Activity of serotonin-containing neurons in nucleus raphe magnus in freely moving cats

Serotonergic neurons were recorded in the nucleus raphe magnus in freely moving cats and were initially identified on-line by their characteristic slow and regular spontaneous activity during quiet waking (3.42 +/- 0.33 spikes/s; mean +/- SE). Discharge rates of these serotonergic neurons were highest during active waking (4.49 +/- 0.40 spikes/s), intermediate during slow-wave sleep (middle: 2.14 +/- 0.23 spikes/s), and lowest during REM sleep (0.20 +/- 0.03 spikes/s). Although these cells fired at a rate 31.3% higher during active waking than during quiet waking, their activity displayed no correlation with phasic elevations of the nuchal EMG or overt body movements. In addition, no relationship was observed between the activity of these neurons during slow-wave sleep and the occurrence of sleep spindles in the cortical EEG or pontogeniculooccipital waves recorded from the lateral geniculate nucleus. Serotonergic neurons of nucleus raphe magnus were also relatively unresponsive to phasic auditory and visual stimuli, with about half of the cells examined showing weak excitatory responses. These neurons did respond, however, to the administration of a small dose of the serotonin specific agonist, 5-methoxy-N,N-dimethyltryptamine (250 micrograms/kg, i.m.) with a mean decrease in unit activity of 73.6 +/- 4.5%. The results of this study are compared with those previously reported for serotonergic neurons in the dorsal raphe nucleus, nucleus centralis superior, and nucleus raphe pallidus of freely moving cats.     

Activity of serotonin-containing neurons in nucleus centralis superior of freely moving cats

Serotonergic neurons were recorded in the nucleus centralis superior (NCS) in freely moving cats and were initially identified on-line by their slow and regular spontaneous activity (mean 2.55 +/- 0.21 spikes/s). Discharge rates of NCS serotonergic neurons were highest during active waking (AW) (mean 2.94 +/- 0.28 spikes/s), decreased during slow-wave sleep (middle of SWS: mean 1.38 +/- 0.18 spikes/s), and were lowest during REM sleep (mean 0.46 +/- 0.13 spikes/s). The activity of serotonergic NCS neurons did not significantly increase during transient elevations of the EMG during AW but did significantly decrease immediately preceding, and during the occurrence of, SWS spindles. These neurons were responsive to phasic auditory and visual stimuli, with most neurons showing excitatory responses. In response to a small dose of the serotonin-specific agonist 5-methoxy-N,N-dimethyltryptamine (50 micrograms/kg, i.m.), NCS serotonergic neurons responded with a mean decrease in unit activity of 43.9 +/- 6.1%. Among the NCS serotonergic neurons a subpopulation differed from the remaining serotonergic neurons in that they showed a much smaller decrease in unit activity across the sleep-wake cycle and responded with an inhibition of activity to phasic auditory and visual stimuli. The results of this study are compared with those previously reported for serotonergic neurons in the dorsal raphe nucleus, nucleus raphe pallidus, and nucleus raphe magnus of freely moving cats.     

Activity of serotonin-containing neurons in the nucleus raphe pallidus of freely moving cats

Serotonergic neurons within nucleus raphe pallidus (NRP) of freely moving cats initially were distinguished by their slow (< 8 Hz), regular discharge and long duration (mean = 2.3 ms) action potentials. The activity of serotonergic NRP neurons was highest during active waking (mean = 4.85 ± 0.37 spikes/s) and gradually slowed, with little change in firing pattern, during the transition from waking through slow wave sleep (middle of SWS: mean = 3.76 ± 0.36 spikes/s). In REM sleep there was a precipitous decrease in firing rate (mean = 0.92 ± 0.23 spikes/s) and loss of discharge regularity. Although there was no significant difference in firing rate between active and quiet waking, discharge rates were significantly increased during transient elevations of the EMG, but these rate increases usually were associated with specific motor behaviors only. The activity of serotonergic NRP neurons during SWS was not related to the occurrence of either sleep spindles in the cortical EEG or PGO waves recorded from the lateral geniculate nucleus. These neurons also were relatively unresponsive to phasic auditory or visual stimuli, with most of the neurons examined showing weak excitatory responses. Activity of all serotonergic NRP neurons tested was suppressed (mean = −81.3 ± 4.3%) by the serotonergic agonist 5-methoxy-N,N-dimethyltryptamine (250 μg/kg, i.m.). The results of this study are compared with those previously reported for serotonergic neurons in the dorsal raphe nucleus of freely moving cats and the issue of homogeneity in central serotonergic systems is discussed.     

