Search

The response of noradrenergic (NE), serotonergic (5-HT), and dopaminergic (DA), neurons to repeated presentations (once/2 s for 64 trials) of phasic auditory (click) and visual (flash) stimuli was examined in freely moving cats. All 3 groups of neurons displayed similar response latencies and somewhat similar durations of excitation to both stimulus modalities. 5-HT neurons in the mesencephalic raphe nuclei showed no decrease in responsiveness across trials to either of the stimuli. DA neurons in the substantia nigra displayed no decrease in responsiveness across trials to the auditory stimulus, but did display an approximately 50% decrease in response to the visual stimulus. NE neurons in the locus coeruleus showed an approximately 50% decrease in responsiveness to both the auditory and visual stimuli. These data are consistent with previous studies showing that the response of many neurons in the brainstem reticular formation habituates to the repetitive presentation of sensory stimuli. They also show that the response of reticular formation neurons are heterogeneous and that they can be subdivided on the basis of their neurochemical identity. Finally, these data provide support for the involvement of NE neurons, and to a lesser extent DA neurons, in various forms of behavioral plasticity.

Extracellular levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were monitored by microdialysis in the hippocampal formation previously denervated of its serotonergic input by an intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT), and in 5,7-DHT denervated hippocampi reinnervated by grafted fetal rat serotonin neurons. Two weeks after 5,7-DHT lesion, baseline 5-HT release was reduced to levels below detection, and KCl- and p-chloro-amphetamine-evoked release was reduced by 90-95%. In the chronically denervated hippocampus (3 months after lesion), baseline 5-HT release had recovered to near-normal levels, but KCl- and p-chloroamphetamine-evoked release remained severely impaired. Addition of the 5-HT re-uptake blocker indalpine to the perfusion medium induced a 5-6-fold increase in serotonin overflow in the normal hippocampus, while the serotonin overflow in the 5,7-DHT denervated hippocampus remained unaffected. The intrahippocampal fetal raphe transplants restored 5-HT release to near-normal levels, not only under baseline conditions but also in the presence of re-uptake blockade. Both KCl- and p-chloroamphetamine-induced release had recovered in the grafted hippocampus and the responses were even greater than those seen in normal animals. In both normal and grafted hippocampus addition of the sodium channel blocker tetrodotoxin reduced 5-HT overflow to the level seen in the denervated hippocampus. The new hippocampal serotonin innervation, established by the grafts, was markedly denser than normal, and the tissue 5-HT and 5-HIAA levels were 3-4-fold higher than normal in the grafted hippocampi. The 5-HIAA level in the perfusate collected from the grafted hippocampi showed a similar increase above normal, whereas 5-HT release was maintained within the normal range, both under baseline conditions and in the presence of re-uptake blockade. The results indicate that the grafted serotonergic raphe neurons are spontaneously active at the synaptic level, despite their ectopic location. The ability of the grafted neurons to maintain 5-HT release within the normal range suggests that local regulatory mechanisms at the terminal level can compensate for abnormalities in the graft-derived innervation density.

Single unit activity of serotonergic neurons in the dorsal raphe nucleus of freely moving cats was recorded in experiments which manipulated central or peripheral motor systems. Unilateral microinjections of the cholinomimetic agent, carbachol, into the pontine tegmentum, produced muscle atonia. During these periods of drug-induced atonia, the activity of serotonergic neurons was reduced 97% below pre-drug baseline rates. In experiments where microinjections of carbachol did not produce muscle atonia, no significant change occurred in serotonergic unit discharge rate. Muscle tonus was also altered by systemic injections of mephenesin, a centrally acting muscle relaxant. A low dose of mephenesin (50 mg/kg) produced mild atonia which was correlated with a 16% reduction in serotonergic neuron discharge rate relative to pre-drug baseline. A higher dose of mephenesin (150 mg/kg) produced complete atonia, during which serotonergic unit activity was reduced by 68% below baseline firing rate. To distinguish between centrally and peripherally induced atonia, we injected either succinylcholine or dantrolene, systemically. These are both drugs whose muscle relaxant properties are known to be mediated by peripheral mechanisms. In neither case was a change in serotonergic unit discharge rate seen following drug-induced atonia. These data demonstrate that manipulation of central, but not peripheral, motor systems can profoundly affect the activity of serotonergic neurons of the dorsal raphe nucleus. Alternate hypotheses are also discussed.

