Pharmacology of new antidepressants

Potential repurposing of known drugs as potent bacterial β-glucuronidase inhibitors

The active metabolite of the chemotherapeutic irinotecan, SN-38, is detoxified through glucuronidation and then excreted into the gastrointestinal tract. Intestinal bacteria convert the glucuronidated metabolite back to the toxic SN-38 using β-glucuronidase (GUS), resulting in debilitating diarrhea. Inhibiting GUS activity may relieve this side effect of irinotecan. In this study, we sought to determine whether any known drugs have GUS inhibitory activity. We screened a library of Food and Drug Administration-approved drugs with a cell-free biochemical enzyme assay using purified bacterial GUS. After triage, five drugs were confirmed to inhibit purified bacterial GUS. Three of these were the monoamine oxidase inhibitors nialamide, isocarboxazid, and phenelzine with average IC(50) values for inhibiting GUS of 71, 128, and 2300 nM, respectively. The tricyclic antidepressant amoxapine (IC(50) = 388 nM) and the antimalarial mefloquine (IC(50) = 1.2 µM) also had activity. Nialamide, isocarboxazid, and amoxapine had no significant activity against purified mammalian GUS but showed potent activity for inhibiting endogenous GUS activity in a cell-based assay using living intact Escherichia coli with average IC(50) values of 17, 336, and 119 nM, respectively. Thus, nialamide, isocarboxazid, and amoxapine have potential to be repurposed as therapeutics to reduce diarrhea associated with irinotecan chemotherapy and warrant further investigation for this use.

Pindolol augmentation of selective serotonin reuptake inhibitors: accounting for the variability of results of placebo-controlled double-blind studies in patients with major depression

Co-administration of pindolol with SSRIs in patients with depression has been suggested as a way to both hasten and augment antidepressant response. Clinical trials have examined the efficacy of this treatment regime and conflicting results have been reported. The present review briefly presents the results of placebo-controlled double-blind trials of pindolol augmentation of SSRIs in patients with major depression, and focuses on factors that may account for the variability of findings. Additionally, a profile of the subset of patients who may most benefit from pindolol augmentation is outlined. Methodological factors such as qualitative differences in definitions of antidepressant response, the timing of pindolol administration and heterogeneous clinical characteristics of patient samples may contribute to the variability in the results of clinical trials to date. Similarly, individual differences in neuropathology, neurophysiology and genotype may also account for some of the inconsistencies in the findings. Finally, the results of recent neuroimaging studies suggest that the 2.5 mg t.i.d. dose of pindolol that has been used in all but one of these investigations may be suboptimal for achieving reliable and significant occupancy of 5-HT1A autoreceptors and may explain the contradictory nature of the results of investigations of pindolol augmentation.

Effectiveness of pindolol plus serotonin uptake inhibitors in depression: a meta-analysis of early and late outcomes from randomised controlled trials

BACKGROUND

Contradictory results on the efficacy of pindolol associated with selective serotonin reuptake inhibitors (SSRIs) in depressive illness have been published and no former review has produced an overall figure of its efficacy. This study aims to review the efficacy and tolerability of pindolol plus SSRIs in depressive illness.

METHODS

A meta-analysis of randomised controlled trials (RCTs) comparing pindolol plus SSRIs with placebo plus SSRIs.

RESULTS

Nine RCTs met inclusion criteria. Outcome favoured pindolol at 2 weeks time (N=5; OR=2.8; 95% CI 1.4-5.7), but not at four to 6 weeks (N=7; OR=1.4; 95% CI 0.8-2.7). Results for early outcome studies were robust to sensitivity analysis. Nineteen more studies, averaging null results, would be needed to change the overall probability (P=0.0001) to a non-significant figure.

CONCLUSIONS

Pindolol seems to hasten the response to SSRIs in depression with a timing window circumscribed to the first weeks of treatment.

An investigation of serotonergic involvement in the regulation of ACTH and corticosterone in the olfactory bulbectomized rat

The bilateral olfactory bulbectomy resulted in significantly higher plasma concentration of corticosterone, but not of ACTH in basal conditions and much higher plasma ACTH and corticosterone concentrations after 15 min of immobilization stress than were observed in sham-operated animals. Daily treatment with fluoxetine-a specific serotonin reuptake inhibitor-(15 mg/kg/day) had no effect on basal ACTH and corticosterone concentrations in OB rats. Fluoxetine treatment caused lower levels of ACTH, but not of corticosterone secretion, in response to immobilization stress. Bulbectomy significantly reducing 5-HT concentration in the amygdala. Stress increased serotonergic activity in the hypothalamus but not in the amygdala of OB rats. Chronic fluoxetine treatment of both unstressed and stressed OB rats resulted in a lower turnover rate in the two structures. Our results suggest that the hypercorticosteronemia observed after bulbectomy in unstressed OB rats is independent of the serotonergic system in both hypothalamus and amygdala. In contrast, they also demonstrate hypothalamic 5-HT changes in the HPA hyperactivity of OB rats in response to stress. Chronic fluoxetine treatment may normalize pituitary ACTH secretion in response to stress, possibly desensitization of the 5-HT receptors in the hypothalamus due to 5-HT being move available at the synapses.

