Chronic imipramine treatment sensitizes 5-HT1A and 5-HT 2 A receptors in the dorsal periaqueductal gray matter: evidence from the elevated T-maze test of anxiety

The dorsal periaqueductal gray matter (DPAG) has been implicated in the mediation of escape, a defensive behavior associated with panic disorder (PD). Chronic treatment with the anti-panic agent imipramine enhances the inhibitory effect on escape evoked by DPAG electrical stimulation of intra-DPAG administration of the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the preferential 5-HT 2 A receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI). In the present study we further explore the hypothesis that sensitization of 5-HT1A and 5-HT 2 A receptors in the DPAG is involved in the anti-panic effect of imipramine. To this end, Wistar rats, subchronically or chronically treated with imipramine, were intra-DPAG injected with 8-OH-DPAT (0.4 or 3.2 nmoles) or DOI (16 nmoles), and tested in the elevated T-maze. In addition to its possible relevance to panic disorder, this test also measures inhibitory avoidance, a behavior that has been associated with generalized anxiety disorder (GAD). The effects of these 5-HT agonists in the DPAG were also investigated in animals chronically injected with buspirone, a drug clinically effective in treating GAD, but not PD. The results showed that intra-DPAG administration of the highest dose of 8-OH-DPAT and of DOI inhibited escape, and this panicolytic-like effect was significantly higher in animals previously treated chronically, but not subchronically, with imipramine. 8-OH-DPAT (0.4 nmole), although not affecting escape in animals systemically treated with saline, had a panicolytic-like effect in those receiving long-term treatment with imipramine. Microinjection of 8-OH-DPAT (3.2 nmoles), but not of DOI, impaired inhibitory avoidance, and this anxiolytic effect did not differ between animals treated with saline or imipramine. Chronic buspirone did not change the effect of 8-OH-DPAT and DOI on inhibitory avoidance and escape. Therefore, chronic imipramine seems to sensitize both 5-HT1A and 5-HT 2 A receptors in the DPAG, strengthening the view that these receptors are involved in the mode of action of anti-panic drugs.     

Serotonin-2A receptor regulation of panic-like behavior in the rat dorsal periaqueductal gray matter: the role of GABA

Rationale  

Electrical stimulation of the dorsal periaqueductal gray (dPAG) evokes escape, a defensive response associated with panic attacks. Stimulation of 5-HT1A or 5-HT2A receptors in this midbrain area equally inhibits escape performance, even though at the molecular level these receptors cause opposite effects, i.e., activation of the former hyperpolarizes the cell membrane, while the latter excites it. A proposal has been made that 5-HT2A receptor agonists exert their inhibitory effect on escape by activating GABAergic interneurons located in the dPAG.     

5-HT2C receptor regulation of defensive responses in the rat dorsal periaqueductal gray

Activation of 5-HT2C receptors in limbic structures such as the amygdala and hippocampus increases anxiety. Indirect evidence obtained with non-selective 5-HT2C-interacting drugs suggests that the same may occur in the dPAG, a brainstem region consistently implicated in the genesis/regulation of panic attacks. In this study we used more selective agonists and antagonists to unveil the role played by dPAG 5-HT2C receptors in the regulation of anxiety- and panic-related defensive behaviors. Our results showed that intra-dPAG microinjection of the endogenous agonist 5-HT (20 nmol) or the 5-HT2C receptor agonists MK-212 (1 and 10 nmol) and RO-600175 (40 nmol) significantly increased inhibitory avoidance acquisition in rats tested in the elevated T-maze, suggesting an anxiogenic effect. 5-HT, but not the two 5-HT2C receptor agonists, inhibited escape performance. In the elevated T-maze, inhibitory avoidance and escape responses have been related to generalized anxiety and panic attacks, respectively. The behavioral effects caused by 5-HT and MK-212 were fully blocked by previous local microinjection of the 5-HT2C receptor antagonist SB-242084. Intra-dPAG injection of MK-212 also failed to affect escape expression in another test relating this behavior to panic, the electrical stimulation of the dPAG. Overall, the results indicate that 5-HT2C receptors in the dPAG are preferentially involved in the regulation of defensive behaviors related to anxiety, but not panic. This finding extends to the dPAG the prominent role that has been attributed to 5-HT2C receptors in anxiety generation.     

