Effects of sustained (+/-)pindolol administration on serotonin neurotransmission in rats

OBJECTIVE: Given reports that (+/-)pindolol, a beta-adrenergic-5-HT1A/1B receptor antagonist, accelerates the onset of the therapeutic effect of certain antidepressant drugs in major depression, the aim of this study was to assess the effect of sustained (+/-)pindolol administration on the sensitivity of pre- and postsynaptic 5-HT1A receptors, terminal 5-HT1B autoreceptors and on overall 5-HT neurotransmission. DESIGN: Prospective animal study. ANIMALS: Sprague-Dawley rats. OUTCOME MEASURES: Modifications of the sensitivity of somatodendritic and postsynaptic 5-HT1A receptors using in vivo electrophysiological paradigms in animals treated with vehicle or (+/-)pindolol (20 mg/kg/day, subcutaneously) through osmotic minipumps for 2 weeks. RESULTS: (+/-)Pindolol attenuated the suppressant effect of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD) on the firing activity of 5-HT neurons, suggesting that (+/-)pindolol antagonized somatodendritic 5-HT1A autoreceptors in the dorsal raphe nucleus. However, following a 2-day washout period, the suppressant effect of LSD was still attenuated, indicating rather a desensitization of 5-HT1A autoreceptors had occurred. In the CA3 region of the dorsal hippocampus, (+/-)pindolol treatment did not modify the responsiveness of postsynaptic 5-HT1A receptors to microiontophoretic applications of 5-HT. Moreover, such a treatment modified neither the effectiveness of the electrical stimulation of 5-HT fibers nor the function of terminal 5-HT autoreceptors. Finally, the administration of the selective 5-HT1A receptor antagonist WAY 100635 (100 micrograms/kg, intravenously) did not increase the firing activity of dorsal hippocampus CA3 pyramidal neurons in rats treated with (+/-)pindolol, thus failing to reveal the enhanced tonic activation of postsynaptic 5-HT1A receptors associated with major classes of antidepressant treatments. CONCLUSION: Prolonged administration of (+/-)pindolol by itself is not sufficient to enhance overall 5-HT neurotransmission; pindolol should therefore not be endowed with intrinsic antidepressant activity. Although pindolol is capable of antagonizing the 5-HT1A autoreceptor upon the initiation of a 5-HT reuptake-blocker treatment, it also induces a desensitization of this 5-HT1A autoreceptor, which could explain why patients do not relapse upon its discontinuation when they continue taking a 5-HT reuptake blocker.     

Effect of neurokinin-I receptor antagonists on the function of 5-HT and noradrenaline neurons

Substance P antagonists have been proposed to be a new class of antidepressants. The present study aimed to determine the effect of the selective non-peptide rat neurokinin-1 (NK1) receptor antagonists WIN 51,708 and CP-96,345 on the firing activity of rat dorsal raphe serotonin (5-HT) and locus coeruleus noradrenaline (NA) neurons. While WIN51,708 (2mg/kg, i.v.) and CP-96,345 (0.15 mg/kg, i.v.) did not modify the firing activity of 5-HT and NA neurons, both antagonists attenuated the suppressant effect of the alpha2-adrenoceptor agonist clonidine on the firing activity of both types of neurons. In contrast, the responsiveness of 5-HT neurons to the i.v. administration of the 5-HT autoreceptor agonist LSD and the 5-HT1A receptor agonist 8-OH-DPAT remained unchanged. These findings suggest that NK1 receptor antagonists affect markedly the NA system via an attenuation of the function of alpha2-adrenoceptors on the cell body of NA neurons and, consequently, may also modulate 5-HT neurotransmission.     

Fenfluramine evokes 5-HT2A receptor-mediated responses but does not displace [11C]MDL 100907: small animal PET and gene expression studies

The in vivo binding of the 5-HT(2A) receptor-selective positron emission tomography (PET) ligand [(11)C]MDL 100907 and its sensitivity to endogenous 5-HT were quantified in rat brain using quad-HIDAC, a novel high-resolution PET camera for small animals. Specific binding of [(11)C]MDL 100907, estimated using volume of interest (VOI) to cerebellum ratios, corresponded well with both the known distribution of 5-HT(2A) receptors and tissue:cerebellum ratios obtained using ex vivo dissection. Specific binding was blocked by predosing with either nonradioactive MDL 100907 (0.2 or 0.4 mg/kg i.v.) or the 5-HT(2A/2C) receptor antagonist ketanserin (2 mg/kg i.v.), but was unaffected in rats pretreated with the 5-HT releasing agent, fenfluramine (10 mg/kg i.p.). In parallel studies, the same dose of fenfluramine was shown to be sufficient to cause an increase in the expression of the immediate early genes (IEG) c-fos and Arc mRNA in cortical regions with high 5-HT(2A) receptor density. This increase was blocked by MDL 100907 (0.2 mg/kg i.v.), confirming a 5-HT(2A) receptor-mediated effect. The results demonstrate that PET with [(11)C]MDL 100907 is insensitive to an increased concentration of synaptic 5-HT, implying that the ligand can be used clinically to monitor 5-HT(2A) receptor function or dysfunction in disease or during therapy, without the need to consider concomitant changes in neurotransmitter concentration.     