Single unit activity of dopaminergic neurons in the substantia nigra was recorded in freely moving cats under a variety of conditions. These neurons displayed their highest discharge rate during active waking (3.68 +/- 0.30 spikes/s), which was 20% greater than their discharge rate during quiet waking (3.07 +/- 0.20). Although these cells fired somewhat faster during active waking, their activity displayed no correlation with phasic EMG changes, and, in general, their activity showed little relationship to overt behavioral changes. As the cat progressed from quiet waking through slow-wave sleep and REM sleep there was no significant change in either the rate or pattern of firing of dopaminergic neurons. In addition, no correlation was observed between the activity of these neurons and either sleep spindles or PGO waves. These neurons did respond, however, to the repeated presentation of a click or light flash with excitation followed by inhibition, with no evidence of habituation. One of the most impressive changes in dopaminergic unit activity was a large decrease in association with orienting responses. This was seen in over 50% of the cells in which this relationship was examined. As the behavioral orientation habituated with repeated stimulus presentation, so did the associated dopaminergic unit suppression. In conclusion, dopaminergic neurons maintain a remarkably constant rate and pattern of firing across a variety of behaviors and states. However, this stability can be dramatically altered under special circumstances, such as during and following orienting responses.

The maturation and growth of fetal serotonergic raphe neurons have been studied immunohistochemically and biochemically between 1 week and 5 months after grafting to the hippocampal formation in 5,7-dihydroxytryptamine-pretreated adult rats. The average number of surviving neurons in each group was 1800, which is equivalent to approximately 20% of the potential number of serotonin neurons contained in the grafted cell suspension. The fetal raphe cells, which were taken from 12-14-day-old embryos, had developed strong serotonin immunoreactivity at 1 week after transplantation, and the number of serotonin cells present at 1 week was similar to that found at later time points. Fiber outgrowth was demonstrable already at 1 week but the serotonin-positive fibers were restricted to the areas close to the graft. Single fibers, however, could be traced for distances of up to 500-800 microns into the host hippocampus and dentate gyrus. At later time points, the graft-derived serotonin-immunoreactive fiber network extended to cover the entire hippocampal formation. At the longest postoperative time point, 7 weeks and 5 months, some of the animals exhibited extensive hyperinnervation patterns throughout the dorsal parts of the hippocampus and the dentate gyrus. Consistent with these immunohistochemical observations, supranormal serotonin levels developed with time after transplantation in the grafted hippocampi from an average of 5% of normal at 1 week, to 28% of normal at 3 weeks, 146% of normal at 7 weeks, and 216% of normal at 5 months. Although the recovery of 5-hydroxyindoleacetic acid (5-HIAA) paralleled that of serotonin (5-HT), the increase in the metabolite concentrations was less than that of the amine, indicating a change in the turnover or metabolism of serotonin in the grafted neurons over time. Thus, the 5-HIAA:5-HT ratio was higher than normal at 3 weeks post-grafting (when the host hippocampus was only partially reinnervated); it was similar to normal at 7 weeks, and it tended to be lower than normal in the hyperinnervated specimens at 5 months' survival. A regression analysis revealed a significant inverse correlation between the hippocampal 5-HT concentration and the 5-HIAA:5-HT ratio in the graft-reinnervated hippocampal formation. In conclusion, the grafted serotonergic raphe neurons, in contrast to other types of aminergic neurons, exhibited a prominent tendency to form extensive hyperiinnervation patterns in the previously dennervated host target.(ABSTRACT TRUNCATED AT 400 WORDS)

