Membrane properties of dissociated trigeminal mesencephalic neurons of the adult rat.

Electrophysiological properties of pseudounipolar trigeminal mesencephalic (Me5) neurons, dissociated from the rat brain, were studied under current-clamp conditions using the whole-cell configuration. Almost all Me5 neurons (37/38, 97%) exhibited a rapid adaptation in response to long depolarizing current pulses. Another firing type, slowly-adapting, was observed in only 3% of neurons (1/38). Most Me5 neurons (42/43) generated an overshooting action potential without a hump on the falling phase, and the remaining neuron (1/43) showed an action potential with a small hump. The action potential of Me5 neurons was reversibly blocked by 1 microM tetrodotoxin (TTX) or by removing Na+ from the bathing medium. When the outward K+ current was suppressed, two types of Ca2+ spikes were revealed. According to characteristic thresholds and sensitivity to inorganic (Ni2+, Cd2+) and organic (nifedipine, omega-conotoxin GVIA) Ca2+ channel blockers, these Ca2+ spikes were identified as T-type LTS (low-threshold spike) and L-type HTS (high-threshold spike). Also, a time-dependent inward rectification was observed in all Me5 neurons. It is concluded that the majority of Me5 neurons are of the rapidly-adapting type and generate a TTX-sensitive Na+ spike with negligible contribution of Ca2+, showing that the electrophysiological properties of Me5 neurons are more similar to those of CNS neurons than to those of PNS ganglion cells which have similar morphological features to Me5 neurons.