Nanomolar concentrations of
neurotensin caused a dose-dependent contraction of the longitudinal muscle layer of the guinea-pig ileum. The contractile activity of
neurotensin was partially blocked by
tetrodotoxin or
atropine, indicating that a component of the
neurotensin-mediated contraction is indirect in nature and likely involves the release of endogenous
acetylcholine from nervous terminals in the myenteric plexus.
Dynorphin and related
peptide fragments also blocked in part the
neurotensin contraction; the potency of this
opioid peptide was about the same as that of
atropine. Other
peptides and
alkaloids tested for ability to block the
neurotensin contractures included the
enkephalins,
beta-endorphin,
normorphine and the ketocyclazocines; all these
opioids inhibited in a dose-dependent fashion the neuronal component of the excitatory effect of
neurotensin. The potency of these compounds to reduce the contractions of
neurotensin showed good correlation with the potency of these agents to depress by 50% the electrically evoked neuromuscular twitches in the same tissue (r = 0.99); in these tests
dynorphin was found to be the most potent of the endogenous
opioid-like
peptides. The
dynorphin blockade was selective to the excitatory effect of
neurotensin because the
opioid peptide did not antagonize the contractile action of
acetylcholine,
histamine,
substance P,
angiotensin II,
bradykinin, Ba++ or K+
ions. In addition,
somatostatin,
vasointestinal peptide,
gastrin or
adenosine did not modify the potency of
neurotensin whereas
thyrotropin releasing hormone and
epinephrine caused a modest doubling of the
neurotensin EC50. The inhibitory action of
dynorphin was reduced in the presence of
naloxone, suggesting that the interaction involved
opiate receptors.
Morphine tolerance was not extended to the inhibitory action of
dynorphin as evidenced by the finding that the potency of dynorphin-(1-13) to block the
neurotensin responses was increased after chronic
morphine exposure. In contrast, the potency of dynorphin-(1-13) was significantly reduced in tissues rendered tolerant to the action of
ketocyclazocine or
ethylketocyclazocine, suggesting that the action of
dynorphin could be partially mediated via occupation of K-
opiate receptors. Thus, a
cholinergic-neuronal component activated by
neurotensin on the myenteric plexus appears to be under the inhibitory influence of
opiate receptors, suggesting that
dynorphin may play a role in the modulation of
cholinergic synapses on the enteric nervous system.