Differential effects of specific opioid receptor agonists on rat pup isolation calls.

The possibility was investigated that specific opioid receptor types might selectively alter the production of isolation-induced ultrasonic vocalizations. Intracisternal injections of mu, delta and kappa opioid receptor agonists were administered to isolated 10-day-old rat pups. The mu receptor agonist [D-Ala2-NMe-Phe4-Gly-ol]-enkephalin (DAMGO) and delta receptor agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE) both reduced the rate of isolation-induced ultrasonic calling in the absence of sedation. The kappa receptor agonist U50,488 had the opposite effect, significantly raising the rate of vocalization. Fourteen-day-old pups, with a larger delta receptor population, showed a greater sensitivity to DPDPE than was seen in the younger animals.     

Multiple opioid receptors mediate feeding elicited by mu and delta opioid receptor subtype agonists in the nucleus accumbens shell in rats

The nucleus accumbens, and particularly its shell region, is a critical site at which feeding responses can be elicited following direct administration of opiate drugs as well as micro-selective and delta-selective, but not kappa-selective opioid receptor subtype agonists. In contrast to observations of selective and receptor-specific opioid antagonist effects upon corresponding agonist-induced actions in analgesic studies, ventricular administration of opioid receptor subtype antagonists blocks feeding induced by multiple opioid receptor subtype agonists. The present study examined whether feeding responses elicited by either putative mu ([D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO)), delta(1) ([D-Pen(2), D-Pen(5)]-enkephalin (DPDPE)) or delta(2) ([D-Ala(2), Glu(4)]-deltorphin (Deltorphin)) opioid receptor subtype agonists administered into the nucleus accumbens shell were altered by accumbens pretreatment with either selective mu (beta-funaltrexamine), mu(1) (naloxonazine), delta(1) ([D-Ala(2), Leu(5), Cys(6)]-enkephalin (DALCE)), delta(2) (naltrindole isothiocyanate) or kappa(1) (nor-binaltorphamine) opioid receptor subtype antagonists. Similar magnitudes and durations of feeding responses were elicited by bilateral accumbens administration of either DAMGO (2.5 microg), DPDPE (5 microg) or Deltorphin (5 microg). DAMGO-induced feeding in the nucleus accumbens shell was significantly reduced by accumbens pretreatment of mu, delta(1), delta(2) and kappa(1), but not mu(1) opioid receptor subtype antagonists. DPDPE-induced feeding in the accumbens was significantly reduced by accumbens pretreatment of mu, delta(1), delta(2) and kappa(1), but not mu(1) opioid receptor subtype antagonists. Deltorphin-induced feeding in the accumbens was largely unaffected by accumbens delta(2) antagonist pretreatment, and was significantly enhanced by accumbens mu or kappa(1) antagonist pretreatment. These data indicate different opioid pharmacological profiles for feeding induced by putative mu, delta(1) and delta(2) opioid agonists in the nucleus accumbens shell, as well as the participation of multiple opioid receptor subtypes in the elicitation and maintenance of feeding by these agonists in the nucleus accumbens shell.     

Opioids modulate cell division in the germinal zone of the late embryonic neocortex.

Opioid effects on cell division in the embryonic cerebral cortex were examined using two experimental approaches: (i) the presence of opioid receptors in the embryonic day 16 mouse neocortex was tested using immunohistochemical techniques; (ii) the values of the indices of [3H]thymidine pulse labelled cells and mitotic indices were estimated in the ventricular zone of the embryonic day 16 mouse neocortex 2.5, 4.5 and 8.5 h after administration to pregnant females of selected opioid receptor agonists or the opioid antagonist naloxone. The immunohistochemical study demonstrated that distinct subpopulations of the ventricular zone cells express mu, delta or kappa opioid receptors. Acute exposure of mouse embryos to mu, delta and kappa opioid receptor agonists or naloxone differentially affects the indices of [3H] thymidine pulse labelled cells and mitotic indices indicating changes in the cell cycle composition. Treatment with the mu opioid receptor agonist D-Ala2-MePhe4, Gly-ol5-enkephalin (DAGO), or the partially selective kappa opioid receptor agonist bremazocine, increased the [3H]thymidine labelling and mitotic indices. In contrast, the delta receptor agonist (D-Ser8)-leucine enkephalin-Thr (DSLET) produced a decrease in the labelled cell indices and mitotic indices. Naloxone provided a biphasic effect: a decrease in the values of labelled cell indices 2.5 h after naloxone administration, followed by an increase in the values of the indices at 4.5 and 8.5 h. These results suggest that the endogenous embryonic/maternal opioid systems are involved in the regulation of cell division in the ventricular zone of the late embryonic cortex.     

