Effects of opioid drugs on capsaicin-sensitive neurones in guinea-pig atria

Nociceptin inhibits capsaicin-sensitive contraction to mesenteric nerve stimulation in the guinea-pig isolated ileum

Mesenteric nerve stimulation (MNS) in the presence of guanethidine and hexamethonium antidromically stimulated extrinsic sensory nerve fibers and cholinergic myenteric motor neurons, resulting in longitudinal muscle contraction in the isolated guinea-pig ileum. Nociceptin (NC) is a recently discovered neuropeptide that structurally resembles an opioid peptide. The aim of the current study was to examine how NC affects the contractile responses to MNS in the isolated guinea-pig ileum, in comparison with an opiate, methionine-enkephalin. These contractions were auxotonically recorded and their amplitude was analyzed. NC (1-100 nM) and methionine-enkephalin (0.1-10 microM) concentration-dependently inhibited the response to MNS (20 Hz, 0.5 ms, supramaximal currents). Naloxone (10 microM) significantly diminished the inhibitory effect of methionine-enkephalin (0.1-10 microM), but did not antagonize the inhibitory effect of NC (1-100 nM). We conclude that NC receptors, distinct from opioid receptors, exist on the capsaicin-sensitive sensory nerve fibers and/or myenteric cholinergic motor neurons in the guinea-pig ileum and that specific antagonists for these NC receptors are not found yet.

Adenosine inhibits efferent function of extrinsic capsaicin-sensitive sensory nerves in the enteric nervous system

Capsaicin (1-3 microM) and electrical stimulation of mesenteric nerves in the presence of hexamethonium and guanethidine antidromically stimulate extrinsic sensory nerve fibers to produce a specific slow depolarizing response of myenteric neurons and a contractile response of muscles in the isolated guinea-pig ileum, mediated by release of substance P and acetylcholine. Adenosine (1-100 microM) inhibited the response to mesenteric nerve stimulation. Adenosine (10-100 microM) suppressed the contractile response to a threshold concentration of capsaicin (1 microM) while leaving the contractile response to a submaximal concentration of substance P (1 nM) and acetylcholine (0.1 microM) intact. Adenosine (1-10 microM) inhibited dose dependently the capsaicin 10 microM)-induced depolarization of myenteric neurons, but did not inhibit the depolarizing response to exogenous substance P. The adenosine P1 receptor antagonist, 8-phenyltheophylline (1-10 microM), antagonized the inhibitory effect of adenosine (1-10 microM) on the mechanical responses. We conclude that adenosine-induced prejunctional inhibition of the mechanical responses is mediated by adenosine P1 receptors.

Silent ischemia: a hypothetical mechanism

Ischemia of visceral organs, especially the heart, is often a painful and potentially life-threatening condition. However, in at least 75% of all cases myocardial ischemia may be "silent" (i.e., without pain or sensation). Yet, the mechanisms responsible for silent ischemia are not well understood. As such, many different theories have been advanced to explain silent ischemia; however, none have been able to adequately explain all of the experimental and clinical findings. This paper proposes a hypothetical mechanism that may help to understand mechanisms of silent ischemia.

Non-adrenergic non-cholinergic (NANC) neural control of the atrial myocardium

1. Current concepts in the regulation of atrial contractility by non-adrenergic non-cholinergic (NANC) sensory nerves are reviewed. 2. There is now evidence that in addition to sympathetic and parasympathetic innervation capsaicin-sensitive sensory nerves contribute to the local control of atrial contractility by releasing NANC transmitters, such as calcitonin gene-related peptide (CGRP). 3. Certain chemical and physical stimuli affect atrial contractility by inducing the release of CGRP from sensory nerves. In addition, as widely recognized for the sympathetic and vagal atrial innervation, NANC neurotransmission is under the inhibitory control of several endogenous modulators. 4. Cardioexcitatory actions of NANC neurotransmission on the atrial myocardium are considered. 5. Pharmacological modulation of NANC neurotransmission and functional evidence for cross-talk between NANC and sympathetic neurones are also discussed.