Synergistic mu-opioid and 5-HT1A presynaptic inhibition of GABA release in rat periaqueductal gray neurons

The periaqueductal gray (PAG) plays a critical role in descending antinociception. In mechanically dissociated rat PAG neurons, pharmacologically separated spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the nystatin-perforated patch technique. Both DAMGO, a specific mu-opioid receptor agonist, and serotonin inhibited mIPSC frequency in a dose-dependent manner without affecting mIPSC amplitude, respectively, in the same PAG neurons. The presynaptic opioid effect was blocked by a specific mu-opioid receptor antagonist, CTOP. The presynaptic serotonergic effect was mimicked by a specific 5-HT(1A) receptor agonist, 8-OH-DPAT, and blocked by the specific antagonist, NAN-190. These opioidergic and serotonergic inhibitions of GABA release employed the similar intracellular mechanism of opening 4-AP-sensitive K(+) channels via GTP-binding proteins (G-proteins). Subthreshold concentrations of DAMGO (3 nM) significantly decreased mIPSC frequency with subthreshold concentrations of serotonin (3 nM) and this effect was completely blocked by pretreatment with N-ethylmaleimide (NEM), a PTX-sensitive G-protein inhibitor. In contrast, maximum doses of DAMGO (10 microM) did not further inhibit mIPSC frequency with maximum doses of serotonin (10 microM). In conclusion, activation of presynaptic mu-opioid and 5-HT(1A) receptors synergistically inhibited GABA release. These results suggest a cellular mechanism within PAG for the analgesic effectiveness of combined therapies using opioids in conjunction with classes of anti-depressants which increase synaptic serotonin levels.     

Effect of imipramine treatments on the 5-HT1A-receptor-mediated inhibition of panic-like behaviours in rats

The escape behaviour induced in rats by injecting D,L-homocysteic acid (DLH) into the dorsal periaqueductal grey area (DPAG) was used as an animal model of panic attacks to investigate the effect of imipramine, a drug used for the treatment of panic disorder, on the sensitivity of 5-HT_1A receptors in the DPAG. Rats given imipramine (10 mg/kg per day SC for 3 weeks or IP for 2–3 days) received 250 nl saline or the 5-HT_1A agonist 8-OH-DPAT (8.6 nmol) into the DPAG 10 min before inducing the escape response with DLH. As expected, 8-OH-DPAT produced a marked decrease in the average speed of the DLH-induced flight response. The short-term treatment with imipramine changed neither the DLH-induced escape behaviour nor the effect of prior 8-OH-DPAT administration on this response. In contrast, long-term treatment with imipramine enhanced the 5-HT_1A-mediated inhibition, as the decrement in the amplitude of the flight response produced by 8-OH-DPAT was 96% after this treatment compared to 41% in controls. The injection of 8-OH-DPAT also significantly decreased the amplitude of the freezing behaviour observed at the end of the flight response in rats given imipramine for 3 weeks, but not in controls. The long-term imipramine treatment, however, did not significantly decrease the amplitude of DLH-induced flight and freezing behaviours in absence of prior 8-OH-DPAT administration. Finally, 8-OH-DPAT failed to inhibit the DLH-induced flight and freezing behaviours in rats withdrawn from imipramine after long-term treatment (10 mg/kg per day × 21 days). It is suggested that an alteration at the level of the DPAG-5-HT_1A receptor system is implicated in the therapeutic and withdrawal effect of imipramine in panic disorder. Received: 15 August 1996/Final version: 24 January 1997     

Effects of fluoxetine and buspirone on the panicolytic-like response induced by the activation of 5-HT1A and 5-HT2A receptors in the rat dorsal periaqueductal gray

