Effects of inositol, 1,4,5-trisphosphate injections into cat spinal motoneurons

Renal afferents and hypertension

The kidney and the autonomic nervous system are linked through renal nerves. Activation of efferent renal sympathetic nerves leads to changes in renal vascular resistance, renin release, and Na(+) and water retention. Evidence also exists indicating that the kidney is not just a target organ of sympathetic activity, but also acts as a sensor. Afferent renal nerves have been shown to carry information from renal chemoreceptors, which respond to changes in the composition of the interstitial fluid environment, and mechanoreceptors, which monitor hydrostatic pressure changes within the kidney, to the central nervous system. These afferent renal nerve inputs alter the activity of central integrative neuronal circuits that normally give rise to command signals that influence the function of effector organs. Renal receptors, through their connections at different levels of the neuraxis, are able to reflexly influence not only cardiovascular function through changes in sympathetic nerve discharge to a variety of vascular beds and the hypothalamic release of vasopressin, but also the function of the kidney. This increased sympathetic activity and hormonal release induced by activation of afferent renal nerves has been implicated in hypertension of diverse etiologies.

Contribution of outward currents to spike-frequency adaptation in hypoglossal motoneurons of the rat

Contribution of outward currents to spike-frequency adaptation in hypoglossal motoneurons of the rat. J. Neurophysiol. 78: 2246-2253, 1997. Spike-frequency adaptation has been attributed to the actions of several different membrane currents. In this study, we assess the contributions of two of these currents: the net outward current generated by the electrogenic Na+-K+ pump and the outward current that flows through Ca2+-activated K+ channels. In recordings made from hypoglossal motoneurons in slices of rat brain stem, we found that bath application of a 4-20 microM ouabain solution produced a partial block of Na+-K+ pump activity as evidenced by a marked reduction in the postdischarge hyperpolarization that follows a period of sustained discharge. However, we observed no significant change in either the initial, early, or late phases of spike-frequency adaptation in the presence of ouabain. Adaptation also has been related to increases in the duration and magnitude of the medium-duration afterhyperpolarization (mAHP) mediated by Ca2+-activated K+ channels. When we replaced the 2 mM Ca2+ in the bathing solution with Mn2+, there was a significant decrease in the amplitude of the mAHP after a spike. The decrease in mAHP amplitude resulted in a decrease in the magnitude of the initial phase of spike-frequency adaptation as has been reported previously by others. However, quite unexpectedly we also found that reducing the mAHP resulted in a dramatic increase in the magnitude of both the early and late phases of adaptation. These changes could be reversed by restoring the normal Ca2+ concentration in the bath. Our results with ouabain indicate that the Na+-K+ pump plays little, if any, role in the three phases of adaptation in rat hypoglossal motoneurons. Our results with Ca2+ channel blockade support the hypothesis that initial adaptation is, in part, controlled by conductances underlying the mAHP. However, our failure to eliminate initial adaptation completely by blocking Ca2+ channels suggests that other membrane mechanisms also contribute. Finally, the increase in both the early and late phases of adaptation in the presence of Mn2+ block of Ca2+ channels lends further support to the hypothesis that the initial and later (i.e., early and late) phases of spike-frequency adaptation are mediated by different cellular mechanisms.

Intracellular Mg2+ inhibits the IP3-activated IK(Ca) in NG108-15 cells. [Why intracellular citrate can be useful for recording IK(Ca)]

Receptor-mediated formation of inositol 1,4,5-trisphosphate (IP3) can induce an outward Ca(2+)-activated K+ current [IK(Ca)] in some neural cells. We have investigated IK(Ca) activated by intracellular injections of IP3 in whole-cell patch-clamped neuroblastoma x glioma hybrid cells. The current could only be recorded reliably using citrate as the anion in the pipette, but not using acetate, aspartate, chloride, fluoride, gluconate or methylsulphate. This could be attributed to buffering of intracellular Mg2+ by citrate. Theoretical calculations suggested free [Mg2+] of 1.0 and 0.07 mM respectively in the acetate- and citrate-based recording solutions. Further, IP3-activated IK(Ca) could be recorded when the free Mg2+ level in the acetate, chloride or methylsulphate solutions was lowered to the range (0.05 mM) calculated for the citrate solution. Thus, raised [Mg2+] blocks IK(Ca). This appeared to be due to inhibition of the response to released Ca2+, since high [Mg2+] also blocked the response to intracellular injections of Ca2+ ions. Mean Mg2+ levels in intact neuroblastoma x glioma hybrid cells measured by Mag-Indo-1/AM fluorescence were estimated to be less than 0.14 mM. We therefore conclude that IP3-induced IK(Ca) is expressed under normal conditions, but may be subject to regulation by intracellular Mg2+.