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A R Gintzler

Publications and source records attributed to A R Gintzler.

At least 19 recordsLinked to original sources
PubMed1992

Stimulus frequency and intensity: critical determinants of opioid enhancement or inhibition of evoked methionine-enkephalin release.

Responses to opioids can be bimodal depending on the concentration of opioid used. For example, low concentrations (nM) enhance whereas higher concentrations (10-100 nM) inhibit the electrically evoked release of enkephalin from the myenteric plexus. The nature of the responsiveness of the enkephalin release process to opioids is also dependent on the intracellular and/or extracellular milieu of enkephalin-containing neurons or other neurons of this plexus. Intracellular levels of cAMP, availability of pertussis toxin- and cholera toxin-sensitive guanine nucleotide binding proteins and intracellular calcium concentration have all been shown to be important determinants of opioid excitatory versus inhibitory actions. The present data indicate that the inhibition of enkephalin release produced by U50,488H or sufentanil is no longer observed when the applied voltage is increased 3- or 2-fold, respectively. Under this condition, a previously inhibitory concentration of opioid produces an enhancement of stimulated enkephalin release. Increasing the frequency of the applied stimulation from 5 to 60 Hz (at a constant voltage) also reverses the sufentanil-induced inhibition to a facilitation of enkephalin release. These data indicate that the intensity (voltage) or frequency of the stimulation that is used to release enkephalin is a critical determinant of the nature of its modulation by opioids. The possible relevance of these findings to known differences in opioid sensitivity between different types of pain is discussed.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗
PubMed1992

Opioid enhancement of evoked [Met5]enkephalin release requires activation of cholinergic receptors: possible involvement of intracellular calcium.

Previous work from this laboratory has shown that the electrically evoked release of enkephalin from the guinea pig myenteric plexus is regulated by an opiate receptor-mediated, concentration-dependent mechanism. Low concentrations (nanomolar) of opioids enhance release, whereas higher concentrations (10-100 nM) inhibit release. Each opioid effect is mediated by a different guanine nucleotide-binding protein. We now demonstrate that activation of cholinergic receptors in the myenteric plexus is a prerequisite for opioid excitatory effects, but not inhibitory effects, on enkephalin release. Pretreatment with the muscarinic cholinergic receptor antagonist atropine abolishes the opioid facilitation of stimulated enkephalin release but does not alter the inhibition of release that is observed with higher concentrations of opioid agonist. Exposure to the calcium ionophore A23187 overcomes the abolishment of opioid enhancement of enkephalin release produced by cholinergic receptor blockade. In tissue treated with both atropine and A23187, the magnitude of the opioid enhancement of release is indistinguishable from that observed in untreated preparations. This suggests that the lack of stimulation-induced generation of elevated cytosolic calcium is responsible for the abolishment of facilitory opioid effects when cholinergic receptors are blocked. The known coupling of muscarinic receptors to phospholipase C activation and the generation of inositol trisphosphate (which elevates cytosolic calcium) could suggest that this second messenger is critical for the manifestation of opioid facilitation of enkephalin release.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗
PubMed1991

Analgesia is produced by uterocervical mechanostimulation in rats: roles of afferent nerves and implications for analgesia of pregnancy and parturition.

Afferent activity from the reproductive tract activates intrinsic pain attenuating processes. For example, analgesia results from vaginocervical mechano-stimulation in nonpregnant rats and occurs during pregnancy and parturition. In the present study, the effect of uterocervical mechanostimulation on pain thresholds was investigated in order to determine whether direct stimulation of the uterine cervix could play a role in the analgesia of pregnancy. Uterocervical mechanostimulation was applied to nonpregnant rats via a silastic disc implanted in the uterus. The disc abutted against the cervix and was attached to a thread externalized through the vaginal orifice. Application of a force of 150 g, but not 100 g, produced a significant increase in tail flick latency (110.4 +/- 40.6%, P less than 0.03). This effect was abolished by pelvic neurectomy, but was not altered by hypogastric neurectomy. Stimulation of the uterine cervix in combined pelvic and hypogastric neurectomy rats produced a decrease in tail flick latency. These results indicate that the analgesia that occurs during pregnancy and/or parturition may result, at least in part, from the uterocervical mechanostimulation that occurs during this condition.

Afferent Pathways↗
PubMed1991

Different G proteins mediate the opioid inhibition or enhancement of evoked [5-methionine]enkephalin release.

