Gamma aminobutyric acid (GABA)-immunoreactive neurons of the paraventricular organ of the bony fish Coregonus albus send dendrites into the third ventricle. Their axons run to the synaptic zone of the infundibular lobe. The dendrites may take up some chemical information from the third ventricle, while the axons communicate it to the neuropil of the hypothalamus perhaps to modify its activity according to the state of the CSF. Serotonin-immunoreactive CSF-contacting neurons in the spinal cord of the hagfish Myxine glutinosa form dendrite terminals in the central canal and bear stereocilia like known mechanoreceptors. The Reissner's fiber runs above the stereocilia and Hows out from the central canal through its caudal opening. Possibly, the fiber keeps open this aperture and ensures the flow of the CSF, which may serve as a mechanoreceptory input for the CSF-contacting neurons. In the pineal recess of hedgehog, CSF-contacting pinealocytes develop enlarged cilia corresponding to the photoreceptor outer segments of submammalian pinealocytes. Potassium pyroantimonate cytochemistry shows a similar localization of calcium ions in the mammalian pinealocyte as in the submammalian photoreceptor ones. Pineal calcifications are present in some birds (goose, duck) and may be connected to the photoreceptory Ca-exchange of the pineal organ. Axonic processes of pinealocytes form synapses on secondary neurons in mammals (hedgehog, rat, cat). Such neurons are also present in human pineals. Axons of these neurons constitute a pinealofugal pathway. In the cat, some of the intrinsic pineal neurons are GABA-immunoreactive, they form axodendritic and axo-axonic synapses (inhibitory?) on immunonegative neurons and pinealocytes, respectively. The presence of multiple synaptic zones strengthens the view that the mammalian pineal is a complex nervous organ being histologically dissimilar to peripheral endocrine glands.
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