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Two methods of evaluating inhibitory sensory processing are prepulse inhibition of acoustic startle (PPI) and gating of auditory evoked potentials. Studies using both methods suggest nicotinic acetylcholinergic receptor modulation of gating, specifically the alpha-bungarotoxin (alpha-BTX) binding site (alpha7 receptor subtype). However, recent assessment of alpha7 null mutant mice failed to demonstrate any effect of the loss of this receptor in either gating paradigm. An alternate approach to assessing the effects of the alpha7 receptor is to reduce its numbers in mature inbred mice, thus, avoiding the twin problems of background and developmental compensation inherent in null mutant mouse studies. Numerous studies have shown that chronic corticosterone (CCS) treatment selectively reduces alpha-BTX binding sites. C3H mice were adrenalectomized and implanted with corticosterone or cholesterol (control) pellets. After 8 days, they were tested in one of the gating paradigms. PPI and auditory gating were significantly diminished in corticosterone-treated mice concomitant with a reduction in alpha-BTX binding in several brain regions. Cholesterol-treated mice had no change in either paradigm. Nicotine treatment (1 mg/kg) produced significant improvement in both paradigms in corticosterone-treated mice. These data agree with previous pharmacological studies suggesting modulation of gating occurs through a nicotinic receptor.

Nicotinic agonist-stimulated efflux of 86Rb+ from mouse brain synaptosomes was monitored continuously by on-line radioactivity detection. The concentration-effect curve following a 5-s stimulation with acetylcholine was biphasic (EC50 = 7.2 and 550 microM). alpha-Bungarotoxin (100 nM) did not inhibit the response, but dihydro-beta-erythroidine (DHbetaE) blocked both phases with differing potency (average IC50 =.22 and 8.9 microM for responses activated by low and high acetylcholine concentrations, respectively). Differential sensitivity DHbetaE inhibition was used to measure stimulation of 86Rb+ efflux by 17 nicotinic agonists, which differed markedly in potency and efficacy. All agonists were more potent at the DHbetaE-sensitive site. Both components were inhibited by the six antagonists tested. Methyllycaconitine and DHbetaE were more potent for the DHbetaE-sensitive component, whereas hexamethonium was more potent at the DHbetaE-resistant component. Both DHbetaE-sensitive and DHbetaE-resistant responses were reduced more than 95% in beta2-null mutant mice, establishing the requirement for the beta2 subunit for both components. Both components were widely, but not identically, distributed throughout the brain. The DHbetaE-sensitive component appears to be identical with agonist-stimulated 86Rb+ efflux described previously and is likely to be mediated by alpha4beta2 receptors. The DHbetaE-resistant component is a novel, active, and widely distributed response mediated by nicotinic receptor(s) that also require the beta2 subunit.

The startle response and adaptability of the startle response (prepulse inhibition and habituation) have been observed in animals. The studies reported here screened 8 inbred mouse strains to determine whether genetic factors influence these behaviors. Strain differences were found in both the sensitivity to acoustic startle and the magnitude of both the auditory and tactile startle as well as the magnitude of prepulse inhibition (PPI) of both tactile and acoustic startle. Neither the 2 startle responses nor the 2 forms of PPI were significantly correlated with one another, suggesting that different genes regulate these 2 forms of startle and PPI. Acoustic-acoustic PPI was significantly correlated, however, with hippocampal auditory gating (TC ratio) suggesting an overlap in the genes that regulate these 2 forms of sensory gating.

Progesterone and its A-ring reduced metabolites are allosteric activators of GABA(A) receptors. The studies reported here examined the effects of these steroids on brain nicotinic receptors using an 86Rb+ efflux assay that likely measures the function of alpha4beta2-type nicotinic receptors and [3H]dopamine release, which may be modulated by an alpha3-containing nicotinic receptor. Both of the A-ring reduced metabolites of progesterone were noncompetitive inhibitors of both assays, whereas progesterone inhibited only the 86Rb+ efflux assay. The 86Rb+ efflux assay was slightly more sensitive than was the dopamine release assay to steroid inhibition. Inhibition developed slowly for both assays (t1/2 = 0.4 min) and was reversed even more slowly (t1/2 = 10-15 min). Steroid addition did not alter either the rate of association of [3H]nicotine binding to brain membranes, nor was equilibrium binding changed. These findings argue that neurosteroids are allosteric inhibitors of brain nicotinic receptors.

