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Nine tachykinin-related peptides (TRPs) have been isolated from the brain and intestine of the cockroach Leucophaea maderae. In the present investigation, two of the nine TRPs, LemTRP 1 and 5, were tested for their ability to stimulate contractions in the foregut of the cockroaches L. maderae and Periplaneta americana in vitro. The two LemTRPs and the related locust peptide locustatachykinin I (LomTK I) induced contractions in the foregut of P. americana in a dose-dependent manner, but had no myostimulatory action in L. maderae. A half-maximal response for the LemTRPs and LomTK I was obtained at 5x10(-9)mol l-1. In both species, the neuropeptide proctolin stimulated foregut contractions. Using an antiserum to LomTK I, we demonstrated that in both species there are LomTK-like-immunoreactive (LomTK-LI) cell bodies and fibers within the ganglia and nerves of the stomatogastric nervous system. However, correlated with the species-specific action of the TRPs, we found efferent LomTK-LI nerve fibers supplying muscle fibers in the foregut of P. americana, but not in L. maderae. In both cockroach species, there is a rich supply of proctolin-immunoreactive fibers to the foregut muscle. Some of the LomTK-LI fibers supplying the P. americana foregut muscle contain co-localized proctolin immunoreactivity. These fibers appear to be derived from a large cell body in the frontal ganglion which also displayed co-localized immunoreactivities. Since TRP-containing neurons are restricted to the nerves and ganglia of the stomatogastric nervous system both in P. americana and L. maderae, TRPs may be involved in the control of foregut movements in both species, but in P. americana the control may be more complex with the additional peripherally projecting LomTK-LI neurons.