Mesencephalic trigeminal neuron responses to γ-aminobutyric acid

Mesencephalic trigeminal neurons are primary sensory neurons which have cell somata located within the brain stem. In spite of the presence of synaptic terminals on and around the cell somata, applications of a variety of neurotransmitter substances in earlier studies have failed to demonstrate responses. Using intracellular recording in a brain slice preparation, we have observed prominent depolarizations and decreases in input resistance in response to applications of γ-aminobutyric acid (GABA) in most recorded mesencephalic trigeminal neurons. Those cells failing to respond were located deeply within the slice, and the low responsiveness was shown to be related to uptake of GABA in the slice. The responses were direct, since they remained during perfusion with a low calcium, high magnesium solution that blocks synaptic transmission. The responses were mimicked by the GABA_A receptor agonist isoguvacine, and blocked by GABA_A receptor antagonists. The GABA_B receptor agonist baclofen evoked no changes in membrane potential or input resistance in neurons exhibiting depolarizations with GABA application. Tests of neuronal excitability during GABA applications indicated that the excitatory effects of the depolarization prevail over the depressant effects of the increase in membrane conductance. In situ hybridization histochemistry indicated that the GABA_A receptors in Me5 cells are comprised of α₂, β₂ and γ₂ subunits.© 1997 Elsevier Science B.V. All rights reserved.