D-Ala2-Met5-enkephalinamide Research Articles

Hypothalamic peptides controlling alcohol intake: Differential effects on microstructure of drinking bouts

Different alcohol drinking patterns, involving either small and frequent drinking bouts or large and long-lasting bouts, are found to differentially affect the risk for developing alcohol-related diseases, suggesting that they have different underlying mechanisms. Such mechanisms may involve orexigenic peptides known to stimulate alcohol intake through their actions in the hypothalamic paraventricular nucleus (PVN). These include orexin (OX), which is expressed in the perifornical lateral hypothalamus, and galanin (GAL) and enkephalin (ENK), which are expressed within as well as outside the PVN. To investigate the possibility that these peptides affect different aspects of consumption, a microstructural analysis of ethanol drinking behavior was performed in male, Sprague–Dawley rats trained to drink 7% ethanol and implanted with guide shafts aimed at the PVN. While housed in specialized cages containing computerized intake monitors (BioDAQ Laboratory Intake Monitoring System, Research Diets Inc., New Brunswick, NJ) that measure bouts of ethanol drinking, these rats were given PVN injections of OX (0.9 nmol), GAL (1.0 nmol), or the ENK analog D-Ala2-met-enkephalinamide (DALA) (14.2 nmol), as compared to saline vehicle. Results revealed clear differences between the effects of these peptides. While all 3 stimulated ethanol intake, they had distinct effects on patterns of drinking, with OX increasing the number of drinking bouts, GAL increasing the size of the drinking bouts, and DALA increasing both the size and duration of the bouts. In contrast, these peptides had little impact on water or food intake. These results support the idea that different peptides can increase ethanol consumption by promoting distinct aspects of the ethanol drinking response. The stimulatory effect of OX on drinking frequency may be related to its neuronally stimulatory properties, while the stimulatory effect of GAL and ENK on bout size and duration may reflect a suppressive effect of these neuronally inhibitory peptides on the satiety-controlling PVN.

Opioids in the perifornical lateral hypothalamus suppress ethanol drinking

The opioid system is known to enhance motivated behaviors, including ethanol drinking and food ingestion, by acting in various reward-related brain regions, such as the nucleus accumbens, ventral tegmental area and medial hypothalamus. There is indirect evidence, however, suggesting that opioid peptides may act differently in the perifornical lateral hypothalamus (PF/LH), causing a suppression of consummatory behavior. Using brain-cannulated Sprague–Dawley rats trained to voluntarily drink 7% ethanol, the present study tested the hypothesis that opioids in the PF/LH can reduce the consumption of ethanol, with animals receiving PF/LH injections of the δ-opioid receptor agonist D-Ala2-met-enkephalinamide (DALA), the μ-receptor agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO), the κ-receptor agonist (±)-trans-U-50,488 methanesulfonate (U-50,488H), or the general opioid antagonist methylated naloxone (m-naloxone). The consumption of ethanol, lab chow, and water was monitored for 4 h after injection. The results showed that the three opioid receptor agonists injected into the PF/LH specifically and significantly reduced ethanol intake, while causing little change in chow or water intake, and the opposite effect, enhanced ethanol intake, was observed with the opioid antagonist. Of the three opioid agonists, the δ-agonist appears to produce the most consistent and long-lasting suppression of consumption. This effect was not observed with injections 2 mm dorsal to this area, focusing attention on the PF/LH as the main site of action. These results suggest that the opioid peptides have a specific role in the PF/LH of reducing ethanol drinking, which is distinct from their more commonly observed appetitive actions in other brain areas. The additional finding, that m-naloxone in the PF/LH stimulates ethanol intake in contrast to its generally suppressive effect in other regions, focuses attention on this hypothalamic area and its distinctive role in contributing to the variable effects sometimes observed with opioid antagonist therapy for alcoholism.

