Stimulation of 5-HT1A receptors in the dorsal raphe ameliorates the impairment of spatial learning caused by intrahippocampal 7-chloro-kynurenic acid in naive and pretrained rats

OBJECTIVE: The present study investigated the effect of stimulating 5-HT(1A) receptors in the dorsal raphe on the impairment of spatial learning caused by intrahippocampal 7-chloro-kynurenic acid (7-Cl-Kyn) in naive rats and in rats familiar with the general requirements of the task. METHODS: A week after implantation of cannulae to give access to the dorsal raphe (DR) and the CA1 region of the dorsal hippocampus, rats started their 5 days acquisition training on a two-platform spatial discrimination task in a water maze. On each acquisition day, WAY 100635 and 8-OH-DPAT alone or in combination were injected into the dorsal raphe (DR) 5 min before intrahippocampal injections of 7-Cl-Kyn which was given 10 min before the training session. Similar experiments were conducted in rats that had been familiarized with the general requirements of the task by pretraining them in the water maze in the absence of distal cues. RESULTS: 7-Cl-Kyn (3 microg/microl), injected bilaterally in the CA1 region of the dorsal hippocampus, impaired choice accuracy with no significant effect on choice latency. Rats treated with 7-Cl-Kyn tended to spend more time swimming close to the pool walls and made more errors of omission than controls in the first two sessions. Administered into the DR, the 5-HT1A receptor agonist 8-OH-DPAT (1 microg/0.5 microl) had no effect on any parameter of rats' performance but antagonized the impairment of choice accuracy caused by intrahippocampal 7-Cl-Kyn. Injected into the DR, 1 microg/0.5 microl WAY 100635, a 5-HT(1A) receptor antagonist, had no effect on rats' performance or on the impairment caused by intrahippocampal 7-Cl-Kyn, but antagonized the effect of 8-OH-DPAT on the 7-Cl-Kyn-induced deficit. The non-mnemonic behavioral disturbances shown by naive rats treated with 7-Cl-Kyn were greatly reduced in pretrained rats which, nevertheless, showed a marked impairment of choice accuracy similar to that of naive rats. As in previous experiments, administration of 1 microg/0.5 microl 8-OH-DPAT in the dorsal raphe antagonized the impairment of choice accuracy caused by intrahippocampal 7-Cl-Kyn without any effect on other parameters of rats' performance. CONCLUSIONS: The results show that stimulation of presynaptic 5-HT(1A) receptors in the dorsal raphe counteracts the deficit in spatial learning caused by a reduced NMDA-mediated excitatory input on pyramidal cells in the hippocampus. The possible mechanisms and the importance of these findings for the symptomatic treatment of memory disorders in man are discussed.     

S 15535, a benzodioxopiperazine acting as presynaptic agonist and postsynaptic 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by intrahippocampal scopolamine

1 The effect of S 15535 (4-benzodioxan-5-yl)1-(indan-2-yl)piperazine), an agonist at presynaptic and antagonist at postsynaptic 5-HT1A receptors, on the impairment of spatial learning caused by intrahippocampal scopolamine in a two-platform spatial discrimination task was studied. 2 Scopolamine (4.0 microg microl-1), injected bilaterally 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 30 min before each training session, S 15535 1.0 (but not 0.3) mg kg-1 did not modify choice accuracy but prevented its impairment by intrahippocampal scopolamine. 4 WAY 100635, a 5-HT1A receptor antagonist, injected into the dorsal raphe at 1.0 microg 0.5 microl-1 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 S 15535 (1.0 mg kg-1) on scopolamine-induced impairment of choice accuracy. 5 The results confirm a previous report (Carli et al., 1998) that stimulation of presynaptic 5-HT1A receptors in the dorsal raphe counteracts the deficit caused by intrahippocampal scopolamine, probably by facilitating the transfer of facilitatory information from the entorhinal cortex to the hippocampus. 6 Drugs that stimulate action on presynaptic 5-HT1A receptors, such as S 15535 and other partial 5-HT1A receptors 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.     

