Low doses of 8-OH-DPAT prevent the impairment of spatial learning caused by intrahippocampal scopolamine through 5-HT(1A) receptors in the dorsal raphe

1. We studied the effects of low doses of 8-OH-DPAT, a 5-HT(1A) receptor agonist, on the impairment of spatial learning caused by scopolamine injected into the CA1 region of the dorsal hippocampus of rats performing a two-platform spatial discrimination task. 2. Bilateral injections of 4 microg (in 1 microl) of scopolamine into the CA1 region of the dorsal hippocampus 10 min before each training session impaired choice accuracy with no effect on choice latency and errors of omission. 3. Administered subcutaneously 20 min before each training session, 8-OH-DPAT 10 and 30 (but not 3) microg kg(-1) did not modify choice accuracy but prevented the impairment by intrahippocampal scopolamine. 4. Injection of 1.0 microg (in 0.5 microl) of WAY 100635, a 5-HT(1A) receptor antagonist, into the dorsal raphe 5 min before scopolamine had no effect on choice accuracy and latency or errors of omission and did not modify the effect of scopolamine, but completely antagonized the effect of 10 and 30 microg kg(-1) 8-OH-DPAT on scopolamine-induced impairment of choice accuracy. 5. The results confirm previous findings that stimulation of presynaptic 5-HT(1A) receptors in the dorsal raphe attenuates the deficit of spatial learning caused by blockade of cholinergic excitatory input on hippocampal pyramidal cells. 6. Drugs that stimulate presynaptic 5-HT(1A) receptors such as 5-HT(1A) receptor partial agonists may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.     

(3)H]-8-OH-DPAT binding in the rat brain raphe area: involvement of 5-HT(1A) and non-5-HT(1A) receptors

1. The 5-HT(1A) agonist 8-OH-DPAT has been shown to have additional 5-HT uptake inhibiting properties. The present work was undertaken to examine further the binding of [(3)H]-8-OH-DPAT in the raphe area of the rat brain, a region rich in 5-HT(1A) receptors and 5-HT uptake sites. 2. 5-HT inhibited [(3)H]-8-OH-DPAT binding in a biphasic manner (pK(i1): 8.82+/-0.01, pK(i2): 6.07+/-0.05, n=4) with the low affinity site representing 36+/-4% of the total population. A biphasic inhibition curve was found also with the 5-HT(1A) antagonist, WAY 100635 (pK(i1): 8.65+/-0.17, pK(i2): 4.26+/-0.38, n=3). In the presence of 1 microM WAY 100635 to mask 5-HT(1A) receptors, 5-HT inhibited [(3)H]-8-OH-DPAT binding in a monophasic manner (pK(i): 6.04+/-0.07, n=3). 3. The affinities of various compounds for sites labelled by [(3)H]-8-OH-DPAT in the presence of 1 microM WAY 100635 and for sites labelled by [(3)H]-citalopram (a selective 5-HT uptake inhibitor) were determined. There was a significant correlation between pK(i) values at 5-HT uptake sites and at non-5HT(1A) sites labelled by [(3)H]-8-OH-DPAT (r=0.80, P<0. 001, n=17), suggesting these latter sites to be 5-HT uptake sites. 4. Whereas the affinities of R(+) and S(-) enantiomers of 8-OH-DPAT for the 5-HT uptake site are similar, R(+)8-OH-DPAT has 10 times higher affinity for the non-5-HT(1A) site than S(-)8-OH-DPAT and was considered as an outlier in the correlation. It is suggested that [(3)H]-8-OH-DPAT labels other, as yet unknown binding sites in the raphe.     

Median raphe, but not dorsal raphe, application of the 5-HT1A agonist 8-OH-DPAT stimulates rat motor activity

Local application of the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in the median raphe of rats caused locomotor stimulation. In contrast, dorsal raphe application of the compound induced flat body posture, which was discontinuous and not dose-dependent, and therefore distinct from that characteristic for postsynaptic 5-HT receptor-mediated behaviour. Injection of 8-OH-DPAT into the dorsal raphe or median raphe caused neither forepaw treading nor head-weaving; stiff tail and sniffing occurred inconsistently. By activating somatodendritic 5-HT1A autoreceptors in the median raphe, 8-OH-DPAT may disinhibit locomotor-enforcing neural pathways that receive 5-HT afferents from this nucleus. The data suggest that median raphe and dorsal raphe 5-HT neurons have different roles in motor control.     