Rationale Administration of 5-hydroxytryptamine (5-HT)_1A and 5-HT_2A receptor agonists into the dorsal periaqueductal gray (DPAG) inhibits escape, a defensive behavior associated with panic attacks. Long-term treatment with the antipanic compound imipramine enhances the DPAG 5-HT_1A- and 5-HT_2A-receptor-mediated inhibition of escape, implicating these receptors in the mode of action of panicolytic drugs. Objectives In the present study, we investigated whether the inhibitory effect on escape elicited by the intra-DPAG injection of 5-HT_1A and 5-HT_2A receptor agonists is also enhanced after treatment with fluoxetine, another widely used antipanic drug. The effects of fluoxetine were compared to those of buspirone, an anxiolytic drug without major effect on panic disorder. Methods Male Wistar rats, subchronically (3–6 days) or chronically (21–24 days) treated with fluoxetine (10 mg/kg i.p.) or chronically treated with buspirone (0.3 mg/kg i.p.), were intra-DPAG injected with 5-HT (20 nmol), the 5-HT_1A receptor agonist (±)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 8 nmol) or the preferential 5-HT_2A receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl) piperazine dihydrochloride (DOI; 16 nmol). The intensity of electrical current that applied to the DPAG-evoked escape behavior was measured before and after the microinjection of these agonists. Results The electrical current necessary to produce escape was increased after the microinjection of the three 5-HT receptor agonists in all groups of animals tested. However, this panicolytic-like effect was significantly higher in animals receiving long-term treatment with fluoxetine. Conclusions The results suggest that facilitation of the 5-HT_1A- and 5-HT_2A-receptor-mediated inhibition of DPAG neuronal activity is implicated in the beneficial effect of antidepressants in panic disorder.     

Chronic effects of triiodothyronine in combination with imipramine on 5-HT transporter, 5-HT(1A) and 5-HT(2A) receptors in adult rat brain.

Triiodothyronine (T3) has been shown to accelerate and potentiate the clinical response to tricyclic antidepressant (TCA) treatment in depressive disorders. The neurobiological mechanisms underlying these therapeutic effects of T3 are still unknown. Since brain serotonin (5-HT) changes have been implicated in the mode of action of TCA drugs, the effects of a chronic (7 or 21 days) administration of imipramine (10 mg/kg/day) and of a low dose of T3 (4 microg/kg/day), given alone or in combination, were investigated on the density of midbrain 5-HT transporters and of hippocampal 5-HT(1A) and cortical 5-HT(2A) receptors in adult Wistar rats. Neither single nor combined administration of imipramine and T3 for 7 days modified the density of 5-HT transporters and of 5-HT(1A) receptors. On day 21, the combination did not change imipramine- or T3-induced decrease in 5-HT transporter density whereas it prevented imipramine-induced increase in 5-HT(1A) receptor density. Whatever the treatment duration, imipramine-T3 combination potentiated imipramine-induced decrease in 5-HT(2A) receptor density. On both day 7 and day 21, T3 given alone had no effects on the density of 5-HT(1A) and 5-HT(2A) receptors. These data indicate that T3 is able to modulate the long-term adaptive changes which occur at the postsynaptic level of 5-HT neurotransmission after antidepressant treatment.     

Regulation of conditioned and unconditioned fear in rats by 5-HT1A receptors in the dorsal periaqueductal gray

Studies on the involvement of 5-HT1-mediated mechanisms in the dorsal periaqueductal gray (dPAG) of animals with past stressful experiences have not been conducted so far. We investigated the role of 5-HT1 receptors in the dPAG of rats previously submitted to contextual fear conditioning. Defensive behaviors induced by activation of the dPAG were assessed by measuring the lowest electric current applied to this structure (threshold) able to produce freezing and escape responses during testing sessions of contextual fear conditioning, in which animals were placed in a context previously paired to footshocks. The 5-HT1A function of the dPAG was evaluated by local injections of 8-OH-DPAT (4 and 8 nmol/0.2 microL) and WAY-100635 (10 nmol/0.2 microL), selective agonist and antagonist of 5-HT1A receptors, respectively. In accordance with previous studies, 8-OH-DPAT increased aversive thresholds (antiaversive effects) but injections of WAY 100635 into the dPAG did not produce significant effects on the aversive thresholds in naive rats. However, the aversive thresholds of animals exhibiting contextual fear remained unchanged with both treatments. Moreover, 8-OH-DPAT and WAY 100635 did not change the dPAG post-stimulation freezing. The present results suggest that the stressful experience of being fear conditioned has an effect on the role of the 5-HT1A receptors in mediating unconditioned fear. Also, the reduction in the regulation of the defensive behaviors by 5-HT1A-mediated mechanisms in the dPAG of these animals may underlie the stress precipitated psychopathology associated with the neural substrates of aversion of the dPAG.     