A comparison of the electrophysiological and pharmacological properties of serotonin-containing neurons in the nucleus raphe dorsalis, raphe medianus and raphe pallidus recorded from mouse brain slices in vitro: role of autoreceptors

The potential role of autoreceptors in regulating the activity of serotonin-containing nucleus raphe dorsalis (RD), raphe medianus (RM) and raphe pallidus (RPA) neurons was examined by recording the activity of these neurons under a variety of conditions both in vivo and in vitro. Raphe neurons recorded in vivo displayed the characteristic slow, rhythmic discharge pattern previously described for rat and cat raphe cells. The activity of these neurons was suppressed in a dose-dependent manner by tryptophan, LSD and chlorimipramine administered intravenously. There were no significant changes in the spontaneous discharge rate of raphe neurons over time when recorded in vitro, even though tissue serotonin and its metabolite, 5-hydroxyindoleacetic acid, decreased dramatically. RPA neurons fired significantly faster than either RD or RM neurons both in vivo and in vitro. Prior depletion of brain serotonin by p-chlorophenylalanine administration resulted in no significant change in raphe unit activity recorded in vitro. Elevation of brain serotonin by monoamine oxidase inhibition produced a total inhibition of raphe unit activity in vitro. Similarly, increasing the concentration of serotonin in the tissue slice by adding serotonin directly to the incubation medium resulted in a profound, though transitory, depression of unit activity. This depressant effect of serotonin was rapidly reversible upon drug wash-out. Serotonin receptor blockers, methiothepin, cypoheptadine, and methysergide, produced no significant change in unit activity. The serotonin reuptake blocker, fluoxetine, produced a total inhibition of raphe unit activity in all three nuclei in vitro. These data suggest that excess serotonin suppresses the activity of raphe neurons, apparently by an action on autoreceptors, but that a deficiency, or normal concentration, of serotonin does not influence the spontaneous activity of these cells. The data also show that RD and RM are much more sensitive to the depressant effects of serotonin than the caudal RPA neurons. More generally, these studies provide a data base for examining the electrophysiological and pharmacological characteristics of serotonergic neurons in the three major serotonin-containing nuclei in mouse brain. The mouse has proven to be a much easier species than the rat to use in these types of studies, based on the finding that mouse brain slices are more viable in vitro than are rat brain slices.     

Time-course variations in tyrosine hydroxylase activity in the rat locus coeruleus after electrolytic destruction of the nuclei raphe dorsalis or raphe centralis.

Time-course variations in tyrosine hydroxylase activity were measured in the locus coeruleus of the albino rat after electrolytic coagulation of either the nucleus raphe dorsalis or the nucleus raphe centralis. Highly significant increases were measured at 4 days after lesioning of the raphe dorsalis (30.33%) and the raphe centralis (81.55%) compared with control values, whereas the activity in groups A9 and A10 was unchanged at this time-point. In conjunction with other experimental evidences, an hypothesis is proposed that the catecholaminergic neurons located in the locus coeruleus are directly and/or indirectly controlled by the serotonin-containing neurons located in the anterior raphe system nuclei.     