Vagal afferent-mediated inhibition of a nociceptive reflex by i.v. serotonin in the rat. II. Role of 5-HT receptor subtypes

In the rat, intravenous (i.v.) serotonin (5-HT) is a noxious stimulus which produces distinct vagal afferent-mediated pseudoaffective responses, a passive avoidance behavior, a vagal afferent-mediated inhibition of the nociceptive tail-flick (TF) reflex and a complex triad of cardiovascular responses. In the present study, we have used a variety of 5-HT receptor antagonists to characterize the receptor subtype(s) in the rat that mediate (1) inhibition of the TF reflex and (2) the cardiovascular responses produced by i.v. 5-HT. 5-HT produced a dose-dependent (3-72 micrograms/kg, i.v.) inhibition of the TF reflex (ED50 = 15.3 +/- 0.7 micrograms/kg). Following administration of the 5-HT2 receptor-selective antagonists ketanserin (50-250 micrograms/kg, i.v.) or xylamidine (10-100 micrograms/kg, i.v.), or the 5-HT3 receptor-selective antagonists ICS 205-930 (50-250 micrograms/kg, i.v.) or MDL 72222 (25-250 micrograms/kg, i.v.), there appeared to be a parallel shift of the 5-HT dose-response curve to the right. Following co-administration of xylamidine (50 micrograms/kg, i.v.) with ICS 205-930 (100 micrograms/kg, i.v.), the 5-HT-induced inhibition of the TF reflex was completely abolished at all doses of 5-HT tested (3-288 micrograms/kg, i.v.). In contrast, administration of the centrally acting 5-HT2 receptor-selective antagonist LY 53857 (10-100 micrograms/kg, i.v.) or the non-specific receptor antagonist methysergide (25-500 micrograms/kg, i.v.) resulted in a dose-dependent, but not parallel shift of the 5-HT dose-response curve to the right. The maximal doses of LY 53857 and methysergide tested (250 micrograms/kg and 500 micrograms/kg, respectively) completely abolished the effects of 5-HT (3-288 micrograms/kg, i.v.). Administration of the alpha 1-adrenoceptor antagonist prazosin (25-100 micrograms/kg, i.v.) failed to alter the 5-HT dose-response curve, indicating that the effects of ketanserin were due to blockage of 5-HT2 receptors rather than alpha 1 receptors. Administration of each of the antagonists also produced marked, but selective effects on components of the complex cardiovascular response to i.v. 5-HT. Each of the 5-HT3 receptor selective antagonists (ICS 205-930 or MDL 72222) produced a dose-dependent attenuation of the Bezold-Jarisch reflex-mediated hypotension and bradycardia, and each of the 5-HT2 receptor selective antagonists (xylamidine, ketanserin or LY 53857) produced a dose-dependent attenuation of the pressor response. The non-specific 5-HT receptor antagonist methysergide produced a dose-dependent attenuation of the 5-HT-induced pressor response.(ABSTRACT TRUNCATED AT 400 WORDS)     

Acute and repeated administration of the selective 5-HT(2A) receptor antagonist M100907 significantly alters the activity of midbrain dopamine neurons: an in vivo electrophysiological study

We examined the effect of the acute and repeated administration of M100907 (formerly MDL 100907), a selective 5-HT(2A) receptor antagonist, on spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) of rats. This was accomplished using in vivo, extracellular single unit recording. The i.v. administration of M100907 (0.01-0.64 mg/kg) did not significantly alter the basal firing rate or pattern of spontaneously active SNC and VTA DA neurons. A single injection of either 0.01 or 0.03 mg/kg i.p. of M100907 did not significantly alter the number of spontaneously active DA neurons in either the SNC or VTA areas. However, 0.1 mg/kg i.p. of M100907 significantly increased the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. A single injection of all doses of M100907 significantly decreased the degree of bursting in VTA DA neurons, whereas the 0.1 mg/kg dose increased the degree of bursting in SNC DA neurons. The repeated administration (one injection per day for 21 days) of 0.03 and 0.1 mg/kg i.p. of M100907 produced a significant decrease in the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. The repeated administration of M100907 did not significantly alter the firing pattern of VTA DA neurons but significantly altered the firing pattern of SNC DA neurons. The results of this study indicate that M100907 administration alters the activity of midbrain DA neurons in anesthetized rats.     