Ca2+ Sensitization of Myocardial Force and Actomyosin ATPase by (D-Ala2, Met5) Enkephalinamide

The presence of proenkephalin mRNA and proenkephalin peptides in cardiac muscle cells suggests the local production of enkephalins in the myocardium. Yet, the effects of these peptides on the function of the contractile proteins are unknown. The effects of (D-Ala2, Met5) enkephalinamide (DALA) on the activity of the actin stimulated Ca, Mg-myosin ATPase in myofibrils and on the contractility and the activity of the related actomyosin ATPase of chemically skinned muscle fibres from pig myocardium were studied. In this article, it is shown that the myofibrillar actomyosin ATPase as well as the contractility and the actomyosin ATPase in skinned fibres are sensitized to Ca2+ ions by DALA. 10(-11) -10(-6) mol/l DALA decrease the effective concentration of Ca2+ stimulating the myofibrillar ATPase activity by 50% (EC50) from 4.0.10(-5) to 1.5.10(-5) mol/l (p < 0.05). The magnesium dependent myosin ATPase activity at low Ca2+ concentration (10(-9) mol/l) is increased. The EC50 values of Ca2+ for both force development and the related actomyosin ATPase activity of skinned fibres are decreased by DALA (10(-11) -10(-5) mol/l) from 2.5.10(-6) to 2.0.10(-6) mol/l (contractions; p < 0.01) and from 2.0.10(-6) to 1.3.10(-6) mol/l (ATPase activity; p < 0.01). The tension cost (ATPase/tension) of the fibres is unchanged by DALA. In conclusion, the results demonstrate a Ca2+ sensitization of the contractile proteins by low concentrations of DALA, indicating a direct regulatory involvement of enkephalins in the regulation of myocardial contractility. These results correspond with the positive inotropic effects of enkephalins in isolated heart muscle cells.

Central d-Ala 2-Met 5-enkephalinamide μ/δ-opioid receptor activation blocks behavioral sensitization to cholecystokinin in CD-1 mice

The present investigation revealed that intraventricular administration of the anxiogenic substance CCK-8S (50 ng) decreased responding for previously rewarding brain stimulation (intracranial self-stimulation; ICSS) and subsequently increased brain stimulation threshold determinations from the dorsal aspects of the ventral tegmental area (VTA) immediately following CCK administration. While central administration of the mixed μ/δ opioid receptor agonist d-Ala 2-Met 5-enkephalinamide (DALA; 1 μg) was ineffective in abrogating CCK induced ICSS deficits during the immediate post-stressor interval, DALA restored ICSS brain stimulation thresholds to basal values 24, 48 and 168 h following CCK challenge. At 18 days following the initial 50 ng CCK-8S and/or DALA challenges, mice were exposed to a previously determined non-anxiogenic dose of CCK-8S (5 ng). Among mice which received an intervening dose of saline following the 50 ng CCK-8S challenge, depressed ICSS responding and elevated brain stimulation thresholds were evident during the immediate (Day 18), 24- (Day 19) and 48-h (Day 20) test sessions relative to mice that received an intervening dose of DALA on Day 1. These data imply that while CCK induces relatively protracted and exaggerated behavioral disturbances, μ/δ opioid-receptor activation may block CCK-induced behavioral sensitization and change the course of psychopathology.

GABA-opioid interactions in the globus pallidus: [D-Ala2]-Met-enkephalinamide attenuates potassium-evoked GABA release after nigrostriatal lesion.

The motor signs of Parkinson's disease have been partly attributed to an overinhibition of the external globus pallidus (GP) that results from hyperactivity of striatopallidal GABA/enkephalinergic neurons. The goals of this study were to measure basal levels of extracellular fluid GABA in the GP of normal cats, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian cats and cats spontaneously recovered from MPTP-induced parkinsonism, and to examine the effects of opioid receptor activation on potassium (K+)-evoked GABA release in the GP in these animals. Basal GP GABA levels were increased 75% from normal in parkinsonian animals 1 week after MPTP administration and returned to control levels in recovered animals 6 weeks after MPTP administration. No significant differences were observed in K+-evoked GABA release across conditions. The opioid receptor agonist [D-Ala2]-Met-Enkephalinamide (DALA) significantly attenuated K+-evoked GABA release in the GP of MPTP-treated symptomatic and recovered cats, but had no significant effect on GABA release in normal animals. These data show that basal GP GABA levels are elevated coincident with expression of parkinsonian signs and return to normal in animals that have functionally compensated for a nigrostriatal lesion. DALA-induced inhibition of pallidal GABA release after a dopamine-depleting lesion, suggests that enkephalin may attenuate GABA release in the GP specifically after striatal dopamine loss.

Expression of functional delta opioid receptors in vascular smooth muscle.