Opioid control of the in vitro release of calcitonin gene-related peptide from primary afferent fibres projecting in the rat cervical cord

In vitro superfusion of slices from the dorsal half of the rat cervical enlargement allowed the measurement of spontaneous, K+ (30 mM)- and capsaicin (0.5 microM)-evoked release of calcitonin gene-related peptide-like immunoreactive material (CGRPLI). The greater part of this immunoreactive material originated in primary afferent fibres since dorsal rhizotomy from C4 to Th2 (8 days before sacrifice) resulted in a 85-90% decrease in CGRPLI release. CGRPLI outflow which persisted after dorsal rhizotomy could still be enhanced by K+-induced depolarization but was no longer sensitive to the stimulatory effect of 0.5 microM capsaicin. Both delta (DTLET, D-Pen2-D-Pen5-enkephalin) and mu (DAGO, PL 017) opioid receptor agonists reduced the K+ evoked release of CGRPLI from the dorsal half of the cervical enlargement. Morphine was also inhibitory but the selective K opioid agonist U 69593 was inactive. As expected from the involvement of delta and mu receptors, the selective opioid antagonist ICI 174864 and naloxone prevented the inhibitory effects of DTLET and DAGO, respectively. These data suggest that opioid-induced presynaptic inhibiton of CGRP-containing primary afferent fibres may be involved in the analgesic effect of intrathecally injected delta and mu opioid agonists in rats.     

Activation of MU opioid receptors in the pre-optic-anterior hypothalamus releases prolactin in the conscious male rat

Microinjection of opioid agonists into the pre-optic-anterior hypothalamus (PO/AHA) was used to determine the identity of the opioid receptor subtype(s) involved in the stimulation of prolactin release. The mu agonist DAGO [(D-Ala2, NMe-Phe4, Gly-o15)-enkephalin] was the only opioid agonist to show dose-dependent release of prolactin, the lowest significant dose being 0.001 nmoles. Neither the specific delta agonist DPDPE [(D-Pen2, D-Pen5)-enkephalin] nor the specific kappa agonist U50,488H [(trans-3,4-dichloro-N-methyl-N-(2-(1-pyrrodinyl)-cyclohexyl)-benz ene acetamide] showed dose-dependent increase of prolactin secretion, or indeed any significant increase in prolactin secretion in the dose range 0.01-1 nmoles and 0.01-10 nmoles respectively. We suggest that mu (and not kappa or delta) opioid receptors in the PO/AHA are involved in the opioid stimulated release of prolactin in the conscious male rat.     

Dissociation of mu and delta opioid receptor-mediated reductions in evoked and spontaneous synaptic inhibition in the rat hippocampus in vitro.

