Behavioural effects of deltorphins in rats

Review of the physiological effects of Phyllomedusa bicolor skin secretion peptides on humans receiving Kambô

Kambô is an Amazonian ritual which includes the application of the defensive secretion of the Phyllomedusa bicolor frog to superficial burns made on the skin of human participants. The secretion, which contains a range of biologically active linear peptides, induces a short purgative experience that is extensively reported by participants to leave them with positive physical, emotional and spiritual after-effects. Various peptides identified in the secretion exert analgesic, vascular, and gastric effects in vivo, and antimicrobial and anti-cancer effects, among others, in vitro. While there has been some investigation into the physiological effects of various individual peptides isolated from the P. bicolor secretion, very little is known about the putative synergistic effects of concurrent administration of the complete substance through the transdermal methods used traditionally in the Kambô ritual. In this review and commentary, the authors summarize the existing biological information from animal research on peptides from the P. bicolor secretion, then consider the evidence in the context of Kambô administration to humans. The presented information suggests that specific peptides are likely to contribute to analogous physiological effects of Kambô in humans. The possibility that beyond their physiological action, the experiential or phenomenological component of these effects may have therapeutic applications is discussed, concluding with a consideration of the feasibility of human clinical research.

Antinociceptive effect of D-Lys(2), Dab(4)N-(ureidoethyl)amide, a new cyclic 1-4 dermorphin/deltorphin analog

BACKGROUND

A preliminary evaluation of antinociceptive activity of a new cyclic dermorphin/deltorphin tetrapeptide analog restricted via a urea bridge and containing C-terminal ureidoethylamid {[H-Tyr-d-Lys(&(1))-Phe-Dab(&(2))-CH2CH2NHCONH2][&(1)CO&(2)]} (cUP-1) revealed a significant and long-lasting increase of pain threshold to thermal stimulation after systemic application. The current studies were aimed at further evaluation of cUP-1 activity in animal models of somatic and visceral pain. The influence of cUP-1 on motor functions was also investigated.

METHODS

The influence of cUP-1 (0.5-2mgkg(-1), iv) on nociceptive threshold to mechanical pressure and analgesic efficacy in formalin and acetic acid-induced writhing tests were estimated. The antinociceptive effect of cUP-1 was compared to that of morphine (MF). The influence of cUP-1 (1, 4 and 8mgkg(-1), iv) on locomotor activity, motor coordination and muscle strength was estimated using open field and rota-rod tests and a grip strength measurement.

RESULTS

Administration of cUP-1 in doses of 1 and 2mgkg(-1) elicited a significant increase of nociceptive threshold to mechanical pressure. MF applied in the same doses induced an antinociceptive effect only at the higher dose (2mgkg(-1)). There were no marked differences between the effect of cUP-1 and MF at each dose, at relative time points. In the writhing test and both phases of the formalin test, cUP-1 showed a significant, dose-dependent antinociceptive effect which did not markedly differ from that of MF. cUP-1 did not significantly affect motor functions of mice.

CONCLUSIONS

Systemic application of cUP-1 elicited a dose-dependent antinociceptive effect. The analgesic efficacy of cUP-1 on mechanical nociception, visceral and formalin-induced pain was comparable to that of MF. cUP-1 did not impair motor functions of mice.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

Effects of group exposure on single injection-induced behavioral sensitization to drugs of abuse in mice

BACKGROUND

Behavioral sensitization in rodents is hypothesized to reflect neuronal adaptations that are related to drug addiction in humans. We evaluated the effects of group exposure on the acute hyperlocomotion and behavioral sensitization induced by four drugs of abuse in C57BL/6 mice: methylenedioxymethamphetamine (MDMA), d-amphetamine, morphine and ethanol.

METHODS

In the priming session, animals received an ip injection of one of the drugs of abuse and were exposed to an open field either individually or in groups of four. Seven days later, we assessed behavioral sensitization in the challenge session. All animals received an ip injection of the same drug and were exposed to the open field in the same social conditions described for the priming session. Locomotion and social interaction were quantified during each session.

RESULTS

Acute MDMA, morphine and ethanol, but not d-amphetamine, increased social interaction. However, group exposure only potentiated MDMA-induced hyperlocomotion. After a challenge injection of each drug, there was no sensitization to the facilitating effect of MDMA, morphine or ethanol on social interaction, but locomotion sensitization developed to all drugs of abuse except ethanol. This sensitization was potentiated by group exposure in MDMA-treated animals, attenuated in morphine-treated animals and not modified in d-amphetamine-treated animals. Acute MDMA enhanced body contact and peaceful following, while acute morphine and ethanol increased social sniffing.

CONCLUSIONS

These results provide preclinical evidence showing that while different drugs of abuse affect different components of social interaction, the neuronal adaptations related to drug dependence can be critically and specifically influenced by group exposure.

Essential role of mu opioid receptor in the regulation of delta opioid receptor-mediated antihyperalgesia

Analgesic effects of delta opioid receptor (DOR) -selective agonists are enhanced during persistent inflammation and arthritis. Although the underlying mechanisms are still unknown, membrane density of DOR was shown to be increased 72 h after induction of inflammation, an effect abolished in mu opioid receptor (MOR) -knockout (KO) mice [Morinville A, Cahill CM, Kieffer B, Collier B, Beaudet A (2004b) Mu-opioid receptor knockout prevents changes in delta-opioid receptor trafficking induced by chronic inflammatory pain. Pain 109:266-273]. In this study, we demonstrated a crucial role of MOR in DOR-mediated antihyperalgesia. Intrathecal administration of the DOR selective agonist deltorphin II failed to induce antihyperalgesic effects in MOR-KO mice, whereas it dose-dependently reversed thermal hyperalgesia in wild-type mice. The antihyperalgesic effects of deltorphin II were blocked by naltrindole but not d-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) suggesting that this agonist was mainly acting through DOR. SNC80-induced antihyperalgesic effects in MOR-KO mice were also attenuated as compared with littermate controls. In contrast, kappa opioid receptor knockout did not affect deltorphin II-induced antihyperalgesia. As evaluated using mice lacking endogenous opioid peptides, the regulation of DOR's effects was also independent of beta-endorphin, enkephalins, or dynorphin opioids known to be released during persistent inflammation. We therefore conclude that DOR-mediated antihyperalgesia is dependent on MOR expression but that activation of MOR by endogenous opioids is probably not required.

Dopamine and the spinal cord in restless legs syndrome: Does spinal cord physiology reveal a basis for augmentation?

