Effects of standardized extracts of St. John's wort on the single-unit activity of serotonergic dorsal Raphe neurons in awake cats: comparisons with fluoxetine and sertraline

St. John's wort is widely used as an herbal remedy for depression. Although its mechanism of action remains unknown, some evidence suggests that St. John's wort might act via brain serotonin (e.g., as a serotonin reuptake inhibitor). To determine whether St. John's wort affects the central serotonergic system, we monitored the discharge rate of serotonin-containing neurons in the dorsal raphe nucleus of awake cats following systemic administration of two clinical preparations of St. John's wort, Jarsin 300 (15-600 mg/kg, p.o.) and Hyperforat (0.5-4.0 ml, i.v.). Both preparations were found to have no effect on neuronal activity. This contrasts sharply with the action of fluoxetine and sertraline (2 mg/kg, p.o.), two selective serotonin reuptake inhibitors (SSRIs), which markedly depressed neuronal activity by increasing the synaptic availability of serotonin at inhibitory somatodendritic 5-HT(1A) autoreceptors. The failure of St. John's wort to depress neuronal activity cannot be attributed to an impairment of the 5-HT(1A) autoreceptor mechanism, since pretreatment with Jarsin 300 (300 mg/kg, p.o.) did not alter the responsiveness of serotonergic neurons to the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10 microg/kg, i.v.). Overall, these findings indicate that the mode of action of St. John's wort is different from that of conventional antidepressant drugs, which elevate brain serotonin and evoke negative feedback control of serotonergic neurons.

Venlafaxine and its interaction with WAY 100635: effects on serotonergic unit activity and behavior in cats

The therapeutic efficacy of antidepressant drugs that inhibit the reuptake of serotonin (5-hydroxytryptamine, 5-HT) may be enhanced by blocking their indirect activation of 5-HT(1A) autoreceptors, which mediate feedback inhibition of serotonergic neuronal activity. In this study, we examined the effects of venlafaxine, a dual 5-HT/noradrenaline reuptake inhibitor, alone and in combination with the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (WAY 100635), on the single-unit activity of serotonergic dorsal raphe neurons and concurrent behavior in freely moving cats. Systemic administration of venlafaxine (0.05-1.0 mg/kg, i.v.) produced a dose-dependent decrease in firing rate (ED(50)=0.19 mg/kg), with virtually complete inhibition of neuronal discharge at the highest dose tested. The subsequent administration of WAY 100635 (0.1 mg/kg, i.v.) rapidly reversed the neuronal suppression produced by venlafaxine and significantly elevated the firing rate above baseline levels. The overshoot in neuronal activity was associated with the onset of an adverse behavioral reaction resembling the 5-HT syndrome resulting from excessive levels of brain 5-HT. The intensity of this reaction paralleled the degree of neuronal restoration induced by WAY 100635, suggesting a causal relationship. Such behavioral responses were either not observed previously, or of a low intensity, when WAY 100635 was combined with selective 5-HT reuptake inhibitors. Overall, these results suggest that the risk of inducing adverse effects, such as the 5-HT syndrome, may be higher with dual 5-HT/noradrenaline reuptake inhibitors than with selective 5-HT reuptake inhibitors, when these agents are combined with a potent 5-HT(1A) autoreceptor antagonist. Possible mechanisms that might account for these differences in drug interaction are discussed.

Single-unit responses of serotonergic medullary and pontine raphe neurons to environmental cooling in freely moving cats