Opposite control mediated by central 5-HT1A and non-5-HT1A (5-HT1B or 5-HT1C) receptors on periaqueductal gray aversion

8-OH-DPAT, a selective 5-HT1A agonist, and mCPP, which has preferential affinity for 5-HT1B and 5-HT1C receptors, were studied for their effects on aversive brain stimulation in rats. Opposite effects were found with these two agonists: D, L-8-hydroxy-N,N-dipropyl-2-aminotetralin HBr (8-OH-DPAT; 0.1-1.0 mg/kg i.p.) dose dependently decreased the threshold for neurostimulation-induced escape behaviour while mCPP (0.1-1.0 mg/kg i.p.) dose dependently increased the threshold. The proaversive effect of 8-OH-DPAT and the antiaversive effect of mCPP suggest that 5-HT1A and non-5-HT1a (5-HT1B or 5-HT1C) receptors play distinct roles in mechanisms of aversion, perhaps at different locations in the CNS.     

Involvement of dorsal raphe nucleus and dorsal periaqueductal gray 5-HT receptors in the modulation of mouse defensive behaviors

Previous findings point to the involvement of the dorsal raphe nucleus (DRN) and dorsal periaqueductal gray (dPAG) serotonergic receptors in the mediation of defensive responses that are associated with specific subtypes of anxiety disorders. These studies have mostly been conducted with rats tested in the elevated T-maze, an experimental model of anxiety that was developed to allow the measurement, in the same animal, of two behaviors mentioned: inhibitory avoidance and one-way escape. Such behavioral responses have been respectively related to generalized anxiety disorder (GAD) and panic disorder (PD). In order to assess the generality of these findings, in the current study we investigated the effects of the injection of 5-HT-related drugs into the DRN and dPAG of another rodent species, mouse, on the mouse defense test battery (MDTB), a test of a range of defensive behaviors to an unconditioned threat, a predator. Male CD-1 mice were tested in the MDTB after intra-DRN administration of the 5-HT_1A receptor antagonist WAY-100635 or after intra-dPAG injection of two serotonergic agonists, the 5-HT_1A receptor agonist 8-OH-DPAT and the 5-HT_2A/2C receptor agonist DOI. Intra-DRN injection of WAY-100635 did not change behavioral responses of mice confronted with a rat in the MDTB. In the dPAG, both 8-OH-DPAT and DOI consistently impaired mouse escape behavior assessed in the MDTB. Intra-dPAG infusion of 8-OH-DPAT also decreased measures of mouse risk assessment in the rat exposure test. In conclusion, the current findings are in partial agreement with previous results obtained with rats tested in the elevated T-maze. Although there is a high level of similarity between the behavioral effects obtained in rats (elevated T-maze) and mice (MDTB and RET) with the infusion of 5-HT agonists into the dPAG, the same is not true regarding the effects of blockade of DRN 5-HT_1A receptors in these rodent species. These data suggest that there may be differences between mice and rats regarding the involvement of the DRN in the mediation of defensive behaviors.     

5-HT1A receptor-mediated activation of G-protein-gated inwardly rectifying K+ current in rat periaqueductal gray neurons

5-hydroxytryptamine (5-HT) has been reported to modulate analgesia produced by opioids or electrical stimulation of the periaqueductal gray (PAG). 5-HT increases K+ conductance and inhibits the firing activity of the PAG neurons. We examined the electrophysiological and pharmacological characteristics of the K+ current involved in 5-HT-induced hyperpolarization of dissociated rat PAG neurons. Among the neurons tested, 5-HT activated inward K+ currents in 30-40%, whilst the remaining 60-70% did not respond to 5-HT. 5-HT activated an inwardly rectifying K+ current (I5-HT) in a concentration- and voltage-dependent manner. I5-HT was mimicked by a 5-HT1A receptor selective agonist, 8-OH-DPAT, and was reversibly blocked by a 5-HT1A receptor antagonist, piperazine maleate, but not by a 5-HT2 receptor antagonist, ketanserin. I5-HT was sensitive to K+ channel blockers such as quinine and Ba2+, but insensitive to 4-aminopyridine, Cs+ and tetraethylammonium. I5-HT was inhibited by GDP(beta)s and was irreversibly activated by GTP(gamma)s. I5-HT was significantly suppressed by N-ethylmaleimide and pertussis toxin, but not by cholera toxin. Second messenger modulators such as staurosporin, forskolin, and phorbol-12-myristate-13-acetate did not alter I5-HT. The present study indicates that 5-HT-induced hyperpolarization of the PAG neurons results from activation of the pertussis toxin-sensitive G-protein-coupled inwardly rectifying K+ currents through 5-HT1A receptors.     

