Combined autoradiographic-immunocytochemical analysis of opioid receptors and opioid peptide neuronal systems in brain

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

The possible involvement of endogenous ligands for mu-, delta- and kappa-opioid receptors in modulating morphine-induced CPP expression in rats

Previous studies suggested that electroacupuncture (EA) can suppress opioid dependence by the release of endogenous opioid peptides. To explore the site of action and the receptors involved, we tried to inject highly specific agonists for mu-, delta- and kappa-opioid receptors into the CNS to test whether it can suppress morphine-induced conditioned place preference (CPP) in the rat. Male Sprague-Dawley rats were trained with 4 mg/kg morphine, i.p. for 4 days to establish the CPP model. This CPP can be prevented by (a) i.p. injection of 3 mg/kg dose of morphine, (b) intracerebroventricular (i.c.v.) injection of micrograms doses of the selective mu-opioid receptor agonist DAMGO, delta-agonist DPDPE or kappa-agonist U-50,488H or (c) microinjection of DAMGO, DPDPE or U50488H into the shell of the nucleus accumbens (NAc). The results suggest that the release of endogenous mu-, delta- and kappa-opioid agonists in the NAc shell may play a role for EA suppression of opiate addiction.

Effects of paraformaldehyde fixation on nicotinic acetylcholine receptor binding in adult and developing rat brain sections

In vitro receptor autoradiography requires unfixed tissue sections, but incubation and washing procedures often result in substantial tissue damage in sections from developing brain, hindering quantitative and qualitative analysis. Formaldehyde fixation greatly preserves morphology. However, fixation can interfere with pharmacological properties of receptors, increase in non-specific background labeling, or even destroy ligand binding sites. Two mild fixation protocols, 0.2% paraformaldehyde (pFA) and pFA vapor fixation, were compared for their ability to improve tissue morphology in postnatal day 7 (P7) brain slices and maintain binding of [125I]-epibatidine and [125I]-alpha-bungarotoxin to heteromeric and homomeric nicotinic acetylcholine receptors, respectively. Fixation greatly improved the ability of P7 brain slices to withstand incubation and washing procedures during binding, resulting in minimal or no loss of tissue after prior 0.2% pFA or vapor fixation, respectively. In adults, distribution pattern of [125I]-epibatidine was identical in fixed and unfixed slices, with no difference in total and non-specific labeling. Distribution of [125I]-alphaBTX labeling was similarly unaffected by 0.2% pFA fixation, but vapor fixation increased total and non-specific binding signal. Thus, mild fixations greatly improve tissue quality during receptor binding procedures and can preserve pharmacological properties of nicotinic acetylcholine receptors. However, different receptors or ligands might exhibit differential sensitivity to fixation protocols.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Modulation of anxiety by μ-opioid receptors of the lateral septal region in mice

Morphine and opiates are known to exert anxiolytic effects, probably by interacting with the GABAergic system. The lateral septum (LS), mainly constituted of GABA neurons, exhibits high densities of mu-opiate receptors and could thus represent one the brain sites where opiates interact with GABAergic transmission to modulate anxiety. We examined the effects of intra-LS morphine injections on measures of anxiety using the elevated plus-maze and hole-board tests. Fos imaging was used to identify neural circuits involved in anxiety modulation. Unilateral intra-LS morphine (100 or 500 ng/100 nl) decreased open-arm exploration in the plus-maze and reduced head-dipping frequency in the hole-board, an anxiogenic-like effect associated with decreased Fos expression in the ventral LS, the dorsal hippocampus and the anterior hypothalamus. Anatomical specificity was assessed by injecting morphine into the medial septum, which failed to produce anxiogenesis. Pre-injection of the mu-opioid receptor antagonist naloxonazine (100 ng/100 nl) into LS reversed morphine-induced anxiogenesis and the associated pattern of Fos expression, indicating a specific recruitment of mu-opioid receptors by morphine. Surprisingly, bilateral morphine injections (20 to 500 ng/100 nl) were not found anxiogenic, perhaps due to their stimulant effect. Taken together, these results suggest that LS mu-opioid receptors participate to the modulation of anxiety.

Micro-opioid receptor preferentially inhibits oxytocin release from neurohypophysial terminals by blocking R-type Ca2+ channels

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

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)).

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.

Control of rat hypothalamic pro-opiomelanocortin neurons by a circadian clock that is entrained by the daily light-off signal

Previous studies have clearly demonstrated that the immediate-early gene, c-fos can regulate, through its protein product Fos, the expression of the pro-opiomelanocortin gene. In the present study, immunohistochemistry for Fos and beta-endorphin was used to assess the basal activity of hypothalamic pro-opiomelanocortin-producing neurons throughout a 12 h light/12 h dark cycle. Here, we showed that Fos is undetectable in most beta-endorphin neurons from late morning until 30 min after light offset in the evening, whereas Fos is spontaneously expressed in these neurons after 1 h following dark onset. The number of beta-endorphin neurons expressing Fos increases continuously during the first half of the dark phase, is maximal at the middle of this phase and decreases through late night and early morning, reaching a nadir 2-3 h after light onset. Acute shifts of lighting parameters allowed us to demonstrate that the light-off signal per se is neither sufficient nor necessary for Fos expression in beta-endorphin neurons. However, when recurrent, this signal is able to entrain Fos expression after a period of adaptation to the new light/dark schedule. Moreover, an expression of Fos in beta-endorphin neurons persists during subjective night in rat exposed to constant light or constant dark for two to three days. Thus, the occurrence of the daily rhythmic increase in the expression of Fos protein in hypothalamic pro-opiomelanocortin neurons exclusively at (subjective) night suggests that these neurons are, most likely, controlled by a (circadian) nocturnal oscillator. Our data also reveal an interesting property of this oscillator: its entrainment by the daily light-to-dark transition signal.

