Calretinin (CR) is a calcium binding protein that is found in subpopulations of brain neurons such as the substantia nigra (SN) and the ventral tegmental area (VTA). We have previously shown that approximately 50% of the SN/VTA dopamine-containing cells colocalize with CR (Isaacs et al., 1994). We also studied the effects of excitatory amino acids (EAA) on the expression of CR and tyrosine hydroxylase (TH) in SN cultures. CR and TH neurons were differentially affected by a 24 h incubation with kainic acid (KA), n-methyl-D-aspartate (NMDA) or (plus minus)-alpha-amino-3-hydroxy-5-methylisozazde-4-propionic acid (AMPA) (100-500 uM). Both cell types expressed glutamate receptors, yet only the TH-ir cell population was significantly reduced after exposure to KA or NMDA. In addition, all TH-ir and CR-ir neurons expressed increased cAMP/Ca2+ response element-binding protein (PCREB)-ir at 10 and 30 min after KA exposure, confirming that both cell populations had functional glutamate receptors. Whereas treated CR cell processes extended for very long distances, TH cells had increased numbers of spoke-like processes emerging directly from cell bodies, many of which had swollen varicosities. It appears that the CR-containing cells are less vulnerable to the EAAs than the TH-containing cells, and may therefore serve a neuroprotective function. We wished to investigate whether the apparent resistance of CR-ir neurons to EAAs was a phenomenon generalizable to all embryonic cell populations containing CR or was region-specific. To this end, we dissected the cortex from E14 rat embryos and exposed the cultures to EAAs for varying lengths of time. Both CR-ir and gama amino butyric acid (GABA)-ir cells contain glutamate receptors, and both CR-ir and GABA cortical neurons are equally susceptible to EAA exposure. Hence, only the mesencephalic CR-ir neurons appear resistant to excitotoxicity induced by KA. Similar studies on olfactory bulb and thalamus primary cultures are underway.