Roles of 5-HT(1A) receptors in the dorsal raphe and dorsal hippocampus in anxiety assessed by the behavioral effects of 8-OH-DPAT and S 15535 in a modified Geller-Seifter conflict model

8-OH-DPAT [8-hydroxy-2-(di-N-propylamino)tetralin], a 5-HT(1A) receptor agonist, and S 15535 (4-benzodioxan-5-yl)1-(indan-2-yl)piperazine, a partial agonist at 5-HT(1A) receptors, were administered into the dorsal raphe nucleus and dorsal hippocampus and their behavioral effects were assessed in a modified Geller-Seifter conflict model. Injected into the dorsal raphe nucleus 8-OH-DPAT, 1 microg but not 0.04 or 0.2 microg 0.5 microl(-1), and S 15535, 2.5 microg but not 0.1 or 0.5 microg 0.5 microl(-1), significantly increased punished responding with no effect on rates of unpunished or time-out responding. WAY 100635, a selective 5-HT(1A) receptor antagonist, injected subcutaneously at 0. 3 mg kg(-1) 30 min before 1 microg 8-OH-DPAT or 2.5 microg S 15535 in the dorsal raphe, completely antagonized their effects on punished responding. At doses ranging from 1 to 10 microg microl(-1) injected into the CA1 region of the dorsal hippocampus neither 8-OH-DPAT nor S 15535 modified punished responding or the rates of time-out. At the highest doses, 8-OH-DPAT significantly reduced unpunished responding whereas S 15535 had the opposite effect. The results suggest that stimulation of 5-HT(1A) receptors in the dorsal raphe nucleus has anxiolytic-like effects whereas stimulation of postsynaptic receptors in the dorsal hippocampus has no anxiolytic or anxiogenic effects, at least judging from changes in rates of punished responding. These results are compatible with the hypothesis that 5-HT(1A) receptor agonists and partial agonists attenuate anxiety by reducing serotonergic transmission in brain areas innervated by the dorsal raphe nucleus.     

Differential coupling of 5-HT1A receptors occupied by 5-HT or 8-OH-DPAT to adenylyl cyclase

Summary Human serotonin (5-hydroxytryptamine, 5-HT)-1A receptors have been transfected in NIH-3T3 cells, and their coupling to adenylyl cyclase was analysed depending on (1) the number of receptor expressed, (2) the experimental conditions used, (3) the nature of the agonists. Two monoclonal cell lines were used, expressing low (45 fmol/mg) and high (500 fmol/mg) levels of 5-HT_1A receptor. Two methods were tested to study the negative coupling of the transfected 5-HT_1A receptors to adenylyl cyclase: (1) measurement of CAMP production in intact cells, (2) measurement of adenylyl cyclase activity in vitro on membrane preparations. Studies on intact cells revealed that an increase in the receptor concentration was followed by (1) an increase in the efficacies of 5-HT, 5-CT (5-carboxamidotryptamine) and 8-hydroxy2-(di-n-propylamino)tetralin (8-OH-DPAT), (2) a 2 to 3-fold increase in the potency of 5-CT and 8-OH-DPAT, but no change in the potency of 5-HT. In membrane preparations, 8-OH-DPAT dose-response curve was shifted leftwards when the receptor concentration became higher whereas the corresponding shift was smaller for 5-HT and absent for 5-CT. Surprisingly, on membrane preparations, 8-OH-DPAT was a partial agonist relative to 5-HT. The relative efficacy of 8-OH-DPAT was lower in the clone expressing the lowest level of receptor. This partial agonist behavior of 8-OH-DPAT could be modulated by the ionic conditions under which the adenylyl cyclase activity was measured. When physiological intracellular concentrations of Na⁺, Mg²⁺ and K⁺ were used, 8-OH-DPAT became an almost full agonist relative to 5-HT.These data indicate that (1) the classical pharmacological models do not exactly fit with characteristics of the negative coupling between transfected 5-HT_1A receptors and adenylyl cyclase; (2) on membranes, the experimental procedures (ionic conditions) can modify this coupling differently depending on the nature of the agonist.Abbreviations 5-HT 5-hydroxytryptamine (serotonin) - 8-OHDPAT 8-hydroxy-2-(di-n-propylamino)-tetraline - 5-CT 5-carboxamidotryptamine - IBMX isobutyl methyl xanthine - BSA bovine serum albumine - EC₅₀ half maximal efficacy - DXM dexamethasone - PTX Bordetella pertussis toxin - G protein GTP-binding protein Correspondence to A. Varrault at the above address     

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.     