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.

An opioid peptide inhibits capsaicin-sensitive vasodilatation in the pig's skin

Microcirculatory effects of electrical stimulation of nerves through a pair of needle electrodes in the skin of anaesthetized pigs were studied by using the laser Doppler flowmetric method. Electrical stimulation (0.3-30 Hz) evoked a short-lasting decrease in capillary blood flux (vasoconstriction) followed by an increase (vasodilatation), of longer duration. Vasoconstriction was inhibited by local guanethidine, but not by capsaicin pretreatment, whereas vasodilatation was blocked by local capsaicin, but not by guanethidine. Both phases of the response were suppressed by local application of tetrodotoxin. Thus, vasoconstriction due to electrical stimulation seems to be of sympathetic origin, while vasodilatation is a result of a release of vasoactive substances from capsaicin-sensitive nerve endings. Vasodilatation due to electrical stimulation was strongly and dose-dependently inhibited by the opioid peptide [D-Met2,Pro5] enkephalinamide, while vasoconstriction remained apparently unchanged. At both doses of the opioid peptide tested (0.03 and 0.15 mumol/kg i.m.) inhibition of vasodilatation was larger at lower than at higher frequencies of stimulation. Guanethidine pretreatment did not influence the inhibitory action of [D-Met2,Pro5] enkephalinamide. Naloxone (1.5 mumol/kg i.m.) reversed or prevented the inhibitory action of the opioid peptide; naloxone on its own did not influence responses due to 0.3-30 Hz stimulation. [D-Met2,Pro5] enkephalinamide (0.15 mumol/kg i.m.) did not influence basal blood flux in the skin, mean arterial blood pressure, respiratory minute volume or respiratory frequency. It was concluded that stimulation of opioid receptors by [D-Met2,Pro5] enkephalinamide is likely to inhibit stimulation-evoked vasodilatation by reducing the release of vasoactive substances from capsaicin-sensitive afferent neurons, an effect that does not depend on functional integrity of sympathetic nerves. Endogenous opioids probably do not modulate the capsaicin-sensitive vasodilatation.

Alpha-adrenoceptor modulation of the efferent function of capsaicin-sensitive sensory neurones in guinea-pig isolated atria

1. Transmural nerve stimulation of guinea-pig atria, obtained from animals pretreated with reserpine (5 mg kg-1, i.p.), in the presence of atropine 1 microM and of the beta-adrenoceptor blocker CGP 20712A 1 microM, induced a positive inotropic effect which was reduced by the calcitonin gene-related peptide (CGRP) antagonist hCGRP-(8-37) and abolished by pretreatment with capsaicin 1 microM. 2. Noradrenaline concentration-dependently (0.01-10 microM) reduced the increase in cardiac contractility induced by transmural nerve stimulation. The inhibitory effect of noradrenaline was antagonized by yohimbine (0.5-1 microM), in a dose-dependent manner. Prazosin (0.5-1 microM) antagonized the effect of noradrenaline and this effect was independent of concentration. 3. In the presence of yohimbine, the lower part of the inhibitory-response curve for noradrenaline was slightly but significantly shifted by prazosin. A similar degree of antagonism was observed in the presence of 1 microM phenoxybenzamine. 4. The selective alpha 2 agonists BHT 920 and clonidine reduced, in the same concentration-range (0.01-1 microM), the cardiac response to transmural nerve stimulation in a yohimbine-sensitive fashion. 5. Phenylephrine (0.1-100 microM) and methoxamine (1-300 microM) also induced an inhibitory effect on transmural nerve stimulation. The effect of phenylephrine was antagonized by yohimbine (1 microM) more efficiently than by prazosin (0.5 microM). 6. These results are in keeping with the presence of inhibitory prejunctional alpha 2-adrenoceptors on cardiac sensory nerve endings which modulate the efferent function of capsaicin-sensitive neurones.

