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The frog is known to have a specific chemoreceptor sensitive to water which is generally called water receptor1,5,7). In order to know the role of this chemoreceptor, we studied the reflex elicited by tap water stimulation of the tongue and found that tap water elicits reflex discharges in the submental and submaxillary muscles and brings about nostril closure by a lever mechanism. When the submental and submaxillar muscles contract, the mandible decreases in width and the prelingual tubercle moves forward. This protrudes the anterior end of the upper jaw forward and closes the nostril. In this lever mechanism of nostril closure, elevation of the mandible should occur simultaneously because the prelingual tubercle is needed to fit into the medial subrostral fossa of the upper jaw and to move upward slightly. In a previous study4), however, we were not able to obtain a jaw-closing reflex in so far as stimulation of the tongue with tap water was concerned. It is known that there are also water receptors in the palate3,6). Thus, we studies the reflex elicited by tap water stimulation of the palate and buccal floor and found that such stimulation elicited a jaw-closing reflex due to activities of the pterygoid and masseter muscles. Materials and Methods Male frogs, Rana nigromaculata and Rana japonica, weighing 15-30 g, were used. Under anesthesia with ethyl ether, the brachial nerves were severed bilaterally and the caudal part of the vertebral canal was destroyed with a needle in order to prevent movement of the animals after recovery from the anesthesia; then, about 0.2ml of 0.5% MS 222 was injected intraperitoneally. After the trigeminal, facial, and glossopharyngeal nerves were dissected according to need under anesthesia, the animals were mounted on a wooden stage and fixed with pins. Reflex activities of the jawand nostrilclosing muscles were examined by recording electromyograms (in cases of the temporal and pterygoid muscles) or by recording neural discharges (in cases of the masseter and submental muscles) with bipolar silver wire electrodes. Myograms and neural discharges were displayed on a cathode-ray oscilloscope and recorded with a camera. Tap water as well as sapid solutions (1M NaCl, 0.5mM quinine-HCl and HC1 at pH 2.5) were used as chemical stimuli. These solutions were applied to the palate, tongue, or buccal floor with a pipet for about 10 seconds. Each test was followed by rinsing with Ringer's solution with no further stimulation for 2 minutes. Tactile stimuli were applied with a small brush. Experiments were carried out at room temperature (2326•Ž) in a damp room.

Jaw-closing reflex elicited by water stimulation of oral mucosa in the frog.