Modulation of gamma-aminobutyric acid (GABA)-mediated inhibition, and glutamate-mediated excitation by highly selective mu and delta opioid agonists was studied using intracellular recordings of CA1 pyramidal neuron synaptic responses in superfused hippocampal slices. Equimolar concentrations of the selective mu agonist, [Tyr-(D-Ala)-Gly-(N-Me-Phe)-Gly-ol]-enkephalin (DAGO), or the delta selective agonist, [D-Pen2,D-Pen5]-enkephalin (DPDPE), reversibly increased the amplitudes of excitatory post-synaptic potentials (EPSPs), evoked by Schaffer collateral/commissural stimulation, without altering the input resistance or resting membrane potential of these CA1 pyramidal neurons. The increased EPSP amplitudes resulting from superfusion with the enkephalin analogs were qualitatively similar to those caused by the GABAA receptor antagonist, bicuculline methiodide (BMI). Specific stimulation/recording protocols and micro-lesions of the slices were used to evoke relatively pure forms of recurrent and feed-forward GABA-mediated inhibitory post-synaptic potentials (IPSPs). The mu opioid agonist DAGO reduced both recurrent and feed-forward IPSPs, while the delta agonist DPDPE had no effect upon these responses. To test the hypothesis that the enhancement of pyramidal neuron EPSPs by delta (and mu) opioids was due to the reduction of an inhibitory potential that was coincident with the EPSP, DPDPE or the mu agonist, DAGO, were applied while recording monosynaptic IPSPs following the elimination of EPSPs by the glutamate receptor antagonists, D,L-2-amino-5-phosphonovalerate (APV) and 6,7-dinitroquinoxaline-2,3-dione (DNQX). The mu agonist, DAGO, reversibly reduced these pharmacologically isolated IPSPs, while the delta agonist, DPDPE, had no effect upon these responses. Despite the fact that the delta agonist, DPDPE, had no effect on recurrent, feed-forward or monosynaptic evoked IPSPs, this enkephalin did reversibly reduce the frequency of spontaneously occurring IPSPs, measured using whole-cell recordings with pipettes containing 65 mM KCl. The mu agonist, DAGO, and the GABAA antagonist, BMI, similarly reduced spontaneous IPSP rates. We conclude from these data that mu and delta opioid receptor activation increases EPSPs via the reduction of a form of GABAergic inhibition that is difficult to characterize, and which may be distinct from conventional feed-forward and recurrent inhibition. Furthermore, delta opioids seem to reduce this form of GABAergic inhibition selectively, while mu opioids reduced this inhibition, and conventional feed-forward and recurrent IPSPs as well.     

Increased delta, but not mu, opiate receptor binding in the medulla oblongata of Long-Evans rats following 5-day water deprivation

Opiate receptors of the mu type were labeled with [125I]D-Ala2,N-Me-Phe4,Met-(O)5-ol-enkephalin (FK-33824). delta receptors were labeled with [125I]D-Ala2-D-Leu5-enkephalin (DADLE) in the presence of excess (N-Me-Phe3,D-Pro4)-morphiceptin (PL017). Since DADLE binds mu and delta receptor sites, and PL017 blocks mu receptors, this protocol improves specific labeling of delta receptors. Quantitative autoradiography showed that chronic dehydration causes no changes in mu receptor binding in the medulla oblongata of Long-Evans rats. However, there is increased delta receptor binding in the solitary, hypoglossal and gracilis nuclei, and the spinal nucleus of trigeminal system of dehydrated animals, suggesting that delta opiate receptors participate in the physiological response to dehydration.     

Glial growth is regulated by agonists selective for multiple opioid receptor types in vitro.

To determine whether one or more opioid receptor types might be preferentially involved in gliogenesis, primary mixed glial cultures derived from mouse cerebra were continuously treated with varying concentrations of opioid agonists selective for mu (mu), i.e., DAGO ([D-Ala2, MePhe4, Gly(ol)5]enkephalin), delta (delta), i.e., DPDPE ([D-PEN2,D-PEN5]enkephalin), or kappa (kappa), i.e., U69,593, opioid receptor types. In addition, a group of cultures was treated with [Met5]-enkephalin, an agonist for delta opioid receptors as well as putative zeta (zeta) opioid receptors. Opioid-dependent changes in growth were assessed by examining alterations in (1) the number of cells in mixed glial cultures at 3, 6, and 8 days in vitro (DIV), (2) [3H]thymidine incorporation by glial fibrillary acidic protein (GFAP) immunoreactive, flat (type 1) astrocytes at 6 DIV, and (3) the area and form factor of GFAP-immunoreactive, flat (type 1) astrocytes. DPDPE at 10(-8) or 10(-10) M, as well as [Met5]-enkephalin at 10(-6), 10(-8), or 10(-10) M, significantly reduced the total number of glial cells in culture; but this effect was not observed with DAGO or U69,593 (both at 10(-6), 10(-8), or 10(-10) M). Equimolar concentrations (i.e., 10(-6) M) of [Met5]enkephalin or U69,593, but not DPDPE or DAGO, suppressed the rate of [3H]thymidine incorporation by GFAP-immunoreactive, flat (type 1) astrocytes. DAGO had no effect on growth, although in previous studies morphine was found to inhibit glial numbers and astrocyte DNA synthesis. [Met5]enkephalin (10(-6) M) was the only agonist to significantly influence astrocyte area. Collectively, these results indicate that delta (and perhaps mu) opioid receptor agonists reduce the total number of cells in mixed glial cultures; while [Met5]enkephalin-responsive (and perhaps kappa-responsive) opioid receptors mediate DNA synthesis in astrocytes. This implies that delta opioid receptors, as well as [Met5]enkephalin-sensitive, non-delta opioid receptors, mediate opioid-dependent regulation of astrocyte and astrocyte progenitor growth. These data support the concept that opioid-dependent changes in central nervous system growth are the result of endogenous opioid peptides acting through multiple opioid receptor types.     