We evaluated smooth muscle from human internal mammary artery and rat aorta for the presence of delta opioid receptors. Radioligand receptor competition studies using the delta-receptor selective agonist, [3H]-[D-Ala2, Met5] enkephalinamide (DAMA) suggested the expression of a high affinity binding site in rat and human blood vessels that was consistent with the delta-2 opioid receptor subtype. Using RT-PCR with primers to the cloned delta opioid receptor (DOR), a cDNA fragment identical to the known DOR sequence was obtained from the smooth muscle cell line, A-10. Stimulation of A-10 cells with DAMA resulted in a significant mobilization of intracellular calcium and membrane depolarization. Exposure of aortic rings denuded of endothelium to DAMA induced a naltrindole-senstive increase in contractile tone. These data demonstrate the presence of a functional DOR in vascular smooth muscle and a direct impact of opioids on vascular contractile tone.

The opioid peptide analog d-Ala 2-Met-enkephalinamide decreases bile flow by a central mechanism☆

The existence of an opioid central pathway that may regulate bile secretion was explored by studying the effect of the intracisternal (i.c.) administration of the opiate d-Ala 2-Met-enkephalinamide (DAME) on bile secretion in anesthetized male rats. The i.c. administration of DAME was associated with a dose-related decrease in bile flow that ranged from 12% to 41%, which was prevented by the opiate antagonist naloxone. Bicarbonate secretion into bile decreased significantly after i.c. DAME. Chemical adrenergic denervation and cholinergic pharmacological blockade with atropine did not prevent the DAME-induced decrease in bile flow. The data support the existence of an opioid-mediated pathway that starts in the brain and that contributes to the regulation of bile secretion.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: Differential effects of intraventricular D-Ala 2-Met 5-enkephalinamide, D-Ala 2, N-Me-Phe 4, Gly-Ol 5-enkephalin, and D-Pen 2, D-Pen 5- enkephalin administration

D-Ala 2-Met 5-enkephalinamide (DALA) (1.0 μg/μl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala 2, N-Me-Phe 4, Gly-Ol 5-enkephalin (DAGO) (0.01 μg/μl) or D-Pen 2, D-Pen 5-enkephalin (DPDPE) (1.0 μg/μl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 μg/μl DAGO or 0.01 μg/μl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 μg/μl DAGO or 1.0 μg/μl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, μ, δ, and μ-δ activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Morphine- and anadamide-stimulated nitric oxide production inhibits presynaptic dopamine release

Morphine and anandamide stimulate the release of nitric oxide (NO) in diverse tissues. The present study examines the consequences of this action on neurotransmitter release in ganglia from two invertebrates: ventral chain ganglia from the leech Hirudo medicinalis and the pedal ganglion from the mussel Mytilus edulis. In these ganglia, preloaded serotonin (5-HT) and dopamine (DA)_can be released by 50 mM KCl. Anandamide, an endogenous cannabinoid substance, suppresses the potassium-stimulated release of [ 3H]DA (80%), but not 5-HT, in a concentration-dependent manner, from the neural tissues of both. The effect of anandamide can be antagonized by pre-exposing the neural tissues of both animals to SR 141716A, a potent cannabinoid receptor antagonist. Prior treatment of the ganglia with N-ω-nitro- l-arginine methyl ester ( l-NAME), a nitric oxide synthase inhibitor, significantly diminishes the inhibitory effect of anandamide. Morphine also inhibits [ 3H]DA release in a naloxone- and l-NAME-sensitive manner. Anandamide and morphine act through separate mechanisms since the respective antagonists show no cross-reactivity. The NO donor, SNAP, depressed the potassium-stimulated release of preloaded [ 3H]DA, but not 5-HT, in the neural ??????? of both animals. d-Ala 2-Met 5 enkephalinamide (DAMA) also inhibited the potassium-stimulated release of [ 3H]DA in a naloxone-sensitive process. However, the effect of DAMA was seen in the presence of l-NAME (10 4 M), indicating that the opioid peptide inhibition of the presynaptic release of DA is not coupled to NO. We postulate that cannabinoids and their endogenous effectors play a prominent role in the regulation of catecholamine release in invertebrates via NO release as is the case for opiate alkaloids.

