Actions of substance P on central neurones and peripheral tissues, and co-existence of 5-hydroxytryptamine and a substance p-like peptide in the heart of the snail Helix aspersa.
The actions of substance P (SP) were tested on central neurones of the snail Helix aspersa, and on the in vitro heart and pharyngeal retractor muscle (PRM). SP was found to show very limited effects in the CNS, but did produce consistent, though weak, inhibition of a group of cells in the right pedal-cerebral connective. On the isolated beating heart 5-hydroxytryptamine (5-HT) produced short latency cardioexcitation, whereas long latency excitation was produced by SP. Electrically-induced contractions of the PRM were found to be inhibited by SP and augmented by the molluscan SP analogue eledoisin, suggesting the presence of separate receptor types in this tissue. Double immunohistochemical staining of sections through the central ganglia for 5-HT and SP revealed that the 2 compounds usually occur in separate neurones, although a few examples showing coexistence were also observed. In the heart, however, all the nerve endings visible were labelled by both antibodies suggesting that two cardioexcitatory compounds, 5-HT and a SP-like peptide, occur together in the same nerve endings in the Helix heart. An assay to measure cAMP levels in homogenized tissue showed that 5-HT and dopamine stimulated cAMP production, whereas production was inhibited by SP, indicating that the actions of cAMP may be mediated via an adenylate cyclase/cAMP system.
Boyd
P J
PJ
Department of Neurophysiology, School of Biochemical and Physiological Sciences, University of Southampton, Southampton SO9 3TU England.
Walker
R J
RJ
Osborne
N N
NN