DOI, a 5-HT2A/2C receptor agonist, attenuates clozapine-induced cortical dopamine release

(±)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 1.25, 2.5 and 5 mg/kg), a serotonin (5-HT)_2A/2C agonist, produced an inverted U-shaped increase in DA release in rat medial prefrontal cortex (mPFC) with a significant effect only at 2.5 mg/kg. This effect was completely abolished by M100907 (0.1 mg/kg), a 5-HT_2A antagonist, and WAY100635 (0.2 mg/kg), a 5-HT_1A antagonist, neither of which when given alone affected dopamine release. DOI (2.5 mg/kg), but not the 5-HT_2C agonist Ro 60-0175 (3 mg/kg), attenuated clozapine (20 mg/kg)-induced mPFC dopamine release. These results suggest that 5-HT_2A receptor stimulation increases basal cortical dopamine release via 5-HT_1A receptor stimulation, and inhibits clozapine-induced cortical dopamine release by diminishing 5-HT_2A receptor blockade.     

NMDA receptors in the midbrain periaqueductal gray mediate hypothalamically evoked hissing behavior in the cat

The present study was designed to test the hypothesis that the descending pathway from the medial hypothalamus to the dorsal periaqueductal gray (PAG) is critical for the expression of defensive rage behavior in the cat and utilizes excitatory amino acids as a neurotransmitter. In the first phase of the study, monopolar stimulating electrodes were implanted into the medial hypothalamus from which defensive rage behavior could be elicited by electrical stimulation. For the entire study, the hissing response was used as a measure of defensive rage behavior. Cannula electrodes were implanted into the PAG from which defensive rage sites could be identified and were later used for microinfusion of the NMDA receptor antagonist,dl-2-amino-7-phosphoheptanoic acid (AP-7), into behaviorally identified sites within the PAG. Initially, intracerbral microinjections of the NMDA receptor antagonist, AP-7 (0.2, 2.0 nmol), which were placed directly into sites within the PAG from which defensive rage had been elicited, blocked the occurrence of hypothalamic hissing. Microinjections of similar doses of AP-7 into the PAG also blocked the facilitatory effects of medial hypothalamic stimulation upon hissing behavior elicited from the PAG. However, microinjections of 2 nmol into the PAG had no effect upon hissing that was also elicited from the region of the injection site. This finding indicates that AP-7 selectively blocks hissing elicited from the medial hypothalamus and that the suppressive effects of AP-7 cannot be the result of anesthetic or other nonselective properties of the drug. The next phase of the study, which employed immunohistochemical, receptor autoradiographic techniques, identified NMDA receptors to be present in highest concentrations in the dorsolateral aspect of the PAG where defensive rage is typically elicited. The final phase of the study, which employed a combination of retrograde labeling procedures following microinjections of Fluoro-Gold into defensive rage sites in the dorsal PAG and the immunocytochemical labeling of glutamatergic neurons, identified large numbers of neurons in the medial hypothalamus that were labeled positively for both Fluoro-Gold and glutamate. The overall findings of this study support the hypothesis that descending fibers of the medial hypothalamus that supply the dorsal aspect of the PAG mediate defensive rage behavior and utilize excitatory amino acids that act upon NMDA receptors within the dorsal PAG.     

Implication of the 5-HT2A and 5-HT2C (but not 5HT1A) receptors located within the periaqueductal gray in the elevated plus-maze test–retest paradigm in mice