This laboratory has previously demonstrated that there is an opiate receptor-mediated, concentration-dependent modulation of the electrically stimulated release of enkephalin from the guinea pig myenteric plexus. Low doses of opioids (nanomolar) enhance release, whereas higher concentrations (10-100 nM) inhibit release. We now demonstrate that the in vivo i.p. administration of the islet-activating protein from pertussis toxin (PTX; 50 micrograms/500 g of body weight) markedly diminishes the potency of mu, delta, or kappa-selective opioids to inhibit the evoked release of enkephalin. In contrast, PTX is without effect on the enhancement of enkephalin release observed after treatment with nanomolar concentrations of the above opioids. Conversely, pretreatment with cholera toxin (CTX; 0.01 nM for 3 hr in vitro) has no effect on the mu, delta, or kappa opioid inhibition of evoked enkephalin release but abolishes the ability of nanomolar concentrations of these agonists to enhance stimulated enkephalin release. These data indicate that different classes of guanine nucleotide-binding proteins (G proteins) appear to mediate the opioid enhancement or inhibition of stimulated enkephalin release. Furthermore, they suggest that a PTX-sensitive G protein (Gi or Go) and a CTX-sensitive G protein (Gs) are integral components of the mechanism that mediates opioid inhibition and opioid enhancement, respectively, of evoked enkephalin release. To our knowledge, this report represents the first demonstration that Gs-coupled opiate receptors (in addition to those that are coupled to Gi) can modulate transmitter release.

3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cycloh↗
PubMed1990

Pain thresholds are elevated during pseudopregnancy.

Increases in pain thresholds have been observed during gestation in a variety of laboratory animals and humans. The specific events that occur during pregnancy that are responsible for this increase in maternal pain threshold are not known. In order to investigate whether or not the changes in the peripheral concentration of sex steroids that are known to occur during gestation contribute to the analgesia of pregnancy, the effect of pseudopregnancy on jump thresholds was determined. Pseudopregnancy is associated with a significant increase in the mean jump threshold. In contrast, rats treated chronically with the opioid antagonist naltrexone do not manifest an increase in jump threshold during the pseudopregnant condition. These results suggest that peripheral sex steroids whose peripheral concentration profile is altered during pseudopregnancy might be important modulators of opioid systems that regulate responsiveness to aversive stimuli.

Animals↗
PubMed1989

Opioids can enhance and inhibit the electrically evoked release of methionine-enkephalin.

The stimulated (40 Hz) release of enkephalin from the myenteric plexus can be modulated by multiple types of opiate receptor. The direction of the modulation is not fixed but is bimodal. Both an inhibition and an enhancement of evoked release is observed depending on the concentration of opioid agonist that is used. Each of these effects can be antagonized by naloxone. Following pretreatment of guinea pig myenteric plexus in vitro with forskolin (0.5 microM) or the lipid soluble cAMP analog 8-(4-chlorphenylthio)-cAMP (8-CPT-cAMP; 100 microM) the inhibition of stimulated Met-enkephalin release that is produced by sufentanil (10(-8) M), [D-Pen2-D-Pen5]enkephalin (DPDPE, 10(-8) M) or dynorphin (10(-7) M) is no longer observed. On the contrary, in forskolin- or 8-CPT-cAMP treated myenteric plexus a previously inhibitory concentration of the above opioids now produces an enhancement of the magnitude of the stimulated Met-enkephalin release. Excitatory responses (enhanced release) to lower concentrations of sufentanil (1 nM) or DPDPE (5 nM) are not affected by pretreatment with the same concentration of forskolin or 8-CPT-cAMP. These data suggest that the ability of opioids to enhance or inhibit evoked enkephalin release is mediated via different biochemical processes (separate second messenger systems). This could imply that the opioid enhancement of enkephalin release is due to a direct facilitation of release and not to disinhibition. The ability of opioids to enhance the release of at least some neurotransmitters should be taken into account when attempting to explain the physiological sequelae of the acute and chronic effects of narcotics.

Action Potentials↗
PubMed1989

Spinal dynorphin involvement in the analgesia of pregnancy: effects of intrathecal dynorphin antisera.

In both rats and humans there is an analgesia associated with pregnancy. This analgesia is spinally mediated and involves the kappa type of opiate receptor. The current study demonstrates that intrathecal administration of high affinity dynorphin antibodies produces a significant reduction in jump thresholds during pregnancy (day 20). The administration of pre-adsorbed antisera fails to produce this effect. These results support the hypothesis that a spinal dynorphin/kappa opiate receptor system is activated during gestation.