Venlafaxine: discrepancy between in vivo 5-HT and NE reuptake blockade and affinity for reuptake sites.

Using an in vivo electrophysiological paradigm, venlafaxine and paroxetine displayed similar potency for suppressing the firing activity of dorsal raphe 5-HT neurons (ED50: 233 and 211 microg/kg i.v., respectively), while venlafaxine was three times less potent than desipramine (ED50: 727 and 241 microg/kg i.v., respectively) to suppress the firing activity of locus coeruleus NE neurons. The selective 5-HT1A receptor antagonist WAY 100635 (100 microg/kg, i.v.) reversed the suppressant effect of venlafaxine and paroxetine on the firing activity of 5-HT neurons and the alpha2-adrenoceptor antagonist piperoxane (1 mg/kg, i.v.) reversed those of venlafaxine and desipramine on the firing activity of NE neurons. The ED50 of venlafaxine on the firing activity of 5-HT neurons was not altered (ED50: 264 microg/kg) in noradrenergic-lesioned rats, while the suppressant effect of venlafaxine on the firing activity of NE neurons was greater in serotonergic-lesioned rats (ED50: 285 microg/kg). Taken together, these results suggest that, in vivo, venlafaxine blocks both reuptake processes, its potency to block the 5-HT reuptake process being greater than that for NE. Since the affinities of venlafaxine for the 5-HT and NE reuptake carriers are not in keeping with its potencies for suppressing the firing activity of 5-HT and NE neurons, the suppressant effect of venlafaxine on the firing activity of 5-HT and NE neurons observed in vivo may not be mediated solely by its action on the [3H]cyanoimipramine and [3H]nisoxetine binding sites. In an attempt to unravel the mechanism responsible for this peculiarity, in vitro superfusion experiments were carried out in rat brain slices to assess a putative monoamine releasing property for venlafaxine. (+/-)Fenfluramine and tyramine substantially increased the spontaneous outflow of [3H]5-HT and [3H]NE, respectively, while venlafaxine was devoid of such releasing properties.     

Action of serotonin in the medial prefrontal cortex: mediation by serotonin3-like receptors.

In the present study, we investigated the effects of various serotonin (5-HT) antagonists on 5-HT's action on medial prefrontal cortical cells (mPFc) using the techniques of single cell recording and microiontophoresis. The microiontophoretic application of 5-HT (10-80 nA) produced a current-dependent suppression of mPFc cell firing and this effect was blocked by the selective 5-HT3 receptor antagonists (+/-)-zacopride, ICS 205930 and granisetron at currents of 5-20 nA. Furthermore, the intravenous (i.v.) administration of (+/-)-zacopride (5-50 micrograms/kg) markedly attenuates the suppressive action of 5-HT on mPFc cell firing. In contrast, the microiontophoresis of 5-HT1 and 5-HT2 receptor antagonists such as (+/-)-pindolol, spiperone, metergoline, and ritanserin (10-20 nA) failed to block 5-HT's effect. In fact, in some cells, spiperone and ritanserin potentiated 5-HT's action and prolonged neuronal recovery. In addition, the intravenous administration of either ritanserin (5-2,000 micrograms/kg) or metergoline (4-2,400 micrograms/kg) failed to alter 5-HT's action. The electrical stimulation of the caudal linear raphe nucleus (CLi) suppressed the spontaneous activity of 83% of the mPFc cells tested by 45 +/- 2%. This suppression was significantly attenuated by the iontophoresis of granisetron (2.5-5 nA) but not by the 5-HT2 and 5-HT1C receptor antagonist ritanserin or the relatively selective 5-HT2 receptor antagonist (+)-MDL 11,939 (10-40 nA). However, the i.v. administration of ritanserin (0.5-1.5 mg/kg) or S-zacopride (0.1 mg/kg) significantly blocked the suppression of mPFc cell firing produced by CLi stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin (1A) receptor ligands act on norepinephrine neuron firing through excitatory amino acid and GABA(A) receptors: a microiontophoretic study in the rat locus coeruleus.