The pathophysiology of restless legs syndrome (RLS) is incompletely understood. L-DOPA, as the precursor of dopamine, as well as dopamine agonists, plays an essential role in the treatment of RLS leading to the assumption of a key role of dopamine function in the pathophysiology of RLS. Periodic limb movements in sleep are a key feature of RLS. They are generated in the spinal cord. Here we review RLS phenomenology on the basis of known dopaminergic influence on spinal control, which has been studied a great deal in recent decades in animals. In particular, we propose that the differential effects of l-DOPA and opioids on early and late flexor reflexes may be linked to the phenomenon of augmentation.

Comparison between delta-opioid receptor functional response and autoradiographic labeling in rat brain and spinal cord

The distribution of delta-opioid receptors (DORs) in the rat central nervous system has been previously characterized by radioligand binding and immunohistochemistry. However, the functional neuroanatomy of DORs has not been mapped in any detail; this is potentially important, because these receptors appear to be primarily cytosolic. Opioid receptors can couple to G(i/o) G proteins, a process that is detected by agonist-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding. The purpose of this study was therefore to determine the distribution of functional DORs, as assessed by [35S]GTPgammaS autoradiographic labeling in response to the DOR agonist deltorphin II. For comparison, adjacent sections were labeled with [125I]deltorphin II or the DOR antagonist [125I]AR-M100613. In all three assays, mu-opioid receptors were blocked pharmacologically. The distributions of [125I]deltorphin II and [125I]AR-M100613 were highly correlated but not identical. Deltorphin II increased [35S]GTPgammaS binding in a concentration-dependent and naltrindole-sensitive manner. The regional [35S]GTPgammaS response to deltorphin II was only moderately predicted by agonist or antagonist radioligand binding (r = 0.67 and 0.50, respectively). [35S]GTPgammaS responses to deltorphin II were strongest in the extended striatum (caudate putamen, nucleus accumbens, olfactory tubercle) and cerebral cortex. In contrast, some areas reported to mediate DOR analgesia (brainstem, spinal cord) possessed a much lower [35S]GTPgammaS response. These findings demonstrate the existence of a partial mismatch between DOR radioligand binding and [35S]GTPgammaS response. This divergence possibly reflects regional heterogeneity in G-protein receptor coupling, or in the subcellular localization of DOR.

Opioid peptides: synthesis and biological properties of [(Nγ-glucosyl,Nγ-methoxy)-α,γ-diamino-(S)-butanoyl]4-deltorphin-1-neoglycopeptide and related analogues

The [D-Ala2]deltorphin 1 sequence in which the aspartic acid residue is replaced by the N gamma-OCH3-alpha, gamma-diamino (S) butanoyl residue was synthesized using the Fmoc-chemistry-based solid phase procedure. The resulting deltorphin analogue was chemoselectively glucosylated by reaction with unprotected D-glucose (Glc). The Asn4-, (2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-galactopyranosyl)-Asn4- and the (2-acetamido-2-deoxy-D-galactopyranosyl)-Asn4-deltorphin I were also prepared for comparison. The affinity of the new compounds for the delta-opioid receptor was expressed by the inhibition constant (Ki) of the binding of the delta-receptor selective ligand [3H]naltrindole (NTI) to rat brain membrane preparations. The in vitro biological activity of the synthetic peptides was compared with that of the mu-opioid receptor agonist dermorphin in guinea pig ileum (GPI) preparations and with that of the delta-opioid receptor agonist deltorphin I in mouse vas deferens (MVD) preparations. The substitution of Asp4 with Asn failed to affect drastically the Ki and IC50 values for delta-sites, suggesting that an electrostatic interaction does not play an essential role in the binding to delta-opioid sites. The steric hindrance of the side chain of the residue in position 4 affects binding to delta-sites. The increase of the Ki value is smaller when the sugar-peptide linkage involves the gamma-nitrogen of the Dab residue in comparison with the Asn amide side chain.

Synthesis, conformation and biological activity of dermorphin and deltorphin I analogues containingN-alkylglycine in place of residues in position 1, 3, 5 and 6

Syntheses are described of new dermorphin and [D-Ala2]deltorphin I analogues in which the phenylalanine, the tyrosine or the valine residues have been substituted by the corresponding N-alkylglycine residues. Structural investigations by CD measurements in different solvents and preliminary pharmacological experiments were carried out on the resulting peptide-peptoid hybrids. The contribution from aromatic side chain residues is prominent in the CD spectra of dermorphin analogues and the assignment of a prevailing secondary structure could be questionable. In the CD spectra of deltorphin analogues the aromatic contribution is lower and the dichroic curves indicate the predominance of random conformer populations. The disappearance of the aromatic contribution in the [Ntyr1,D-Ala2]-deltorphin spectrum could be explained in terms of high conformational freedom of the N-terminal residue. The kinetics of degradation of the synthetic peptoids digestion by rat and human plasma enzymes were compared with that of [Leu5]-enkephalin. The binding to opioid receptors was tested on crude membrane preparations from CHO cells stably transfected with the mu- and delta-opioid receptors. The biological potency of peptoids was compared with that of dermorphin in GPI preparations and with that of deltorphin I in MVD preparations. All the substitutions produced a dramatic decrease in the affinity of the peptide-peptoid hybrids for both the mu- and delta-opioid receptors. Nval5 and/or Nval6 containing hybrids behaved as mu-opioid receptor agonists and elicit a dose-dependent analgesia (tail-flick test) when injected i.c.v. in rats.

Immunoreactive neurons in the brain of two mouse strains after incubation with an antiserum recognizing Asp-Val-Val-Gly.NH2 (DVVG), the C-terminal fragment of (D-Ala2)-deltorphin I

D-Ala(2)-deltorphin I (DADTI) is a heptapeptide amide first extracted from frog skin that displays a high selectivity and affinity for delta opioid receptors. Previous studies using a polyclonal antiserum specific for its C-terminal tetrapeptide-amide (DVVG) have already described in rat and mouse brain the presence of immunoreactive neurons, most of them belonging to the mesencephalic dopaminergic neurons. C57BL/6J (C57) and DBA/2J (DBA) are two inbred strains of mice well known for showing marked genotype-dependent differences for phenotypes related to differential brain dopamine functioning. Brain specimens of both inbred mouse strains were frozen, cut and immunostained using the same antiserum. Some sections were also double immunostained with monoclonal anti-tyrosine hydroxylase (TH). DVVG-immunoreactive neurons were observed among both dopaminergic and non-dopaminergic neurons. DVVG- and TH-immunoreactive neurons were observed among the dopaminergic A8, A9 and A10 mesencephalic nuclei. They were on average 21.9% more numerous in DBA than in C57 mice. DVVG-immunoreactive nerve fibres could be seen in limbic, striatal, cortical and thalamic areas. The distribution patterns of DVVG-IR and TH-IR nerve fibres differed most conspicuously within the infralimbic, prelimbic and cingulate cortices, forming a dense network in DBA but rare in C57 mice. Non-dopaminergic DVVG-immunoreactive neurons did not differ significantly in the two strains. Our finding that the number and distribution pattern of this dopaminergic neuronal subpopulation differed in the two mouse strains could provide morphological support for the known behavioural differences between the DBA and C57 strains under normal and experimental conditions.