Dimerization of 8-OH-DPAT increases activity at serotonin 5-HT1A receptors

[³⁵S]GTPγS binding responses can be used to measure differences between the intrinsic activity of ligands at human 5-hydroxytrypamine-_1A (h 5-HT_1A) receptors expressed in recombinant cell lines. The maximal [³⁵S]GTPγS binding response to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was lower than that to 5-HT in a recombinant C6-glial membrane preparation and dependent on the GDP concentration: it was attenuated by about 60% vs 5-HT by increasing the concentration of GDP from 0.3 to 30 and 300 μM. Whereas dimerization of 8-OH-DPAT almost did not affect its potency at h 5-HT_1A receptors (pEC₅₀: 7.45 and 7.40 for 8-OH-DPAT and its dimer at 30 μM GDP), it increased efficacy at h 5-HT_1A receptors. The maximal response to the 8-OH-DPAT dimer was systematically greater than the response to 8-OH-DPAT and identical to that to 5-HT; moreover in contrast to the 8-OH-DPAT monomer, the maximal response to the dimer was unaffected by increasing the GDP concentration. An enhanced [³⁵S]GTPγS binding response (44 to 63% vs 8-OH-DPAT) was also observed in the hippocampus, lateral septum, dorsal raphe and cingulate cortex of guinea-pig brain sections using autoradiography of 5-HT_1A receptor-activated G-proteins. Hence, the 8-OH-DPAT dimer shows increased efficacy at 5-HT_1A receptors compared to 8-OH-DPAT. The differential regulation of the maximal agonist responses by GDP suggests that the [³⁵S]GTPγS binding responses to these two ligands could be mediated by different G-protein subtypes upon activation of the 5-HT_1A receptor. Received: 23 June 1998 / Accepted: 29 July 1998     

5-HT7 receptor stimulation by 8-OH-DPAT counteracts the impairing effect of 5-HT(1A) receptor stimulation on contextual learning in mice

The principal 5-HT(1A) receptor agonist 8-Hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) impairs several different types of learning. Besides 5-HT(1A) receptors, 8-OH-DPAT stimulates 5-HT(7) receptors, but it is not known whether 5-HT(7) receptors contribute to the impairments. The 5-HT(7) receptor antagonist (2R)-1-[(3-Hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl] pyrrolidine (SB-269970) was combined with 8-OH-DPAT to dissociate 5-HT(1A) from 5-HT(7) receptor-mediated effects, in the passive avoidance task for emotional learning. SB-269970 intensified impairments caused by 8-OH-DPAT. SB-269970 alone had no effect on memory performance, but moderately decreased retention under suboptimal learning conditions. These findings indicate that 5-HT(7) receptor stimulation by 8-OH-DPAT counteracts 5-HT(1A) receptor-mediated impairments in hippocampal-dependent contextual learning.     

Evidence that the hyperphagic response to 8-OH-DPAT is mediated by 5-HT1A receptors

The 5-HT_1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) at dose of 1 mg/kg s.c. increased food intake in free feeding rats. 8-OH-DPAT-induced feeding was blocked by metergoline which has comparable affinity for 5-HT_1A, 5-HT_1B, 5-HT_1C and 5-HT₂ receptors. This is consistent with the hyperphagia being mediated by an action at 5-HT receptors. Evidence against the involvement of 5-HT₂ or 5-HT₃ receptors was provided by the lack of effect of methysergide, ketanserin, MDL 72222 and ICS 205930 on the feeding response. Blockade of the hyperphagia by (−)- but not (+)pendolol which stereoselectively interacts with 5-HT₁ receptors indicated an involvement of this receptor type. The lack of effect of ketanserin suggest that the 5-HT_1C site is not involved as it has high affinity for both 5-HT₂ and 5-HT_1C receptors. Blockade of the hyperphagia by spiperone suggest mediation by 5-HT_1A rather than 5-HT_1B receptors. Although spiperone also blocks dopamine and ₂-adrenoreceptors, involvement of these sites is unlikely as neither the DA antagonist haloperidol nor the ₂-adrenoceptor antagonist idazoxan blocked 8-OH-DPAT-included feeding. These results indicate that 8-OH-DPAT-induced feeding is mediated by 5-HT_1A receptors.     

