Biphasic effect of pentobarbitone on chick myotube nicotinic receptor channel kinetics

Pentobarbital has curare-like effects on adult-type nicotinic acetylcholine receptor channel currents

Pentobarbital (PB) is widely used as a short-term sedative and anticonvulsive drug with a side-effect of relaxing muscle tone. We investigated block of nicotinic acetylcholine receptor (nAChR) channel currents by PB using the patch-clamp technique in combination with an ultrafast system for solution exchange. As a preparation, recombinant rat adult-type nAChR channels transiently expressed in HEK293 cells were used. Appli-cation of 1 mM acetylcholine to small cells or outside-out patches showed a transient current with fast activation and desensitization kinetics. Adding PB to the acetylcholine-containing solution resulted in a decrease of the time constant of current decay and of the peak current amplitude starting at concentrations >0.01 mM PB. Preincubation of nAChR channels with PB led to a decrease of the peak current amplitude without alteration of activation and desensitization kinetics caused by competitive block of nAChR channels. In conclusion, similar to the effect of d-Tubocurarine, block of nAChR channel currents by PB can be explained by a combination of open-channel and competitive block.

IMPLICATIONS

The interaction between adult-type nicotinic acetylcholine receptors, acetylcholine, and pentobarbital was biophysically investigated by using the patch-clamp technique in combination with tools for ultrafast solution exchange. PB elicited open-channel block and competitive block of nicotinic acetylcholine receptor channel currents, whereas the latter seems to be effective in clinically relevant concentrations.

Evidence for cooperativity between nicotinic acetylcholine receptors in patch clamp records

It is often assumed that ion channels in cell membrane patches gate independently. However, in the present study nicotinic receptor patch clamp data obtained in cell-attached mode from embryonic chick myotubes suggest that the distribution of steady-state probabilities for conductance multiples arising from concurrent channel openings may not be binomial. In patches where up to four active channels were observed, the probabilities of two or more concurrent openings were greater than expected, suggesting positive cooperativity. For the case of two active channels, we extended the analysis by assuming that 1) individual receptors (not necessarily identical) could be modeled by a five-state (three closed and two open) continuous-time Markov process with equal agonist binding affinity at two recognition sites, and 2) cooperativity between channels could occur through instantaneous changes in specific transition rates in one channel following a change in conductance state of the neighboring channel. This allowed calculation of open and closed sojourn time density functions for either channel conditional on the neighboring channel being open or closed. Simulation studies of two channel systems, with channels being either independent or cooperative, nonidentical or identical, supported the discriminatory power of the optimization algorithm. The experimental results suggested that individual acetylcholine receptors were kinetically identical and that the open state of one channel increased the probability of opening of its neighbor.

Modulatory action of PACAP27 on NMDA receptor channel activity in cultured chick cortical neurons

The modulatory effect of PACAP27 on NMDA receptor channel activity in cultured chick cortical neurons was investigated using the outside-out recording mode of the patch clamp technique. Channel opening frequency elicited by 20 microM NMDA, or 20 microM NMDA plus 1 microM glycine, was potentiated in the presence of 100 nM PACAP27 and inhibited with 1000 nM PACAP27. These effects were reversible on washout and reduced when glycine concentration was increased to 10 microM, but were not affected by the PACAP antagonist PACAP6-27 (1 microM) or the GTP inhibitor GDP-beta-S (100 microM). It is suggested that PACAP27 may exert its modulatory action on NMDA receptor channel activity through the glycine site(s).

PACAP38 modulates activity of NMDA receptors in cultured chick cortical neurons

The outside-out recording mode of the patch-clamp technique was used to study modulatory effects of pituitary adenylate cyclase-activating polypeptide (PACAP38) on N-methyl--aspartate (NMDA) receptor activity in cultured chick cortical neurons. Biphasic concentration-dependent effects of PACAP38 on channel opening frequency induced by NMDA (20 microM) and glycine (1 microM) were found, with low concentrations (0.5-2 nM) of PACAP38 increasing activity and higher concentrations (10-1,000 nM) causing inhibition. These effects were reversible, reduced with higher concentrations of glycine (2-10 microM) but not by 200 microM NMDA, and inhibited by 10 microM 7-chlorokynurenic acid. In addition, 1 microM PACAP6-38 (a PACAP antagonist) inhibited channel activity due to 20 microM NMDA and 1 microM glycine by 66%, and this inhibition was reduced to 13% in the additional presence of 2 nM PACAP38. These observations suggest that PACAP38 has a direct modulatory effect on the NMDA receptor that is independent of intracellular second messengers and probably mediated through the glycine coagonist site(s).