The kunitz protease inhibitor form of the amyloid precursor protein (KPI/APP) inhibits the proneuropeptide processing enzyme prohormone thiol protease (PTP). Colocalization of KPI/APP and PTP in secretory vesicles

Proteolytic processing of proenkephalin and proneuropeptides is required for the production of active neurotransmitters and peptide hormones. Variations in the extent of proenkephalin processing in vivo suggest involvement of endogenous protease inhibitors. This study demonstrates that "protease nexin 2 (PN2)," the secreted form of the kunitz protease inhibitor (KPI) of the amyloid precursor protein (APP), potently inhibited the proenkephalin processing enzyme known as prohormone thiol protease (PTP), with a Ki,app of 400 nM. Moreover, PTP and PN2 formed SDS-stable complexes that are typical of kunitz protease inhibitor interactions with target proteases. In vivo, KPI/APP (120 kDa), as well as a truncated form of KPI/APP that resembles PN2 in apparent molecular mass (110 kDa), were colocalized with PTP and (Met)enkephalin in secretory vesicles of adrenal medulla (chromaffin granules). KPI/APP (110-120 kDa) was also detected in pituitary secretory vesicles that contain PTP. In chromaffin cells, calcium-dependent secretion of KPI/APP with PTP and (Met)enkephalin demonstrated the colocalization of these components in functional secretory vesicles. These results suggest a role for KPI/APP inhibition of PTP in regulated secretory vesicles. In addition, these results are the first to identify an endogenous protease target of KPI/APP, which is developmentally regulated in aging and Alzheimer's disease.

Electrophysiological and behavioral effects of Tyr-D-Arg-Phe-Sar on locus coeruleus neurons of the rat

The effect of Tyr-D-Arg-Phe-Sar (TAPS), a mu-selective tetrapeptide analog of dermorphin, was studied in the rat both in vitro, using slices of the locus coeruleus, and in vivo, after microinjection into the locus coeruleus. In electrophysiological studies, TAPS (1-100 nM) was able to inhibit spontaneous firing, cause hyperpolarization of the membrane potential and reduce the input resistance of neurons of the locus coeruleus, suggesting that there was an effect on the potassium channels. Based on the inhibition of the spontaneous firing rate, the average IC50 for TAPS was calculated to be 1.9 nM, a value lower than that reported for dermorphin or morphine. The TAPS-induced effects were antagonized by naloxone, with a dissociation equilibrium constant of 1.96 +/- 0.14 nM. The results indicate that TAPS binds to mu-opioid receptors on the cell membrane of neurons of the locus coeruleus to cause its inhibitory actions. In behavioral study, TAPS was microinjected bilaterally via chronically implanted cannulae into the locus coeruleus of non-anesthetized rats and its effects on locomotor activity determined. TAPS, at concentrations of 1 microM and 10 microM, but not of 0.1 microM, induced hypolocomotion/sedation and the effect was significantly reversed by naloxone (5 mg/kg i.p.). Taken together, these data suggest that TAPS has an inhibitory effect on neurons of the locus coeruleus and produces hypolocomotive/sedative effects in vivo.

General, mu and kappa opioid antagonists in the nucleus accumbens alter food intake under deprivation, glucoprivic and palatable conditions

Ventricular microinjection studies found that whereas mu (beta-funaltrexamine, B-FNA), mu1 (naloxonazine) and kappa (nor-binaltorphamine, Nor-BNI) opioid receptor antagonists, but not delta antagonists, reduce deprivation-induced intake, kappa and mu, but not mu1 or delta antagonists reduce both 2-deoxy-D-glucose (2DG) hyperphagia and sucrose intake. Since opioid agonists stimulate spontaneous food intake in the accumbens, the present study examined whether administration of either naltrexone, B-FNA or Nor-BNI in the accumbens altered intake under deprivation (24 h), glucoprivic (2DG: 500 mg/kg, i.p.) or palatable sucrose (10%) conditions. Naloxonazine's effects in the accumbens were also evaluated for deprivation-induced intake. Deprivation-induced intake was significantly decreased over 4 h by naltrexone (5-20 micrograms, 44%), B-FNA (1-4 micrograms, 55%) and Nor-BNI (4 micrograms, 31%) but not naloxonazine (10 micrograms) in the accumbens. 2DG hyperphagia was significantly decreased by naltrexone (10-20 microgram, 79%), B-FNA (1-4 micrograms, 100%) and NOR-BNI (104 micrograms, 75%) in the accumbens. Sucrose intake was significantly decreased by naltrexone (50 micrograms, 27%) and B-FNA (1-4 micrograms, 37%), but not NOR-BNI in the accumbens. These data suggest that mu receptors, and particularly the mu2 binding site in the accumbens are responsible for the opioid modulation of these forms of intake in this nucleus, and that this control may be acting upon the amount of intake per se.