5-HT1A receptors in the dorsal hippocampus mediate the anxiogenic effect induced by the stimulation of 5-HT neurons in the median raphe nucleus.

We evaluated the involvement of dorsal hippocampus (DH) 5-HT1A receptors in the mediation of the behavioral effects caused by the pharmacological manipulation of 5-HT neurons in the median raphe nucleus (MRN). To this end, we used the rat elevated T-maze test of anxiety. The results showed that intra-DH injection of the 5-HT1A/7 agonist 8-OH-DPAT facilitated inhibitory avoidance, an anxiogenic effect, without affecting escape. Microinjection of the 5-HT1A antagonist WAY-100635 was ineffective. In the elevated T-maze, inhibitory avoidance and escape have been related to generalized anxiety and panic disorders, respectively. Intra-MRN administration of the excitatory amino acid kainic acid, which non-selectively stimulates 5-HT neurons in this brain area facilitated inhibitory avoidance and impaired escape performance, but also affected locomotion. Intra-MRN injection of WAY-100635, which has a disinhibitory effect on the activity of 5-HT neurons in this midbrain area, only facilitated inhibitory avoidance. Pre-administration of WAY-100635 into the DH blocked the behavioral effect of intra-MRN injection of WAY-100635, but not of kainic acid. These results indicate that DH 5-HT1A receptors mediate the anxiogenic effect induced by the selective stimulation of 5-HT neurons in the MRN.     

Somatodendritic 5-HT1A receptors are critically involved in the anxiolytic effects of 8-OH-DPAT

In the rat shock-induced ultrasonic vocalization test, the anxiolytic effects of the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) obtained after systemic (IP) and intracerebral injection into the dorsal raphe nucleus (DRN) were selectively abolished by pretreatment with the 5-HT_1A receptor antagonist WAY-100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide trihydrochloride]. This blockade was demonstrated both after systemic and DRN application of WAY-100635. Therefore, it is concluded that the anxiolytic effects of 8-OH-DPAT are mediated by activation of somatodendritic 5-HT_1A receptors.     

Anti-aggressive effects of agonists at 5-HT1B receptors in the dorsal raphe nucleus of mice

Rationale In rodents, serotonin 1B (5-HT_1B) agonists specifically reduce aggressive behaviors, including several forms of escalated aggression. One form of escalated aggression is seen in mice that seek the opportunity to attack another mouse by accelerating their responding during a fixed interval (FI) schedule. Responses preceding the opportunity to attack may reflect aggressive motivation. Objective This study investigated the effects of two 5-HT_1B receptor agonists on the motivation to fight and the performance of heightened aggression. Materials and methods Male mice were housed as “residents” and performed nose-poke responses on an FI 10-min schedule with the opportunity to briefly attack an “intruder” serving as the reinforcer. In the first experiment, the 5-HT_1B receptor agonist, CP-94,253 (0–10 mg/kg, IP), was given 30 min before the FI 10 schedule. To confirm that CP-94,253 achieved its effects via 5-HT_1B receptors, the 5HT_1B/1D receptor antagonist, GR 127935 (10 mg/kg, IP) was administrated before the agonist injection. In the second experiment, the 5-HT_1B agonist CP-93,129 (0–1.0 μg) was microinjected into the dorsal raphe 10 min before the FI 10 schedule. Results The agonists had similar effects on all behaviors. CP-94,253 and CP-93,129 significantly reduced the escalated aggression towards the intruder at doses lower than those required to affect operant responding. The highest doses of CP-94,253 (10 mg/kg) and CP-93,129 (1.0 μg) decreased the rate and accelerating pattern of responding during the FI 10 schedule; lower doses were less effective. GR 127935 antagonized CP-94,253’s effects on all other behaviors, except response rate. Conclusions These data extend the anti-aggressive effects of 5-HT_1B agonists to a type of escalated aggression that is rewarding and further suggest that these effects are associated with actions at 5-HT_1B receptors in the dorsal raphe.     

