No significant differences were observed between sP drinking and control rats in retention latencies in the passive avoidance test, in radial arm-maze performance or in basal levels of hippocampal ACh release. These results show that long-term ethanol consumption by sP rats is not associated with cognitive impairments or with alterations in the hippocampal cholinergic function. To the extent that chronic ethanol intoxication can be considered a causal factor in the development of memory and neurochemical alterations, these results suggest that sP rats self-regulate ethanol consumption so as to avoid intoxication. These findings may challenge the notion that sP rat lines can be considered a valid model of human alcoholism.

Ethanol withdrawal is a physiopathological state associated with increased number and function of N-methyl-D-aspartate glutamate receptors. We assessed the effect of N-methyl-D-aspartate receptor stimulation on the extracellular levels of glutamate in vivo by the focal application of N-methyl-D-aspartate in the striatum of dependent rats following withdrawal from chronic treatment with ethanol. In control, chronic sucrose-treated rats, 800 microM N-methyl-D-aspartate increased glutamate levels to 268% of baseline values. In ethanol-withdrawn animals, 12 h after interruption of the chronic treatment, the application of N-methyl-D-aspartate increased glutamate levels to 598% of baseline values. In ethanol-intoxicated rats N-methyl-D-aspartate was ineffective. Concentration-response curves showed that in ethanol withdrawn animals N-methyl-D-aspartate was five-fold more potent than in controls. In withdrawn animals, the non-competitive N-methyl-D-aspartate receptor antagonist dizocilpine (1.0 mg/kg i.p.) or ethanol (5 g/kg i.g.) markedly reduced the N-methyl-D-aspartate-induced increase in glutamate levels. These results are consistent with the up-regulation of N-methyl-D-aspartate receptors by chronic ethanol and add biochemical evidence for the presence of N-methyl-D-aspartate receptors facilitating glutamate release through a positive feedback mechanism. The glutamate-induced, N-methyl-D-aspartate receptor-mediated elevations of extracellular glutamate may constitute a neurochemical substrate for the neuropathological alterations associated with alcoholism.

In this review first we evaluate evidence on the role of the neurobiological alterations induced by chronic ethanol consumption in the development of ethanol tolerance, dependence and withdrawal. Secondly, we describe the neuropathological consequences of chronic ethanol on cognitive functions and on brain structures. Chronic alcohol consumption can induce alterations in the function and morphology of most if not all brain systems and structures. While tolerance mechanisms are unlikely to contribute to the neuroadaptive changes associated with ethanol dependence, it is otherwise clear that repeated high, intoxicating doses of ethanol trigger those neuroadaptive processes that lead to dependence and contribute to the manifestation of the abstinence syndrome upon withdrawal. An unbalance between inhibitory and excitatory neurotransmission is the most prominent neuroadaptive process induced by chronic ethanol consumption. Due to the diffuse glutamatergic innervation to all brain structures, the neuroadaptive alterations in excitatory neurotransmission can affect the function of most if not all of neurotransmitter systems. The expression of the withdrawal syndrome is the major causal factor for the onset and development of the neuropathological alterations. This suggests a link between the neuroadaptive mechanisms underlying the development of ethanol dependence and those underlying the functional and structural alterations induced by chronic ethanol. In animals and humans, specific alterations occur in the function and morphology of the diencephalon, medial temporal lobe structures, basal forebrain, frontal cortex and cerebellum, while other subcortical structures, such as the caudate nucleus, seem to be relatively spared. The neuropathological alterations in the function of mesencephalic and cortical structures are correlated with impairments in cognitive processes. In the brain of alcoholics, the prefrontal cortex and its subterritories seem particularly vulnerable to chronic ethanol, whether Korsakoff's syndrome is present or not. Due to the role of these cortical structures in cognitive functions and in the control of motivated behavior, functional alterations in this brain area may play an important role in the onset and development of alcoholism.

