Functional role of cerebrospinal fluid contacting cells in the spinal cord and hypothalamus

Abstract

Cerebrospinal fluid-contacting (CSF-c) cells are found lining the walls of the central canal and the ventricles throughout the vertebrate phylum, but their function has remained elusive. The aim here was to investigate the physiological role of CSF-c cells in the spinal cord and hypothalamus. We have identified two types of CSF-cells in the lamprey spinal cord located by the lateral part of the wall of the central canal, and that both send projections to the lateral margin of the spinal cord. Type 1 cells, with ciliated bulb-like protrusions into the central canal, co-express somatostatin and GABA, have neuronal properties and receive synaptic input. Type 2 cells, with flat endings in contact with the CSF, express taurine but not somatostatin or GABA and have passive membrane properties. They may constitute a form of radial glia (Paper I).

The next important question, not yet addressed for CSF-c neurons (type 1), was which type of stimuli that may represent the physiological mode of activation. CSF-c neurons respond to graded mechanical stimulation provided by very brief fluid jets that elicit receptor potentials and at somewhat larger amplitude trigger action potentials (paper II). However, the same cells also respond to small changes of the extracellular pH (Paper II, III). The responses to mechanical stimuli and to acidic pH are both mediated by ASIC3 (an acid-sensing ion channel) present also in other sensory terminals, whereas the alkaline response is mediated by PKD2L1 channels. The activity of the individual spinal CSF-c neurons was markedly enhanced at both alkaline and acidic pH with a U-shaped discharge pattern and a minimum frequency around pH 7.4 (Paper II, III).

A change of pH also affects the rate of activity in the locomotor network. Acidic as well as alkaline pH reduce the locomotor burst rate, and somatostatin applied extracellularly has a similar effect. Since CSF-c neurons are the only neurons that express somatostatin in the spinal cord, this allows for the possibility that they are responsible for the slowing of the locomotor activity induced by pH deviations. We could then show that the effect of pH changes on the locomotor network was indeed blocked by an antagonist of the somatostatin receptor sst2. The data thus indicate that the CSF-c neurons represent an intraspinal system for detecting any deviation of pH that can result from for instance a high level of neuronal activity, and the net effect will be to reduce the level of motor activity.

As somatostatin-expressing CSF-c neurons are also found in the periventricular area of the hypothalamus, the next goal was to investigate whether these hypothalamic CSF-c neurons have similar properties as their spinal counterparts (paper IV). The hypothalamic CSF-c neurons also have bulb-like, ciliated protrusions into the CSF along the third ventricle and co-express GABA and somatostatin. They also respond to changes of the extracellular pH with a U-shaped response curve. As in their spinal counterparts, ASIC3 mediates the response to acidic pH in hypothalamic CSF-c neurons. The alkaline response, however, does not appear to depend on PKD2L1 channels, since these channels are not expressed in hypothalamic CSF-c neurons, and thus must rely on an as yet unidentified channel (Paper IV). The hypothalamic CSF-c neurons ramify extensively in the hypothalamus and forebrain, and when activated they will cause a release of somatostatin in these areas presumably affecting circuits that control different aspects of behavior, thereby possibly counteracting a deviation in pH and thus contributing to homeostasis.

Taken together, both spinal and hypothalamic CSF-c neurons serve as pH sensors, thereby providing a novel homeostatic module for the regulation of pH in the CNS, in addition to the regulation exerted by the respiratory and renal systems.