It was previously shown that the excitatory effect of the 5-HT(1A) agonist 8-OH-DPAT on firing activity of locus coeruleus (LC) norepinephrine (NE) neurons and the inhibitory action of the 5-HT(1A) antagonist WAY 100,635 are dependent on the presence of 5-HT neurons, whereas the inhibitory action of the 5-HT(2) agonist DOI is not. Using in vivo extracellular unitary recordings performed in anesthetized rats, iontophoretic applications of the excitatory amino acid antagonist kynurenate attenuated the enhancement in firing produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the enhancement produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the GABA(A) receptor antagonist bicuculline. 8-OH-DPAT (10-60 microg kg(-1), i.v.) produced a dose-dependent enhancement in the firing activity of NE neurons that was abolished in the presence of kynurenate application. The selective 5-HT(1A) receptor antagonist WAY 100,635 (100 microg kg(-1), i.v.) suppressed NE firing which was reversed by the selective 5-HT(2A) antagonist MDL 100,907 (200 microg kg(-1), i.v.). In the presence of bicuculline, the inhibitory effect of WAY 100,635 was blunted. These results suggest that WAY 100,635 mainly attenuates NE neuron firing by blocking inhibitory 5-HT(1A) receptors on glutamatergic neurons, thereby enhancing glutamate release and activating excitatory amino acid receptors, possibly of the kainate subtype, on 5-HT terminals. The ensuing increased 5-HT release would then act on excitatory 5-HT(2A) receptors on GABA neurons that would ultimately mediate the inhibition of NE neurons. The prevention of the excitatory action of 8-OH-DPAT on NE neuron firing by kynurenate is also consistent with this neurocircuitry.     

Effect of the selective noradrenergic reuptake inhibitor reboxetine on the firing activity of noradrenaline and serotonin neurons

Reboxetine is a non-tricyclic antidepressant with selective noradrenergic (NA) reuptake-blocking effects. The effects of acute and sustained administration of reboxetine, on the firing activity of locus coeruleus NA neurons and dorsal raphe 5-HT neurons, were assessed using in vivo extracellular unitary recording in rats anaesthetized with chloral hydrate. Reboxetine (0.1-1.25 mg/kg, i.v.) dose-dependently decreased the firing activity of NA neurons (ED50 = 480 +/- 14 microg/kg). A 2-day treatment with reboxetine at 1.25, 2.5, 5, or 10 mg/kg per day (using osmotic minipumps implanted subcutaneously) produced significant decreases of 52%, 68%, 81%, and 83%, respectively, of NA firing activity. When the reboxetine treatment (2.5 mg/kg per day) duration was prolonged to 7 days, a 66% decrease in NA firing activity was observed which further decreased to 80% after 21 days of treatment. In contrast, 5-HT neuron firing rate remained unaltered following short- and long-term reboxetine treatments. The suppressant effect of the alpha2-adrenoceptor agonist clonidine on the firing activity of NA neurons was unchanged in long-term reboxetine-treated rats, but its effect on the firing activity of 5-HT neurons was blunted. The enhancement of NA firing activity by the 5-HT1A agonist 8-OH-DPAT was abolished in long-term reboxetine-treated rats, whereas, the inhibitory effect of the 5-HT2 agonist DOI was attenuated by about three-fold. In conclusion, sustained NA reuptake blockade by reboxetine lead to profound alterations in the function of NA neurons and of 5-HT receptors modulating their firing activity.     

Pindolol excites dopaminergic and adrenergic neurons, and inhibits serotonergic neurons, by activation of 5-HT1A receptors

Pindolol accelerates the clinical actions of selective serotonin reuptake inhibitors (SSRIs) in man, and modulates extracellular levels of monoamines in corticolimbic structures in rats. Herein, we examined its influence upon electrical activity of serotonergic, dopaminergic and adrenergic perikarya in the dorsal raphe nucleus (DRN), ventral tegmental area (VTA) and locus coeruleus (LC) of anaesthetized rats. In analogy to the serotonin1A (5-HT1A) agonist, 8-OH-DPAT (-100%), pindolol dose-dependently (0.063- 1.0 mg/kg) decreased (-70%) the firing rate of serotonergic neurons. The inhibitory action of pindolol was abolished by the selective 5-HT1A antagonist, WAY-100,635 (0.031 mg/kg). In contrast, 8-OH-DPAT (+26%) and pindolol (0.25-4.0 mg/kg, +60%) dose-dependently increased the firing rate of dopaminergic cells. Of 57 neurons recorded (pindolol, 2.0 mg/kg), 36 (63%) were excited, 11 (19%) were unaffected and 10 (18%) were inhibited. This variable influence could be attributed to regularly firing neurons in the parabrachial subdivision, inasmuch as all neurons in the paranigral subnucleus were excited. The facilitation of firing by pindolol was accompanied by an increase in burst firing throughout the VTA. Both the increases in burst firing and in firing rate were reversed by WAY-100,635 (0.031 mg/kg). Finally, the electrical activity of adrenergic neurons was dose-dependently enhanced by 8-OH-DPAT and pindolol (+99% and +83%, respectively). WAY-100,635 reversed this excitation and, itself, inhibited the activity of adrenergic neurons. In conclusion, via engagement of 5-HT1A receptors, pindolol inhibits serotonergic, and activates dopaminergic and adrenergic, neurons in anaesthetized rats. Such actions may contribute to its influence upon mood, both alone and in association with antidepressant agents.     

