Metallothionein (MT) protein and mRNA levels were monitored following exposure of rat neonatal primary astrocyte cultures to methylmercury (MeHg). MT-I and MT-II mRNAs were probed on northern blots with an [alpha-32P]dCTP-labeled synthetic cDNA probe specific for rat MT mRNA. MT-I and MT-II mRNAs were detected in untreated cells, suggesting constitutive MT expression in these cells. The probes hybridize to a single mRNA with a size appropriate for MT, approximately 550 and 350 bp for MT-I and MT-II, respectively. Expression of MT-I and MT-II mRNA in astrocyte monolayers exposed to 2 x 10(-6) M MeHg for 6 h was increased over MT-I and MT-II mRNA levels in controls. Western blot analysis revealed a time-dependent increase in MT protein synthesis through 96 h of exposure to MeHg. Consistent with the constitutive expression of MTs at both the mRNA level and the protein level, we have also demonstrated a time-dependent increase in MT immunoreactivity in astrocytes exposed to MeHg. The cytotoxic effects of MeHg were measured by the rate of astrocytic D-[3H]aspartate uptake. Preexposure of astrocytes to CdCl2, a potent inducer of MTs, completely reversed the inhibitory effect of MeHg on D-[3H]aspartate uptake that occurs in MeHg-treated astrocytes with constitutive MT levels. Associated with CdCl2 treatment was a time-dependent increase in astrocytic MT levels. In summary, astrocytes constitutively express MTs; treatment with MeHg increases astrocytic MT expression, and increased MT levels (by means of CdCl2 pretreatment) attenuate MeHg-induced toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)