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.     

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.     

Mediation of the antidepressant-like effect of 8-OH-DPAT in mice by postsynaptic 5-HT1A receptors.

1. The 5-hydroxytryptamine (5-HT)1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) has been evaluated in a mouse model for detecting potential antidepressants (Porsolt test). The effects of various receptor antagonists, lesions of brain monoaminergic neurones and chronic drug treatments on this 8-OH-DPAT-induced response have also been determined. 2. 8-OH-DPAT (0.3-10.0 mg kg-1, s.c.) dose-dependently increased the mobility of mice in the Porsolt test. Other selective 5-HT1A receptor ligands (0.3-30 mg kg-1, s.c.) either mimicked the 8-OH-DPAT response (ipsapirone, at 10 and 30 mg kg-1, s.c.) or were inactive (buspirone and gepirone). However, each of these compounds (< or = 100 mg kg-1, p.o.) inhibited the response to 8-OH-DPAT (3 mg kg-1, s.c.) when given concurrently. 3. The putative 5-HT1A antagonists, spiroxatrine (1-30 mg kg-1, p.o.), (+/-)-pindolol (30 mg kg-1, p.o.) and methiothepin (3-10 mg kg-1, p.o.), each attenuated the 8-OH-DPAT (3 mg kg-1, s.c.)-induced increase in mobility. 4. The dopamine D1 receptor antagonist, SCH 23390 (3-10 mg kg-1, p.o.), weakly reversed the 8-OH-DPAT response.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

WAY 100635, a 5-HT1A receptor antagonist, prevents the impairment of spatial learning caused by blockade of hippocampal NMDA receptors.

We studied the effect of WAY 100635, a 5-HT1A receptor antagonist, on the impairment of spatial learning caused by intrahippocampal injection of 3-((R)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), a competitive NMDA receptor antagonist, in a two-platform spatial discrimination task. CPP, 3 and 10 ng/microl, administered bilaterally into the CA1 region of the dorsal hippocampus 10 min before each training session, dose-dependently reduced choice accuracy in the two-platform spatial discrimination task with little or no effect on choice latency and errors of omission. A volume of 10 ng/microl intrahippocampal CPP did not affect choice accuracy or latency of a non-spatial visual discrimination task. Subcutaneous doses of 0.3 and 1 mg/kg WAY 100635 did not modify the choice accuracy, but prevented the impairment caused by 10 ng/microl intrahippocampal CPP. A dose of 20 ng/microl WAY 100635 into the dorsal hippocampus prevented the deficit caused by 10 ng/microl CPP administered in the same region. The results suggest that blockade of 5-HT1A receptors can compensate the loss of NMDA-mediated excitatory input to pyramidal cells in the hippocampus. These findings may have clinical relevance for the symptomatic treatment of memory disorders associated with reduced glutamate transmission mediated by NMDA receptors.     

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.     

Circadian rhythm in the response of central 5-HT1A receptors to 8-OH-DPAT in rats

Circadian rhythm in the behavioral responsiveness to the selective 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was studied in rats. Rats were subcutaneously injected with 8-OH-DPAT at one of the following times of day: 0000, 0400, 0800, 1200, 1600, 2000 hours. The postsynaptic 5-HT_1A receptor behavioral syndrome, that is, forepaw treading, head weaving, and flat body posture, were measured after the administration of 8-OH-DPAT. Circadian rhythms were found in each of the behavioral responses to 8-OH-DPAT. Peak responses were observed in the mid-dark phase (1200 hours) while the weakest responses were observed in the midlight phase (0000 hours). In a subsequent experiment, 8-OH-DPAT was administered intracerebroventricularly during the mid-dark phase and the mid-light phase. The behavioral responses to the drug in the middark phase were significantly higher than those in the mid-light phase. These results suggest that the function of central postsynaptic 5-HT_1A receptor exhibits circadian rhythm.     

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.     

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.