Regulation of gene expression represents a major component in antidepressant drug action. The effect of antidepressant treatments on the function of cAMP-responsive element binding protein (CREB), a transcription factor that regulates expression of several genes involved in neuroplasticity, cell survival, and cognition, has been extensively studied. Although there is general agreement that chronic antidepressants stimulate CREB function, conflicting results suggest that different effects may depend on drug type, drug dosage, and different experimental paradigms. CREB function is activated by a vast array of physiological stimuli, conveyed through a number of signaling pathways acting in concert, but thus far the effects of antidepressants on CREB have been analyzed mostly with regard to the cAMP-protein kinase A pathway. A growing body of data shows that other major pathways, such as the calcium/calmodulin-dependent kinase and the mitogen-activated kinase cascades, are involved in activity-dependent regulation of gene expression and may also be implicated in the mechanism of action of antidepressants. In this article the available evidence is reviewed with an attempt to identify the reasons for experimental discrepancies and possible directions for future research. Particularemphasis is given to the regulation of brain-derived neurotrophic factor (BDNF), a CREB-regulated gene, which has been implicated in both the pathophysiology and pharmacology of mood disorders. The array of different results obtained by various groups is analyzed with an eye on recent advancements in the regulation of BDNF transcription, in an attempt to understand better the mechanisms of drug action and dissect molecular requirements for faster and more efficient antidepressant treatment.

Signaling Pathways Regulating Gene Expression, Neuroplasticity, and Neurotrophic Mechanisms in the Action of Antidepressants: A Critical Overview

There is mounting evidence that a family of guanosine triphosphate binding proteins (G proteins) play an obligatory role in the transduction of a vast array of extracellular, receptor-detected signals across cell membranes to intracellular effectors. The author reviews the literature dealing with G protein coupling to second messenger generation, the role of G proteins in regulating both the convergence and divergence of neurotransmitter action in the CNS, and the involvement of G proteins in a variety of clinical conditions, with an emphasis on psychiatric conditions and their treatments. G proteins from the basis of signal integration in the CNS, endowing the neuron with a large degree of functional diversity. Abnormalities in the function and/or expression of G proteins have been implicated in a variety of pathophysiologic states, and a number of currently available psychotropic drugs affect G proteins. The understanding of the mechanisms by which G proteins modulate neuronal activity may be one of the keys to understanding the functioning and the complexities of the nervous system. Given their widespread, critical roles in the regulation of neuronal function, it seems likely that G proteins are involved in the pathophysiology of various major psychiatric illnesses. The development of novel, site-specific drugs with primary G protein targets remains an exciting prospect for the future.

G proteins: implications for psychiatry.

Experimental animal and peripheral blood cell studies point to guanine nucleotide regulatory (G) protein disturbances in bipolar affective disorder. We have previously reported elevated prefrontal cortex Gs alpha protein in bipolar affective disorder and have now extended these preliminary observations in a larger number of subjects, assessing the brain regional specificity of these changes in greater detail, determining the functional biochemical correlates of such changes, and evaluating their diagnostic specificity. Membrane G protein (Gs alpha, Gi alpha, Go alpha, and G beta) immunoreactivities were estimated by western blotting in postmortem brain regions obtained from 10 patients with a DSMIII-R diagnosis of bipolar affective disorder and 10 nonpsychiatric controls matched on the basis of age, postmortem delay, and brain pH. To examine whether there were functional correlates to the observed elevated Gs alpha levels, basal and GTP gamma S- and forskolin-stimulated cyclic AMP production was determined in the same brain regions. Compared with controls, Gs alpha (52-kDa species) immunoreactivity was significantly (p < 0.05) elevated in prefrontal (+36%), temporal (+65%), and occipital (+96%) cortex but not in hippocampus (+28%), thalamus (-23%), or cerebellum (+21%). In contrast, no significant differences were found in the other G protein subunits (Gi alpha, Go alpha, G beta) measured in these regions. Forskolin-stimulated cyclic AMP production was significantly increased in temporal (+31%) and occipital (+96%) cortex but not in other regions. No significant differences were apparent in basal or GTP gamma S-stimulated cyclic AMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral cortex Gs alpha protein levels and forskolin-stimulated cyclic AMP formation are increased in bipolar affective disorder.

Until recently, research on the neurochemical basis of affective disorders (AD) and schizophrenia (SCZ) focused on detecting postulated disturbances in presynaptic neurotransmitter release and metabolism, or postsynaptic receptor function. New insights into the molecular mechanisms involved in the propagation of neurotransmitter signals across biological membranes and in the regulation of neuronal responses have allowed the development of novel hypotheses, which may explain the altered postsynaptic neuroreceptor responsivity thought to be integral to the pathophysiology of these disorders. In this review we evaluate evidence from both basic science and clinical research implicating disturbances in postreceptor signal transduction in the pathophysiology and pharmacotherapy of AD and SCZ. Specific findings regarding potential postreceptor sites of pathophysiology are highlighted in each of these disorders, together with the growing body of data on the possible postreceptor loci of psychotropic drug action, especially lithium and antidepressants.

CNS signal transduction in the pathophysiology and pharmacotherapy of affective disorders and schizophrenia

Adenylyl cyclase activity and G-protein subunit levels in postmortem frontal cortex of suicide victims.

In an attempt to resolve a unified postreceptor mechanism of action for antidepressant therapy, rats were treated with amitriptyline, desipramine or iprindole. Chronic treatment with these antidepressant drugs increased guanylylimidodiphosphate-[Gpp(NH)p-], NaF-, or forskolin-activated adenylate cyclase in synaptic membranes prepared from cerebral cortexes of treated rats. Gpp(NH)p-dependent inhibition of adenylate cyclase was unaffected. Maximal binding of the photoaffinity GTP analog azidoanilido-GTP (AAGTP) to the adenylate cyclase stimulatory (Gs alpha) and inhibitory (Gi alpha) G proteins was unaffected by antidepressant treatment. The chemical elimination of Gs (low pH treatment) eliminated all differences between control and antidepressant-treated groups. Further, nonneural tissues from rats receiving chronic antidepressants showed no changes in adenylate cyclase activity or AAGTP binding. The results of these studies suggest that chronic antidepressant administration promoted increased coupling between Gs and catalytic unit of adenylate cyclase. Thus, the molecular locus of antidepressant action may reside at the stimulatory GTP-binding protein, Gs.

Coupling of the stimulatory GTP-binding protein Gs to rat synaptic membrane adenylate cyclase is enhanced subsequent to chronic antidepressant treatment

H Ozawa

Papers

Coupling of the stimulatory GTP-binding protein Gs to rat synaptic membrane adenylate cyclase is enhanced subsequent to chronic antidepressant treatment