Effects of the 5-HT7 receptor antagonist SB-269970 on rat hormonal and temperature responses to the 5-HT1A/7 receptor agonist 8-OH-DPAT

The physiological function of 5-HT(7) receptors is not yet fully determined. This study was designed to characterize the involvement of 5-HT(7) receptor in rat body temperature regulation and in adrenocorticotropic hormone (ACTH) and corticosterone secretion. In the first part of our study, acute administration of SB-269970 (0.1-1 mg/kg, i.p.), a potent and selective 5-HT(7) receptors antagonist, dose-dependently prevented 5-HT(1A/7) receptor agonist 8-OH-DPAT (0.1 mg/kg, s.c.)-induced hypothermia and when the 5-HT(1A) receptor antagonist WAY-100,635 was co-injected with SB-269970, a reduction of the latter hypothermia was obtained in an additive manner. In contrast, 1 mg/kg (i.p.) of SB-269970 failed to prevent 8-OH-DPAT (0.5 mg/kg, s.c.)-induced increase of ACTH and corticosterone plasma levels. In conclusion, the present results unveil an additive effect of both 5-HT(1A) and 5-HT(7) receptors in core body temperature regulation.     

Effects of 5-HT in globus pallidus on haloperidol-induced catalepsy in rats

Histological, behavioral and electrophysiological studies have suggested that 5-HT may regulate motor function by affecting globus pallidus neurons activity. In this study, the effects of 5-HT in globus pallidus on haloperidol-induced catalepsy and its possible receptor mechanisms were examined in rats using bar tests. Bilateral microinjection of 5-HT (10 μM) into globus pallidus significantly attenuated haloperidol-induced catalepsy. This anticataleptic effect was completely counteracted by selective 5-HT_1B receptors antagonist SB-224289 (10 μM), while partly reversed by selective 5-HT₄ receptors antagonist GR-113808 (1 μM). In addition, the selective 5-HT₇ receptors antagonist SB-269970 (1 μM) partly reversed the anticataleptic effect of 5-HT only at the incipient period after the intrapallidal injection. In conclusion, 5-HT in globus pallidus could attenuate haloperidol-induced catalepsy via multiple receptor mechanisms.     

5-HT1A, but not 5-HT2 and 5-HT7, receptors in the nucleus raphe magnus modulate hypoxia-induced hyperpnoea

Aim: In the present study, we assessed the role of 5‐hydroxytryptamine (5‐HT) receptors (5‐HT_1A, 5‐HT₂ and 5‐HT₇) in the nucleus raphe magnus (NRM) on the ventilatory and thermoregulatory responses to hypoxia. Methods: To this end, pulmonary ventilation (V_E) and body temperature (T_b) of male Wistar rats were measured in conscious rats, before and after a 0.1 μL microinjection of WAY‐100635 (5‐HT_1A receptor antagonist, 3 μg 0.1μL^?1, 56 mm ), ketanserin (5‐HT₂ receptor antagonist, 2 μg 0.1μL^?1, 36 mm ) and SB269970 (5‐HT₇ receptor antagonist, 4 μg 0.1 μL^?1, 103 mm ) into the NRM, followed by 60 min of severe hypoxia exposure (7% O₂). Results: Intra‐NMR microinjection of vehicle (control rats) or 5‐HT antagonists did not affect V_E or T_b during normoxic conditions. Exposure of rats to 7% O₂ evoked a typical hypoxia‐induced anapyrexia after vehicle microinjections, which was not affected by microinjection of WAY‐100635, SB269970 or ketanserin. The hypoxia‐induced hyperpnoea was not affected by SB269970 and ketanserin intra‐NMR. However, the treatment with WAY‐100635 intra‐NRM attenuated the hypoxia‐induced hyperpnoea. Conclusion: These data suggest that 5‐HT acting on 5‐HT_1A receptors in the NRM increases the hypoxic ventilatory response.     

