Melatonin reversal of DOI-induced hypophagia in rats; possible mechanism by suppressing 5-HT(2A) receptor-mediated activation of HPA axis

Serotonin type 2A (5-HT_2A) receptor-mediated neurotransmitter is known to activate hypothalamic–pituitary–adrenal (HPA) axis, regulate sleep–awake cycle, induce anorexia and hyperthermia. Interaction between melatonin and 5-HT_2A receptors in the regulation of the sleep–awake cycle and head-twitch response in rat have been reported. Previous studies have shown that melatonin has suppressant effect on HPA axis activation, decreases core body temperature and induces hyperphagia in animals. However, melatonin interaction with 5-HT_2A receptors in mediation of these actions is not yet reported. We have studied the acute effect of melatonin and its antagonist, luzindole on centrally administered (±)-1-(2,5-dimethoxy-4-iodophenyl) 2-amino propane (DOI; a 5-HT_2A/2C agonist)-induced activation of HPA axis, hypophagia and hyperthermia in 24-h food-deprived rats. Like ritanserin [(1 mg/kg, i.p.) 5-HT_2A/2C antagonist], peripherally administered melatonin (1.5 and 3 mg/kg, i.p.) did not affect the food intake, rectal temperature or basal adrenal ascorbic acid level. However, pretreatment of rats with it significantly reversed DOI (10 μg, intraventricular)-induced anorexia and activation of HPA axis. But the hyperthermia induced by DOI was not sensitive to reversal by melatonin. Mel₁ receptor subtype antagonist luzindole (5 μg, intraventricular) did not modulate the DOI effect but antagonized the melatonin (3 mg/kg, i.p.) reversal of 5-HT_2A agonist response. The present data suggest that melatonin reversal of DOI-induced hypophagia could be due to suppression of 5-HT_2A mediated activation of HPA axis.     

Stimulation of 5-HT2A receptors in the paraventricular hypothalamus attenuates neuropeptide Y-induced hyperphagia through activation of corticotropin releasing factor

The effect of 5-HT₁ and 5-HT₂ receptor agonists administered into the paraventricular hypothalamus was studied on the hyperphagia caused by neuropeptide Y (NPY) injected into the same area. The 5-HT_2A/2C receptor agonist DOI (10–20 nmol/0.5 μl) significantly reduced NPY overeating while the 5-HT_1A/1B receptor agonist RU 24969 (3.5–14 nmol/0.5 μl) and the 5-HT_1B/2C receptor agonist mCPP (5–20 nmol/0.5 μl) had no such effect. The 5-HT_2A receptor antagonist spiperone (5 μg/0.5 μl) and the corticotropin releasing factor antagonist α-helical-CRF^9–41 (0.5–1 μg/0.5 μl completely antagonized the effect of 10 nmol DOI.     

In vivo effects of activation and blockade of 5-HT2A/2C receptors in the firing activity of pyramidal neurons of medial prefrontal cortex in a rodent model of Parkinson's disease

In the present study, we examined changes in the firing rate and firing pattern of pyramidal neurons in medial prefrontal cortex (mPFC), and the effects of 5-HT_2A/2C receptor agonist DOI and antagonist ritanserin on the neuronal firing in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta by using extracellular recording. The unilateral lesion of the nigrostriatal pathway significantly increased the mean firing rate of pyramidal neurons compared to sham-operated rats, and the firing pattern of these neurons also changed significantly towards a more bursty one. Systemic administration of DOI (20–320 μg/kg, i.v.) increased the mean firing rate of pyramidal neurons in sham-operated and the lesioned rats. The excitation was significant only at doses higher than 160 μg/kg and 320 μg/kg in sham-operated and the lesioned rats, respectively. In addition, the local application of DOI, 5 μg, in mPFC inhibited the firing rate of pyramidal neurons in sham-operated rats, while having no effect on firing rate in the lesioned rats. After treatment with GABA_A receptor antagonist picrotoxinin, the local application of DOI, at the same dose, increased the mean firing rate of the neurons in sham-operated rats; however, DOI did not alter the firing activity of the neurons in the lesioned rats. These results indicate that the lesion of the nigrostriatal pathway leads to hyperactivity of pyramidal neurons in mPFC, and the decreased response of pyramidal neurons to DOI, suggesting dysfunction of 5-HT_2A and 5-HT_2C receptors on pyramidal neurons and GABAergic interneurons in the 6-OHDA-lesioned rats.     

