Neural excitation of the larval firefly photocyte: slow depolarization possibly mediated by a cyclic nucleotide.

1. In firefly larvae, extracellular recordings from the light organ nerve show that a volley of action potentials elicits a glow of an intact animal. 2. Intracellular recordings from the photocytes show that they respond to nerve stimulation with a slow, graded depolarization which precedes light emission. The depolarization begins about 0-5 s after the nerve is stimulated; it peaks about 1 s after stimulation; and subsides about 2-5 s after the stimulus. The glow increases fastest when the photocyte depolarization is at its peak and lasts 5-15 s. 3. Photocyte depolarization is associated with a decrease in the input resistance of the cell. 4. Adrenergic receptors in the light organ are pharmacologically similar to vertebrate alpha-receptors. 5. Phophodiesterase inhbitors, aminophylline and theophylline, cause the light organ to glow, suggesting that cyclic nucleotides may mediate the effect of the adrenergic nerve transmitter.