Distribution of 5-HT7 receptors in the human brain: a preliminary autoradiographic study using [3H]SB-269970

The distribution of the 5-HT7 receptor was analyzed using the selective 5-HT7 receptor antagonist radioligand [3H]SB-269970 and human brain whole hemisphere autoradiography. The results indicated that 5-HT7 receptors are most abundantly localized in the anterior thalamus and in the dentate gyrus. Other regions containing intermediate levels of 5-HT7 receptors included the hypothalamus, anterior cingulate gyrus, hippocampus, amygdala and certain brainstem nuclei. The distribution pattern obtained in this study supports the involvement of this receptor subtype in affective behavior and cognition.     

Central 5-HT7 receptors are critical for reflex activation of cardiac vagal drive in anaesthetized rats

5-Hydroxytryptamine (5-HT; serotonin)-containing neurones contribute to reflex activation of parasympathetic outflow in a number of species, but the 5-HT receptors mediating these effects have yet to be fully determined. The present experiments demonstrate that central 5-HT₇ receptors are involved in the vagal bradycardia evoked during the cardiopulmonary reflex, baroreflexes and the chemoreflex, as well as other autonomic changes caused by these reflexes. The experiments examined the effects of the selective 5-HT₇ receptor antagonists SB-269970 and SB-656104 on these reflexes. For the cardiopulmonary reflex, when compared to time-matched vehicle control experiments, intracisternal application of SB-269970 (30–300 μg kg⁻¹, i.c. ) dose-dependently attenuated the evoked bradycardia. At the highest dose, SB-269970 also attenuated the reflex hypotension and sympathoinhibition. The structurally different 5-HT₇ receptor antagonist SB-656104 (100 μg kg⁻¹, i.c. ) similarly attenuated the reflex bradycardia and hypotension. SB-269970 (100 μg kg⁻¹, i.c. ) also attenuated the bradycardias evoked by electrical stimulation of aortic nerve afferents and the baroreflex evoked by the pressor response to phenylephrine (3–25 μg kg⁻¹, i.v. ). The gain of the baroreflex was also significantly attenuated (0.15 ± 0.06 versus 0.34 ± 0.06 ms mmHg⁻¹). Finally, SB-269970 (100 μg kg⁻¹, i.c. ) significantly attenuated both the bradycardia and sympathoexcitation evoked by the chemoreflex. These data indicate that central 5-HT₇ receptors play an important facilitatory role in the reflex activation of vagal outflow to the heart.     

Elucidating the role of 5-HT(1A) and 5-HT(7) receptors on 8-OH-DPAT-induced behavioral recovery after experimental traumatic brain injury

8-OH-DPAT is a 5-HT(1A/7) receptor agonist that enhances behavioral recovery after traumatic brain injury (TBI). This study is a first attempt to decipher whether the benefits induced by 8-OH-DPAT after TBI are mediated by 5-HT(1A) or 5-HT(7) receptors. A single i.p. injection of 8-OH-DPAT (0.5 mg/kg) alone or co-administered with either the 5-HT(1A) or 5-HT(7) receptor antagonists WAY 100635 (0.5 mg/kg) or SB 269970 HCl (2.0 mg/kg), respectively, or vehicle control (1.0 mL/kg) was given 15 min after cortical impact or sham injury. Function was assessed by established motor and cognitive tests. No difference in motor performance was observed among the TBI groups. Spatial acquisition was enhanced, relative to vehicle controls, by 8-OH-DPAT alone and when co-administered with WAY 100635, but not when combined with SB 269970 HCl. These data imply that 5-HT(1A) receptor antagonism does not abate the 8-OH-DPAT-induced cognitive benefits, but 5-HT(7) receptor antagonism does, which suggests that the 8-OH-DPAT-induced benefits in this single administration paradigm may be mediated more by 5-HT(7) versus 5-HT(1A) receptors. Evaluation of a specific 5-HT(7) receptor agonist will further elucidate the contribution of 5-HT(1A) and 5-HT(7) receptors on behavioral recovery conferred by acute 8-OH-DPAT treatment after TBI.     

