A persistent sodium current in acutely isolated histaminergic neurons from rat hypothalamus

Histamine neurons acutely dissociated from the tuberomammillary nucleus of the rat hypothalamus were studied in whole-cell and cell-attached patch-clamp experiments. Electrophysiological properties of dissociated cells were found to be similar to those recorded in slice experiments using microelectrodes. Tuberomammillary neurons fired spontaneously and this activity persisted when Cs+ (1.5 mM) was added to, or when K+ was removed from the extracellular solution. In whole-cell experiments a persistent tetrodotoxin-sensitive inward current was recorded. In cell attached recordings voltage-gated sodium channels displayed either normal or non-inactivating behavior. These results provide a further analysis of the properties of histaminergic neurons and indicate that spontaneous activity is intrinsic to individual neurons. Evidence for a non-inactivating tetrodotoxin-sensitive sodium current is presented. Single channel recordings indicate that this current is the result of non-inactivating behavior of sodium channels. Such a current is well suited for biasing tuberomammillary neurons toward spontaneous activity.

The binomial distribution and the evidence for independent action of ion channels

Calculations are presented demonstrating a class of hypothetical channel interactions which cannot be recognized by means of the commonly used binomial test. On the contrary, ion channels which interact in such a way will be identified with the help of the binomial test as independent. Thus, the binomial test cannot be used to prove the absence of interactions. Furthermore, in cases in which existing interactions are recognized by the binomial test, estimation of the strength of the interactions may be inaccurate.

Pb2+ modulates the NMDA-receptor-channel complex

The actions of Pb2+ on NMDA channel currents of acutely dissociated hippocampal CA1- and CA3-neurones from adult rats activated by aspartate plus glycine (asp/gly) were examined. A fast reversible and a slow irreversible response to Pb2+ were found. Pb2+ applied simultaneously with asp/gly decreased an inward current. The threshold concentration was below 2 microM, the current was reduced > 90% at concentrations over 100 microM. The decrease of the asp/gly activated current showed no voltage dependence. Opening of NMDA channels was not necessary for Pb(2+)-action, as preincubation in 50 microM Pb(2+)-containing external solution for several seconds dramatically reduced the response to asp/gly/Pb2+. This effect was reversed within 2 to 5 s of wash. Presence of Pb2+ or asp/Pb2+ or glycine/Pb2+ in the external solution did not prevent recovery of the NMDA receptor/channel complex from desensitization. Prolonged perfusion of a cell with the asp/gly/Pb(2+)-containing external solution resulted in an irreversible decrease of the asp/gly current, whereas the amplitude of the asp/gly/Pb2+ response did not change over the duration of an experiment. We conclude that Pb2+ modulates NMDA channel activity via interaction with the NMDA/glycine receptor: as a result the channel current decreases.

Desensitization of N-methyl-D-aspartate receptors in neurons dissociated from adult rat hippocampus

Desensitization of the N-methyl-D-aspartate (NMDA) receptor-channel complex was studied in isolated rat hippocampal neurons using a fast drug application system. 1) Desensitization rate was slower at more negative membrane potentials and when external [Ca2+] was lowered. 2) In the presence of 10 microM glycine, 2-amino-5-phosphonovalerate neither induced desensitization nor prevented recovery from it. 3) Preincubation in 500 microM aspartate or 10 microM glycine alone elicited desensitization only weakly or not at all. 4) Aspartate appeared to bind at its receptor site in the absence of glycine, and vice versa. It is proposed that, for the NMDA receptor, channel opening is necessary for the occurrence of desensitization and, thus, that desensitization involves structural changes in the channel-lining section of the protein rather than the glycine or NMDA binding sites.