Agonist-stimulation of cerebral phosphoinositide turnover following long-term treatment with antidepressants

CNS Target Identification and Validation: Avoiding the Valley of Death or Naive Optimism?

There are many challenges along the path to the approval of new drugs to treat CNS disorders, one of the greatest areas of unmet medical need with a large societal burden and health-care impact. Unfortunately, over the past two decades, few CNS drug approvals have succeeded, leading many pharmaceutical companies to deprioritize this therapeutic area. The reasons for the failures in CNS drug discovery are likely to be multifactorial. However, selecting the most biologically plausible molecular targets that are relevant to the disorder is a critical first step to improve the probability of success. In this review, we outline previous methods for identifying and validating novel targets for CNS drug discovery, and, cognizant of previous failures, we discuss potential new strategies that may improve the probability of success of developing novel treatments for CNS disorders.

Diverse antidepressants increase CDP-diacylglycerol production and phosphatidylinositide resynthesis in depression-relevant regions of the rat brain

Background

Major depression is a serious mood disorder affecting millions of adults and children worldwide. While the etiopathology of depression remains obscure, antidepressant medications increase synaptic levels of monoamine neurotransmitters in brain regions associated with the disease. Monoamine transmitters activate multiple signaling cascades some of which have been investigated as potential mediators of depression or antidepressant drug action. However, the diacylglycerol arm of phosphoinositide signaling cascades has not been systematically investigated, even though downstream targets of this cascade have been implicated in depression. With the ultimate goal of uncovering the primary postsynaptic actions that may initiate cellular antidepressive signaling, we have examined the antidepressant-induced production of CDP-diacylglycerol which is both a product of diacylglycerol phosphorylation and a precursor for the synthesis of physiologically critical glycerophospholipids such as the phosphatidylinositides. For this, drug effects on [³H]cytidine-labeled CDP-diacylglycerol and [³H]inositol-labeled phosphatidylinositides were measured in response to the tricyclics desipramine and imipramine, the selective serotonin reuptake inhibitors fluoxetine and paroxetine, the atypical antidepressants maprotiline and nomifensine, and several monoamine oxidase inhibitors.

Results

Multiple compounds from each antidepressant category significantly stimulated [³H]CDP-diacylglycerol accumulation in cerebrocortical, hippocampal, and striatal tissues, and also enhanced the resynthesis of inositol phospholipids. Conversely, various antipsychotics, anxiolytics, and non-antidepressant psychotropic agents failed to significantly induce CDP-diacylglycerol or phosphoinositide synthesis. Drug-induced CDP-diacylglycerol accumulation was independent of lithium and only partially dependent on phosphoinositide hydrolysis, thus indicating that antidepressants can mobilize CDP-diacylglycerol from additional pools lying outside of the inositol cycle. Further, unlike direct serotonergic, muscarinic, or α-adrenergic agonists that elicited comparable or lower effects on CDP-diacylglycerol versus inositol phosphates, the antidepressants dose-dependently induced significantly greater accumulations of CDP-diacylglycerol.

Conclusion

Chemically divergent antidepressant agents commonly and significantly enhanced the accumulation of CDP-diacylglycerol. The latter is not only a derived product of phosphoinositide hydrolysis but is also a crucial intermediate in the biosynthesis of several signaling substrates. Hence, altered CDP-diacylglycerol signaling might be implicated in the pathophysiology of depression or the mechanism of action of diverse antidepressant medications.

Electroconvulsive shock increases alpha 1b- but not alpha 1a-adrenoceptor binding sites in rat cerebral cortex

Repeated administration of electroconvulsive shock (ECS) increases [3H]prazosin binding to alpha 1-adrenoceptors in rat cerebral cortex. In contrast, [3H]WB4101 binding in cortex has been reported to be unchanged after ECS. [3H]Prazosin labels two alpha 1-adrenoceptor subtypes, termed alpha 1a and alpha 1b, whereas [3H]WB4101 labels the alpha 1a subtype preferentially. The purpose of this study was to determine whether ECS increases one or both alpha 1-adrenoceptor subtypes in rat cerebral cortex. We found that treatment of rats with ECS once daily for 10-12 days increased [3H]prazosin binding in cortex by about 25% but did not significantly alter [3H]WB4101 binding to alpha 1-adrenoceptors. Measurement of alpha 1a and alpha 1b receptors by competition analysis of the selective alpha 1a antagonist 5-methylurapidil against [3H]prazosin and measurement of [3H]prazosin binding in homogenates preincubated with chlorethylclonidine, which alkylates alpha 1b binding sites, also indicated that the ECS-induced increase in alpha 1-adrenoceptors is confined to the alpha 1b subtype. In contrast to its effect on [3H]prazosin binding, ECS did not increase phosphoinositide hydrolysis as measured by [3H]inositol 1-phosphate accumulation in slices of rat cerebral cortex stimulated by either norepinephrine or phenylephrine. The failure of ECS to increase [3H]inositol 1-phosphate accumulation stimulated by phenylephrine, which is a partial agonist for this response, suggests that spare receptors do not account for the apparent absence of effect of ECS on alpha 1-adrenoceptor-mediated phosphoinositide hydrolysis.

Antidepressants and brain neurochemistry

Most antidepressant drugs prescribed today have been available for decades. Nonetheless, their mechanism of action in treating depression has remained elusive. On the basis of neurochemical studies in laboratory animals, hypotheses explaining their therapeutic effects have been formulated. The most attractive of these theories involves antidepressant-induced changes in the sensitivity of certain catecholamine and serotonergic receptors in the brain. Support for this hypothesis from clinical studies has been difficult to obtain. Pharmacologic studies of antidepressant drugs, however, indicate the involvement of blockade of neuronal uptake systems for norepinephrine and serotonin and blockade of many receptors for neurotransmitters. These properties of antidepressants can explain some of their adverse effects and certain interactions with other drugs.

Further evidence for an anomalous regulation of cortical 5-HT2 receptors: studies with 5-HT precursors

Rats pretreated with either phenelzine (50 mg/kg 18 h and 12 mg/kg 90 min previously) or carbidopa (25 mg/kg 30 min previously) then given tryptophan or 5-hydroxytryptophan (5-HTP), developed wet-dog shakes which were more intense after 5-HTP. Cortical 5-HT2 receptors were significantly reduced within 3 h of tryptophan (150 or 500 mg/kg) given to phenelzine-treated rats. 5-HTP either did not affect (150 mg/kg) or slightly increased (350 mg/kg) 5-HT2 receptor numbers in rats pretreated with carbidopa. The response to 5-HTP (350 mg/kg) was not altered by pretreatment with alpha-methyl-p-tyrosine, haloperidol, propranolol or yohimbine.

Lithium and serotonin function: implications for the serotonin hypothesis of depression

Lithium enjoys wide clinical use in the treatment of affective disorders, but the mechanism of its action in these conditions is still controversial. Recent studies have shown that lithium can interact with other antidepressant drugs to enhance their efficacy, perhaps by specific effects on serotonin (5-HT) function. A large body of independent evidence suggests that 5-HT function is abnormal in depression. This review documents preclinical evidence of lithium's effects on 5-HT function at the levels of precursor uptake, synthesis, storage, catabolism, release, receptors, and receptor-effector interactions. The weight of this evidence suggests that lithium's primary actions on 5-HT may be presynaptic, with many secondary postsynaptic effects. Studies in humans, using very different methodological approaches, generally suggest that lithium has a net enhancing effect on 5-HT function. These actions of lithium may serve to correct as-yet unspecified abnormalities of 5-HT function involved in the pathogenesis of depression.