Sigma receptors are putative targets for neuroprotection following
ischemia; however, little is known on their mechanism of action. One of the key components in the demise of neurons following ischemic injury is the disruption of intracellular
calcium homeostasis. Fluorometric
calcium imaging was used to examine the effects of
sigma receptor activation on changes in intracellular
calcium concentrations ([Ca(2+)](i)) evoked by in vitro
ischemia in cultured cortical neurons from embryonic rats. The
sigma receptor agonist, 1,3-di-o-tolyl-guanidine (DTG), was shown to depress [Ca(2+)](i) elevations observed in response to
ischemia induced by
sodium azide and
glucose deprivation. Two
sigma receptor antagonists,
metaphit [1-(1-(3-isothiocyanatophenyl)-cyclohexyl)-
piperidine] and
BD-1047 (N-[2-3,4-dichlorophenyl)-ethyl]-N-methyl-2-(dimethylamino)
ethylamine), were shown to blunt the ability of DTG to inhibit
ischemia-evoked increases in [Ca(2+)](i), revealing that the effects are mediated by activation of
sigma receptors and not via the actions of DTG on nonspecific targets such as
N-methyl-d-aspartate receptors. DTG inhibition of
ischemia-induced increases in [Ca(2+)](i) was mimicked by the sigma-1 receptor-selective agonists,
carbetapentane, (+)-
pentazocine and
PRE-084 [2-(4-morpholinethyl) 1-phenylcyclohexanecarboxylate hydrochloride], but not by the sigma-2-selective agonist,
ibogaine, showing that activation of sigma-1 receptors is responsible for the effects. In contrast, DTG,
carbetapentane, and
ibogaine blocked spontaneous, synchronous
calcium transients observed in our preparation at concentrations consistent with
sigma receptor-mediated effects, indicating that both sigma-1 and sigma-2 receptors regulate events that affect [Ca(2+)](i) in cortical neurons. Our studies show that activation of
sigma receptors can ameliorate [Ca(2+)](i) dysregulation associated with
ischemia in cortical neurons and, thus, identify one of the mechanisms by which these receptors may exert their neuroprotective properties.