Analgesia↗
PubMed1989

Regional brain glucose utilization during and following chronic naltrexone administration: preliminary observations in rat brain.

Experiments were conducted to determine some of the metabolic correlates of tonic opioid activity in the central nervous system under conditions previously examined for changes in monoamine levels. The glucose metabolic rates in seven brain regions were determined by autoradiographic visualization of 14C-deoxyglucose incorporation in female rats after 8 days of chronic exposure to naltrexone pellets and 10 days after pellet removal. Autoradiographs were analyzed on a region-by-region basis to correspond to areas previously dissected and analyzed for changes in monoamine content under similar experimental conditions. Chronic administration of naltrexone resulted in a significant decrease in the metabolic activity of neurons in the striatum. Other brain areas examined under this condition were not significantly affected. Ten days following pellet removal, 14C-deoxy-glucose incorporation was indistinguishable from that determined in placebo treated rats in all brain regions examined. These results indicate that tonic opioid input is an important determinant of metabolic activity in the striatum. In addition, these results indicate that conditions previously shown to alter regional content of monoamines do not necessarily produce concomitant changes in regional glucose utilization.

Animals↗
PubMed1989

Minimum-structure enkephalin analogues incorporating L-tyrosine, D(or L)-phenylalanine, and a diamine spacer.

In order to test the theory that high mu-activity of opioid peptides could be elicited by the presence of an amino-terminal L-Tyr residue and a Phe aromatic ring held in the proper relative spatial disposition, a novel series of hybrid retro peptides were prepared in which L-Tyr was linked to N-acyl Phe through a variety of diamine spacers. These compounds were evaluated for opioid agonist and antagonist activity in the guinea pig ileum (GPI) in vitro assay. Analogues containing a 1,2-ethanediamine spacer, which conferred a Tyr-Phe separation distance closest to that found in Phe3 opioid peptides, were more potent agonists than the corresponding analogues containing a 1,3-propanediamine spacer. Agonist activity was observed for both L-Phe and D-Phe analogues, consistent with the known activity for both Phe stereochemistries for certain Phe3 opioid peptide analogues. Concerning the diamine spacer, conformational constraints imposed by 4-aminopiperidine and 4-(aminomethyl)piperidine as well as the presence of a hydroxyl group eliminated activity, but the presence of gem-dimethyl substitution next to the nitrogen attached to Tyr increased activity substantially for the D-Phe derivatives. Removal of the N-acetyl group from Phe did not eliminate activity. Naloxone Ke values determined for six of the most potent analogues are indicative of predominantly mu-agonism, but the D-Phe compounds 3a and 6a (1.4-2.1 nM) appear to be more mu-selective than the L-Phe compounds 2b, 3b, 5b, and 6b (3.3-4.4 nM), even though the latter are more potent agonists. Compounds 3a and 3b, which were found to be 10 and 21 times more potent, respectively, than morphine in the GPI, are two of the most structurally simple yet potent opioid peptide analogues described to date.

Animals↗
PubMed1988

Spinal kappa-opiate receptor involvement in the analgesia of pregnancy: effects of intrathecal nor-binaltorphimine, a kappa-selective antagonist.

In a variety of laboratory animals as well as humans, pregnancy has been associated with an activation of a maternal opioid system(s) and with a concomitant elevation in the threshold for maternal responsiveness to aversive stimuli. This analgesia is mediated via the activation of spinal opiate receptors and does not require an intact peripheral opioid system(s). The recently developed kappa-selective opioid antagonist, nor-binaltorphimine (nor-BNI), significantly reduces the threshold for reflexive jumping in response to electric foot shock when administered to the lumbar intrathecal (i.t.) space of pregnant rats (day 20 of gestation). In contrast, i.t. nor-BNI when administered to non-pregnant rats as well as systemic (i.p.) administration of an intrathecally effective dose of nor-BNI to pregnant rats is without effect on the jump thresholds. These data indicate that the kappa-type of opiate receptor mediates, at least in part, the analgesia observed during gestation, thus providing an important physiological function for this receptor type.

Analgesia↗
PubMed1988

Effect of chronic naltrexone administration and its withdrawal on the regional activity of neurons that contain norepinephrine, dopamine and serotonin.

