Effects of Nicotinic and Muscarinic Agents on an Identified Motoneurone and its Direct Afferent Inputs in Larval Manduca Sexta

ABSTRACT The principal locomotory appendages of larval Manduca sexta, the prolegs, bear at their tips an array of mechanosensory hairs (the planta hairs). Each of the single sensory neurones associated with a planta hair sends an axon into the ganglion of the same segment where the afferent terminals make synaptic contact with interneurones and motoneurones. Electrical stimulation of a single afferent elicits a monosynaptic excitatory postsynaptic potential (EPSP) in PPR, one of the motoneurones controlling the prolegs. We have used this synapse to study the pharmacology of sensory transmission in M. sexta. The following observations were made. The EPSP was reversibly inhibited in a dose-dependent fashion by the cholinergic antagonists d-tubocurarine, atropine and mecamylamine, indicating that the planta hair afferent neurones use acetylcholine (ACh) as a neurotransmitter. α-Bungarotoxin (αBGTX) also suppressed the EPSP but required concentrations above l×10−6moll-1. PPR depolarized in response to ionophoretic or bath application of cholinergic agonists, but compared to motoneurones of a non-nicotine-resistant insect such as the cockroach, PPR was relatively insensitive to nicotine. Application of N-methyl nicotinamide (NMN) to the nerve cord to inhibit putative alkaloid pumps in the central nervous system (CNS) only weakly potentiated PPR’s response to nicotine. This suggests that such pumps do not markedly contribute to PPR’s nicotine resistance. PPR’s responses to nicotinic agents showed several pharmacological differences from those reported for other insects, indicating that the nicotinic ACh receptors of M. sexta may be specifically adapted to accommodate a nicotine-rich diet. During the application of muscarinic agonists to isolated abdominal ganglia, the firing rate of motoneurones, as monitored in the ventral nerve, increased dramatically. PPR responded to muscarinic agents even during synaptic blockade, suggesting that muscarinic receptors may be present on PPR itself. The main effect of muscarine on PPR was to lower its spike threshold. Bath-applied muscarinic agents also affected the afferent-evoked EPSP in a manner consistent with the presence of another group of receptors that, when stimulated, act presynaptically to inhibit the release of ACh from the sensory terminals. These apparent pre- and postsynaptic actions of muscarinic agents are the first reported findings of muscarinic physiology in an identified insect neurone and its synaptic inputs.