Modulation by opioids and by afferent sensory neurones of prostanoid protection of the rat gastric mucosa

1. Pretreatment with capsaicin, to deplete sensory neuropeptides from primary afferent neurones or the administration of morphine (9 mg kg-1, i.v.), which can inhibit neuropeptide release, augmented gastric mucosal injury induced by a 5 min challenge with intragastric ethanol in the rat, as assessed by macroscopic and histological evaluation. 2. Morphine administration substantially attenuated the protective actions of the prostaglandin analogue 16,16 dimethyl prostaglandin E2 (dm PGE2; 0.5-20 micrograms kg-1, p.o.) against ethanol-induced damage. This reduced degree of protection by dmPGE2 was not however, the consequence of the enhanced level of damage. 3. These actions of morphine in reducing prostaglandin protection against mucosal injury were abolished by pretreatment (5 min) with naloxone (1 mg kg-1, i.v.) or the peripherally acting opioid antagonist, N-methyl nalorphine (6 mg kg-1, i.v.). 4. Capsaicin pretreatment (2 weeks before study), likewise attenuated the protective actions of dmPGE2, although to a lesser degree than did morphine. 5. These findings, thus implicate the involvement of capsaicin- and opioid-sensitive afferent neurones in the processes by which exogenous prostanoids can protect the gastric mucosa from damage.

The inhibitory effect of opioid and α2-adrenoceptor agonists on cardiac sensory neurones is pertussis toxin-insensitive

The role of pertussis toxin-sensitive G proteins on the alpha 2-adrenoceptor and mu-opioid receptor-mediated inhibition of the efferent function of capsaicin-sensitive neurones was investigated in guinea-pig atria pretreated with guanethidine. In the presence of atropine, CGP 20712A (2-hydroxy-5-(2-[hydroxy-3-(4-[(1-methyl- 4-trifluormethyl)1H-imidazol-2-yl]-phenoxy)propyl]aminoethoxyl+ ++)-benzamide) and prazosin, [D-Ala2,NMe-Phe4,Gly5-ol]enkephalin (DAGO, 0.1-3 microM) and 2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo(4,5-d)azepine (BHT 920, 0.01-1 microM) reduced the positive inotropic effect induced by transmural stimulation of preparations obtained from control and from pertussis toxin-treated animals. These results suggest that pertussis toxin-sensitive G proteins are not involved in the inhibitory regulation of the efferent function of capsaicin-sensitive nerve terminals in cardiac tissue induced by alpha 2 and opioid receptor stimulation.

ATP modulates the efferent function of capsaicin-sensitive neurones in guinea-pig isolated atria

1. The effect of adenosine triphosphate (ATP) and its stable analogues, alpha, beta-methylene-ATP and beta, gamma-methylene-ATP, on the efferent function of capsaicin-sensitive non-adrenergic, non-cholinergic (NANC) nerves was tested in guinea-pig isolated atria. 2. Transmural nerve stimulation of atria isolated from reserpine-pretreated guinea-pigs, in the presence of 1 microM atropine and 0.3 microM CGP 20712A, induced a transient positive inotropic effect attributable to calcitonin gene-related peptide (CGRP) release from NANC nerve endings. 3. ATP (1-30 microM) concentration-dependently reduced the cardiac response to transmural nerve stimulation, without affecting the inotropic response to 10 nM exogenous CGRP. The inhibitory effect of ATP was competitively antagonized by the P1-purinoceptor antagonist, 8-phenyltheophylline (8-PT, 1 microM), but was unaffected by the P2-purinoceptor antagonist, suramin (100 microM). 4. beta, gamma-methylene-ATP in the same concentration range as ATP, inhibited the cardiac response to transmural nerve stimulation. The inhibitory effect of beta, gamma-methylene ATP was antagonized by 1 microM 8-PT. The desensitizing agonist for P2-purinoceptors, alpha, beta-methylene ATP did not induce any inhibitory effect either on the cardiac response to transmural nerve stimulation or on the inhibitory effect curve for ATP. 5. The inhibitory effect of ATP on the NANC neurotransmission was inconsistently modified in the presence of 10 microM alpha, beta-methylene-adenosine diphosphate, an inhibitor of the 5'-ectonucleotidases. 6. These results demonstrate that ATP modulates the efferent function of cardiac NANC nerve endings through prejunctional inhibitory receptors belonging to the P1 type. The metabolic conversion of ATP to adenosine does not seem to be a pre-requisite for the ATP agonist activity.