Limbic influence on the periaqueductal gray: A single unit study in the awake squirrel monkey

Single neurons of the periaqueductal gray (PAG) were studied during electrical stimulation of the amygdala and hippocampus. Fifty-one percent (34/67) of the units sampled throughout the rostrocaudal extent of the PAG were found to have a limbic influence. PAG neurons were characterized by low spontaneous firing rates (X¯= 4.94 spikes/sec). Units responded to basolateral amygdala stimulation primarily with short duration excitatory responses having a mean latency of 30 ms (range: 13.3–110 ms). Responses to corticomedial and lateral amygdala stimulation produced different patterns of activation including complex excitatory and inhibitory sequences. Only 10 units (15%) sampled in PAG responded to hippocampal stimulation with excitatory or tonic-inhibitory responses. The majority of responsive units (8) were to anterior hippocampal stimulation (latency range: = 20–75 ms). High frequency (9 Hz) basolateral amygdala stimulation recruited responses with increases in the probability of firing and a decrease in initial latency and latency variability.     

Discharge patterns of cat raphe neurons during sleep and waking

Discharge patterns of 21 single neurons of median and magnus raphe have been chronically recorded with tungsten microelectrodes stereotaxically placed in freely behaving unrestrained, unanesthetized cats. These neurons in the median and magnus raphe fired slowly during slow-wave sleep (SWS, 5.66 ± 4.24/sec), faster during awake (AWA, 11.11 ± 7.56/sec) and fastest during paradoxical sleep (PS, 15.87 ± 8.46/sec). Student'st-test (single-tailed) indicated that differences between SWS and AWA, SWS and PS were highly significant, and was significant between AWA and PS. The ratio of the firing rate of PS to SWS varied from 1.4 to 5 for the entire group. During PS the raphe units had a tendency to fire in burst. Interspike interval and joint interval distribution histograms (JIDH) also showed pronounced differences between the patterns of raphe activity during PS and SWS. Twelve units recorded from raphe magnus began to discharge more rapidly about 10–20 sec before the onset of the hippocampal theta rhythm characteristic of PS. This enhanced unit activity decreased concomitantly with the cessation of theta activity. These data indicate that neurons in raphe magnus might participate in the generation and maintenance of theta activity in dorsal hippocampus (DH) during PS. The lower single unit activity during SWS in comparison to AWA seems to disagree with Jouvet's ‘active serotonergic control of the SWS’; the increased raphe magnus activity prior to and during theta rhythm in DH gives some support to his ‘possible participation of the serotonergic caudal raphe system in PS genesis’.     

Subcutaneous administration of nicotine changes dorsal raphe serotonergic neurons discharge rate during REM sleep

In the present study nicotine (0.1 mg/kg, s.c.) increased discharge rate of putative dorsal raphe (DRN) serotonergic neurons of behaving rats during REM sleep (362.61%), without any significant change during waking and non-REM sleep. Since serotonergic DRN neurons gate PGO onset, these results suggest that nicotine-induced suppression of PGO spikes during REM sleep previously reported is achieved through stimulation of dorsal raphe serotonergic cells.     

Raphe unit activity during REM sleep in normal cats and in pontine lesioned cats displaying REM sleep without atonia

Previous studies have shown that the activity of serotonin-containing raphe neurons in cats is almost completely suppressed during rapid eye movement (REM) sleep. However, since raphe unit activity is known to be grossly correlated with the level of behavioral arousal or tonic motor activity, this decrease in activity during REM sleep may be simply due to the fact that tonic EMG activity or motoric output is at a minimum. On the other hand, raphe unit activity may be related to the state (i.e. REM sleep) of the organism. To test these competing hypotheses, in the present study we compared raphe unit activity in normal cats with that in cats that display REM sleep without atonia (produced by bilateral lesions of the pontine tegmentum). These lesioned cats manifest episodes which, by all criteria, appear to be REM sleep except that they display overt behavior, presumably because the mechanism normally responsible for producing atonia has been disrupted. Although the activity of raphe neurons in lesioned cats during REM sleep without atonia was significantly below that seen in these cats during waking, the level of activity was often impressive. This is especially true when those animals that displayed the greatest degree of tonic motor activity during REM sleep (group IV animals) are considered separately. In these cats, the depression was only 40.5% below their quiet waking level, whereas in lesioned cats displaying less tonic motor activity (Group II animals), raphe discharge rate was 65.6% below their quiet waking level. The discharge rate of raphe neurons during REM sleep in lesioned cats was more than 6-fold greater than that seen in normal animals. These data, in conjunction with other recent results from our laboratory, suggest that the decrease in raphe unit activity during REM sleep is largely a concomitant of the atonia which characterizes that state. These data are discussed within the general context of the relationship between raphe unit discharge and the activity of central motor systems.     