Opiate microinjections in the locus coeruleus area of the cat enhance slow wave sleep

The effects on sleep/wakefulness states of morphine, morphiceptin (specific mu agonist), DPDPE (delta agonist) and U-50,488H (kappa agonist) microinjections in the Locus coeruleus area (LC) were studied in cats. Morphine (0.8-1.75 nmols in 50 nl of saline) and morphiceptin (1.75 nmols) in LC significantly increased the total time spent in slow wave sleep (SWS) and the mean duration of SWS episodes. Prior naloxone administration blocked the morphine hypnogenic effects. The total time spent in SWS was unaffected by delivery of equimolar doses of DPDPE or U-50,488H in LC; however, the mean duration of the SWS episodes increased significantly after U-50,488H microinjections in LC. Thus, when acting in the LC, opiates have a SWS-enhancing effect and this effect appears to be mediated by mu receptors, although kappa receptors may have a subsidiary action.

Immunocytochemical localization of delta opioid receptors in mouse brain

An affinity-purified anti-peptide antibody generated against the carboxy-terminal region of the delta opioid receptor was used to localize delta opioid receptors in mouse brain. delta Opioid receptor immunoreactivity was found in axons and nerve terminals in regions of the olfactory bulb, hippocampal formation, cerebral and cerebellar cortex, midbrain and hindbrain. The immunocytochemical distribution correlated well, though not completely with autoradiographic distribution of delta opioid receptors in mouse brain using either [3H][2-D-penicillamine, 5-D-penicillamine]-enkephalin (DPDPE) or [3H]naltrindole. Confocal microscopy of double-labeled tissue provided direct evidence that delta opioid receptors are principally expressed on GABAergic terminals in the hippocampus. These anatomical findings complement extensive physiological studies to provide a more detailed description of endogenous opioid circuitry.

Effect of anterior hypothalamic deafferentation on the negative feedback of gonadal steroids on luteinizing hormone pulse frequency in the ewe

Three experiments were performed to determined the effect of anterior hypothalamic deafferentation (AHD) on the inhibitory actions of estradiol and progesterone on luteinizing hormone (LH) pulse frequency in ovariectomized ewes during the anestrous season. The first experiment tested the effects of AHD on LH secretion in the absence of gonadal steroids. AHD was accomplished by a 180 degree knife cut (Halasz knife) placed at the posterior border of the optic chiasm (n = 4). Control ewes received sham cuts or no surgery (n = 6). LH pulses were monitored in blood samples taken every 12 min for 4 hr before and after surgery. AHD reduced LH pulse amplitude (pre 7.4 +/- 2.9 ng/ml; post 1.2 +/- 0.3 ng/ml), but had no effect on LH pulse frequency (pre 3.5 +/- 0.3/4 hr; post 3.3 +/- 0.3/4 hr) in ovariectomized ewes. In the second experiment all ewes were given a Silastic implant containing estradiol (1 cm long) and 2 d later bled at 12 min intervals for 4 hr before and after administration of pimozide (PIM, 0.08 mg/kg), a dopamine antagonist. Estradiol decreased LH pulse frequency in controls (2.8 +/- 0.4 to 1.5 +/- 0.3/4 hr) and this was reversed by PIM (2.5 +/- 0.4/4 hr). In contrast, estradiol did not decrease pulse frequency in AHD ewes (3.3 +/- 0.3 to 2.8 +/- 0.5/4 hr) and PIM did not increase it (2.7 +/- 0.7/4 hr). In the third experiment, all ewes were given Silastic implants containing progesterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a multi-compartmental histochemical organization of the nucleus accumbens in the rat

In the present study, the compartmental organization of the nucleus accumbens was investigated by comparing the pattern of leu-enkephalin immunoreactivity with that of the opioid receptor ligand, naloxone, an established marker for the compartmental organization of the neostriatum. Both patterns have a nonhomogeneous, patch-like appearance throughout the rostrocaudal extent of the nucleus and show a good, mutual correspondence. In the core of the nucleus accumbens as well as in the border region between the nucleus accumbens and the caudate-putamen, leu-enkephalin-rich areas are in register with opioid receptor-dense areas. In the shell region the precise relationship between the enkephalin and the naloxone patterns could not be established. A comparison of the connectivity patterns and neurochemical characteristics of the opioid receptor-dense compartments in the nucleus accumbens with those in the caudate-putamen reveals major discrepancies between these two striatal subdivisions. We therefore conclude that, rather than a bicompartmental patch/striosome-matrix organization, the nucleus accumbens has a multicompartmental organization.

