GABA(A) receptor activation by
muscimol has sex and age specific effects on substantia nigra reticulata (SNR)-mediated control of
generalized seizures.
GABA(A) receptor agonists depolarize or hyperpolarize neurons depending upon the level of expression of the neuronal specific
potassium chloride contransporter KCC2. We studied KCC2
mRNA expression in the SNR as a function of sex and age and correlated KCC2 expression with the in vivo and in vitro effects of
muscimol. Methods included in situ hybridization,
gramicidin-perforated patch clamp and
fura-2 AM imaging of acute SNR slices. KCC2
mRNA expression increased between postnatal days (PN) 15 and 30 in both sexes, and reached adult levels in males by PN30. Female PN15 and PN30 SNR neurons contained more KCC2
mRNA compared with age-matched males. In male PN14-17 rats, bath application of the
GABA(A) receptor agonist muscimol in acute SNR slices depolarized neurons and increased intracellular
calcium concentration ([Ca(2+)](i)). Furthermore, acute in vivo administration of
muscimol upregulated, whereas blockade of L-type voltage sensitive
calcium channels with
nifedipine downregulated KCC2
mRNA. In contrast, in female PN14-17 rats, bath application of
muscimol hyperpolarized SNR neurons and did not alter [Ca(2+)](i). In vivo
muscimol administration acutely downregulated KCC2
mRNA expression whereas
nifedipine had no effect. The lower expression of KCC2
mRNA in infantile male SNR neurons may explain why
muscimol-induced depolarization and [Ca(2+)](i) increases occur only in males. Consequently,
GABA(A) receptor activation selectively upregulates the expression of
calcium-regulated genes, such as KCC2, in male SNR, promoting the sexual differentiation of the SNR.