Fluorescence histochemistry of monoamine-containing cell bodies in the brain stem of the squirrel monkey (Saimiri sciureus). 3. Serotonin-containing groups

The distribution and cytological characteristics of the serotonin-containing cell bodies in the brain stem of the squirrel monkey (Saimiri sciureus) are described using the fluorescence histochemical technique of Falck and Hillarp for the demonstration of monoamines. Eight groups of serotonin-containing neurons were found, located predominantly in the midline raphe region of the brain stem within defined nuclei, such as the nucleus raphe obscurus, nucleus raphe pallidus, nucleus raphe magnus, nucleus raphe pontis, nucleus raphe dorsalis and nucleus centralis superior. Portions of some of the groups also extend laterally in the brain stem, particularly at the levels of the upper pons and midbrain. The appearance and distribution of the serotonin-containing groups of cell bodies in the squirrel monkey were found to be somewhat similar to that described for the rat, but differed in at least one major respect from that reported for the cat.     

Spinal reflexes and lateral geniculate nucleus activity during sleep: Quantitative relationships

The relationship between monosynaptic spinal reflex (MSR) activity and monophasic spike activity of the lateral geniculate nucleus (LGN) was studied in unanesthetized, unrestrained cats during spontaneous sleep. Relative to MSR modulation during slow-wave sleep (SWS), modulation during paradoxical sleep (PS) was toward decreased reflex amplitude within 500 ms subsequent to the onset of isolated LGN spikes (t₀). These modulations, although statistically significant, were not particularly pronounced and were not always consistent for all animals. Reflex amplitude decreased in the 1 s following onset of either small (three to four spikes) or larger (≥ 5 spikes) spike bursts, but this reduction was significant only for the latter group. Consistent changes in reflex amplitude did not outlast brief spike bursts even when additional isolated spikes occurred in the extended sample interval (2.2 e post-t₀). Extended modulation (5 s post-t₀) of reflex activity was detected for bursts of ≥ 14 spikes. Whereas previous reports indicated variations in MSR activity only in association with bursts of LGN spikes, the present data suggest state-dependent modulation of such activity for isolated and low-frequency bursts as well as high-frequency spike bursts. A possible role for the caudal raphe nuclei in the generation of low frequency pontogeniculate (PGO) activity and related reflex variations is postulated, and bases for discrepancies between phasic MSR modulation during synchronized sleep in man and cat are explored.     

Dissociations between the effects of hallucinogenic drugs on behavior and raphe unit activity in freely moving cats

The hypothesis that the action of hallucinogenic drugs is mediated by a depression of the activity of brain serotonergic (raphe) neurons was testedby examining the behavioral effects of several hallucinogenic drugs while concurrently monitoring the activity of raphe neurons in freely moving cats. LSD produced a dose-dependent decrease in raphe unit activity and a dose-dependent increase in certain behaviors (e.g. limb flick and abortive groom), and the peak of the behavioral and unit changes were temporally correlated. However, there were three important dissociations between the behavioral and electrophysiological effects of LSD. Firstly, low doses of LSD produced only small decreases in raphe unit activity but significant behavioral changes. Secondly, the duration of LSD-induced behavioral changes significantly outlasted the depression of raphe unit activity. And thirdly, raphe neurons were at least as responsive to LSD during tolerance as they were in the nontolerant condition. Psilocin produced a dose-dependent decrease in raphe unit activity, while the behavioral changes were not dose-related. However, the peak behavioral changes corresponded to the maximal depression of raphe unit activity. The phenylethylamine hallucinogens. DOM and mescaline, both produced large behavioral changes but no overall effect on raphe neurons. Following administration of DOM or mescaline, some raphe units showed a significant increase, while some showed a significant decrease, othrs showed a significant increase, while some phenylethylamine hallucinogens may exert a depressant effect upon a subset of serotonin-containing neurons, and an amphetamine-like excitatory effect upon an other subset of these neurons. Consistent with previous studies, all hallucinogens produced a high concentration of slow waves in the cortical EEG. Following administration of LSD or psilocin, the appearance of slow waves in the EEG was often associated with a transitory decrease in unit activity, while this was not observed for the phenylethylamine hallucinogens. The present data, in conjunction with recent data from other laboratories, suggest that the serotonin hypothesis of hallucinogenic drug action should be re-evaluated.     

