Central pre- and postsynaptic monoamine receptors in antidepressant therapy

The effect of sympathetic antagonists on the antidepressant action of alprazolam

Alprazolam is an anti-anxiety drug shown to be effective in the treatment of depression. In this study, the effect of sympathetic receptor antagonists on alprazolam-induced antidepressant action was studied using a mouse model of forced swimming behavioral despair. The interaction of three sympathetic receptor antagonists with benzodiazepines, which may impact the clinical use of alprazolam, was also studied. Behavioral despair was examined in six groups of albino mice. Drugs were administered intraperitoneally. The control group received only a single dose of 1% Tween 80. The second group received a single dose of alprazolam, and the third group received an antagonist followed by alprazolam. The fourth group was treated with imipramine, and the fifth group received an antagonist followed by imipramine. The sixth group was treated with a single dose of an antagonist alone (atenolol, a β1-selective adrenoceptor antagonist; propranolol, a non selective β-adrenoceptor antagonist; and prazocin, an α1-adrenoceptor antagonist). Results confirmed the antidepressant action of alprazolam and imipramine. Prazocin treatment alone produced depression, but it significantly potentiated the antidepressant actions of imipramine and alprazolam. Atenolol alone produced an antidepressant effect and potentiated the antidepressant action of alprazolam. Propranolol treatment alone produced depression, and antagonized the effects of alprazolam and imipramine, even producing depression in combined treatments.In conclusion, our results reveal that alprazolam may produce antidepressant effects through the release of noradrenaline, which stimulates β2 receptors to produce an antidepressant action. Imipramine may act by activating β2 receptors by blocking or down-regulating β1 receptors.

Differences in activation of the dorsal raphe nucleus depending on performance of suicide

The serotonergic system has been implicated in the pathogenesis of mood disorders as well as in suicidal behavior. It is unknown, however, whether raphe neurons, which are mostly serotonergic, show altered activity in patients with mood disorders who complete suicide as compared to those without suicidal behavior. In order to measure cellular markers of serotonergic activity in the dorsal raphe nucleus in brains of 12 people with mood disorders and of 12 controls (C), stereological measurements were carried out of nucleolar organizer regions (AgNORs) and of serotonergic neuron numbers. Six patients died from suicide (S) and the other six patients died from natural causes (NS). Results were assessed using ANOVA and post hoc Tukey-HSD tests looking for effects of diagnostic group (S, NS, C). Results show that in the rostral subnuclei of the dorsal raphe there was a significant effect of diagnostic group on the ratios of the nucleolar organizer regions to nuclear area (NOR ratio) and a nearly significant effect on numbers of serotonergic neurons. Post hoc tests revealed larger values for those dependent variables in S compared to NS. Dose equivalents of antidepressants correlated positively with NOR ratios and numbers of serotonergic neurons in the rostral part of the dorsal raphe. In conclusion, the present data suggest that there are functional differences in the dorsal raphe of patients with mood disorders depending on suicidal behavior. Antidepressants appear to contribute to cellular activation in the rostral part of the dorsal raphe.

Shared mechanisms of antidepressant and antiepileptic treatments: drugs and devices

Many treatments for the epilepsies and affective disorder share the properties of seizure suppression and mood stabilization. Moreover, affective disorders and the epilepsies appear to share partially similar pathogenic mechanisms. A component of the shared predisposition appears to arise from noradrenergic and serotonergic deficits. Increasing evidence supports the hypothesis that noradrenergic and/or serotonergic elevation is a mechanism of therapeutic benefit shared by most antidepressants and many antiepileptic medications. Medication induced alterations in GABAergic, glutamatergic, and CRH (corticotropin releasing hormone) containing neurons may also contribute to the shared therapeutic properties of antidepressant and antiepileptic medications.

Cortical alpha-adrenoceptor downregulation by tricyclic antidepressants in the rat brain

The aim of the present study was to examine the effect of chronic tricyclic antidepressants (TCAs) treatment on the density of alpha-adrenoceptors in the rat brain. Density of alpha1- and alpha2-adrenoceptors was measured in cortex and hippocampus of rats treated with imipramine (IMI, 5mg/kg body weight), desipramine (DMI, 10mg/kg body weight), clomipramine (CMI, 10mg/kg body weight) and amitriptyline (AMI, 10mg/kg body weight), for 40 days, using [3H]prazosin and [3H]clonidine, respectively. The density of cortical alpha1-adrenoceptors was significantly decreased with IMI (46%), DMI (21%), CMI (50%) and AMI (67%) treatment, without altering the affinity of the receptor. The density of cortical alpha2-adrenoceptors was also significantly decreased with DMI (69%), CMI (81%) and AMI (80%) treatment, without affecting the affinity for [3H]clonidine. The density of hippocampal alpha1-adrenoceptors was significantly decreased only with AMI treatment (47%), without affecting the affinity for [3H]prazosin. However, no change in hippocampal alpha2-adrenoceptor density was observed with any of these TCAs. The results suggest that chronic antidepressant (AD) treatment downregulates the cortical, but not hippocampal, alpha1- and alpha2-adrenoceptors in rat brain. The region-specific downregulation of alpha1- and alpha2-adrenoceptors density, which occur after prolonged AD treatment, may underline the therapeutic mechanism of action.

