Single unit responses from the isolated posterior semicircular canal of the bullfrog (Rana catesbeiana) in the study of vestibular pharmacology (neurotransmitter, cholinergic, gaba ergic)
Description
A preparation of the isolated posterior semicircular canal of the bullfrog was developed to study the pharmacology of the peripheral vestibular system. The preparation was viable for over 12 hours. A piezoelectric microdisplacement device was developed for hydraulic stimulation of the cannulated semicircular canal. Drugs were perfused in the solution bathing the preparation. The lag time of the perfusion system was less than 45 sec. Stable single unit recordings were maintained for more than 1 hour The evoked response of single units in the posterior ampullar nerve was used to monitor the effect of drugs perfused onto the preparation. The hydraulic stimulus used to evoke activity was a bipolar triangle wave of fluid displacement. The average of 20 evoked responses (ER-20) was used to characterize the response of the units before and after drug perfusions. The evoked response was a more reliable and sensitive indicator of pharmacological activity than spontaneous activity. A computer system was developed for on-line analysis of the average evoked response with peristimulus time histograms and interspike interval histograms Gamma aminobutyric acid (GABA) reduced the ER-20 at a concentration of 10 mM and had no activity at lower concentrations. Picrotoxin had no effect up to a concentration of 100 uM. These results indicate that GABA cannot be the afferent transmitter in the semicircular canal of the bullfrog. Carbachol had an unexpected excitatory effect. The excitatory effect declined during prolonged perfusions, suggesting receptor desensitization. A dose-response curve indicated the ED(,50) for the excitatory effect of carbachol was between 5 and 10 uM. A low concentration of the nicotinic antagonist mecamylamine (5 uM) did not block the excitatory effect of carbachol (50 uM). In concentrations greater than 5 uM, mecamylamine alone reduced the evoked response. Since there is no tonic release of acetylcholine from efferent nerves in the isolated canal, this must be a direct action of mecamylamine A working model of the peripheral vestibular system was developed to reconcile the conflicting pharmacological data in vestibular organs. The identity of the afferent transmitter remains enigmatic. The excitatory effect of carbachol may be related to the excitatory effect of efferent vestibular stimulation