Electrophysiological evidence for a functional interaction between 5-HT1A and 5-HT2A receptors in the rat medial prefrontal cortex: An lontophoretic study

In this study, we examined the interaction of 5-HT_1A and 5-HT_2A receptors in the rat medial prefrontal cortex (mPFc) using the techniques of extracellular single unit recording and microiontophoresis. The iontophoresis of the selective 5-HT_1A receptor agonist (±)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) produced a current-dependent suppression (2.5-20 nA) of the basal firing rate of spontaneously active mPFc cells. The iontophoretic (5-10 nA) and systemic administration (0.1-0.5 mg/kg, i.v. ) of the 5-HT_2A/5-HT_2C receptor antagonist ritanserin and the selective 5 HT_2A receptor antagonist MDL 28727 significantly potentiated and prolonged 8-OHDPATs suppressant action. In addition, the systemic administration of another selective 5-HT_2A antagonist MDL 100907, but not its less active enantiomer MDL 100009, also potentiated and prolonged 8-OHDPATs action. The potentiating effect of the 5-HT_2A receptor antagonists on the action of 8-OHDPAT is specific in that neither the iontophoresis of ritanserin nor MDL 28727 altered the suppressant action produced by the iontophoresis of the 5-HT₃ receptor agonist 2-methylserotonin onto mPFc cells. Moreover, the suppressant action of 8-OHDPAT was not altered by the systemic administration of the selective 5-HT₃ receptor antagonist granisetron (0.1-0.5 mg/kg, i.v.). On the other hand, the iontophoresis of a low current (0.5 nA) of the 5-HT_2A,2C receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) potentiated the excitation induced by the iontophoresis of 1-glutamate on quiescent mPFc cells. The iontophoresis of 8-OHD-PAT at a current that had no effect on the firing rate of 1-glutamate activated when administered alone significantly attenuated the excitatory action produced by the iontophoresis of DOI. Overall these results confirm and extend the hypothesis that there is an interaction between 5-HT_1A and 5-HT_2A receptors in the mPFc at the neuronal level. © 1994 Wiley-Liss, Inc.     

Biochemical and electrophysiological evidence that RO 60-0175 inhibits mesolimbic dopaminergic function through serotonin(2C) receptors

In vivo microdialysis and electrophysiological techniques were used to elucidate the role of the 5-HT(2) receptor family on the control of mesolimbic dopaminergic system exerted by serotonin (5-HT). Administration of RO 60-0175 (1 mg/kg, i.p.), a selective 5-HT(2C) receptor agonist, significantly decreased dopamine (DA) release by 26+/-4% (below baseline) 60 min after injection. Moreover, RO 60-0175 (80-320 microg/kg, i.v.) dose-dependently decreased the basal firing rate of DA neurons in the ventral tegmental area (VTA), reaching its maximal inhibitory effect (53.9+/-15%, below baseline) after the dose of 320 microg/kg. The selective 5-HT(2C) receptor antagonist SB 242084 completely blocked the inhibitory action of RO 60-0175 on accumbal DA release and on the firing rate of VTA DA cells. On the contrary, both (+/-)-DOI, a mixed 5-HT(2A/2C) receptor agonist, and the selective 5-HT(2B) agonist BW 723C86, did not affect either DA release in the nucleus accumbens or the firing rate of VTA DA cells. Taken together, these data confirm that central 5-HT system exerts an inhibitory control on the mesolimbic DA system and that 5-HT(2C) receptors are involved in this effect.     

