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

Dopaminergic and serotonergic mechanisms in the modulation of pain: In vivo studies in human brain

Here we review the literature assessing the roles of the brain dopaminergic and serotonergic systems in the modulation of pain as revealed by in vivo human studies using positron emission tomography. In healthy subjects, dopamine D₂/D₃ receptor availability particularly in the striatum and serotonin 5-HT_1A and 5-HT_2A receptor availabilities in the cortex predict the subject's response to tonic experimental pain. High availability of dopamine D₂/D₃ or serotonin 5-HT_2A receptors is associated with high pain intensity, whereas high availability of 5-HT_1A receptors associates with low pain intensity. Chronic neuropathic pain is associated with high striatal dopamine D₂/D₃ receptor availability, for which low endogenous dopamine tone is a plausible explanation, although a compensatory increase in striatal dopamine D₂/D₃ receptor density may also contribute. In contrast, chronic musculoskeletal pain is associated with low baseline availability of striatal dopamine D₂/D₃ receptors. In healthy subjects, brain serotonin 5-HT_1A as well as dopamine D₂/D₃ receptor availabilities associate with the subject's response criterion rather than the capacity to discriminate painful thermal stimuli suggesting that these neurotransmitter systems act mainly on non-sensory rather than sensory factors of thermally induced pain experience. Additionally, 5-HT_1A receptor availability predicts the subject's discriminative ability but not response criterion for non-painful tactile test stimuli, while no such correlation is observed with dopamine D₂/D₃ receptors. These findings suggest that dopamine acting on striatal dopamine D₂/D₃ receptors and serotonin acting on cortical 5-HT_1A and 5-HT_2A receptors contribute to top-down pain regulation in humans.

Noxious stimulation excites serotonergic neurons: a comparison between the lateral paragigantocellular reticular and the raphe magnus nuclei

The present study was designed to record electrophysiological responses to graded noxious thermal stimuli of serotonergic and nonserotonergic neurons in the lateral paragigantocellular reticular (LPGi) and the raphe magnus (RMg) nuclei in rats. All of the neurons recorded were juxtacellularly filled with neurobiotin and identified with double immunofluorescent labeling for both neurobiotin and serotonin. Under halothane anesthesia (0.75%), noxious thermal stimuli ⩾48°C activated almost all (88%) of the serotonergic neurons located within the LPGi (n=16). The increase in firing was clear (3.4±0.3spike/s: mean of responses above the population median) and sustained during the whole application of strong thermal noxious stimuli, with a high mean threshold (48.3±0.3°C) and large receptive fields. Recording of serotonergic neurons in the RMg (n=21) demonstrated that the proportion of strongly activated (>2spike/s) neurons (19% vs 59% for the LPGi) as well as the magnitude of the activation (2.1±0.4spike/s: mean of responses above the population median) to thermal noxious stimuli were significantly lower than in the LPGi (P<.05). Within the boundaries of both the LPGi and the RMg (B3 group), nonserotonergic neurons were also predominantly excited (75%) by noxious stimuli, and the resulting activation (7.9±1.2spike/s) was even greater than that of serotonergic neurons. Thermal noxious stimuli-induced activation of LPGi serotonergic cells probably plays a key role in serotonin-mediated modulations of cardiac baroreflex and transmission of nociceptive messages occurring under such intense noxious conditions.

Effects of general anaesthetics on 5-HT neuronal activity in the dorsal raphe nucleus

The ascending 5-HT system has been and continues to be the subject of much research. The majority of in vivo electrophysiological and neurochemical studies of 5-HT function in rodents have been conducted in animals under anaesthesia - usually chloral hydrate or urethane. However, the effects of anaesthetics, on 5-HT function have not been systematically investigated. Here we used in vitro electrophysiology in dorsal raphe slices, to determine the effects of anaesthetically relevant concentrations of chloral hydrate (100 μM and 1 mM), urethane (10 and 30 mM), pentobarbitone (10 and 100 μM) and ketamine (10, 100 and 300 μM) on regulators of 5-HT firing activity. We examined i) basal firing (driven by α(1) adrenoceptors), ii) the excitatory response to N-methyl-d-aspartate (NMDA), iii) the 5-HT(1A) autoreceptor-mediated inhibitory response to 5-HT and iv) the GABA(A) receptor-mediated inhibitory response to 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridinyl-3-ol (THIP, gaboxadol). Pentobarbitone selectively enhanced the response to THIP. Ketamine decreased basal firing, attenuated the response to NMDA, and enhanced responses to both 5-HT and THIP. Chloral hydrate had marginal effects on basal firing, slightly attenuated the NMDA response, and enhanced both the 5-HT and THIP responses. Urethane increased basal firing, decreased the NMDA response, increased the response to THIP, but had no effect on the 5-HT response. Our data indicate that all anaesthetics tested significantly affect the regulators of 5-HT neuronal function. These findings will aid in the interpretation of previous reports of in vivo studies of the 5-HT system and will allow researchers to make a rational selection of anaesthetic for future studies.