Medullary mu and delta opioid receptors modulate mesencephalic morphine analgesia in rats

Supraspinal opioid analgesia is mediated in part by connections between the midbrain periaqueductal gray (PAG) and rostral ventral medulla (RVM) which includes the nuclei raphe magnus and reticularis gigantocellularis. Serotonergic 5HT2 and 5HT3 receptor subtypes appear to participate in this pathway since general and selective serotonergic antagonists microinjected into the RVM significantly reduced morphine analgesia elicited from the PAG. Since both an enkephalinergic pathway between the PAG and RVM and intrinsic enkephalinergic cells in the RVM exist, the present study evaluated the abilities of general (naltrexone), mu-selective (beta-funaltrexamine: B-FNA) and delta 2-selective (naltrindole) opioid receptor subtype antagonists microinjected into the RVM to alter morphine (2.5 micrograms) analgesia elicited from the PAG as measured by the tail-flick and jump tests. Mesencephalic morphine analgesia was significantly reduced after pretreatment in the RVM with naltrexone (1-10 micrograms), B-FNA (0.5-5 micrograms) or naltrindole (0.5-5 micrograms). Naltrexone in the RVM failed to alter basal nociceptive thresholds and none of the opioid antagonists were effective in reducing mesencephalic morphine analgesia when they were microinjected into placements lateral or dorsal to the RVM. These data indicate that mu and delta 2 opioid receptors in the RVM modulate the transmission of opioid pain-inhibitory signals from the PAG.

Electron microscopic localization of photoaffinity-labelled delta opioid receptors in the neostriatum of the rat

The distribution of delta opioid receptors, selectively labelled in vitro with the photoaffinity probe monoiodo azido-DTLET ([D-Thr2,pN3Phe4, Leu5]enkephaly-Thr6), was analyzed by light and electron microscopic radioautography in sections from rat neostriatum. Preliminary experiments indicated that up to 65% of specific 125I-azido-DTLET binding to rat striatal sections was still detectable following prefixation of the brain with 0.5% glutaraldehyde. These experiments also showed that up to 20-30% of the specifically bound radioactivity was covalently linked following ultraviolet irradiation and was thereby retained in tissue during subsequent postfixation and dehydration steps. Accordingly, the topographic distribution of the covalently attached azido-DTLET molecules was similar to that seen in fresh frozen sections and characteristic of that previously described for delta sites. Light and electron microscopic examination of the label in prefixed, striatal sections irradiated with ultraviolet light revealed that a significant proportion of specifically bound 125I-azido-DTLET molecules was intraneuronal. Specifically, 16% of the labelled binding sites were found in dendrites, 12% in perikarya and 4% in axon terminals. These results suggest that an important proportion of delta opioid binding sites labelled in the neostriatum correspond to receptors that are undergoing synthesis, transport and/or recycling. They also imply that a major fraction of delta sites are associated with intrastriatal neurons, as opposed to afferent axons. Approximately 44% of the labelled binding sites were associated with neuronal plasma membranes. Although most of these were found at the level of axodendritic (20%) and dendrodendritic (7%) appositions, comparison of the labelling incidence of these two compartments with their frequency of occurrence in tissue suggested that delta sites are fairly widely dispersed along neuronal plasma membranes. Only a small proportion (smaller than that of mu or kappa sites labelled in the same region) was associated with synaptic specializations. These results support the concept that delta receptors correspond to molecular entities that are distinct from mu and kappa sites and suggest that delta ligands act primarily nonjunctionally on the plasma membrane of striatal neurons.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphine effects in brainstem-transected cats: I. EEG and 'sleep-wakefulness' in the isolated forebrain

In order to examine the prosencephalic mechanisms that might sustain the effects of opiates on EEG and sleep-wakefulness, the actions of morphine sulfate on the EEG and the pupil size were examined in the chronically isolated forebrain of brainstem-transected cats. Single morphine doses (0.5, 2.0 or 3.0 mg/kg, i.p.) administered to these animals produced a long-lasting EEG desynchronization in the isolated forebrain which was associated with pupil mydriasis. The specificity of these morphine effects was shown by the fact that naloxone blocks both the EEG and pupillary effects of the drug. After morphine, spontaneous synchronized EEG with delta waves normally seen in the isolated forebrain preparation was suppressed for 6-18 h, followed by a strong rebound. Both the suppression and rebound in synchronization with delta waves occurred in a dose-dependent manner. The duration of these effects closely paralleled previously reported morphine effects on non-rapid eye movement (NREM) sleep in intact cats. Therefore, in relation to the effects of morphine on EEG and sleep-wakefulness in intact animals, this study suggests that: (1) Morphine suppression of NREM sleep and the subsequent arousal state of the animal are mediated by prosencephalic structures; (2) the generation of the typical neocortical EEG slow burst activity produced by opiates depends on lower brainstem structures.