The selective serotonin (5-HT)1A receptor ligand, S15535, displays anxiolytic-like effects in the social interaction and Vogel models and suppresses dialysate levels of 5-HT in the dorsal hippocampus of freely-moving rats

RATIONALE: The benzodioxane, S15535, possesses low intrinsic activity and marked selectivity at 5-HT1A receptors, hippocampal populations of which are implicated in anxious states. OBJECTIVE: Herein, we examined its potential anxiolytic actions in relation to its influence upon extracellular levels of 5-HT in the dorsal hippocampus of freely-moving rats. Its effects were compared with those of other anxiolytic agents: the 5-HT1A agonists, buspirone and 8-hydroxy-2-(di-n-propylamino)-tetralin HBr (8-OH-DPAT), the 5-HT2C antagonist, SB206,553 and the benzodiazepine, diazepam. METHODS: Potential anxiolytic actions were evaluated in the Vogel conflict paradigm (increase in punished responses) and the social interaction (SI) test (increase in active SI) in rats. Extracellular levels of 5-HT were determined by microdialysis. RESULTS: In analogy to diazepam. S15535 increased punished responses in the Vogel test. This action was dose dependently expressed over a broad (16-fold) dose range. Buspirone and 8-OH-DPAT were likewise active, but yielded highly biphasic dose-response curves. SB206,553 was dose dependently active in this model. In the SI test, S15535 similarly mimicked the anxiolytic-like effect of diazepam and was active over a broad dose range. Buspirone and 8-OH-DPAT again showed biphasic dose-response curves, as did SB206,553. In both the Vogel and SI tests, the anxiolytic-like effects of S15535 were abolished by the selective 5-HT1A receptor antagonist, WAY100,635, which was inactive alone. S15535 exerted its anxiolytic-like effects with a more pronounced separation to motor-disruptive doses than the other drugs. Finally, S15535 suppressed dialysate levels of 5-HT in the dorsal hippocampus, an action abolished by WAY100,635. Buspirone, 8-OH-DPAT and diazepam, but not SB206,553, also reduced 5-HT levels. CONCLUSION: Likely reflecting its distinctive ability to selectively and preferentially activate pre- versus postsynaptic 5-HT1A receptors, S15535 suppresses hippocampal 5-HT release and displays marked anxiolytic-like effects over a broad dose range in the relative absence of motor perturbation.     

Localisation of mRNA for h5-HT1B and h5-HT1D receptors in human dorsal raphe

In the mammalian mesencephalon, virtually all serotoninergic neurons are located in the raphe nuclei and the adjacent reticular formation. Pharmacological evidence obtained in rodents suggests that terminal and somatodendritic autoreceptors controlling serotonin (5-hydroxytryptamine, 5-HT) release belong to the 5-HT1B/D subtype of receptors, whereas somatodendritic autoreceptors controlling neuronal cell firing are predominantly of the 5-HT1A subtype. This study investigated the presence of h5-HT1D and h5-HT1B receptor mRNA within the subdivisions of the dorsal raphe of post-mortem human brains by means of in situ hybridisation. Although differences in the labelling intensity, which may be caused by different pre- and/or post-mortem conditions, were obvious among the specimens, all brains expressed both the h5-HT1D and the h5-HT1B mRNA in dorsal raphe neurons. In comparison to h5-HT1D mRNA, expression of h5-HT1B mRNA was slightly more abundant. Information on the existence and localisation of h5-HT1D and h5-HT1B receptors in human dorsal raphe neurons confirms that both subtypes may serve an autoreceptor function in humans. This finding is of pharmacological relevance since these receptors are potential new targets for therapeutic interventions in psychiatric disorders such as depression and anxiety.     

Role of 5-HT(1A) and 5-HT(2) receptors of dorsal and median raphe nucleus in tolerance to morphine analgesia in rats

Several studies indicate that central serotonergic neurons have important role in morphine analgesia and tolerance. The aim of this study was to investigate possible role of 5-HT(1A) and 5-HT(2) receptors in dorsal and median raphe nucleus on development of tolerance to analgesic effect of morphine using hot plate test. Chronic injection of 5-HT(1A) receptor agonist 8-OH-DPAT (8-hydroxy-2-[di-n-propylamino]tetralin) (2, 4 and 8 mug/rat/day) to dorsal raphe nucleus (DRN) delayed tolerance to morphine analgesia, whereas injection of the same doses of 8-OH-DPAT to the median raphe nucleus (MRN) did not alter tolerance to morphine. In addition, chronic administration of ketanserin (1.5, 3 and 6 mug/rat/day), as a 5-HT(2) receptors antagonist, in DRN and MRN did not produce any significant effect. We conclude that 5-HT(1A) receptors of DRN are involved in tolerance to antinociceptive effect of morphine. However, the exact mechanism of interaction between serotonergic and opioidergic systems is not clear and remains to be elucidated.     