Slow excitatory post-synaptic potentials in myenteric AH neurons of the guinea-pig ileum are reduced by the 5-hydroxytryptamine7 receptor antagonist SB 269970

Serotonin (5-HT) is a key modulator of neuronal excitability in the central and peripheral nervous system. In the enteric nervous system, 5-HT causes a slow depolarization in the intrinsic sensory neurons, but the receptor responsible for this has not been correlated with known gene products. The aim of this study was to determine whether the newly characterized 5-HT7 receptor may participate in the 5-HT-mediated depolarization of, and synaptic transmission to, the intrinsic sensory neurons of the guinea-pig ileum. Intracellular electrophysiological recordings were made from intrinsic sensory neurons identified as myenteric AH neurons from guinea-pig ileum. 5-HT (5 microM) applied to the cell body evoked both a fast depolarization (5-HT3 mediated) and/or a slow depolarization (5-HT1P-like). The 5-HT1/5/7 receptor agonist 5-carboxamidotryptamine (5-CT) (5 microM) evoked only a slow depolarization. When the fast depolarization evoked by 5-HT was blocked with granisetron (1 microM, 5-HT3 receptor antagonist), only a slow depolarization remained; this was abolished by the 5-HT7 receptor antagonist SB 269970 (1 microM, control: 14+/-2 mV, granisetron+SB 269970: -1+/-2 mV). The slow depolarization evoked by 5-CT was also significantly reduced by SB 269970 (control: 14+/-1 mV, SB 269970: 5+/-2 mV) suggesting a 5-HT7 receptor was activated by exogenous application of 5-CT and 5-HT. Slow excitatory postsynaptic potentials evoked by stimulating descending neural pathways (containing serotonergic fibers) were reduced by SB 269970 (control: 8+/-3 mV, SB 269970: 3+/-1 mV). However, SB 269970 had no effect on slow excitatory postsynaptic potentials evoked by stimulation of circumferential (tachykinergic) pathways (control: 7+/-1 mV, SB 269970: 6+/-1 mV). These data are consistent with the presence on enteric AH neurons of functional 5-HT7 receptors that participate in slow synaptic transmission.     

Serotonergic mechanisms on breathing modulation in the rat locus coeruleus

The locus coeruleus (LC) is a noradrenergic nucleus that plays an important role in the ventilatory response to hypercapnia. This nucleus is densely innervated by serotonergic fibers and contains high density of serotonin (5-HT) receptors, including 5-HT_1A and 5-HT₂. We assessed the possible modulation of respiratory response to hypercapnia by 5-HT, through 5-HT_1A and 5-HT₂ receptors, in the LC. To this end, we determined the concentrations of 5-HT and its metabolite 5-hydroxyindole-3-acetic acid (5-HIAA) in the LC after hypercapnic exposure. Pulmonary ventilation (V e, plethysmograph) was measured before and after unilateral microinjection (100 nL) of WAY-100635 (5-HT_1A antagonist, 5.6 and 56 mM), 8-OHDPAT (5-HT_1A/7 agonist, 7 and 15 mM), Ketanserin (5-HT_2A antagonist, 3.7 and 37 mM), or (±)-2,5-dimethoxy-4-iodoamphetaminehydrochloride (DOI; 5-HT_2A agonist, 6.7 and 67 mM) into the LC, followed by a 60-min period of 7% CO₂ exposure. Hypercapnia increased 5-HTIAA levels and 5-HIAA/5-HT ratio within the LC. WAY-100635 and 8-OHDPAT intra-LC decreased the hypercapnic ventilatory response due to a lower tidal volume. Ketanserin increased CO₂ drive to breathing and DOI caused the opposite response, both acting on tidal volume. The current results provide evidence of increased 5-HT release during hypercapnia in the LC and that 5-HT presents an inhibitory modulation of the stimulatory role of LC on hypercapnic ventilatory response, acting through postsynaptic 5-HT_2A receptors in this nucleus. In addition, hypercapnic responses seem to be also regulated by presynaptic 5-HT_1A receptors in the LC.     

