Effects of low and high doses of selective sigma ligands: further evidence suggesting the existence of different subtypes of sigma receptors

Several high affinity sigma (sigma) ligands, such as DTG, JO-1784, (+)-pentazocine, BD-737 and L-687,384, administered at low doses act as agonists by potentiating N-methyl-D-aspartate (NMDA)-induced activation of pyramidal neurons in the CA3 region of the rat dorsal hippocampus. This potentiation is dose-dependent at doses between 1 and 1000 micrograms/kg, IV but bell-shaped dose-response curves are obtained. Other sigma ligands like haloperidol, BMY-14802, (+)3-PPP and NE-100 administered at low doses act as sigma antagonists, since they do not modify the NMDA response but suppress the potentiation of the NMDA response induced by sigma agonists. Because high doses of the sigma agonists do not potentiate the NMDA response, the present experiments were undertaken to assess if, at high doses, these sigma ligands could also act as sigma antagonists and suppress the potentiation induced by low doses of sigma agonists. High doses of DTG, JO-1784, BD-737, and L-687,384, administered acutely, had an effect similar to that of low doses of haloperidol, by suppressing and preventing the potentiation induced by low doses of DTG, JO-1784, BD-737, L-687,384 and (+)-pentazocine. High doses of (+)-pentazocine suppressed the effect of a low dose of (+)-pentazocine but did not affect the potentiation induced by a low dose of the other sigma agonists. The potentiation induced by a low dose of a sigma 1 agonist was not further increased by the subsequent administration of another low dose of a sigma 1 agonist. All together, these results strongly suggest that more than two subtypes of sigma receptors exist in the CNS.