A single exposure to the elevated plus-maze test (EPM) increases open arms avoidance and reduces or abolishes the anxiolytic-like effect of benzodiazepines assessed during a second trial, a phenomenon defined as “one-trial tolerance” (OTT). It has been emphasized that the dorsal portion of the midbrain periaqueductal gray (dPAG) plays a role on this enhanced aversion phenomenon in maze-experienced rodents. Given that intra-dPAG injections of a wide range of serotonergic 5-HT_1A, 5-HT_2A and 5-HT_2C receptor agonists produce anxiolytic-like effects in maze-naïve rodents, the present study examined the effects of the 5-HT_1A receptor agonist 8-OH-DPAT (5.6 and 10.0 nmol in 0.15 µl) the preferential 5-HT_2A receptor agonist DOI (2.0 and 8.0 nmol in 0.1 µl) and the preferential 5-HT_2C receptor agonist MK-212 (21.2 and 63.6 nmol in 0.1 µl) microinjected into the dPAG prior to Trial 1 and Trial 2 on the behaviour of mice in the EPM. Test sessions were recorded and subsequently scored for anxiety-like behaviour (percentage of open arms entries and time) as well as general locomotor activity (closed arm entries). The results showed a lack of 8-OH-DPAT (5.6 and 10.0 nmol) effect on the behaviour of maze-naïve and maze-experienced mice, while intra-dPAG microinfusions of DOI (8 nmol) reduced anxiety-like behaviour only in maze-experienced mice that had received a similar treatment prior to Trial 1. Furthermore, intra-dPAG MK-212 (63.6 nmol) showed an anxiolytic-like effect on both Trial 1 and Trial 2. Importantly, these effects were observed in the absence of any significant change in closed arm entries, the parameter considered to be a valid index of locomotor activity in the plus-maze. These results support the dPAG as a crucial structure involved in the neurobiology of the OTT phenomenon as well as accounting the role of the 5-HT_2A and 5-HT_2C receptors located within this midbrain structure on the emotional state induced by EPM test and retest paradigm mice.     

Effects of 8-OH-DPAT, a 5-HT1A receptor agonist, and DOI, a 5-HT2A/2C agonist, on monosynaptic transmission in spinalized rats

We investigated the effect of the 5-HT_1A receptor agonist (±)-8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) and the 5-HT_2A/2C receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on monosynaptic transmission in spinalized rats. 8-OH-DPAT significantly inhibited the excitation of α-motoneurons evoked by monosynaptic transmission without a direct effect on α-motoneuron excitation. DOI potentiated the excitation of α-motoneurons by both direct stimulation and monosynaptic transmission. These results indicate that activation of 5-HT_1A receptors inhibits monosynaptic transmission, whereas activation of 5-HT_2A/2C receptors enhances it.     

Indications of pre- and post-synaptic 5-HT1A receptor interactions in feeding behavior and neuroendocrine regulation

This bipartite study uses behavioral and biochemical means to explore the involvement of both pre- and post-synaptic 5-HT_1A receptors in the control of food intake and neuroendocrine regulation. In the pharmacological study, the administration of the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT; 60 μg/kg b.wt., i.p.) to rats caused a significant increase in 2 h intake of a high carbohydrate (CARB)/sugar diet (P < 0.05) during the relatively inactive feeding period of the late light cycle. No significant change was detected in the intake of Purina laboratory chow at 2 h, or of the intake of either diet at 4 h and 24 h after 8-OH-DPAT administration. Injection of 8-OH-DPAT induced a drop in insulin levels in rats maintained on high CARB/sugar diets only (−90%; P<0.05). It also caused an increase in circulating glucose levels in both high CARB/sugar (240%; P<0.01) and chow fed (123%; P<0.05) rats; it did so more intensely in high CARB/sugar-fed rats. In the biochemical study, radioligand binding techniques were used to assess 5-HT_1A receptor density in the hypothalamus, as well as the relationship between 5-HT_1A receptors and circulating levels of insulin and glucose. Chronic and acute administration (25 mg/kg b.wt./5 injections, and 50 mg/kg b.wt., respectively, i.p.) of the potent hypoglyce mic agent tolbutamide (TOL) caused a significant increase in 5-HT_1A receptor density (+243% and +132.6%, respectively; P<0.05) in the medial hypothalamus but not in the lateral hypothalamus, as compared to vehicle-treated rats. Chronic glucose replacement therapy showed a trend towards reversing the depressed circulating glucose levels as well as the medial hypothalamic 5-HT_1A receptor density to control levels. These studies indicate that the pre-synaptic mechanism of 8-OH-DPAT-induced hyperphagia may require specific circulating levels of insulin and glucose, which are regulated via post-synaptic 5-HT_1A receptors.     

