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Distribution and effects of PACAP, VIP, nitric oxide and GABA in the gut of the African clawed frog Xenopus laevis
Department of Zoophysiology, Göteborg University, Box 463, S-405 30 Göteborg, Sweden
e-mail: c.olsson{at}zool.gu.se
Accepted 28 January 2002
The distribution and possible effects on gastrointestinal motility of
pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive
intestinal polypeptide (VIP), nitric oxide and 
-amino-butyric acid
(GABA) were investigated in the African clawed frog (Xenopus laevis)
using immunohistochemistry and in vitro strip preparations. PACAP-
and VIP-immunoreactive nerve fibres were common in the myenteric plexus as
well as in the longitudinal and circular muscle layers all along the
gastrointestinal tract. Double labelling demonstrated a close correlation
between PACAP and VIP immunoreactivities, indicating that the two
neurotransmitters are colocalised within the enteric nervous system.
Occasionally, PACAP- and VIP-positive nerve cell bodies were seen in the
myenteric or submucous plexa. In addition, VIP immunoreactivity coexisted with
helospectin immunoreactivity. Nitric oxide synthase (NOS)-immunoreactive nerve
cells were found in the myenteric plexus at an average density for the whole
gastrointestinal tract of 4584±540 cells cm-2. The
NOS-immunoreactive nerve cells were usually multipolar with an average size of
11.3±3.7 x 23.2±6.6 µm. Some NOS-immunoreactive nerve
fibres were VIP-immunoreactive but not all VIP-positive fibres showed NOS
immunoreactivity. GABA immunoreactivity was found in nerve fibres and nerve
cells in the myenteric plexus of all regions of the gut. Few
GABA-immunoreactive nerve fibres were VIP-immunoreactive. PACAP 27, VIP,
sodium nitroprusside (a nitric oxide donor; NaNP) and GABA caused similar
responses on spontaneously contracting circular preparations of the cardiac
stomach of X. laevis. The mean force developed was decreased, mainly
by a reduction in resting tension, while the amplitude of contractions was not
necessarily affected. The NOS inhibitor NG-nitro-L-arginine methyl
ester (L-NAME) increased the mean force developed, indicating a nitrergic tone
in the preparations. In contrast, PACAP 27, VIP, NaNP, GABA and L-NAME had no
significant effect on longitudinal strip preparations from the duodenum. These
results indicate that PACAP, VIP, nitric oxide and GABA, which are known to be
important inhibitory neurotransmitters in other vertebrates, are widely spread
in the enteric nervous system of Xenopus laevis and may be involved
in the inhibitory control of gastric motility. Although no effect of PACAP,
VIP, nitric oxide or GABA on the longitudinal strips of the duodenum was seen
in this study, this does not rule out the possibility that they might play an
important role in controlling intestinal motility as well.
Key words: amphibian, immunohistochemistry, gut motility, enteric nervous system, VIP, PACAP, nitric oxide, GABA, Xenopus laevis
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