Role of cholinergic and GABAergic systems in the feedback inhibition of dorsal raphe 5-HT neurons

Several observations indicate that 5-HT1A receptors found on a long neuronal feedback loop, originating from the medial prefrontal cortex, regulate 5-HT neuronal firing. In the present study, the muscarinic (M) receptor antagonists atropine and scopolamine as well as the M2 receptor antagonist AF-DX 116, but not the preferential M1 receptor antagonist pirenzepine, reduced the suppressant effect of the 5-HT1A receptor agonist 8-OH-DPAT on the spontaneous firing activity of rat dorsal raphe 5-HT neurons. Moreover, AF-64A-induced lesions of cholinergic neurons directly in the medial prefrontal cortex and after its i.c.v. injection attenuated the effect of 8-OH-DPAT. Finally, the NMDA receptor antagonist (+)MK-801 and the GABA(B) receptor antagonist SCH-50911, but not the GABA(A) receptor antagonist (-)bicuculline, dampened the latter response. The present study unveiled a key role for the cholinergic and GABAergic systems in the feedback inhibition of dorsal raphe 5-HT neurons.     

Evidence for serotonin receptor subtypes involvement in agmatine antidepressant like-effect in the mouse forced swimming test

This study investigated the involvement of 5-HT(1) and 5-HT(2) receptors in the antidepressant-like effect of agmatine in the mouse forced swimming test (FST). Pretreatment with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, intraperitoneally (i.p.), an inhibitor of serotonin synthesis, for 4 consecutive days), methysergide (5 mg/kg, i.p., a serotonin (5-HT) antagonist), pindolol (32 mg/kg, i.p., a 5-HT(1A/1B) receptor/beta-adrenoceptor antagonist), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg, subcutaneously (s.c.), a selective 5-HT(1A) receptor antagonist), 1-(2-methoxyphenyl)-4[-(2-phthalimido)butyl]piperazine) (NAN-190; 0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane; 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) antagonist) or ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), prevented the effect of agmatine (10 mg/kg, i.p.) in the FST. A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5 mg/kg, i.p.), WAY 100635 (0.03 mg/kg, s.c.), (+)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT; 0.01 mg/kg, i.p., a 5-HT(1A) receptor agonist), R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI; 1 mg/kg, i.p., a preferential 5-HT(2A) receptor agonist), or fluoxetine (10 mg/kg, i.p., a selective serotonin reuptake inhibitor, SSRI) but not with isamoltane (2.5 mg/kg, i.p.), ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or ketanserin (5 mg/kg, i.p.). Taken together, the results firstly demonstrate that agmatine antidepressant-like effects in the FST seem to be mediated, at least in part, by an interaction with 5-HT(1A/1B) and 5-HT(2) receptors.     

Effects of serotonergic 5-HT1A and 5-HT1B ligands on ventral pallidal neuronal activity

To clarify the role of the 5-HT system in limbic outputs, the present study compared the effects of the 5-HT1A agonist 8-OH-DPAT and the 5-HT1B agonist CP-94253 with the non-selective 5-HT agonist TFMPP on the firing rate of ventral pallidal (VP) neurons recorded in chloral hydrate-anesthetized rats. 8-OH-DPAT (0.25-256 microg/kg i.v.) dose-dependently enhanced (9/26 neurons) or suppressed (8/26) activity, and the 5-HT1A antagonist (+)WAY-100135 often attenuated these responses. TFMPP (0.011-1.453 mg/kg i.v.) dose-dependently reduced the firing rate of 7/8 VP neurons tested. In contrast, CP-94253 (0.013-12.8 mg/kg i.v.) had little or no effect. In sum, these data suggest that the 5-HT1A receptor appears to be particularly important in influencing limbic outputs mediated via the VP.     

5-HT1A and 5-HT1B receptors control the firing of serotoninergic neurons in the dorsal raphe nucleus of the mouse: studies in 5-HT1B knock-out mice