Phenotypes of mice with invalidation of cholecystokinin (CCK(1) or CCK(2)) receptors

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract, but also found in high density in the mammalian brain. This peptide has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK interacts with nanomolar affinites with two different receptors designated CCK(1) and CCK(2). Primarily, the functional role of these binding sites in the brain and the periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. However, several studies have yielded conflicting data. Knockout mice provide unique opportunities to analyse diverse aspects of gene function in vivo. This review highlights recent progress in our understanding of the role of CCK(1) and CCK(2) receptors obtained by using mice with genetic invalidation of CCK(1) or CCK(2) receptors or natural CCK receptors mutants. The limits of this approach is discussed and some results were compared to those obtained by pharmacological blockade of CCK receptors by selective antagonists.

Fetal alveolar epithelial cells contain [D-Ala(2)]-deltorphin I-like immunoreactivity: delta- and mu-opiate receptors mediate opposite effects in developing lung

Opiate-like peptides can regulate many cellular functions. We now map [D-Ala(2)]deltorphin I (DADTI)-like immunoreactivity (DADTI-LI) in developing mouse lung and analyze potential functional roles. Most DADTI-LI-positive cells were alveolar cells negative for prosurfactant protein (proSP)-C immunoreactivity. Peak numbers of DADTI-LI-positive cells occurred on embryonic Day 18, decreasing postnatally. To analyze developmental effects of DADTI, e17-18 lung explants were treated with [D-Ala(2)]deltorphin II (DADTII, soluble DADTI analogue, delta-receptor-specific) versus dermorphin (mu-receptor-specific). Type II pneumocyte differentiation, assessed by [(3)H]choline incorporation into saturated phosphatidylcholine and proSP-C immunostaining, was inhibited by DADTII but stimulated by dermorphin. Cell proliferation, measured as [(3)H]-thymidine incorporation and proliferating cell nuclear antigen immunostaining, was stimulated by DADTII and inhibited by dermorphin. All effects were dose-dependent. DADTII-inhibited choline incorporation was reversed by the delta-blocker, naltrindole. Unexpectedly, DADTII-stimulated thymidine incorporation was augmented by naltrindole and reversed by naloxone (mu-blocker). Although dermorphin-stimulated choline incorporation was appropriately blocked by binaltorphimine, dermorphin-inhibited thymidine incorporation was reversed by delta, kappa-, or mu-blockers. The delta- and mu-receptor messenger RNAs occurred pre- and postnatally, whereas kappa-receptor transcripts occurred mainly prenatally. All three receptor proteins were present in epithelial and mesenchymal cells in e18 lung. Thus, DADTI-LI from proSP-C-immunonegative alveolar cells could regulate development via both direct and indirect effects involving multiple opiate receptors.

delta-Opioid modulation of social interactions in juvenile mice weaned at different ages

The environmental stimulus of weaning has been shown to affect both the developmental expression of social behavior and the maturation of the opioid delta-receptors' subpopulation in altricial rodents. The aim of this study was to address both these issues by using the social interaction paradigm. Separate groups of male and female mice were randomly assigned to three different weaning ages -- early (Wean-15), regular (Wean-20), and delayed (Wean-25) -- and assessed when 30 days old under intraperitoneal administration of the selective delta-opioid agonist SNC80 (0, 0.1, or 0.3 mg/kg). Wean-15 male and female subjects were much more involved in investigating the partner as well as the cage environment compared to the regular Wean-20 group. An increased social investigation was also found as a consequence of delayed weaning in the female group. The neurobehavioral changes induced by the manipulation of weaning age were also reflected in an altered responsivity to the effects of SNC80 administration. The drug-induced increase in the expression of investigative and affiliative social interactions was further magnified by early weaning. A delayed weaning time was instead associated with reduced sensitivity to the drug, which suggests a delayed maturation of the system. As a whole, the present results indicate that the time of weaning is able to markedly affect the expression of social interactions of adolescent mice by possibly exerting a direct modulatory role on the development of the still plastic delta-opioid system.

Pharmacology of amphibian opiate peptides

In 1980 the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists, and named dermorphins. Since 1987 deltorphins, a family of highly selective delta-opioid peptides were identified either by cloning of the cDNA from frog skins or isolation of the peptides. The distinctive feature of opioid peptides is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe. The discovery of the amphibian opiate peptides, provided new insights into the functional role of the mu- and delta-opiate systems. It also provided models for novel analgesics with enhanced therapeutic benefits and reduced toxicity.

Antihyperalgesic effects of delta opioid agonists in a rat model of chronic inflammation

Opioid receptors in the brain activate descending pain pathways to inhibit the nociceptive response to acute noxious stimuli. The aim of the present study was to clarify the role of supraspinal opioid receptors in modulating the nociceptive response to persistent inflammation in rats. Subcutaneous administration of 50 microl of complete Freund's Adjuvant (CFA) into the plantar surface of the hindpaw induced a significant decrease in paw withdrawal latency to thermal stimuli (P<0.01) at 24 h post-injection. Intracerebroventricular (i.c.v.) administration of the mu opioid receptor agonists, DAMGO and morphine, and the delta opioid receptor agonists, deltorphin II and SNC80, significantly reversed the hyperalgesic response associated with peripheral inflammation in a dose-dependent manner (P<0.0001). The mu and delta agonists also significantly attenuated the antinociceptive response to acute thermal stimulation in rats (P<0.001). However, deltorphin II and SNC80 were less potent, and in the case of SNC80 less efficacious, in modulating the response to acute thermal nociception in comparison to hyperalgesia associated with persistent inflammation. These results indicate that mu and delta opioid receptors in the brain modulate descending pain pathways to attenuate the nociceptive response to acute thermal stimuli in both normal and inflamed tissues. The heightened response to delta agonists in the hyperalgesia model suggests that delta opioid receptors in the brain are promising targets for the treatment of pain arising from chronic inflammation.

