[The function of sigma receptors--electrophysiological approach]

The functions of sigma (sigma) receptors were reviewed based on electrophysiological studies. Systemic administration of sigma ligands reportedly produces a variety of effects on dopamine (DA) neurons. In the mesolimbic DA system, (+)SKF-10,047 suppressed activities of the ventral tegmental area. In the substantia nigra, DTG also suppressed these activities, while BMY-14802 increased the activity of neurons. In the cerebellum, however, activities of Purkinje cells were suppressed by locally applied DTG, with probable involvement of the catecholaminergic system. This effect on Purkinje cells may explain the action of sigma ligands on the motor system. In the hippocampus, neuronal activities were inhibited by SR31742A in vivo (CA3 region) and by (+)SKF10,047 in vitro (CA1 region). DTG at high concentration (1 mM) completely suppressed population spikes (PS) in the CA1 region of hippocampal slice preparations. In our experiments, a novel sigma ligand, OPC-24439, suppressed PS in CA1 at concentrations of 1-100 microM. However, NMDA-induced neuronal firings in CA3 in vivo were augmented by low doses of DTG in a haloperidol-sensitive manner, but high doses were ineffective. In contrast, non-NMDA responses were not affected by DTG. In addition, several sigma ligands having no effect on the NMDA response have been reported. In addition, endogenous ligands, neuropeptide Y and dehydroepiandrosterone, augmented the NMDA-induced firing. In whole-cell patch clamp recording, NMDA-induced currents were suppressed by a relatively higher concentration of DTG in a concentration-dependent manner, while non-NMDA responses were only slightly suppressed. These findings suggest that effects of sigma ligands on NMDA receptor responses are biphasic, and sigma ligands may modulate memory and learning and suppress neuronal death by anoxia. In addition, sigma ligands are also reported to suppress Ca2+ channels in hippocampal culture neurons and induce current by closing K+ channels in NCB-20 cells. Thus, sigma receptors may be involved in the modulation of a variety of neurons that relate to psychiatric function and plasticity.