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In studies on superfused slices obtained from the rat hippocampus, we extracellularly recorded field EPSP (fEPSPs), pharmacologically isolated components of these fEPSP, which were related to activation of synaptic NMDA-glutamate receptors, and neuronal responses of the dentate gyrus, which were mediated by activation of extrasynaptic NMDA-glutamate receptors. Recordings were performed in junctions formed by fibers of the medial perforant pathway and dendrites of granular cells of the gyrus dentatus. In the course of the development of behavioral depression in rats, which was caused by zoosocial isolation or chronic injection of dexamethasone, the amplitude of fEPSPs decreased, on average, by 19.8 and 26.0%, respectively, while the NMDA components of fEPSP increased by 28.6 and 33.4%; the rise in the amplitude of neuronal responses recorded in the gyrus dentatus, which were mediated by the activation of extrasynaptic NMDA receptors (by 75.4 and 92.3%), was clearly expressed. In intact rats, chronic (over 14 days) injections of tricyclic antidepressants, imipramine and amitriptyline, as well as of an atypical antidepressant, maprotiline, resulted in an increase in the fEPSP amplitude by 22.1 to 25.9%. At the same time, the amplitude of NMDA components of fEPSP dropped by 27.0 to 29.1%, while the amplitude of responses mediated by the activation of extrasynaptic NMDA-glutamate receptors decreased by 46.1 to 49.8%. Changes in the neuronal responses in the gyrus dentatus, which were mediated by the activation of NMDA-glutamate receptors, are related to an increase or decrease in the number of such receptors. It is supposed that an increase in the total number of NMDA-glutamate receptors under conditions of behavioral depression results in a deterioration of the energy supply of cerebral neurons, while an increase in the extrasynaptic pool of NMDA-glutamate receptors leads to suppression of biosynthesis of neurotrophins and to injury of neurons. As a general result, informational processes in the brain are subjected to significant negative influences. Chronic injections of antidepressants promote a permanent rise in the concentration of noradrenaline and serotonin in extracellular environments and the activation of monoamine receptors positively conjugated with adenylate cyclase; suppression of expression of informational RNAs, which encode subunits of NMDA-glutamate receptors, and a decrease in the number of these receptors. These events improve the functional state of neurons.
In studies on superfused slices obtained from the rat hippocampus, we extracellularly recorded field EPSP (fEPSPs), pharmacologically isolated components of these fEPSP, which were related to activation of synaptic NMDA-glutamate receptors, and neuronal responses of the dentate gyrus, which were mediated by activation of extrasynaptic NMDA-glutamate receptors. Recordings were performed in junctions formed by fibers of the medial perforant pathway and dendrites of granular cells of the gyrus dentatus. In the course of the development of behavioral depression in rats, which was caused by zoosocial isolation or chronic injection of dexamethasone, the amplitude of fEPSPs decreased, on average, by 19.8 and 26.0%, respectively, while the NMDA components of fEPSP increased by 28.6 and 33.4%; the rise in the amplitude of neuronal responses recorded in the gyrus dentatus, which were mediated by the activation of extrasynaptic NMDA receptors (by 75.4 and 92.3%), was clearly expressed. In intact rats, chronic (over 14 days) injections of tricyclic antidepressants, imipramine and amitriptyline, as well as of an atypical antidepressant, maprotiline, resulted in an increase in the fEPSP amplitude by 22.1 to 25.9%. At the same time, the amplitude of NMDA components of fEPSP dropped by 27.0 to 29.1%, while the amplitude of responses mediated by the activation of extrasynaptic NMDA-glutamate receptors decreased by 46.1 to 49.8%. Changes in the neuronal responses in the gyrus dentatus, which were mediated by the activation of NMDA-glutamate receptors, are related to an increase or decrease in the number of such receptors. It is supposed that an increase in the total number of NMDA-glutamate receptors under conditions of behavioral depression results in a deterioration of the energy supply of cerebral neurons, while an increase in the extrasynaptic pool of NMDA-glutamate receptors leads to suppression of biosynthesis of neurotrophins and to injury of neurons. As a general result, informational processes in the brain are subjected to significant negative influences. Chronic injections of antidepressants promote a permanent rise in the concentration of noradrenaline and serotonin in extracellular environments and the activation of monoamine receptors positively conjugated with adenylate cyclase; suppression of expression of informational RNAs, which encode subunits of NMDA-glutamate receptors, and a decrease in the number of these receptors. These events improve the functional state of neurons.
To date, the mechanisms of action of antidepressants at the molecular and subcellular levels have been relatively well understood. Traditional antidepressants suppress the activity of either monoamine translocases or monoamine oxidase A. Under conditions, of chronic introduction, antidepressants provide a gradual increase in the level of monoamines in the extracellular space of the forebrain structures, which is partly due to the desensitization of monoamine receptors on somatodendrite membranes and membranes of axon terminals. Intensification of the effects of monoamines on the forebrain structures resulting from chronic introduction of antidepressants is accompanied by facilitation in the signal pathway of a secondary messenger (cAMP), activation of the gene apparatus in the neurons, and the enhancement of expression of neurotrophins; these effects lead to a trend toward normalization of the functional state of the neurons impaired in depression. Concepts on the mechanisms of the antidepressive activity of antagonists of NMDA receptors and blockers of receptors of modulatory neuropeptides are discussed. In this review, we also discuss the data on disorders in recycling of inositol; the antimaniacal effect of mood stabilizers can be based on correction of this process. A description of the regulation of activity of cerebral monoaminergic neurons and release of monoamines in their projection is also attached.
Rhythmic stimulation of nerve-muscle preparation of frog sternal muscle bathed in low-Ca(2+) saline increased the release of neurotransmitter (facilitation) and modified the shape of extracellular response of nerve terminal (decreased phase III amplitude). Iberiotoxin and 4-aminopyridine modified the dynamics these processes. We conclude that inactivation of potential-dependent K(+)-channels and activation of calcium-dependent K(+)-channels in frog motor nerve terminals during rhythmic activity modulate Ca(2+) influx into nerve terminals and contribute into facilitation of neurotransmitter secretion. The degree of these mechanisms depends on the rate of synaptic rhythmic activity.
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