Ontogeny of (D-Ala(2))-deltorphin I-like immunoreactive neurones in foetal rat brain

Foetal rat brain from embryonic day (ED) 12-22 was immunohistochemically studied to describe the time of first appearance and further distribution patterns of (D-Ala(2))-deltorphin-I-immunoreactive (DADTI-IR) nerve elements. The primary antiserum used in this study was a polyclonal antibody against DADTI previously used in adult and postnatal rat brain mapping. DADTI-IR nerve elements first appeared in the neuroepithelium of ventral mesencephalon on ED 13. From there, positive cell bodies migrated towards the mantle layer until they invaded the whole ventral mesencephalic tegmentum. They then reached their definitive position, corresponding to a subpopulation of the A8, A9 and A10 dopaminergic neurones that had been constantly observed also in the adult age. From ED 15-17, DADTI-positive nerve fibres appeared in the medial forebrain bundle, the neostriatum anlage, the accumbens nucleus, the olfactory tubercle, the fasciculus retroflexus, and the prefrontal cortex. All these locations have also been found in adult rats. From ED 14 onwards, transient DADTI-IR somata and nerve fibres were observed in retinal neuroepithelium, optic pathways as far as the superior colliculus, CA3 hippocampal field, reticular formation in the medulla oblongata. All these locations gradually disappeared either before birth (medulla oblongata) or within the first 3 weeks after birth. These results suggest that the DADT-like molecule recognised by our antibody has during the embryonic development a regulatory function in neuronal growth and differentiation.

Effects of antisense oligonucleotides on brain delta-opioid receptor density and on SNC80-induced locomotor stimulation and colonic transit inhibition in rats

1. To reduce the density of delta-opioid receptor protein, five antisense phosphorothioate oligodeoxynucleotides (aODN), targeting the three exons of rat delta-opioid receptor mRNA (DOR), were injected twice daily for 4 days or continuously infused for 7 days into brain lateral ventricles (i.c.v.) of Sprague-Dawley rats. Rats acting as controls were infused or injected with a mismatch sequence (mODN) of each aODN. The density of opioid receptors in rat brain membranes was measured by saturation binding experiments using selective ligands for delta, mu and kappa opioid receptors. 2. aODNs injected twice a day for 4 days left rat brain delta-opioid receptor density unchanged. The ODN targeting the DOR nucleotide sequence 280 - 299 (aODN280 - 299, exon 2), decreased brain delta-opioid receptor density significantly more than aODNs targeting exon 1 (aODN239 - 258), exon 2 (aODN361 - 380), or exon 3 (aODN741 - 760) (to 52% vs 79, 72, and 68%). None of the aODNs to the DOR changed the brain density of mu- or k-opioid receptors. 3. When in a novel environment (but not when kept in their home cages), the locomotor activity of aODN280 - 299 treated rats was significantly lower than that of saline or mODN treated rats. The delta-opioid agonist SNC80 (5 mg kg-1, s.c.) significantly and potently stimulated locomotion and delayed colonic propulsion in saline- and mODN-infused rats, but left motor behaviour and colonic transit of delta-knockdown rats unchanged. 4. The baseline nociceptive threshold and the antinociceptive response to morphine were unchanged in delta-knockdown rats.

Morphine treatment during juvenile isolation increases social activity and opioid peptides release in the adult rat

The consequences of juvenile isolation and morphine treatment on general activity, social activity and endogenous opioid release during a social interaction test were investigated in the adult rat. Rats were either isolated or socially housed during weeks 4 and 5 of age and treated daily during this isolation period subcutaneously with either saline or morphine. Directly after a social interaction test at 10 weeks of age, rats were injected with [3H]-diprenorphine and subsequently prepared for in vivo autoradiography. The autoradiographic technique was used to visualise neuroanatomical changes in opioid receptor occupancy, probably reflecting changes in opioid peptide release, as a result of social activity. Juvenile isolation increased general activity during the social interaction test, an effect which was accompanied by a reduction of opioid receptor occupancy in many brain areas, suggesting an increased opioid peptide release as a consequence of socially-induced general activity. Morphine treatment in isolated rats caused an increase in adult social activity and enhanced opioid peptide release in some cortical regions and the ventral tegmental area as compared to saline treated rats. Both social activity and opioid receptor occupancy were unaffected by morphine treatment in non-isolated rats. The present study underscores the role of opioid systems in adult social behaviors as a consequence of juvenile isolation. The results suggest a relationship between social activity and opioid peptide release during social contact. Increased social activity seems to be accompanied by elevated opioid peptide release in distinct brain areas after morphine treatment during juvenile isolation.

Differential knockdown of delta-opioid receptor subtypes in the rat brain by antisense oligodeoxynucleotides targeting mRNA

Two antisense oligodeoxynucleotides (A-ODN), targeting delta-opioid receptor mRNA (DOR) and two mismatch ODN sequences (mODN) were continuously infused for 24 days into the lateral brain ventricles of Wistar rats. The density of delta-opioid receptors in rat brain homogenates was measured by saturation binding experiments using four selective ligands, two agonists ([D-Ala2, Glu4]-deltorphin and DPDPE) and two antagonists (Dmt-Tic-OH and naltrindole), and by immunoblotting SDS solubilized receptor protein. In brain membranes of mODN or saline-infused rats, the rank order of delta-opioid receptor density, calculated by Bmax values of the four delta-opioid receptor ligands, was: [D-Ala2, Glu4]deltorphin approximately Dmt-Tic-OH approximately naltrindole (86-118 fmo/mg protein) > DPDPE (73.6+/-6.3 fmol/mg protein). At the end of the 24 day infusion of A-ODN targeting DOR nucleotide sequence 280299 (A-ODN280-299), the Bmax of DPDPE (62.4+/-3.2 fmol/mg protein) was significantly higher than that of Dmt-Tic-OH (31.5+/-3.9 fmol/mg protein). Moreover, both the Kd value for DPDPE saturation binding and the Ki value for Dmt-Tic-OH displacement by DPDPE were halved. In contrast, an A-ODN treatment targeting exon 3 (A-ODN741-760) decreased the specific binding of [D-Ala2, Glu4]deltorphin and Dmt-Tic-OH significantly less (67%-81%) than the binding of DPDPE (53%), without changes in DPDPE Ki and KD values. No A-ODN treatment modified the specific binding of the micro-opioid agonist DAMGO and of the k-selective opioid receptor ligand U69593. On the Western blot of solubilized striatum proteins, A-ODN(280-299) and A-ODN(741-760) downregulated the levels of the DOR protein, whereas the corresponding mODN were inactive. The 24-day infusion of A-ODN(280-299) inhibited the rat locomotor response to [D-Ala2, Glu4]deltorphin but not to DPDPE. Intracerebroventricular (i.c.v.) infusion of A-ODN(741-760) reduced the locomotor responses to both delta-opioid receptor agonists, whereas mODN infusion never affected agonist potencies. In conclusion, these results demonstrate that 24-day continuous i.c.v. infusion of A-ODN targeting the nucleotide sequence 280-299 of DOR can differentially knockdown delta1 and delta2 binding sites in the rat brain.

