GABA-immunoreactive intrinsic and -immunonegative secondary neurons in the cat pineal organ.

The pineal organ of the cat was studied by postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. Two polyclonal rabbit GABA antisera were used with light microscopic peroxidase and electron microscopic immunogold techniques. A considerable number of intrinsic neurons are scattered in the proximal portion of the pineal organ. Some of the nerve cells were GABA-immunoreactive; other neurons as well as pinealocytes and glial/ependymal cells were immunonegative. A few GABA-immunoreactive neurons behave like CSF-contacting neurons by penetrating the ependymal lining of the pineal recess. GABA-immunoreactive neurons were more frequently found in the subependymal region. Small bundles of thin immunoreactive unmyelinated and thick immunoreactive myelinated nerve fibers occurred in the proximal pineal, especially near the habenular commissure. There were synapses of various types between GABA-immunoreactive and -immunonegative fibers. Myelinated immunoreactive axons seemed to loose their sheaths after entering the organ. Axon-like processes of pinealocytes terminated on dendrites of immunonegative neurons present near the posterior and habenular commissures. The axons of these neurons were found to join the commissural fibers and may represent a pinealofugal pathway conducting information originating from pinealocytes. The pinealocytic axons forming ribbon-containing synapses on dendrites of secondary neurons speak in favor of the sensory-cell nature of the pinealocytes. The pinealopetal myelinated GABA-immunoreactive axons and the intrinsic "GABA-ergic" neurons are proposed to inhibit the action of intrapineal neurons on which the pinealocytic axons terminate.