Sleep/waking effects following intrathecal administration of the 5-HT1A agonist 8-OH-DPAT alone and in combination with the putative 5-HT1A antagonist NAN-190 in rats

Sleep, waking, and EEG power spectra were investigated in rats after intrathecal (IT) administration of a 5-HT_1A agonist and a 5-HT_1A antagonist. Total slow wave sleep (TSWS) was increased and waking was decreased over the 8-h recording period after the 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (38 nmol). Within TSWS, SWS1 was unchanged while SWS-2 tended to be increased. The 5-HT_1A antagonist 1-[2-Methoxyphenyl)-4-(4-(2-phthalimido)-butyl]piperazine hydrobomide (NAN-190) did not change and sleep/waking stages. Combined treatment with 8-OH-DPAT and NAN-190 increased variance. Following the combination, sleep and waking were not significantly different from control. SWS-2 tended to be reduced compared to the effect of 8-OH-DPAT alone. There were no systematic changes in neither waking nor TSWS fronto-frontal or fronto-parietal EEG power spectrum after any of the treatments, indicating that sleep quality was not changed. The results confirm earlier data suggesting that in the spinal cord, stimulation of 5-HT1A receptors have a dampening effect on transmission of sensory information, leading to deactivation and thereby increased sleep tendency. The reason why the 8-OH-DPAT effect was not clearly antagonized by the putative 5-HT1A antagonist NAN-190, may be due to the generally weak antagonistic and also partial agonistic effect of NAN-190 as reported in the literature.     

Effects of triiodothyronine and fluoxetine on 5-HT1A and 5-HT1B autoreceptor activity in rat brain: regional differences

The thyroid hormone triiodothyronine (T3) augments and accelerates the effects of antidepressant drugs. Although the majority of studies showing this have used tricyclics, a few studies have shown similar effects with the selective serotonin re-uptake inhibitor (SSRI) fluoxetine. In this study we investigated the effects of fluoxetine (5 mg/kg), T3 (20 μg/kg) and the combination of these drugs, each administered daily for 7 days, on serotonergic function in the rat brain, using in vivo microdialysis. Fluoxetine alone induced a trend towards desensitization of 5-HT_1A autoreceptors as shown by a reduction in the effect of 8-OH-DPAT to lower 5-HT levels in frontal cortex, and desensitized 5-HT_1B autoreceptors in frontal cortex. The combination of fluoxetine and T3 induced desensitization of 5-HT_1B autoreceptors in hypothalamus. Since there is evidence linking hypothalamic function and depression, we suggest that this effect may partly account for the therapeutic efficacy of the combination of an SSRI and T3.     

Increased midbrain 5-HT1A receptor number and responsiveness in cholestatic rats

Midbrain somatodendritic 5-HT_1A autoreceptors play a central inhibitory role in the regulation of serotonergic neurotransmission. Given that serotonergic neurotransmission appears to be altered in experimental cholestatic liver disease we examined alterations in midbrain 5-HT_1A autoreceptor binding and physiological responses in rats with experimental cholestatic liver disease in comparison to non-cholestatic controls. Using a standard receptor binding assay cholestatic rats exhibited an increase in midbrain 5-HT_1A receptor number but no change in receptor affinity compared to controls. Midbrain 5-HT_1A receptor mRNA expression as determined by semiquantitative RT–PCR was similar in cholestatic and non-cholestatic animals. In addition, cholestatic rats exhibited enhanced 5-HT_1A autoreceptor-mediated hypothermic and hyperphagic responses compared to non-cholestatic controls after the administration of the highly specific 5-HT_1A receptor agonist LY293284. These findings indicate that experimental cholestatic liver injury is associated with enhanced 5-HT_1A autoreceptor-mediated physiological responsiveness in the setting of increased midbrain 5-HT_1A receptor number but not affinity.     