Electrical stimulation of the medial prefrontal cortex caused glutamate release in the ventral tegmental area (VTA) of freely moving animals. Cathodal stimulation was given through monopolar electrodes in 0.1-ms pulses at an intensity of 300 microA and frequencies of 4-120 Hz. Glutamate was measured in 10-min perfusate samples by HPLC coupled with fluorescence detection following precolumn derivatization with o-phthaldialdehyde/beta-mercaptoethanol. The stimulation-induced glutamate release was frequency dependent and was blocked by the infusion of the sodium channel blocker tetrodotoxin (10 microM) through the dialysis probe. The stimulation also induced bilateral Fos-like immunoreactivity in ventral tegmental neurons, with a significantly greater number of Fos-positive cells on the stimulated side. These findings add to a growing body of evidence suggesting that the medial prefrontal cortex regulates dopamine release in the nucleus accumbens via its projection to dopamine cell bodies in the VTA.

Extracellular glutamate was measured by microdialysis in the striatum of ethanol-dependent, freely behaving rats following withdrawal from chronic ethanol treatment. Within 12 h from withdrawal, extracellular glutamate rose to 255% of that in control, chronic sucrose-treated rats. Glutamate output remained elevated for the subsequent 12 h and returned to control levels within 36 h from the interruption of the treatment. The changes in glutamate were time-locked to the overt physical signs of withdrawal. In 12-h ethanol-withdrawn rats an ethanol challenge suppressed the withdrawal signs and reduced the extracellular glutamate. The NMDA receptor antagonist, dizocilpine, reduced both the physical signs of withdrawal and glutamate output. In contrast, diazepam reduced the withdrawal signs but failed to change the glutamate levels. These findings suggest that the increased extraneuronal glutamate reflects overactivity of excitatory neurotransmission during withdrawal. Furthermore, they provide a biochemical rationale for the use of NMDA receptor antagonists and ethanol itself in the treatment of ethanol withdrawal syndrome.

Dizocilpine administration enhances dopamine metabolism in the rat striatum, nucleus accumbens, olfactory tubercle, and prefrontal cortex. Concomitant with increased metabolism is enhanced tyrosine hydroxylase and aromatic L-amino acid decarboxylase activities in the striatum and increased mRNA for the two enzymes in the midbrain. Activation of dopaminergic neurons may, in part, explain increased locomotor activity in normal animals and the ability of dizocilpine to potentiate the antiparkinsonian action of L-3,4-dihydroxyphenylalanine in an animal model.

Exposure of rate to the behavioral despair test (an animal model of depression) for 40 min resulted in a long-lasting depletion of mesolimbic dopamine output to about 40% of baseline values. The decrease in extracellular dopamine was partially prevented by chronic pretreatment with imipramine (20 mg/kg per day i.p. for 21 days). The results suggest that a fall in mesolimbic dopamine output may be associated with depressive states and indicate that changes in the functional status of the dopamine system contribute to the mechanism of action of imipramine.

Activity of the mesolimbic dopaminergic system was investigated in rats withdrawn from chronic ethanol administration by single-cell extracellular recordings from dopaminergic neurons of the ventrotegmental area, coupled with antidromic identification from the nucleus accumbens, and by microdialysis-technique experiments in the nucleus accumbens. Spontaneous firing rates, spikes per burst, and absolute burst firing but not the number of spontaneously active neurons were found drastically reduced; whereas absolute and relative refractory periods increased in rats withdrawn from chronic ethanol treatment as compared with chronic saline-treated controls. Consistently, dopamine outflow in the nucleus accumbens and its acid metabolites were reduced after abruptly stopping chronic ethanol administration. All these changes, as well as ethanol-withdrawal behavioral signs, were reversed by ethanol administration. This reversal suggests that the abrupt cessation of chronic ethanol administration plays a causal role in the reduction of mesolimbic dopaminergic activity seen in the ethanol-withdrawal syndrome. Results indicate that during the ethanol-withdrawal syndrome the mesolimbic dopaminergic system is tonically reduced in activity, as indexed by electrophysiological and biochemical criteria. Considering the role of the mesolimbic dopaminergic system in the reinforcing properties of ethanol, the depressed activity of this system during the ethanol-withdrawal syndrome may be relevant to the dysphoric state associated with ethanol withdrawal in humans.