8-OH-DPAT acts on both 5-HT1A and 5-HT7 receptors to induce hypothermia in rodents

Studies using selective drugs and knockout mice have demonstrated that the 5-HT(7) receptor plays an instrumental role in serotonin-induced hypothermia. There is also evidence supporting an involvement of the 5-HT(1A) receptor, although mainly from studies using 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT(1A/7) receptor agonist. Here we studied the effects of 8-OH-DPAT and selective antagonists for the 5-HT(1A) and 5-HT(7) receptors on body temperature in rats, wild-type (5-HT(7)(+/+)) mice and knockout (5-HT(7)(-/-)) mice. At lower doses (0.3-0.6 mg/kg, i.p.), 8-OH-DPAT decreased body temperature in 5-HT(7)(+/+) mice but not in 5-HT(7)(-/-) mice. At a higher dose (1 mg/kg, i.p.) 8-OH-DPAT induced hypothermia in both 5-HT(7)(-/-) and 5-HT(7)(+/+) mice. The 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide (WAY-100135) (10 mg/kg, i.p.) inhibited the effect of 8-OH-DPAT at all doses in rats and mice. In 5-HT(7)(+/+) mice the selective 5-HT(7) receptor antagonist (R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol (SB-269970) (10 mg/kg, i.p.) fully inhibited the hypothermia induced by 0.3 mg/kg 8-OH-DPAT, but not that of higher doses. In rats, SB-269970 caused a 60% inhibition of the hypothermia induced by 0.3 mg/kg 8-OH-DPAT. Thus, both 5-HT(7) and 5-HT(1A) receptors are involved in a complex manner in thermoregulation, with the 5-HT(7) receptor being more important at lower, possibly more physiological, concentrations.     

Effects of local application of 5-HT and 8-OH-DPAT into the dorsal and median raphe nuclei on core temperature in the rat

The core temperature of male Wistar rats was measured after local application of 5-HT (10 µg) or 8-OH-DPAT (5 µg) into the dorsal (DR) or the median raphe (MR) nuclei. The core temperature was measured by a rectal thermistor probe, 20 and 60 min after the injection procedure started. The injected volume was 0.5 µl and injections were made by means of 31G needles, at a rate of 0.33 µl min^–1. The raphe nuclei were approached at 30° in order to avoid penetration of the cerebral aqueduct or to avoid the DR with injections aimed for the MR. The application of 5-HT or the 5-HT_1A agonist 8-OH-DPAT into the DR produced a marked decrease in core temperature, whereas injections into the MR had no effect. These results demonstrate an important role for the DR in temperature regulation in the rat. The fact that the 5-HT_1A agonist 8-OH-DPAT produced a decrease in core temperature, together with the observation that administration of the 5-HT₁ antagonist (–)pindolol antagonized the 5-HT as well as the 8-OH-DPAT-induced decrease, indicates the involvement of DR 5-HT_1A receptors in rat thermoregulation.     

Electrophysiological evidence for postsynaptic 5-HT(1A) receptor control of dorsal raphe 5-HT neurones.

Postsynaptic 5-hydroxytryptamine(1A) (5-HT(1A)) receptors have been proposed to participate in the control of dorsal raphe 5-HT neurone activity. To further investigate this hypothesis we performed single-unit extracellular recordings in anaesthetized rats. Pertussis toxin (2 microg/4 microl/day; 2 days, 24-72 h before the experiment) was applied close to the dorsal raphe nucleus to uncouple somatodendritic 5-HT(1A) autoreceptors from their effector system. After this treatment the spontaneous firing rate was higher (approximately +60% P<0.005) than in the vehicle-pretreated group. In addition, intravenous administration of 8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT) inhibited 5 out of 11 cells of the pertussis toxin-pretreated group (ED(50)=1.65+/-0.94 microg/kg), whereas in the vehicle-pretreated group, all tested cells were inhibited (ED(50)=1.87+/-0.39 microg/kg). Local administration of 8-OH-DPAT did not affect cells (n=12) in pertussis toxin-pretreated rats, even at doses much higher than those needed to completely inhibit 5-HT cells in vehicle-pretreated rats (ED(50)=3.34+/-0.62 fmol). These results confirm the involvement of distal postsynaptic 5-HT(1A) receptors in the control of 5-HT neurone activity in the dorsal raphe nucleus. However, this control does not appear to be exerted on all 5-HT neurones, but rather on a subpopulation of them.     

