1. Application of the muscarinic agonist oxotremorine-M (oxo-M) to isolated abdominal ganglia of larval Manduca sexta excited an identified proleg retractor motoneuron called PPR. This excitation consisted of a persistent depolarization and an increased tendency to generate action potentials. Previous work has established that the action of oxo-M is probably mediated by muscarinic acetylcholine receptors (mAChRs) on PPR and that oxo-M mimics an afferent-induced long-lasting depolarization called the slow excitatory postsynaptic potential (sEPSP). 2. Action potentials in the ganglion could be blocked by applying tetrodotoxin (TTX) in the bath saline. Under these conditions all excitatory postsynaptic potentials in PPR were also blocked, but the depolarizing action of oxo-M was unaffected. In the absence of background activity PPR could be voltage clamped using a single-electrode switching clamp to study the currents underlying the response to oxo-M. 3. At a membrane potential of -50 mV, application of oxo-M to the ganglion in the bath saline (3-6 x 10(-7) M) or by brief (20-40 ms) pulses from a micropipette into the neuropil(1 X 10(-5) h(-1)) evoked an apparently inward current called I-ox. The mean peak current change in response to pulses was -0.80 +/- 0.04 nA (n = 48 preparations). 4. The voltage dependence of I-ox was determined by subtracting the current-voltage relationship for PPR in control saline from that during a response to oxo-M. I-ox was maximal near the resting potential of PPR (-45 to -40 mV), decreasing slightly with hyperpolarization and strongly with depolarization. 5. Peak I-ox was directly dependent on the bath Na+ concentration. Complete replacement of Na+ with N-methyl-D-glucamine in the saline blocked I-ox. Changes in the bath K+ concentration (extracellular K+ concentration, [K+](o)) had only a small effect on I-ox. Reducing [Cl-](o) from 140 to 74.5 mM had no significant effect on I-ox during a 15-min exposure. Intracellular injections of Cl- from a KCl-containing electrode also had no measurable effect on I-ox. 6. Changes in the bath Ca2+ concentration above or below 2 mM inhibited I-ox. Furthermore, the divalent cations Ni2+, Co2+ Mg2+, and Ba2+ at millimolar concentrations and the Ca2+ channel blocking agents nifedipine and Cd2+ at micromolar concentrations inhibited I-ox. 7. These results suggest that mAChRs on PPR activate an inward current that is persistent, TTX insensitive, voltage dependent and carried predominantly by Na+. However, the results cannot eliminate the possibility that changes in K+ or Cl- conductances might also be involved. The effects of Ca2+ and Ca2+ channel blocking agents suggest that Ca2+ is also required for activating, maintaining, or controlling the effects of muscarinic stimulation. This novel insect current may underlie the previously described sEPSP and may serve to regulate the excitability of PPR in response to afferent stimulation.