Dermorphin and deltorphin glycosylated analogues: synthesis and antinociceptive activity after systemic administration

In the present paper we describe the synthesis of some dermorphin and deltorphin analogues beta-O- and alpha-C-glycosylated on the C-terminal amino acid residue and report their opioid receptor affinity and selectivity as well as their analgesic potency after subcutaneous injection in mice.

What peptides these deltorphins be

The deltorphins are a class of highly selective delta-opioid heptapeptides from the skin of the Amazonian frogs Phyllomedusa sauvagei and P. bicolor. The first of these fascinating peptides came to light in 1987 by cloning of the cDNA of from frog skins, while the other members of this family were identified either by cDNA or isolation of the peptides. The distinctive feature of deltorphins is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe, comparable to dermorphin, which is the prototype of a group of mu-selective opioids from the same source. The D-amino acid and the anionic residues, either Glu or Asp, as well as their unique amino acid compositions are responsible for the remarkable biostability, high delta-receptor affinity, bioactivity and peptide conformation. This review summarizes a decade of research from many laboratories that defined which residues and substituents in the deltorphins interact with the delta-receptor and characterized pharmacological and physiological activities in vitro and in vivo. It begins with a historical description of the topic and presents general schema for the synthesis of peptide analogues of deltorphins A, B and C as a means to document the methods employed in producing a myriad of analogues. Structure activity studies of the peptides and their pharmacological activities in vitro are detailed in abundantly tabulated data. A brief compendium of the current level of knowledge of the delta-receptor assists the reader to appreciate the rationale for the design of these analogues. Discussion of the conformation of these peptides addresses how structure leads to further hypotheses regarding ligand receptor interaction. The review ends with a broad discussion of the potential applications of these peptides in clinical and therapeutic settings.

The role of mu and kappa opioid receptors within the periaqueductal gray in the expression of conditional hypoalgesia

The periaqueductal gray (PAG) is a midbrain structure involved in the modulation of pain and expression of classically conditioned fear responses. Non-selective opioid antagonists applied to the PAG block the expression of hypoalgesia in rats exposed to a Pavlovian signal for shock. This study was conducted to determine the anatomical and pharmacological specificity of the PAG's role in conditional hypoalgesia. Rat subjects received injections of either the mu opioid antagonist CTAP (6.6 nMol), the kappa opioid antagonist Nor-binaltorphimine (Nor-BNI, 6.6 nMol) or saline. Injections were made into either the dorsolateral (dlPAG) or ventrolateral (vlPAG) PAG prior to the presentation of an auditory stimulus that had previously been paired with foot shock while measuring nociception with the radiant heat tail flick (TF) test. Elevation in TF latency in response to the auditory stimulus was blocked only by administration of CTAP into the vlPAG. These results suggest that conditional hypoalgesia (CHA) is subserved by mu but not kappa opioid receptors located in the vlPAG but not the dlPAG.

The neurobiology of social play behavior in rats

Social play behavior is one of the earliest forms of non-mother-directed social behavior appearing in ontogeny in mammalian species. During the last century, there has been a lot of debate on the significance of social play behavior, but behavioral studies have indicated that social play behavior is a separate and relevant category of behavior. The present review provides a comprehensive survey of studies on the neurobiology of social play behavior. Evidence is presented that opioid and dopamine systems play a role in the reward aspect of social play behavior. The role of cholinergic, noradrenergic and opioid systems in attentional processes underlying the generation of social play behavior and the involvement of androgens in the sexual differentiation of social play behavior in rats is summarized. It is concluded that there is not only behavioral, but also neurobiological evidence to suggest that social play behavior represents a separate category of behavior, instead of a precursor for adult social, sexual or aggressive behavior.

Use of antisense oligodeoxynucleotide to determine delta-opioid receptor involvement in [D-Ala2]deltorphin II-induced locomotor hyperactivity

Intracerebroventricularly (i.c.v.)-administered [D-Ala2]deltorphin II (20 micrograms) produced a marked locomotor hyperactivity in male ICR mice. The locomotor hyperactivity induced in response to i.c.v. [D-Ala2]deltorphin II (20 micrograms) was suppressed by pretreatment with naltriben (NTB, 10 micrograms) but not 7-benzylidene naltrexone (BNTX, 1 microgram) and D-Phe-Cys-Tyr-D-Try-Orn-Thr-Phe-Thr-NH2 (CTOP, 100 ng). The influence of antisense oligodeoxynucleotide to delta-opioid receptor mRNA (delta-AS oligo) or a mismatch oligodeoxynucleotide (MM oligo) on the locomotor hyperactivity induced by [D-Ala2]deltorphin II was determined. Groups of mice pretreated i.c.v. with delta-AS oligo (1 microgram), MM oligo (1 microgram) or saline (4 microliters) once a day for 3 days, were injected i.c.v. [D-Ala2]deltorphin II (10 or 20 micrograms) and the locomotor response to [D-Ala2]deltorphin II was measured. The locomotor hyperactivity of i.c.v. [D-Ala2]deltorphin II (10 or 20 micrograms) were significantly suppressed by i.c.v. pretreatment with delta-AS oligo but not MM oligo. The present results indicate that pretreatment with delta-AS oligo suppresses mouse locomotor hyperactivity produced by stimulation of delta 2-opioid receptors in the brain.

Antinociceptive and behavioral effects of synthetic deltorphin analogs

A possible correlation of behavioral, antinociceptive and cataleptic responses with central delta- and mu-opioid receptor stimulation was tested for in the rat by i.c.v. injections of some synthetic deltorphin analogs. At doses ranging from 0.1 to 3.0 nmol/rat, the selective delta-opioid receptor agonist, [D-Ala2,Glu4]deltorphin (Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2), induced a dose-dependent stereotyped pattern of locomotor activity, reaching the maximum in the first 30 min; doses higher than 30 nmol induced early and fleeting antinociception. The replacement of Glu4 by Gly, Ala, Val, His or Asn yielded peptides with a lower delta-selectivity because of a gain in mu-affinity. [D-Ala2,Ala4]deltorphin (0.14-4.0 nmol) induced negligible behavioral stimulation but a rapidly appearing and long-lasting analgesia and catalepsy. The other four synthetic peptides induced biphasic effects: low dosages stimulated locomotion whereas higher doses initially suppressed, then increased locomotor activity. At doses ranging from 1 to 70 nmol all the peptides induced analgesia and catalepsy. In experiments examining the locomotor and antinociceptive effects induced by 14 nmol of [D-Ala2,Gly4]deltorphin in rats pretreated with mu and delta antagonists, the non-selective mu-opioid receptor antagonist, naloxone (1 mg/kg i.p.), reduced analgesia and abolished the initial hypolocomotion. The delta-selective antagonist, naltrindole (10 mg/kg i.p.), abolished locomotor activity without affecting analgesia. The mu1 -selective antagonist, naloxonazine (10 mg/kg i.v.), seemed to prolong analgesia and immobility. Hence this peptide appears to activate, in addition to delta-receptors, mainly the opioid receptor mu2-subtype, which mediates catalepsy in the rat. We suggest that the mu2- and delta-opioid receptors of the rat brain modulate locomotor behavior by activating functionally opposed responses. [D-Ala2,Ala4]deltorphin had an antinociceptive and cataleptic potency higher than would have been expected from its mu-affinity. A possible explanation might be a mu/delta-opioid receptor interaction.