Brain serotonin has long been implicated in the regulation of body temperature, although its precise role is not completely understood. The present study examined the effects of environmental cooling (4-8 degrees C for 2 or 4h) on the single-unit activity of serotonergic neurons recorded in the medullary raphe nuclei obscurus and pallidus and in the pontine dorsal raphe nucleus of freely moving cats. These neuronal groups have primarily descending projections to the spinal cord and ascending projections to the forebrain, respectively. Cold exposure induced shivering and piloerection, but no appreciable changes in core temperature. Of the medullary serotonergic cells studied (n=14), seven were activated and seven were unresponsive to cold exposure. For the responsive cells, the mean increase and peak effect in unit activity relative to baseline were 31% and 46%, respectively. Of the seven cold-responsive cells, the activity of four was monitored when the animals were transferred back to room temperature (23 degrees C). Within 15-30 min, the activity of these cells returned to baseline. In contrast, none of the dorsal raphe nucleus cells studied (n=14) displayed a significant change in neuronal activity during cold exposure, suggesting that these neurons do not receive afferent input from cold-sensitive cutaneous receptors or participate in thermoregulatory responses evoked by low ambient temperatures.Overall, these results suggest that a subset of medullary serotonergic neurons play a role in physiological mechanisms underlying cold defense (e.g. increases in motor output and/or autonomic outflow). On the other hand, the lack of responsiveness of serotonergic dorsal raphe nucleus neurons to cold exposure does not support a specific role for these cells in thermoregulation.

Pindolol suppresses serotonergic neuronal activity and does not block the inhibition of serotonergic neurons produced by 8-hydroxy-2-(di-n-propylamino)tetralin in awake cats

Clinical studies have shown that pindolol can enhance the effects of antidepressant drugs, presumably by acting as an antagonist at somatodendritic 5-hydroxytryptamine (5-HT)(1A) autoreceptors, which regulate the firing rate of central serotonergic neurons. The current study characterized the action of pindolol on the single-unit activity of serotonergic neurons in the dorsal raphe nucleus of freely moving cats. (+/-)-Pindolol produced a dose-dependent inhibition of neuronal activity after i.v. (ED(50) = 0.25 mg/kg) and s.c. (ED(50) = 1.23 mg/kg) administration. The active enantiomer (-)-pindolol (1 mg/kg i.v.) also suppressed neuronal activity (maximal decrease, 88%). Upon p.o. administration, (+/-)-pindolol (10 mg/kg) produced a marked, long-acting suppression of neuronal activity similar to that observed after s.c. administration. In all cases, the reduction in firing rate produced by pindolol was completely reversed by low doses of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)cycloh exanecarboxamide (WAY-100635) (0.1 mg/kg i.v. or 0.2 mg/kg s.c.), a selective 5-HT(1A) antagonist. Systemic administration of (-)-tertatolol (1-5 mg/kg i.v.), another beta-adrenoceptor blocker/putative 5-HT(1A) antagonist, had no significant effect on neuronal activity. The ability of i.v. (+/-)-pindolol (0.1-1 mg/kg) to reverse the suppression of serotonergic neuronal activity produced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10 microg/kg i.v.), a selective 5-HT(1A) agonist, also was examined. (+/-)-Pindolol had no appreciable effect on the action of 8-OH-DPAT. In contrast, the 5-HT(1A) antagonist drugs WAY-100635 (0.1 mg/kg i.v. ), 4-fluoro-N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl benzamide (0.1 mg/kg, i.v.), N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylprop anamid e [(S)-WAY-100135] (0.5 mg/kg i.v.), and (-)-tertatolol (1-5 mg/kg i. v.) reversed the effect of 8-OH-DPAT to varying degrees. Overall, these results indicate that pindolol acts as an agonist rather than an antagonist at 5-HT(1A) autoreceptors in awake animals.

Pindolol, a putative 5-hydroxytryptamine(1A) antagonist, does not reverse the inhibition of serotonergic neuronal activity induced by fluoxetine in awake cats: comparison to WAY-100635