Chronic effects of imipramine and lithium on postsynaptic 5-HT1A and 5-HT1B sites and on presynaptic 5-HT3 sites in rat brain

The effects of chronic treatment with imipramine, a tricyclic antidepressant, or lithium, an antimanic-depressive illness drug, on postsynaptic serotonin-1A (5-HT1A) and 5-HT1B sites and on presynaptic 5-HT3 sites in the frontal cortex and hippocampus from rat brains were studied. Chronic i.p. administration (21 days) of imipramine reduced the maximum number of binding sites (Bmax) for postsynaptic 5-HT1A as monitored by the radioligands 3H-5-HT or 3H-8-hydroxy-2-(di-n-propylamino)tetralin (3H-8-OH-DPAT), but did not change the Bmax for postsynaptic 5-HT1B and presynaptic 5-HT3 in either the frontal cortex or the hippocampus. Chronic i.p. administration (21 days) of lithium reduced the Bmax for postsynaptic 5-HT1A sites in the hippocampus, but not in the frontal cortex. There was a specific difference between imipramine and lithium regarding the inhibitory effect on postsynaptic 5-HT1A sites in the frontal cortex. In addition, lithium decreased the affinity of presynaptic 5-HT3 sites in the hippocampus. These findings may be also consistent with the inhibitory effect of lithium on presynaptic autoreceptors, which results in an increase of 5-HT release. It is concluded that enhanced 5-HT neurotransmission which develops during chronic treatment with imipramine or lithium seems tob e related to the down-regulation of postsynaptic 5-HT1A receptors in addition to postsynaptic 5-HT2 receptors, which may also have an important role in the antidepressant effects of these drugs.     

Enhanced reactivity of 5-HT1A receptors in the rat dorsal periaqueductal gray matter after chronic treatment with fluoxetine and sertraline: evidence from the elevated T-maze

Behavioral evidence indicates that sensitization of 5-HT1A and 5-HT2A receptors in the dorsal periaqueductal gray (DPAG) may underlie the therapeutic effect of serotonin reuptake inhibitors (SRIs) in panic disorder. These results were obtained from studies using animal models that associate escape behavior with panic attacks, such as the elevated T-maze. In this test, chronic administration of the non-selective SRI imipramine enhances the inhibitory effect on escape caused by the intra-DPAG injection of the 5-HT1A receptor agonist 8-OH-DPAT. We here evaluated the generality of this finding by investigating the effect of chronic administration of two selective SRIs (SSRIs), fluoxetine and sertraline, on the reactivity of the rat DPAG 5-HT1A receptors. The results showed that both SSRIs inhibited escape behavior in the elevated T-maze, suggestive of a panicolytic-like effect. Whereas intra-DPAG injection of a low dose of 8-OH-DPAT (0.4nmol) had no effect on escape in control animals, it significantly enhanced the inhibitory effect caused by the SSRIs on this response. Microinjection of 8-OH-DPAT in SSRI-treated rats also inhibited the acquisition of inhibitory avoidance, another defensive response measured by the elevated T-maze. The results indicate that chronic administration of fluoxetine and sertraline sensitizes 5-HT1A receptors in the DPAG. Overall, they support the view that facilitation of 5-HT1A receptor-mediated neurotransmission in the DPAG is implicated in the therapeutic effect of SRIs on panic disorder.     

Effects of triiodothyronine and imipramine on basal 5-HT levels and 5-HT(1) autoreceptor activity in rat cortex

Clinical studies have shown that triiodothyronine (T3) both augments and accelerates the therapeutic response to antidepressant drugs, particularly tricyclics. There is evidence that this effect is mediated by the serotonergic system. We show here that T3 administered daily for 7 days over the range 0.02-0.5 mg/kg increases basal serotonin (5-hydroxytryptamine, 5-HT) levels, as measured by in vivo microdialysis in rat cortex, in a dose-dependent fashion. All the doses of T3 examined reduced 5-HT(1A) autoreceptor activity, as measured by the effect of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.05 mg/kg s.c.) to decrease 5-HT levels in frontal cortex. T3 administered daily for 14 days at 0.02 mg/kg also reduced 5-HT(1B) autoreceptor activity, as measured by the effect of locally administered 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93129, 10 microM) to decrease 5-HT levels. In animals administered imipramine (10 mg/kg/day by osmotic minipump) concurrently with T3 injections, no further changes in either 5-HT(1A) or 5-HT(1B) autoreceptor activity were seen. We suggest that the effect of T3 to accelerate the therapeutic actions of antidepressant drugs may be due to a combination of the actions of T3 at autoreceptors and the actions of the drugs at postsynaptic 5-HT(1A) receptors.     