Augmentative effect of spinosin on pentobarbital-induced loss of righting reflex in mice associated with presynaptic 5-HT1A receptor

Objectives

This study investigated whether spinosin potentiates pentobarbital-induced loss of righting reflex (LORR) in mice via 5-HT1A receptors.

Methods

Our primary endpoint for sedation was LORR. In addition, the basal rectal temperature was measured.

Key findings

The results demonstrated that the 5-HT1A agonist 8-OH-DPAT (s.c.) induced reductions in duration of LORR at 0.1, 0.5 and 1.0 mg/kg (P &lt; 0.01), and prolongation of LORR latency at 0.5 and 1.0 mg/kg (s.c., P &lt; 0.01) in pentobarbital (45 mg/kg, i.p.)-treated mice. This effect of 8-OH-DPAT was antagonized either by 5-HT1A antagonist p-MPPI (5 mg/kg, i.p.) or by spinosin (15 mg/kg, i.g.) with significance, respectively. Co-administration of spinosin and p-MPPI both at ineffective doses (spinosin at 5.0 mg/kg, i.g. and p-MPPI at 1.0 mg/kg, i.p.) showed significant augmentative effects in reducing latency to LORR, and increasing LORR duration (P &lt; 0.01) in pentobarbital-treated mice. On the other hand, spinosin inhibited 8-OH-DPAT-induced hypothermia, which has been generally attributed to the activation of somatodendritic 5-HT1A autoreceptors in mice.

Conclusions

Based on our previous results and the present data, it should be presumed that presynaptic 5-HT1A autoreceptor mechanisms may be involved in the inhibitory effect of spinosin on 8-OH-DPAT-induced hypothermia and also in the potentiating effect of spinosin on pentobarbital-induced LORR in mice.

High-frequency stimulation of the subthalamic nucleus inhibits the firing of juxtacellular labelled 5-HT-containing neurones

High-frequency stimulation (HFS) of the subthalamic nucleus (STN) is an established neurosurgical therapy for movement disability in advanced Parkinson's disease (PD), but some patients experience psychiatric side-effects like depression. In a previous electrophysiological study, we observed that HFS of the STN inhibited a population of neurones in the rat dorsal raphe nucleus (DRN), with firing properties characteristic of 5-HT neurones. The present study extended these findings to a second population of neurones, and combined extracellular recording with juxtacellular-labelling to investigate the chemical identity of the neurones affected by HFS. Bilateral HFS (130 Hz, 100-200 μA, 5 min) of the STN inhibited (26.0±2.9%) the firing of 37/74 DRN neurones displaying a slow, regular firing pattern. Slower firing neurones were more strongly inhibited than those firing faster. Importantly, 10 inhibited DRN neurones were juxtacellular-labelled with neurobiotin, and all neurones contained 5-HT as shown by post-mortem 5-HT immunocytochemistry. A minority of slow firing DRN neurones (18/74) were activated by STN HFS (37.9±8.3%) which was not observed previously. Of these neurones, three were juxtacellular-labelled and one was 5-HT immunopositive. Also a small number of DRN neurones (19/74) did not respond to HFS, four of which were juxtacellular-labelled and all contained 5-HT. These data show that individual chemically-identified 5-HT-containing neurones in the DRN were modulated by STN HFS, and that the majority were inhibited but some were activated and some failed to respond. These data extend previous findings of modulation of the 5-HT system by STN HFS but suggest a destabilisation of the 5-HT system rather than simple inhibition as indicated previously. Although the mechanism is not yet known, such changes may contribute to the psychiatric side-effects of STN stimulation in some PD patients.