Interaction between the mu-agonist dermorphin and the delta-agonist [D-Ala2, Glu4]deltorphin in supraspinal antinociception and delta-opioid receptor binding

1. In rats, the interaction between the mu-opioid agonist dermorphin and the delta-opioid agonist [D-Ala2, Glu4]deltorphin was studied in binding experiments to delta-opioid receptors and in the antinociceptive test to radiant heat. 2. When injected i.c.v., doses of [D-Ala2, Glu4]deltorphin higher than 20 nmol produced antinociception in the rat tail-flick test to radiant heat. Lower doses were inactive. None of the doses tested elicited the maximum achievable response. This partial antinociception was accomplished with an in vivo occupancy of more than 97% of brain delta-opioid receptors and of 17% of mu-opioid receptors. Naloxone (0.1 mg kg-1, s.c.), and naloxonazine (10 mg kg-1, i.v., 24 h before), but not the selective delta-opioid antagonist naltrindole, antagonized the antinociception. 3. In vitro competitive inhibition studies in rat brain membranes showed that [D-Ala2, Glu4]deltorphin displaced [3H]-naltrindole from two delta-binding sites of high and low affinity. The addition of 100 microM Gpp[NH]p produced a three fold increase in the [D-Ala2, Glu4]deltorphin Ki value for both binding sites. The addition of 10 nM dermorphin increased the Ki value of the delta-agonist for the high affinity site five times. When Gpp[NH]p was added to the incubation medium together with 10 nM dermorphin, the high affinity Ki of the delta-agonist increased 15 times. 4. Co-administration into the rat brain ventricles of subanalgesic doses of dermorphin and [D-Ala2, Glu4]deltorphin resulted in synergistic antinociceptive responses. 5. Pretreatment with naloxone or with the non-equilibrium mu-antagonists naloxonazine and beta-funaltrexamine completely abolished the antinociceptive response of the mu-delta agonist combinations. 6. Pretreatment with the delta-opioid antagonists naltrindole and DALCE reduced the antinociceptive response of the dermorphin-[D-Ala2, Glu4]deltorphin combinations to a value near that observed after the mu-agonist alone. At the dosage used, naltrindole occupied more than 98% of brain delta-opioid receptors without affecting mu-opioid-receptors. 7. These data suggest that in the rat tail-flick test to radiant heat, mu- and delta-opioid agonists co-operate positively in evoking an antinociceptive response. Although interactions between different opioid pathways cannot be excluded, in vitro binding results indicate that this co-operative antinociception is probably mediated by co-activation of the delta-opioid receptors at the cellular level by the mu- and delta-agonist.

Characterization of supraspinal antinociceptive actions of opioid delta agonists in the rat

Supraspinally mediated antinociception has been clearly established for agonists acting via both micro- and delta-opioid receptors. The present experiments were undertaken to further characterize the role of supraspinal opioid delta receptors in the mediation of antinociception in rats and to examine the possible role of putative delta1- and delta2-opioid receptors in the antinociceptive effect. Cannulae directed at the right lateral ventricle, the periaqueductal gray (PAG), or the medullary reticular formation (MRF) were implanted in adult male, Sprague-Dawley rats for the microinjection of [D-Ala2,Glu4]deltorphin (delta2 agonist), [D-Pen2,D-Pen5]enkephalin (DPDPE, delta1 agonist), [D-Ser2,Leu5,Thr6]enkephalin (DSLET, mixed delta/micro agonist) or morphine (reference micro-opioid). Pretreatments (24 h prior to agonist microinjection) were made with the putative delta1 and delta2 antagonists, [D-Ala2,Leu5,Cys6]enkephalin (DALCE) and [D-Ala2,Cys4]deltorphin (Cys-DELT) and antinociception was measured in the 55 degrees C hot plate (HP) and 52 degrees C and 55 degrees C (low and high intensity) warm-water tail-flick (TF) tests. Data were converted to percent maximal possible effect (%MPE). Intracerebroventricular (i.c.v.) administration of DPDPE produced less than a 50%MPE in the HP test whereas [d-Ala2,Glu4]deltorphin produced Cys-DELT sensitive antinociception of up to 92% MPE. Neither i.c.v. agonist was effective in the TF assays, and both agonists were without effect in the PAG. [D-Ala2,Glu4]deltorphin microinjected into the MRF produced Cys-DELT sensitive antinociception of 60 and 47% MPE in the HP and low-intensity TF tests, respectively, but was not effective in the 55 degrees C TF test; DPDPE did not produce antinociception when microinjected at this site. Microinjection of DSLET in the MRF produced significant antinociception in all three assays. Morphine produced antinociception following i.c.v. administration or microinjection into the PAG in all tests. Microinjection of morphine into the MRF produced antinociception in the HP and 52 degrees C, but not 55 degrees C, TF tests. Morphine anticociception was not antagonized by either DALCE or Cys-DELT. These data demonstrate that supraspinal delta-opioid receptors can be activated to elicit antinociception in the rat and that opioid delta2 receptors predominate in this effect. Further, these effects may occur predominately via inhibition of supraspinally organized behavior without activation of descending systems such as those mediating the TF response in the rat.

Dopamine depletion augments endogenous opioid-induced locomotion in the nucleus accumbens using both mu 1 and delta opioid receptors

The aim of this study is to analyze further the opioid receptor subtypes involved in the augmentation of behavioral activity after dopamine depletion in the nucleus accumbens of rats. Initially, the opioid receptors involved in the augmentation of locomotion produced by endogenous opioids were evaluated by microinjection of kelatorphan, an inhibitor of proteolytic enzymes that inactivates enkephalin, with or without specific antagonists for mu 1 or delta-opioid receptors, naloxonazine or naltrindole, respectively. Kelatorphan produced a dose-dependent increase in horizontal photocell counts and vertical movements. At all doses examined the behavioral response was augmented in rats sustaining accumbal dopamine lesions. The augmentation in dopamine-depleted rats was partially blocked by naloxonazine or naltrindole. Since the motor stimulant response to intra-accumbens microinjection of the delta-opioid agonist, [D-penicillamine2,5]-enkephalin, was not augmented in a previous study, we tested the behavioral response to a new endogenous delta-opioid agonist, [D-Ala2] deltorphin I. The locomotor response to deltorphin was slightly augmented in dopamine-depleted rats. These data suggest that the augmentation in the motor response elicited by endogenous opioids after dopamine lesions in the nucleus accumbens involves both mu 1, and delta-opioid receptors.