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.     

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.     

A 5-HT1B receptor agonist inhibits light-induced suppression of pineal melatonin production

Serotonin (5-HT) modulates the phase adjusting effects of light on the mammalian circadian clock through the activation of presynaptic 5-HT_1B receptors located on retinal terminals in the suprachiasmatic nucleus (SCN). The current study was conducted to determine whether activation of 5-HT_1B receptors also alters photic regulation of nocturnal pineal melatonin production. Systemic administration of the 5-HT_1B receptor agonist TFMPP attenuated the inhibitory effect of light on pineal melatonin synthesis in a dose-related manner with an apparent ED₅₀ value of 0.9 mg/kg. The effect of TFMPP on light-induced melatonin suppression was blocked by the 5-HT₁ receptor antagonist, methiothepin, but not by the 5-HT_1A antagonist, WAY 100,635, consistent with the involvement of 5-HT_1B receptors. The results are consistent with the interpretation that activation of presynaptic 5-HT_1B receptors on retinal terminals in the SCN attenuates the effect of light on pineal melatonin production, as well as on circadian phase.     

DOI disrupts prepulse inhibition of startle in rats via 5-HT2A receptors in the ventral pallidum

Serotonin (5-HT)_2A receptors are known to be involved in prepulse inhibition of the startle response (PPI), a measure of sensorimotor inhibition that is deficient in schizophrenia, Huntington's disease, and obsessive compulsive disorder. In the present studies, the localization of the 5-HT_2A receptors responsible for modulating PPI was investigated using central injections of the hallucinogenic 5-HT₂ agonist DOI and the novel 5-HT_2A antagonist MDL 100,907. 5-HT_2A receptors are densely localized in the nucleus accumbens (NAC) and the ventral pallidum (VP), areas known to be important components of neural circuitry that mediates the ventral forebrain modulation of PPI. In the present studies, it was found that the infusion of DOI (0.0–5.0 μg/0.5 μl) into the VP disrupted PPI without having effects on startle reactivity. In contrast, similar infusions into the NAC had no effect on PPI or startle reactivity. The infusion of MDL 100,907 into the VP was found to increase PPI by itself and to attenuate the PPI-disruptive effects of systemically administered DOI. It is concluded that 5-HT_2A receptors within the VP are important for the modulation of PPI, presumably through interactions at intrinsic GABAergic or cholinergic interneurons.     

Studies on the involvement of the dopaminergic system in the 5-HT2 agonist (DOI)-induced premature responding in a five-choice serial reaction time task

The present experiments investigated whether the enhanced premature (impulsive) responding induced by DOI, [(±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride], a 5-HT_2A/2C receptor agonist, is mediated by activation of the dopaminergic system and if this effect of DOI occurs in the nucleus accumbens. Therefore, the effects of a dopamine (D_1/2) receptor antagonist given alone or combined with DOI were examined on the performance of rats in a five-choice serial reaction time (5-CSRT) task. Secondly, the effects of DOI in nucleus accumbens core and shell were studied, in order to find the target brain area for DOI-induced premature responding. The results indicate that DOI (0.1 mg/kg, subcutaneously) increases the number of premature responses, as found previously. α-Flupenthixol (0.03 mg/kg), a D_1/2 dopamine receptor antagonist, and raclopride (0.015 mg/kg), a D₂ receptor antagonist, attenuated the DOI-induced enhancement in premature responding. SCH 23390 (0.005 mg/kg), a selective D₁ receptor antagonist with little affinity to 5-HT₂ receptors totally blocked the effect of DOI. Those doses of DA antagonists did not significantly decrease premature responding when given alone. On the other hand, higher doses of all of these dopamine antagonists increased the number of omissions and decreased the number of ITI hole responses. In contrast to subcutaneous administration, direct injections of DOI (1, 3, and 10 μg bilaterally) to the nucleus accumbens shell or core had no effect on premature responding. These results suggest that the activation of the dopamine system mediates, at least in part, the effect of a 5-HT₂ agonist on premature responding, but the nucleus accumbens is not the primary site for this action.     

