Pharmacological characterization of alpha 2-adrenoceptor regulated serotonin release in the rat hippocampus

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Differential α2A- and α2C-adrenoceptor protein expression in presynaptic and postsynaptic density fractions of postmortem human prefrontal cortex

BACKGROUND

Three different α₂-adrenoceptor (α₂-AR) subtypes have been described. The α_2A-AR and α_2C-AR subtypes are highly expressed in the human prefrontal cortex, where they modulate neurotransmission. However, due to the lack of subtype-selective ligands, the physiological relevance of both subtypes has not been fully resolved.

AIMS

In this context, the aim of the present study was to characterize the protein expression of both α₂-AR subtypes, in different synaptic fractions of postmortem human prefrontal cortex.

METHODS

A subcellular fractionation of the samples was performed and the protein expression of α_2A- and α_2C-ARs was measured in presynaptic membranes and postsynaptic density fractions by Western blot.

RESULTS

The results revealed that the α_2A-AR subtype is mainly located postsynaptically (95±3%) whereas the remaining 5±3% is in the presynapse. Conversely, the α_2C-AR subtype showed a similar distribution between pre- and postsynaptic membranes, with a slightly higher percentage present in the presynapse (60±2% vs. 40±2%).

CONCLUSIONS

These findings could explain some contradictory effects reported for α₂-AR agonists and antagonists in the human prefrontal cortex. Furthermore, the present data could contribute to elucidating the therapeutic potential of selectively targeting α_2A- or α_2C-AR subtypes.

Mirtazapine

Mirtazapine is one of antidepression which is used mainly in the treatment of depression, moreover, it is sometimes used in the treatment of anxiety disorders, insomnia, nausea, and vomiting, and to produce weight gain when desirable. The action of mirtazapine is an antagonist of certain adrenergic and serotonin receptors, and, furthermore, the drug is used strong as antihistamine, and it is occasionally defined as a noradrenergic and specific serotonergic antidepressant (NaSSA). The comprehensive profile of mirtazapine gives more detailed information about nomenclature, formulae, elemental analysis, and appearance. In addition, the numerous methods of drug synthesis are summarized. Also the profile covers the physicochemical properties as: the value of pK_a, drug solubility, melting point, X-ray powder diffraction, and analysis methods for example: (compendial, electrochemical, spectroscopic, and method of chromatographic). Besides that, the profile covered pharmacological profile and clinical pharmacokinetics in subtitle's (absorption, distribution, metabolism, and elimination). About 100 references were given as a proof of the above-mentioned studies.

Delusional disorder, somatic type treated with aripiprazole--mirtazapine combination

Delusional disorder, somatic type (DDST) is a rare psychiatric disorder and the treatment is mostly based on observations, due to the lack of well-organized studies. Initially, antipsychotics and especially pimozide were considered to be the pharmacological approach of choice but, subsequently, tryciclic anti-depressants and selective serotonin re-uptake inhibitors (SSRIs) were also suggested to be effective, implicating the serotonergic system in the pathophysiology of the disorder. We present the case of a female with DDST, who responded to aripiprazole-mirtazapine combination, a finding that is in accordance with the initial approach of this disorder as a part of the schizophrenic spectrum, but also supports the hypothesis of serotonin dysfunction in DDST.

Effects of co-administration of antidepressants and monoamine oxidase inhibitors on 5-HT-related behavior in rats

5-hydroxytryptamine (5-HT) syndrome is a dangerous condition of 5-HT excess that can occur during co-administration of a monoamine oxidase (MAO) inhibitor and an antidepressant. We investigated the effects of acute administration of MAO inhibitors and subchronic administration of tricyclic and heterocyclic antidepressants, and a serotonin-noradrenaline reuptake inhibitor (SNRI) on 5-HT syndrome in rats treated with 5-hydroxytryptophan (5-HTP). The irreversible and non-selective MAO inhibitor pargyline, and the reversible and selective MAO-A inhibitor clorgyline, produced increases in 5-HT syndrome in the 5-HTP-treated rats, while subchronic co-administration of imipramine partly intensified and partly attenuated the syndrome, whereas milnacipran only attenuated the syndrome. Co-administration of mianserin partly intensified and partly attenuated the syndrome but the attenuating effect was dominant. Administration of the irreversible and selective MAO-B inhibitor selegiline did not produce any increase in 5-HT syndrome in the 5-HTP-treated rats, compared with the saline control. These data suggest that non-selective MAO and selective MAO-A inhibitors can induce 5-HT syndrome in humans when co-administered with antidepressants. Furthermore, the risk of 5-HT syndrome may be lower with the selective MAO-B inhibitor selegiline.