Facilitatory effects of WAY-100635, a 5-HT1A receptor antagonist, on lordosis in female rats

Effects of the serotonin (5-HT) receptor 1A antagonist, WAY-100635, on lordosis in female rats were examined. Ovariectomized rats were implanted with a silicon tube containing estradiol and behavioral tests were performed. Next, 5, 10 or 20 mg/kg bw WAY-100635 or saline was injected subcutaneously in female rats with a lordosis quotient (LQ) from 10 to 30 and the behavioral test was performed again. As a result, the mean LQs in 10 or 20 mg WAY-100635-treated groups were higher than in the saline-treated group (P < 0.01 and P < 0.05, 10 or 20 mg groups versus saline, respectively). In the experiment on the time-course of change in LQ after injection with 10 mg WAY-100635, the mean LQ was increased (P < 0.01, versus saline) 15 min after the injection and high levels persisted for 1 h. This finding shows that WAY-100635 has the potency to enhance lordosis behavior acutely in female rats with a low estrous state. In order to investigate relationships between the 5-HT(1A) receptor and the GABA(B) receptor in regulating lordosis, 10 mg baclofen, a GABA(B) receptor agonist, was injected and this was followed 1 h later by the injection of 10 mg/kg WAY-100635. Mean LQ decreased after the injection of baclofen (P < 0.0001, versus placebo-treated control), but the decrease in LQs was not reversed by injection with WAY-100635.     

Modulation of locomotor activity by multiple 5-HT and dopaminergic receptor subtypes in the neonatal mouse spinal cord

Recently, it has been shown that bath-applied 5-HT can elicit fictive locomotion from perinatal mouse preparations. Since 5-HT acts on multiple receptor subtypes, the focus of this study was to examine which receptor families contribute to the genesis and modulation of locomotor activity. Blockade of 5-HT(2) (ketanserin or N-desmethylclozapine) or 5-HT(7) receptors (SB-269970) could reversibly block or modulate the locomotor-like pattern. A 5-HT(2) agonist (alpha-methyl-5-HT) was shown to be capable of activating the rhythm. Bath application of 5-HT(7) agonists (5-CT) generally led to a tonic increase in neurogram discharge, accompanied by bouts of rhythmic activity. Blockade of dopaminergic receptors (D(1) [R-(+)-SCH-23390 or LE 300]/D(2) [(+/-)-sulpiride or L-741,626] ) could reversibly disrupt the rhythm and most effectively did so when the D(1) and D(2) antagonists were added together. Conversely, 5-HT(2) and D(1)/D(2) agonists can interact to evoke locomotor activity. Overall, our data show that, in the neonatal mouse preparation, 5-HT evoked locomotion is partly dependent on activation of 5-HT(2), 5-HT(7), and dopaminergic receptor subtypes.     

5-HT acting on 5-HT(1/2) receptors does not participate in the in vitro hypoxic respiratory depression

The involvement of serotoninergic mechanisms in the central respiratory depression produced by hypoxia was studied in the newborn rat brainstem-spinal cord preparation. The respiratory frequency measured by the C4 ventral root activity was recorded. 5-HT (30 microM) superfusion elicited a rapid increase in respiratory frequency, prevented by a treatment with methysergide (a 5-HT(1/2) receptor antagonist) (40 microM). To investigate the possible participation of 5-HT in hypoxic respiratory depression, this concentration of methysergide was added to the bathing medium during hypoxia. Methysergide did not modify the decrease in respiratory frequency produced by hypoxia. In order to ensure that other 5-HT subtype receptors were not involved in hypoxic respiratory depression, 5-HT was added to the bath during hypoxic-methysergide tests; no effect on respiratory frequency was observed. These results suggest that in the newborn rat brainstem-spinal cord preparation, serotoninergic mechanisms are not involved in the elaboration of the in vitro respiratory response to hypoxia.     