Evidence that 5-HT1A receptors are involved in the adrenaline-releasing effects of 8-OH-DPAT in the conscious rat

Summary 8-Hydroxy-2-(di-n-propylamino)tetralin (8OH-DPAT) is a 5-HT_1A receptor-selective agonist that has recently been reported to trigger adrenal catecholamine release and hyperglycemia. The aim of this study was to analyze in the conscious rat whether the 5-HT_1A receptor subtype is involved in these effects.8-OH-DPAT (0.1–1 mg/kg, i.v.) evoked dose-dependent increases in plasma adrenaline and glucose concentrations. Increases in plasma adrenaline levels peaked 5 min after administration of 8-OH-DPAT; in contrast, plasma glucose levels rose throughout the 20 min period of analysis. Prior administration of (–)pindolol, a beta-adrenoceptor antagonist that blocks 5-HT_1A receptors, markedly diminished the rise in plasma adrenaline levels and abolished the hyperglycemia triggered by 8-OHD-PAT. On the other hand, neither the selective beta 1-adrenoceptor antagonist, betaxolol, the selective beta 2-adrenoceptor antagonist, ICI 118.551, nor the 5-HT₂ receptor antagonist, ketanserin, affected 8-OH-DPAT-induced increases in plasma adrenaline levels. In addition, except for ICI 118.551 pretreatment, which delayed the hyperglycemic effect of 8-OH-DPAT, none of these antagonists affected the rise in glycaemia evoked by 8-OHD-PAT.The data suggest that the adrenaline-releasing and a major part of the hyperglycemic effects of 8-OH-DPAT are mediated by activation of 5-HT_1A receptors. Send offprint requests to F. Chaouloff at the above address     

Chronic lithium treatment enhances the postsynaptic 5-HT1A receptor-mediated 5-HT behavioral syndrome induced by 8-OH-DPAT in rats via catecholaminergic systems

The effects of antimanic agents, including lithium, carbamazepine, clonazepam and zotepine, on the postsynaptic 5-HT_1A receptor-mediated behavioral and hypothermic responses induced by 8-OH-DPAT in rats, and on [³H]8-OH-DPAT binding to 5-HT_1A receptors in the rat hippocampus were examined. Treatment with lithium (3 mEq/kg, IP) for 14 days enhanced forepaw treading, one component of the 5-HT behavioral syndrome induced by 8-OH-DPAT, and this enhancement by lithium was abolished by catecholamine depletion with reserpine or -methyl-p-tyrosine, but not by 5-HT depletion withp-chlorophenylalanine. These data suggest that lithium enhances 5-HT_1A-mediated behavior via catecholaminergic systems. In contrast, chronic lithium treatment did not alter the hypothermic response to 8-OH-DPAT in untreated rats, as well as in rats treated with reserpine. These findings strengthen the suggestion that lithium has no direct influence on the postsynaptic 5-HT_1A-mediated response. Other antimanic agents had no effect on either forepaw treading or hypothermia induced by 8-OH-DPAT. Radioligand binding studies using [³H]-8-OH-DPAT demonstrated that chronic lithium treatment, but not other antimanic agents, caused 5-HT_1A receptor down-regulation in rat hippocampus. A discrepancy therefore exists between 5-HT_1A receptor down-regulation and unaltered 5-HT_1A-mediated behavioral and hypothermic responses in catecholamine-depleted rats after chronic lithium treatment.     