The ability of pindolol to enhance the clinical antidepressant response to selective serotonin reuptake inhibitors (SSRIs) is generally attributed to a blockade of the feedback inhibition of serotonergic neuronal activity mediated by somatodendritic 5-hydroxytryptamine (5-HT)(1A) autoreceptors. The current study examined the ability of pindolol to restore the single-unit activity of serotonergic dorsal raphe nucleus neurons in awake cats after acute treatment with the SSRI fluoxetine. The effects of pindolol were compared with those of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide (WAY-100635), a selective 5-HT(1A) receptor antagonist. Systemic administration of fluoxetine (0.5 and 5 mg/kg i. v.) decreased neuronal firing rates to approximately 50 and 1%, respectively, of baseline levels. The subsequent administration of cumulative doses of (+/-)-pindolol (0.1-5 mg/kg i.v.) failed to reverse the neuronal inhibition produced by either dose of fluoxetine. In addition to lacking efficacy as an antagonist in these experiments, (+/-)-pindolol produced an additional decrease in neuronal activity in animals pretreated with the low dose of fluoxetine. The active enantiomer, (-)-pindolol (1 mg/kg i.v.), also was ineffective in restoring neuronal activity after fluoxetine. In contrast, systemic administration of WAY-100635 completely reversed the effect of fluoxetine (5 mg/kg) at low doses (0.025 mg/kg i.v.), and further elevated the firing rate of these neurons above prefluoxetine baseline levels. Overall, these results indicate that pindolol, unlike WAY-100635, lacks appreciable antagonist activity at 5-HT(1A) autoreceptors. Thus, the clinical efficacy of pindolol in augmenting the antidepressant response to SSRIs, such as fluoxetine, may be unrelated to a restoration of serotonergic neuronal activity.

Activity of serotonergic neurons in behaving animals

Brain serotonergic neurons display a distinctive slow and regular discharge pattern in behaving animals. This activity gradually declines across the arousal-waking sleep cycle, becoming virtually silent during rapid eye movement sleep. The activity of these neurons, in both the pontine and medullary groups, is generally unresponsive to a variety of physiological challenges or stressors. However, these neurons are activated in association with increased muscle tone/tonic motor activity, especially if the motor activity is in the repetitive or central pattern generator mode. We interpret these data within the following theoretical framework. The primary function of the brain serotonergic system is to facilitate motor output. Concurrently, the system coordinates autonomic and neuroendocrine function with the present motor demand, and inhibits information processing in various sensory pathways. Reciprocally, when the serotonin system is briefly inactivated (e.g., during orientation to salient stimuli), this disfacilitates motor function and disinhibits sensory information processing. It is within this context that serotonin exerts its well-known effects on pain, feeding, memory, mood, etc.

Pindolol increases extracellular 5-HT while inhibiting serotonergic neuronal activity

The effects of pindolol, a beta-adrenoceptor blocker/putative 5-hydroxytryptamine (5-HT)1A/1B antagonist, on both the single-unit activity of serotonergic neurons in the dorsal raphe nucleus (DRN) and extracellular 5-HT levels in the caudate nucleus, were examined in freely moving cats. Administration of (+)-pindolol (1 and 10 mg/kg, s.c.) decreased neuronal activity and increased 5-HT levels in a dose- and time-dependent manner. The subsequent administration of WAY-100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cycloh exanecarboxamide] (0.2 mg/kg, s.c.), a selective 5-HT1A receptor antagonist, blocked pindolol-induced neuronal suppression and potentiated 5-HT output. These results indicate that pindolol may be acting at the level of the nerve terminal to increase 5-HT.

The 5-HT1A receptor antagonist p-MPPI blocks 5-HT1A autoreceptors and increases dorsal raphe unit activity in awake cats

The effects of the putative 5-HT1A receptor antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzam ide (p-MPPI) were examined on the activity of serotonergic dorsal raphe nucleus neurons in freely moving cats. Systemic administration of p-MPPI produced a dose-dependent increase in firing rate. This stimulatory effect of p-MPPI was evident during wakefulness (when serotonergic neurons display a relatively high level of activity), but not during sleep (when serotonergic neurons display little or no spontaneous activity). p-MPPI also blocked the ability of the 5-HT1A receptor agonist 8-hydroxy-(2-di-n-propylamino)tetralin (8-OH-DPAT) to inhibit serotonergic neuronal activity. This antagonism was evident both as a reversal of the neuronal inhibition produced by prior injection of 8-OH-DPAT and as a shift in the potency of 8-OH-DPAT following p-MPPI pretreatment. Overall, these results in behaving animals indicate that p-MPPI acts as an effective 5-HT1A autoreceptor antagonist. The increase in firing rate produced by p-MPPI supports the hypothesis that autoreceptor-mediated feedback inhibition operates under physiological conditions.