Opioid control of the release of Met-enkephalin-like material from the rat spinal cord.

The possible control by opioids of the release of Met-enkephalin-like material (MELM) from the rat spinal cord was investigated in vitro and in vivo. Superfusion of slices of the dorsal zone of the lumbar enlargement with the mu selective agonists DAGO or PL 017 or the delta selective agonist DTLET produced a significant reduction in the K(+)-evoked MELM release from these tissues. These effects persisted in the presence ot tetrodotoxin, as expected from their mediation through presynaptically located opioid autoreceptors. Furthermore, the inhibitory effect of DAGO and PL 017, but not that of DTLET, was prevented by the preferential mu antagonist naloxone. Conversely, the effect of DTLET was prevented by the delta antagonist naltrindole but not by naloxone. In vivo experiments performed in halothane-anaesthetized rats have shown that the intrathecal perfusion of DAGO and DTLET significantly depressed the spontaneous MELM outflow from the whole spinal cord. In contrast to these mu and delta agonists, the kappa selective agonist U 50488 H did not affect the in vivo- and only slightly reduced (at a very high concentration: 50 microM) the in vitro-release of MELM from the rat spinal cord. These data indicate that both mu and delta opioid autoreceptors are involved in a local presynaptic autoinhibitory control of MELM release in the rat dorsal horn.     

Analysis of opioid receptor subtype antagonist effects upon mu opioid agonist-induced feeding elicited from the ventral tegmental area of rats

The present study examined opioid receptor(s) mediation of feeding elicited by mu opioid agonists in the ventral tegmental area using general or selective opioid antagonist pretreatment. Naltrexone as well as equimolar doses of selective mu and kappa, but not delta opioid antagonists in the ventral tegmental area significantly reduced mu agonist-induced feeding, indicating a pivotal role for these receptor subtypes in the full expression of this response.     

Delta opiate receptors are involved in the endopioid-induced myoclonic contractions

Morphine, the prototype mu opiate receptor agonist, decreased the spontaneous and [D-Ala2]-Met-enkephalinamide (DALA)-induced myoclonic contractions (MC) of submandibular muscles in the anaesthetized rat. The proposed kappa receptor agonists ketocyclazocine, ethylketocyclazocine and bremazocine failed to induce MC. In addition, bremazocine inhibited the spontaneous and DALA induced MC. Cyclazocine, the so-called sigma opiate receptor agonist, had a weak potency in generation of MC, but without step dose response tendency. The most potent opioid peptide in inducing the MC and electrocortical (ECoG) epileptic pattern was the delta opiate receptor agonist [D-Ala2,D-Leu5]-enkephalin (DADL). All drugs were administered intraventricularly. The results indicate that myoclonic phenomena induced by DADL and probably by other endopioids are mediated by delta opiate receptors in the rat brain. It is suggested that the combined ECoG and EMG method used in this study offers an opportunity to define further the biological role of opiate receptors and to identify the potential delta opiate receptor acting drugs, which might provide a new approach to the therapy of some seizure disorders.     

Opioids intrinsically inhibit the genesis of mouse cerebellar granule neuron precursors in vitro: differential impact of mu and delta receptor activation on proliferation and neurite elongation

Although opioids are known to affect neurogenesis in vivo, it is uncertain the extent to which opioids directly or indirectly affect the proliferation, differentiation or death of neuronal precursors. To address these questions, the intrinsic role of the opioid system in neurogenesis was systematically explored in cerebellar external granular layer (EGL) neuronal precursors isolated from postnatal mice and maintained in vitro. Isolated neuronal precursors expressed proenkephalin-derived peptides, as well as specific mu and delta, but negligible kappa, opioid receptors. The developmental effects of opioids were highly selective. Morphine-induced mu receptor activation inhibited DNA synthesis, while a preferential delta2-receptor agonist ([D-Ala2]-deltorphin II) or Met-enkephalin, but not the delta1 agonist [D-Pen2, D-Pen5]-enkephalin, inhibited differentiation within the same neuronal population. If similar patterns occur in the developing cerebellum, spatiotemporal differences in endogenous mu and delta opioid ligand-receptor interactions may coordinate distinct aspects of granule neuron maturation. The data additionally suggest that perinatal exposure to opiate drugs of abuse directly interfere with cerebellar maturation by disrupting normal opioid signalling and inhibiting the proliferation of granule neuron precursors.     

