1. End‐plate currents produced by nerve‐released acetylcholine and iontophoretically applied acetylcholine and carbachol have been recorded from voltage‐clamped frog cutaneous pectoris neuromuscular junctions made visible with Nomarski differential interference contrast optics. 2. The effectiveness of agonists ‐ that is, the end‐plate conductance change produced by a given dose‐has been determined as a function of post‐junctional membrane potential. 3. As the post‐junctional membrane potential is made more negative, nerve‐released acetylcholine becomes less effective whereas iontophoretically‐applied agonists become more effective. 4. This voltage dependence of agonist effectiveness is mediated neither by end‐plate current iontophoresis of agonist into the cleft nor through electric field effects on the esterase. 5. Influences of membrane potential on the opening and closing of end‐plate channel gates can account quantitatively for the voltage‐dependent effectiveness of both nerve‐released and iontophoretically applied agonist.

The terminal nerve is an anterior cranial nerve that innervates the lamina propria of the chemosensory epithelia of the nasal cavity. The function of the terminal nerve is ambiguous, but it has been suggested to serve a neuromodulatory role. We tested this hypothesis by exposing olfactory receptor neurons from mudpuppies (Necturus maculosus) to a peptide, gonadotropin releasing hormone (GnRH), that is found in cells and fibers of the terminal nerve. We used voltage-clamped whole-cell recordings to examine the effects of 0. 5-50 micrometer GnRH on voltage-activated currents in olfactory receptor neurons from epithelial slices. We found that GnRH increases the magnitude, but does not alter the kinetics, of a tetrodotoxin-sensitive inward current. This increase in magnitude generally begins 5-10 min after initial exposure to GnRH, is sustained for at least 60 min during GnRH exposure, and recovers to baseline within 5 min after GnRH is washed off. This effect occurred in almost 60% of the total number of olfactory receptor neurons examined and appeared to be seasonal: approximately 67% of neurons responded to GnRH during the courtship and mating season, compared with approximately 33% during the summer, when the sexes separate. GnRH also appears to alter an outward current in the same cells. Taken together, these data suggest that GnRH increases the excitability of olfactory receptor neurons and that the terminal nerve functions to modulate the odorant sensitivity of olfactory receptor neurons.

SUMMARY1. Frog neuromuscular junctions were viewed with Nomarski optics and voltage clamped. Agonist was applied ionophoretically and agonist concentrations were measured using a micro-electrode sensitive to quaternary amines.2. The dose-response relationship was studied using the agonists carbamylcholine, suberyldicholine and hydroxyphenyl-propyltrimethylammonium.3. With all of these agonists, it appeared that the ACh receptor could be active when either one or two agonist binding sites were occupied. The receptor was much more likely to be active when both sites were occupied. Agonist dissociation constants and receptor activation probabilities were estimated by non-linear regression techniques for several possible receptor activation schemes.

Single-channel currents from acetylcholine receptor channels of garter snake neuromuscular junctions were recorded using the patch-clamp technique. Low concentrations of acetylcholine or carbamylcholine induced populations of single current events whose amplitudes and durations had unimodal distributions. The probability with which channel opening transitions occurred was time dependent, so that it was more probable for channels to open during the several hundred microseconds following a closing transition than during any later equivalent interval. The time-dependent distributions of duration and opening-transition probability were fitted by a sequential, reversible kinetic model in which the agonist binding steps occur before, and separately from, channel activation. This description allowed estimates to be obtained of both the opening (approximately 750s-1) and closing (approximately 500s-1) transition rates of these channels and of the mean lifetimes of the open- (approximately 2 ms) and the closed-channel state (approximately 200 mus) to which the open state was reversibly related.

A B S T R A C T Olfactory receptor neurons isolated from embryonic, neonatal, and adult mice were studied using the patch-clamp technique. Several distinct types of ion channels were characterized in patches of membrane from the neuronal soma and the dendritic knob of receptor neurons, including a 130-pS Ca++-activated K + channel with voltage-dependent kinetics, an 80-pS Ca ++-activated K + channel with voltage-insensitive kinetics, a 25-pS K + channel with properties similar to inward rectifiers, and a 40-pS K + channel that was activated and then inactivated by rapid depolarization. Evidence of large-conductance (>200 pS) CI-channels, which were Ca ++ insensitive and increasingly active at depolarizing membrane potentials, and voltage-activated Ca ++ channels (16 pS) was also obtained. From K + channel activity recorded from cell-attached patches, the intracellular [Ca ++] was inferred to be below 0.1 aM, and the membrane potential was inferred to be approximately -50 mV. The receptor neurons had high input resistances, and action potentials could be elicited by picoampere amounts of depolarizing current. The receptor neurons responded to applied odorant molecules and to forskolin with increases in membrane conductance. These results provide a description of the membrane properties of olfactory receptor neurons and a basis for understanding their electrical activity and response to odorants.