Neuropeptides in insect sensory neurones: tachykinin-, FMRFamide- and allatotropin-related peptides in terminals of locust thoracic sensory afferents

Sensory afferents in the thoracic ganglia of the locust Locusta migratoria were labelled with antisera to different neuropeptides: locustatachykinins, FMRFamide and allatotropin. The locustatachykinin-immunoreactive (LTKIR) s ensory fibres were derived from the legs and entered the ventral sensory ne uropil of each of the thoracic ganglia via nerve 5. In the thoracic neuropi l, the LTKIR sensory fibres formed a distinct plexus of terminations ventra lly in the ipsilateral hemisphere. The peripheral cell bodies of the sensor y neurones could not be revealed, but lesion experiments indicated that ori gin of the LTKIR fibres was the tarsus of each leg. Possibly the thin fibre s are from tarsal chemoreceptors. Double labelling immunocytochemistry reve aled that all the LTKIR sensory fibres contained colocalized FMRFamide immu noreactivity. A larger population of sensory fibres reacted with antiserum to moth (Manduca sexta) allatotropin. By means of double labelling immunocy tochemistry, we could show that the LTKIR fibres constituted a subpopulatio n of the larger set of allatotropin-like immunoreactive fibres. Thus some s ensory fibres may contain colocalized peptides related to locustatachykinin s, FMRFamide-related peptide(s) and allatotropin-like peptide. A separate n on-overlapping small set of sensory fibres in nerve 5 reacted with an antis erum to serotonin. Sensory fibres of the other nerves of the ventral nerve cord, including the abdominal ganglia, did not react with the peptide antis era. Since acetylcholine is the likely primary neurotransmitter of insect s ensory fibres, it is possible that the peptides and serotonin are colocaliz ed with this transmitter and serve modulatory functions in a subset of the leg afferents. (C) 1999 Elsevier Science B.V. All rights reserved.