A. Postorino

Evidence against purines being neurotransmitters of non-adrenergic, non-cholinergic nerves in rat duodenum

The possible involvement of purines in the non-adrenergic non-cholinergic (NANC) relaxation of rat duodenum was studied using an isometric-isovolumic preparation. Purines (adenosine, AMP, ADP, ATP) induced a concentration-dependent, tetrodotoxin (TTX)-insensitive, fall in both endoluminal pressure and isometric tension. The relaxation induced by adenosine and by 2-chloroadenosine was selectively antagonized by 8-phenyltheophylline (1, 10 nM, 0.5 microM) and the ATP-induced relaxation was opposed by alpha, beta-methylene ATP (10 microM) and by reactive blue 2 (10 microM). Electrical field stimulation (EFS) caused TTX-sensitive inhibitory effects similar to those induced by ATP. None of the purinergic antagonists used were capable of affecting the EFS-induced relaxation. Our results indicate that both P1 and P2 purinoreceptors are present in muscle of the rat duodenum and are not involved in the NANC relaxation.

Motility pattern of isolated rat proximal colon and excitatory action of neurotensin

The investigation concerned the effects of neurotensin on mechanical activity of isolated rat proximal colon. An isometric-isovolumic preparation was used. Colonic segments showed spontaneous contractile activity, consisting of regular changes in both endoluminal and isometric tension. Neurotensin (1 pM to 0.1 microM) induced a concentration-dependent tonic contraction of both circular and longitudinal muscle accompanied by high frequency oscillatory activity. Desensitization of the neurotensin receptors antagonized the contractile activity of neurotensin. The excitatory effects of neurotensin were partially blocked to the same degree by tetrodotoxin and atropine, indicating that a component of the neurotensin-mediated contraction involves the release of endogenous acetylcholine. The tetrodotoxin-resistant component of the neurotensin-induced effect seems to be due to a direct action on the smooth muscle cells.

Neurotensin: dual effect on the motor activity of rat duodenum

The effects of neurotensin on mechanical activity of rat duodenum were investigated using an isometric-isovolumic preparation. Neurotensin (1 pM to 10 nM) induced a concentration-dependent, tetrodotoxin (TTX)-insensitive fall in both endoluminal pressure and isometric tension. At higher concentrations of neurotensin (1 nM to 1 microM) the relaxation was followed by a concentration-dependent TTX-insensitive contraction, detected only by an increase in endoluminal pressure. Different concentrations of neurotensin were required to desensitize the relaxant and the contractile actions of the neuropeptide. The relaxation was antagonized by apamin, while the contractile response was blocked by nifedipine. Neurotensin, when tested separately on longitudinal and circular muscular strips, caused relaxation of the longitudinal strips. Circular strips showed contractions in response to neurotensin, following an inhibitory phase, if the strips were spontaneously or pharmacologically activated. The results suggest the presence of two sets of neurotensin receptors with a differential localization between the two muscular layers in rat duodenum.

Tachykinins mediate noncholinergic excitatory neural responses in the circular muscle of rat proximal colon

Using the sucrose-gap technique, we attempted to assess a role for tachykinins (TKs) in mediating noncholinergic excitatory junction potential (EJP) and contraction, in the circular muscle of rat proximal colon. Excitatory responses were evoked by submaximal electrical field stimulation (EFS) in the presence of atropine (1 microM), guanethidine (1 microM), indomethacin (10 microM), and N(omega)-nitro-L-arginine methyl ester (L-NAME) (100 microM). The NK1 receptor antagonist, SR 140,333 (up to 3 microM) or the NK2 receptor antagonists, SR 48,968 and MEN 10,627 (up to 5 microM) produced a partial inhibition of the excitatory responses to EFS. The co-administration of the selective NK1 and NK2 receptor antagonists produced additive effects on the responses to EFS. Selective NK1 receptor agonist, [Sar9, Met (O2)11]-substance P, induced depolarization and contraction, antagonized by SR 140,333, but not by NK2 receptor antagonists. NK2 receptor agonist, [betaAla8]-neurokinin A (4-10), also produced electrical and mechanical excitatory effects that were antagonized by SR 48,968 or MEN 10,627, but not by the NK1 receptor antagonist. Our results provide evidence that, in circular muscle of rat colon, endogenous tachykinins are the main excitatory transmitters for nonadrenergic, noncholinergic (NANC) excitation and their action is mediated by both NK1 and NK2 receptors.