The effects of bilateral intranigral microinjection of selective opioid agonists on behavioral responses to noxious thermal stimuli.

This study examined the effects of bilateral intranigral microinjection of selective opioid agonists on the tail-flick and hot-plate antinociception tests. The principal findings are: (1) the mu-selective agonist D-Ala2, N-Me-Phe4, Gly5-ol-enkephalin (DAGO) had antinociceptive effects on both tests which were reversible by beta-funaltrexamine (beta-FNA: a mu-selective antagonist) and naloxone (a non-selective opioid antagonist); (2) the antinociceptive potency of DAGO injected into the nigra is comparable to its potency in the periaqueductal gray; (3) intranigral D-Pen2, D-Pen5-enkephalin (a delta-selective agonist), U-50, 488H and dynorphin A-(1-13) (kappa-selective agonists) had no antinociceptive effects; (4) antinociceptive effects were produced by the mixed delta/mu agonists D-Thr2-leucine enkephalin-Thr (DTLET) and D-Ser2-leucine enkephalin-Thr (DSLET); (5) the effect of DTLET on the hot-plate but not the tail-flick test was reversed by Cys2, Tyr3, Orn5, Pen7-amide (CTOP; a mu-selective antagonist), beta-FNA, and naloxone, but not by the delta-selective antagonist naltrindole. Based on the potent antinociceptive effects of DAGO, the complete lack of such effects by the highly selective delta and kappa agonists, and the antagonism of DTLET by CTOP and beta-FNA, it is concluded that the antinociceptive effects of intranigral opioid agonists are mediated by mu receptors.     

Analysis of dopamine receptor antagonism upon feeding elicited by mu and delta opioid agonists in the shell region of the nucleus accumbens

The nucleus accumbens (NAcc) has been implicated as an important reward site for the mediation of unconditioned reinforcers such as food. Although both mu-selective and delta-selective opioid agonists in the NAcc induce spontaneous and palatable feeding, these effects are mediated by multiple opioid receptor subtypes within the nucleus. A role for dopaminergic mediation of feeding in the NAcc is based upon selective antagonist-induced suppression of feeding induced by systemic amphetamine. The present study investigated whether feeding elicited by infusion of either mu ([D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin) or delta(2) ([D-Ala(2), Glu(4)]-deltorphin) opioid receptor subtype agonists in the shell region of the NAcc would be modified by intra-accumbens pretreatment with equimolar (12-100 nmol) doses of either D(1)-selective (SCH23390) or D(2)-selective (raclopride) antagonists. Both opioid agonists displayed comparable magnitudes and durations of feeding responses in the NAcc. SCH23390 significantly and dose-dependently reduced mu agonist-induced feeding in the NAcc with significant reductions noted following the two higher, but not two lower doses. In contrast, raclopride pretreatment produced inconsistent effects upon mu agonist-induced feeding with limited actions across doses and test times. Further, neither SCH23390 nor raclopride pretreatment in the NAcc affected feeding elicited by the delta(2) opioid agonist. These data indicate that the role of dopamine receptors in mediating opioid-induced feeding within the shell region of the NAcc is both dependent upon the dopamine receptor subtype that was blocked (D(1) vs. D(2)) as well as the opioid receptor subtype which was being stimulated mu vs. delta(2)).     