Electrophysiological and neurochemical characterization of the effect of repeated treatment with milnacipran on the rat serotonergic and noradrenergic systems

The present study was undertaken to elucidate the effects of repeated treatment with milnacipran, a serotonin (5-HT) and noradrenaline (NA) reuptake inhibitor (SNRI), on the synaptic plasticity in the hippocampal CA1 field, focusing on the interaction between the serotonergic and noradrenergic system. Repeated treatment with milnacipran (30 mg/kg, i.p. after 30 mg/kg, p.o. x 14 days) completely restored the suppression of the long-term potentiation (LTP) induced by single milnacipran treatment (30 mg/kg, i.p.). Single and repeated milnacipran increased to a similar extent extracellular NA in the hippocampus. Single milnacipran increased extracellular 5-HT and this effect tended to be enhanced by repeated treatment. The restoration of LTP and facilitation of the 5-HT level were not shown after repeated treatment with a selective 5-HT reuptake inhibitor (SSRI) fluvoxamine (30 mg/kg, p.o. x 14 days). These results suggest that milnacipran-induced restoration of LTP suppression is responsible for the enhancement of 5-HT neurotransmission, which appears to be associated with noradrenergic neuronal activity. In addition, the 5-HT1A receptor agonist tandospirone-induced suppression of LTP was completely blocked by repeated treatment with milnacipran, indicating the possibility that this reversal effect is due to the functional changes in postsynaptic 5-HT1A receptors. Taken together, the present data suggest that the interaction between the serotonergic and noradrenergic mechanism play an important role in the modulation of synaptic plasticity caused by repeated treatment with milnacipran, which may be implicated in the therapeutic effects of SNRI on psychiatric disorders.

Effect of noradrenergic system on the anxiolytic-like effect of DOI (5-HT2A/2C agonists) in the four-plate test

RATIONALE

Selective serotonin reuptake inhibitors and serotonin and noradrenaline reuptake inhibitors demonstrated an anxiolytic-like effect in the four-plate test (FPT). (+/-)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI; a 5-HT2A receptor agonist) also possessed strong anxiolytic-like effect in the same test. A 5-HT2A mechanism seems to be implicated in the mechanism of action of both antidepressants and DOI in this test. On the other hand, the alpha-adrenergic ligands have also demonstrated an activity in other models of anxiety. A previous study demonstrated that the alpha2-adrenoceptor agonists abolished the anxiolytic-like effect of antidepressants.

OBJECTIVES

The aim of the present study was to evaluate the role of noradrenergic system on the regulation of 5-HT2 receptors implicated in the DOI anxiolytic-like activity in the FPT.

METHODS

First, the effect of noradrenergic and serotonergic lesions on DOI anxiolytic-like activity was studied in the FPT. Second, the effect of co-administration of alpha-adrenoceptor ligands and DOI was evaluated in the same test.

RESULTS

The noradrenergic and serotonergic lesions had no effect on DOI (1 mg/kg) anti-punishment activity in the FPT. Adrafinil 0.25 and 4 mg/kg (an alpha1-adrenoceptor agonist), prazosin 0.5 and 2 mg/kg (an alpha1-adrenoceptor antagonist) and idazoxan 1 and 4 mg/kg (an alpha2-adrenoceptor antagonist) did not modify the activity of DOI. Clonidine 0.06 mg/kg, guanabenz 0.125 and 0.5 mg/kg (two alpha2-adrenoceptor agonists) and guanfacine 0.06 and 0.125 mg/kg (a specific alpha2A-adrenoceptor agonist) completely abolished DOI-induced increase in punished passages.

CONCLUSION

These results indicate that the DOI seems to act on the 5-HT2 receptors post-synaptically located. The effect of DOI is regulated by the alpha2-adrenoceptors.