Striatal extracellular levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were monitored with the microdialysis technique during electrical stimulation of the lateral habenula-dorsal raphe (LHb-NRD) pathway in halothane anaesthetized rats. A new double-loop probe, with an improved recovery factor, was implanted into the head of the caudate-putamen and perfused with Ringer solution containing 1 microM of the 5-HT uptake blocker indalpine. Samples were collected every 15 min and analyzed with HPLC coupled to fluorimetric detection. Low frequency stimulation of the LHb (1.5 and 3 Hz, 0.5 mA) produced no detectable changes in striatal indole levels, whereas 15 Hz stimulation induced a 70% increase in 5-HT release. This effect was most likely mediated by a direct LHb-NRD link, since it persisted after ibotenic acid lesions of the interpeduncular nucleus (which is the major projection area for the medial habenular nucleus), but was completely abolished after transection of the fasciculus retroflexus, which carries the axons of the LHb-NRD pathway. The possible identity of the transmitter operating in the LHb-NRD pathway was investigated by NRD injections of kynurenic acid, a potent blocker of excitatory amino acid transmission, and by NRD injections of the GABA antagonist bicuculline. Kynurenic acid (300 nl, 50 mM) did not by itself induce any detectable changes in spontaneous indole output, but completely blocked the effect of LHb stimulation. Injection of bicuculline (300 nl, 2 mM) increased the striatal 5-HT output by about 70%, and potentiated the effect of LHb stimulation by a further 50%. In none of the experiments performed in this study were there any significant changes in striatal 5-HIAA output. These data are compatible with the idea that excitatory amino acids in the LHb-NRD pathway are involved in the regulation of striatal 5-HT release, and that this influence is modulated by GABAergic synaptic activity at the level of the NRD.

The in vitro culture approach was combined with the cell suspension grafting technique to examine whether the maturation of dopamine (DA) neurones in vitro imposed similar limitations on their ability to survive grafting as when they are allowed to develop in situ in the fetus. The functional capacity, survival and growth of DA neurones from 2.5- and 7-day-old cultures, grafted to rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway, was compared with similar grafts freshly prepared from fetal donors of embryonic days 14, 16 and 20. Grafts of freshly dissociated mesencephalic DA neurones, taken from embryonic day 14-16 donors and 2.5-day-old cultures, generally survived well and markedly reduced amphetamine-induced rotational asymmetry in the recipient rats. However, when cultured for 7 days prior to grafting, or when taken from 20-day-old fetuses, the mesencephalic DA neurones survived very poorly and the grafts did not have any functional effects. Plating of aliquots of cell suspension used for grafting indicated that the survival rate of dissociated DA neurones is in the same order of magnitude when grown in vitro (about 2 DA neurones per 1000 cells) as when grafted in vivo to the rat striatum (about 1-5 DA neurones per 1000 cells). When the number of surviving grafted DA neurones was plotted against the behavioural effects of the grafts, a threshold number of around 100-200 DA neurones was found necessary to obtain a marked reduction (greater than 50%) in amphetamine-induced rotational asymmetry. Moreover, the survival of 300-500 DA neurones seemed to produce a 'ceiling effect' beyond which additional surviving DA neurones gave rise to little or no further effect on the amphetamine-induced rotational behaviour.

Quantitative analyses of the single unit responses to sensory stimuli were made in caudate nucleus neurons in the unanesthetized freely moving cat. Over half of the cells were responsive to repetitive presentation of simple sensory stimuli. Typically, neurons gave similar responses to both auditory (click) and visual (flash) stimuli. This study confirms previous reports of the polysensory response characteristics of caudate nucleus neurons, and extends these observations to the freely moving animal.