Several previous studies have shown that 1 to 2 weeks of treatment with ethanol elicits tolerance to several effects produced by ethanol and cross-tolerance to nicotine-induced hypothermia. Similarly, short-term, high-dose nicotine treatment produces tolerance to nicotine and cross-tolerance to ethanol-induced hypothermia. In the studies reported here, C57BL/6 mice were force-fed ethanol, nicotine, or an ethanol/nicotine combination in the drinking water for 6 months. All of the chronic drug-treated mice developed tolerance to ethanol as measured by open-field activity, body temperature, and sleep-time tests. Ethanol tolerance is due, in part, to enhanced metabolism and reduced CNS sensitivity in the two ethanol-treated groups but only to reduced CNS sensitivity in the nicotine-treated group. Similar levels of tolerance to nicotine developed in those two groups that were nicotine-treated, but no tolerance to nicotine was seen in those animals treated with ethanol alone. The tolerance to nicotine may be related to an upregulation of brain (cortex, hippocampus, and hypothalamus) [3H]-nicotine binding, but ethanol tolerance is not readily explained by changes in the number of the brain high affinity nicotine binding sites.

Previous studies have shown that chronic corticosterone (CCS) treatment via subcutaneous pellets elicits reduced sensitivity to many actions of nicotine in mice as well as decreased brain alpha-bungarotoxin (alpha-BTX) binding. We report here the time courses of altered sensitivity to nicotine, as measured by acoustic startle, Y-maze crossing and rearing activities, heart rate, and body temperature, and alpha-BTX binding during and after CCS treatment. CCS treatment resulted in rapid decreases in sensitivity to nicotine for four of the five responses that were measured, as well as rapid changes in alpha-BTX binding. Sensitivity to nicotine returned to control levels within 3 days following pellet removal, but alpha-BTX binding returned to control levels in most brain regions 9-11 days after pellet removal. Because the restoration of control sensitivity to nicotine occurred long before alpha-BTX binding returned to control levels, it seems likely that factors other than changes in alpha-BTX binding cause chronic CCS-induced changes in sensitivity to nicotine.

The possible contribution of non-nicotinic cation channels to nicotine-stimulated 86Rb+ efflux was investigated. Dendrotoxin, charybdotoxin and apamin did not affect nicotine-stimulated 86Rb+ efflux. Although 4-aminopyridine and tetraethyl-ammonium completely inhibited nicotine-stimulated 86Rb+ efflux, the concentrations required to achieve this inhibition differed markedly from those required to inhibit K(+)-stimulated 86Rb+ efflux. Cs+ reduced basal 86Rb+ efflux, but did not affect nicotine-stimulated efflux. Tetrodotoxin completely inhibited veratridine-stimulated 86Rb+ efflux (IC50 = 3.4 nM) and partially inhibited (about 42%) nicotine-stimulated efflux (IC50 = 1.3 nM). Saxitoxin also completely inhibited veratridine-stimulated efflux (IC50 = 19.5 nM) and partially inhibited (about 47%) nicotine-stimulated efflux (IC50 = 5.4 nM). Tetrodotoxin had no significant effect on the EC50 for nicotine, but decreased the efflux rate and reduced modestly the rate of desensitization measured with prolonged exposure to nicotine. Tetrodotoxin (50 nM) also inhibited maximal nicotine-stimulated 86Rb+ efflux to approximately the same extent in each of ten brain regions examined. Nicotine-stimulated 86Rb+ efflux measured in the presence or absence of 100 nM tetrodotoxin is correlated with the density of [3H]nicotine, but not [3H]saxitoxin, binding sites in 12 regions, suggesting that the density of nicotinic receptors, not the number of Na+ channels, is the rate-limiting step for nicotine-stimulated 86Rb+ efflux. Therefore, activation of Na+ channels secondary to the stimulation of nicotinic receptors seems to contribute to the nicotine-stimulated 86Rb+ efflux observed in mouse brain synaptosomes. In contrast, no evidence for K+ channel modulation of nicotine-stimulated 86Rb+ efflux from thalamic synaptosomes was obtained.