The characteristics of the spontaneous firing of serotoninergic neurons in the dorsal raphe nucleus and its control by serotonin (5-hydroxytryptamine, 5-HT) receptors were investigated in wild-type and 5-HT1B knock-out (5-HT1B-/-) mice of the 129/Sv strain, anaesthetized with chloral hydrate. In both groups of mice, 5-HT neurons exhibited a regular activity with an identical firing rate of 0.5-4.5 spikes/s. Intravenous administration of the 5-HT reuptake inhibitor citalopram or the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) induced a dose-dependent inhibition of 5-HT neuronal firing which could be reversed by the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xane carboxamide (WAY 100635). Both strains were equally sensitive to 8-OH-DPAT (ED50 approximately 6.3 microgram/kg i.v.), but the mutants were less sensitive than wild-type animals to citalopram (ED50 = 0.49 +/- 0.02 and 0.28 +/- 0.01 mg/kg i.v., respectively, P < 0.05). This difference could be reduced by pre-treatment of wild-type mice with the 5-HT1B/1D antagonist 2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carbox yli c acid [4-methoxy-3-(4-methyl-piperazine-1-yl)-phenyl]amide (GR 127935), and might be accounted for by the lack of 5-HT1B receptors and a higher density of 5-HT reuptake sites (specifically labelled by [3H]citalopram) in 5-HT1B-/- mice. In wild-type but not 5-HT1B-/- mice, the 5-HT1B agonists 3-(1,2,5, 6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP 94253, 3 mg/kg i.v.) and 5-methoxy-3-(1,2,3, 6-tetrahydropyridin-4-yl)-1H-indole (RU 24969, 0.6 mg/kg i.v.) increased the firing rate of 5-HT neurons (+22.4 +/- 2.8% and +13.7 +/- 6.0%, respectively, P < 0.05), and this effect could be prevented by the 5-HT1B antagonist GR 127935 (1 mg/kg i.v.). Altogether, these data indicate that in the mouse, the firing of 5-HT neurons in the dorsal raphe nucleus is under both an inhibitory control through 5-HT1A receptors and an excitatory influence through 5-HT1B receptors.     

Effects of short-term serotonin depletion on the efficacy of serotonin neurotransmission: electrophysiological studies in the rat central nervous system

The effects of short-term serotonin (5-HT) depletion by p-chlorophenylalanine (PCPA) on the firing activity of dorsal raphe nucleus 5-HT neurons, on the responsiveness of dorsal hippocampus pyramidal neurons to microiontophoretically applied 5-HT and on the efficacy of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of CA3 dorsal hippocampus pyramidal neurons were assessed in chloral hydrate-anesthetized rats. PCPA (250 mg/kg/day i.p. for 2 days) reduced the 5-HT content of the dorsal hippocampus by 90%. However, the number of spontaneously active 5-HT neurons per microelectrode trajectory through the dorsal raphe or their average rate of firing was unaltered. The effect of afferent 5-HT pathway stimulation was reduced in only 40% of treated rats, whereas the sensitivity of CA3 pyramidal neurons to microiontophoretic 5-HT was not modified. The function of the terminal 5-HT autoreceptor was assessed using methiothepin, an autoreceptor antagonist. Methiothepin (1 mg/kg, i.v.) significantly enhanced the efficacy of the stimulation in PCPA-treated rats, although the degree of enhancement was much less than in controls. A greater reduction of the effectiveness of the stimulation was obtained by increasing the dose of PCPA (350 mg/kg/day i.p. for 2 days). This regimen reduced the 5-HT content of the dorsal hippocampus by 95%. In these rats, the sensitivity of the terminal 5-HT autoreceptor was assessed by increasing the frequency of the stimulation from 1 to 5 Hz. This procedure reduced to a similar extent the efficacy of the stimulation in treated and control rats, suggesting that the reduced effectiveness of methiothepin in enhancing 5-HT synaptic transmission in PCPA-treated rats is due to a lower degree of activation of the terminal 5-HT autoreceptor. The present results showing that the 350 mg/kg/day regimen of PCPA, but not the 250 mg/kg/day regimen, reduced the efficacy of the stimulation of the ascending 5-HT pathway suggest that a greater than 90% depletion is required to affect 5-HT neurotransmission significantly. The reduced level of activation of terminal 5-HT autoreceptors in rats treated with the lower dose of PCPA may facilitate the release of the remaining 5-HT per stimulation-triggered action potential, ensuring a virtually unaltered synaptic efficacy.     

5-HT4 receptors exert a frequency-related facilitatory control on dorsal raphé nucleus 5-HT neuronal activity

We investigated, using single-unit recordings in chloral hydrate-anaesthetized rats, the role of serotonin 4 (5-HT4) receptors in the control of dorsal raphé nucleus (DRN) 5-HT neuron activity. About one-half (36) of the 76 neurons recorded were affected by either the preferential 5-HT4 agonist cisapride (500 and 1000 micro g/kg, i.v.) or the selective 5-HT4 antagonist, GR 125487 (200- 2000 micro g/kg, i.v.). Responding neurons displayed a significantly higher mean basal firing rate (1.93 +/- 0.1 Hz) than non-responders (1.31 +/- 0.1 Hz). The firing rate of responding 5-HT neurons was enhanced dose-dependently by cisapride (+47 and +94% at 500 and 1000 micro g/kg, respectively), an effect abolished by GR 125487 (500 micro g/kg) and reduced by the 5-HT4 antagonist, SDZ 205557 (500 micro g/kg, i.v). Conversely, GR 125487 induced a dose-dependent inhibition of responders activity, which was almost completely suppressed at the dose of 2000 micro g/kg. In a separate set of experiments, the selective 5-HT4 agonist, prucalopride (500 micro g/kg, i.v), increased the firing activity (+35%) of 5-HT neurons displaying a high basal firing rate; subsequent injection of GR 125487 (500 micro g/kg, i.v.) suppressed this effect. These results indicate that 5-HT4 receptors exert both a tonic and a phasic, positive, frequency-related control on DRN 5-HT neuronal activity. The existence of such a control might open new avenues for therapeutic research in the antidepressant field.     