Serotonin receptor subtypes involved in vagus nerve stimulation-induced phrenic long-term facilitation in rats

Episodic vagus nerve stimulation (VNS) induces phrenic long-term facilitation (LTF, a persistent augmentation of phrenic nerve activity after the stimulation ends), sensitive to the serotonin 5-HT(1,2,5,6,7) receptor antagonist methysergide and similar to that elicited by episodic hypoxia or carotid sinus nerve stimulation. This study examined the effect of ketanserin (5-HT(2) antagonist) or clozapine (5-HT(2,6,7) antagonist) on VNS-induced LTF in anesthetized, vagotomized, paralyzed and ventilated rats to determine which receptor subtype(s) is involved. Three episodes of 5 min VNS (50 Hz, 0.1 ms, approximately 500 microA) with 5 min intervals elicited phrenic LTF in control (amplitude: 38% above baseline at 60 min post-VNS) and ketanserin (2 mg x kg(-1), i.p.) pre-treated rats (45%), but not clozapine (3 mg x kg(-1)) rats (8%). These data suggest that unlike hypoxia-induced LTF (5-HT(2) receptor-dependent), VNS-induced LTF requires non-5-HT(2) serotonin receptors, perhaps 5-HT(6) and/or 5-HT(7) subtype(s).     

The role of 5-HT1A receptors in the proliferation and survival of progenitor cells in the dentate gyrus of the adult hippocampus and their regulation by corticoids

These experiments explore the role of 5-HT1A receptors in the regulation of cell proliferation in the dentate gyrus of the intact and adrenalectomized adult rat. Depleting 5-HT with p-chlorophenylalanine (300 mg/kg initially followed by 100 mg/kg/day) or stimulating 5-HT1A receptors with 8-OH-DPAT (1 mg/kg or 2 mg/kg, s.c. injections twice daily) for 14 days had no effect on cell proliferation as measured by Ki-67 or BrdU (5-bromo-3-deoxyuridine) immunocytochemistry in the dentate gyrus. However, combined treatment with p-chlorophenylalanine followed by 8-OH-DPAT significantly increased cell proliferation compared with p-chlorophenylalanine alone. Micro-injection of the 5-HT neurotoxin 5,7-dihydroxytryptamine into the fimbria-fornix (3.0 microg/side) and the cingulate bundle (1.8 microg/side) depleted hippocampal 5-HT locally but did not change cell proliferation 3 weeks after the surgery. However, 8-OH-DPAT (1 mg/kg, twice daily) stimulated cell proliferation in the dentate gyrus of hippocampal 5-HT-depleted rats compared with controls. These results suggest that 5-HT(1A) modulates cell proliferation in the hippocampus by a direct post-synaptic effect. Previous studies demonstrate that adrenalectomy increases hippocampal 5-HT1A receptor expression and binding, and thus we investigated whether the effect of adrenalectomy on cell proliferation and survival was dependent on the activity of the 5-HT1A receptors. In contrast to the null effect following twice-daily s.c. injection, 8-OH-DPAT (2.0 mg/kg/day) delivered by s.c. osmotic pumps increased proliferation in intact rats. The 5-HT1A antagonist WAY-100635 (1.5 mg/kg/day also delivered by osmotic pump) by itself did not alter cell proliferation, confirming that reduced serotonin activity does not change proliferation, but blocked the effect of 8-OH-DPAT. However, WAY-100635 could not block the stimulating action of adrenalectomy cell proliferation. 5-HT1A mRNA expression was not altered in the hippocampus by adrenalectomy. Thus, the effect of adrenalectomy on cell proliferation and survival is not 5-HT1A dependent, despite the interaction between 5-HT1A and corticosterone.     