Nicotine tolerance and dependence are key aspects of tobacco addiction; however, the cellular mechanisms underlying these phenomena are poorly understood. Adrenal chromaffin cells release catecholamines upon exposure to nicotine and with repeated exposure this response exhibits nicotine tolerance. Using bovine adrenal chromaffin cells in culture, we have demonstrated acute and chronic nicotine tolerance at doses relevant to that in the blood and tissues of smokers (10(-7) M to 10(-6) M). Chromaffin cells are preexposed to low doses of nicotine for time periods ranging from 10 min to 7 days and then subsequently challenged with a maximally stimulating dose of nicotine (10(-5) M) for 10 min, all at 37 degrees C. Preexposure to nicotine results in a depression of 45Ca uptake and catecholamine release upon subsequent nicotine challenge. Acute tolerance or desensitization of nicotine-stimulated catecholamine release begins to occur in minutes after preexposure to 10(-6) M nicotine at 37 degrees C. The depression of catecholamine release upon preexposure to nicotine is both dose and temperature dependent and is not seen with potassium-evoked release. Chronic exposure to 10(-7) M nicotine for 3 days led to a depression of the secretory response to approximately 70% of control responses. There was a trend toward recovery of full response by days 5 and 7 of 10(-7) M nicotine preexposure. Nearly complete depression of the nicotine-evoked release occurs within the first day of exposure to 10(-6) M nicotine and persists for at least a week of nicotine exposure at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo microdialysis was used to monitor bilaterally the release of dopamine and its metabolites 3,4-dihydroxyphenyl acetic acid (DOPAC) and homovanillic acid (HVA) in the striata of both anesthetized and awake, freely moving female rats. Under baseline conditions, an asymmetry in dopamine release was reciprocally related to an asymmetry in DOPAC. Baseline dopamine and DOPAC asymmetries were predictive of the preferred direction of amphetamine-induced rotation: the striatum having higher dopamine and lower DOPAC was contralateral to the preferred direction of rotation. Amphetamine (D-amphetamine sulfate, 1.25 mg/kg) enhanced dopamine release and decreased DOPAC and HVA; the increase in dopamine was greater in the ipsilateral striatum. Effects in anesthetized and awake rats were similar. Variations in rotation and in the dopamine asymmetry after amphetamine were correlated across time within individual awake rats.

8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is a selective 5-HT1A serotonin agonist. Derivatives of 8-OH-DPAT with amine substituents larger or more bulky than n-propyl appear to be inactive in a presynaptic biochemical assay measuring agonist-induced feedback inhibition of 5-HT synthesis but have never been examined in brain binding assays. A series of N-phenylalkyl derivatives of 8-methoxy-2-aminotetralin was evaluated at [3H]-8-OH-DPAT-labeled 5-HT1A sites in rat brain hippocampal membranes. All of the phenylalkyl derivatives displayed significant affinity for these sites and, of the agents examined, the 3-phenylpropyl 8-hydroxy analogue appears to be optimal and had an affinity (Ki = 1.9 nM) comparable to that of 8-OH-DPAT (Ki = 1.2 nM). In addition, the presence of an oxygen-containing substituent at the 8-position of the tetralin ring is not necessary for good affinity, and secondary amines and tertiary amines displayed equal affinity at central 5-HT1A binding sites. 5-HT1A sites are found both pre- and postsynaptically; thus, differences observed in the biochemical assay as compared to the results of the present binding study could be due to different structural requirements of these two receptors. This seems unlikely, however, because there was little difference in the affinities of several selected analogues for striatal versus hippocampal binding sites. Because we have now demonstrated that amine substituents larger than propyl, and an unsubstituted 8-position, are well tolerated by central 5-HT1A sites, future studies aimed at the development of new serotonergic tetralin analogues need not be limited to N-propyl or 8-hydroxy derivatives of 2-aminotetralin.