Potentiating effect of spinosin, a C-glycoside flavonoid of Semen Ziziphi spinosae, on pentobarbital-induced sleep may be related to postsynaptic 5-HT(1A) receptors

Previous results have suggested that spinosin, a C-glycoside flavonoid of Semen Ziziphi spinosae, potentiates pentobarbital-induced sleep via the serotonergic system. The present study investigated whether spinosin potentiates pentobarbital-induced sleep via serotonin-1A (5-hydroxytryptamine, 5-HT(1A)) receptors. The results demonstrated that spinosin significantly augmented pentobarbital (35 mg/kg, i.p.)-induced sleep in rats, reflected by reduced sleep latency and increased total sleep time, non-rapid eye movement (NREM) sleep time, and REM sleep time. With regard to NREM sleep duration, spinosin mainly increased slow-wave sleep (SWS). Additionally, spinosin (15mg/kg, i.g.) significantly antagonized 5-HT(1A) agonist 8-OH-DPAT (0.1mg/kg, i.p.)-induced reductions in total sleep time, NREM sleep, REM sleep, and SWS in pentobarbital-treated rats. These results suggest that spinosin may be an antagonist at postsynaptic 5-HT(1A) receptors because these effects of 8-OH-DPAT were considered to be mediated via postsynaptic 5-HT(1A) receptors. Moreover, co-administration of spinosin and the 5-HT(1A) antagonist 4-iodo-N-{2-[4-(methoxyphenyl)-1-piperazinyl]ethyl}-N-2-pyridinylbenzamide (p-MPPI), at doses that are ineffective when administered alone (spinosin 5mg/kg, p-MPPI 1mg/kg), had significant augmentative effects on pentobarbital-induced sleep, reflected by reduced sleep latency and increased total sleep time, NREM sleep, and REM sleep. In contrast to the attenuating effects of p-MPPI on REM sleep via presynaptic 5-HT(1A) autoreceptors, 15mg/kg spinosin significantly increased REM sleep. These results suggest that the effect of spinosin on REM sleep in pentobarbital-treated rats may be related to postsynaptic 5-HT(1A) receptors.

Increased extracellular 5-HT but no change in sleep after perfusion of a 5-HT1A antagonist into the dorsal raphe nucleus of rats

AIM

The 5-HT(1A) receptor antagonist 4-Iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI) (10 microM) was perfused into the dorsal raphe nucleus (DRN) to study simultaneously the effects of the drug on the DRN and frontal cortex extracellular serotonin (5-hydroxytryptamine, 5-HT) levels and concurring behavioural states.

METHODS

Waking, slow wave sleep and rapid eye movement sleep were determined by polygraphic recordings during microdialysis perfusion and extracellular sample collection. The samples were analysed by microbore high-performance liquid chromatography coupled with electrochemical detection for analysis of 5-HT.

RESULTS

p-MPPI perfusion into the DRN (n = 6) produced a sixfold 5-HT increase in the DRN during all behavioural states. The increased 5-HT level was most likely related to the blockage of 5-HT(1A) receptors in the DRN by p-MPPI. No significant effect was seen on sleep.

CONCLUSION

Despite the dramatic increase in DRN extracellular 5-HT produced by p-MPPI, only a transient and nonsignificant effect on sleep was recorded. It is suggested that the usual coupling between 5-HT level and behavioural state may be lost when an excessive serotonergic output is pharmacologically achieved.

5-HT1A receptors in endogenous regulation of neuropathic hypersensitivity in the rat

The role of medullary and spinal 5-HT(1A) receptors in endogenous regulation of neuropathic hypersensitivity was studied. When administered in the rostroventromedial medulla or subcutaneously, WAY-100635, a 5-HT(1A) receptor antagonist, attenuated mechanical hypersensitivity in rats with a spinal nerve injury. Thermal or mechanical nociception outside of the injured area was not influenced by medial medullary or subcutaneous administration of WAY-100635. Intrathecal administration of WAY-100635 had no significant effect on pain-related behavior. Suppression of mechanical hypersensitivity induced by medial medullary administration of WAY-100635 was reversed by intrathecal administration of WAY-100635 or atipamezole, an alpha2-adrenoceptor antagonist, but not by naloxone, an opioid receptor antagonist. The results indicate that endogenous release of 5-HT, via action on medial medullary 5-HT(1A) receptors, tonically suppresses descending inhibition in neuropathic animals. Following medial medullary administration of a 5-HT(1A) receptor antagonist, descending pain regulatory pathways are disinhibited. This leads to selective attenuation of neuropathic hypersensitivity, due to action on spinal 5-HT(1A) receptors and alpha2-adrenoceptors.

