Local application of neurotensin to abdominal organs triggers cardiovascular reflexes in guinea pigs: possible mechanisms

Non-steroidal anti-inflammatory drugs and spinal nociceptive processing

Non-steroidal anti-inflammatory drugs have a direct action on spinal nociceptive processing in vivo with a relative order of potency which correlates with their capacity as inhibitors of cyclooxygenase activity. However, recent clinical surveys and new in vivo evidence strongly suggest that for some of these agents, centrally mediated analgesia may also be achieved by additional mechanisms, which are independent of prostaglandin synthesis inhibition. In this review we explore the likelihood for such mechanisms following an extensive survey of existing data. The implications of these mechanisms are discussed in the light of our current understanding of spinal nociceptive processing.

Cardiovascular and abdominal motor responses evoked by intravenous neurotensin in guinea pigs

The intravenous (IV) infusion of neurotensin (NT) in anesthetized guinea pigs was found to elicit dose-dependent increases of systemic blood pressure (BP) and of heart rate (HR), accompanied by abdominal motor responses consisting in transient, twitch-like contractions of the abdominal wall (TAWC), and a slowly developing, relatively sustained increase of the basal abdominal wall tension (AWT). The TAWC responses were inhibited in animals pretreated with pancuronium, morphine, clonidine, and CP-96,345 [a neurokinin (NK) antagonist], were potentiated by naloxone, but were not modified by atropine or prior (30 s) intraperitoneal (IP) injection of lidocaine. The BP increases caused by IV NT were reduced by clonidine and by IP lidocaine only. The HR increases were attenuated by morphine and clonidine only. Increases of the basal AWT were resistant to all drug treatments and were attributed to a passive stretch of the abdominal wall caused by cecal distension. No defecation was observed in any of the animals given IV NT. These results were interpreted as an indication that the pressor and TAWC responses to IV NT represent an integrated nociceptive response likely to be triggered in part by NT-induced activation of abdominal visceral afferents. A NK acting through NK-1 receptors may participate in TAWC responses to IV NT.

Behavioral responses elicited by intraperitoneal neurotensin in guinea pigs

The potential behavioral responses to single intraperitoneal (IP) injection of neurotensin (NT) were examined in conscious, freely moving guinea pigs. Most animals injected with NT solvent (i.e., controls) or NT solution with low NT concentration (i.e., 5.4 nM) exhibited little or no particular behavioral reaction. On the other hand, 50 to 75% of the animals given IP injection of NT with 54, 540, or 5400 nM of NT exhibited motor responses characterized by a transient episode of clonic body jerks. Vocalization responses were observed in less than 25% of the animals given IP NT. Animal pretreatment with morphine reduced the incidence of motor and vocalization responses. The results were interpreted as an indication that IP injection of moderately concentrated NT solution (i.e., greater than or equal to 54 nM) is probably a weak noxious stimulus in conscious guinea pigs.

Haemodynamic and abdominal motor reflexes elicited by neurotensin in anaesthetized guinea-pigs

1. Single intraperitoneal (i.p.) injections of neurotensin (NT) (0.14- 140 nmol kg-1) in anaesthetized guinea-pigs were found to trigger transient abdominal wall contractions (TAWC) accompanied by relatively sustained increases of systemic blood pressure (BP) and heart rate (HR). The modification of the latter NT effects by various drugs and surgical manipulations was examined to obtain some insight into the nature of, and possible relationship between, these responses. 2. The abdominal motor response (i.e. TAWC) to i.p. NT (14 nmol kg-1) was inhibited by prior i.v. injection of the guinea-pigs with pancuronium (0.27 mumol kg-1), morphine (1.5 and 15 mumol kg-1), clonidine (0.34 mumol kg-1), by concomitant i.p. injection of procaine 2% w/v, or by acute spinalization. It was potentiated by naloxone (2.8 and 28 mumol kg-1), but not affected by i.v. injection of autonomic drugs (i.e. pentolinium, prazosin, yohimbine and atropine), by capsaicin desensitization, or by acute bilateral cervical vagotomy. In spinalized animals a sustained abdominal wall contraction (SAWC) was unmasked, which was resistant to i.v. morphine, clonidine or baclofen but suppressed by i.v. pancuronium or i.p. lignocaine 2% w/v. 3. Haemodynamic responses to i.p. NT were not affected by i.v. pancuronium, morphine, naloxone, atropine, or by vagotomy. They were inhibited by i.v. pentolinium or clonidine (BP, HR), i.v. prazosin (BP), i.p. procaine 2% w/v (BP, HR), capsaicin desensitization or acute spinalization (BP, HR). Yohimbine (i.v.) potentiated BP and HR increases caused by i.p. NT.4. These results suggest that: (1) the haemodynamic and TAWC responses to i.p. NT in this animal model, are two independent, neurally-mediated reflexes which are likely to originate from the abdominal cavity and require a functionally intact spinal cord for their full expression; (2) the neural pathways subserving both types of responses appear to be different from each other. The nature and time-response characteristics of the reflexes caused by i.p. NT, raise the possibility that i.p. NT is a noxious stimulus, at least in guinea-pigs.

Effect of initial blood pressure level on cardiovascular reflexes caused by intraperitoneal neurotensin

The reflex changes of systemic blood pressure (BP) and heart rate (HR) induced by intraperitoneal (IP) injections of neurotensin-containing solutions (NTCS) were measured in anesthetized guinea pigs whose BP was raised up by intravenous (IV) infusions of either angiotensin II (ATII) or noradrenaline (NA), and compared to those observed in control animals (i.e., saline-infused guinea pigs with low initial BP level). The amplitudes of reflex pressor and tachycardic responses to IP NTCS were either not affected or reduced in animals whose BP was raised with ATII or NA. However, no hypotensive effect was observed, following IP NTCS, in any of the animals tested. The results indicate that high initial BP levels tend to attenuate the reflex increases of BP and HR caused by IP NTCS, the level of inhibition being largely determined by the type of vasopressor agent utilized to raise up the BP. They also suggest that the initial BP level is unlikely to be an important factor in determining whether the activation of capsaicin-sensitive visceral afferents will produce hypotensive or hypertensive effects in this animal model.

Further observations on the mechanism of the cardiovascular reflexes caused by exposure of the peritoneum to neurotensin

Intraperitoneal (IP) injections of either 1, 3 or 9 ml of neurotensin-containing solutions (NTCS) with 5.4, 54, 540 or 5400 nM of neurotensin (NT) were found to cause concentration-dependent, but volume-independent, increases of blood pressure (BP) and heart rate (HR) in anesthetized, close-abdomen guinea pigs. The duration of both effects varied between 15 to 30 min depending both on the NT concentration and volume of NTCS utilized. Indirect evidence suggested that NT inactivation within the peritoneal cavity contributed to shorten the duration of NT effects. Animal pretreatment with a ganglion blocker, adrenoceptor antagonists, clonidine or capsaicin, reduced the BP and HR increases caused by IP injection of NTCS whereas both effects were either unaffected or slightly potentiated by animal pretreatment with atropine, morphine or captopril. Addition of a local anesthetic to NTCS inhibited the hemodynamic effects of NT whereas acute bilateral cervical vagotomy was without significant effect. These results suggest that NT has the ability to trigger cardiovascular reflexes following its IP injection in guinea pigs. The activation of peritoneal, sympathetic, capsaicin-sensitive primary afferents appears to be at the basis of these reflexes, the amplitudes of which seem poorly related to the volume of NTCS utilized (at least within the range of volume examined).