Serotonin and motor activity

The activity of brain serotonergic neurons in both the pontine-mesencephalic and medullary groups is positively correlated with the level of behavioral arousal and/or the behavioral state. This, in turn, appears to be related to the level of tonic motor activity, especially as manifested in antigravity muscles and other muscle groups associated with gross motor activity. In addition, a subset of serotonergic neurons displays a further increase in activity in association with repetitive, central pattern generator mediated responses. Accumulating evidence indicates that this relation to motor activity is related both to the co-activation of the sympathetic nervous system and to the modulation of afferent inputs.

Effects of sleep deprivation on serotonergic neuronal activity in the dorsal raphe nucleus of the freely moving cat

Total sleep deprivation (TSD) for one or more nights produces a rapid antidepressant response in humans. Since most pharmacological treatments for depression increase brain serotonin neurotransmission, the purpose of the present study was to determine whether TSD increases the activity of serotonergic neurons in the dorsal raphe nucleus (DRN) in cats. Cats were prevented from sleeping by the experimenter, who monitored the behavioral state of each animal on a polygraph. Firing rates during quiet waking (QW) and active waking (AW) were obtained throughout a 24-h sleep deprivation period and subsequent 6-h recovery period. During the experiments, unit activity was also recorded during exposure to loud white noise, which elicited strong behavioral arousal. The inhibitory response of serotonergic DRN neurons to systemic administration of the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was determined before and after TSD to assess possible changes in 5-HT1A autoreceptor sensitivity. TSD increased mean firing rates by as much as 18% during both AW and white noise exposure. Maximal effects were observed after 15 h of TSD for AW, and after 18 h for white noise. QW firing rates also tended to be elevated throughout TSD. Firing rates for all conditions during the recovery period were not significantly different from baseline. The neuronal inhibition produced by 8-OH-DPAT was significantly diminished after TSD. Overall, these results indicate that TSD increases the firing rate of serotonergic DRN neurons during AW and arousal. This effect may be attributable to a decrease in the sensitivity of 5-HT1A autoreceptors. These findings are consistent with the hypothesis that TSD exerts its antidepressant action, at least in part, through an activation of brain serotonergic neurons.

Single-unit responses of serotonergic dorsal raphe neurons to specific motor challenges in freely moving cats

Serotonin has been hypothesized to play an important role in the central control of motor function. Consistent with this hypothesis, virtually all serotonergic neurons within the medullary nuclei raphe obscurus and raphe pallidus in cats are activated in response to specific motor challenges. To determine whether the response profile of serotonergic neurons in the midbrain is similar to that observed in the medulla, the single-unit activity of serotonergic dorsal raphe nucleus cells was studied during three specific motor activities: treadmill-induced locomotion, hypercarbia-induced ventilatory response and spontaneous feeding. In contrast to the results obtained for medullary raphe cells, none of the serotonergic dorsal raphe cells studied (n=26) demonstrated increased firing during treadmill-induced locomotion. A subset of serotonergic dorsal raphe cells (8/36) responded to the hypercarbic ventilatory challenge with increased firing rates that were directly related to the fraction of inspired carbon dioxide, and a non-overlapping subset of cells (6/31) was activated during feeding. All feeding-on cells demonstrated a rapid activation and de-activation coincident with feeding onset and offset, respectively. Although the proportions of serotonergic cells activated by hypercarbia or feeding in the dorsal raphe nucleus were similar to those found in the medullary raphe, there were several major distinctions in the response characteristics for the two cell groups. In contrast to the medullary serotonergic neurons, only a minority of dorsal raphe nucleus serotonergic neurons responded to a motor challenge. Overall, the above results suggest very different roles for the midbrain and medullary serotonergic neurons in response to motor activities.