Nicotine-induced alterations in peripheral tissue concentrations of native and cryptic Met- and Leu-enkephalin

This study examined the peripheral tissue distribution of native and cryptic Met- and Leu-enkephalin, and regulation of tissue enkephalins by nicotine. Met- and Leu-enkephalin concentrations showed widespread variation in tissue concentration and degree of processing. HPLC characterization of homogenate of spleen revealed that both native and cryptic immunoreactive Met-enkephalin are comprised of two peaks, one representing authentic Met-enkephalin pentapeptide and the other its sulfoxide. Subacute repeated administration of nicotine 0.1 mg/kg ip, six times at 30 min intervals, increased native Met- and Leu-enkephalin in adrenal medulla without affecting cryptic Met- and Leu-enkephalin concentrations, consistent with increased processing of larger peptides to Met- and Leu-enkephalin. Subacute nicotine decreased splenic concentrations of native and cryptic Met-enkephalin and native Leu-enkephalin, consistent with increased release of Met- and Leu-enkephalin from spleen and decreased synthesis of proenkephalin A or inadequate processing of larger peptides to enkephalin pentapeptides in spleen to compensate for the increased release during this period. HPLC characterization revealed that nicotine-induced decrease in native Met-enkephalin in spleen resulted from reductions in both pentapeptide and its sulfoxide. Nicotine also increased native Met-enkephalin in jejunum, decreased cryptic Met-enkephalin in heart atrium, increased native Leu-enkephalin in anterior pituitary and decreased cryptic Leu-enkephalin in jejunum. Nicotine may produce some of its effects through alterations in release of enkephalins from peripheral tissues.

Multiple neurochemical and behavioral consequences of stressors: implications for depression

Animal models of clinical depression have frequently focused on the contribution of stressors to the induction of behavioral impairments and pharmacological intervention in the amelioration of these disturbances. Stressors provoke various behavioral disturbances and influence the activity of central neurotransmitters implicated in depression. It is our contention that those variables which favor the provocation of amine depletions or prevent the development of a neurochemical adaptation will increase vulnerability to behavioral disturbances. It is essential to consider, however, that marked interindividual and interstrain differences exist in the behavioral and neurochemical response to stressors, and in the effectiveness of antidepressant treatments.

Compartmental organization of the ventral striatum of the rat: immunohistochemical distribution of enkephalin, substance P, dopamine, and calcium-binding protein

In the caudate-putamen of the rat a patch/matrix organization can be recognized on the basis of the immunohistochemical distribution of several markers, which include enkephalin, substance P, dopamine, and calcium-binding protein. In the present experiments the distributional relations of these markers were investigated in the nucleus accumbens. The distribution of enkephalin fibers shows different inhomogeneities according to their location in the nucleus. Rostrally, heavily labeled areas stand out against a moderately stained background, whereas caudally, in medial and ventral parts of the nucleus, lightly stained areas delineate regions in the moderately stained neuropil. In the distribution of substance P, areas with high staining intensity were observed in the medial and ventral parts of the nucleus accumbens. Inhomogeneities in the distribution of strong dopamine immunoreactivity consist of weakly immunoreactive areas throughout the rostrocaudal extent of the nucleus accumbens and extremely heavily labeled areas in the medial and ventral parts of the nucleus. Calcium-binding protein immunoreactivity can only be detected in dorsal parts of the nucleus. The generally intense immunostaining for calcium-binding protein is interspersed with "blanks" of weak immunoreactivity. The heavily and moderately labeled enkephalin areas each maintain specific relations with inhomogeneities in the distribution of substance P, dopamine, and calcium-binding protein. Rostrally, the heavily labeled enkephalin areas coincide with areas strongly immunostained for calcium-binding protein and with lightly stained areas in the dopamine and substance P immunoreactivity patterns. In the same region lightly stained areas in the enkephalin distribution match heavily labeled substance P areas. Caudally, in the border region of the nucleus accumbens and the caudate-putamen, the heavily labeled enkephalin areas are either related to "blanks" or to the intense staining regions in the calcium-binding protein immunoreactivity distribution. The moderately labeled enkephalin areas caudomedially in the nucleus accumbens are in register with the heavily labeled regions in the distribution of substance P and with the extremely heavily labeled regions in the distribution of dopamine. Relations with connectivity are discussed and the inhomogeneities are compared to those in the caudate-putamen. It is concluded that in the ventral striatum either one patch and one matrix compartment exist with different immunohistochemical relationships or there are several compartments with different immunohistochemical characteristics and different input-output relations.

Immunocytochemical study of enkephalin-like cell bodies in the thalamus of the rat

The distribution of enkephalin-like cell bodies in the thalamus of the rat was studied by means of intratissular injections of colchicine and using an indirect immunoperoxidase technique. The densest clusters of immunoreactive perikarya were observed in the nuclei geniculatum lateralis ventralis, medialis dorsalis, centralis lateralis, centralis medialis and anterior ventralis. Whereas the nuclei praetectalis lateralis, lateralis posterior, habenularis lateralis, parataenialis (its caudal part), parafascicularis, centrum medianum, reuniens and ventralis medialis had the lowest density. In other thalamic nuclei geniculatum mediale, paraventricularis and parataenialis (its rostral part) the density of enkephalin-like cell bodies was intermediate. These results suggest that the intratissular injection of colchicine is the better way of administration of the drug in order to study the distribution of peptidergic cell bodies in the mammalian CNS. The similarities and differences found in the distribution of enkephalinergic cell populations in the thalamus of different mammals are discussed.