Behavioral effects of 8-OH-DPAT injections into pontine and mesencephalic areas containing 5-HT-immunoreactive perikarya in the pigeon

This study examined the distribution of 5-HT-immunoreactive perikarya (5-HT-IRp) and the effects of local injections of 8-OH-DPAT into 5-HT-IRp-containing pontine and mesencephalic regions on feeding and drinking behaviors in free-feeding pigeons. When infused into the midline 5-HT-IRp-containing areas, 8-OH-DPAT (6.1 nmol) reliably elicited drinking and, to a lesser extent, feeding responses during the first hour after injection. These responses were significantly higher than the ingestive indexes observed (1) after vehicle (ascorbic acid 0.1%, 200 nl) injections at the same sites and (2) after 8-OH-DPAT injections into adjacent sites devoid of 5-HT-IRp. Increases in drinking were proportionally higher than those observed in feeding and a significant negative correlation was observed between water and food after midline 8-OH-DPAT injections. Similar dipsogenic responses were observed after injections of different 8-OH-DPAT doses (0.6, 2.0, and 6.1 nmol). Pretreatment with local injections of p-MPPI (an antagonist of 5-HT1A receptors) attenuated the ingestive responses evoked by 8-OH-DPAT injections. Injections of 8-OH-DPAT into lateral 5-HT-IRp-containing sites evoked only inconsistent and weak ingestive responses. These results indicate that 5-HT1A receptor-mediated circuits located in the midline superior raphe system of the pigeon may play an important role in mechanisms controlling water intake, similar to that observed in mammals.

Moderate differences in circulating corticosterone alter receptor-mediated regulation of 5-hydroxytryptamine neuronal activity

Circulating glucocorticoid levels vary with stress and psychiatric illness and play a potentially important role in regulating transmitter systems that regulate mood. To determine whether chronic variation in corticosterone levels within the normal diurnal range altered the control of 5-hydroxytryptamine (5-HT) neuronal activity, male rats were adrenalectomized and implanted with either a 2% or 70% corticosterone/cholesterol pellet (100 mg). Two weeks later, the regulation of 5-HT neuronal activity in the dorsal raphe nucleus was studied by in vitro electrophysiology. At this time, serum corticosterone levels approximated the low-point (2%) and mid-point (70%) of the diurnal range. The excitatory response of 5-HT neurones to the alpha1-adrenoceptor agonist phenylephrine (1-11 microM) was significantly greater in the 2% group compared to the 70% group. By contrast, the inhibitory response to 5-HT (10-50 microM) was significantly lower in the 2% group compared to the 70% group. Thus, chronic variation in circulating corticosterone over a narrow part of the normal diurnal range causes a shift in the balance of positive and negative regulation of 5-HT neurones, with increased alpha 1-adrenoceptor-mediated excitation and reduced 5-HT-mediated autoinhibition at lower corticosterone levels. This shift would have a major impact on control of 5-HT neuronal activity.

Modulation of serotonergic projection from dorsal raphe nucleus to basolateral amygdala on sleep-waking cycle of rats

Putative serotonergic dorsal raphe nucleus (DRN) neurons display a dramatic role in the modulation of behavior. However, it is not clear how this modulation is mediated. The present study investigated the modulatory effects of serotonergic projection of the DRN to the basolateral amygdala (BLA) on the sleep-waking cycle using polysomnograph (PSG) in rats. DRN microinjection of kainic acid (KA) caused insomnia immediately. From the third day, however, slow wave sleep (SWS) and paradoxical sleep (PS) increased markedly. DRN microinjection of p-chlorophenylalanine (PCPA, once a day for 2 days), which inhibits the synthesis of serotonin (5-HT), led to similar effect to KA administration. The percent of sleep-wakefulness began to change on the third day after PCPA microinjection into the DRN, and the effect was most significant on the sixth day. The percent of sleep-wakefulness started to resume on the seventh day. SWS and PS were reduced after excitation of DRN neurons by microinjection of L-glutamate (L-Glu) into the DRN. Preapplication of the nonselective 5-HT receptor antagonist methysergide (MS) into bilateral BLA blocked the effect of DRN microinjection of L-Glu. Furthermore, bilateral BLA microinjection of 5-hydroxytryptophan (5-HTP), the precursor of 5-HT, on the sixth day after microinjection of PCPA into the DRN, could reverse the effect of PCPA microinjection. These results indicate that the modulation of the DRN on sleep is partially mediated by the serotonergic projection of the DRN to the BLA.