Social isolation and social interaction alter regional brain opioid receptor binding in rats

Endogenous opioid systems have been implicated in the consequences of social isolation and in the regulation of social behavior, although their precise role is not clear. There is not much information on a possible locus in the brain at which opioids exert their effects on social behavior. In an effort to address this issue we analyzed regional opioidergic activity upon social isolation-induced social interaction using in vivo autoradiography. Animals were either socially isolated for 7 days or group housed, and tested singly or in a dyadic encounter. Subsequently, a tracer dose of [3H]diprenorphine was administered and in vivo autoradiographic analysis was performed. Seven days of social isolation caused changes in both social behavior (dyadic encounters) and non-social behavior (singly tested animals). Opioid receptor binding was increased in the medial prefrontal cortex and the parafascicular area in isolates, suggesting that social isolation may evoke an upregulation of opioid receptors in these areas. Social interaction increased opioid binding in the parafascicular area of non-isolated rats. In substantia nigra para compacta and ventral tegmental area binding was increased upon social isolation, and social interaction decreased opioid binding in isolates, but these changes failed to reach significance. These observed local changes in opioid receptor binding suggest a role for opioid systems in discrete areas in the consequences of social isolation and the regulation of social behavior in rats.

Mu- and kappa-opioid receptor-mediated opioid effects on social play in juvenile rats

Previously, morphine has been shown to influence social play behavior in rats on two levels. An increasing effect on social play was interpreted as an effect on the rewarding aspects of social play. A lower dose of morphine abolished the effects of an unfamiliar environment on social play, supposedly by affecting the integration of environmental stimuli. In the present study the effects of receptor-specific opioid drugs on social play and measures of social behavior unrelated to play were investigated. Fentanyl, a mu-opioid receptor agonist, seemingly mimicked both effects of morphine. The mu-opioid receptor antagonist, beta-funaltrexamine, decreased social play, although a low dose of this drug increased it. BUBUC (Tyr-D-Cys(StBu)-Gly-Phe-Leu-Thr(OtBu)) and naltrindole, a delta-opioid receptor agonist and delta-opioid receptor antagonist, respectively, had no effects on social behavior. The kappa-opioid receptor agonist, U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide), dose dependently suppressed all measures of social behavior. The kappa-opioid receptor antagonist, nor-binaltorphimine, abolished the effect of an unfamiliar environment on social play. These studies suggest that the opioidergic effect on social play is mediated through mu- and kappa-opioid receptor systems.

Blockade of cocaine-induced conditioned place preference: relevance to cocaine abuse therapeutics

Conditioned place preference/aversion testing is a behavioral method believed capable of measuring the affective (positive, neutral or negative) properties of psychoactive drugs. Cocaine injections in rats reliably produces a positive place preference. Drugs that attenuate or block this effect of cocaine have obvious potential for developing treatments to address cocaine addiction as well as to add to the scientific understanding of the mechanism of cocaine's action at the cellular level. To date, six drugs have been reported to block the expression of a cocaine-induced conditioned place preference (CPP) and this review evidences the cocaine-induced CPP blockage by the two potent L-type calcium channel blockers, isradipine and nifedipine, the two serotonin-3 receptor antagonists, MDL72222 and ICS205-930, the delta opioid receptor selective antagonist naltrindole, and lastly, a mixed opioid agonist-antagonist buprenorphine. Additional evidence relating to the blockade of other cocaine behavioral effects by these putative blockers is addressed, where appropriate, from studies employing other procedures such as drug stimulus discrimination, self-administration, electrical brain stimulation and increases in locomotor activity. The significance of these findings is discussed in the context of their relevance to the development of treatment regimens to allow for cessation of cocaine abuse.

Immunohistochemical demonstration of [D-Ala2]deltorphin-I in amacrine cells of rat retina

In an immunohistochemical study, a specific antiserum raised against [D-Ala2]deltorphin-I (DADTI), a highly selective ligand for delta opioid receptors, was used to demonstrate immunoreactive structures in the rat retina. [D-Ala2]Deltorphin-I immunoreactivity occurred in a subpopulation of the retinal amacrine cells situated in the inner nuclear layer. Their stained processes were mainly distributed in the sublaminae 1 and 3 of the inner plexiform layer. A few positive cells, probably displaced amacrine cells, were also seen in the ganglion cell layer. Double immunostaining revealed that 12.8% of DADTI-immunoreactive cells costored GABA and 27.7% costored neuropeptide Y, whereas only few DADTI-positive cells colocalized tyrosine hydroxylase (0.3%) and almost no other peptides. These findings suggest that some retinal amacrine cells possess DADTI-like molecule(s), possibly acting as neurotransmitter(s) or neuromodulator(s).

Delta-opioid receptor antagonists attenuate motor activity induced by amphetamine but not cocaine

Naloxone and naltrindole attenuate the locomotor response to amphetamine, implicating delta-opioid receptors in the opioid-antagonist/amphetamine interaction. To determine the role of delta-opioid receptor subtypes in this phenomenon, rats were pretreated with the following selective antagonists administered intracisternally: naltrindole, [D-Ala2,Leu5,Cys6]enkephalin (DALCE, delta 1 receptor selective), naltrindole-5'-isothiocyanate (delta 2 receptor selective). Cumulative dose-response curves to amphetamine were constructed (saline, 0.1, 0.4, 1.6 and 6.4 mg/kg s.c.), with injections every 30 min. Naltrindole was also tested against cumulative doses of cocaine (saline, 3.0, 10, 30 and 56 mg/kg i.p.). Gross and fine motor activity were recorded for 20 min, commencing 10 min postinjection. Amphetamine and cocaine dose dependently increased both gross and fine movements. Naltrindole (10 micrograms) attenuated the gross but not fine activity response to amphetamine, but 10 or 30 micrograms failed to influence the response to cocaine. Naltrindole-5'-isothiocyanate (30 micrograms) attenuated slightly but significantly the gross activity response to amphetamine, whereas DALCE (30 micrograms) was without effect. However, a combination of 10 micrograms each of DALCE and naltrindole-5'-isothiocyanate markedly attenuated the amphetamine-induced increases in gross movements without altering fine activity. These data provide further evidence for the involvement of delta-opioid receptors in the modulation of behavioral effects of amphetamine; both delta 1- and delta 2-opioid receptors appear to play a role. The differential effects of opioid antagonists on locomotor activity stimulated by amphetamine and cocaine suggests differences in the mechanism of action of these drugs not previously appreciated.