Study of mechanisms of calcitonin analgesia in mice: Involvement of 5-HT3 receptors

The analgesic effect of calcitonin when serotonin (5-HT) concentration is increased and the involvement of some 5-HT receptors were studied using the writhing test in mice. 5-hydroxytryptophan (5-HTP) administration increased both 5-HT levels in the central nervous system (CNS) and calcitonin analgesia. The 5-HT_1A agonist (±)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) diminished calcitonin analgesia, this effect being antagonised by the 5-HT_1A antagonist (WAY 100, 135). As the stimulation of 5-HT_1A autoreceptors reduces the turnover of 5-HT, the effect of 8-OH-DPAT on calcitonin analgesia may be attributed to this decrease. The 5-HT_2A–2C agonist (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI) diminished calcitonin analgesia. A sub-analgesic dose of the 5-HT_2A antagonist ketanserin failed to prevent this effect. The 5-HT₃ agonist (±)-2-methyl-5-hydroxytryptamine maleate (2-methyl-5-HT) potentiated calcitonin analgesia, whereas it was significantly reduced by the 5-HT₃ antagonist tropisetron. The effect of 2-methyl-5-HT on calcitonin analgesia was also reversed by tropisetron, This result suggests that the 5-HT₃ receptor may play an important role in the relationship between calcitonin and the serotonergic system. Tropisetron also reversed the analgesia induced by calcitonin plus 5-HTP corroborating importance of the 5-HT₃ receptors.     

Effects of the 5-HT3 receptor agonist 1-(m-chlorophenyl)-biguanide in the rat kindling model of epilepsy

The present study was conducted to identify serotonin (5-HT) receptor subtypes involved in the development of amygdala (AM) kindling. We used 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT_1A agonist, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT₂ agonist, both of which were injected subcutaneously 15 min prior to each daily electrical stimulation to the rat AM. Treatment with 8-OH-DPAT (1 mg/kg) slightly suppressed behavioral and electrographic seizure development during the course of kindling. In contrast, DOI (1 mg/kg) strongly facilitated kindling development and reduced the number of stimulations needed to produce generalized seizures. These facilitatory effects of DOI were completely blocked by pretreatment with a 5-HT₂ antagonist ketanserin. The present results suggest that the activation of 5-HT_1A receptors can retard the development of AM kindling, whereas 5-HT₂ receptors play a facilitatory role in this developmental seizure process.     

Involvement of 5-HT1B receptors in the anticonflict effect of m-CPP in rats

Summary The anticonflict activity of m-CPP, a non-selective agonist of 5-HT receptors, was studied in the drinking conflict test in rats. m-CPP administered in doses of 0.125–0. 5 mg/kg increased the number of punished licks, the maximum effect having been observed after a dose of 0.25 mg/kg. The anticonflict effect of m-CPP (0.25 mg/kg) was antagonized by the non-selective 5-HT antagonist metergoline (1–4 mg/kg) and by the -adrenoceptor blocker SDZ 21009 (2 and 4 mg/kg) with affinity for 5-HT_1A and 5-HT_1B receptors. On the other hand, the 5-HT_1A receptor antagonist NAN-190 (0.5 and 1 mg/kg), the 5-HT₂ receptor antagonist ritanserin (0.25 and 0.5 mg/kg), and the -blockers betaxolol (8 mg/kg) and ICI 118,551 (8 mg/kg) with no affinity for 5-HT receptors did not affect the effect of m-CPP. The effect of m-CPP was not modified, either, in animals with the 5-HT lesion produced by p-chloroamphetamine.These results suggest that the anticonflict effect of m-CPP described above results from stimulation of 5-HT_1B receptors — most probably these which are located postsynaptically.     

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-HT_1A agonist and antagonist drugs. Administration of low doses of the selective 5-HT_1A agonists 8-OH-DPAT (5–20 μg/kg) and ipsapirone (20–100 μg/kg) produced rhythmic slow activity (theta) in the hippocampal EEG within 30 s. Similar effects were observed with BMY 7378 (20 and 100 μg/kg), which acts as an agonist at presynaptic (somatodendritic) 5-HT_1A receptors and as an antagonist at postsynaptic 5-HT_1A 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 μg/kg) was greatly attenuated by spiperone (1 mg/kg), a highly effective 5-HT_1A 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-HT_1A receptors, and subsequent inhibition of serotonin neurotransmission, facilitates the appearance of hippocampal theta activity in awake cats.     

