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Intracerebroventricular administration of ET-1 (12.5 and 25 pmol/rat) and ET-3 (25 and 50 pmol/rat) induced the loss of the righting reflex and barrel-rolling behaviour in rats. The C-terminal hexapeptide ET(16–21) did not produce these effects. The automated recording of motor activity revealed that, unlike ET(16–21), both ET-1 and ET-3 evoke a dose-regulated increase of the immobility time. Pretreatment with peptidase inhibitors did not modify the activity of subthreshold doses of peptides. Since ET(16–21) displays full agonist properties at ETb but does not interact with ETa receptors, the present data suggest that the behavioural syndrome induced by centrally administered ET-1 and ET-3 may be mediated by stimulation of ETa receptors.

We used in vivo, in vitro studies and immunohistochemistry to elucidate the mechanisms activated by tachykinin NK 1 receptors in evoking inhibitory motor response in the guinea-pig small intestine. In vivo, the selective NK 1 receptor agonist GR 73,632 produced a dose-dependent suppression of the distension-induced duodenal contractions, and a decrease of basal tone. These effects were reduced by pretreatment with the NK 1 receptor antagonist SR 140,333. In L-Nω-nitro-L-arginine methylesther hydrochloride-pretreated animals, the suppressant effect of GR 73,632 on duodenal contractions was reduced, whereas the relaxation of the basal tone was unaffected. In vitro, GR 73,632 evoked a biphasic response consisting of a transient, tetrodotoxin-sensitive inhibitory effect followed by tetrodotoxin-resistant contractions. SR 140,333 blocked both inhibitory and excitatory motor responses induced by GR 73,632. NK 1 immunoreactivity was localized to myenteric and submucosal neurons and to interstitial cells of Cajal in the deep muscular plexus of the small intestine. NK 1 receptor-expressing neurons had Dogiel type I morphology and many of them were β-nicotinamide adenine phosphate dinucleotide-diaphorase-positive, indicating they are inhibitory neurons. In conclusion, in the guinea-pig small intestine, NK 1 receptor stimulation evokes a myogenic excitatory motor response and a neurogenic inhibitory motor response that involves, at least in part, a nitrinergic pathway.

The existence of a septide-sensitive subtype of the tachykinin NK 1 receptor has been recently proposed. In the rat isolated urinary bladder, the non-peptide NK 1 receptor antagonist RP 67,580 exhibits a higher affinity towards septide (pK B 7.57) than towards [Sar 9 ]substance P sulfone (pK B 7.00). In this study we have investigated the pharmacological profile of the non-mammalian tachykinin physalaemin, of the synthetic NK 1 receptor agonist GR 73,632 (δ-aminovaleryl[LPro 9 , NMeLeu 10 ]substance P(7–11)) and of [Glu(OBzl) 11 ]substance P in relation to the putative existence of a septide-sensitive receptor. The activity of [Glu(OBzl) 11 ]substance P at the NK 1 , NK 2 and NK 3 receptor was assayed in the guinea-pig ileum NK 1 receptor assay (EC 50 26 nM), in the rabbit pulmonary artery NK 2 receptor assay (weak agonist activity) and in the rat portal vein NK 3 receptor assays (no appreciable activity up to 1 μM). GR 73,632, [Glu(OBzl) 11 ]substance P and physalaemin, all produced concentration-dependent contractions of the rat isolated urinary bladder, with EC 50 values of 17, 79, and 9 nM, respectively. The responses to the three agonists were very slightly or not modified by the NK 2 receptor antagonist SR 48,968 (1 μM). RP 67,580 (0.3-3 μM) produced a concentration-dependent rightward shift of the curve to GR 73,632, [Glu(OBzl) 11 ]substance P and physalaemin without producing depression of their maximal response. Schild plot analysis indicated the competitive nature of the antagonism. The affinity (pK B ) of RP 67,580 towards physalaemin, GR 73,632 and [Glu(OBzl) 11 ]substance P was 7.12, 7.56 and 7.95, respectively. These findings indicate that the C-terminal derivative of substance P GR 73,632 and the undecapeptide [Glu(OBzl) 11 ]substance P preferentially stimulate the septide-sensitive NK 1 receptor in the rat urinary bladder, suggesting that the N-terminal sequence of substance P does not necessarily contribute to a preferential interaction with the ‘classical NK 1 ' receptor. [Glu(OBzl) 11 ]substance P appears as a better ligand than septide itself at the septide-sensitive NK 1 receptor.

The ability of the selective tachykinin NK1 and NK2 receptor agonists, [Sar9] substance P (SP) sulfone and [beta Ala8] neurokinin A (NKA), respectively, and neurokinin B (NKB) to stimulate urinary bladder contractions was determined in urethane-anaesthetized rats with intact bladder innervation and in animals with acute, bilateral ablation of pelvic ganglia. In addition, tachykinin receptors mediating the response to the agonists were characterized by means of the non-peptide NK1 and NK2 receptor selective antagonists, RP 67,580 and SR 48,968 respectively. In both experimental conditions (normal and ganglionectomized), the three tachykinin agonists induced a dose-dependent increase in intravesical pressure, however reflex bladder contractions were produced by the agonists only in animals with intact bladder innervation. RP 67,580 (10 mumol/kg, i.v.) reduced the response to [Sar9]SP sulfone (50 pmol/rat) in both preparations without modifying the effects induced by the NK2 receptor agonist. On the other hand, SR 48,968 (1 mumol/kg, i.v.) antagonized responses induced by [beta Ala8] NKA (50 pmol/rat) but not those evoked by [Sar9] SP sulfone. In animals with intact urinary bladder innervation, the effect of NKB (50 pmol/rat) was inhibited by SR 48,968 (1 mumol/kg, i.v.) but not by RP 67,580; on the contrary, in rats with ablation of pelvic ganglia, the direct bladder contraction induced by NKB was reduced by RP 67,580 (10 mumol/kg, i.v.) but not by SR 48,968. We conclude that NKB induces reflex and direct bladder muscle contractions by stimulating NK2 and NK1 receptors, respectively.