Characterization of stimulation-produced analgesia from the nucleus tractus solitarius in the rat

Electrical stimulation of the commissural region of the nucleus tractus solitarius (NTS) inhibits the tail-flick reflex evoked by noxious heat. This antinociception can be measured in the awake or pentobarbital anesthetized rat at current intensities that do not induce overt behavioral side effects. Glutamate microinjections into the NTS, but not immediately surrounding the NTS, also inhibit the tail-flick reflex, demonstrating that activation of NTS cell bodies, and not fibers of passage, mediates antinociception from this region. In contrast, morphine microinjections into the NTS have no effect on the tail-flick reflex in anesthetized rats. These findings provide further evidence that the NTS is involved in the modulation of nociception.

Immunocytochemical demonstration of opioid receptors in selected rat brain areas and neuroblastoma x glioma hybrid (NG108-15) cells using a monoclonal anti-idiotypic antibody

A monoclonal anti-idiotypic opioid receptor antibody was used for the light-microscopic visualization of opioid receptors in several brain structures and monolayer cultures of a neuroblastoma x glioma hybrid cell-line (NG108-15). The antibody proved to be specific, displaying affinity for mu greater than delta much greater than kappa opioid receptors. Receptor distribution in the brain areas studied was in agreement with previous autoradiographic analyses; of particular interest, high densities of immunoreactive opioid receptors were found in the perikarya and in the initial parts of the axons and dendrites; light microscopy did not allow an exact determination of the subcellular localization of opioid receptors, but the immunoreactivity seemed to be associated with the plasma membrane and to be present within the cytoplasm as well. Similar observations were made for the cell bodies and neurites of NG108-15 cells. The methodology described potentially permits the study of opioid receptor distribution in discrete brain areas under different physiological and pharmacological conditions and of the ontogeny of these receptors; in addition, it may help to find a morphological basis for events such as receptor internalization and recycling.

Direct inhibition by opioid peptides of neurones located in the ventromedial nucleus of the guinea pig hypothalamus

In slices of guinea pig brain, intracellular recordings were obtained from neurones of the ventromedial nucleus of the hypothalamus (VMH). [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO), an agonist selective for mu-opioid receptors, caused an inhibition of spontaneous firing activity and a membrane hyperpolarization. This effect was reversible, concentration-dependent and could be blocked by naloxone. DAGO directly inhibited VMH neurones since its effect persisted when the slice was perifused with a solution that blocks synaptic transmission. The hyperpolarization induced by DAGO was associated with a marked decrease in membrane input resistance and it was reversed in polarity at membrane potentials 30-40 mV more negative than the resting potential. A chloride current did not contribute to the hyperpolarization brought about by DAGO. We conclude that DAGO inhibits VMH neurones, probably by opening membrane potassium channels.

Microinjected morphine suppresses the activity of locus coeruleus noradrenergic neurons in freely moving cats

Microinjection of morphine (1.0 microgram/0.1 microliter) produced a significant suppression (approximately 60%) of the single unit activity of locus coeruleus noradrenergic neurons in freely moving cats. This effect was reversible by systemic administration of the opioid receptor antagonist, naloxone (1.0 mg/kg i.v.). The microinjection of naloxone (1.0 microgram/0.1 microliter), however, was without effect on the spontaneous activity of noradrenergic neurons in the locus coeruleus. Non-noradrenergic neurons recorded in the same vicinity showed no consistent response to the microinjection of morphine. These results suggest that the direct effect of opioids in the locus coeruleus is an inhibition of noradrenergic neuronal activity. Furthermore, it appears that opioid influences upon these neurons are not tonically active.

Systemic naloxone administration potentiates locus coeruleus noradrenergic neuronal activity under stressful but not non-stressful conditions

When administered during non-stressful, quiet-waking conditions, i.v. naloxone (1.0 mg/kg) had no effect on the activity of locus coeruleus (LC) noradrenergic neurons in behaving cats. In contrast, the activation of LC noradrenergic unit activity produced by restraint stress was greatly potentiated by the same dose of naloxone. Indices of behavioral distress, vocalization and struggling, were also found to be significantly increased in animals given naloxone during stress. These results suggest that endogenous opioids have a moderating influence upon the level of activity of the LC which operates only under specific conditions such as stress.

Nicotine-induced alterations in brain regional concentrations of native and cryptic Met- and Leu-enkephalin

