GABA- and glycine-mediated inhibitory postsynaptic potentials in neonatal rat rostral ventrolateral medulla neurons in vitro

Abstract

Whole-cell patch recordings were made from rostral ventrolateral medulla neurons of two in vitro preparations: (i) brainstem–spinal cords of two- to five-day-old rats, and (ii) coronal brainstem slices of eight- to 12-day-old rats, and the inhibitory synaptic activities in these neurons have been studied. In brainstem–spinal cord preparations, Lucifer Yellow was diffused into the recording neurons at the end of experiments. Medullary neurons were characterized as: (i) spinally projecting by the appearance of an antidromic spike following electrical stimulation of the spinal tract between T2 and T3 segments, and (ii) adrenergic by the detection of phenylethanolamine- N-methyltransferase immunoreactivity in Lucifer Yellow-filled neurons. Of the 13 spinally projecting and phenylethanolamine- N-methyltransferase-positive medullary neurons, focal stimulation elicited in the presence of glutamate receptor antagonists an inhibitory postsynaptic potential in nine neurons. Inhibitory synaptic potentials were reversibly eliminated by the GABA A receptor antagonist bicuculline (10–20 μM) in six of nine neurons, by the glycine receptor antagonist strychnine (0.1–1 μM) in two and by a combination of bicuculline and strychnine in one neuron. In brainstem slice preparations, focal stimulation elicited three types of synaptic potential: (i) an excitatory postsynaptic potential, (ii) an inhibitory postsynaptic potential and (iii) a biphasic synaptic potential consisting of an excitatory synaptic potential followed by an inhibitory synaptic potential. Inhibitory synaptic potentials had a reversal potential between −70 and −80 mV, reversed their polarity in a low (6.7 mM) Cl −–Krebs' solution, and suppressed or blocked by either bicuculline or strychnine or both. Elimination of inhibitory synaptic potentials unmasked in some cells an excitatory synaptic potential or enhanced the excitatory synaptic potential component in medullary neurons with a biphasic response, indicating a marked convergence of excitatory and inhibitory inputs onto a single neuron. A population of medullary neurons appeared to be pacemaker neurons whereby they discharged spontaneously. When discharges were suppressed by membrane hyperpolarization, focal stimulation elicited inhibitory synaptic potentials in 8/23 neurons tested. Our results suggest that inhibitory synaptic potentials in medullary neurons are mediated by either GABA and/or glycine which open primarily Cl − channels. The prevalence of inhibitory synaptic potentials in medullary neurons indicates an essential role of inhibitory transmission in controlling the input and output ratio of these neurons.