Homeostasis of Extracellular Fluid in Retinas of Invertebrates and Vertebrates. In: Autrum H, Ottoson D, Perl ER, Schmidt RF, Shimazu H, Willis WD, editors. Progress in Sensory Physiology 6. Berlin: Springer Berlin Heidelberg; 1986. pp. 105-38. [DOI: 10.1007/978-3-642-70411-6_3]

Changes in [K+]0 at the vitreal surface compared with those around receptors in the isolated rabbit retina

Isolated rabbit retinas were superfused from the receptor side with a plasma-saline mixture kept at 35 degrees C. The vitreal side was exposed to an atmosphere of humidified warm oxygen. In one study the second-order neuronal activity was suppressed with aspartate and glutamate; in another study transmission was not blocked. When all neurons were active, [K+]0 around receptors was 4.5 +/- 0.4 mM in the dark. During a long (60s) exposure to light stimulus, [K+]0 dropped to 73% of the dark value and reaccumulated to 80%. At the vitreal surface, [K+]0 in the dark was 4.7 +/- 0.8 mM. During the 60s light stimulus, [K+]0 increased transiently, dropped to 83% of the dark value, then increased again to 91%. A continuous decrease of [K+]0 at the vitreal surface during long light stimuli concurrent with the increase of [K+]0 around receptors would indicate that the spatial buffering capability of the Müller cells contributes to the reaccumulation of potassium. Such a decrease, however, was not detected. After the blockage of transmission, [K+]0 values did not vary significantly from those after light stimulus in unblocked preparations. In the dark, [K+]0 was 5.2 +/- 0.9 mM at the vitreal surface and 4.6 +/- 0.4 mM around the receptors.

Increase in glial intracellular K+ in drone retina caused by photostimulation but not mediated by an increase in extracellular K+

The predominant glial cells of the drone retina (outer pigment cells) respond to an increase in extracellular [K+] (Ko) by a net uptake of K+; thus, they contribute to bringing Ko back toward its baseline value. The authors report herein that there is also a different mechanism by which light stimulation of the retina causes an increase in intracellular free [K+] in the glial cells. In superfused retinal slices, after 5-10 minutes of continuous illumination at physiological intensities, extracellular [K+] often fell back to below its original level in the dark. This fall can be explained by increased activity of the Na/K pump in the photoreceptors and diffusion of K+ down their axons. Despite the absence of raised Ko, K+-selective microelectrodes in glial cells recorded a small increase in intracellular [K+] that was maintained for the duration of the illumination; i.e. a change occurred in the glia that was not mediated by an increase in Ko. The increase in intracellular [K+] is not mediated by illumination of the screening pigment in the glia. Unless the increase is caused by illumination of some other, unknown, pigment in the glia, the results show that some unidentified signal (that is not K+) passes from the photoreceptors to the glia.

Functions of glial cells in the retina of the honeybee drone

In the retina of the honey bee drone, Apis mellifera male, physiological interactions between glial cells and neurons (the photoreceptors) are exceptionally clear-cut and amenable to investigation. The principal glia (outer pigment cells) contribute to the homeostasis of extracellular [K+] and [Na+] by 1) spatial buffering of K+ and 2) net uptake of K+ and Cl-. The glia supply carbohydrate metabolic substrate to the neurons; only the glia take up and phosphorylate glucose. Neuronal activity 1) modifies glycogen metabolism in the glia, and 2) can be signalled to the glia in the absence of elevated extracellular [K+].

