Effects of strychnine, bicuculline, and picrotoxin on inhibition of hypoglossal motoneurons

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Abstract
Postsynaptic potentials, evoked by lingual or hypoglossal nerve stimulation, were recorded from hypoglossal motoneurons of the cat with glass microelectrodes. Lingual nerve‐evoked inhibitory postsynaptic potentials (LIPSPs) were recorded in 98% of hypoglossal motoneurons. Hypoglossal nerve stimulation caused a hyperpolarzing potential following the antidromic spike in all hypoglossal motoneurons tested. This potential was unaffected by depolarizing or hyperpolarizing currents, could not be evoked at a stimulus strength less than that which was threshold for the antidromic action potential, and did not change in shape or amplitude at stimulus strengths which were above threshold for antidromic invasion. This hyperpolarpolarizing potential was therefore considered to be an afterhyperpolarization. However, hypoglossal nerve‐induced inhibitory postsynaptic potentials were recorded from hypoglossal units which had characteristics of interneurons, thus suggesting the presence of afferent fibers in the hypoglossal nerve.The hypoglossal nerve‐induced afterhyperpolarization was not affected by strychnine, bicuculline, or picrotoxin. The LIPSP was antagonized by strychnine but unaffected by bicuculline or picrotoxin. The results suggest that inhibition of hypoglossal motoneurons via the lingual nerve is more likely to be mediated by glycine than gamma‐aminobutyric acid (GABA) and is therefore similar to the strychnine‐sensitive postsynaptic inhibition of spinal motoneurons.