Serotonin 5-HT2 receptors activate local GABA inhibitory inputs to serotonergic neurons of the dorsal raphe nucleus

The purpose of the present study was to characterize the synaptic currents induced by bath-applied serotonin (5-HT) in 5-HT cells of the dorsal raphe nucleus (DRN) and to determine which 5-HT receptor subtypes mediate these effects. In rat brain slices, 5-HT induced a concentration-dependent increase in the frequency of inhibitory postsynaptic currents (IPSCs) in 5-HT neurons recorded intracellularly in the ventral part of the DRN (EC₅₀: 86 μM); 5-HT also increased IPSC amplitude. These effects were blocked by the GABA_A receptor antagonist, bicuculline (10 μM) and by the fast sodium channel blocker, TTX, suggesting that 5-HT had increased impulse flow in local GABAergic neurons. DAMGO (300 nM), a selective μ-agonist, markedly suppressed the increase in IPSC frequency induced by 5-HT (100 μM) in the DRN. A near maximal concentration of the selective 5-HT_2A antagonist, MDL100,907 (30 nM), produced a large reduction (70%) in the increase in IPSC frequency induced by 100 μM 5-HT; SB242,084 (30 nM), a selective 5-HT_2C antagonist, was less effective (24% reduction). Combined drug application suppressed the increase in 5-HT-induced IPSC frequency almost completely, suggesting involvement of both 5-HT_2A and 5-HT_2C receptors. Unexpectedly, the phenethylamine hallucinogen, DOI, a partial agonist at 5-HT_2A/2C receptors, caused a greater increase (+334%) in IPSC frequency than did 5-HT 100 μM (+80%). This result may be explained by an opposing 5-HT_1A inhibitory effect since the selective 5-HT_1A antagonist, WAY-100635, enhanced the 5-HT-induced increase in IPSCs. These results indicate that within the DRN–PAG area there may be a negative feedback loop in which 5-HT induces an increase in IPSC frequency in 5-HT cells by exciting GABAergic interneurons in the DRN via 5-HT_2A and, to a lesser extent, 5-HT_2C receptors. Increased GABA tone may explain the previous observation of an indirect suppression of firing of a subpopulation of 5-HT cells in the DRN induced by phenethylamine hallucinogens in vivo.     

Electrophysiological characterization of 5-HT receptors on rat petrosal neurons in dissociated cell culture

The petrosal ganglion supplies chemoafferent pathways via the glossopharyngeal (IXth) nerve to peripheral targets which release various neurotransmitters including serotonin (5-HT). Here, we combined rapid 5-HT application with patch clamp, whole-cell recording to investigate whether 5-HT receptors are expressed on isolated petrosal neurons (PN), cultured from 7–12 day-old rat pups. In responsive cells, the dominant effect of 5-HT was a rapid depolarization associated with a conductance increase in 43% of the neurons (53/123); however, in a minority population (6%; 8/123), 5-HT caused membrane depolarization associated with a conductance decrease. In the former group, 5-HT produced a transient inward current (I_5-HT) in neurons voltage-clamped near the resting potential (-60 mV); the effect was mimicked by the 5-HT₃ receptor-specific agonist, 2-methyl-5-HT, suggesting it was mediated by 5-HT₃ receptors. Further, I_5-HT was selectively inhibited by the 5-HT₃ receptor-specific antagonist MDL72222 (1–10 μM), but was unaffected by either 5-HT₁/5-HT₂ receptor antagonist, spiperone, or by 5-HT₂ receptor-specific antagonist, ketanserin (50–100 μM). I_5-HT displayed moderate inward rectification and had a mean reversal potential (±S.E.M.) of −4.3±6.6 mV (n=6). Application of 5-HT (dose range: 0.1–100 μM) produced a dose–response curve that was fitted by the Hill equation with EC₅₀=3.4 μM and Hill coefficient=1.6 (n=8). The activation phase of I_5-HT (10 μM 5-HT at −60 mV) was well fitted by a single exponential with mean (±S.E.M.) time constant of 45±30 ms (n=6). The desensitization phase of I_5-HT was best fitted by a single exponential with mean (±S.E.M.) time constant of 660±167 ms (n=6). Fluctuation analysis yielded an apparent mean single-channel conductance (±S.E.M) of 2.7±1.5 pS (n=4) at −60 mV. In the minority (6%) population of neurons which responded to 5-HT with a conductance decrease, the depolarization was blocked by the 5-HT₂ receptor antagonist, ketanserin (50 μM). Taken together, these results suggest that 5-HT₃ receptors are the major subtype expressed by rat petrosal neurons, and therefore are candidates for facilitating chemoafferent excitation in response to 5-HT released from peripheral targets.     