Effects of salbutamol upon performance on an operant screen for antidepressants

The beta adrenergic (beta) agonist salbutamol increased reinforcement rates and decreased response rates on a differential-reinforcement-of-low-rate (DRL) 72-S schedule. These changes in DRL 72-S schedule performance are also produced by most clinically used antidepressants. The effects of salbutamol on a DRL 72-S schedule were dose-dependently antagonized by the beta antagonist metoprolol, but not changed by the 5HT antagonist methysergide. Additionally, neither salbutamol nor the antagonism of salbutamol by metoprolol caused disruption of DRL 72-S schedule performance. These results indicate that stimulation of beta receptors, and not of 5HT receptors, mediates salbutamol antidepressant-like effects on a DRL 72-S schedule.

Effects of psychotropic drugs on the behavior and neurochemistry of olfactory bulbectomized rats

Since its first characterization as a model for the detection of antidepressant drugs (van Riezen et al., 1976) a large body of data now supports the view that olfactory bulbectomy produces changes in animal behavior that are reversed by chronic treatment with antidepressants. The behavioral deficits seen in olfactory bulbectomized rats (such as irritability, deficits in acquisition of avoidance and of appetitive motivated conditioning and hyperactivity in a new environment) are probably the results of a reduced ability to adapt to environmental changes. These behavioral changes, their biochemical consequences and the effects of treatments with psychotropic drugs are reviewed. These studies suggest that the olfactory bulbectomized rat is a model of depression useful to detect antidepressant drugs.

The psychopharmacological era: notes toward a history

Cultural and economic influences on the psychopharmacological era are reviewed, in an attempt to bring into focus what has been happening in psychopharmacology for the past thirty years. It is argued that the belief that clinical advances are made through the heroic achievements of disinterested scientists is a simplistic view that may militate against future significant discoveries. Such discoveries, it is argued, still come about for the most part serendipitously, despite a widespread belief that psychopharmacology has become a rational science.

Chronic tranylcypromine treatment induces functional alpha 2-adrenoceptor down-regulation in rats

The effects of chronic (28 d Alzet 2002 osmotic mini-pumps) administration of antidepressant drugs on the functional sensitivity of alpha 2-adrenoceptors and on monoamine oxidase activity has been assessed. Tranylcypromine, 4-fluorotranylcypromine and clorgyline (0.5-1.0 mg kg-1) induced a decrease in the motor-suppressant effects of clonidine (50 micrograms kg-1) observed at 9-10 and 23-24 days of drug administration. These effects were associated with marked decreases in type A (clorgyline) or type A and B monoamine oxidase activity (tranylcypromine and 4-fluorotranylcypromine). The results indicate that alpha 2-adrenoceptor down-regulation is an early emergent feature of adaptive changes in noradrenaline systems induced by prolonged exposure to tranylcypromine.

Neurochemical and electrophysiological events underlying stress-induced depression in an animal model

This paper has described an animal (rodent) model of depression in which depression-like characteristics are produced by exposure of animals to stressful events that they cannot control. This model, called "stress-induced depression," appears to be mediated by stress-induced changes in brain norepinephrine (NE), and evidence now indicates that a large depletion of NE in the Locus Coeruleus region of the brain stem (LC) is critical for producing the behavioral disturbance seen in this model. To explain the functional significance of NE depletion in the LC, it has been suggested that this change results in decreased stimulation, (i.e., functional blockade) of alpha-2 receptors that normally inhibit firing of LC neurons. Consequently, LC neurons should be disinhibited in stress-induced depression. The final part of this paper describes findings from electrophysiological measurement of LC activity. Prior to measuring changes occurring in stress-induced depression, studies examined the basic role played by alpha-2 receptors in LC activity. These receptors were found to regulate the responsivity of LC neurons to excitatory stimuli, so that pharmacological blockade of these receptors increased the firing of LC neurons to excitatory input. When LC activity was then measured in stressed animals showing behavioral depression, responsivity of LC neurons to excitatory input was elevated in comparison to animals not exposed to stress. Also, administration of an alpha-2 blocking drug could not increase responsivity of LC neurons in stressed animals, which further indicated that alpha-2 receptors in the LC region were functionally blocked in these animals. Finally, the amount that LC neurons increased their firing to an excitatory stimulus (i.e., an index of alpha-2 receptor blockade) was highly correlated with the amount of behavioral depression seen in an activity test conducted just prior to electrophysiological measurement. These electrophysiological findings indicate that LC neurons are disinhibited in stress-induced depression, and that this disinhibition is particularly characterized by increased responsivity of LC neurons to excitatory input, which is indicative of alpha-2 receptor blockade. These findings further support the view that the LC is involved in stress-induced depression, and are consistent with a proposed mechanism that attributes behavioral disturbance in the model to disinhibition of LC neurons arising from subnormal activation of inhibitory alpha-2 receptors.

Rhythm and blues. Neurochemical, neuropharmacological and neuropsychological implications of a hypothesis of circadian rhythm dysfunction in the affective disorders

Current views on the organisation and functions of the circadian rhythm system are outlined. Evidence is presented supportive of the notion that the pathophysiology of the affective disorders involves a disruption of circadian rhythms and that the primary locus of action of agents effective in the affective disorders is on the circadian rhythm system. Potential disruptions of this system are enumerated. Such a hypothesis, it is argued, might potentially unite the disparate neurochemical and neuroendocrinological findings emerging in both depression and mania. There are in addition neuropsychological and nosological implications of such a framework, which may help bridge the divide between molecular and behavioural approaches to research on the affective disorders which are outlined.