PHARMACOLOGICAL PROPERTIES OF CAT'S COLLICULAR AUDITORY NEURONS

Abstract

Effects of various drugs on the collicular auditory neurons were examined by means of pressure microinjection and microelectrophoresis methods.
1. Acetylcholine had stimulating action on 67% of the nuerons tested. Most of them showed a relatively fast onset and a slow recovery.
2. Monosodium-L-glutamate excited 85% of the neurons tested. The action was exerted with a relatively fast time course and by smaller doses than those required for acetylcholine.
3. L-aspartic acid also had an excitatory action. It was clearly much weaker than L-glutamate.
4. Eserine sulphate stimulated 60% of the neurons tested.
5.γ-Aminobutyric acid exerted a potent inhibitory action on both spontaneous discharges and tone-induced responses. The recovery required a much longer time than that needed for the onset of the effect.
6. Glycine caused a powerful depression comparable to that of GABA. The neurons tested were of a fast time course of action.
7. Atropine sulphate, dihydro-β-erythroidine hydrobromide and d-tubocurarine blocked tone-induced excitation. Atropine had the strongest action among these cholinergic antagonists.
8. Disinhibitory effect by picrotoxin was observed with 43% of the “on” type neurons examined, that is, “on” responses were changed to sustained responses. The action of picrotoxin was time-limited in duration to 40-80 msec and mainly affected the discharges during the beginning of the stimulation period.
9. Strychnine sulphate was found to have no effect upon “on” type neurons.
10. 5-Hydroxytryptamine was also found to have no effect.
11. Irreversible blocking effect was observed with tetrodotoxin.
12. It is suggested that the receptor sites of the collicular auditory neurons are cholinergic in nature for mediating the excitatory process. GABA may be a principal candidate as an inhibitory transmitter which may produces “on” type responses in the inferior colliculus.