5-HT(1A) receptor antagonist p-MPPI attenuates acute ethanol effects in mice and rats

The effect of a selective 5-HT(1A) antagonist, 4-(2'-methoxy-)phenyl-1-[2'-(N-2"-pyridinyl)-p-iodobenzamino-]ethyl-piperazine (p-MPPI), on acute ethanol-induced hypothermia, sleep and suppression of acoustic startle reflex in C3H/He mice and Wistar rats was studied. Administration of p-MPPI at the doses of 0.4, 0.7 and 1.0 mg/kg reduced in a dose-dependent manner the ethanol-induced hypothermia and the sleep time and attenuated the ethanol-induced decrease of acoustic startle reflex magnitude in mice. Similar p-MPPI (0.4 mg/kg) effects on ethanol-induced sleep and hypothermia were obtained in rats. It was concluded that 5-HT(1A) receptors were involved in the mechanisms of the ethanol-induced hypothermia and sleep, and that 5-HT(1A) antagonist increased acute ethanol tolerance.

Increased REM sleep after intra-dorsal raphe nucleus injection of flesinoxan or 8-OHDPAT: prevention with WAY 100635

The effects of 8-OHDPAT and flesinoxan, two selective 5-HT(1A) receptor agonists, and of WAY 100635, a selective 5-HT(1A) receptor antagonist, on spontaneous sleep were studied in adult rats implanted for chronic sleep recordings. The serotonergic ligands were microinjected directly into the dorsal raphe nucleus (DRN). Direct administration of flesinoxan (25.0-50.0 ng) into the DRN induced a significant increment of REM sleep (REMS) during the second and third 2 h of recording. Microinjection of 8-OHDPAT (50.0 ng) induced similar effects on REMS during the second 2 h of recording. On the other hand, intra-DRN injection of WAY 100635 (12.5-50.0 ng) significantly reduced REMS during the second 2 h recording period. REM sleep values had also decreased significantly during the first 2 h of recording after the 50 ng dose. Pretreatment with WAY 100635 (25.0 or 50.0 ng) prevented the increase of REMS induced by flesinoxan (25.0 ng) during the second two recording hours. Our findings support the proposal that activation of somatodendritic 5-HT(1A) receptors in the DRN increases REMS, whereas their blockade induces the opposite effect.

On the antinociceptive effect of fluoxetine, a selective serotonin reuptake inhibitor

Antidepressant drugs are reported to be used as co-analgesics in clinical management of migraine and neuropathic pain. The mechanism through which they alleviate pain remains unknown. The present study explores the possible mechanism of a selective serotonin reuptake inhibitor (SSRI) fluoxetine-induced antinociception in animals. Acetic acid-induced writhing, hot plate and tail-flick test were used to assess fluoxetine-induced antinociception. Fluoxetine (5-20 mg kg(-1), i.p.) produced a significant and dose-dependent antinociceptive effect against acetic acid-induced writhing in mice. Fluoxetine (20 mg kg(-1)) also exhibited antinociceptive effect in tail flick as well as hot plate assays. Further, i.c.v. administration of fluoxetine showed significant antinociception against writhing test in rats. However, fluoxetine (1 microg/10 microl/rat, i.c.v.) did not exhibit any antinociceptive effect in serotonin-depleted animals. Further, pindolol (10 mg kg(-1), i.p.) enhanced fluoxetine-induced antinociceptive effect. The antinociceptive effect of fluoxetine was sensitive to blockade by naloxone (5 mg kg(-1), i.p.) and naltrexone (5 mg kg(-1), i.p.). These data suggest that fluoxetine-induced antinociception involves both central opioid and the serotoninergic pathways.

The selective 5-HT(1A) receptor antagonist p-MPPI antagonizes sleep--waking and behavioural effects of 8-OH-DPAT in rats

Systemic administration of the selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT) increases waking and reduces slow wave sleep (SWS) and rapid eye movement (REM) sleep in the freely moving rat. The selective 5-HT(1A) antagonist 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethyl-piperazine (p-MPPI) induces a dose-related decrease in REM sleep. The present study examined p-MPPI's potential as an antagonist of the sleep and waking responses elicited by 8-OH-DPAT. Also, the experiments explored the ability of p-MPPI to block behavioural reactions of the 5-HT syndrome induced by 8-OH-DPAT, and whether p-MPPI induced any behavioural effects of its own. This study demonstrated that pre-treatment with p-MPPI (5 mg/kg intraperitoneal (i.p.)) 30 min before 8-OH-DPAT (0.375 mg/kg subcutaneously (s.c.)) reduced the effect of 8-OH-DPAT on waking and REM sleep. Also, p-MPPI (5 and 10 mg/kg i.p.) reduced the effect of 8-OH-DPAT on locomotion and partially or completely antagonized hindlimb abduction and flat body posture. No overt behavioural change was produced by p-MPPI alone. Thus, p-MPPI behaved as a true 5-HT(1A) antagonist.