Spinal pharmacology of antinociception produced by microinjection of mu or delta opioid receptor agonists in the ventromedial medulla of the rat

This study examined the role of spinal GABAergic, serotoninergic and alpha(2) adrenergic receptors in the antinociception produced by the microinjection of equi-antinociceptive doses of selective opioid receptor agonists in the nucleus raphe magnus (NRM) or the nucleus reticularis gigantocellularis pars alpha (NGCpalpha) of the rat. Rats were pretreated with intrathecal administration of either the GABA(A) receptor antagonist bicuculline, the GABA(B) receptor antagonist CGP35348, the serotonin(1/2) receptor antagonist methysergide, the alpha(2) adrenergic receptor antagonist yohimbine or saline. Ten minutes later, either the delta(1) opioid receptor agonist [D-Pen(2,5)]enkephalin (DPDPE), delta(2) opioid receptor agonist [D-Ala(2),Glu(4)]deltorphin (DELT) or mu opioid receptor agonist [D-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) was microinjected into the NRM, NGCpalpha or sites in the medulla outside these two regions. The increase in tail-flick latency produced by microinjection of DPDPE into the NRM or NGCpalpha was antagonized by intrathecal pretreatment with either methysergide or yohimbine. Intrathecal pretreatment with CGP35348 antagonized the antinociception produced by microinjection of DPDPE in the NRM, whereas bicuculline antagonized the antinociception produced by microinjection of DPDPE in the NGCpalpha. The increase in tail-flick latency produced by microinjection of DELT into the NGCpalpha, but not the NRM was antagonized by intrathecal pretreatment with yohimbine or CGP35348. Intrathecal pretreatment with methysergide or bicuculline did not antagonize the antinociception produced by microinjection of DELT into either the NRM or the NGCpalpha. The increase in tail-flick latency produced by microinjection of DAMGO in the NRM was antagonized by intrathecal pretreatment with methysergide or CGP35348, but not by bicuculline or yohimbine. Taken together, these results support the hypothesis that the antinociception produced by activation of delta(1), delta(2) or mu opioid receptors in the rostral ventromedial medulla is mediated by different neural substrates.     

Involvement of median raphe nucleus 5-HT1A receptors in the regulation of generalized anxiety-related defensive behaviours in rats

Changes in 5-HT_1A receptor-mediated neurotransmission at the level of the median raphe nucleus (MRN) are reported to affect the expression of defensive responses that are associated with generalized anxiety disorder (e.g. inhibitory avoidance) but not with panic (e.g. escape). The objective of this study was to further explore the involvement of MRN 5-HT_1A receptors in the regulation of generalized anxiety-related behaviours. Results of experiment 1 showed that intra-MRN injection of the 5-HT_1A/7 receptor agonist 8-OH-DPAT (0.6 nmol) in male Wistar rats impaired the acquisition of inhibitory avoidance, without interfering with the performance of escape in the elevated T-maze test of anxiety. Pre-treatment with the 5-HT_1A receptor antagonist WAY-100635 (0.18 nmol) fully blocked this anxiolytic-like effect. As revealed by experiment 2, intra-MRN injection of 8-OH-DPAT (0.6, 3 or 15 nmol) also caused anxiolytic effect in rats submitted to the light–dark transition test, another animal model that has been associated with generalized anxiety. In the same test, intra-MRN injection of WAY-100635 (0.18, 0.37 or 0.74 nmol) caused the opposite effect. Overall, the current findings support the view that MRN 5-HT neurons, through the regulation of 5-HT_1A somatodendritic autoreceptors, are implicated in the regulation of generalized anxiety-associated behaviours.     

Serotonergic and noradrenergic facilitation of the visceromotor reflex evoked by urinary bladder distension in rats with inflamed bladders

Bladder inflammation resulting from intravesical administration of zymosan significantly enhances the visceromotor reflex (VMR) evoked by urinary bladder distension (UBD). The present study examined whether intrathecal (i.t.) administration of receptor antagonists to either noreprinephrine (NE) or serotonin (5-HT) altered this enhancement effect. I.t. administration of the non-specific 5-HT receptor antagonist methysergide (30 μg), the 5-HT₃ receptor antagonist ondansetron, or the 5-HT_1A receptor antagonist WAY 100635 eliminated the enhancement effect produced by intravesical zymosan and also tended to reduce electromyographic (EMG) responses to UBD in non-inflamed rats. I.t. administration of either the non-specific NE receptor antagonist phentolamine (30 μg) or the α₁ antagonist WB 4101 also eliminated the enhancement effect, whereas i.t. administration of the α₂ antagonist yohimbine failed to significantly affect the enhancement effect. The effects of phentolamine and methysergide were not mediated by changes in bladder compliance. This is the first study to demonstrate that bladder hypersensitivity resulting from bladder inflammation is partly mediated by 5-HT and NE facilitatory effects. Based on these and previous findings we conclude that the net nociceptive response to bladder distension under conditions of bladder inflammation represents a complex interaction of facilitatory influences of spinal 5-HT and NE, and inhibitory influences of spinal opioids.     

