Spectral sensitivity and mechanism of interaction between inhibitory and excitatory responses of photosensory pineal neurons.

The characteristics and distribution of chromatic-type neurons in the photosensory pineal organ of the river lamprey, Lampetra japonica, were investigated electrophysiologically. Neuronal activity was inhibited by light of short wavelengths and excited by middle to long wavelengths. The maximum sensitivities of the inhibitory and excitatory responses were at about 380 nm and 540 nm respectively. The spike activity of the neurons during steady illumination for a 10-min period was measured. Although a flash of short-wavelength light caused a strong inhibition in the neuron, this effect was not sustained during 10 min of photic stimuli. It was found that the inhibitory effect continued when excitatory (middle-wavelength) light was delivered together with inhibitory (short-wavelength) light. The result supports the hypothesis of photoregeneration in the pineal photoreceptor, which occurs when photoreceptors having high sensitivity to short wavelengths receive middle-wavelength light. Contrary to the inhibitory response, the excitatory one caused by middle wavelengths continued during stimulation. Spike frequency of the neuron was determined by the spectral composition of the light. Since environmental light contains both inhibitory and excitatory components, the neuron would keep both sensitivities during the daytime and could measure the variation in the spectral composition. Judging from the recording sites, the chromatic-type neurons are distributed in the peripheral part of the pineal organ.