Sleep and waking following microdialysis perfusion of the selective 5-HT1A receptor antagonist p-MPPI into the dorsal raphe nucleus in the freely moving rat

The aim of this study was to examine the involvement of the dorsal raphe nucleus (DRN) presynaptic serotonergic 5-HT1A autoreceptors on sleep and waking parameters, in particular rapid eye movement (REM) sleep. In a previous study, the systemic administration of the selective 5-HT1A receptor antagonist p-MPPI reduced REM sleep in a dose-dependent manner suggesting a blockade of the 5-HT1A autoreceptors. In the present study, a blockade by microdialysis perfusion of 10 microM and 100 microM of p-MPPI for 7 h into the DRN in freely behaving rats influenced vigilance state only to a small extent. The administration of 10 microM of p-MPPI induced a reduction of total REM sleep mainly due to a suppression of REM sleep during the third 2 h period of the recording of sleep and waking. Perfusion of 100 microM of p-MPPI decreased total transition type sleep (TRANS) but the effect on REM sleep did not reach significance. There was no change in waking or slow wave sleep (SWS) following any of the doses. The data suggest that 5-HT1A receptor-mediated mechanisms in the DRN may be only moderately important in the serotonergic modulation of REM sleep.

5-HT(1A) receptor mutant mice exhibit enhanced tonic, stress-induced and fluoxetine-induced serotonergic neurotransmission

Mutant mice that lack serotonin(1A) receptors exhibit enhanced anxiety-related behaviors, a phenotype that is hypothesized to result from impaired autoinhibitory control of midbrain serotonergic neuronal firing. Here we examined the impact of serotonin(1A) receptor deletion on forebrain serotonin neurotransmission using in vivo microdialysis in the frontal cortex and ventral hippocampus of serotonin(1A) receptor mutant and wild-type mice. Baseline dialysate serotonin levels were significantly elevated in mutant animals as compared with wild-types both in frontal cortex (mutant = 0.44 +/- 0.05 n M; wild-type = 0.28 +/- 0.03 n M) and hippocampus (mutant = 0.46 +/- 0.07 n M; wild-type = 0.27 +/- 0.04 n M). A stressor known to elicit enhanced anxiety-like behaviors in serotonin(1A) receptor mutants increased dialysate 5-HT levels in the frontal cortex of mutant mice by 144% while producing no alteration in cortical 5-HT in wild-type mice. There was no phenotypic difference in the effect of this stressor on serotonin levels in the hippocampus. Fluoxetine produced significantly greater increases in dialysate 5-HT content in serotonin(1A) receptor mutants as compared with wild-types, with two- and three-fold greater responses being observed in the hippocampus and frontal cortex, respectively. This phenotypic effect was mimicked in wild-types by pretreatment with the serotonin(1A) antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (p-MPPI). These results indicate that deletion of central serotonin(1A) receptors results in a tonic disinhibition of central serotonin neurotransmission, with a greater dysregulation of serotonin release in the frontal cortex than ventral hippocampus under conditions of stress or increased interstitial serotonin levels.

Effect of 8-OH DPAT and ketanserin on the ventilatory acclimatization to hypoxia in awake goats