A critical review of 5-HT brain microdialysis and behavior

Serotonin (5-HT) has been implicated in many central nervous system-mediated functions including sleep, arousal, feeding, motor activity and the stress response. In order to help establish the precise role of 5-HT in physiology and behavior, in vivo microdialysis studies have sought to identify the conditions under which the release of 5-HT is altered. Extracellular 5-HT levels have been monitored in more than fifteen regions of the brain during a variety of spontaneous behaviors, and in response to several physiological, environmental, and behavioral manipulations. The vast majority of these studies found increases (30-100%) in 5-HT release in almost all brain regions studied. Since electrophysiological studies have shown that behavioral arousal is the primary determinant of brain serotonergic neuronal activity, we suggest that the increase in 5-HT release seen during a wide variety of experimental conditions is largely due to one factor, namely an increase in behavioral arousal/motor activity associated with the manipulation.

5-HT1A agonists induce hippocampal theta activity in freely moving cats: role of presynaptic 5-HT1A receptors

Electrical activity in the dorsal hippocampus was recorded in freely moving cats in response to intravenous administration of 5-HT1A agonist and antagonist drugs. Administration of low doses of the selective 5-HT1A agonists 8-OH-DPAT (5-20 micrograms/kg) and ipsapirone (20-100 micrograms/kg) produced rhythmic slow activity (theta) in the hippocampal EEG within 30 s. Similar effects were observed with BMY 7378 (20 and 100 micrograms/kg), which acts as an agonist at presynaptic (somatodendritic) 5-HT1A receptors and as an antagonist at postsynaptic 5-HT1A receptors. Power spectral analyses showed that all three compounds produced a dose-dependent increase in the EEG power occurring in the theta frequency band (3.5-8.0 Hz) as a proportion of total power from 0.25 to 30.0 Hz (relative theta power). The increase in relative theta power produced by 8-OH-DPAT (20 micrograms/kg) was greatly attenuated by spiperone (1 mg/kg), a highly effective 5-HT1A autoreceptor antagonist. Administration of spiperone alone had no significant effect on relative theta power. These results are discussed in relationship to the effects of these drugs on serotonergic neuronal activity. Our results suggest that preferential activation of presynaptic 5-HT1A receptors, and subsequent inhibition of serotonin neurotransmission, facilitates the appearance of hippocampal theta activity in awake cats.

WAY-100635, a potent and selective 5-hydroxytryptamine1A antagonist, increases serotonergic neuronal activity in behaving cats: comparison with (S)-WAY-100135

We reported previously that pharmacological blockade of somatodendritic 5-hydroxytryptamine (5-HT)1A autoreceptors with spiperone, a nonselective 5-HT1A antagonist, increases the spontaneous firing rate of central serotonergic neurons in awake cats. The present study examined the effects of systemic administration of two reportedly selective 5-HT1A receptor antagonists, (S)-WAY-100135 {N-tert-butyl-3-[4-(2-methoxyphenyl) piperazin-1-yl]-2-phenylpropanamide} and its more potent analog WAY-100635 {N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide}, on the single-unit activity of serotonergic neurons in the dorsal raphe nucleus of freely moving cats. In addition, we assessed the antagonist action of these compounds at the 5-HT1A autoreceptor by examining their ability to block the inhibition of serotonergic neuronal activity produced by systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin, a highly selective 5-HT1A agonist. Administration of (S)-WAY-100135 (0.025-1.0 mg/kg i.v.) moderately depressed neuronal activity at all doses tested. In contrast, administration of WAY-100635 (0.025-0.5 mg/kg i.v.) significantly increased neuronal activity. The stimulatory action of WAY-100635, like that of spiperone, was evident during wakefulness (when serotonergic neurons typically display a relatively high level of activity) but not during sleep (when serotonergic neurons display little or no spontaneous activity). Pretreatment with (S)-WAY-100135 (0.5 mg/kg i.v.) weakly attenuated the inhibitory action of 8-hydroxy-2-(di-n-propylamino)tetralin. In contrast, WAY-100635 at doses as low as 0.1 mg/kg i.v. completely blocked the action of 8-hydroxy-2-(di-n-propylamino)tetralin. The antagonist action of WAY-100635 at 5-HT1A autoreceptors closely paralleled its ability to increase neuronal activity. Overall, WAY-100635 appears to act as a selective 5-HT1A antagonist, whereas (S)-WAY-100135 does not. The results obtained with WAY-100635 confirm our previous findings obtained with spiperone and further support the hypothesis that 5-HT1A autoreceptor-mediated feedback inhibition operates under physiological conditions.

