Labyrinthine sensory cells: is a monoamine the chemical transmitter?

In sensory cells of the frog labyrinth, reserpine and guanethidine have proved able to affect the rnorphology of the synaptic apparatus, in particular of synaptic bars, to a substantial degree. Further pharmacological tests have also enabled some authors to suggest on the basis of morphological evidence that a monoamine, in all likelihood a catecholamine, may be responsible for chemically operating the junctions between pair cells and afferent nerve fibers. This hypothesis has been tested in our laboratory by electrophysiological methods, and the negative results we recorded are briefly presented in this report. Isolated preparations of posterior semicircular canals in Tyrode solution were obtained from frogs (Rana esculenta) weighing 20-25 g, following a method described elsewhere. Suitable cupula movements were produced by means of a mìcrosyringe, whose piston was driven by a small loudspeaker. Action potentials in the ampullary nerve were picked up by suction electrodes. Both spontaneous discharges and responses to sinusoidal cupula deflections were recorded. The effects of reserpine and guanethidine were tested on two groups, each comprising 10 frogs. The drugs were administered by precisely following the scheme suggested by Osborne and Thornhill. After this treatment, which made the animals become obviously myotic, isolated preparations of posterior semicircular canals were removed by careful dissectìon. Fatigue tests were carried out by prolonged sinusoidal stimulations of suitable amplitude and frequency (0.1-5 Hz) for up to 1 h. The nerve discharges recorded were compared with those elicited from a group af 5 control preparations but revealed no significant difference . The same was true even after treatment periods 24 times longer and with doses 5-10 times greater than those employed by Osborne and Thornhill. Possible involvement of catecholamines in synaptic transmission at the periphery of the vestibular apparatus was also investigated subsequently by testing the effects of dibenamine and D(-) INPEA in vitro at concentrations of 10-6-10-4 M, these drugs being specific α- and β-blocking agents respectively. The receptor responses to sinusoidal stimulations (0.5 Hz for 20 s) were recorded at 3 min intervals before exposure to the drugs, during a 30 min period of exposure, and after washing. Neither dibenamine nor D(-) INPEA affected evoked discharges from the ampullary nerve to any degree. Since it is highly improbable that there are any significant differences in the mechanisms of labyrinthine receptors as between different kinds of European frogs, this investigation highlights an embarassing contrast between electrophysiolocal cvidence and morphological data. The importance of synaptic bars in cytoneural transmission in the labyrinthine sensory system is not diminished by these results. Nevertheless the hypothesis that a monoamine may act as the transmitter appears not to be supported by electrophysiological data. The nature of the chemical transmitter in labyrinthine sensory cells ís therefore still far from being identified, and the catecholamine hypothesis appears to be highly questionable.