Central neuronal response to the activation of osmoreceptors in the osphradium of Aplysia.

The physiology of osphradium (the pallial organ situated near the branchiae in most mollusca with the exception of the higher cephalopoda) is poorly known. Since the work of Copeland (1918) on Busy con canaliculatvm, it has been generally accepted that the osphradium serves a chemoreceptive function. This conception has been shared by Henschel (1932), Brock (1936), Brown & Noble (1960) Michelson (i960) and Wolper (1950); all of whom reached this conclusion by means of experiments in which behavioural responses to a variety of stimuli were observed before and after the removal of the osphradium. A different and more direct approach was attempted by Bailey & Laverack (1963, 1966) on the carnivorous gastropod Buccinwn undatum. The osphradium of this animal was put in contact with mussel extracts or with solutions of different aminoacids, and excitatory and inhibitory responses to this stimulation were measured in single cells of the central ganglia. It was concluded from this work that the osphradium is used by these carnivorous gastropods in the detection of their prey. Such an hypothesis does not seem tenable, however, for a non-carnivorous gastropod such as Aplysia. Frings & Frings (1965) have shown that whereas the buccal and tentacular sensory organs were involved in the localization and identification of food (seaweed), the osphradium did not appear to be engaged in this activity; a selective destruction of the osphradium by cauterization did not modify the response of the animal to the presentation of food. In the present study we have endeavoured, using isolated preparation, to analyse the responses of identified cells of the visceral ganglion to different chemical and physical stimuli applied to the osphradium in Aplysia. Evidence has been found for the presence of receptors which are sensitive to the diminution of osmotic pressure of the medium bathing the osphradium. Excitation of these osmoreceptors produces in an identifiable cell the phenomenon of inhibition of long duration, which phenomenon had been evoked in earlier studies by stimulation of certain nerves afferent to the ganglion, especially by the stimulation of the branchial nerve which joins the visceral ganglion to the osphradium (Tauc, 1958). A preliminary note of the work reported below has already been published (Stinnakre & Tauc, 1966).