Differential effects of NK1 receptors in the midbrain periaqueductal gray upon defensive rage and predatory attack in the cat

This study utilized anatomical and behavioral-pharmacological methods to determine the role of NK(1)-Substance P receptors in the midbrain periaqueductal gray (PAG) in defensive rage behavior in cats. For behavioral pharmacological experiments, monopolar stimulating electrodes were implanted in the medial hypothalamus for elicitation of defensive rage behavior and cannula-electrodes were implanted in the PAG for microinjections of receptor compounds. Microinjections of the NMDA antagonist, AP-7 (2 nmol), into the dorsal PAG blocked defensive rage elicited by medial hypothalamic stimulation, thus establishing the PAG as a synaptic region that receives hypothalamic inputs linked to defensive rage behavior. Microinjections of the NK(1) agonist, GR73632, into the same injection sites facilitated defensive rage in a dose-dependent manner, and also induced spontaneous hissing in five cats. The effects of GR73632 were reduced by pretreatment of the PAG with the NK(1) antagonist, GR82334 (16 nmol), microinjected into the same sites. Microinjections of GR73632 (8 nmol) into the PAG also suppressed predatory attack elicited by stimulation of the lateral hypothalamus. Immunohistochemical methods utilized to detect Substance P and Fos immunoreactivity revealed that neurons in the PAG activated after defensive rage-inducing medial hypothalamic stimulation lie in the same region as Substance-P-immunoreactive processes. Fos immunoreactivity was highest in the dorsomedial aspect of the rostral PAG after medial hypothalamic stimulation. Cats that were unstimulated or that exhibited predatory attack after lateral hypothalamic stimulation had low c-fos expression levels in the PAG. Substance P immunoreactivity was high throughout the dorsal PAG. The results indicate that NK(1) receptors in the PAG potentiate defensive rage and suppress predatory aggression in the cat.     

NK1 receptors in the medial hypothalamus potentiate defensive rage behavior elicited from the midbrain periaqueductal gray of the cat

Defensive rage in the cat occurs naturally in response to a threat and is also elicited by electrical or chemical stimulation over the rostro-caudal extent of the medial hypothalamus and dorsolateral aspect of the periaqueductal gray (PAG). This behavior is mediated over a descending projection from the hypothalamus to the midbrain PAG. The underlying hypothesis for the present study was that medial hypothalamic defensive rage neurons are excited in two ways: by NK(1) receptors and by an ascending input from the PAG. The first aspect of this hypothesis was tested by eliciting defensive rage by electrical stimulation of the PAG and then microinjecting a selective NK(1) agonist and antagonist into the hypothalamus. Microinjections of 16 or 12 nmol/0.25 microl of the NK(1) agonist, GR 73632, resulted in facilitation of defensive rage. These facilitatory effects were then blocked by pretreatment with the NK(1) antagonist, GR 82334. However, microinjections of GR 82334 alone had no effect. The second aspect of the hypothesis was tested by stimulating defensive rage sites in the PAG and using immunohistochemical methods to test for the presence of c-Fos in the hypothalamus. The results revealed the presence of c-Fos immunoreactivity in the medial but not lateral hypothalamus. Overall, the findings indicate that NK(1) receptors in the medial hypothalamus facilitate defensive rage elicited from PAG neurons whose axons project back to the medial hypothalamus. The likely ethological significance of the ascending input is that it allows for potentiation and prolongation of defensive rage in response to a threatening stimulus.     