The changes in extracellular concentrations of noradrenaline (NA) in the prefrontal cortex of morphine-dependent rats were studied by microdialysis following an acute morphine challenge and during naloxone-precipitated withdrawal. Animals were implanted with morphine- or placebo-containing pellets for 5 days. In control rats a challenge dose of morphine (5 mg/kg s.c.) induced a maximum decrease in NA output of about 45% of pre-drug levels. In contrast, morphine challenge had no effect on extraneuronal NA concentrations in morphine-implanted animals. In control animals, naloxone (2 mg/kg i.p.) produced no behavioral effect nor changed NA levels. However, in morphine-dependent animals naloxone suddenly increased extraneuronal NA by 175% of baseline dialysate levels in the first sample after the injection and precipitated a morphine-withdrawal symptomatology that paralleled the changes in NA output. Thus, chronic morphine treatment in rats results in the development of tolerance to the acute inhibitory effect of morphine on extraneuronal NA and is associated with a stimulation of prefrontocortical NA output during naloxone-precipitated withdrawal.

The administration of ethanol (2 g/kg, i.p.) or of the non-competitive antagonist(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycloepten-5,1 0-imine maleate (MK-801; 1 mg/kg, i.p.) induced a decrease in the extracellular concentrations of glutamate, as studied by microdialysis in the striatum of awake rats. Moreover, ethanol and MK-801 completely prevented the increase in extraneuronal glutamate concentration induced by the focal application of N-methyl-D-aspartate (NMDA). The present results suggest that ethanol suppresses glutamate release through an inhibition of NMDA glutamate receptors in the rat striatum.

The effect of ethanol challenge on the extracellular concentrations of dopamine, 3,4-dihydroxy-phenylacetic acid and homovanillic acid was studied in the ventral striatum of rats repeatedly treated with ethanol. Ethanol-treated animals (1 g/kg i.p. twice a day for 12 days) developed marked tolerance to the behavioral signs of ethanol intoxication when challenged with ethanol (2 g/kg i.p.). However, in ethanol-treated animals the increased output of dopamine and metabolites after ethanol challenge (1 or 2 g/kg i.p.) was not statistically different from that observed in saline-treated rats. These results indicate that tolerance does not develop to the ethanol-induced stimulation of dopamine release and support the hypothesis that activation of the mesolimbic dopamine system contributes to the reinforcing properties of ethanol.

Ethanol (EtOH) administration is considered to elicit its reinforcing properties by stimulating dopaminergic (DA) transmission in the mesolimbic system. Accordingly, (EtOH) activates dopamine neuronal firing in the Ventro-Tegmental Area (VTA) and DA output in the nucleus accumbens. Concomitantly, EtOH reduces the firing rate of Pars Reticulata (PR) neurons which are thought to exert an inhibitory control over DA neurons. Further, chronic ingestion of EtOH produces tolerance to its sedative effects as to the depressant effect on PR neurons but no tolerance to the DA stimulating action. Moreover the NMDA antagonist MK-801, but not SL-820715, stimulates DA firing, suggesting that this effect is not a general characteristic of NMDA receptor antagonists and questioning the possibility that NMDA-receptor blockade may underlie EtOh-induced activation of DA-ergic transmission. The results indicate that activation of the mesolimbic DA tract is essential in the rewarding properties of EtOH and that neither GABA-ergic inhibition nor NMDA-receptor blockade by EtOH, are causally linked to the EtOH-induced activation of DA-ergic transmission.

The electrophysiological activity of mesoaccumbens dopaminergic neurons was monitored during the ethanol-withdrawal syndrome in ethanol-dependent and in control rats. Spontaneous firing was reduced by about half in ethanol-dependent rats as compared to controls. Likewise, the number of spikes/burst was also reduced in ethanol-dependent rats. These results are consistent with the reduction in dopamine release observed during ethanol-withdrawal syndrome and may provide the basis for the aversive effects of the ethanol-withdrawal syndrome.

Withdrawal of rats from chronic ethanol, morphine, cocaine and amphetamine resulted in a marked reduction in extracellular dopamine (DA) concentration in the ventral striatum as measured by microdialysis. Following ethanol and naloxone-precipitated morphine withdrawal, the time course of DA reduction paralleled that of the withdrawal symptomatology. On the other hand, following discontinuation of chronic cocaine, DA reduction was delayed by over 24 h but persisted for several days. After amphetamine withdrawal the fall in DA occurred more rapidly but the reduction also persisted for several days. The administration of the NMDA receptor antagonist, MK-801, to rats withdrawn from chronic ethanol, morphine or amphetamine, but not from chronic cocaine, readily reversed the fall in DA output. The reduction in extracellular DA during ethanol withdrawal was also reversed by SL 82.0715, another NMDA receptor antagonist.