We have previously reported that broad-spectrum serotonergic blockade increased the acute hypoxic ventilatory response in awake goats. The purpose of the present study was to examine the putative serotonin (5-HT) receptor subtype(s) that may have contributed to this response. Following the administration of the selective 5-HT(1A)-receptor agonist, 8-hydroxy-(2-di-n-propylamino) tetralin (8-OH DPAT, 0.1 mg x kg(-1)i.v.), there was an increase in normoxic expired minute ventilation (V(E)) that was due to an increased breathing frequency. V(E) increased during hypoxia but the change in V(E) (Delta V(E)) associated with hypoxic exposure was not different from the Delta V(E) of saline treated goats. The combination of 8-OH DPAT and a selective 5-HT(2A/2C) receptor antagonist, ketanserin (0.1 and 1.0 mg x kg(-1)i.v., respectively), also increased normoxic V(E) but did not alter the hypoxia induced Delta V(E). Both 8-OH DPAT alone and in combination with ketanserin attenuated the change in V(E) associated with sustained hypoxia but neither was able to attenuate the increased hypoxic ventilatory response that occurs following acclimatization. The augmented acute hypoxic ventilatory response that we previously reported does not appear to be mediated via the activation of the 5-HT(1A) receptor or through the combination of 5-HT(1A) activation and 5-HT(2A/2C) blockade. The results of this study further suggest that while 5-HT may modulate hypoxic ventilation it does not appear to be necessary for the development of ventilatory acclimatization to hypoxia.

Short-term modulation of the exercise ventilatory response in goats: effects of 8-OH-DPAT and MPPI

Increased respiratory dead space increases the exercise ventilatory response, a response known as short-term modulation (STM). We hypothesized that STM results from a spinal, serotonin (5-HT)-dependent mechanism. Because 5-HT(1A) autoreceptors on caudal brain stem raphe neurons inhibit 5-HT release, we hypothesized that 5-HT(1A)-receptor agonists would inhibit, whereas 5-HT(1A)-receptor antagonists would enhance, STM. Ventilatory and arterial blood-gas measurements were made at rest and during exercise (4.0-4.5 km/h, 5% grade) in goats with the respiratory mask alone or with increased dead space (0.20-0.25 liter), before and after intravenous administration of the 5-HT(1A)-receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.1 mg/kg) or the antagonist 4-iodo-N-(2-[4-(methoxyphenyl)-1-piperazinyl]ethyl)-N-2-pyridinylbenz amide (MPPI; 0.08 mg/kg). 8-OH-DPAT increased the slope of the arterial PCO(2) vs. metabolic CO(2) production relationship and decreased the ventilation vs. metabolic CO(2) production relationship during exercise with increased dead space (not with the mask alone), indicating an impairment of STM. In contrast, MPPI had minimal effects on any measured variable. Although nonspecific effects of 8-OH-DPAT cannot be ruled out, impaired STM is consistent with the hypothesis that STM requires active raphe serotonergic neurons and 5-HT release.

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.

Sleep-wake effects following the selective 5-HT(1A) receptor antagonist p-MPPI in the freely moving rat

The 5-HT(1A) receptors appear to play an important role in the serotonergic modulation of sleep and waking. Both presynaptic somatodendritic 5-HT(1A) autoreceptors and postsynaptic 5-HT(1A) heteroreceptors may be involved. The present study addressed the question of whether the selective 5-HT(1A) receptor antagonist 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethy l-p iperazine (p-MPPI) affected sleep and waking and whether such an effect would be dose-related. Polygraphic recording of sleep and waking in freely moving rats was employed following control injection and three doses of p-MPPI (1, 5 and 10 mg/kg i.p. in a balanced order design. Waking was increased and deep slow wave sleep decreased, while rapid eye movement (REM) sleep was suppressed over the first 6 h following injection, compared to after control injection. REM sleep was also suppressed following 10 mg/kg i.p. of p-MPPI as compared to following 1 mg/kg i.p. of p-MPPI. The interpretation of the effects is complex and the effects are not easily compatible with a simple model for serotonergic sleep-waking modulation. However, the REM sleep reduction probably reflects p-MPPIs ability to block the presynaptic 5-HT(1A) autoreceptors, increasing the firing activity in the serotonergic neurones and possibly inhibiting serotonin sensitive REM sleep active neurones.

Discharge modulation of rat dorsal raphe neurons during sleep and waking: effects of preoptic/basal forebrain warming

