Differential expression of pre- and postsynaptic GABA(B) receptors in rat substantia nigra pars reticulata neurones

Abstract

Whole-cell recordings were made from substantia nigra pars reticulata in rat midbrain slices to study the functional expression of pre- and postsynaptic GABA(B) receptors in GABA output neurones. Baclofen (up to 300 μM) dose-dependently activated a weak current which was insensitive to tetrodotoxin and Ca -free solution but blocked by Ca and 2-OH-saclofen. The maximum current activated by baclofen (30 μM) was 43.0 ± 4.5 pA (n = 27), representing only 23% of that in dopamine neurones. Baclofen (1-30 μM) also reduced the frequency of the GABA(A) receptor-mediated miniature inhibitory postsynaptic currents while the distribution of their amplitudes was unaffected. This presynaptic effect of baclofen, prominent at a concentration as low as 1 μM, was sensitive to 2-OH-saclofen and occluded by Cd , but was unaffected by Ba . The results suggest a predominant role of the presynaptic GABA(B) receptors in substantia nigra pars reticulate. The relative abundance of pre- and postsynaptic GABA(B) receptor subtypes in this brain region may also be important in mediating the anticonvulsant effect of baclofen in rats. 2+ 2+ 2+ 2+