The distribution of cryptic forms (larger enkephalin-containing peptides) in neostriatum, hypothalamus, spinal cord T3-L1 and neurointermediate lobe of pituitary were determined by radioimmunoassay. Optimal conditions for enzymic hydrolysis of the cryptic enkephalins by trypsin and carboxypeptidase B were established. The proportion of total Met- and Leu-enkephalin represented by native pentapeptide varied markedly among these central nervous system regions. Also, the distributions of native and cryptic Met-enkephalin were distinct from that of Leu-enkephalin. Chromatographic separation by HPLC of immunoreactive Met-enkephalin peptides revealed only two peaks corresponding to Met-enkephalin and Met-enkephalin sulfoxide in rather equal amounts. Hydrolysis of cryptic Met-enkephalin also produced only two HPLC-separable peaks of immunoreactive Met-enkephalin, again corresponding to Met-enkephalin and Met-enkephalin sulfoxide. Bioactivity of cryptic striatal Met-enkephalin after hydrolysis was demonstrated by antinociception and catalepsy in rats following its intracerebroventricular injection. Repeated short-term administration of nicotine, 0.1 mg/kg IP six times at 30 min intervals, produced significant increases in native and cryptic Met-enkephalin in striatum, consistent with an increase in neuronal release of Met-enkephalin together with increases in synthesis and processing of proenkephalin A in this brain region. This regimen of nicotine also decreased levels of native Met-enkephalin and of both native and cryptic Leu-enkephalin in neurointermediate lobe, consistent with nicotine-induced release of both proenkephalin A- and prodynorphin-derived peptides from neurointermediate lobe.

A possible interface between autonomic function and pain control: opioid analgesia and the nucleus tractus solitarius

Opioid peptides appear to be important neurochemical mediators in central nervous system mechanisms of analgesia, cardiovascular control, and many endocrinological responses to stress. The nucleus tractus solitarius (NTS), a brain region expressing all 3 opioid peptide families, is also associated with regulation of autonomic and endocrine functions. We now report that electrical stimulation of the NTS causes pronounced analgesia in rats. This analgesia appears to involve opioids and is pharmacologically dissociable from the hemodynamic changes elicited by NTS stimulation. These results suggest the NTS as a neural substrate for inter-relationships between stress, cardiovascular function, alterations in respiration, and pain sensitivity.

The effects of nitrous oxide on the secretory activity of pro-opiomelanocortin peptides from basal hypothalamic cells attached to cytodex beads in a superfusion in vitro system

Dispersed cells from adult rat basal hypothalami, attached to Cytodex-3 microcarrier beads, were placed in a column and superfused with aerated high glucose media or media enriched with variable concentrations of nitrous oxide with oxygen. beta-Endorphin and alpha-MSH content was measured in the effluent collected during superfusion and demonstrated a near constant baseline release. Nitrous oxide, 60% (P less than 0.025) and 80% (P less than 0.02), caused significant increases in release of beta-endorphin. Potassium chloride (50 mM) caused a significant increase in release (P less than 0.007) of beta-endorphin whereas saline and 30% nitrous oxide did not. Neither nitrous oxide-enriched media nor potassium chloride had any statistically significant effect on alpha-MSH release. The increase in beta-endorphin secretory activity during exposure to nitrous oxide demonstrates that nitrous oxide may have a stimulatory effect on central pro-opiomelanocortin neurons.

Heterogeneous distribution and upregulation of mu, delta and kappa opioid receptors in the amygdala

Levels of mu, delta and kappa opioid receptors in 4 subnuclei of the rat amygdala were determined by quantitative autoradiography following chronic treatment with naloxone or saline. A different distribution of each receptor subtype was observed, with mu binding greatest in the lateral nucleus (La), delta greatest in the basolateral (Bl), and kappa greatest in the medial (Me). Levels of all 3 receptors were very low in the central nucleus. Receptor upregulation following chronic naloxone treatment was also anatomically heterogeneous. Increases in mu receptors were statistically significant in the Me, Bl and La, while increases in delta and kappa receptors were significant only in the Bl.

Effects of bestatin on the central cardiovascular regulatory mechanisms in the rat

We evaluated the effects of bestatin, the specific aminopeptidase-B and leucine aminopeptidase inhibitor, on the central cardiovascular regulatory mechanisms in Sprague-Dawley rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Intracerebroventricular injection of bestatin (100 or 200 nmol/5 microliters) consistently elevated the basal systemic arterial pressure and heart rate. At the same time, this degradative enzyme blocker increased the sensitivity of the baroreceptor reflex responses as well as the efficacy of the modulatory actions of the medullary nucleus reticularis gigantocellularis on these reflexes. We speculate that enhancing the tonic activities of the endogenous neuropeptides in the brain by protecting them from their catabolic enzymes may affect the central cardiovascular regulatory machinery by modifying the operations of the baroreceptor feedback controls and their modulatory mechanisms.

Opioid receptors in rat neostriatum: radioautographic distribution at the electron microscopic level