Effects of electrical stimulation of the lateral habenula on single-unit activity of raphe neurons.

Recent anatomical studies with horseradish peroxidase injections into the anterior raphe have demonstrated that the nucleus raphe dorsalis in the rat receives a major afferent input from the lateral habenula (LHb). The present study examined electrophysiologically the effects of electrical stimulation of the LHb on the spontaneous activity of midbrain and anterior pontine raphe units in anesthetized rats. The results showed that: (a) LHb stimulation (1 or 10 Hz, 0.5 to 1.0 mA) suppressed the activity of most raphe units, with the effects outlasting the duration of the stimulation in some instances; the raphe cells which showed periods of suppression during LHb stimulation were both those of the classical serotonin type (N = 26), characterized by slow regular baseline firing rates, and other raphe cells (N=52) with faster baseline rates (to 60/s); (b) inhibition of unit activity was much less pronounced for non-raphe cells lateral to the midline; (c) anatomical control stimulation points dorsal to the LHb did not alter raphe unit activity; and (d) the pathway from the habenula to the raphe may involve a dorsal route. After a knife cut through the superior colliculus-central gray at the level of the interpeduncular nucleus, the effects of habenular stimulation were substantially reduced. Conversely, stimulation of the superior colliculus just posterior to the habenula (presumably containing descending fibers from the habenula) markedly suppressed raphe unit activity. In summary, the present electrophysiologic findings were consistent with the view that activation of habenular afferent fibers to the raphe exerted a major inhibitory influence on the spontaneous activity of midbrain and pontine raphe neurons. Considerably smaller effects were exerted on lateral reticular cells. A dorsal pathway may be involved in mediating the habenular effects on raphe activity.     

An ascending serotonergic pain modulation pathway from the dorsal raphe nucleus to the parafascicularis nucleus of the thalamus

(1) Three types of spontaneously active neurons were found in the parafascicularis (PF) nucleus of the thalamus of the rat: slow firing units (0.5–10 spikes/s), bursting units (2–5 spikes/burst in 10–20 ms, one burst every 1–2 s) and fast firing units (15–40 spikes/s). A similar population of neurons was found in the PF of rats treated with 5,7-dihydroxytryptamine (5,7-DHT), a serotonin neurotoxin.

(2) Noxious tail pinch (TP) caused 68% of the PF neurons to increase their firing rates to 242% of their initial baseline activity, while non-noxious touch stimulation failed to induce a response. In the 5,7-DHT-treated rats, TP caused 85% of the neurons in the PF to increase their firing rates to 581% of their initial baseline activity and 22% of the neurons increased their firing in response to touching the tail. Both the number of cells responding (P < 0.05) and the percentage increase (P < 0.001) were statistically greater in serotonin-depleted rats than in controls. This indicates that serotonin (5-HT) has a tonic inhibitory influence on responses to both noxious and non-noxious sensory stimuli.

(3) In control rats, electrical stimulation of the dorsal raphe nucleus (DR) decreased the firing rates of PF neurons. In contrast, the same DR stimulation induced an increase in PF firing rates during stimulation in serotonin-depleted rats and this increase in firing rates remained several seconds after cessation of stimulation. This indicates that the DR may use at least two different neurotransmitters in its projections to forebrain structures.

