Electrophysiological examination of a non-olfactory, non-gustatory chemosense in the searobin,Prionotus carolinus

Summary

• 1. Electrophysiological responses to chemical stimuli were recorded from the spinal nerves innervating the free fin rays of the searobin,Prionotus carolinus. These modified pectoral fin rays are capable of detecting chemical stimuli although they contain neither taste buds nor olfactory receptors.

• 2. Squid extract elicited a vigorous response and of the 24 compounds which have been found in squid extract (Mackie 1982) 15 were tested. Ten of these compounds elicited increases in nerve activity and 5 were nonstimulatory (Table 1). Betaine (trimethylglycine) was the most stimulatory compound tested (Table 1) with a threshold of 10^−6.2 mol/l.

• 3. Response magnitude increased with increasing concentration over approximately 2–3 log units for the most potent stimuli and saturated at higher concentrations (Figs. 4, 6).

• 4. Elimination of the methyl groups from betaine led to an increase in threshold concentration and a decrease in relative effectiveness. Substitution of a neutral hydroxymethyl group for the primary carboxyl group of betaine (choline) eliminated the response entirely (Table 1).

• 5. Single unit responses to chemical stimuli were similar to the multiunit integrated responses in terms of relative effectiveness, threshold, and concentration-response curves.

• 6. Vigorous responses to tactile and proprioceptive stimuli were obtained from all of the animals tested, indicating that the searobin fin rays are sensitive to mechanical as well as chemical stimulation.