In cats, putative serotonergic neurons (PSNs) recorded from the dorsal raphe nucleus (DRN) across the sleep-wake cycle exhibit the so-called rapid eye movement sleep-off (REM-off) discharge pattern. Since, the sleep-wake discharge patterns of DRN neurons in behaving rats is poorly known, the present study examined this neuronal populations. The PSNs recorded in this study exhibited: (1) progressive decrease in discharge rate from waking to NREM to REM sleep; (2) long action potential duration, and (3) reduction of discharge rate after systemic administration of a selective 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT). Evidence supports the hypothesis that NREM sleep is modulated by thermoregulatory mechanisms localized in the preoptic area and adjacent basal forebrain (POA/BF). We previously reported that POA/BF warming suppresses the discharge of wake-promoting neurons in the posterior hypothalamus and the basal forebrain. Since the DRN is one component of the brainstem arousal system and receives projections from POA/BF, we examined the effects of local POA/BF warming by 1.5-2.0 degrees C during waking on the discharge of DRN neurons. POA/BF warming reduced the discharge in 14 of 19 PSNs and in 12 of 17 other wake-related neurons in the DRN. DRN neuronal discharge reduction occurred without accompanying EEG frequency or behavioral changes. These results suggest that PSNs recorded in DRN in unrestrained and unanesthetized rats exhibit a "wake-active REM-off" discharge pattern and further support the hypothesis that the POA/BF warm-sensitive hypnogenic system induces sleep by a coordinated inhibition of multiple arousal systems including that modulated by the DRN.

Serotonin and the sleep/wake cycle: special emphasis on microdialysis studies

Several areas in the brainstem and forebrain are important for the modulation and expression of the sleep/wake cycle. Even if the first observations of biochemical events in relation to sleep were made only 40 years ago, it is now well established that several neurotransmitters, neuropeptides, and neurohormones are involved in the modulation of the sleep/wake cycle. Serotonin has been known for many years to play a role in the modulation of sleep, however, it is still very controversial how and where serotonin may operate this modulation. Early studies suggested that serotonin is necessary to obtain and maintain behavioral sleep (permissive role on sleep). However, more recent microdialysis experiments provide evidence that the level of serotonin during W is higher in most cortical and subcortical areas receiving serotonergic projections. In this view the level of extracellular serotonin would be consistent with the pattern of discharge of the DRN serotonergic neurons which show the highest firing rate during W, followed by a decrease in slow wave sleep and by virtual electrical silence during REM sleep. This suggests that during waking serotonin may complement the action of noradrenaline and acetylcholine in promoting cortical responsiveness and participate to the inhibition of REM-sleep effector neurons in the brainstem (inhibitory role on REM sleep). The apparent inconsistency between an inhibitory and a facilitatory role played by serotonin on sleep has at least two possible explanations. On the one hand serotonergic modulation on the sleep/wake cycle takes place through a multitude of post-synaptic receptors which mediate different or even opposite responses; on the other hand the achievement of a behavioral state depends on the complex interaction between the serotonergic and other neurotransmitter systems. The main aim of this commentary is to review the role of brain serotonin in relation to the sleep/wake cycle. In particular we highlight the importance of microdialysis for on-line monitoring of the level of serotonin in different areas of the brain across the sleep/wake cycle.

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 agonist Bay x 3702 inhibits apoptosis induced by serum deprivation in cultured neurons

We examined whether the highly selective 5-HT1A receptor agonist (-)-(R)-2-[4-[[(3,4-dihydro-2H-1-benzopyran-2-yl)methyl]-amino]butyl]-11 ,2-benz-isothiazol-3(2H)-one 1,1-dioxide monohydrochloride (Bay x 3702) could inhibit neuronal apoptosis induced by serum deprivation. In primary cultures of chick embryonic neurons and in mixed neuronal/glial cultures from neonatal rat hippocampus, Bay x 3702 (1 microM) rescued serum-deprived neurons from apoptosis. The antiapoptotic effect of Bay x 3702 (1 microM) was blocked in chick neurons by the selective 5-HT1A receptor antagonists 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazin]ethyl]-N-2-pyridinyl-be nzamide hydrochloride (p-MPPI, 10 microM) and 4-[3-benzotriazol-1-propyl]-1-(2-methoxyphenyl)-piperazine (BPMP, 10 microM) as well as by anti-nerve growth factor (anti-NGF) antibodies and in rat neurons by N-[2-4-(2-methoxy)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexane-carbo xamide trihydrochloride (WAY 100635, 10 microM). We found only under control conditions (medium with serum), but not in serum-deprived cultures, that NGF secretion was 6-fold increased by Bay x 3702 (1 microM) compared to untreated cultures. Additionally, Bay x 3702 (4 microg/kg i.v.), infused within a period of 4 h, significantly increased the NGF content of the rat hippocampus, but not of the striatum. In summary, our data suggest that Bay x 3702 inhibited growth factor withdrawal-induced apoptosis by the stimulation of 5-HT1A receptors and that the NGF signalling pathway is involved in the mechanism of action.