Diazepam increases melatonin secretion of photosensitive pineal organs of trout in the photopic and mesopic range of illumination.

The pineal organ of teleost fish receives photic information directly through specialized photoreceptor cells that transmit their light response to second-order neurons and respond also with an endocrine light-dependent melatonin signal. In the present study we have analyzed the action of diazepam, a full agonist of the benzodiazepine receptor, on the photic regulation of the endocrine melatonin response of cultured trout pineal organs. Melatonin release of explanted pineal organs was clearly dependent on the irradiance of incident light with a maximum change during mesopic illuminations. Addition of diazepam to the superfusion medium significantly increased melatonin production in the mesopic and partly in the photopic range of illumination, without showing clear effects in the dark-adapted organ. Flumazenil, a central acting benzodiazepine antagonist, slightly reduced melatonin secretion. The action of diazepam appears to be comparable to a dark-pulse in the mesopic range of illuminations.