A subgroup of dorsal raphe serotonergic neurons in the cat is strongly activated during oral-buccal movements

A subgroup of approximately 25% of dorsal raphe nucleus serotonergic neurons in cat was strongly activated in association with oral-buccal movements, such as chewing, licking, and grooming. The mean magnitude of increase in neuronal activity for these cells was approximately 100% above the spontaneous waking level. However, some of these cells were activated by as much as 200-300%. The neuronal activation frequently preceded the initiation of the movement and stopped abruptly in association with either pauses in the motor sequence or with its cessation. Most of the neurons in this subgroup were also strongly and preferentially activated by somatosensory stimuli applied to the head, neck, and face. During orientation to a strong or novel stimulus, the activity of these neurons fell silent for periods of 1-5 s. These data and results from our previous studies of medullary raphe neurons are discussed within the context of the general role of serotonin in tonic and central pattern generator-related motor activity.

Response of serotonergic caudal raphe neurons in relation to specific motor activities in freely moving cats

Serotonergic neuronal responses during three specific motor activities were studied in nuclei raphe obscurus (NRO) and raphe pallidus (NRP) of freely moving cats by means of extracellular single-unit recordings. Responses to treadmill-induced locomotion were primarily excitatory, with 21 of 24 neurons displaying increased firing rates, directly related to treadmill speed. Individual regression analyses determined three response patterns: maximal activation at low speed (0.25 m/sec), augmentation of neuronal activity only at high treadmill speed (0.77 m/sec), and a linear increase. A smaller fraction of NRO and NRP serotonergic neurons (6 of 27) also responded to hypercarbic ventilatory challenge with increased firing rates. The magnitude of neuronal response was dependent upon the fraction of inspired CO2 and was related to ventilatory motor output, specifically, inspiratory amplitude. A subgroup of neurons responsive to hypercarbia in wakefulness demonstrated significant reductions in neuronal response to hypercarbia in slow-wave sleep. Finally, unit activity for 12 of 29 cells increased in response to spontaneous feeding, displaying two distinct patterns of neuronal response in relation to onset and termination of feeding: rapid activation and deactivation versus a gradual increase and decrease. More than half of the cells studied under all three conditions were responsive to more than one motor task. These results indicate that serotonergic caudal raphe neurons are responsive to specific motor system challenges, with many neurons responsive to multiple motor tasks, and that the responsiveness of serotonergic neurons to at least one motor task, hypercarbic ventilatory challenge, is state dependent.

Single-unit responses of serotonergic dorsal raphe neurons to 5-HT1A agonist and antagonist drug administration in behaving cats

Single-unit activity of serotonergic neurons in the dorsal raphe nucleus was recorded in free-moving cats in response to i.v. administration of 5-hydroxytryptamine (5-HT)1A agonist and antagonist drugs. The 5-HT1A agonist drugs 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), ipsapirone, buspirone and 5-methoxy-N,N-dimethyltryptamine produced a rapid, dose-dependent inhibition of neuronal activity. 8-OH-DPAT (ED50 = 1.5 micrograms/kg) was approximately 45 times more potent than ipsapirone, buspirone or 5-methoxy-N,N-dimethyltryptamine (ED50 range = 6.0-6.8 micrograms/kg) in producing inhibition, and all drugs were more effective when cats were inactive (e.g., drowsiness) than during periods of behavioral arousal (e.g., active waking). Administration of the 5-HT1A autoreceptor antagonist spiperone (0.25 and 1 mg/kg) produced a rapid, dose-dependent increase in the firing rate, suggesting that under physiological conditions serotonergic neurons are controlled by tonic feedback inhibition. This effect was evident during wakefulness (a period of relatively high neuronal activity), but not during sleep (a period of relatively low neuronal activity). Spiperone also blocked the inhibitory action of 8-OH-DPAT in a dose- and time-dependent manner. There was a strong positive correlation between the magnitude of spiperone-induced neuronal activation and blockade of 8-OH-DPAT-induced neuronal suppression. These effects of spiperone cannot be attributed to its dopaminergic D2 or serotonergic 5-HT2 antagonist properties, because administration of haloperidol and ritanserin produced no increase in neuronal activity and did not block the action of 8-OH-DPAT. These results confirm the marked sensitivity of serotonergic dorsal raphe nucleus neurons to selective 5-HT1A agonist compounds in unanesthetized animals and suggest that 5-HT1A somatodendritic autoreceptors exert a tonic inhibitory influence on the firing rate of these neurons during periods of behavioral activation, but not during periods of behavioral quiescence.