Dibenzoazepine analogues: the electrophysiological properties of JL3, a potential atypical antidepressant

JL3, 10-(4-methylpiperazin-1-yl)pyrido[4,3-b][1,4]benzothiazepine, has potent antidepressant-like activity in Porsolt's test in mice. Therefore, its influence on the electrical activity of central monoaminergic neurons was investigated in rats anaesthetized with chloral hydrate. JL3 induced a marked decrease of the firing rate of dorsal raphe serotonergic neurons (ID50 = 3.87 +/- 0.57 mg kg-1) and of locus coeruleus noradrenergic neurons (ID50 = 2.63 +/- 0.35 mg kg-1). The drug did not modify the electrical activity of A10 dopaminergic neurons. JL3 does not block amine uptake but it has affinity for 5-HT1A and 5-HT2 receptors. It is speculated that serotonergic mechanisms could play a role in the electrophysiological effects of JL3.

Zimeldine: a review of its effects on ethanol consumption

This review evaluates the literature and describes an extensive series of experiments which examined the effects of zimeldine , its metabolite norzimeldine and other serotonin and norepinephrine reuptake inhibitors on voluntary ethanol consumption in rats. The results of these experiments indicate that drugs which specifically inhibit serotonin reuptake are capable of decreasing voluntary ethanol consumption. The behavioral mechanism through which these drugs exert their effects seems to be extinction of the primary reinforcing properties of alcohol. These effects seem to be partially attenuated both by drugs which modulate the norepinephrine system as well as by the serotonin postsynaptic receptor blocker methergoline. The data presented in this review are discussed in terms of the involvement of the serotonin and norepinephrine systems in the mechanism of action of these drugs. In addition, several alternative hypotheses concerning the nature of the phenomenon are offered. Finally, the implications of these data for the possible development of a treatment procedure for problem drinkers is discussed.

Effects of the two antidepressant drugs mianserin and indalpine on the serotonergic system: single-cell studies in the rat

Several antidepressant treatments enhance serotonergic neurotransmission. The present electrophysiological studies were undertaken to assess the effect of mianserin and indalpine, two antidepressant drugs with different pharmacological profiles, on serotonergic neurotransmission. In a first series of experiments, the responsiveness of hippocampal pyramidal neurons to microiontophoretic applications of serotonin (5-HT), norepinephrine (NE) and gamma-aminobutyric acid (GABA) was assessed following mianserin, imipramine (5 mg/kg/day IP) or saline administration for 14 days. At 48 h after the last dose of mianserin, responsiveness to 5-HT was increased whereas that to NE and GABA was not modified. The degree of sensitization to 5-HT was the same as that produced by imipramine. Acute IV administration of mianserin (up to 10 mg/kg) did not decrease the firing rate of dorsal raphe 5-HT neurons. In a second series of experiments, long-term administration of indalpine (5 mg/kg/day IP for 14 days) did not modify the responsiveness of hippocampal pyramidal neurons to microiontophoretically applied 5-HT, NE and GABA whereas imipramine treatment (5 mg/kg/day IP) increased selectively their sensitivity to 5-HT when compared to indalpine-treated rats. In keeping with its potent reuptake-blocking property, acute IV indalpine produced a marked decrease in the firing rate of dorsal raphe 5-HT neurons (ED50 0.33 mg/kg). The firing rate of dorsal raphe 5-HT neurons was assessed following 2-, 7- and 14-day treatments with indalpine (5 mg/day IP). After 2 days, the firing rate of 5-HT neurons was greatly reduced, after 7 days it had recovered partially and after 14 days it had returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Acutely administered antidepressants possess a multiplicity of pharmacological actions. However, the fact that agents possessing similar pharmacological actions are devoid of antidepressant activity, together with the lack of correlation between doses required for acute pharmacological effects and clinical efficacy, suggest that the mechanism(s) of action of antidepressants cannot be directly attributed to the acute pharmacological properties of the drugs. The lag phase in onset of clinical effectiveness emphasizes the importance of adaptive changes following chronic antidepressant administration. A rapidly accelerating trend in attempting to delineate the precise molecular mechanisms of action of antidepressants is the shift in emphasis following chronic antidepressant therapies from alterations in uptake, storage, synthesis and release of neurotransmitters to adaptive changes in receptor functioning. These adaptations occur both pre- and postsynaptically. Examples of the former are alpha 2 and DA presynaptic receptors, both being down-regulated by certain forms of chronic antidepressant therapy. The fact that the NE-coupled adenylate cyclase system in rat brain slices is down-regulated by tricyclics, atypical antidepressants, MAO inhibitors and ECT emphasizes the importance of the system. Electrophysiological and behavioral studies point to the up-regulation of central alpha 1 and 5-HT receptor functioning following long-term antidepressant therapy. In contrast to the beta-adrenoceptor, these findings cannot be correlated with data from radioligand binding studies. In general central alpha 1-adrenoceptor binding remains unaltered. This is also true for 5-HT1 binding whereas cortical 5-HT2 binding is both increased and decreased depending on the type of antidepressant therapy being investigated. The relationship of these adaptive changes to the clinical efficacy of antidepressants in man is not clear since there is generally a lack of good models for studying human central receptor functioning. A review of current data from animal studies would tend to disfavour the view that all forms of antidepressant therapy possess a common mechanism of action. Perhaps multiple intervention sites exist. The introduction and evaluation of agents possessing a specificity of pharmacological action will undoubtedly aid psychotherapeutic research. The knowledge that peptides and 'classical' neurotransmitters can co-exist in the same neurone will undoubtedly generate studies of the significance and importance of the co-transmitter function of peptides in the mechanisms of action of antidepressant therapies.

