Mediobasal hypothalamic neurons are excited by the iontophoretic application of sodium.
In the course of studies on the responsiveness of mediobasal hypothalamic neurons to the iontophoretic application of cortisol, it was found that positive currents applied to a sodium chloride (1 M) barrel alone, but not to a choline chloride (1 M) barrel, frequently increased the firing of these neurons. Subsequently, systematic examination demonstrated that out of 102 MBH neurons 52 (51%) increased their firing by at least 30% with application of NaCl, using currents no greater than 10 nA. No such effect was obtained in response to Na application from a dilute solution (0.05 or 0.1 M). When glutamate was absent from the electrodes, the incidence of Na+ sensitivity fell to 17%, despite the routine use of backing currents to the glutamate barrel. K+ ions were more active than Na+ ions in producing excitation. When Na+ sensitivity was found, however, Na+ effects were produced by currents greater than K+ currents producing equivalent excitation. Like glutamate, K+ ions were capable of greatly enhancing responses to Na+. Comparison was made between cortisol and Na+ sensitivity in 70 MBH neurons; 28 cells responded to both, and 24 of them were inhibited by cortisol. Thus Na+ sensitivity is a frequent characteristic of MBH neurons inhibited by cortisol, and was present in 83% of cortisol-sensitive cells in this region. Iontophoresis of Na+ is commonly used as a control in pharmacological studies of the nervous system. Even more common is the case of concentrated NaCl solutions for recording. These procedures may not be as inert as previously thought, particularly in the hypothalamus.