Effects of the putative 5-hydroxytryptamine1A antagonists BMY 7378, NAN 190 and (-)-propranolol on serotonergic dorsal raphe unit activity in behaving cats

Recent evidence from our laboratory has demonstrated that blockade of somatodendritic 5-hydroxytryptamine (5-HT)1A autoreceptors by systemic administration of spiperone increases the firing rate of central serotonergic neurons in awake cats. The present study examines the effects of three other putative 5-HT1A antagonists (BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro [4,5]decane-7,9-dione), NAN 190 [1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine) and (-)-propranolol) on the single-unit activity of serotonergic neurons recorded in the dorsal raphe nucleus of free-moving cats. Systemic administration of the phenylpiperazine derivatives BMY 7378 (5-100 micrograms/kg i.v.) and NAN 190 (5-250 micrograms/kg i.v.) produced a rapid, dose-dependent inhibition of neuronal activity with BMY 7378 being approximately twice as potent as NAN 190 (ED50 = 15.3 micrograms/kg vs. 34.2 micrograms/kg). The suppression of neuronal activity produced by both compounds was greatly attenuated by spiperone (1 mg/kg i.v.). Systemic administration of (-)-propranolol (2 and 4 mg/kg i.v.) produced a modest suppression of serotonergic neuronal activity which did not appear to be dose-related. The ability of BMY 7378, NAN 190 and (-)-propranolol to block the suppression of neuronal activity produced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A agonist, was also examined. Pretreatment with these compounds had no significant effect on the inhibitory response of serotonergic neurons to 8-OH-DPAT challenge. These results indicate that BMY 7378 and NAN 190 act as agonists rather than antagonists at the somatodendritic 5-HT1A autoreceptor.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT and motor control: a hypothesis

The activity of 5-HT-containing neurons in the brain is activated preferentially in association with motor output in cats. This is especially apparent during changes in muscle tone and during responses mediated by central pattern generators; such as chewing, locomotion and respiration. These and other data support the hypothesis that the primary functions of the 5-HT system in the brain are to facilitate motor output and concurrently inhibit sensory information processing. This hypothesis is applicable phylogenetically, from invertebrates to mammals.

Single-unit and physiological analyses of brain norepinephrine function in behaving animals

In behaving cats, the single-unit activity of locus coeruleus noradrenergic neurons is strongly activated by a variety of challenges (stressors). For example, exposing cats to a dog or to loud white noise, dramatically increases the activity of these neurons and simultaneously produces strong activation of the sympathetic nervous system. Similarly, glucoregulatory, thermoregulatory, and cardiovascular challenges also coactivate noradrenergic neurons and the sympathetic nervous system. A related research program utilized a simple brainstem response (the monosynaptic jaw closure reflex) to explore the physiological significance of this response of brain noradrenergic neurons. Conditions which activate these neurons were also shown to potentiate the elicited jaw closure-reflex response. Importantly, when the noradrenergic input to the motor side of this reflex pathway was destroyed with a neurotoxin, the conditions which previously potentiated the reflex were now ineffective. These data represent the first demonstration that the release of norepinephrine, at a specific site, and under physiological conditions, facilitates behavioral output in the intact organism.

