Cerebrospinal fluid-contacting neurons, ciliated perikarya and "peptidergic" synapses in the magnocellular preoptic nucleus of teleostean fishes

The magnocellular preoptic nucleus of fishes (Anguilla anguilla, Amiurus nebulosus, Cyprinus carpio, Carassius auratus, Ctenopharyngodon idella, Cichlasoma nigrofasciatum) has been studied by light and electron microscopy. Two kinds of neurons Were found: a) large, electron-dense, Gomori-positive cells with moderate acetylcholinesterase (AChE) positivity which contain granulated vesicles of 1400 to 2200 A (in average 1600 to 1800 A), adn b) small, strongly AChE-positive, electron-lucent neurons containing granulated vesicles of 900 to 1200 A. The nerve cells are supplied with axo-somatic and axo-dendritic synapses. These are formed by axon terminals containing either 1. synaptic vesicles of 500 A, or 2. synaptic vesicles of 500 A and dense-core vesicles of 600 to 800 A, or 3. synaptic vesicles of 600 A and granulated vesicles of up to 1100 A, or 4. synaptic vesicles of about 400 A and granulated vesicles of up to 1800 A. The presence of "peptidergic" and numerous other synapses shows the complexity of the organization and afferentation of the magnocellular preoptic nucleus. In the eel, both types of nerve cells form dendritic terminals within the cerebrospinal fluid (CSF). These CSF contacting dendrites are supplied with 9 x 2 + 0 cilia. In the other species investigated, only some large neurons build up intraventricular endings. The ependymofugal process of the CSF contacting neurons enters the preoptic-neurohypophysial tract. Perikarya of both the large and the small cells may give rise to single, paired or multiple 9 x 2 + 0 cilia extending into the intercellular space. The number of CSF contacting neurons is reciprocal to the number of perikarya with intercellular cilium. These latter cells may represent modified, more differentiated forms of the CSF contacting neurons. We think that atypical cilia protruding into the intercellular space may have the same significance for the intercellular fluid as the cilia of the intraventricular dendrites of the CSF contacting neurons for the CSF.