A method is described that permits the simultaneous quantitation of norepinephrine (NE), dopamine (DA) serotonin (5-HT) and their respective major metabolites, 3-methoxy-4-hydroxy phenylglycol (MHPG), 3-methoxytyramine (3-MT), dihydroxyphenyl acetic acid (DOPAC) and 5-hydroxyindole acetic acid (5-HIAA) in discrete brain regions. The ratio of MHPG/NE, DOPAC/DA and 5-HIAA/5-HT was used to assess the effects of the chronic administration of the narcotic antagonist, naltrexone, and its withdrawal on the regional activity of neurons that contain NE, DA and 5-HT respectively. Chronic administration of naltrexone (8 days) is associated with a significant increase in the ratio of 5-HIAA/5-HT and DOPAC/DA in the frontal cortex and dorsal hippocampus respectively. Under this condition the thalamic concentration of 3-MT in 4 of 8 animals is also significantly elevated. In contrast, the mesolimbic forebrain exhibited a decrease in the MHPG/NE ratio (4 out of 8 animals). One day following naltrexone pellet removal the above ratios, as well as the mean content of 3-MT in the thalamus, returned to control values. At this time the content of 3-MT in the thalamus (5 of 5 animals) and frontal cortex (3 of 9 rats) was appreciably elevated, while its content in the dorsal hippocampus was significantly reduced (6 of 9 rats). These data suggest that the activity of several central monoaminergic neuronal systems are regulated by an opioid input that is tonically active.

Animals↗
PubMed1988

Meal-stimulated release of methionine-enkephalin into the canine jejunal lumen.

Application of enkephalins to the luminal surface of the bowel augments intestinal absorption. However, to date, endogenous enkephalins have not been demonstrated within intestinal luminal fluid. To determine whether enkephalins are present in the intestinal lumen, five adult dogs had 25-cm chronic jejunal Thiry-Vella loops constructed. Dogs were studied in the awake, fasted state. Jejunal loops were perfused with isoosmotic, neutral Krebs buffer containing protease inhibitors. After basal sampling, the dogs received a high fat meat meal. Collections were made during the meal and for 60 min postprandially. Luminal met-enkephalin levels were determined by radioimmunoassay and confirmed by HPLC. HPLC separation of luminal samples demonstrated two immunoreactive peaks which co-eluted with pure met-enkephalin and met-enkephalin-sulfoxide. Basal met-enkephalin outputs averaged 52 +/- 13 ng/min. The meal significantly increased mean luminal met-enkephalin output to 137 +/- 71 ng/min. During the initial 20-min postprandial period, output remained elevated (180 +/- 73 ng/min), after which it returned to basal levels. We conclude that met-enkephalin is present in the jejunal lumen, and that luminal release of this opioid is augmented by a meal.

Animals↗
PubMed1987

Effects of hypophysectomy and dexamethasone treatment on plasma beta-endorphin and pain threshold during pregnancy.

During pregnancy, rats and humans show an increase in pain threshold that is mediated by an endorphin system. In order to determine whether plasma beta-endorphin and/or other factors of pituitary origin are involved in pregnancy-induced analgesia in the rat, the effects of hypophysectomy (day 12 of pregnancy) or pharmacological suppression of pituitary function via dexamethasone administration (day 14-21 of pregnancy) were investigated. Hypophysectomy did not affect either the magnitude of the increase or the pattern of change in pain threshold despite the resulting decrease in stress-induced plasma beta-endorphin concentrations. However, the observed effect of the surgical and/or postsurgical procedure on pain threshold confounded unequivocal interpretation of these results. Pharmacological suppression of pituitary function with dexamethasone (2 micrograms/ml), a non-invasive procedure, also produced a significant decrease in resting plasma beta-endorphin levels. As was observed for surgical removal of the pituitary gland, this treatment did not produce a significant alteration in the magnitude of the increase in jump threshold. Furthermore, no correlation was found between plasma beta-endorphin concentrations and jump threshold values on day 21 of pregnancy. These results indicate that the pituitary gland does not play an essential role in the maintenance of opioid analgesia during pregnancy. It is suggested that pregnancy-induced analgesia depends on central rather than peripheral opioid systems.

Animals↗
PubMed1987

Spinal cord mediation of the opioid analgesia of pregnancy.