(4) In control rats, the TP stimulation induced an increase in firing rates of PF neurons while DR stimulation attenuated the excitation induced by TP stimulation. In serotonin-depleted rats, DR stimulation and TP both caused an increase in firing rates. This effect was not additive indicating that there may be a serotonergic projection from the DR to the PF which modifies responses to somatosensory stimuli.

(5) The inhibitory effects elicited by electrical stimulation were limited to the immediate area of the DR. Stimulation of the adjacent reticular formation 1 mm lateral to the DR produced the opposite effect, an increase in firing rate often accompanied by driven spike activity in the PF.

Firing of 'possibly' cholinergic neurons in the rat laterodorsal tegmental nucleus during sleep and wakefulness.

To clarify functional roles of mesopontine cholinergic neurons as a component of an activating system, single neuronal activity in the laterodorsal tegmental nucleus (LDT) of undrugged rats, whose head was fixed painlessly, was recorded along with cortical EEG and neck EMG. Activity of some dorsal raphe (DR) neurons was also recorded for comparison. Most of the animals had been sleep-deprived for 24 h. Observation was made only on neurons generating broad spikes, presumed from previous studies to be cholinergic or monoaminergic. The position of recorded neurons was marked by Pontamine sky blue ejected from the glass pipette microelectrode, and was identified on sections processed for NADPH diaphorase histochemistry which specifically stained cholinergic neurons. According to their firing rates during wakefulness (AW), slow-wave sleep (SWS) and paradoxical sleep (PS), 46 broad-spike neurons in the LDT were classified into 4 groups: (1) neurons most active during AW and silent during PS (some of these neurons might be serotonergic rather than cholinergic, as all the 9 neurons in the DR); (2) neurons most active during PS and silent during AW; (3) neurons equally more active during AW and PS than SWS; and (4) others mainly characterized by transiently facilitated activity at awakening and/or onset of PS. Neurons of groups 2 and 3 were the major constituents of the LDT. In most neurons change in firing preceded EEG change, except at awakening from PS. These results suggest that: (1) the LDT is composed of cholinergic neurons with heterogenous characteristics in relation to sleep/wakefulness; and (2) some tegmental cholinergic neurons play a privotal role in induction and maintenance of PS.     

Firing of ‘possibly’ cholinergic neurons in the rat laterodorsal tegmental nucleus during sleep and wakefulness

To clarify functional roles of mesopontine cholinergic neurons as a component of an activating system, single neuronal activity in the laterodorsal tegmental nucleus (LDT) of undrugged rats, whose head was fixed painlessly, was recorded along with cortical EEG and neck EMG. Activity of some dorsal raphe (DR) neurons was also recorded for comparison. Most of the animals had been sleep-deprived for 24 h. Observation was made only on neurons generating broad spikes, presumed from previous studies to be cholinergic or monoaminergic. The position of recorded neurons was marked by Pontamine sky blue ejected from the glass pipette microelectrode, and was identified on sections processed for NADPH diaphorase histochemistry which specifically stained cholinergic neurons. According to their firing rates during wakefulness (AW), slow-wave sleep (SWS) and paradoxical sleep (PS), 46 broad-spike neurons in the LDT were classified into 4 groups: (1) neurons most active during AW and silent during PS (some of these neurons might be serotonergic rather than cholinergic, as all the 9 neurons in the DR); (2) neurons most active during PS and silent during AW; (3) neurons equally more active during AW and PS than SWS; and (4) others mainly characterized by transiently facilitated activity at awakening and/or onset of PS. Neurons of groups 2 and 3 were the major constituents of the LDT. In most neurons change in firing preceded EEG change, except at awakening from PS. These results suggest that: (1) the LDT is composed of cholinergic neurons with heterogenous characteristics in relation to sleep/wakefulness; and (2) some tegmental cholinergic neurons play a pivotal role in induction and maintenance of PS.     