Surgical anticipatory stress manifests itself in immunocyte desensitization: Evidence for autoimmunoregulatory involvement

The immunocyte behavior (conformational changes and locomotion in response to signal molecule challenge) in patients about to undergo elective cardiac surgery was studied to elucidate the effect of psychological anticipatory stress on the immune system. Granulocytes and monocytes from 10 patients and 35 non-surgical controls were examined. Computer-assisted microscopic image analysis, capable of measuring cellular conformational and velocity changes, was used to measure the responsiveness of these immunocytes to peptidergic and cytokine stimulation. Immunocyte desensitization would appear to account for the reduction in their abilities to respond to chemotaxic challenge associated with the pre-cardiac surgery state. Their abilities to respond to d-Ala2-Met-enkephalinamide (DAMA) were observed only at much higher concentrations than previously reported (10–11 M vs. 10-9 M prior to surgery). This finding, together with the observed decrease in adrenocorticotropin levels compared to non-surgical controls, suggests that neutral endopeptidase activity was elevated just prior to surgery. Indeed, neutral endopeptidase activity is statistically elevated in the pre-cardiac surgery state. Furthermore, glucocorticoid levels remained constant, within normal resting limits, in both groups. Thus, surgical anticipatory stress may manifest itself, in part, as a desensitization of various immunocytes. Thus, a psychological anticipatory stress response may be a precipitant of the desensitization. Although this desensitization seemed not to involve the entire hypothalamic-pituitary-adrenal axis, the data suggest that psychological anticipatory stress may initially involve and influence autoimmunoregulation.

Peripheral excitatory effects of two enkephalinase inhibitors, acetorphan and thiorphan, and an enkephalin analogue, [D-Ala2-Met5]-enkephalinamide, on uterine motility in periparturient rats in vivo and in vitro.

The effects of two enkephalinase inhibitors, acetorphan and thiorphan, and the enkephalin analogue [D-Ala2-Met5]-enkephalinamide (DAMEA), on spontaneous uterine contractions were studied at day 21 of pregnancy in rats following treatment in vivo or in vitro. Acetorphan (10 mg kg-1) and thiorphan (1 mg kg-1), immediately after their i.v. administration, increased the duration of spontaneous contractions 3.4- and 4.6-fold, respectively, but did not modify the maximum amplitude. Similarly, thiorphan (40 mumol l-1) increased the duration of contractions when administered in vitro. Thiorphan was ineffective during the first 30 min when given into the cerebral ventricles (50 micrograms per rat). These results suggest that the enkephalinase inhibitors are acting via a peripheral opioid pathway; and this conclusion is supported by the observation that thiorphan potentiated the stimulatory effect of a submaximal dose of DAMEA administered in vitro. The excitatory effects of DAMEA and the enkephalinase inhibitors were blocked by naloxone. This antagonistic effect of naloxone on uterine motility in the periparturient rat uterus, induced by either acetorphan and thiorphan or DAMEA, seems to be regulated by peripheral opiate receptors. Naloxone (10 mg kg-1 s.c.) increased both the amplitude and duration of uterine motility in vivo; however, naloxone (26 mumol l-1 and 52 mumol l-1) produced a paradoxical dose-dependent biphasic effect in vitro.

Delta opioid agonists inhibit proliferation of highlypurified murine CD4+ and CD8+ T-cells.

A substantial body of evidence demonstrates that opiates and opioid peptides modulate immune function. The present study used highly purified murine CD4+ and CD8+ T-cells to determine the effects of delta opioid receptor (DOR) agonists on proliferation. Splenic T-cells, obtained from male or female C57BL/6 or CD1 mice, were separated by a fluorescence activated cell sorter. Cells were stimulated to proliferate in serum free medium by cross-linking the T-cell receptor using plate-coated anti-CD3-epsilon, 3H-thymidine uptake was determined at 48 hours. Previous experiments had shown that deltorphin and [D-Ala2]-met-enkephalinamide (DAME), at concentrations from 10(-11) to 10(-7) M, dose dependently inhibited the proliferation of CD4+ and CD8+ T-cells obtained from female C57BL/6 or CD1 mice. Similarly, the experiments herein demonstrate that proliferation of CD4+ T-cells from female CD1 mice was inhibited by 2,5 DPDP-enkephalin (DPDP-E), in direct relation to dose. In contrast, the anti-proliferative response of cells from C57BL/6 mice demonstrated an inverse relationship to dose. At 10(-11) M, the most effective dose of DPDP-E studied, 3H-thymidine uptake was inhibited by 50%. The selective DOR antagonist, naltrindole (10(-12) M), abolished this. DAME was used to compare the effects of DOR agonists on CD8+ T-cells from both strains of female mice. 3H-Thymidine uptake was dose-dependently inhibited to a similar degree in both strains; 10(-7) M DAME maximally reduced proliferation by 70%. DAME had similar effects on both CD8+ and CD4+ T-cells from male mice, and its inhibitory effect was markedly attenuated after 72 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of accumbens DALA microinjections on brain stimulation reward and behavioral activation in intact and 6-OHDA treated rats.