Morphine can produce analgesia via spinal kappa opioid receptors in the absence of mu opioid receptors

Previous studies have demonstrated the virtual lack of analgesia in mu opioid receptor knockout mice after systemic administration of morphine. Thus, it has been suggested that analgesic actions of morphine are produced via the mu opioid receptor, despite its ability to bind to kappa and delta receptors in vitro. However, it is not clear whether the results of these studies reflect the effect of morphine in the spinal cord. In the present study, we report study of the analgesic actions of spinally-administered morphine and other opioid receptor agonists in mu opioid receptor knockout and wild type mice. Morphine produced a dose-dependent antinociceptive effect in the tail flick test in the knockout mice, although higher doses were needed to produce antinociception than in wild type mice. The antinociceptive effect of morphine was completely blocked by naloxone (a non-selective opioid antagonist) and nor-binaltorphimine (nor-BNI, a selective kappa-opioid receptor antagonist), but not by naltrindole (a selective delta-opioid receptor antagonist). U-50,488H (a selective kappa-opioid receptor agonist) also produced a dose-dependent antinociceptive effect in knockout mice but presented lower analgesic potency in knockout mice than in wild type mice. Analgesic effects of [d-Pen2,d-Pen5]enkephalin (DPDPE, a selective delta-opioid receptor agonist) were observed in wild type mice but abolished in knockout mice. SNC80 (a selective delta-opioid receptor agonist) was not antinociceptive even in wild type mice. The present study demonstrated that morphine can produce thermal antinociception via the kappa opioid receptor in the spinal cord in the absence of the mu opioid receptor. Lower potency of U50,488H in mu opioid receptor knockout mice suggests interaction between kappa and mu opioid receptors at the spinal level.     

Effects of guanyl nucleotides and ions on kappa opioid binding

Displacement curve analyses demonstrated that GTP and its nonhydrolyzable analog, GPP(NH)P, inhibited the binding of [3H]dihydromorphine (mu agonist), [3H]D-Ala2-D-Leu5-enkephalin (delta agonist), and [3H]ethylketocyclazocine (general agonist) but not [3H]diprenorphine (general antagonist). Using a paradigm to block mu and delta sites with specific cold ligands, [3H]ethylketocyclazocine labeled kappa sites which were less GTP sensitive than sites labeled by mu and delta agonists. Further, Na+ and Mg++, important in inhibitory adenylate cyclase systems, also decreased both unblocked and mu-/delta-blocked [3H]ethylketocyclazocine binding. Scatchard analyses revealed that the inhibitory effects of GTP result in a decrease in affinity without a significant change in binding capacity, and that the kappa component of [3H]ethylketocyclazocine binding was less sensitive to the effects of GTP than binding sites labeled by mu or delta agonists. In comparison to the effects of GTP, Na+ decreased binding affinity but increased the binding capacity of the kappa component. These data also suggest that the inhibitory effects of Na+ and GTP on binding affinity are not additive. Association and dissociation plots revealed that although both components of binding may be involved in these affinity changes, the dissociation rate represents the more significant factor. These data suggest that [3H]ethylketocyclazocine binding to kappa sites is GTP and Na+ sensitive. However, it should be noted that [3H]ethylketocyclazocine binding to kappa sites is less sensitive to GTP than its binding to other opiate sites, and that this kappa binding is differentially affected by Na+. The significance of these characteristics with regard to the effect of kappa opiates on adenylate cyclase activity remains to be determined.     

Naltrexone-induced opiate receptor supersensitivity

Chronic administration of the long-lived narcotic antagonist naltrexone resulted in a marked increase in brain opiate receptors. Similar changes in receptor density were observed for binding of the putative mu agonist [3H]dihydromorphine, the mu antagonist [3H]naloxone, the putative delta ligand [3H]D-Ala2,D-Leu5-enkephalin and [3H]etorphine. In addition, the sensitivity of agonist binding to guanyl nucleotide inhibition increased significantly. In contrast, no such changes in opiate binding were observed following acute administration of naltrexone. The increase in opiate receptor number following chronic naltrexone was highest in the mesolimbic and frontal cortex areas, and lowest in the dorsal hippocampus and periaqueductal gray. These results indicate a degree of plasticity in the opiate receptor system that may correlate with specific functional pathways.     