[d-Ala2]deltorphin I-like immunoreactivity in the adult rat brain: immunohistochemical localization

Using a specific antiserum recently raised against [D-Ala2]deltorphin I (DADTI: Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2), a highly selective ligand for delta-opioid receptors, we have previously demonstrated the occurrence of positive immunostaining in several structures of mouse brain. We describe here the neuroanatomical distribution patterns of DADTI-immunoreactive neuronal bodies, axons, and tanycytes in rat brain. Positive neuronal somata were localized mainly in the ventral mesencephalon, including the ventral tegmental area and the pars compacta of the substantia nigra. A minor population of positive somata was found in the pars reticulata and pars lateralis of the substantia nigra, raphe nuclei, supramammillary nucleus, and retrorubral reticular nucleus. All these regions, except for the supramammillary nucleus, contain dopamine cell bodies. Intensely stained positive nerve fibers could be traced along the medial forebrain bundle. Dense positive terminals were seen in the neostriatum, nucleus accumbens shell, olfactory tubercle, septal areas, cingulate, and medial prefrontal cortex. Double-immunostaining study revealed that, in the substantia nigra, almost all (97.8%) DADTI-positive neurons colocalized with tyrosine hydroxylase (TH), and the doubly stained cells occupied about one-third (29.1%) of the total population of TH-positive neurons. Only a few DADTI/TH-positive cells also stained for 28-kDa calbindin D, although many neurons double-stained for 28-kDa calbindin D and TH. In contrast, the supramammillary nucleus contained a number of DADTI-positive cells, which nearly always stained positively for 28-kDa calbindin D but did not stain for TH. The association of DADTI-like immunoreactivity with certain dopaminergic pathways seems of particular interest. A small population of DADTI-immunostained tanycytes was present in the ventral part of the third ventricle wall.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Spinal antinociceptive effects of [D-Ala2]deltorphin II, a novel and highly selective delta-opioid receptor agonist

Pharmacological assays in isolated tissues and binding tests have recently shown that two peptides, with the sequence Tyr-D-Ala-Phe-Asp-(or Glu)- Val-Val-Gly-NH2, isolated from skin extracts of Phyllomedusa bicolor and named [D-Ala2]deltorphin I and II, respectively, possess a higher affinity and selectivity for delta-opioid receptors than any other known natural compound. Since much evidence supports the role of spinal delta-opioid sites in producing antinociceptive effects, we investigated whether analgesia might be detected by direct spinal cord administration of [D-Ala2]deltorphin II (DADELT II) in the rat. The thermal antinociceptive effects of intrathecal DADELT II and dermorphin, a potent mu-selective agonist, were compared at different postinjection times by means of the tail-flick test. The DADELT II produced a dose-related inhibition of the tail-flick response, which lasted 10-60 min depending on the dose and appeared to be of shorter duration than the analgesia produced in rats after intrathecal injection of dermorphin (20-120 min). The analgesic effect of infused or injected DADELT II was completely abolished by naltrindole, the highly selective delta antagonist. These results confirm the involvement of delta receptors in spinal analgesic activity in the rat.

Antidepressant-type effects of endogenous enkephalins protected by systemic RB 101 are mediated by opioid delta and dopamine D1 receptor stimulation

The role of endogenous enkephalins in behavioural control in mice was investigated by i.v. injection of RB 101 (N-[(R,S)-2-benzyl-3[(S)(2-amino-4- methylthio)butyl dithio]-1-oxopropyl]-L-phenylalanine benzyl ester). RB 101 is a recently reported systemically active mixed inhibitor prodrug of the two enzymes which metabolize the enkephalins neutral endopeptidase 24.11 and aminopeptidase N. RB 101 (2.5-10 mg/kg) induced a dose-dependent long-lasting hyperlocomotion and attenuated the conditioned suppression of motility in mice placed in an environment where they had received footshocks 24 h before. In addition, RB 101 decreased the duration of immobility in the forced swim test. All these actions of RB 101 were antagonized by the selective delta antagonist, naltrindole, supporting the preferential involvement of delta opioid receptors in these enkephalin-controlled behavioural responses. The effects induced by RB 101 were also suppressed by prior administration of the selective dopamine D1 antagonist, SCH 23390, but not by the D2 antagonist, sulpiride. Attenuation of the conditioned suppression of motility was associated with increased striatal dihydroxyphenylacetic acid (DOPAC)/dopamine (DA) and homovanillic acid (HVA)/DA ratios, both effects being antagonized by naltrindole. This latter compound is also efficient to inhibit the effect of imipramine in the mouse forced swim test. Taken together, these results support the occurrence of tonic and phasic controls of mood-related behaviour by endogenous enkephalins through delta and D1 receptor stimulation and suggest a possible future use of these mixed inhibitors as new antidepressants.

Hypothermic effect of D-Ala-deltorphin II, a selective delta opioid receptor agonist

The natural heptapeptide D-Ala-deltorphin II, the most selective agonist for delta-receptors currently available, was used to study the role of brain delta opioid receptors in the control of body temperature. In rats placed in a cold ambient temperature (4 degrees C), intracerebroventricular injections of D-Ala-deltorphin II produced a significant hypothermia. In animals at an ambient temperature of 22 degrees C, only the highest dose employed induced a slight fall in body temperature. At a warm temperature (34 degrees C), D-Ala-deltorphin II induced no significant changes in body temperature. D-Ala-deltorphin II-induced hypothermia, unaffected by naloxone, was significantly reduced by the selective delta receptor antagonist naltrindole. These findings indicate that D-Ala-deltorphin II produces its hypothermic effects at a supraspinal delta receptor, and support the involvement of delta receptors in central control of body temperature.

Long-term sensitization to the activation of cerebral delta-opioid receptors by the deltorphin Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2 in rats exposed to morphine

In experiments to evaluate responses to the activation of cerebral delta-opioid receptors, repeated daily injection of the selective delta-opioid agonist Tyr-D-Ala-Phe-Glu-Val-Val-Gly-NH2 ([D-Ala2]deltorphin II) into rat brain resulted in the development of tolerance, whereas repeated daily injection or continuous infusion of morphine resulted in sensitization to the behavioral activating effects of the delta-opioid agonist. Although the rats did not modify their spontaneous locomotor activity after morphine withdrawal, they became markedly hyperresponsive to the locomotor and stereotypy-producing effects of a challenge dose of the delta-opioid agonist. Sensitization to activation of delta-opioid receptors persisted for at least 60 days after discontinuing morphine treatment. These results show that the development of tolerance and long-term sensitization to opioids involves delta-opioid as well as mu-opioid receptors.