Interaction between 5-HT(1A) and 5-HT(1B) receptors: effects of 8-OH-DPAT-induced hypothermia in 5-HT(1B) receptor knockout mice

To test for adaptive compensatory changes that may have occurred in the functional activity of somatodendritic 5-HT(1A) receptors during the development of constitutive "knockout" mice lacking the 5-HT(1B) receptor subtype (5-HT(1B) -/- KO), we assayed for decrease in body temperature induced by an acute subcutaneous injection of the 5-HT(1A) receptor agonist, 8-hydroxy 2(di-n-propyl(amino)tetralin (8-OH-DPAT), either alone or in the presence of a selective 5-HT(1A) receptor antagonist, N-[4-(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide (WAY 100635). We compared dose-response curves, time course study, calculated ED(50) values (potency), maximal response to 8-OH-DPAT (efficacy) as well as measurements of the dose-dependent blockade of this response by WAY 100635 between wild-type controls and mutant mice. We found a higher efficacy of 8-OH-DPAT-induced hypothermia in 5-HT(1B) -/- KO compared to wild-type mice suggesting that an adaptive thermoregulatory process involving the functional activity of somatodendritic 5-HT(1A) receptors is altered in mutant mice lacking 5-HT(1B) receptors.     

Facilitation by 8-OH-DPAT of passive avoidance performance in rats after inactivation of 5-HT(1A) receptors

1. The effect on membrane currents of infection of mouse neuroblastoma NA cells with rabies virus was studied by using the whole-cell patch clamp technique. 2. Three types of membrane currents, namely voltage-dependent Na+ current (I(Na)), delayed rectifier K+ current (I(K-DR)) and inward rectifier K+ current (I(K-IR)) were elicited in uninfected cells. 3. In cells 3 days after infection with the virus, no detectable change was observed in morphology and membrane capacitance, but I(Na) and I(K-IR) were significantly decreased in amplitude without any appreciable difference in the time course of current activation and inactivation. The voltage-dependence of I(Na) activation was significantly shifted in the positive direction along the voltage axis with a decreased slope. I(K-DR) remained almost unaltered after the viral infection. 4. The resting membrane potential, measured with a physiological K+ gradient across the cell membrane, was decreased (depolarized) after the viral infection. The depolarization was associated with the decreased amplitude of I(K-IR). 5. These results suggest that infection of mouse neuroblastoma NA cells with rabies virus causes reduction of functional expression of ion channels responsible for I(Na) and I(K-IR), and provide evidence for possible involvement of the change in membrane properties in the pathogenesis of rabies disease.     

Time course of the effects of adrenalectomy and corticosterone replacement on 5-HT1A receptors and 5-HT uptake sites in the hippocampus and dorsal raphe nucleus of the rat brain: an autoradiographic analysis

Previous studies have shown that adrenalectomy (ADX) increases the binding of³H-DPAT to 5-HT_1A receptors in the hippocampus (HIP) and this effect is partially overcome by corticosterone (CORT) replacement. The present study investigated the time course of the effects of ADX with or without CORT replacement on serotonin (5-HT) pre- and postsynaptic systems in the HIP and dorsal raphe nucleus (DR) by quantitative autoradiography. In the HIP, ADX for 7, 10 or 14 days caused a significant increase in³H-DPAT binding in the CA₁ region (pyramidal layer), CA_2,3 region (molecular and pyramidal layers) and in the dentate gyrus (molecular and granular layers) which returned to control levels when measurements were made 35 days post-ADX. A decrease in³H-DPAT binding was observed 14 days after ADX in the DR but not in the median raphe nucleus (MR). Although replacement with CORT did not lead to a reversal in³H-DPAT binding at early time points, binding was restored to control levels 7–28 days after CORT replacement in all regions of the HIP. In the DR, CORT did not cause a reversal in³H-DPAT binding at any of the time points examined. In contrast to the effects seen on the 5-HT_1A receptor subtype, no significant change was noted on the binding of³H-CN-IMI to uptake sites for 5-HT in the HIP or DR after ADX or CORT replacement. The results of this study indicate that long-term alterations in the HPA axis lead to changes in the 5-HT_1A receptor system that are both region-specific and time-dependent.     

A single dose of 8-OH-DPAT reduces raphe binding of [3H]8-OH-DPAT and increases the effect of raphe stimulation on 5-HT metabolism.