The effect of μ, δ, and ε opioid receptor agonists on heart rate and blood pressure of the pithed rat

Opioid peptides and opioid receptors are found in the hearts of various species. Opioid peptides were also shown to modulate norepinephrine inducing changes in atrial rate, in vitro. Since we have recently shown a predominance of kappa and delta receptors in the rat atria, we found it of interest to study the role of highly selective opioid agonists on spontaneous and sympathetically stimulated heart rate. The pithed, artificially ventillated rat was used in these studies. D-Ala2-D-Leu5-enkephalin (DADL), was used as an delta-agonist, D-Ala2-MePhe4-Gly-ol5-enkephalin (DAGO) as a highly selective mu-agonist; Dynorphin A (1-17) as a kappa-agonist and beta-endorphin (beta-END) as a mixed epsilon-delta-mu agonist. Naloxone was used as an opiate antagonist. None of the above opioid peptides changed the basal blood pressure and heart rate at 1-100 nmol/kg except Dyn A-(1-17) which produced a brief depressor response (-15 +/- 2 mmHg, p less than 0.01). Stimulation of the spinal cord (50 v, 1 msec, 1 Hz, 30 sec) produced consistant pressor and cardiac accelerating responses. None of the opioid peptides studied blocked or enhanced the increase in blood pressure or heart rate produced by spinal cord stimulation. The depressor effect of the high dose of Dyn A-(1-17) was not blocked by naloxone. These results suggest that mu, delta or kappa opioid receptors in the rat heart have no role in the regulation of basal or sympathetically driven heart rate. Our data also suggest no role for these opioid receptors in modulation of basal arterial tone or norepinephrine-induced arteriolar constriction.     

gamma-Aminobutyric acid receptor subtype antagonists differentially alter opioid-induced feeding in the shell region of the nucleus accumbens in rats.

Food intake is significantly increased by administration of mu-selective opioid agonists into the nucleus accumbens, particularly its shell region. Pretreatment with either opioid (mu, delta(1), delta(2) or kappa(1)) or dopaminergic (D(1)) receptor antagonists in the nucleus accumbens shell reduce mu opioid agonist-induced feeding. Selective GABA(A) (muscimol) and GABA(B) (baclofen) agonists administered into the nucleus accumbens shell each stimulate feeding which is respectively and selectively blocked by GABA(A) (bicuculline) and GABA(B) (saclofen) antagonists. The present study investigated whether feeding elicited by the mu-selective opioid agonist, [D-Ala(2),NMe(4),Gly-ol(5)]-enkephalin in the nucleus accumbens shell was decreased by intra-accumbens pretreatment with an equimolar dose range of either GABA(A) or GABA(B) antagonists, and further, whether general opioid or selective GABA antagonists decreased feeding elicited by GABA(A) or GABA(B) agonists in the nucleus accumbens shell. Feeding elicited by the mu-selective opioid agonist was dose-dependently increased following intra-accumbens pretreatment with GABA(A) (bicuculline) antagonism; this enhancement was significantly blocked by pretreatment with general or mu-selective opioid antagonists. In contrast, mu opioid agonist-induced feeding elicited from the nucleus accumbens shell was dose-dependently decreased by GABA(B) (saclofen) antagonism. Neither bicuculline nor saclofen in the nucleus accumbens shell altered baseline food intake. Whereas muscimol-induced feeding elicited from the nucleus accumbens shell was reduced by bicuculline and naltrexone, but not saclofen pretreatment, baclofen-induced feeding elicited from the nucleus accumbens shell was reduced by saclofen, but not by bicuculline or naltrexone. These data indicate that GABA(A) and GABA(B) receptor subtype antagonists differentially affect feeding elicited by mu opioid receptor agonists within the nucleus accumbens shell in rats.     

Beta-endorphin fragments (DT gamma E and DE gamma E) reduce opiate withdrawal in guinea pig ileum

The effect exerted by two beta-endorphin fragments (DTgammaE and DEgammaE) was investigated on the acute opioid dependence induced by mu, kappa and delta receptor agonists in vitro. After a 4-min in vitro exposure to morphine (less selective mu agonist), DAGO (highly selective mu agonist), U50-488H (highly selective kappa agonist) and beta-endorphin (selective mu-delta agonist), a strong contracture of guinea pig isolated ileum was observed after the addition of naloxone. This effect was also observed when rabbit isolated jejunum was pretreated with deltorphin (highly selective delta agonist). DTgammaE or DEgammaE injection treatment before or after morphine, DAGO, U50-488H, beta-endorphin or deltorphin were able to both prevent and reverse the naloxone-induced contracture after exposure to the opioid agonists in a concentration-dependent fashion. Our results indicate that both DTgammaE or DEgammaE are able to reduce significantly opioid dependence in vitro, suggesting an important functional interaction between beta-endorphin fragments and opioid dependence induced by mu, kappa and delta receptors.