The effects of bilateral nucleus accumbens microinjections of d-ala-met-enkephalinamide (DALA) were assessed in behavioral activation and lateral hypothalamic self-stimulation (LHSS) rate-frequency curve-shift paradigms in normal and accumbens 6-OHDA (4.0 micrograms) treated rats. Microinjections of DALA (2.5 micrograms/microliters) in the behavioral activation paradigm had little effect on normal activity; however, DALA administered to 6-OHDA treated rats produced a significant overall increase in locomotion. The 6-OHDA DALA-induced locomotion effect peaked at 2 weeks after 6-OHDA treatment and then returned to baseline levels by week 5 post-treatment. Using LHSS, DALA tested over a range of doses (2.5, 5, 10, 20 micrograms/microliters) displayed a weak biphasic reward effect only at the highest dose, which was characterized by an initial suppression followed by an elevation. DALA significantly depressed initial operant motor/performance in LHSS in a dose dependent fashion. Microinjections of the normally ineffective low dose of DALA (2.5 micrograms/microliters) following accumbens 6-OHDA treatment produced a significant LHSS reward decrease 2 weeks post-treatment, while LHSS motor/performance was relatively unaffected. Results are discussed in terms of opiate-dopamine and limbic-motor interactions.

Effect of sorbin on duodenal absorption of water and electrolytes in the rat

Sorbin is a newly isolated intestinal peptide that has been purified because of its ability to induce water absorption. The effects that sorbin and some synthetic peptides corresponding to its C-terminal sequence have on duodenal absorption of water, chloride, and sodium were studied in comparison with the effects of vasoactive intestinal peptide (VIP), [D-Ala,Met]-enkephalinamide (DAMA), and angiotensin II. The technique of an in situ ligated duodenal loop in the rat was used for all peptides. Under the experimental conditions used, a low basal secretion of water, chloride, and sodium was obtained; VIP induced an increase of the secretion, whereas DAMA induced an absorption, both in the nanomolar dose range. Angiotensin II and sorbin induced an absorption in the picomolar dose range. The most effective doses of sorbin peptides but not of angiotensin induced the lowest final concentrations of Na+ and Cl− obtainable without inducing secondary water secretion. All synthetic peptides containing the C-terminal heptapeptide of sorbin were active in the picomolar dose range. Contrary to angiotensin, they had no effect on blood pressure.

Opioid inhibition of GABA release from presynaptic terminals of rat hippocampal interneurons

Opiates and the opioid peptide enkephalin can cause indirect excitation of principal cortical neurons by reducing inhibitory synaptic transmission mediated by GABAergic interneurons. The mechanism by which opioids mediate these effects on interneurons is unknown, but enkephalin hyperpolarizes the somatic membrane potential of a variety of neurons in the brain, including hippocampal interneurons. We now report a new, more direct mechanism for the opioid-mediated reduction in synaptic inhibition. The enkephalin analog d-Ala 2-Met 5-enkeph-alinamide (DALA) decreases the frequency of miniature, action potential-independent, spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) without causing a change in their amplitude. Thus, we conclude that DALA inhibits the action potential—independent release of GABA through a direct action on interneuronal synaptic terminals. In contrast, DALA reduces the amplitude of action potential—evoked, GABA-mediated IPSCs, as well as decreases their frequency. This suggests that the opioid-mediated inhibition of non—action potential—dependent GABA release reveals a mechanism that contributes to reducing action potential—evoked GABA release, thereby decreasing synaptic inhibition.

Intraventricular administration of D-Ala 2-Met 5-enkephalinamide induces rapid recovery of responding for electrical brain stimulation from the ventral tegmental area following uncontrollable footshock

The effects of uncontrollable footshock were assessed in CD-1 mice responding for electrical brain stimulation (ICSS) from the dorsal aspects of the ventral tegmental area (VTA). Uncontrollable footshock provoked a marked reduction of responding for ICSS immediately, 24 h and 168 h following initial stresser exposure. Intraventricular administration of the neuropeptide, D-Ala 2-Met 5-enkephalinamide (DALA) (1 μg/μl delivered in a 1-μl volume) following induction of reward alterations from the dorsal VTA provoked a rapid recovery of responding for ICSS in the immediate poststressor interval. This ameliorative effect of the neuropeptide deteriorated 24 h after initial exposure to uncontrollable footshock and was absent 1 week later. These data are discussed with respect to stressor-induced anhedonia in the mesocorticolimbic system.