It has been demonstrated that during pregnancy and labor in rats and humans there is an opioid-mediated elevation in the threshold for responsiveness to aversive stimuli which reaches a maximum at term. Acute administration of the opiate antagonist, naltrexone, into the lumbar intrathecal (i.t.) space of pregnant rats (day 20 of gestation) significantly reduces the threshold for reflexive jumping in response to electric footshock. The i.t. administration of the inactive stereoisomer of a closely related narcotic antagonist, (+)-naloxone, is devoid of any effect on pain threshold. No effect on the pain threshold is observed following intrathecal saline administration to pregnant rats, i.t. naltrexone administration to non-pregnant rats or following systemic administration of an intrathecally effective dose of naltrexone to pregnant rats. These data indicate that the analgesia observed during gestation is mediated, at least in part, via a spinal opioid pathway which is activated by some aspect of the pregnant condition.

Animals↗
PubMed1987

Evoked release of methionine enkephalin from tolerant/dependent enteric ganglia: paradoxical dependence on morphine.

Experiments were performed in order to determine whether the state of tolerance to and dependence upon opiates is associated with changes in one or more of the characteristics of the electrically induced release of methionine enkephalin from enteric ganglia. Acute morphine pretreatment substantially reduces the magnitude of the evoked release of this peptide from opiate-naive ilea. However, the rate of the evoked release of enkephalin from morphine-pretreated, tolerant/dependent preparations is indistinguishable from that observed for untreated, naive ilea. Paradoxically, 15 min after acute in vitro withdrawal of morphine form such preparations, the presence of morphine appears to be prerequisite for the manifestation of electrically evoked release of methionine enkephalin. The evoked release of this peptide from ilea 60 min after withdrawal is no longer dependent upon morphine. Moreover, the magnitude of the increase in the rate of enkephalin release from these preparations is almost double that observed for opiate-naive ilea. These data indicate that the manifestation of opiate tolerance/dependence for the release of methionine enkephalin from enteric ganglia comprises several adaptive processes, the consequences of which can be observed at different stages of withdrawal.

Animals↗
PubMed1986

Short-term nerve stimulation increases enkephalin production and content in the guinea pig myenteric plexus.

Methionine-enkephalin content in the guinea pig myenteric plexus was determined before and after acute, short-term electrical or chemical stimulation. Stimulation at 20 Hz for 30 s or exposure to high potassium, the calcium channel agonist, CGP28 392, or the narcotic antagonist, (-)-naloxone, resulted in a significant increase in the content of myenteric methionine-enkephalin. The increase produced by electrical stimulation is dependent upon functional sodium channels and the presence of extracellular calcium. These results indicate that tissue levels of enkephalin are not fixed but can fluctuate in response to nerve stimulation and suggest a mechanism whereby the rate of production of this opioid peptide is coupled to neuronal activity. Furthermore, the ability of (-)-naloxone but not (+)-naloxone to almost double myenteric enkephalin content suggests that the neurons in which this increase occurs are under tonic modulation (direct or indirect) by opioids.

Animals↗
PubMed1986

Direct analysis of the release of methionine-enkephalin from guinea pig myenteric plexus: modulation by endogenous opioids and exogenous morphine.

The in vitro release of methionine-enkephalin (met-enkephalin) from two longitudinal muscle myenteric plexus strips from guinea pig ileum has been obtained during continuous superfusion and quantitated directly using a radioimmunoassay specific for this opioid peptide. Electrical stimulation (5-80 Hz) produced a significant increase in the rate of release of met-enkephalin the magnitude of which was not dependent on the frequency of stimulation. Analysis of the release of met-enkephalin per pulse as a function of the frequency of stimulation indicated that the release of this opioid peptide from the myenteric plexus is inversely proportional to the frequency of stimulation. Electrically evoked release (40 Hz) of met-enkephalin was reduced by greater than 80% by substituting CoCl2 for CaCl2 or by pretreatment with tetrodotoxin (1 microgram/ml for 15 min). Evoked release was also reduced substantially by pretreatment with morphine (1 microM, 1.5 min). Alternatively, pretreatment of naive longitudinal muscle myenteric plexus strips with the opiate antagonist (-)-naloxone caused a significant increase in the rate of met-enkephalin release in the absence of electrical stimulation. In contrast, (+)-naloxone was devoid of any activity. These data, in combination with indirect pharmacological experiments, strongly indicate that met-enkephalin functions as a neurotransmitter in the enteric nervous system. Moreover, the activity of neurons that transmit via met-enkephalin appears to be under opioid regulation.

Animals↗