Ethanol elicited a biphasic effects on the extracellular noradrenaline (NA) concentrations in the rat frontal cortex, as assessed by microdialysis in awake animals. A low dose of ethanol (0.2 g/kg i.p.) raised NA output to about 160% of baseline levels. In contrast, a dose of 2 g/kg inhibited NA output to about 70% of pre-drug levels. These results suggest that the decrease in cortical NA output may reflect the sedative-hypnotic properties of ethanol at high doses, whereas the stimulation of extraneuronal NA may represent a biochemical correlate of the arousal and increased alertness elicited by low doses of ethanol.

Rats maintained for 10 days on a 10% ethanol solution as the sole source of fluid developed marked tolerance to ethanol-induced loss of righting reflex, but no tolerance to the stimulatory effects on mesolimbic dopaminergic system. An ethanol challenge stimulated both the electrical activity of A10 dopaminergic cells and dopamine output in the ventral striatum of behaviourally tolerant animals and of controls to the same extent. These results are compatible with the hypothesis that an increased dopamine neurotransmission in the limbic system participates in the reinforcing effect of ethanol.

Twelve hypertensive subjects were treated for 2 weeks with the dihydropyridine calcium channel antagonist nicardipine (40 mg daily) according to a double-blind, placebo-controlled study protocol. Nicardipine treatment significantly decreased systolic and diastolic blood pressure and increased plasma noradrenaline levels measured at supine rest. However, the treatment significantly inhibited the physiological increase of circulating neurotransmitter following sympathetic stimulation induced by orthostatism. These results suggest that dihydropyridine-sensitive calcium channels may modulate the noradrenaline release from nerve terminals of the peripheral sympathetic nervous system.

Withdrawal of rats from chronic ethanol (2-5 g/kg, every 6 hr for 6 days) resulted in withdrawal symptomatology and dramatic fall in extracellular dopamine (DA) in the ventral striatum as measured by microdialysis. The changes in DA output paralleled the withdrawal symptomatology and both phenomena were reversed by a challenge ethanol dose (5 g/kg orally). The results suggest that the decrease in DA output may be responsible for the aversive symptoms of withdrawal.

The neurotoxin MPP+ potently inhibited the striatal binding of [3H]-tyramine, a putative marker for the vesicular transporter of dopamine, and provoked a massive in vivo release of striatal dopamine. Tetrabenazine, an established ligand for the vesicular catecholamine carrier, potently inhibited [3H]-tyramine binding, tyramine-provoked striatal efflux of dopamine and the fast component of MPP(+)-induced dopamine release. It is concluded that MPP+ in the striatum, besides interacting with additional intracellular targets, avidly binds at a vesicular site functionally involved with the outward transport of dopamine.

The stimulatory effect of cocaine on locomotor activity and on dopamine efflux from the ventral striatum was studied in rats chronically treated with the antidepressant imipramine. Chronic imipramine (20 mg/kg per day for 3 weeks) potentiated by about 2-fold cocaine-stimulated motor activity and extracellular dopamine concentrations. The results indicate that chronic imipramine facilitates mesolimbic dopamine neurotransmission by potentiating the mechanism which is thought to mediate the rewarding effects of cocaine.

The effects of daily treatment with GM1 ganglioside (30 mg/kg s.c.) from birth to day 30, on striatal pre- and postsynaptic markers of the dopaminergic system in euthyroid- and 32 day-old hypothyroid rats were studied. The purpose was to assess whether GM1 could prevent the extensive, hypothyroidism-provoked impairment of dopaminergic neurotransmission. Neonatal administration of GM1 well counteracted the hypothyroidism-related deficits in striatal synaptosomal uptake of [3H]dopamine and in membrane binding of [3H]tyramine, a putative marker for the vesicular carrier of dopamine. In the hypothyroid striatum, the decrease of concentrations of DOPAC and HVA, the loss of [3H]SCH-23,390-labelled D1-receptors and the decrease of basal- or dopamine-stimulated, D1-mediated activity of adenylate cyclase were not prevented by GM1. Although somatic and neurobehavioural aberrations of hypothyroids were not at all or only partially ameliorated, a slight improvement of the thyroid status was suggested by less decreased levels of serum thyroxine (T4) after treatment with GM1. The ganglioside-driven selective recovery of the transport and storage process of [3H]dopamine might result either from a chronically-exerted stimulation by GM1 on the NA/K- and Mg-ATPase activities, thus reflecting on the ATPase-dependent neuronal and vesicular transport processes of dopamine or from a GM1-promoted maturation of the otherwise retarded functionality of dopaminergic nerve endings in the neonatal hypothyroid striatum.

