The photosensory function of the pineal organ of the pike (Esox lucius L.) Correlation between structure and function

Summary

Electrical recordings from the exposed pineal organ of the pike (Esox lucius L.) were performed in order to localize the photoreceptive structures. Extracellular recordings showed a maintained activity of nerve fibers from the pineal tract and of single neurons from the distal region of the pineal organ. At increasing levels of steady exposure to white light, the impulse frequency decreased. Illumination of the organ with wavelengths between 380 and 710 nm resulted in an inhibition of the spike activity (achromatic response), associated with slow graded responses (electropinealogram, EPG). Sensitivity curves exhibited maxima at 530 and 620 nm in the light adapted, and one maximum at 530 nm in the dark adapted organ. In rare occasions, inhibitory (λ_max 380 nm) and excitatory (λ_max 620 nm) responses were recorded from single ganglion cells (chromatic response). Some observations (dark adaptation curves; intensity-duration relationship) suggest that the spike potentials and graded responses are probably not generated by the same structures. Moreover, slow potentials without spike potentials were recorded from isolated medial regions of the pineal where no nerve cells are observed.

The pineal organ of the pike appears to be a functional photoreceptive organ that may act as a dosimeter of solar radiation, and as an indicator of day-length. The morphological differentiation of its epithelium is closely related to its function, no electrical activity being propagated from the medial region to the brain.