5-HT1A/1B, 5-HT6, and 5-HT7 serotonergic receptors recruitment in tonic-clonic seizure-induced antinociception: Role of dorsal raphe nucleus

Pharmacological studies have been focused on the involvement of different neural pathways in the organization of antinociception that follows tonic-clonic seizures, including 5-hydroxytryptamine (5-HT)-, norepinephrine-, acetylcholine- and endogenous opioid peptide-mediated mechanisms, giving rise to more in-depth comprehension of this interesting post-ictal antinociceptive phenomenon. The present work investigated the involvement of 5-HT_1A/1B, 5-HT₆, and 5-HT₇ serotonergic receptors through peripheral pretreatment with methiothepin at doses of 0.5, 1.0, 2.0 and 3.0 mg/kg in the organization of the post-ictal antinociception elicited by pharmacologically (with pentylenetetrazole at 64 mg/kg)-induced tonic-clonic seizures. Methiothepin at 1.0 mg/kg blocked the post-ictal antinociception recorded after the end of seizures, whereas doses of 2.0 and 3.0 mg/kg potentiated the post-ictal antinociception. The nociceptive thresholds were kept higher than those of the control group. However, when the same 5-hydroxytryptamine receptors antagonist was microinjected (at 1.0, 3.0 and 5.0 μg/0.2 μL) in the dorsal raphe nucleus, a mesencephalic structure rich in serotonergic neurons and 5-HT receptors, the post-ictal hypo-analgesia was consistently antagonized. The present findings suggest a dual effect of methiothepin, characterized by a disinhibitory effect on the post-ictal antinociception when peripherally administered (possibly due to an antagonism of pre-synaptic 5-HT_1A serotonergic autoreceptors in the pain endogenous inhibitory system) and an inhibitory effect (possibly due to a DRN post-synaptic 5-HT_1B, 5-HT₆, and 5-HT₇ serotonergic receptors blockade) when centrally administered. The present data also suggest that serotonin-mediated mechanisms of the dorsal raphe nucleus exert a key-role in the modulation of the post-ictal antinociception.     

Autoradiographic mapping of 5-HT1B and 5-HT1D receptors in the post mortem human brain using [H]GR 125743

The distribution of 5-HT_1B and 5-HT_1D receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [³H]GR 125743. [³H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [³H]GR 125743 binding was observed, reflecting low densities of 5-HT_1B/1D receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT_1D receptors and the second using SB 224289 to inhibit 5-HT_1B receptor binding. The autoradiograms indicated that in the human brain, the 5-HT_1B receptor is much more abundant than the 5-HT_1D receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [³H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT_1B receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT_1D receptor in the human brain.     

5-HT2 receptor antagonists and migraine therapy

5-Hydroxytryptamine (5-HT; serotonin) has been implicated in the pathophysiology of migraine, and several drugs with potent 5-HT₂ receptor blocking activity (methysergide, pizotifen, cyproheptadine and mianserin) have been recognized as being clinically effective in migraine prophylaxis, although the selective 5-HT₂ receptor antagonist ketanserin (the principal agent used to identify 5-HT₂ receptor-mediated actions) seems to be ineffective in migraine. Pizotifen is the most widely used 5-HT₂ receptor antagonist in migraine prophylaxis, because of its superior efficacy compared with cyproheptadine, and because the incidence and severity of adverse effects with pizotifen is lower compared with methysergide and mianserin. These agents have additional antagonistic effects at histamine H₁, muscarinic cholinergic, ₁-adrenergic, ₂-adrenergic and dopamine receptors, but drugs which are selective for these non-5-HT receptors appear to be of no benefit in migraine. Actions mediated by 5-HT₂ receptors which could be of relevance to migraine comprise cranial vasoconstriction, increased cranial capillary permeability and platelet aggregation, and some central nervous system effects and neuroendocrine functions. Although pizotifen, cyproheptadine and mianserin are considered to be relatively specific for 5-HT₂ receptors, these agents and methysergide all share a high affinity for 5-HT_1C binding sites; ketanserin, however, has little affinity for these sites, thus activation of 5-HT_1C receptors may be an important step in the pathogenesis of migraine. It is not yet known which 5-HT_1C receptor-mediated actions of 5-HT are relevant to migraine, but some behavioural actions and cranial vasodilatation via release of endothelium-derived relaxing factor may be involved. If 5-HT_1C rather than 5-HT₂ receptor-mediated actions are important, then other 5-HT₂ receptor antagonists with a high affinity at 5-HT_1C binding sites, such as LY 53857, metergoline, mesulergine, ritanserin and SCH 23390, may also prove to be effective in migraine. The efficacy of methysergide may also depend on other 5-HT₁-like receptor-mediated actions such as cranial vasoconstriction, and inhibition of cranial vascular neurogenic inflammation. The efficacy of these agents implies that 5-HT is causally involved in at least some of the underlying pathophysiology of migraine.     