Milnacipran, a serotonin and noradrenaline reuptake inhibitor, suppresses long-term potentiation in the rat hippocampal CA1 field via 5-HT1A receptors and alpha 1-adrenoceptors

Pharmacological characteristics of a serotonin (5-HT) and noradrenaline reuptake inhibitor (SNRI), milnacipran, in modulation of the synaptic plasticity were investigated. Milnacipran (30 mg/kg, i.p.) suppressed the long-term potentiation (LTP) in the hippocampal CA1 field of anesthetized rats. Milnacipran-induced suppression was reversed by pretreatment with the selective 5-HT1A receptor antagonist WAY 100635 (0.1 mg/kg, i.v.) or the alpha1-adrenoceptor antagonist prazosin (1 and 10 microg/rat, i.c.v.). The alpha2-adrenoceptor antagonist idazoxan (5 mg/kg, i.p.) did not influence the milnacipran-induced synaptic responses. These data suggest that the inhibitory effects of milnacipran on LTP induction are mediated via both 5-HT1A receptors and alpha1-adrenoceptors. In other words, functional interaction between the serotonergic and noradrenergic neuronal systems is involved in alteration of the hippocampal synaptic plasticity, which may be implicated in the SNRI-induced therapeutic effect on psychiatric disorders.

Zuclopenthixol facilitates memory retrieval in rats: possible involvement of noradrenergic and serotonergic mechanisms

Although disturbed memory function often coexists with psychosis, the cognitive effects of antipsychotic medications with diverse pharmacodynamic properties are rarely investigated. The neurocognitive profile of zuclopenthixol, a thioxanthene dopaminergic antagonist and a conventional neuroleptic agent, has yet to be investigated despite the effect of the drug on a variety of neurotransmitter systems involved in mediation of learning and memory processes. In this study, the effect of zuclopenthixol was tested on memory retrieval 24 h after training using an inhibitory avoidance task in rats. Acute administration of zuclopenthixol (0.7 and 1.4 mg/kg i.p.) before retrieval testing increased step-through latency during the test session. The same doses of zuclopenthixol did not affect the ambulatory activity of rats in the openfield test and therefore the facilitatory effect of the drug on memory function could not be confounded with any motoric properties. This study also investigated the effect of zuclopenthixol on cortical and hippocampal monoaminergic neurotransmitters' levels together with acetylcholinesterase enzyme (AChE) activity, both of which are known to be important in control of cognitive function. Administration of zuclopenthixol (0.7 and 1.4 mg/kg i.p.) neither affected dopamine (DA) level nor AChE activity in rat cortex and hippocampus. On the other hand, the lower dose of zuclopenthixol elevated cortical norepinephrine (NE) level, while the higher dose elevated both cortical and hippocampal NE level together with hippocampal serotonin (5-HT) level. These results may suggest the involvement of adrenergic and serotonergic mechanisms in the facilitatory effect of zuclopenthixol on retrieval memory. Zuclopenthixol may therefore be a better alternative than other commonly used antipsychotic medications reported to impair cognitive function of schizophrenic patients.

Calcium-dependent inhibition of synaptosomal serotonin transport by the alpha 2-adrenoceptor agonist 5-bromo-N-[4,5-dihydro-1H-imidazol-2-yl]-6-quinoxalinamine (UK14304)

Termination of serotonergic transmission is the function of the plasma membrane 5-hydroxytryptamine (serotonin, 5-HT) transporter (SERT), which is also a high-affinity target in vivo for antidepressants, amphetamines, and cocaine. Studies show that SERT is regulated by protein kinase- and phosphataselinked pathways. In contrast, receptor-linked modulation of SERT is only minimally defined. Because noradrenergic stimulation is reported to influence 5-HT release, we explored possible presynaptic adrenoceptor-mediated regulation of SERT. In mouse forebrain synaptosomes, alpha2-adrenoceptor agonists, particularly 5-bromo-N-[4,5-dihydro-1H-imidazol-2-yl]-6-quinoxalinamine (UK14304), triggered a concentration- and time-dependent decrease in 5-HT transport. In contrast, 5-HT uptake was unaffected by pharmacological alpha1-adrenoceptor activation. Kinetically, UK14304 significantly decreased the apparent substrate affinity, Km without altering transport capacity, Vmax. At concentrations of UK14304 supporting maximal inhibition of SERT in synaptosomes, no effect on SERT in transfected cells was observed, suggesting that UK14304 acts indirectly to reduce SERT activity. The effect of UK14304 on 5-HT uptake was not shared by other Na+ and Cl--dependent transporters. UK14304-mediated inhibition of SERT function was yohimbine-sensitive, as was inhibition triggered by norepinephrine, and was abolished in the absence of added Ca2+. Moreover, UK14304 effects were attenuated by voltage-sensitive Ca2+ channel antagonists, consistent with a role for Ca2+ in UK14304 effects. In agreement with altered 5-HT transport activity in vitro, in vivo chronoamperometry studies revealed that UK14304 significantly prolonged 5-HT clearance. Our findings suggest that UK14304 modulates SERT function in vitro and in vivo via signaling pathways, possibly supported by an influx of Ca2+ through voltage-sensitive Ca2+ channels.

