Excitation and inhibition of marginal layer and interstitial interneurons in cat nucleus caudalis by mechanical stimuli.

Abstract

In cats anesthetized with urethane, the caudal medulla was stabilized in preparation for intracellular recording from interstitial neurons in the descending tract of the trigeminal nerve and from neurons in lamina I of nucleus caudalis. Glass micropipets (10-50 M ohms) were advanced from the surface to a maximum depth of 350 micrometer. When DC potential shifts occurred, it was found that mechanical stimuli to the face generated bursts of positive-going spikes, followed in some cases by inhibitory postsynaptic potentials (IPSPs). Subdermal electrical stimulation of the face in each receptive field almost always enabled the same neuron to be driven electrically. Recordings were classified as from primary afferent fibers or from interneurons. Primary fibers had a purely positive spike, with a latency varying by no more than 0.05 msec, and could follow stimulation at 500 Hz. The mean latency for the fibers was 1.87 +/- 0.06 msec (n=75), and their absolute refractory period was 0.42 +/- 0.02 msec (n=36). Recordings were classed as from interneurons if there was an IPSP or the latency was at least 4 msec, with a variation of latency of at lest 0.5 msec. Responses thought to be monosynaptically driven had a mean latency of 2.09 +/0 0.07 msec (n=32) and could follow pairs of stimuli at a mean minimum interval of 0.70 +/- 0.06 msec (n=20). Responses thought to be polysynaptically driven had a mean latency of 7.9 +/- 1.08 msec (n=49) and a mean interstimulus interval of 2.96 +/- 0.84 (n=20). Most responses were generated by brushing the face (n=87), some by pressure on the face (n=25), and a few by pinching the skin of the face (n=6). Interneuron responses were most commonly recorded in the first 200 micrometer of the descending tract, and this position was confirmed by the injection of pontamine sky blue and the examination of frozen sections. The recordings were thought to be from dendrites of marginal and interstitial cells or the somas of interstitial cells. The IPSPs which followed spike potentials could only follow stimuli at 10 Hz or less. The failure appeared to be at a primary afferent synapse upon an inhibitory interneuron. Collision tests between mechanically evoked and electrically evoked responses showed long-lasting inhibition of the response to electrical stimulation after collision. Presynaptic inhibition exerted on the primary afferent excitation was suggested as the explanation.