Evidence that substance P does not mediate slow synaptic excitation within the myenteric plexus

ELECTRICAL stimulation of presynaptic fibres to the so-called AH1
or type II2 myenteric neurones in guinea pig small intestine evokes
a slow excitatory postsynaptic potential (e.p.s.p.) characterised
by long-lasting depolarisation associated with increased membrane
resistance and augmented excitability3. Two substances have been
implicated as possible neurotrans-mitters for the slow e.p.s.p.
Katayama and North reported that application of substance P to
myenteric neurones mimicked the slow e.p.s.p.4, and J.D.W. and
C.J.M. presented several lines of evidence for serotonin as the
transmitter substance5,6. We now report that methysergide, a drug
which abolishes both the slow e.p.s.p. and the action of exogenous
serotonin5,6, does not affect the action of substance P on guinea
pig myenteric neurones. The results suggest that substance P is
unlikely to be the neuro-transmitter which mediates the slow
e.p.s.p.