Multiple classes of photoreceptors and neurons in the frontal organ of Rana pipiens.

An attempt has been made to characterize the photoreceptors and neurons of the frontal organ of Rana pipiens and the synaptic contacts of these cells, in order to provide morphological correlates for previous physiological findings. Neurons, glial cells, and photoreceptors with two different appearances can be seen in normal anatomical preparations. The photoreceptors make ribbon synaptic contacts onto both neurons and other photoreceptors. Retrograde transport of either cobalt or horseradish peroxidase through the frontal nerve results in labeling of neurons which are often located near the exit point of the frontal nerve from the frontal organ. Some of the dendritic processes of these neurons ramify among the neurons themselves, while others ramify among the photoreceptors. In addition, the frontal organ contains two photoreceptor types which synaptically contact both neurons and other photoreceptors. In some frontal organs labeled with cobalt (but never in peroxidase-labeled preparations) certain photoreceptors were also labeled. The labeled photoreceptors were probably the result of transcellular cobalt movement from labeled neurons, and not the result of some photoreceptors having axons in the frontal nerve. These results are discussed in conjunction with physiological evidence that chromatic interactions occur within single photoreceptors, each of which contains a single visual pigment with two physiologically active states.