Evidence for two types of firing pattern during the sleep-waking cycle in the reticular thalamic nucleus of the cat

The activity of 23 neurons was recorded extracellularly in the nucleus reticularis thalami (RT) of four chronically implanted cats. Patterns of discharge were studied and their relationship to wakefulness (W), slow-wave sleep (SS), and paradoxical sleep (PS) were examined. The firing pattern was analyzed using zero-order (firing rate), first-order (interspike interval histogram), and second-order (joint interval histogram and autocorrelogram histogram) statistics. The SS bursting pattern was investigated taking into account the duration of the intervals between the bursts, the number of spikes per burst, and the duration of the bursts. Zero- and first-order characterizations during W and PS were found to be comparable. However, the joint interval histogram revealed a state-specific pattern during W for 74% of the cells. This pattern was characterized by a nonrandom occurrence of some categories of adjacent intervals. This was not found during PS. Two-thirds of the cells recorded, called “fast” neurons, exhibited firing rates higher than 15 spikes/s during W and PS. Those remaining, called “slow” neurons, showed a mean discharge rate lower than 10 spikes/s. In SS “fast” neurons fired long-duration bursts with interburst intervals generally shorter than 1 s. Conversely, “slow” neurons discharged shortduration bursts interrupted by long interburst intervals (greater than 1 s). Nevertheless, both types of cells had the same number of spikes within the burst and a similar intraburst pattern. Approximately half of the cells studied depicted a “slow rhythm” in the autocorrelogram histogram. Its periodicity was independent of firing rates and behavioral states.     

Dopamine-containing ventral tegmental area neurons in freely moving cats: activity during the sleep-waking cycle and effects of stress

The activity of dopamine-containing ventral tegmental area (VTA) units was recorded by means of movable 32- and 64-microns-diameter insulated Nichrome wires in freely moving cats. The VTA units displayed a slow, somewhat irregular activity during quiet waking (mean 3.63 +/- 0.41 spikes/s) and showed no significant change in activity during slow-wave sleep or REM sleep. Although VTA unit activity was somewhat higher and more erratic during active waking, there was no relationship between unit discharge and phasic movement. These neurons were inhibited (-87%) by small doses of apomorphine (1.0 mg/kg, i.p.) and excited (+43%) by small doses of haloperidol (0.5 mg/kg, i.p.). The stress of a conditioned emotional reaction (CER) paradigm resulted in a significant increase in the discharge rate of VTA neurons (+39%), compared with the quiet-waking baseline. The CER paradigm increased plasma glucocorticoids by 74%. Neurochemical studies revealed that the CER paradigm resulted in a significant decrease of dopamine in the limbic forebrain (-31%), whereas both homovanillic acid (+47%) and dihydroxyphenylacetic acid (+43%) concentrations were increased. No significant changes in dopamine metabolism were observed in the striatum under the CER situation. These data have implications in relation to the role of stress and dopamine in mediating certain psychiatric disorders.     

Raphe dorsalis-spinal cord cografts in oculo: electrophysiological evidence for an excitatory serotonergic innervation of transplanted spinal neurons

Intraocular replicas of descending serotonergic bulbospinal pathways were constructed by means of sequential intraocular grafting of nucleus raphe dorsalis and spinal cord. Using extracellular recordings we have studied the functional connections between such double grafts. Superfusion of single spinal cord grafts with serotonin causes an increase in spontaneous activity. This excitation is reversibly blocked by the specific 5-hydroxytryptamine (5-HT) antagonist metergoline. Stimulation of the raphe part of nucleus raphe dorsalis-spinal cord double grafts causes a long-lasting excitation of the spinal neurons similar to that seen in single spinal cord grafts given serotonin. The electrically induced excitation could also be reversibly blocked with metergoline. It is concluded that serotonin-containing nerves from grafts of nucleus raphe dorsalis are not only morphologically organotypic, but also form functional contacts with neurons in cografted spinal cord. The results further support an excitatory or modulatory role of the descending spinal serotonergic pathways and demonstrate that functional contacts can be established between isolated CNS grafts when 5-HT fibers invade immature or mature spinal cord tissue.