Effects of carbon dioxide on extracellular potassium accumulation and volume in isolated frog spinal cord

A 6-10-fold increase in pCO2 in the superfusing Ringer solution increased the volume of the extracellular space (ECS) and changed the spatial distribution and amplitude of the extracellular K+ accumulation which resulted from dorsal root stimulation. Using the increase in tetraethylammonium concentration [( TEA+]) resulting from iontophoretic injection of that ion in the extracellular fluid as an indication of the volume of the ECS, it was found that in high pCO2 the ECS volume in spinal dorsal horn increased by more than 60%. In addition, in the presence of raised pCO2 we also observed the following: (1) The rate of diffusion of TEA+ into the dorsal horn increased. (2) The accumulation of K+ evoked by single or tetanic stimulation of the dorsal root was less. (3) The clearance of K+ was slowed down. (4) The regions where K+ accumulated were more restricted. (5) The K+ evoked depolarization of the primary afferent fibres decreased. (6) In contrast to TEA+, the rate of diffusion of K+ into the dorsal horn decreased. The effects of an increase in pCO2 on K+ accumulation and clearance appear to result from an increase in ECS volume and a possible decrease in glial electrical coupling which interferes with glial spatial buffering of K+.

Synaptic and neuronal-glial plasticity in the adult oxytocinergic system in response to physiological stimuli

Magnocellular oxytocinergic neurons in the hypothalamus offer a striking example of a mammalian neuronal system whose basic architecture and synaptic circuitry can be reversibly modified in adulthood. During parturition, lactation and prolonged osmotic stimulation, glial coverage of oxytocinergic neurons markedly diminishes and their surfaces are left in extensive juxtaposition; concurrently, there is formation of new synapses, which are predominantly GABAergic and which couple two or more oxytocinergic neurons simultaneously. These structural changes do not permanently modify the anatomy of the system since upon cessation of stimulation, neuronal juxtapositions and shared synapses disappear, to reappear upon new stimulation. At present, we can only speculate about the cellular mechanisms and factors responsible for these reversible neuroanatomical changes. However, oxytocin itself appears to be of primary importance since it can induce similar anatomical changes when chronically infused into the third ventricle.

Model of potassium dynamics in the central nervous system

A one-dimensional numerical model of potassium dynamics in the central nervous system is developed. The model incorporates the following physiological processes in computing spatial and temporal changes in extracellular K+ concentration, [K+]o: 1) the release of K+ from K+ sources into extracellular space, 2) diffusion of K+ through extracellular space, 3) active uptake of K+ into cells and blood vessels, 4) passive uptake of K+ into a cellular distribution space, and 5) the transfer of K+ by K+ spatial buffer current flow in glial cells. The following tissue parameters can be specified along the single spatial dimension of the model: 1) the volume fraction and tortuosity of extracellular and glial cell spaces, 2) the volume fraction of the cellular distribution space, 3) rate constants of active uptake and passive uptake processes, and 4) glial cell membrane conductance. The model computes variations in [K+]o and current flow through glial cells for three tissue geometries: 1) planar geometry (the retina and the surface of the brain), 2) cylindrical geometry (tissue surrounding a blood vessel), and 3) spherical geometry (tissue surrounding a point source of K+). For simple sources of K+, the performance of the model matches that predicted from analytical equations. Simulations of previous ion dynamics experiments indicate that the model can accurately predict ion diffusion and K+ current flow in the brain. Simulations of electroretinogram generation and K+ siphoning onto blood vessels suggest that unanticipated K+ dynamics mechanisms may be operating in the central nervous system.

Postnatal development of radial glial (Müller) cells of the rabbit retina

Radial glial (Müller) cells were isolated from postnatal rabbit retinae by enzymatic dissociation in papain-containing solution, air-dried, and submitted to Pappenheim's panoptic stain. Morphometric data of these cells were evaluated by light microscopy. During postnatal development, the cells become substantially thicker and shorter, their nuclei lose the rod shape and move more toward scleral layers, and the nucleus-cytoplasm volume relation decreases. Whereas the cell volume increases from birth on, substantial outgrowth of fine side branches within the plexiform layers fails to occur before electrical activity is established there, i.e. after postnatal day 9. A model is proposed relating the growth of sheath-bearing glial processes to local protein synthesis stimulated by external K+ accumulation due to neuronal activity. Early myelinated nerve fibers are suggested to bear mechanical resistance to growing radial glial processes thus causing a splitting of these processes when they enter developing nerve fiber layers.