Serotonin reverses dominant social status

Social stress from aggressive interaction is expressed differently in specific brain regions of dominant and subordinate male Anolis carolinensis. Prior to aggressive behavior, the outcome is predictable via the celerity of postorbital coloration: Dominant males exhibit more rapid eyespot darkening. Serotonergic activation is manifest rapidly (1 h) in hippocampus, nucleus accumbens and brainstem of subordinate males, and is expressed more rapidly in dominant males. Amygdalar serotonergic activation responds rapidly (1 h) in dominant males, but is expressed slowly (1 w) and chronically in subordinate males. We hypothesized that chronic (1 w) serotonin elevation, manipulated by the selective serotonin reuptake inhibitor sertraline, would decrease aggressiveness and result in subordinate status. Dominant status was established in pairs of male A. carolinensis. The pairs were separated and treated with sertraline or vehicle. Sertraline was given in food to either the dominant or the subordinate male, both males or neither male for 1 week. Pairs were reintroduced, and behavior and social status recorded. When both dominant and subordinate males were treated with sertraline (or vehicle), or when subordinate males alone were treated with sertraline, previously established social relationships remained unchanged or became associative. However, when dominant males alone were treated with sertraline, their social status was reversed (43%) or negated (57%). Latency to eyespot darkening was significantly retarded in dominant males treated with sertraline, and aggressive displays and attacks were reduced. Chronic 5-HT elevation is consistent with subordinate status. Social status and aggressive disposition do not appear to be immutable, but may be changed by neuroendocrine mechanisms that mediate adaptation to environmental conditions like stress.

Monoaminergic activities of limbic regions are elevated during aggression: influence of sympathetic social signaling

A visual social signal inhibiting aggression is coincident with limiting serotonergic and noradrenergic activity in subiculum, hippocampus, nucleus accumbens, medial amygdala, but not lateral amygdala, septum, and hypothalamus. Darkening of postorbital skin in the lizard Anolis carolinensis is stimulated by sympathetic activation of beta(2)-adrenergic receptors via adrenal catecholamines, and occurs more rapidly in dominant males during social interaction. Eyespot darkening functions as a social signal limiting aggressive interaction. To assess the effect of this social signal on telencephalic activity of monoamines, males were painted postorbitally with green or black paint, and exposed to a mirror. Serotonergic and noradrenergic turnover, as estimated by ratios of catabolite to transmitter, were elevated in the subiculum, hippocampus, nucleus accumbens, and medial amygdala of animals in which the eyespots were masked by green paint. Conversely, dopaminergic activity in these brain regions was lower in males with hidden eyespots (painted green). Hiding the eyespot evoked significantly increased aggressive activity toward the mirror image. Furthermore, changes in monoaminergic turnover were coincident with altered aggressive behavior, suggesting a relationship between them. Changes of monoaminergic activity were not observed in the septum, lateral amygdala, or hypothalamus, when males with eyespots permanently marked (black) were compared with those with eyespots hidden (painted green). Stimulated (serotonergic and noradrenergic) or inhibited (dopaminergic) activity due to social signal and aggression are confined to regions of the brain similarly activated during social stress, and do not constitute a generalized activation of monoaminergic systems.

alpha2-Adrenoceptor antagonists reverse the 5-HT2 receptor antagonist suppression of head-twitch behavior in mice