The effect of different calcium antagonists on cocaine-induced dopamine (DA) release in the striatum, as measured by brain microdialysis in freely moving rats, and on cocaine-induced motor stimulation was studied. While two dihydropyridine calcium antagonists, nimodipine (20 mg/kg) and isradipine (2.5 mg/kg), prevented cocaine-induced DA release and motor stimulation, the diphenylalkylamine-type calcium antagonist flunarizine (20 mg/kg) strongly potentiated both effects of cocaine. Moreover, two calcium antagonists, verapamil (20 mg/kg) and diltiazem (20 mg/kg), were ineffective. The results indicate that various classes of calcium antagonists differ in their interaction with the effects of cocaine in the CNS and suggest that dihydropyridine calcium channel antagonists might be clinically useful for the treatment of cocaine abuse.

Pretreatment with flunarizine (20 mg/kg), a diphenylalkylamine calcium channel antagonist, markedly potentiated cocaine (10 mg/kg)-induced dopamine (DA) output from the ventral striatum, as measured by microdialysis in freely moving rats. Moreover, flunarizine enhanced cocaine-induced motor stimulation. Pretreatment with the D2 receptor antagonist (-)-sulpiride (25 mg/kg) potentiated cocaine-induced DA output but, unlike flunarizine, inhibited cocaine-induced motor stimulation. When added to striatal membrane preparations, flunarizine displaced [3H]spiperone binding with a Ki of about 100 nM. Since recent evidence indicates that the effects of cocaine are calcium-dependent and are prevented by dihydropyridine calcium antagonists, these findings suggest that the paradoxical potentiating effect of flunarizine is probably due to an interaction with the DA system and is not due to its calcium antagonist property.

The concentrations of dopamine (DA) and of the excitatory amino acids (EAAs) glutamate (Glu) and aspartate (Asp) were measured in dialysates from the striatum of awake rats in order to study the link between the release of DA and of EAAs induced by the infusion of 1-methyl-4-phenylpyridinium ion (MPP+). DA and EAAs were detected simultaneously by HPLC-EC. The infusion of MPP+ at the concentration of 1 mM elevated DA levels in the perfusates, but did not affect EAA release. However, MPP+ at 10 mM maximally stimulated Glu and Asp release to 230- and 68-fold of baseline, respectively. In this condition, pretreatment with the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (5 mg/kg, i.p.) prevented the MPP(+)-induced EAA release. In contrast, MK-801 had no effect on DA release induced either by 1 or 10 mM MPP+. These results suggest that MPP(+)-induced DA and EAA release are independently regulated processes. In addition, the finding that MK-801 inhibits MPP(+)-induced EAA release suggests that EAAs may act on NMDA receptors to stimulate their own release through a positive-feedback mechanism.

We considered the drug-induced circling behaviour of rats monolaterally lesioned with kainic acid (KA) as a marker of the dopamine (DA) concentration in the synaptic space. D-Amphetamine produced a dose-related increase in the DA concentration of the dialysate from an intact striatum and a proportional number of ipsilateral circlings. Pargyline or L-dihydroxyphenylalanine (L-Dopa), alone or in combination with benserazide, increased the concentration of DA to a similar or even higher level than d-amphetamine, but failed to elicit a circling response. Apomorphine given after these drugs at the peak of DA accumulation always induced circling behaviour. The results suggest that released DA may undergo different inactivation processes before reaching the dialysis probe, and that these processes may be differentially affected by drug treatments. Alternatively, it may be suggested that DA can be released into the synaptic space, in a functional manner, or into the interstitial fluid, from where it cannot reach the synaptic receptors.