Modulation by adrenergic transmitters of the efferent function of capsaicin-sensitive nerves in cardiac tissue

In atrial preparations obtained from reserpine-pre-treated guinea-pigs, incubated in the presence of 1 microM atropine plus 1 microM CGP 20712A (a beta 1 blocking drug), a positive inotropic effect due to CGRP release from capsaicin-sensitive sensory neurons was induced by electrical field stimulation (EFS). This response was concentration-dependently reduced by noradrenaline (0.01-3 microM), neuropeptide Y (NPY, 3-300 nM) and adenosine triphosphate (ATP, 1-30 microM). On the other hand, the overflow of [3H]-noradrenaline from sympathetic nerve terminals induced by EFS in isolated atria obtained from normal untreated animals was not modified in 10 nM calcitonin gene-related peptide (CGRP). Substance P (SP) and neurokinin A (NKA), at concentrations ranging from 0.01 to 1 microM did not affect the cardiac response to field stimulation of adrenergic terminals of atrial tissue. These findings demonstrate that all the co-transmitters stored in adrenergic nerve terminals have a modulatory role on the efferent function of cardiac capsaicin-sensitive sensory neurons, while cardiac adrenergic neurotransmission is not influenced by the peptidergic transmitters released from sensory neurons.

Differential effects of neuropeptide Y and opioids on neurogenic responses of the perfused rat mesentery

In perfused rat mesentery transmural nerve stimulation activates both adrenergic and capsaicin-sensitive sensory nerves. When adrenergic nerves were blocked with guanethidine and smooth muscle tone was increased, transmural nerve stimulation caused a dilator response which was attenuated by tetrodotoxin and abolished by capsaicin. Indomethacin increased the vasodilator response to transmural nerve stimulation, but did not affect the dilation to calcitonin gene-related peptide. Neuropeptide Y (NPY) potentiated vasoconstrictor responses to transmural nerve stimulation, but suppressed capsaicin-sensitive vasodilation, an effect which was unaltered by indomethacin. Opioid agonists selective for mu, delta or kappa receptors, DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin), DPDPE ([D-Pen2,D-Pen5]enkephalin) and ethylketocyclazocine, had no effect on the vasoconstrictor response to transmural nerve stimulation. DAMGO and DPDPE significantly inhibited vasodilator responses to transmural nerve stimulation, but ethylketocyclazocine was without effect. After treatment with indomethacin, DAMGO still inhibited the vasodilator response, but DPDPE was no longer effective. Prejunctional control of transmitter release by NPY or opioids is dependent on the specific nerve type as well as, in some cases, the participation of endogenous prostaglandins.

Characterization of opioid receptors modulating the function of capsaicin-sensitive neurons in guinea-pig atria

Transmural nerve stimulation of isolated atria, obtained from reserpine-pretreated guinea-pigs, in the presence of atropine and the beta 1-adrenoceptor-blocking drug CGP 20712A, induced a positive inotropic effect. [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO), [D-Ala2,D-Leu5]enkephalin (DADLE), morphine and dynorphin dose dependently reduced the cardiac response to transmural nerve stimulation. The delta receptor selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE), and the kappa receptor agonist, U50488, were unable modify the response. The inhibitory effect of all the active opioid agonists was antagonized by naloxone but not by the selective delta and kappa opioid receptor antagonists, ICI 174.864 and MR 2266. These results suggest the presence on sensory nerve terminals of inhibitory opioid receptors belonging to the mu, but not to the delta and kappa subtypes.