Opposing electrophysiological actions of 5-HT on noncholinergic and cholinergic neurons in the rat ventral pallidum in vitro

The ventral pallidum in rat is a basal forebrain structure that contains neurons that project in the limbic striatopallidal circuitry and magnocellular cholinergic corticopetal neurons. Because 5-hydroxytryptamine (5-HT) terminals on dorsal raphe projections form close appositions with these neurons, we made patch-clamp recordings in immature rat brain slices to determine whether they are modulated by postsynaptic 5-HT receptors. Inward currents were predominantly induced by 5-HT in noncholinergic neurons, which were distinguished from cholinergic neurons by immunohistochemical and electrophysiological criteria. The inward current induced by 5-HT was mimicked and occluded when adenylyl cyclase was stimulated with forskolin, and was almost abolished when h-currents in noncholinergic neurons were blocked with cesium. Consistent with 5-HT(7) receptor activation of h-curents by cAMP in other brain regions, we found inward currents were mimicked by the mixed 5-HT(1)/5-HT(7) agonists 5-methoxytryptamine, and by 5-carboxamidotryptamine (5-CT), which was more potent than 5-HT. In contrast, 5-HT(1) preferring 8-OH-DPAT was a weak partial agonist, and the 5-HT(1)-selective antagonist pindolol had no effect. However, despite this profile, antagonists that bind at the 5-HT(7) receptor only partly reduced the agonist inward current (SB-269970 and clozapine), or had no effect (mianserin and pimozide). We found in cholinergic neurons that 5-HT predominantly induced hyperpolarizing currents, which were carried by potassium channels, and were smaller than currents induced by 8-OH-DPAT and 5-CT. We conclude from this study that ascending 5-HT projections from the dorsal raphe could have direct and opposite effects on the activities of neurons within the limbic striatopallidal and cholinergic corticopetal circuitry in the ventral pallidum.     

The lateral habenula regulates defensive behaviors through changes in 5-HT-mediated neurotransmission in the dorsal periaqueductal gray matter

Chemical stimulation of the lateral nucleus of the habenula (LHb), an area implicated in the regulation of serotonergic activity in raphe nuclei, affects the acquisition of inhibitory avoidance and escape expression of rats submitted to the elevated T-maze test of anxiety. Here, we investigated whether facilitation of 5-HT-mediated neurotransmission in the dorsal periaqueductal gray (dPAG) accounts for the behavioral consequences in the elevated T-maze induced by chemical stimulation of the LHb. The dPAG in the midbrain, which is innervated by 5-HT fibers originating from the dorsal raphe nucleus (DRN), has been consistently implicated in the genesis/regulation of anxiety- and fear-related defensive responses. The results showed that intra-dPAG injection of WAY-100635 or ketanserin, 5-HT_1A and 5-HT_2A/2C receptor antagonists, respectively, counteracted the anti-escape effect caused by bilateral intra-LHb injection of kainic acid (60 pmol/0.2 μl). Ketanserin, but not WAY-100635, blocked kainic acid's facilitatory effect on inhibitory avoidance acquisition. Overall, the results suggest that the pathway connecting the LHb to the DRN is involved in the control of 5-HT release in the dPAG, and facilitation of 5-HT-mediated neurotransmission in the latter area distinctively impacts upon the expression of anxiety- and fear-related defensive behaviors. While stimulation of 5-HT_1A receptors selectively affects escape performance, 5-HT_2A/2C receptors modulate both inhibitory avoidance and escape.     