Aromatic L-amino acid decarboxylase (AAAD) activity of rat retina increases when animals are placed in a lighted environment from the dark. The increase of activity can be inhibited by administering the selective dopamine D1 receptor agonist SKF 38393, but not the selective D2 agonist quinpirole, or apomorphine. Conversely, in the dark, enzyme activity can be enhanced by administering the selective D1 antagonist SCH 23390 or haloperidol, but not the selective D2 antagonist (-)-sulpiride. Furthermore, in animals exposed to room light for 3 h, the D1 agonist SKF 38393 reduced retinal AAAD activity, and this effect was prevented by prior administration of SCH 23390. In contrast, quinpirole had little or no effect when administered to animals in the light. Kinetic analysis indicated that the apparent Vmax for the enzyme increases with little change in the apparent Km for the substrate 3,4-dihydroxyphenylalanine or the cofactor pyridoxal-5'-phosphate. We suggest that dopamine released in the dark tonically occupies D1 receptors and suppresses AAAD activity. When the room light is turned on, D1 receptors are vacated and selective D1 agonists can either prevent the rise of AAAD or reverse light-enhanced AAAD activity.

Foot-shock produced a more than 2-fold increase in noradrenaline (NA) release from the frontal cortex of freely moving rats. The effect of acute stress was almost completely prevented by the administration of diazepam (5 mg/kg i.p.). Diazepam alone inhibited cortical NA release, the maximal inhibition (-57%) being observed 90 min after the injection. Cortical NA release therefore appears to be a reliable index of central noradrenergic activity in response to stressful conditions.

Oral administration of the antithyroid drug methimazole (50 mg/kg per day) to rats during the last six days of pregnancy, and subsequent daily s.c. injection of methimazole (20-30 mg/kg) to their pups from birth to postnatal day 30 provoked hormonal and somatic alterations resembling (with all caution to any association between rodent and human data) those of congenital hypothyroidism. The steady-state concentrations of striatal dopamine were similar in hypothyroid and euthyroid, 32-day-old rats, while the levels of the dopamine metabolites 3,4-dihydroxyphenylacetic and homovanillic acids were markedly decreased in hypothyroidism. The results of this and our earlier study [Vaccari A. and Gessa G. L. (1989) Neurochem. Res. 14, 949-955] show that the maximal synaptosomal uptake of [3H]dopamine, an index for the density of nigrostriatal dopaminergic terminals, and the maximum number of membrane [3H]tyramine binding sites, reflecting the concentration of the vesicular transporter for dopamine, were decreased in the hypothyroid striatum. There was also a loss of those D1-type dopaminergic receptors claimed to be located on neurons intrinsic to the striatum, and, consequently, dopamine-stimulated, D1-regulated adenylate cyclase activity was depressed. It is suggested that individual dopaminergic nerve endings in the neonatal hypothyroid striatum must contain more dopamine, owing to some loss of pertinent innervation and, therefore, to the presence of less vesicular transport sites for dopamine. Hypothyroidism-related decreases in the maximum number of striatal D1- and, reportedly, D2-receptors, plus the impairment of D1-coupled second messenger activity, may play a role in the derangement of those neurobehavioural patterns where a dopaminergic regulation is putatively implied.

The administration of bromocriptine and lisuride to rats caused a decrease in striatal dopamine (DA) synthesis, as measured by 3,4-dihydroxy-phenylalanine (DOPA) accumulation after decarboxylase inhibition. DOPA formation was inhibited by a maximum of about 60% of control values by bromocriptine and lisuride, 5.0 and 0.3 mg/kg, respectively. Both compounds showed very similar time-courses for the effect and failed to modify DOPA accumulation during the first 30 min. Pretreatment with (-)-sulpiride (50 mg/kg i.p.), a specific D2-receptor blocker, completely prevented the inhibitory effect of bromocriptine and lisuride on DOPA accumulation. Finally, both compounds significantly reversed the gamma-butyrolactone (GBL) (700 mg/kg i.p.)-induced DOPA accumulation at doses (0.25 and 0.015 mg/kg, respectively) that were inactive in normal rats. The data suggest that bromocriptine and lisuride act as agonists on D2-presynaptic autoreceptors which have different sensitivity to the agonist according to the basal firing rate of DA neurons.