Plexus Preparation Of Guinea-pig Ileum Research Articles

Histamine H(3) receptors mediate inhibition of noradrenaline release from intestinal sympathetic nerves.

1. The present study investigates whether presynaptic histamine receptors regulate noradrenaline release from intestinal sympathetic nerves. The experiments were performed on longitudinal muscle-myenteric plexus preparations of guinea-pig ileum, preincubated with [(3)H]-noradrenaline. 2. In the presence of rauwolscine, electrically-induced [(3)H]-noradrenaline release was inhibited by histamine or R-alpha-methylhistamine, whereas it was unaffected by pyridylethylamine, impromidine, pyrilamine, cimetidine, thioperamide or clobenpropit. The inhibitory effects of histamine or R-alpha-methylhistamine were antagonized by thioperamide or clobenpropit, but not by pyrilamine or cimetidine. In the absence of rauwolscine, none of these drugs modified the release of [(3)H]-noradrenaline. 3. The modulatory action of histamine was attenuated by pertussis toxin and abolished by N-ethylmaleimide. Tetraethylammonium or 4-aminopyridine enhanced the evoked tritium outflow and counteracted the inhibitory effect of histamine. However, the blocking effects of tetraethylammonium and 4-aminopyridine were no longer evident when their enhancing actions were compensated by reduction of Ca(2+) concentration in the superfusion medium. 4. Histamine-induced inhibition of tritium output was enhanced by omega-conotoxin or low Ca(2+) concentration, whereas it was not modified by nifedipine, forskolin, rolipram, phorbol myristate acetate, H7 or lavendustin A. 5. The present results indicate that presynaptic H(3) receptors, located on sympathetic nerve endings, mediate an inhibitory control on intestinal noradrenergic neurotransmission. It is suggested that these receptors are coupled to G(i)/G(o) proteins which modulate the activity of N-type Ca(2+) channels through a direct link, thus reducing the availability of extracellular Ca(2+) at the level of noradrenergic nerve terminals.

Antagonistic activities of N-3389, a newly synthetized diazabicyclo derivative, at 5-HT 3 and 5-HT 4 receptors

The antagonistic activities of compound N-3389 ( endo-3,9-dimethyl-3,9-diazabicyclo[3,3,1]non-7-yl 1 H-indazole-3- carboxamide dihydrochlo ride) at 5-HT 3 and 5-HT 4 receptors were examined using in vitro and in vivo assays. N-3389 showed potent 5-HT 3 receptor antagonistic activities in a radioligand binding assay (p K i = 8.77), against 2-methyl-5-HT (2-Me-5-HT)-induced bradycardia in rats (ED 50 = 0.73 μg/kg i.v., 38 μg/kg p.o.) and against 2-Me-5-HT-induced contraction in longitudinal muscle myenteric plexus preparations of guinea-pig ileum (IC 50 = 3.2 × 10 −8 M). As preliminary to investigating the effect of N-3389 on 5-HT 4 receptors, we examined the contraction induced by 5-HT in guinea-pig ileum preparations. We confirmed that 5-HT (10 −8 − 10 −5 M) induced biphasic contractions in the preparations.Furthermore, 5-HT 3 receptor antagonism inhibited the late phase of the contraction induced by high concentrations of 5-HT (3 × 10 −6 − 10 −5 M), whereas 5-HT 4 receptor antagonism inhibited the early phase of the contraction induced by low concentrations of 5-HT (10 −8 − 10 −6 M). N-3389 (10 −7 − 10 −5 M) inhibited both phases of contraction induced by 5-HT. In addition, N-3389 (3 × 10 −7 −3 × 10 −6 M) was found to inhibit the increase of electrically stimulated twitch responses induced by 5-HT (10 −8 M) in longitudinal muscle myenteric plexus preparation of the guinea-pig ileum. These results suggest that N-3389 acts as a 5-HT 3 and 5-HT 4 receptor antagonist.

Inhibitory effects of opioids in a circular muscle-myenteric plexus preparation of guinea-pig ileum.

The actions of opioids were examined in a strip preparation of the external muscle and myenteric plexus of the guinea-pig ileum cut parallel to the circular muscle. Contractions of the circular muscle induced by electrical stimulation of myenteric neurons were depressed in a concentration-dependent manner by the mu agonists, morphine and DAGO, and by the kappa agonist, U-50,488H. The concentrations of morphine, DAGO and U-50,488H which depressed nerve-mediated contractions by 50% (IC50) were 86 nM, 11 nM and 5.0 nM, respectively. The equilibrium dissociation constants (KD) for naloxone as an antagonist of the inhibitory effects of DAGO and of U-50,488H were 5.6 nM and 29.4 nM, respectively. In contrast to the potent inhibitory effects of mu and kappa agonists, the delta-selective agonist, D-Pen-L-Pen, produced only weak inhibition of nerve-mediated contractions. Even at a concentration of 3 microM, there was less than 50% inhibition, which was not antagonised by the delta receptor antagonist, ICI 174864. The experiments indicate that both mu and kappa opioid receptors are present on the myenteric neurons supplying the circular muscle and that delta receptors are either absent or ineffectively activated.

Neurotensin receptors on the ileum of the guinea-pig: evidence for the coexistence of inhibitory and excitatory receptors.

Neurotensin caused a complex muscular response of the longitudinal muscle-myenteric plexus preparation of guinea-pig ileum: picomoles of neurotensin produced inhibition while larger concentrations caused an inhibitory effect followed by a delayed dose-dependent contraction. The inhibitory phase of the neurotensin-induced muscular activity was not modified by tetrodotoxin but was potently antagonized in a non-competitive manner by apamin, a bee venom toxin. The contractile component was blocked by tetrodotoxin but not by apamin. These toxins were used to dissect the neurotensin muscular response into an inhibitory phase and an excitatory component. It was possible to further characterize the two neurotensin muscular components by their kinetics of desensitization. The inhibitory neurotensin response showed a fast rate of desensitization and presented a relatively low spare receptor capacity. In contrast, desensitization to the excitatory action of neurotensin was much slower, the excitatory receptors apparently having a larger spare receptor capacity. Desensitization to the action of neurotensin was selective for the neuropeptide not altering the contractile activity of substance P, angiotensin II, bradykinin, histamine or acetylcholine. These results strongly suggest the presence of two subsets of neurotensin receptors in the ileum: the inhibitory set probably localized at the postsynaptic effector level and excitatory neurotensin receptors probably of neuronal origin whose function is probably to modulate the release of neurotransmitters. The physiological implications of these two subtypes of neurotensin receptors in the control of gastrointestinal motility are discussed.