Pertussis-toxin insensitive enkephalin inhibition of adenylyl cyclase in lacrimal acinar cells

The mechanism by which the inhibitory effect of d-ala 2-met-enkephalinamide (DALA) on lacrimal acinar adenylyl cyclase is exerted was assessed in membrane preparations by a cAMP protein binding assay. Inhibition by the analogue was GTP-dependent with a significant enhancement of the inhibitory effect by GTP. While pretreatment of membranes with either cholera or pertussis toxin resulted in stimulation of adenylyl cyclase activity, modification of the G iα subunit by pertussis-toxin catalyzed ADP-ribosylation did not effect the hormonal inhibition of adenylyl cyclase. Incubation of membranes with manganese, however, prevented the inhibitory action of DALA in addition to enhancing basal and forskolin-stimulated adenylyl cyclase activity. The results suggest that the inhibitory effect of DALA in lacrimal acinar cells is exerted via a mechanism other than pertussis-toxin sensitive coupling of the receptor to adenylyl cyclase through G i. The mechanism may be effected through a pertussis-toxin insensitive G protein, through an interaction with G i that is pertussis-toxin insensitive, or through an interaction with the catalytic subunit of adenylyl cyclase.

TRH-enkephalin interactions in the amygdaloid complex during gastric stress ulcer formation in rats

The formation of gastric stress ulcers was studied as a function of interactions between thyrotropin releasing hormone (TRH) and endogenous opioids in the central amygdalar nucleus (CEA) in rats. Bilateral microinjections of TRH (1 or 10 μg) into the CEA produced dose-related aggravations in cold restraint stress (CRS, 3 h at 4°C) — induced gastric ulcer formation. Similar stress ulcer facilitating effects were also seen with intra-CEA injections of the opioid antagonist, naloxone (1 or 10 μg). On the other hand, the enkephalin analog, d-Ala 2-metenkephalinamide (DAMEA, 1, 10 or 20 μg) produced dose-dependent attenuations in gastric stress pathology, the effects being most marked with the latter two doses. Pretreatment of rats with intra-CEA naloxone (1 μg) (a) antagonized the gastric cytoprotective effects of DAMEA (20 μg) and (b) further aggravated the ulcerogenic response of TRH (1 μg), without influencing significantly the TRH (10 μg) effect. Further, when DAMEA (20 μg) was administered intra-CEA just after TRH (10 μg), the stress ulcer facilitating effects of the latter was neutralized. The results indicate that TRH-enkephalin interactions are possible at the level of the CEA during CRS-induced gastric ulcer formation.

Conditioned place preference produced by infusion of Met-enkephalin into the medial preoptic area

The conditioned place preference procedure was used to evaluate the rewarding properties of d-Ala 2-Met 5-enkephalinamide (DALA) after bilateral infusion into the medial preoptic area. Doses of 60,250 and 1000 ng/cannula were used. It was found that all doses of DALA produced place preference. This suggests that the medial preoptic area is a structure where opioid reward is produced in doses as low as those required in already established reward systems. The significance of this in relation to sexual reward is discussed.

Opioid modulation of Pavlovian learning in rabbits: Involvement of sublenticular pathways.

Intravenous naloxone enhanced, and D-Ala2-Met-enkephalinamide (DALA) impaired, Pavlovian conditioned heart rate discrimination in rabbits (Oryctolagus cuniculus) during initial training trials, compared with saline; naloxone also delayed subsequent extinction of the discrimination. These effects of the opioid treatments on discrimination were abolished by parasagittal knife-cut lesions in the sublenticular substantia innominata that did not, themselves, impair discrimination. Both naloxone and DALA decreased the magnitude and altered the topography of bradycardiac conditioned responses, and the lesions also abolished these effects of the opioid treatments, but they did not alter the effect of naloxone to decrease bradycardiac orienting response magnitude. These findings suggest that fibers in the sublenticular area mediate specifically the effects of systemic opioids on associative functions during Pavlovian cardiac conditioning and extinction.