8-Hydroxy-(di-n-propylamino)tetralin (8-OH-DPAT) has antidepressant-like effects in rats and selectively reduces presynaptic 5-HT1A function a day after administration. In the present study, the effect of 8-OH-DPAT (1 mg/kg s.c.) pretreatment on presynaptic (raphe nuclei) and postsynaptic (frontal cortex and hippocampus) [3H]8-OH-DPAT binding was studied. Bmax values were markedly reduced in the raphe, but not in the hippocampus and frontal cortex. Kd values were unchanged. Electrical stimulation of the dorsal raphe (300 microA, 1 ms, 20 Hz, 30 min) significantly increased 5-hydroxyindoleacetic acid in the frontal cortex, but not in the amygdala or the nucleus accumbens and caused smaller increases in the rest of the brain. The increase in the frontal cortex was significantly potentiated one day after giving 8-OH-DPAT. These results confirm the ability of 8-OH-DPAT to desensitise presynaptic 5-HT1A receptors and suggest that this may lead to a loss of feedback control so that, on neuronal stimulation, the increase of 5-HT function is enhanced. This effect may underlie the antidepressant-like action of 8-OH-DPAT pretreatment, i.e. its ability to oppose restraint-induced defects in locomotion on placement in an open field one day later. A requirement of presynaptic 5-HT for this behavioural effect is consistent with its prevention by the 5-HT synthesis inhibitor parachlorophenylalanine.     

Effects in the X-maze anxiety model of agents acting at 5-HT1 and 5-HT2 receptors

Three 5-HT agonists produced a dose-related fall in open/total arm entry ratio in the elevated X-maze model of anxiety at doses which did not affect total entries. The relative potency, 8-hydroxy-2-(di-n-propylamino)tetra lin (8-OH-DPAT)5-methoxy-N,N-dimethyltryptamine (5-MeODMT)>=5-methoxy-3(tetrahydropyridin-4-yl)1H-indole (RU 24969), was unrelated to the occurrence of wet dog shakes and suggests that 5-HT₁ rather than 5-HT₂ receptors may be involved. However, the 5-HT₂ receptor antagonists ritanserin, ketanserin and seganserin caused an anxiolytic-like increase in entry ratio, although only ritanserin produced this effect across the dose range tested. +/-Pindolol, an antagonist at 5-HT₁ receptors, showed a biphasic dose-response curve with a fall in entry ratio at one high dose. The effect of a submaximal dose of 8-OH-DPAT was prevented by pindolol but not by a similarly anxiolytic dose of ritanserin or diazepam. A higher dose of diazepam caused intense muscle hypotonia in combination with 8-OH-DPAT. Since open/total entry ratio appears to represent choice, rather than suppression or delay, of a response, the effects seen may indicate involvement of 5-HT receptors in anxiety separately from any change in the ability to withhold a response. The precise role of each receptor subtype, however, remains to be determined.     

Effect of 5-HT1A receptor agonists in two models of anxiety after dorsal raphe injection

The purpose of the present study was two-fold. Firstly, to present a more comprehensive analysis of the disinhibitory effects of 5-HT_1A receptor agonists after discrete dorsal raphe (DRN) injections (Higgins et al. 1988). Secondly, the effects of the 5-HT_1B receptor agonist CGS12066B and the 5-HT_1B/1C agonist mCPP were examined following injection into this nucleus. The increases in social interaction (SI) induced by intra-raphe injections of 8-OH DPAT (0.02–1 μg), buspirone (0.04–0.2 μg), ipsapirone (0.2 μg) and gepirone (0.2–1 μg) under a high light unfamiliar paradigm (HLU) were typically due to increased bout frequency, duration and a higher incidence of sniff, follow, allogroom behaviour. These increases were qualitatively similar to those seen in control animals tested under low light/familiar (LLF) conditions, thus supporting the belief that the drug-induced increases in SI reflected decreases in anxiety. Furthermore, at doses effective under the HLU condition, 8-OH DPAT, buspirone and gepirone failed to modify SI under conditions of minimal suppression (LLF paradigm). At doses which significantly increased punished responding in a water-lick conflict test 8-OH DPAT, ipsapirone and gepirone tended to also increase unpunished rates of drinking. However, in drug untreated rats, prior habituation to the test apparatus also increased unpunished drinking, suggesting some neophobia-induced suppression. At a comparatively high dose, the 5-HT_1B agonist CGS12066B (2.5 μg), but not the putative 5-HT_1B/1c agonist mCPP (0.5–12.5 μg), increased SI under the HLU condition. Considered along-side the other compounds described in this report, the relative potency of CGS12066B may be reflective of a 5-HT_1A receptor interaction. Together, these data support the proposal that the DRN is an important site through wich 5-HT_1A receptor agonists express their anxiolytic actions.