Increased Motor-Impairing Effects of the Neuroactive Steroid Pregnanolone in Mice with Targeted Inactivation of the GABA(A) Receptor gamma 2 Subunit in the Cerebellum

Endogenous neurosteroids and neuroactive steroids have potent and widespread actions on the brain via inhibitory GABA(A) receptors. In recombinant receptors and genetic mouse models their actions depend on the alpha, beta, and delta subunits of the receptor, especially on those that form extrasynaptic GABA(A) receptors responsible for non-synaptic (tonic) inhibition, but they also act on synaptically enriched gamma 2 subunit containing receptors and even on alpha beta binary receptors. Here we tested whether behavioral sensitivity to the neuroactive steroid agonist 5 beta-pregnan-3 alpha-ol-20-one is altered in genetically engineered mouse models that have deficient GABA(A) receptor mediated synaptic inhibition in selected neuronal populations. Mouse lines with the GABA(A) receptor gamma 2 subunit gene selectively deleted either in parvalbumin-containing cells (including cerebellar Purkinje cells), cerebellar granule cells, or just in cerebellar Purkinje cells were trained on the accelerated rotating rod and then tested for motor impairment after cumulative intraperitoneal dosing of 5 beta-pregnan-3 alpha-ol-20-one. Motor impairing effects of 5 beta-pregnan-3 alpha-ol-20-one were strongly increased in all three mouse models in which gamma 2 subunit-dependent synaptic GABA(A) responses in cerebellar neurons were genetically abolished. Furthermore, rescue of postsynaptic GABA(A) receptors in Purkinje cells normalized the effect of the steroid. Anxiolytic/explorative effects of the steroid in elevated plus maze and light:dark exploration tests in mice with Purkinje cell gamma 2 subunit inactivation were similar to those in control mice. The results suggest that, when the deletion of gamma 2 subunit has removed synaptic GABA(A) receptors from the specific cerebellar neuronal populations, the effects of neuroactive steroids solely on extrasynaptic alpha beta or alpha beta delta receptors lead to enhanced changes in the cerebellum-generated behavior.Peer reviewe