The response of dopaminergic neurons of the substantia nigra pars compacta to auditory clicks continuously presented across the sleep-wake cycle was studied in cats. The initial excitatory followed by inhibitory response to the click which occurred during quiet waking diminished as the cat progressed into slow-wave sleep and was absent during REM sleep. Upon awakening from REM sleep, dopamine neurons once again displayed an excitatory/inhibitory response to the clicks, implying that the decrease across the sleep-wake cycle was not attributable to long-term habituation.

Single-unit activity of dopaminergic neurons in the substantia nigra was recorded in freely moving cats during a variety of conditions designed to shed light on the hypotheses that these neurons are involved in the regulation of arousal-stress and/or selective attention. Both aversive and non-aversive arousing experimental conditions were used, including tail pinch, immersion of feet in ice-water, white noise, inaccessible food, feeding, grooming, inaccessible rats, and somatosensory stimulation. None of these conditions had an effect on tonic neuronal discharge rate. However, these neurons did exhibit brief excitatory and inhibitory responses to phasic auditory or visual stimuli presented when the cat was sitting quietly. These responses were dramatically attenuated if these stimuli were presented during the aforementioned conditions of behavioral arousal. This sharply contrasts with the inability of these same conditions to influence spontaneous discharge rate. The sensitivity of this neuronal sensory response to the concurrent behavioral condition supports the hypothesis that these neurons are involved in attentional processes or selective responding. The lack of responsiveness of these neurons to a variety of arousal/stress manipulations supports the hypothesis that dopaminergic neurons play a permissive, rather than an active, role in these processes.

The purpose of the present experiments was to study the effects of a combined cholinergic and serotonergic denervation of the rat forebrain on spatial learning using the Morris water maze task. Experiment 1 compared the acute effects of a radiofrequency lesion of the septum, an intraventricular 5,7-dihydroxytryptamine (5,7-DHT) lesion, and a combined septal plus 5,7-DHT lesion. Although the 5,7-DHT lesion alone did not produce any significant deficits in the water maze task, the lesion greatly potentiated the learning impairments produced by the septal lesion. Thus, the rats with both lesions combined showed severe difficulties in finding the platform and they did not develop any place navigational search strategy. This effect was not dependent on any effect on swimming ability or locomotor activity. The long-term effects of the combined septal and 5,7-DHT lesion was investigated in experiment 2, where the rats were tested in the water maze both 5 and 24-25 weeks after surgery. In this experiment, the rats showed the same severe deficits in spatial learning in both tests, showing that the impairments remain for long periods and after extended training. The results show that a combination of a cholinergic and a serotonergic denervation of the rat forebrain produces pronounced impairments in spatial learning in the Morris water maze task, and that this effect is long-lasting. This indicates that the recently proposed serotonergic deficit in patients with Alzheimer's disease may contribute importantly to the cognitive disabilities in these patients.

The activity of dopaminergic neurons in the substantia nigra was recorded from freely moving cats during feeding and satiety, and following injections of glucose. At no time during feeding or in the postprandial satiety period was there a significant increase or decrease in firing rate of these neurons relative to baseline. Additionally, no change in firing rate was observed following injections of glucose (300, 500 and 1000 mg/kg) or glucose in combination with insulin (300 mg/kg glucose and 0.8 units/kg insulin).

The septal/diagonal band (SDB) area, obtained from a 9- to 10-week-old aborted human fetus, was grafted to the hippocampal formation of adult, immunosuppressed rats subjected to an aspirative lesion of the fimbria-fornix. Nineteen weeks after transplantation, microscopical analysis revealed large, partly acetylcholinesterase (AChE)-positive grafts in the hippocampus in 3 of the 5 recipients. The AChE-positive grafts gave rise to a reinnervation of the host hippocampus and an AChE-positive lamination of the different hippocampal subfields with the same characteristics as the normal septum-derived innervation. Immunological evaluation of host sera revealed that all rats were immunized by the graft. This indicates that grafted human cholinergic SDB neurons can respond to or interact with factors that regulate and guide the innervation of the rat hippocampus.