Pindolol occupancy of 5-HT(1A) receptors measured in vivo using small animal positron emission tomography with carbon-11 labeled WAY 100635

Positron emission tomography (PET), following an intravenous injection of [carbonyl-(11)C]WAY 100635, was used to image central 5-HT(1A) receptors in rat following pretreatment with graded doses of (-)-pindolol (0.001-3 mg/kg, i.v.). The use of PET had advantages over ex vivo radioligand binding methods in that it produced parametric image volumes and reduced errors due to inter-rat variability. Time-radioactivity curves from regions of interest (ROI) acquired from individual rats enabled the estimation of specific binding of the radioligand using a compartmental model with reference tissue input. Binding potential (BP) of [(11)C]WAY 100635 was estimated for frontal cortex and hippocampus (postsynaptic), and midbrain raphe nuclei (presynaptic). In the latter ROI, pindolol dose-dependently decreased BP. The saturation curve could be fitted to a single-site model up to the lowest dose of pindolol used, giving an ED(50) (dose to cause 50% occupancy) value of 0.26 +/- 0. 05 mg/kg, and inclusion of control (nonpindolol-treated) rats did not affect the fit. In contrast, in cortex and hippocampus ROI, low doses of pindolol caused an increase in BP compared with controls. Pindolol doses greater than approximately 0.1 mg/kg, resulted in a dose-dependent decrease in BP, and ED(50) values in cortex and hippocampus were estimated as 0.44 +/- 0.13 and 0.48 +/- 0.12 mg/kg, respectively. The increase in [(11)C]WAY 100635 binding at low pindolol doses is feasibly related to a decrease in basal receptor occupancy following reduced release of endogenous 5-HT. Considering the apparently greater potency of pindolol at the midbrain raphe ROI, this effect could be mediated via agonist activity at the autoreceptor.     

5-HT(1A) agonist potential of pindolol: electrophysiologic studies in the dorsal raphe nucleus and hippocampus.

BACKGROUND: The ability of pindolol to block 5-HT(1A) autoreceptors on serotonin-containing neurons in the raphe nuclei is thought to underlie the clinical reports of enhanced efficacy and rate of improvement in depressed patients treated with pindolol/selective serotonin reuptake inhibitor (SSRI) combinations. Selectivity for somatodendritic 5-HT(1A) autoreceptors is a crucial requirement, as blockade of postsynaptic 5-HT(1A) sites may jeopardize the therapeutic response. Previous investigators have probed the effects of pindolol on serotonergic dorsal raphe cell firing in animal species; here we confirm their findings and extend them to include observations on postsynaptic 5-HT(1A) receptors in the hippocampus. METHODS: Extracellular single-unit recordings were made in rats using standard electrophysiologic techniques. Firing rates of serotonin-containing neurons in the dorsal raphe nucleus and CA3 hippocampal pyramidal neurons were monitored and the effects of pindolol given alone or in combination with an SSRI (fluoxetine) or a 5-HT(1A) antagonist (WAY-100,635) were determined. RESULTS: Pindolol inhibited the firing rates of serotonergic dorsal raphe neurons in a dose-dependent manner. Recovery to baseline firing rates was gradual, but this inhibition could be acutely reversed by WAY-100,635. A range of pindolol doses failed to block the inhibitory effects of fluoxetine on dorsal raphe cell firing. In the hippocampus, pindolol also inhibited cell firing as a function of dose, although these effects were insensitive to WAY-100,635 treatment. CONCLUSIONS: The ability of pindolol to inhibit serotonergic dorsal raphe cell firing is indicative of its agonist potential and is consistent with previous studies. The lack of observable antagonism of the SSRI-induced slowing of raphe unit activity casts doubt on the suitability of this mechanism of action to account for the positive findings in clinical studies utilizing pindolol/SSRI combinations. The 5-HT(1A)-independent inhibition of hippocampal CA3 cell firing by pindolol suggests that this compound invokes multiple pharmacologic actions, all of which need to be assimilated into any proposed mechanism of action.