Single-unit responses of serotonergic neurons to vasoactive drug administration in behaving cats

Single-unit activity of serotonergic neurons in the dorsal raphe nucleus (DRN), heart rate (HR), and arterial blood pressure were recorded in freely moving cats during spontaneous behavior and in response to systemic administration of vasoactive drugs. The activity of serotonergic neurons varied in association with behavioral arousal but was unrelated to spontaneous fluctuations in HR and blood pressure. Bolus administration of phenylephrine hydrochloride and sodium nitroprusside (15-20 micrograms/kg iv) produced a rapid transient increase (35 mmHg) and decrease (49 mmHg), respectively, in mean arterial pressure (MAP). Infusion of phenylephrine and sodium nitroprusside (100 micrograms/ml) produced sustained hypertension (avg MAP 166 mmHg) and hypotension (avg MAP 49 mmHg), respectively. The activity of serotonergic neurons was not significantly altered in response to phenylephrine or sodium nitroprusside administration. Furthermore, no significant changes in unit activity were observed after hydralazine administration (1 mg/kg iv) despite prolonged reflex activation of sympathetic outflow. Thus the activity of DRN serotonergic neurons was unrelated to transient alterations in blood pressure and baroreceptor activity. These results suggest that changes in the activity of serotonergic DRN neurons are not involved in physiological mechanisms underlying reflex alterations in sympathetic (and parasympathetic) outflow invoked by hypertension and hypotension.

The role of brain serotonin. A neurophysiologic perspective

In behaving animals, the activity of brain serotonergic neurons is closely tied to the sleep-wake-arousal cycle: highest firing rate during active waking or arousal; intermediate level of discharge during quiescent states and slow wave sleep; and virtual silence during rapid-eye-movement sleep. Environmental stressors, such as exposure to white noise or physical restraint, and physiologic stressors, such as induction of a febrile response or hypoglycemia, do not activate these cells above the baseline level observed during active waking. Continuing this line of investigation, we have utilized in vivo brain microdialysis in order to determine whether there could be a dissociation between level of serotonergic neuronal activity and release of serotonin at the nerve terminal. Our data indicate that under a variety of the above conditions, neuronal activity and release are not dissociated. We have recently discovered a group of serotonergic neurons whose activity is strongly linked to various oral-buccal activities, such as feeding, grooming, etc. In general, we propose that the brain serotonergic system exerts a modulatory influence over its target structures so as to coordinate their activity with the organism's sleep-wake-arousal state (level of behavioral arousal).

Single-unit responses of serotonergic neurons to glucose and insulin administration in behaving cats

Extracellular single-unit activity of serotonergic neurons in the dorsal raphe nucleus (DRN) was recorded in response to glucose loading and insulin administration in conscious, freely moving cats. Serotonergic neurons were identified based on their discharge characteristics, activity across states of behavioral arousal, response to systemic administration of serotonin autoreceptor agonists, and histological localization to the DRN. The spontaneous activity of serotonergic neurons varied in association with behavioral state, reaching their highest level during arousal and their lowest level during rapid-eye-movement sleep, when cells typically stopped firing. The activity of serotonergic DRN neurons was not significantly altered by a glucose load (500 mg/kg iv) that produced an approximately 3.5-fold increase in blood glucose levels. Furthermore, serotonergic DRN neuronal activity was not significantly altered after insulin administration (2-4 IU/kg iv), which lowered blood glucose approximately 50% below control levels, or after the rapid reversal of hypoglycemia by subsequent glucose administration. These results indicate that the activity of serotonergic DRN neurons is unrelated to alterations in blood glucose and is not sensitive to elevations of endogenous circulating insulin levels or to exogenous insulin administration. Furthermore, changes in the activity of serotonergic DRN neurons does not appear to be a component of glucoregulatory mechanisms invoked by either hyper- or hypoglycemia. Overall, these results do not support a role for serotonergic DRN neurons in glucoregulation in the cat.