Wenbo Zhang & Thomas E Fisher
Department of Physiology, University of Saskatchewan, Saskatoon, Canada

https://doi.org/10.36866/pn.63.22

When the osmolality of our blood increases, our magnocellular neurosecretory cells (MNCs) start to burst (electrophysiologically speaking, that is). These neurones fire few action potentials when our blood is hypo­osmolar, but progressively increase their rate of firing as the osmolality increases (Bourque & Oliet, 1997). This is part of an important homeostatic mechanism that maintains our body fluid balance because these cells release either vasopressin (VP), which causes the retention of water in the kidneys, or oxytocin (OT), which causes (in some species) excretion of sodium. The release of these two hormones into the circulation is increased as the firing rate of the MNCs is increased. VP­releasing MNCs, furthermore, adopt a pattern of firing composed of bursts of action potentials lasting tens of seconds alternating with rest periods of roughly equal length. This pattern of firing, known as phasic firing, maximizes VP release.

The electrophysiological mechanisms underlying the osmosensitive changes in firing rate and pattern are incompletely understood. Although some of the increase in firing rate is due to an increase in excitatory inputs onto the MNCs, the MNCs are also inherently osmosensitive. Patch clamp experiments have shown that the MNC cell bodies express non-selective stretch inactivated cation channels (SICs) whose activity is determined by osmotically-evoked changes in cell volume (Oliet & Bourque, 1993). When exposed to hypo-osmolar solutions, the MNCs stretch, causing the SICs to close. When the external solution is hyper-osmolar, however, the MNCs shrink, which relieves the membrane tension and allows the SICs to open. This allows cations to flow into the cell, causing it to depolarize and become more responsive to excitatory inputs. This process is central to explaining how the MNCs increase their firing rate in response to increases in osmolality.

The initiation of a burst depends on a Ca2+ dependent depolarizing afterpotential (DAP) that is activated by action potentials and that increases the likelihood of subsequent action potentials (Roper et al. 2004). The amplitude of the DAP summates during repetitive firing, leading to the sustained depolarization that underlies the phasic burst. Ca2+ dependent K+ currents are also activated by action potentials and these currents may be involved in slowing firing during a burst. Although activation of Ca2+ dependent currents is known to terminate bursts in other types of neurons, they do not appear to play such a role in MNCs. Measurements of Ca2+ levels during a burst suggest that the activation of Ca2+ dependent K+ currents should be maximized well before the termination of the phasic bursts (Roper et al 2004). The mechanisms underlying the termination of the long phasic bursts of MNCs are therefore not entirely clear. One current model posits that somatodendritic release of the K-opioid peptide dynorphin during a burst acts on the MNCs in an autocrine fashion to suppress the DAP, thereby leading to burst termination (Brown & Bourque, 2004).

It is also possible, however, that osmotic activation of a current or currents other than the SIC might be involved in the transition to burst firing. Since MNCs exposed to high osmolality may go through a stage of rapid continuous firing before converting to a phasic firing pattern (Bourque & Renaud, 1984), there may be a separate process that is necessary for the adoption of burst firing. We therefore sought to determine whether there are any other ion channels modulated by changes in osmolality. As reported in the August issue of The Journal of Physiology, we recently identified another current in isolated MNCs that increases as a function of osmolality (Liu et al. 2005). Unlike the SIC, this osmosensitive current (the OC) is also voltage sensitive and is activated during voltage clamp by steps to potentials greater than -60 mV. Although the amplitude of the OC varies from cell to cell, there is a marked increase in amplitude (approximately two-fold) in greater than 60% of cells tested. The OC is therefore the first voltage gated current identified in the MNCs that is sensitive to osmolality. The slow activation of this current, and the lack of inactivation, suggest that it could increase in amplitude slowly over the course of a burst of action potentials. The OC might thus be expected to influence the MNC firing rate and/or pattern.

The ionic selectivity of the OC has not yet been established, but our evidence suggests that the current may be a K+ current. Although the current may be carried by Na+ in the absence of K+, the addition of even a small quantity of K+ into the internal recording solution results in block of Na+ flux. When K+ is then added to the external solution, voltage steps result in an inward flux of K+ . These data suggest that the OC may be a voltage dependent K+ current. Pharmacological experiments support this conclusion. The OC can be blocked by either Ba2+ or Cs+ ions, both of which are known K+ channel blockers. The OC can be activated, however, in the presence of a large concentration of TEA, suggesting that it is relatively insensitive to this K+ channel blocker.

What could be the function of a K+ current that turns on as osmolality is increased? It appears paradoxical that a K+ current would be activated by a stimulus that causes an increase in the output from these cells (i.e. the release of VP). The answer to this question may be that the OC has a role in mediating the transition into phasic bursting. By slowly activating during repetitive firing, the OC might act as a brake that temporarily silences a cell, thereby preparing it to respond to excitatory inputs with another burst of action potentials. Differential activation of the SIC and the OC might contribute to a variety of firing patterns, such as slow irregular or fast continuous firing, or firing in short or phasic bursts.

We have therefore identified an osmosensitive current that is likely to be a K+ current and that may be involved in the regulation of MNC firing in response to changes in the external osmolality. We are presently trying to identify the current in the hope of finding specific modulators that would help in identifying its physiological role. It remains to be seen whether the OC, like the SIC, is mechanosensitive. Alternatively, the current may be modulated by changes in second messenger systems mediated by changes in osmolality. Further characterization of the OC may help to understand the osmosensitive regulation of VP and OT release and the transitions between firing patterns observed in the MNCs.

Acknowledgements

Work in our laboratory has been supported by the Canadian Institutes of Health Research and the Saskatchewan Health Research Foundation through the CIHR Regional Partnership Program by an operating grant and by a New Investigator Award to TEF.

References

Bourque CW & Oliet SH (1997). Osmoreceptors in the central nervous system. Annu Rev Physiol 59, 601-619.

Bourque CW & Renaud LP (1984). Activity patterns and osmosensitivity of rat supraoptic neurones in perfused hypothalamic explants. J Physiol (Lond) 349, 631-642.

Brown CH & Bourque CW (2004). Autocrine feedback inhibition of plateau potentials terminates phasic bursts in magnocellular neurosecretory cells of the rat supraoptic nucleus. J Physiol 557, 949-960.

Liu XH, Zhang W & Fisher TE (2005). A novel osmosensitive voltage gated cation current in Rat Supraoptic Neurones. J Physiol 568, 61­68.

Oliet SH & Bourque CW (1993). Mechanosensitive channels transduce osmosensitivity in supraoptic neurons. Nature 364, 341­343.

Roper P, Callaway J & Armstrong W (2004). Burst initiation and termination in phasic vasopressin cells of the rat supraoptic nucleus: a combined mathematical, electrical, and calcium fluorescence study. J Neurosci 24, 4818-4831.