Antagonism by SR 48692 of mechanical responses to neurotensin in rat intestine

1 The effects of SR 48692 on neurotensin (NT)‐induced mechanical responses were investigated in rat duodenum and proximal colon by use of isometric, isovolumic preparations. 2 SR 48692 inhibited the relaxant responses to NT in duodenal circular and longitudinal muscle. It also antagonized the NT‐induced contractile effects in duodenal circular muscle and in proximal colon (both muscular layers). 3 From Schild analysis the pA2 value for SR 48692 was 8.2 in tissues where NT induced relaxant effects and 7.5 in tissues where NT induced contractile effects and the slope of the regression line was not significantly different from unity, indicating competitive antagonism. 4 SR 48692 did not antagonize the duodenal relaxant effect induced by noradrenaline and the contractile response to carbachol or substance P in duodenum and colon. 5 Our results demonstrate that SR 48692 selectively antagonizes the mechanical actions of NT in rat intestine and confirm the existence of specific NT receptors. Receptors that subserve a relaxant effect seem to be related, but not identical, to those that mediate contractile effects.

On the purinergic system in rat duodenum: existence of P1 and P2 receptors on the smooth muscle.

In rat duodenum, in vitro, in the presence of atropine and guanethidine, ATP administration caused a tetrodotoxin-insensitive relaxation followed by a rebound contraction. A similar response was obtained also after electrical field stimulation (EFS) of non-adrenergic, non-cholinergic (NANC) nerves. alpha, beta-methylene-TP and theophylline antagonized the response to ATP, but they failed to affect the noradrenaline- and EFS-induced relaxation. These results suggest that P1 and P2 receptors are present in rat duodenum, but their activation is not responsible for the inhibitor effects due to the NANC nerves.

Electrical stimulation of glossopharyngeal nerve and oesophageal EMG response in the pigeon

The effects of the efferent glossopharyngeal nerve stimulation, on EMG activity of the pigeon cervical oesophagus, were studied. In control animals, stimulation caused a biphasic response characterized by an intra-stimulus excitatory component followed by a post-stimulus inhibitory one. The EMG response to glossopharyngeal stimulation appeared simultaneously throughout the cervical oesophagus. A bell-shaped mechanical wave was detected relating to the electrical excitatory component. Atropine administration antagonized the excitatory component, while the inhibitory one persisted. It occurs intra-stimulus, and its duration is increased, compared to control ones. A reduction in the oesophageal resting pressure was observed relating to the electrical inhibitory component. Hexamethonium caused complete disappearance of any EMG response to glossopharyngeal stimulation, as well as suppression of mechanical responses. The comparison between the EMG responses to swallow and to efferent glossopharyngeal stimulation suggests that in pigeon cervical oesophagus: primary peristalsis is central in origin; a dual system of glossopharyngeal fibres, excitatory and inhibitory, carries the central control for oesophageal motility; these excitatory and inhibitory fibres supply the oesophageal muscle via intramural neurons; the synaptic arrangement of the inhibitory pathway is more complex than the excitatory one.

Evidence that adenosine is not involved in the non-adrenergic non-cholinergic relaxation in the rat duodenum

In rat isolated duodenal segments, adenosine induced, in the presence of atropine and guanethidine, a dose-dependent, long-lasting (about 20 s), tetrodotoxin (TTX)-resistant relaxation both in endoluminal pressure and in isometric tension. Electrical field stimulation (EFS) induced, in the presence of atropine and guanethidine, a TTX-sensitive short-lasting (about 6 s) relaxation followed by a sustained rebound contraction. Theophylline, a P1 receptor antagonist, at the concentration of 100 microM caused a marked inhibition of the adenosine-induced relaxation, while the EFS-induced relaxation was not modified. Our results suggest that adenosine induces relaxation of the rat duodenal smooth muscle acting on P1 receptors localized at muscular level. However, differences in the morphology and in the sensitivity to theophylline between adenosine- and EFS-induced relaxation ruled out adenosine as neurotransmitter of the non-adrenergic, non-cholinergic inhibitory system.

Excitatory effects of opiates on the spontaneous EMG activity in pigeon oesophagus

The effects and the mechanism of action of morphine, methionine-enkephalin and leucine-enkephalin were examined in transverse muscular strips from pigeon oesophagus. All the opiates produced a concentration-dependent excitatory effect on the spontaneous EMG activity, characterized mainly by an increase in the spike burst frequency. The maximal excitatory response to morphine and opioid peptides was fully antagonized by naloxone and tetrodotoxin, significantly reduced by atropine and it was not affected by guanethidine pretreatment. Treatment of pigeons with reserpine abolished the excitatory effects induced by opiates. The above results suggest the existence of specific opioid receptors in pigeon oesophagus. Opiates have no direct action on smooth muscle cells, increasing the EMG activity via excitatory both cholinergic and non-adrenergic non-cholinergic neurons. The hypothesis of a possible involvement of serotonergic interneurons might be advanced.

Evidence for extrinsic control of oesophageal primary peristalsis

The rôle of both peripheral and central mechanism in the control of primary peristalsis was studied in pigeon cervical oesophagus. The results from the transection of oesophageal muscular wall and of extrinsic nerves suggest that: primary peristalsis is programmed centrally. extrinsic motor input is carried in glossopharyngeal nerves and distributed separately at each oesophageal level through intramural neurons. intramural neurons do not seem capable of propagating the peristaltic sequence irrespective of the central control.