Pharmacological studies on the mechanism of cutaneous inhibition on the masseteric monosynaptic reflex

Abstract

The masseteric monosynaptic reflex is inhibited with a latency of about 8 msec by conditioning stimulation of the superficial radial nerve (so called the cutaneous inhibition). The mechanism of inhibition was pharmacologically investigated using the three indicators: the masseter MSR at masseter nerve, field EPSP simultaneously recorded in trigeminal motoneuron pool evoked after single shock to the mesencephalic tract nucleus of trigeminal nerve and IPSP recorded (extra-or intracellularly) in the same motoneuron pool produced by the stimulation of the superfical radial nerve in the pentobarbital anesthetized or precollicularly decerebrated cat. The following points warrant consideration: 1) The cutaneous inhibition was prolonged about two times longer than control in time course and the amplitude of this MSR and the field EPSP was diminished after administration of physostigmine 0.15 mg/kg. 2) The field IPSP (N1) was increased in amplitude and became 1.4 times of the control, in addition to that, a new negative going potential (N2) appeared with a delay of about 30 msec following administration of physostigmine which was clearly distinguished from the N1 potential. The prolonged time curses of the cutaneous inhibition of MSR and field EPSP was corresponded to the total duration of N1 and N2 potential. 3) 1 mg/kg of atropine counteracted to these physostigmine effects upon not only the cutaneous inhibition but also the diminution of the MSR. 4) Several other drugs related to acetylcholine were tested. Dihydro-β-erythroidine, d-tubocurarine, gallamine triethiodide did not change the cutaneous inhibition and mecamylamine showed a slight influence upon it. Mephenesine blocked the cutaneous inhibition. Strychnine reversed the field IPSP to the positive potentials. 5) It was found that some reticulospinal neurons located in the Nucleus reticularis gigantocellularis with slow conduction velocity and responsive to the cutaneous nerve shock were sensitive to physostigmine and their enhanced activity was antagonized by atropine. These data suggests, that acetylcholine is related with the synaptic transmission pathway inducing the IPSP in the masseter motoneurone from the cutaneous nerve.