Effects of adrenalectomy and type I or type II glucocorticoid receptor activation on 5-HT1A and 5-HT2 receptor binding and 5-HT transporter mRNA expression in rat brain

We evaluated the effects of adrenalectomy (ADX) and replacement with glucocorticoid receptor agonists on serotonin (5-HT) 5-HT_1A and 5-HT₂ receptor binding in rat brain. 5-HT_1A receptor binding was increased in the CA2–CA4 and the dentate gyrus of the hippocampus 1 week after ADX. This effect was prevented by the systemic administration of aldosterone (10 μg/μl/h) but not by RU28362 (10 μg/μl/h). No significant effect was observed on 5-HT₂ receptor binding in rat cortex. The expression of 5-HT transporter mRNA was unchanged in the raphe nucleus as measured by in situ hybridization.     

Crucial role of the 5-HT2C receptor, but not of the 5-HT2A receptor, in the down regulation of stimulated dopamine release produced by pressure exposure in freely moving rats

Helium pressure of more than 2 MPa is a well known factor underlying pressure-dependent central neuroexcitatory disorders, referred to as the high-pressure neurological syndrome. This includes an increase in both serotonin (5-HT) and dopamine (DA) release. The relationship between the increase in 5-HT transmission produced by helium pressure and its effect on DA release has been clarified in a recent study, which have first demonstrated that the helium pressure-induced increase in DA release was dependent on some 5-HT receptor activation. In the present study, we examined in freely moving rats the role of 5-HT_2A and 5-HT_2C receptors in the increase in DA release induced by 8 MPa helium pressure. We used the 5-HT_2A receptor antagonist ketanserin and the 5-HT_2C receptor agonist m-CPP which have been demonstrated to reduce DA function. Because neither ketanserin is an ideal 5-HT_2A receptor antagonist nor m-CPP an ideal 5-HT_2C receptor agonist, additional experiments were made at normal pressure to check up on the selectivity of ketanserin and m-CPP for 5-HT_2A and 5-HT_2C receptors, respectively. Administration of m-CPP reduced both DA basal level and the helium pressure-induced increase in DA release, whereas administration of ketanserin only showed a little effect on the increase in DA release produced by high helium pressure. These results suggest that the 5-HT_2C receptor, but not the 5-HT_2A receptor, would play a crucial role in the helium pressure-induced increase in DA release. This further suggests that helium pressure may simultaneously induce an increase in 5-HT transmission at the level of 5-HT_2A receptors and a decrease in 5-HT transmission at the level of 5-HT_2C receptors.     