Stimulation of the lateral hypothalamus produces antinociception mediated by 5-HT1A, 5-HT1B and 5-HT3 receptors in the rat spinal cord dorsal horn

The lateral hypothalamus is part of an efferent system that modifies pain at the spinal cord dorsal horn, but the mechanisms by which lateral hypothalamus-induced antinociception occur are not fully understood. Previous work has shown that antinociception produced from electrical stimulation of the lateral hypothalamus is mediated in part by spinally projecting 5-hydroxytryptamine (5-HT) neurons in the ventromedial medulla. To further examine the role of the lateral hypothalamus in antinociception, the cholinergic agonist carbamylcholine chloride (125 nmol) was microinjected into the lateral hypothalamus of female Sprague-Dawley rats and nociceptive responses measured on the tail-flick and foot-withdrawal tests. Intrathecal injections of the selective 5-HT1A, 5-HT1B, 5-HT3 receptor antagonists, WAY 100135, SB-224289, and tropisetron, respectively, and the non-specific antagonist methysergide, were given. Lateral hypothalamus stimulation with carbamylcholine chloride produced significant antinociception that was blocked by WAY 100135, tropisetron, and SB-224289 on both the tail-flick and foot-withdrawal tests. Methysergide was not different from controls on the tail flick test, but increased foot-withdrawal latencies compared with controls. These results suggest that the lateral hypothalamus modifies nociception in part by activating spinally projecting serotonin neurons that act at 5-HT1A, 5-HT1B, and 5-HT3 receptors in the dorsal horn.     

Radioligand binding analysis of knockout mice reveals 5-hydroxytryptamine(7) receptor distribution and uncovers 8-hydroxy-2-(di-n-propylamino)tetralin interaction with alpha(2) adrenergic receptors

In the present autoradiographic study, we took advantage of 5-hydroxytryptamine(7) (5-HT(7)) receptor knockout mice to analyze the brain distribution of 5-HT(7) receptor binding sites using [(3)H]5-carboxamidotryptamine (5-CT; a 5-HT(1A/1B/1D/5/7) receptor ligand) and [(3)H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; a 5-HT(1A/7) receptor ligand). Low to moderate densities of [(3)H]5-CT (2 nM) binding sites insensitive to pindolol (10 microM, for 5-HT(1A/1B) receptor blockade) and GR-127935 (1 microM; for 5-HT(1D) receptor blockade) were observed in wild-type mice (mainly in thalamus and hypothalamus) but not in 5-HT(7) receptor knockout mice. Surprisingly, moderate to high densities of [(3)H]8-OH-DPAT (10 nM) binding sites insensitive to pindolol (10 microM) remained in 5-HT(7) receptor knockout mouse brain. These non-5-HT(1A), non-5-HT(7) binding sites were found to be adrenergic alpha(2A) receptor binding sites. In alpha(2A) receptor knockout mice low to moderate densities of [(3)H]8-OH-DPAT binding sites insensitive to pindolol but sensitive to the selective 5-HT(7) receptor antagonist SB-269970 (300 nM) were observed mainly in thalamus and hypothalamus. Therefore, in addition to 5-HT(1A) and 5-HT(7) binding sites, [(3)H]8-OH-DPAT also binds to alpha(2A) receptor binding sites in wild-type mouse brain. [(3)H]8-OH-DPAT (in the presence of pindolol and 1 microM RX-821002 for alpha(2) receptor blockade) and [(3)H]5-CT (in the presence of pindolol and GR-127935) bind to a similar receptor binding population corresponding to 5-HT(7) binding sites. Detailed anatomical mapping of 5-HT(7) receptor binding sites in wild-type mouse brain was then performed using both radioligands in the presence of suitable pharmacological agents for non-5-HT(7) receptor binding sites blockade. The mapping revealed binding sites consistent with the mRNA distribution with the highest densities found in anterior thalamic nuclei.