Inhibitory effects of tandospirone, a 5-HT1A agonist, on medial vestibular nucleus neurons responding to lateral roll tilt stimulation in rats

An electrophysiological study was performed using chloral hydrate-anesthetized rats to determine whether tandospirone, a 5-HT_1A agonist, affects neuronal activities of the medial vestibular nucleus (MVN), since serotonergic innervation and 5-HT_1A receptors are present in this nucleus. Tandospirone applied microiontophoretically at a current of 20–60 nA caused an inhibition of tilt-induced firing of α-type neurons, which showed increased and decreased firing with lateral tilt ipsilateral and contralateral to the recording site, respectively, along with that of β-type neurons which exhibited the reverse responses to ipsilateral and contralateral tilt stimulation. The inhibition was antagonized during simultaneous, iontophoretic application of WAY-100635 (20–60 nA), a 5-HT_1A receptor antagonist, although WAY-100635 alone rarely affected spontaneous or tilt-induced firing in either type of neurons. These results suggest that tandospirone acts on a 5-HT_1A receptor to inhibit transmission of otolith information to α- and β-type MVN neurons.     

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.     

The 5-HT1A agonist 8-OH-DPAT increases the number of spike-wave discharges in a genetic rat model of absence epilepsy

The effects of the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) on the epileptiform activity has been investigated in adult WAG/RIJ rats. Either intraperitoneal (0.1–0.5 mg/kg) or intracerebroventricular (2–20 μg/rat) administration of 8-OH-DPAT caused marked, dose-dependent increases in the number and mean cumulative duration of spike-wave discharges. These effects were attenuated by NAN-190, a 5-HT_1A receptor antagonist. These data indicate that serotonergic system regulates the epileptiform activity in this genetic model of human absence epilepsy.     

Stimulation of spinal 5-HT(2A/2C) receptors potentiates the capsaicin-induced in vivo release of substance P-like immunoreactivity in the rat dorsal horn

Stimulation of spinal serotonin (5-HT)_2A/2C receptors has previously been reported to lead to either a pro-nociceptive or an anti-nociceptive response. Behavioral data have indicated that the pro-nociceptive effect is related to the release of substance P (SP). The aim of this in vivo microdialysis study was to investigate if stimulation of spinal 5-HT_2A/2C receptors by the selective agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) induces spontaneous or capsaicin-evoked increase in the release of SP-like immunoreactivity (SP-LI) in the rat dorsal horn. A dose of capsaicin (25 μM in the perfusion medium administered for 30 min), which did not lead to a significant release of SP-LI on its own, induced a significant increase of greater than 4-fold of the SP-LI level following spinal application of 50 nmol DOI. Higher (500 nmol) or lower (5 nmol) doses of DOI failed to induce a similar effect. In rats with a peripheral inflammation, induced by carrageenan, capsaicin (25 μM) induced a non-significant increase of SP-LI. A significant 8-fold increase of the SP-LI level was detected following administration of 50 nmol DOI in combination with capsaicin. The effect of DOI, which was completely prevented by co-administration of the 5-HT_2A receptor antagonist ketanserin in control animals without peripheral inflammation, was only partly blocked in animals with carrageenan induced peripheral inflammation. In conclusion, stimulation of 5-HT_2A/2C receptors facilitates the capsaicin-evoked release of SP-LI in the dorsal horn in both animals with and without carrageenan-induced unilateral inflammation. The observation that the highest dose of DOI failed to induce SP-LI release may be due to an inhibitory postsynaptic action at this dose.     

