Actual problems of the cerebrospinal fluid-contacting neurons

Abstract

Cerebrospinal fluid (CSF)‐contacting neurons form a part of the circumventricular organs of the central nervous system. Represented by different cytologic types and located in different regions, they constitute a CSF‐contacting neuronal system, the most central periventricular ring of neurons in the brain organized concentrically according to our concept. Because the central nervous system of deuterostomian echinoderm starfishes and the prochordate lancelet is composed mainly of CSF‐contacting‐like neurons, we hypothesize that this cell type represents ancient cells, or protoneurons, in the vertebrate brain. Neurons may contact the ventricular CSF via their dendrites, axons, or perikarya. Most of the CSF‐contacting nerve cells send their dendritic processes into the ventricular cavity, where they form ciliated terminals. These ciliated endings resemble those of known sensory cells. By means of axons, the CSF‐contacting neurons also may contact the external CSF space, where the axons form terminals of neurohormonal type similar to those known in the neurohemal areas. The most simple CSF‐contacting neurons of vertebrates are present in the terminal filum, spinal cord, and oblongate medulla. The dendritic pole of these medullospinal CSF‐contacting neurons terminates with an enlargement bearing many stereocilia in the central canal. These cells are also supplied with a 9 × 2 + 2 kinocilium that may contact Reissner's fiber, the condensed secretory material of the subcommissural organ. The Reissner's fiber floating freely in the CSF leaves the central canal at the caudal open end of the terminal filum in lower vertebrates, and open communication is thus established between internal CSF and the surrounding tissue spaces. Resembling mechanoreceptors cytologically, the spinal CSF‐contacting neurons send their axons to the outer surface of the spinal cord to form neurosecretory‐type terminals. They also send collaterals to local neurons and to higher spinal segments. In the hypothalamic part of the diencephalon, neurons of two circumventricular organs, the paraventricular organ and the vascular sac, of the magnocellular neurosecretory nuclei and several parvocellular nuclei, form CSF‐contacting dendritic terminals. A CSF‐contacting neuronal area also was found in the telencephalon. The CSF‐contacting dendrites of all these areas bear solitary 9 × 2 + 0 cilia and resemble chemoreceptors and developing photoreceptors cytologically. In electrophysiological experiments, the neurons of the paraventricular organ are highly sensitive to the composition of the ventricular CSF. The axons of the CSF‐contacting neurons of the paraventricular organ and hypothalamic nuclei terminate in hypothalamic synaptic zones, and those of magno‐ and parvocellular neurosecretory nuclei also form neurohormonal terminals in the median eminence and neurohypophysis. The axons of the CSF‐contacting neurons of the vascular sac run in the nervus and tractus sacci vasculosi to the nucleus (ganglion) sacci vasculosi. Some hypothalamic CSF‐contacti...

A mammalian neural tissue opsin (Opsin 5) is a deep brain photoreceptor in birds

“…S4). Interestingly, projections from the PVO to the median eminence and the pituitary gland have been reported in several other species (18).…”

“…Our present findings may have implications for evolutionary and developmental biology, because the retinal and pineal photosensory cells structurally resemble these neurons-e.g., they send dendritic processes into the pineal lumen and the photoreceptor space of the retina, and both are derived from diverticles of the third ventricle of the diencephalon (18). Although immunoreactivities for most of the opsins are generally observed in the photoreceptor outer segments or cilia-like structures of the modified photoreceptor cells, immunoreactivity of Opsin 5 was also observed in the axons of the external zone of the median eminence as well as in the cell bodies of the PVO.…”

“…As Opsin 5 appears to activate the Ca 2+ -Cl − channel via the Gq protein, it is possible that not only Opsin 5 localized in the cell bodies but also Opsin 5 localized in the axons activate membrane currents. It is well established that the CSF-contacting neurons in the PVO contain serotonin or dopamine (18). Neurotransmitters used by Opsin 5 neurons remain to be identified in the future.…”

“…S1). The PVO is composed of three layers of neurons arranged in parallel to the surface of the third ventricle (18). The neuronal perikarya of the first layer are located either in the ependyma or just underneath it (i.e., the subependyma).…”

Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles

“…OPN5 was located within the quail PVO, a caudal hypothalamic structure also present in the photosensitive tadpole preparation. Moreover, the PVO cells of other species have been shown to contain dopamine, noradrenaline, and/or serotonin (34), all known modulators of locomotion in Xenopus (35-37). An interesting example is the three-spined stickleback, which has large dopaminergic neurons in the PVO forming a contiguous group with the dopaminergic neurons of the PT (38).…”