A comparative study of the co-localisation of three different families of
neuropeptides, viz. allatostatins of the Y/FXFGL-NH(2) type,
Manduca sexta allatostatin (Mas-AS) and
allatotropin, in the frontal
ganglion of lepidopteran larvae has been carried out by means of immunocytochemistry and confocal
laser scanning microscopy. The simultaneous application of three types of
fluorochrome-conjugated
antibodies reveals triple co-localisation in an anterodorsal pair of neurones in the frontal
ganglion of the noctuids Heliothis virescens and Lacanobia oleracea. There is no evidence of differential axonal transport, since all parts of these neurones show complete co-localisation of all three
peptides. Prominent axons of the ganglionic neurones project in the recurrent nerve to the foregut and stomodeal valve. Over the crop, lateral and sub-lateral branches follow the course of circular muscle fibres and terminate in varicosities. All three
neuropeptides have previously been shown to be myoregulatory on the foregut; the Y/FXFGL-NH(2) allatostatins and Mas-AS are inhibitory, whereas
allatotropin is excitatory. The morphological evidence of co-localisation of physiologically antagonistic
peptides within the same terminals suggests that an extremely complex mechanism controls the contractile activities of the foregut. A posterodorsal pair of neurones in the frontal
ganglion have prominent axons projecting via the frontal connectives to the brain and in the recurrent nerve to the stomodeal valve where extensive branching suggests control over the valve movements. Studies of another noctuid, Spodoptera frugiperda, and the sphingid, M. sexta, show interesting variations in the co-localisation phenomenon.