Risperidone attenuates and reverses hyperthermia induced by 3,4-methylenedioxymethamphetamine (MDMA) in rats

3,4-Methylenedioxymethamphetamine (MDMA, “ecstasy”) is a widely used recreational drug. Despite an increase in the number of fatalities related to its use, no definite therapeutic method has been established thus far. In the present study, risperidone's ability to attenuate MDMA-induced hyperthermia and its mechanism of action were investigated in rats. The pharmacological effect of MDMA was evaluated using microdialysis. In the body temperature experiment, administration of risperidone before and after MDMA administration significantly suppressed MDMA-induced hyperthermia in a dose-dependent fashion. Furthermore, risperidone completely inhibited MDMA-induced hyperthermia at a low ambient temperature. Moreover, pretreatment with ritanserin, ketanserin, or R-96544, all of which are 5-HT_2A-receptor antagonists, significantly prevented MDMA-induced hyperthermia. On the other hand, pretreatment with WAY-100635 (a 5-HT_1A receptor antagonist), SB 206553 (a 5-HT_2B/2C receptor antagonist), or SB 242084 (a 5-HT_2C receptor antagonist) did not prevent MDMA-induced hyperthermia. Pretreatment with haloperidol, which blocks the dopamine (DA) receptors D₂ and D₁, significantly prevented MDMA-induced hyperthermia. However, sulpiride and L-741626, which are D₂ receptor blockers, did not prevent MDMA-induced hyperthermia. Pretreatment with SCH 23390 (a D₁ receptor antagonist) significantly prevented MDMA-induced hyperthermia. Furthermore, postadministration of ritanserin, haloperidol, and SCH23390 reversed MDMA-induced hyperthermia. These results demonstrate that the mechanism underlying the suppression of MDMA-induced hyperthermia by risperidone is primarily based on the drug's potent 5-HT_2A receptor blocking effect, and to a lesser extent, on its D₁ receptor blocking effect. A microdialysis study showed that when MDMA (10 mg/kg) was subcutaneously (s.c.) injected into the rats, the DA and serotonin (5-HT) levels in the anterior hypothalamus of the rats increased approximately 10- and 50-fold, respectively, as compared to their preadministration levels. These increases in the DA and 5-HT levels after MDMA injection were significantly suppressed by pretreatment with risperidone (0.5 mg/kg). This suggested that both the DA and 5-HT systems were involved in the induction of hyperthermia by MDMA. Taken together, the present study's results indicate that risperidone may be an effective drug for the treatment of MDMA-induced hyperthermia in humans.     

The effect of serotonin1A receptor agonism on antipsychotic drug-induced dopamine release in rat striatum and nucleus accumbens

Serotonin (5-HT)_1A receptor agonism may be of interest in regard to both the antipsychotic action and extrapyramidal symptoms (EPS) of antipsychotic drugs (APD) based, in part, on the effect of 5-HT_1A receptor stimulation on the release of dopamine (DA) in the nucleus accumbens (NAC) and striatum (STR), respectively. We investigated the effect of R(+)-8-hydroxy-2-(di-n-propylamino)-tetralin (R(+)-8-OH-DPAT) and n-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-n-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY100635), a selective 5-HT_1A receptor agonist and antagonist, respectively, on basal and APD-induced DA release. In both STR and NAC, R(+)-8-OH-DPAT (0.2 mg/kg) decreased basal DA release; R(+)-8-OH-DPAT (0.05 mg/kg) inhibited DA release produced by the 5-HT_2A/D₂ receptor antagonists clozapine (20 mg/kg), low dose risperidone (0.01 and 0.03 mg/kg) and amperozide (10 mg/kg), but not that produced by high dose risperidone (0.1 and 1.0 mg/kg) or haloperidol (0.01–1.0 mg/kg), potent D₂ receptor antagonists. This R(+)-8-OH-DPAT-induced inhibition of the effects of clozapine, risperidone and amperozide was antagonized by WAY100635 (0.05 mg/kg). WAY100635 (0.1–0.5 mg/kg) alone increased DA release in the STR but not NAC. The selective 5-HT_2A receptor antagonist M100907 (1 mg/kg) did not alter the effect of R(+)-8-OH-DPAT or WAY100635 alone on basal DA release in either region. These results suggest that 5-HT_1A receptor stimulation inhibits basal and some APD-induced DA release in the STR and NAC, and that this effect is unlikely to be mediated by an interaction with 5-HT_2A receptors. The significance of these results for EPS and antipsychotic action is discussed.