Involvement of serotonin-mediated neurotransmission in the dorsal periaqueductal gray matter on cannabidiol chronic effects in panic-like responses in rats

Rationale

Cannabidiol (CBD) is a non-psychotomimetic constituent of Cannabis sativa plant that promotes antianxiety and anti-panic effects in animal models after acute systemic or intra-dorsal periaqueductal gray (DPAG) administration. However, the effects of CBD repeated administration, and the possible mechanisms involved, in animal models of anxiety- and panic-related responses remain poorly understood.

Objective

The present study evaluates the role of the serotonergic neurotransmission within the DPAG in the modulation of escape responses of rats chronically treated with CBD.

Methods

Male Wistar rats received acute or repeated (5 mg/Kg/daily/21 days) administration of CBD and were submitted to the elevated T-maze (ETM). We also investigated if CBD effects on the ETM depend on facilitation of 5-HT1A-mediated neurotransmission in the DPAG. To this latter aim, we verified if these effects would be prevented by intra-DPAG injection of the 5-HT1A receptor antagonist WAY100635 (0.37 nmol/0.2 μL). Also, we verified, by in vivo microdialysis, if CBD chronic treatment increases serotonin (5-HT) release and, by quantitative polymerase chain reaction, if there are changes in 5HT-1A or 5HT-2C mRNA expression in DPAG.

Results

The results showed that repeated but not acute peripheral administration of CBD decreases escape responses in the ETM, suggesting a panicolytic effect. This treatment did not change 5HT-1A or 5-HT-2C receptor mRNA expression nor modify serotonin extracellular concentrations in the DPAG. CBD effects were prevented by DPAG injection of the 5-HT1A receptor antagonist.

Conclusions

Together, these findings suggest that repeated treatment with CBD induces anti-panic effects by acting on 5-HT1A receptors in DPAG.     

5-HT1C receptors in the serotonergic control of periaqueductal gray induced aversion in rats

The functional role of brain 5-HT and 5-HT receptor subtypes in periaqueductal gray (PAG) induced aversion has been investigated in rats. Antiaversive effects were found with the serotonin agonists TFMPP, mCPP and DOI but not with RU 24969 which was found to facilitate PAG aversion. The first three serotonin agonists share potent 5-HT_1C activity while RU 24969 differs with a high 5-HT_1A activity. Proaversive effects were found with the mixed 5-HT_1C/5-HT₂ antagonists cyproheptadine and ritanserin; this effect was already reported for the mixed 5-HT_1C/5-HT₂ antagonists metergoline and mianserin and is opposite to the effects of the selective 5-HT₂ antagonists ketanserin, pirenperone, trazodone and spiperone. The antiaversive effects of mCPP (1 mg/kg) could be prevented by pretreatment of the animals with mianserin (1 and 10 mg/kg). These results suggest that 5-HT_1C receptors play an important role in the serotonergic control of PAG aversion. 5-HT_1C receptor activation seems to mediate antiaversive effects whereas acute 5-HT_1C receptor blockade appears to facilitate PAG aversion. Functional interactions take place between several receptor types in the in vivo control of PAG aversion, where 5-HT_1C receptors appear to play a predominant function.     

5-Hydroxytryptamine induced excitation and inhibition in the subthalamic nucleus: action at 5-HT(2C), 5-HT(4) and 5-HT(1A) receptors

Extracellular single-unit recordings in mouse brain slices were used to determine the effect of exogenously applied 5-HT on STN neurones. Recordings were made from 74 STN cells which fired action potentials at a regular rate of 7.19+/-0.5 Hz. In 61 cells (82%), 5-HT application increased STN neurone firing rate (10 microM, 180+/-16.8%, n=35) with an estimated EC(50) of 5.4 microM. The non-specific 5-HT(2) receptor agonist alpha-methyl 5-HT (1-10 microM) mimicked 5-HT induced excitations (15 cells). These excitations were significantly reduced by pre-perfusion with the specific 5-HT(2C) receptor antagonist RS102221 (500 nM, 9 cells) and the 5HT(4) antagonist GR113808 (500 nM, 7 cells). In 6 cells (8%) 5-HT induced biphasic responses where excitation was followed by inhibition, while in 7 cells (9%) inhibition of firing rate was observed alone. Inhibitory responses were reduced by the 5-HT(1A) antagonist WAY100135 (1 microM, 4 cells). No inhibitory responses were observed following alpha-methyl 5-HT applications. Both the excitations and inhibitions were unaffected by picrotoxin (50 microM, n=5) and CNQX (10 microM, n=5) indicative of direct postsynaptic effects. Thus, in STN neurones, 5-HT elicits two distinct effects, at times on the same neurone, the first being an excitation which is mediated by 5-HT(2C) and 5-HT(4) receptors and the second an inhibition which is mediated by 5-HT(1A) receptors.     

