DIFFERENTIAL-EFFECTS OF EXCITATORY AMINO-ACIDS ON MESENCEPHALIC NEURONS EXPRESSING EITHER CALRETININ OR TYROSINE-HYDROXYLASE IN PRIMARY

In mesencephalic primary cultures derived from E14 rat embryos, calret inin- and tyrosine hydroxylase-immunoreactive neurons comprised 2% and 5% of the total cell population, respectively, at 6-7 days in vitro. The number of calretinin-immunoreactive neurons was unchanged after a 12- or 24-h exposure to 500 mu M kainic acid (KA), but a 50% cell loss was detected after a 48-h exposure to KA. Tyrosine hydroxylase-immuno re active neurons demonstrated a 50% and 67% cell loss at 24- and 48-h exposures to 500 mu M KA. A 500 mu M N-methyl-D-aspartic acid (NMDA) incubation for 24 h had no effect on calretinin-immunoreactive cell nu mber, but did significantly reduce tyrosine hydroxylase-immunoreactive cell numbers by 26%. In tyrosine hydroxylase-immunoreactive cells, ex posure to KA appeared to stimulate the retraction of the neuritic tree and to cause somatic swelling. In contrast, calretinin-immunoreactive neurons developed larger and more complex neuritic trees after a 24-h exposure to 500 mu M KA but not NMDA. Immunohistochemical colocalizat ion studies revealed that all tyrosine hydroxylase-immunoreactive and the majority of calretinin-immunoreactive neurons expressed the glutam ate receptor subunits GluR2-R3. Very low levels of NMDAR1 receptor sub units were detected on cells in this culture and GluR4 receptor subuni ts were not detectable. Our experiments showed that glutamate receptor s present in both calretinin- and tyrosine hydroxylase-immunoreactive cells were functional, since phosphorylated cAMP/Ca2+ response element -binding protein levels were increased in both cell types after 10 or 30 min exposures to 500 mu M KA. The present results indicate that in the mesencephalic cultures tyrosine hydroxylase-immunoreactive cells a re more vulnerable to KA excitotoxicity than calretinin-immunoreactive neurons.