The alpha2-adrenoceptor agonist clonidine, as well as 5-HT2 receptor antagonists, reportedly suppress 5-HT2 receptor-mediated head-twitch behavior. We investigated the effect of alpha2-adrenoceptor antagonists on the suppressive action of 5-HT2 receptor antagonists in mice pretreated with the noradrenaline toxin 6-hydroxydopamine (6-OHDA) or the 5-HT synthesis inhibitor p-chlorophenylalanine (p-CPA). In normal mice, idazoxan (0.08-0.2 mg/kg, IP) or yohimbine (0.2-2.0 mg/kg, IP), both alpha2-adrenoceptor antagonists, had no effect on the head-twitch response caused by 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; 16 mg/kg, IP), but idazoxan significantly enhanced the response at 0.5 mg/kg. On the other hand, these alpha2-adrenoceptor antagonists, at doses that had no effect on the basal number of head-twitches (idazoxan 0.2 mg/kg and yohimbine 0.5 mg/kg), significantly attenuated not only the suppressive effect of clonidine (0.01 mg/kg, IP) on head-twitch response but also that of the 5-HT2 receptor antagonist ritanserin (0.03 mg/kg, IP). Moreover, idazoxan (0.2 mg/kg) also significantly reversed the inhibition by 0.01 mg/kg (IP) ketanserin, a selective 5-HT2 receptor antagonist. Pretreatment with 6-OHDA plus nomifensine but not with p-CPA significantly attenuated the effect of idazoxan (0.2-0.5 mg/kg) on the ritanserin inhibition of the head-twitch response. Prazosin, an alpha1-adrenoceptor antagonist, dose-dependently suppressed the response, and the effect of prazosin (1.25 mg/kg) was significantly attenuated by 0.5 mg/kg idazoxan. These results indicate that endogenous noradrenaline is involved in the apparent antagonistic interaction between selective alpha2-adrenoceptor antagonists and 5-HT2 receptor antagonists in the head-twitch response, and suggest that noradrenaline stimulation of alpha1-adrenoceptors may be involved in this apparent antagonism.

Functional interaction between serotonin and other neuronal systems: focus on in vivo microdialysis studies

In this review, the functional interactions between serotonin (5-HT) and other neuronal systems are discussed with the focus on microdialysis studies in the mammalian brain (mainly rats). 5-HT release is negatively regulated not only by somatodendritic 5-HT1A and terminal 5-HT1B (5-HT1D) autoreceptors but also by alpha 2-adrenergic and mu-opioid heteroreceptors that are located on serotonergic nerve terminals. 5-HT by itself is involved in the inhibitory effects of noradrenaline release and the facilitatory regulation of dopamine release via multiple 5-HT receptors. Acetylcholine release appears to be regulated by inhibitory 5-HT1B heteroreceptors located on cholinergic nerve terminals. Long-term treatment with 5-HT-uptake inhibitors and noradrenaline-uptake inhibitor produces desensitization of 5-HT1A autoreceptors and alpha 2-heteroreceptors, respectively, which may be related therapeutically to the delayed onset of the effects of antidepressants. Some microdialysis studies have predicted that the combination of a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist might produce much greater availability of 5-HT in the synaptic cleft in terms of much faster induction of subsensitivity of 5-HT1A autoreceptors. Clinical trials based on this hypothesis have revealed that combination therapy with a 5-HT-uptake inhibitor and 5-HT1A-autoreceptor antagonist ameliorated the therapeutic efficacy in depressive patients. Taken together, neurochemical approaches using microdialysis can contribute not only to clarification of the physiological role of the serotonergic neuronal systems but also might be a powerful pharmacological approach for the development of therapeutic strategies.

Changes in the regulation of 5-hydroxytryptamine release by alpha2-adrenoceptors in the rat hippocampus after long-term desipramine treatment

In vivo microdialysis was used to measure the effects of long-term treatment of rats with desipramine upon the regulation by alpha2-adrenoceptors of serotonin (5-hydroxytryptamine, 5-HT) release from the serotonergic neurons in the hippocampus. Rats were injected with saline or desipramine, 10 mg/kg, i.p., every 12 h for 14 days. When added to the perfusion solution, brimonidine, an alpha2-adrenoceptor agonist, significantly inhibited the K+-evoked release of 5-HT in the hippocampi of saline-treated, control rats. This action of brimonidine was prevented by pretreating the rats with idazoxan, an alpha2-adrenoceptor antagonist. Long-term desipramine treatment significantly reduced the inhibitory effect of brimonidine upon the K+-evoked 5-HT release. With long-term administration of desipramine, noradrenaline content in the hippocampi was significantly decreased as compared with that of the control rats, whereas the basal noradrenaline concentration in the dialysate was significantly increased. On the other hand, both the 5-HT content of the hippocampus and the basal 5-HT concentration in the dialysate were significantly increased. The present study suggests that long-term administration of desipramine causes a functional subsensitivity of the presynaptic alpha2-adrenoceptors that regulate serotonergic neuronal function in the rat hippocampus. It also supports the concept that changes in the sensitivity of alpha2-adrenoceptors that regulate neurotransmitter release play an important role in the mechanism of antidepressant drug action.