Chronic clomipramine alters presynaptic 5-HT(1B) and postsynaptic 5-HT(1A) receptor sensitivity in rat hypothalamus and hippocampus, respectively

Clomipramine is a tricyclic antidepressant drug with a high affinity for the serotonin (5-HT) uptake site or transporter. Electrophysiological experiments have provided evidence that repeated administration of clomipramine induces an increase in the sensitivity of postsynaptic 5-HT(1A) receptors in the hippocampus. We have studied the effects of clomipramine, administered to rats at a dose of 10mg/kg/day for 28 days by osmotic minipumps, on presynaptic 5-HT(1A) and 5-HT(1B) autoreceptors in the hypothalamus, and on postsynaptic 5-HT(1A) receptors in the hippocampus, by using in vivo microdialysis to measure 5-HT and cyclic adenosine monophosphate (cAMP) levels. Postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus was determined by means of a neuroendocrine challenge procedure. Although the sensitivity of presynaptic 5-HT(1A) autoreceptors, as measured by the effect of a subcutaneous (s.c.) injection of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.2mg/kg or 50 microg/kg) to reduce 5-HT levels, did not change, there was a reduction in sensitivity of presynaptic 5-HT(1B) receptors, as measured by the effect of an injection of the 5-HT(1B/1D) antagonist GR 127935 (5mg/kg, s.c.) to increase 5-HT levels. This effect probably accounted for the increase in basal 5-HT levels observed in the hypothalamus after chronic clomipramine administration. Postsynaptic 5-HT(1A) receptor sensitivity in the hippocampus, measured by the effect of 8-OH-DPAT to increase cAMP levels in the dialysate, was increased after chronic clomipramine. Animals that had received daily intraperitoneal injections of 10mg/kg clomipramine for 28 days did not show a change in postsynaptic 5-HT(1A) receptor sensitivity in the hypothalamus as measured by the ability of 8-OH-DPAT (50 microg/kg, s.c.) to stimulate secretion of corticosterone. Taken together with the results of previous experiments involving the cerebral cortex, these in vivo results show that chronic clomipramine exerts effects on both pre- and postsynaptic serotonin receptors, but that these effects are highly region-specific.     

DOI disruption of prepulse inhibition of startle in the rat is mediated by 5-HT(2A) and not by 5-HT(2C) receptors

Prepulse inhibition (PPI) of the startle response is used as a measure of sensorimotor inhibitory processes. Deficits in PPI have been found in patients with schizophrenia, obsessive compulsive disorder (OCD) and Huntington's disease. PPI can also be disrupted in animals through manipulations that augment serotonergic activity, such as administration of the serotonin (5-HT) agonists 8-OH-DPAT, RU 24969 and DOI. In the present experiment the identity of the 5-HT receptor subtype that mediates the DOI-induced disruption of PPI was examined. Dose-response studies revealed that the novel 5-HT(2A) antagonist, MDL 100,907 (0.01, 0.1 and 1.0mg/kg, s.c.), but not the new 5-HT(2C) antagonist SDZ SER 082 (0.125, 0.25 and 0.5mg/kg, s.c.), prevented the loss of PPI induced by DOI (0.25 or 0.5mg/kg, s.c.). The results support the hypothesis that the 5-HT(2A) receptor is involved in the modulation of sensorimotor gating. Because deficits in PPI are used as a model of sensorimotor gating abnormalities found in schizophrenia, the present study supports the view that MDL 100,907 may be a novel atypical antipsychotic. Studies of the serotonergic substrates of PPI may provide a model of the possible serotonergic role in the sensorimotor gating abnormalities in schizophrenia and OCD patients.     

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.