The distribution of mu-opioid receptors, selectively labeled in vitro with a monoiodinated Met-enkephalin analog [( 125I]FK 33-824), was analyzed by light and electron microscopic radioautography in sections from the neostriatum of the rat. In the light microscope, patches of high receptor densities were detected amidst a moderately labeled matrix. The number of silver grains, as counted in 1-micron thick plastic-embedded sections, was 3 times greater inside the patches than in the intervening matrix. In both compartments, the proportion of labeled binding sites associated with the neuropil was significantly higher (greater than 70%) than that associated with nerve cell bodies or myelinated fascicles. Quantitative analyses of electron microscopic radioautographs revealed that the majority of silver grains corresponding to specifically bound [125I]FK molecules originated from radioactive sources associated with apposed neuronal membranes. Of the total number of specific binding sites, 53% was associated with axodendritic, 18% with axoaxonic and 3% with axosomatic interfaces. The occurrence of multiple labeled foci along the plasma membrane of certain perikarya and dendrites suggested that some of the binding sites might be associated with somato/dendritic elements. The high incidence of labeling along axoaxonic interfaces indicated that others were linked to the membrane of axons and/or axon terminals. A major finding of the present study was that only a small proportion of specific FK binding sites (7% of total) was associated with synaptic junctions. Labeled synapses were primarily of the asymmetric type and were found predominantly on dendritic branches and spines. A few were observed on nerve cell bodies. Labeled symmetric synapses were rare and encountered exclusively on dendritic branches. The high frequency with which specifically labeled binding sites were found to be associated with neuronal interfaces involving axonal processes strongly suggests that even if non-junctional these binding sites correspond to functional receptors. Whether these receptors are activated by endogenous ligand molecules released by the labeled terminals themselves or from terminals located at a distance from the labeled interfaces remains to be determined.

An endogenous ligand for the sigma opioid binding site

It had been suggested that phencyclidine (PCP) and sigma opioids exert their similar psychotomimetic effects through a common receptor. Recently, however, there have been several reports demonstrating significant differences between the binding of PCP and SKF 10,047, a sigma opioid agonist, which suggests that there may be distinct PCP and sigma opioid receptors. If these differences in binding represent different receptors, then there may be different endogenous ligands for each receptor. Using porcine brains, which have already been used to isolate and purify an endogenous ligand for the PCP receptor, another factor has been isolated that inhibited the binding of [3H]-(+)SKF 10,047 and not the binding of [3H]-PCP. This factor appears to be a peptide or protein because incubation of the active fraction with pronase, a nonspecific peptidase, eliminated the ability of the porcine fractions to inhibit the binding of [3H]-(+)SKF 10,047. These findings suggest the existence of an endogenous ligand for sigma opioid receptors, which is different from the previously identified endogenous ligand for PCP receptors.

Pharmacological and anatomical evidence of selective mu, delta, and kappa opioid receptor binding in rat brain

While the distribution of opioid receptors can be differentiated in the rat central nervous system, their precise localization has remained controversial, due, in part, to the previous lack of selective ligands and insensitive assaying conditions. The present study analyzed this issue further by examining the receptor selectivity of [3H]DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol), [3H]DPDPE (2-D-penicillamine-5-D-penicillamine-enkephalin), [3H]DSLET (Tyr-D-Ser-Gly-Phe-Leu-Thr) and [3H](-)bremazocine, and their suitability in autoradiographically labelling selective subpopulations of opioid receptors in rat brain. The results from saturation, competition, and autoradiographic experiments indicated that the three opioid receptor subtypes can be differentiated in the rat brain and that [3H]DAGO and [3H]DPDPE selectively labelled mu and delta binding sites, respectively. In contrast, [3H]DSLET was found to be relatively non-selective, and labelled both mu and delta sites. [3H]Bremazocine was similarly non-selective in the absence of mu and delta ligands and labelled all three opioid receptor subtypes. However, in the presence of 100 nM DAGO and DPDPE, concentrations sufficient to saturate the mu and delta sites, [3H]bremazocine did label kappa sites selectively. The high affinity [3H]bremazocine binding sites showed a unique distribution with relatively dense kappa labelling in the hypothalamus and median eminence, areas with extremely low mu and delta binding. These results point to the selectivity, under appropriate conditions, of [3H]DAGO, [3H]DPDPE and [3H]bremazocine and provide evidence for the differential distribution of mu, delta, and kappa opioid receptors in rat brain.

Antinociceptive profiles of opioid peptide agonists in a rat tooth pulp stimulation procedure

The analgesic activity of the prototypic opioid peptides for the mu (D-Ala2-Me-Phen4-Gly-ol5-enkephalin [DAGO]) kappa (Dynorphin 1-13), delta (D-Ala2-D-Leu5-enkephalin [DADLE]), or epsilon (beta-endorphin) receptor was assessed in a rat tooth pulp stimulation procedure. All opioid peptides tested and the opioid alkaloid U50, 488H (kappa receptor agonist) significantly elevated response thresholds. The rank order of potency based on the Minimum Effective Dose values was beta-endorphin greater than DAGO = dynorphin A (1-13) amide greater than DADLE greater than dynorphin A (1-13) greater than U50,488H. Based on absolute magnitude, the rank order of dose response slopes was DAGO greater than U50,488H greater than dynorphin A (1-13) amide greater than beta-endorphin greater than DADLE. Dynorphin A (1-13) produced the shallowest dose response slope and the magnitude of response threshold was the lowest for all compounds tested. Finally, the general conclusion that mu agonists are effective against noxious stimuli derived from thermal, chemical, and mechanical is extended by our data to include electrical sources derived from tooth pulp stimulation; kappa agonists are effective against noxious stimuli derived from chemical, mechanical, and electrical sources (tooth pulp stimulation) and delta agonists are effective analgesics against thermal, chemical and electrical stimuli (tooth pulp stimulation).