The 5-HT1A and 5-HT2A/2C receptor antagonists WAY-100635 and ritanserin do not attenuate -fenfluramine-induced Fos expression in the brain

-Fenfluramine is a serotonin (5-hydroxytryptamine, 5-HT) releaser and reuptake inhibitor. It is used to study the neurochemical control of feeding and has been used to treat obesity. It has also been employed as a pharmacological tool to study changes in serotonergic function in psychiatric patients. Brain sites activated by -fenfluramine via the release of 5-HT have been mapped via the expression of the immediate early gene c-fos. Studies in our laboratory have indicated that -fenfluramine induces Fos in the hypothalamus and cortex through 5-HT release. The present study investigated whether 5-HT released by -fenfluramine induces Fos expression in the brain by activating 5-HT_1A or 5-HT_2A/2C receptors. Rats were pretreated either with WAY-100635, a 5-HT_1A antagonist, or ritanserin, a 5-HT_2A/2C antagonist, prior to -fenfluramine injection. Blockade of either 5-HT_1A or 5-HT_2A/2C receptors was not sufficient to suppress the Fos response to -fenfluramine in any region of the brain examined, including the cingulate cortex, frontal cortex, caudate–putamen, paraventricular nucleus of the hypothalamus, amygdala, and brainstem. These results indicate that Fos response elicited by -fenfluramine may be mediated by other receptors, in addition to the 5-HT_1A or 5-HT_2A/2C receptors.     

Sleep effects following intrathecal administration of the 5-HT1A agonist 8-OH-DPAT and the NMDA antagonist AP-5 in rats

The modulating effect of an intrathecally (i.t.) administered 5-HT_1A agonist and an NMDA antagonist on sleep, waking and EEG power spectra was investigated in rats. The 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (38 nmol) increased total slow wave sleep (TSWS) and decreased waking over the 8 h recording period. The TSWS increase was mostly due to an increase in SWS1. Sleep latency to SWS1 was also reduced. The NMDA antagonist dl-2-amino 5-phosphonovaleric acid (AP-5) (31.5 nmol) reduced waking. SWS1 was increased, but TSWS was not changed. An increase in REM sleep was seen during the last part of the recording. Combined treatment with 8-OH-DPAT and AP-5 reduced waking and increasd TSWSS. No change in REM sleep was seen. There were no systematic changes in either waking, TSWS or REM fronto-frontal or fronto-parietal EEG power spectrum after any of the treatments. The results suggest that in the spinal cord stimulation of 5-HT_1A receptors have a dampening effect on transmission of sensory information, leading to deactivation and thereby increased possibilities for sleep induction. Blockade of the NMDA receptors may also lead to a small dampening of sensory transmission with similar consequences.     

5-HT1A and 5-HT2 receptors differentially regulate the excitability of 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro

We have used intracellular recording techniques to examine the effects of 5-hydroxytryptamine (5-HT, serotonin) on 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro. Bath-applied 5-HT (30–300 μM) had two opposing effects on the membrane excitability of these cells, reflecting the activation of distinct 5-HT receptor subtypes. As demonstrated previously in the rat, 5-HT evoked a hyperpolarization and inhibition of 5-HT neurones, which appeared to involve the activation of an inwardly rectifying K⁺ conductance. This hyperpolarizing response was blocked by the 5-HT_1A receptor-selective antagonist WAY-100635 (30–100 nM). In the presence of WAY-100635, 5-HT induced a previously unreported depolarizing, excitatory response of these cells, which was often associated with an increase in the apparent input resistance of the neurone, likely due to the suppression of a K⁺ conductance. Like the hyperpolarizing response to 5-HT, this depolarization could be recorded in the presence of the Na⁺ channel blocker tetrodotoxin. In addition, the response was not significantly attenuated by the α₁-adrenoceptor antagonist prazosin (500 nM), indicating that it is not due to the release of noradrenaline, or to the direct activation of α₁-adrenoceptors by 5-HT. The 5-HT₃ receptor antagonist granisetron (1 μM) and the 5-HT₄ receptor antagonist SB 204070 (100 nM) failed to reduce the depolarizing response to 5-HT; however, ketanserin (100 nM), mesulergine (100 nM) and lysergic acid diethylamide (1 μM) significantly reduced or abolished the depolarization, indicating that this effect of 5-HT is mediated by 5-HT₂ receptors.