Ce. Rick et Mg. Lacey, RAT SUBSTANTIA-NIGRA PARS RETICULATA NEURONS ARE TONICALLY INHIBITED VIA GABA(A), BUT NOT GABA(B), RECEPTORS IN-VITRO, Brain research, 659(1-2), 1994, pp. 133-137
Extracellular single unit recordings were made from substantia nigra p
ars reticulata (SNr) neurones in slices of rat brain. Cells fired spon
taneous action potentials at 11.4 +/- 0.8 Hz. The GABA, receptor agoni
st isoguvacine (1-10 mu M) reduced firing rate in a concentration-depe
ndent manner [50% of maximal inhibition (IC50) with 3.2 mu M], as did
the GABA(B) agonist baclofen (0.3-10 mu M; IC50 1.4 mu M). The GABA(A)
antagonist bicuculline (30 mu M) not only blocked the action of isogu
vacine, but also increased the basal firing rate to 187.5 +/- 12.6% of
control. The GABA(B) antagonist CGP 55845A (0.1 mu M), while blocking
the inhibitory action of baclofen, was without effect on spontaneous
firing rate, as was strychnine (10 mu M), the antagonist of glycine an
d taurine, and also Met-enkephalin (10 mu M) Tiagabine (50 mu M), the
blocker of GABA uptake, caused an inhibition of firing which could be
reversed with bicuculline (30 mu M) but not CGP 55845A (1 mu M) We con
clude that the firing rate of SNr neurones is under tonic inhibition b
y GABA in vitro, which can be relieved by antagonists of GABA(A), but
not GABA(B) receptors, and enhanced by blockade of GABA reuptake. The
source of this GABA tone is likely to be from recurrent axon collatera
ls of SNr neurones themselves.