MUSCARINIC ACETYLCHOLINE-RECEPTORS MODULATE THE EXCITABILITY OF AN IDENTIFIED INSECT MOTONEURON

1. With the use of an isolated, perfused ganglion preparation from the tobacco hornworm, Manduca sexta, we have examined the responses of an identified proleg retractor motoneuron (designated PPR) to trains of stimuli delivered to sensory branches of the ventral nerve (VN). 2. Tr ains of stimuli (50 Hz, 100 ms to 5 s) delivered to the proleg sensory nerve, VNA, caused PPR to depolarize and initiate a bout of spiking a ctivity that outlasted the stimulus. A fast component of this response was due to monosynaptic input from planta hair sensory neurons, actin g on nicotinic acetylcholine receptors (nAChRs). The fast response to VNA stimulation was abolished when the ganglion was treated with the n icotinic antagonist, mecamylamine, leaving a slow, long-lasting depola rization of PPR, which we have called the slow excitatory postsynaptic potential (sEPSP). 3. A sEPSP could be evoked by stimulation of all t he major subbranches of VNA ipsilateral to PPR's cell body. Small sEPS Ps were also evoked by stimulation of all but one of the major contral ateral subbranches of VNA. 4. During a sEPSP the spike threshold of PP R became more negative. This increase in excitability was not correlat ed with changes in membrane potential measured at the cell body, and t here was no detectable change in input resistance. We conclude that th e spike-threshold change reflects either a depolarization electrically remote from the cell body, or a change in PPR's spike initiation prop erties that are not reflected in the membrane potential. 5. Both the s EPSP and the associated change in PPR's spike threshold were blocked b y several muscarinic antagonists. Scopolamine was effective at concent rations > 2 x 10(-7) M, atropine at concentrations > 1 x 10(-6) M, and pirenzepine at 5 x 10(-5) M. 4-Diphenylacetoxy-N-methylpiperidine (4- DAMP), methoctramine, hexahydrosiladifenidol, and AF-DX 116 were all p oor antagonists. 6. Bath application of the muscarinic agonist oxotrem orine-M (oxo-M) at concentrations >3 x 10(-7) M increased the spontane ous spiking activity of PPR and other proleg motoneurons. In PPR, this increased activity was accompanied by a small depolarization and a mo re negative spike threshold, both of which were inhibited by 1 X 10(-7 ) M scopolamine. 7. At concentrations > 6 x 10(-1) M, bath-applied oxo -M depolarized PPR even when spike activity in the ganglion was blocke d with tetrodotoxin. During such spike blockage, pressure ejection of brief puffs of oxo-M into the neuropil evoked a long-lasting depolariz ation of PPR that resembled the sEPSP. This depolarization was insensi tive to mecamylamine but could be inhibited by scopolamine and pirenze pine. 8. These results suggest that, in addition to the activation of postsynaptic nAChRs, proleg sensory neurons can also modulate the exci tability of motoneurons through a separate pathway with muscarinic pha rmacology. The muscarinic receptors (mAChRs) in this pathway are proba bly present on PPR itself. This system provides an opportunity to stud y the functional role of mAChRs in insects and the modulation of neuro n excitability.