Sensitivity in vivo of central alpha 2- and opiate receptors after chronic treatment with various antidepressants

Recent studies suggest that the antidepressant activity could be explained by gradually developing modifications in the sensitivity of some central monoaminergic receptors. As concerns alpha 2-receptors, some largely indirect informations suggest that a subsensitivity develops following chronic treatment with desipramine or imipramine. However it is not clear whether this effect is shared by other antidepressants. In view of the important role of these receptors in the regulation of noradrenergic neuro-transmission, single cell recording and microiontophoretic techniques were used in this work to systematically assess locus coeruleus (L.C.) alpha 2-receptor sensitivity following acute and chronic administration of various antidepressants. The responsiveness of L.C. alpha 2-receptors to iontophoretically applied clonidine was studied simultaneously with the responsiveness of L.C. opiate receptors to iontophoretically applied morphine in order to test the specificity in the putative modifications induced by the antidepressants. The compounds studied were desipramine (DMI), imipramine (IMI), clomipramine (CIMI), zimelidine (ZIM), mianserin (MIAN), iprindole (IPR) and chlorpromazine (CPZ). Long-term treatment with DMI, IMI and ZIM but not with the other clinically effective antidepressant drugs induced a decrease in the responsiveness of L.C. neurons to iontophoretically applied clonidine. None of the drugs tested altered the responsiveness of these neurons to iontophoretically applied morphine. Consequently the therapeutic effectiveness of antidepressant drugs can not be generally related to modulation of the sensitivity of central alpha 2- or opiate receptors.

Some current problems related to the mode of action of antidepressant drugs

An important relationship between antidepressant drugs and the monoamine transmitters of the brain is generally recognized. Major groups of established antidepressants are inhibitors of monoamine inactivation (by reuptake or monoamine oxidase) or monoamine precursors. However, the tricyclic antidepressants have many actions in addition to the blockade of monoamine reuptake, e.g. blockade of muscarinic, alpha-adrenergic and histaminic receptors and general membrane effects which may lead to cardiotoxicity. The discovery that certain antihistaminic agents devoid of condensed ring systems are fairly potent inhibitors of monoamine reuptake led to the synthesis of zimelidine, a potent and selective 5-HT reuptake inhibitor with little affinity for receptors and almost devoid of cardiotoxicity. Careful pharmacological studies and clinical trials with such selective monoaminergic agents are of considerable value for testing the possible role of monoaminergic mechanisms in various aspects of human and animal behaviour and for central nervous disorders, including depression.

Increased brain serotonergic and noradrenergic activity after repeated systemic administration of the beta-2 adrenoceptor agonist salbutamol, a putative antidepressant drug

Subchronic (5 mg/kg SC, twice daily for 14 days) but not acute administration of the beta-2-adrenoceptor agonist salbutamol to rats caused a significant increase in the accumulation of 5-hydroxytryptophan in the limbic forebrain, the corpus striatum and the cerebral cortex when measured during 30 min after inhibition of L-amino acid decarboxylase by NSD 1015 (100 mg/kg IP). Simultaneously assayed tryptophan concentrations in the same brain regions were not affected. These results indicate an increase in the in vivo rate of tryptophan hydroxylation in the brain, produced by subchronic salbutamol administration. The effect of salbutamol treatment on brain catecholamine(CA) utilization was estimated by studying the disappearance of CA in the brain after inhibition of tyrosine hydroxylase by alpha-methyltyrosine methyl ester (H 44/68), 250 mg/kg IP, 3.5 h before sacrifice. Subchronically but not acutely administered salbutamol caused both a significant increase in endogenous noradrenaline (NA) levels and an increase NA utilization. Dopamine levels and turnover were, however, not altered by either acute or subchronic treatment. The activation, probably centrally elicited, of brain NA and 5-hydroxytryptamine systems by the subchronic salbutamol regimen supports the concept of beta-adrenoceptor mediated regulation of brain monoamine systems, and could contribute to the clinically reported antidepressant activity of beta-2-adrenoceptor agonists.