Endomorphins 1 and 2 reduce relaxant non-adrenergic, non-cholinergic neurotransmission in rat gastric fundus

Effects of remifentanil on gastric tone

OBJECTIVES

Opioids are well known for impairing gastric motility. The mechanism is far from clear and there is wide interindividual variability. The purpose of this study was to evaluate the effect of remifentanil on proximal gastric tone.

MATERIALS AND METHODS

Healthy volunteers were studied on two occasions and proximal gastric tone was measured by a gastric barostat. On the first occasion (n=8), glucagon 1 mg IV was given as a reference for a maximal relaxation of the stomach. On the second occasion (n=9), remifentanil was given in incremental doses (0.1, 0.2 and 0.3 microg/kg/min) for 15 min each, followed by a washout period of 30 min. Thereafter, remifentanil was readministered, and 10 min later glucagon 1 mg was given. Mean intragastric bag volumes were calculated for each 5-min interval.

RESULTS

Glucagon decreased gastric tone in all subjects. Remifentanil had a marked effect on gastric tone; we found two distinct patterns of reactions with both increases and decreases in gastric tone and, during the remifentanil infusion, glucagon did not affect gastric tone.

CONCLUSIONS

Remifentanil induced changes in gastric tone with both increases and decreases. The effect of remifentanil on gastric tone is probably dependent on the current state of the systems involved.

From MIF-1 to endomorphin: the Tyr-MIF-1 family of peptides

The Tyr-MIF-1 family of small peptides has served a prototypic role in the introduction of several novel concepts into the peptide field of research. MIF-1 (Pro-Leu-Gly-NH(2)) was the first hypothalamic peptide shown to act "up" on the brain, not just "down" on the pituitary. In several situations, including clinical depression, MIF-1 exhibits an inverted U-shaped dose-response relationship in which increasing doses can result in decreasing effects. This tripeptide also can antagonize opiate actions, and the first report of such activity also correctly predicted the discovery of other endogenous antiopiate peptides. The tetrapeptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH(2)) not only shows antiopiate activity, but also considerable selectivity for the mu-opiate binding site. Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH(2)) is an even more selective ligand for the mu receptor, leading to the discovery of two more Tyr-Pro tetrapeptides that have the highest specificity and affinity for this site. These are the endomorphins: endomorphin-1 is Tyr-Pro-Trp-Phe-NH(2) and endomorphin-2 is Tyr-Pro-Phe-Phe-NH(2). Tyr-MIF-1 proved, contrary to the then prevailing dogma, that peptides can be saturably transported across the blood-brain barrier by a quantifiable transport system. Unexpectedly, the Tyr-MIF-1 transporter is shared with Met-enkephalin. In the era in which it was doubtful whether a peripheral peptide could exert CNS effects, the Tyr-MIF-1 family of peptides also explicitly showed that they can exert more than one central action that persists longer than their half-lives in blood. These peptides clearly illustrate that the name of a peptide restricts neither its actions nor its conceptual implications.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

The role of endogenous opioids in the control of gastrointestinal motility: predictions from in vitro modelling

Gastrointestinal motility can be assessed in vitro by investigating the effects of drugs or gene knockouts on intestinal propulsion, and on neurone-mediated responses evoked by electrical field stimulation (EFS). The latter predominantly measure enteric motor activity and can detect prokinetic activity of exogenous agents. Some evidence suggests that naloxone has prokinetic activity when evaluated for an ability to modulate responses to EFS, but the effects are inconsistent across different species or intestinal regions. Models of intestinal peristalsis measure an integrated sensory-motor nerve function and possess more intact neuro-neuronal connections. In such preparations, the effects of naloxone also suggest a prokinetic property but again, this is inconsistent. By contrast, consistent prokinetic activity of naloxone is apparent in models where peristalsis is compromised by drug-induced suppression of motor nerve activity or by modulation of endogenous processes using receptor antagonists or inappropriate intraluminal distension. These data suggest that endogenous opioids play little or no role in normal intestinal physiology, but suppress intestinal motility when motor function is compromised. Consequently, drugs that antagonize opioid receptors may exert prokinetic activity in conditions where intestinal motility is reduced, such as constipation. Further work is required to elucidate the opiate receptor(s) involved.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Prejunctional modulation of non-adrenergic non-cholinergic (NANC) inhibitory responses in the isolated guinea-pig gastric fundus

The inhibitory neurotransmission of the stomach was investigated in isolated guinea-pig gastric fundus. In preparations treated with guanethidine (1 micro mol L-1) and p-fluoro-hexahydro-sila-difenidol (1 micro mol L-1), electrical stimulation evoked neurogenic inhibitory responses not modified by hexamethonium (100 micro mol L-1), suggesting that inhibitory postganglionic non-adrenergic non-cholinergic (NANC) nerve fibres are involved. The nitric oxide (NO)-synthase inhibitor Nomega-nitro-l-argininine-methyl-ester hydrochloride (1-100 micro mol L-1) and the soluble guanylyl cyclase inhibitor ODQ (0.1-3 micro mol L-1) also abolished such relaxant response, suggesting the involvement of NO/Cyclic Guanosine 3',5' monophosphate (cGMP) system as the final mechanism of muscle relaxation. The alpha2-adrenoceptor agonist, UK 14 304 (10 nmol L-1-10 micro mol L-1) did not influence the electrical field stimulation (EFS)-evoked NANC responses. These latter responses were also refractory to a variety of receptor agonists and antagonists, acting at Gamma Aminobutyric Acid (GABA), serotonin 5HT1a, opioid micro , delta and kappa, muscarinic M1 and M2, histamine H2 and H3 and cannabinoid receptors. The NANC response was insensitive to the P/Q-type Ca2+-channel blocker omega-agatoxin TK (1 nmol L-1-0.1 micro mol L-1), but partially inhibited by the N-type Ca2+-channel blocker omega-conotoxin GVIA (0.1 nmol L-1-0.1 micro mol L-1), and by the L-type Ca2+-channel blockers nifedipine and calcicludine (0.1 nmol L-1-0.1 micro mol L-1). These data suggest that the NANC relaxation of the isolated guinea-pig gastric fundus is mediated by NO as the final inhibitory (neuro)transmitter at the longitudinal smooth muscle cells. The mechanism(s) promoting NO production is/are Ca2+-dependent, but apparently insensitive to presynaptic modulation. Both N- and L-type channels seem to occur in nitrergic nerve endings, where they contribute to trigger NO diffusion at the synaptic cleft.