Similar involvement of several brain areas in the antinociception of endogenous and exogenous opioids

Endogenous opioid peptides in the descending pain modulatory circuit

The opioid epidemic has led to a serious examination of the use of opioids for the treatment of pain. Opioid drugs are effective due to the expression of opioid receptors throughout the body. These receptors respond to endogenous opioid peptides that are expressed as polypeptide hormones that are processed by proteolytic cleavage. Endogenous opioids are expressed throughout the peripheral and central nervous system and regulate many different neuronal circuits and functions. One of the key functions of endogenous opioid peptides is to modulate our responses to pain. This review will focus on the descending pain modulatory circuit which consists of the ventrolateral periaqueductal gray (PAG) projections to the rostral ventromedial medulla (RVM). RVM projections modulate incoming nociceptive afferents at the level of the spinal cord. Stimulation within either the PAG or RVM results in analgesia and this circuit has been studied in detail in terms of the actions of exogenous opioids, such as morphine and fentanyl. Further emphasis on understanding the complex regulation of endogenous opioids will help to make rational decisions with regard to the use of opioids for pain. We also include a discussion of the actions of endogenous opioids in the amygdala, an upstream brain structure that has reciprocal connections to the PAG that contribute to the brain's response to pain.

Allostery at opioid receptors: modulation with small molecule ligands

Opioid receptors are 7-transmembrane domain receptors that couple to heterotrimeric G proteins. The endogenous ligands for opioid receptors are peptides which bind to the orthosteric site on the receptors. The μ-opioid receptor is the target for opioid analgesics, while the δ-opioid receptor has been suggested as a target for pain management, migraine and depression. Similarly, κ-opioid receptors are involved in pain and depression and nociceptin receptors in pain and mood behaviours. However, exogenous orthosteric ligands for opioid receptors cause a myriad of on-target side effects. Recently, selective allosteric ligands for μ- and δ-opioid receptors have been described. These compounds bind to a site on the receptor distinct from the orthosteric site. Occupation of this allosteric site leads to modulation of orthosteric ligand binding affinity and/or efficacy. Allosteric modulators may be positive, negative or silent (neutral) (PAMs, NAMs or SAMs respectively). PAMs may have in vivo activity by enhancing the activity of exogenous drugs or endogenous opioid peptides. Enhancing endogenous opioid peptide activity maintains the temporal and spatial distribution of these molecules but improves, and potentially qualitatively changes, activity at their cognate receptors which could limit side effects compared with traditional opioid drugs. In this review, we describe the rationale and promise for the development of allosteric modulators for opioid receptors, the discovery of selective allosteric modulators, the identification of potential allosteric sites on opioid receptors and the mode of action of the modulators.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Nocturnal sleep architecture disturbances in early methadone treatment patients

The subjective and objective sleep patterns of patients with opioid dependence have been previously reported, but the sleep characteristics of patients in early methadone treatment, especially the objective sleep patterns, remain largely unexamined. This study was designed to explore the nocturnal sleep structure of patients on early methadone treatment. Twenty male methadone treatment (MT) patients and 20 male age- and body mass index-matched controls were assessed with overnight limited polysomnography. Subjective sleep was assessed using the Pittsburgh Sleep Quality Index (PSQI) and the Epworth Sleepiness Scale (ESS). Compared with healthy controls, MT patients had lower sleep efficiency, shorter total sleep time, more awakenings and shorter slow wave sleep (SWS). The PSQI and ESS scores in MT patients were significantly higher than in the controls. ESS scores of the patients were significantly associated with the SWS. The findings indicate that patients in early MT have poor sleep quality and abnormal sleep architecture.

Arginine vasopressin antinociception in the rat nucleus raphe magnus is involved in the endogenous opiate peptide and serotonin system

Arginine vasopressin (AVP) in the nucleus raphe magnus (NRM) has been implicated in antinociception. This communication was designed to investigate which neuropeptide and neurotransmitter are involved in AVP antinociception in the rat NRM. The results showed that (1) in the NRM perfuse liquid, pain stimulation could increase the concentrations of AVP, leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek), beta-endorphin (beta-Ep), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), but not change the concentrations of dynorphinA(1-13) (DynA(1-13)), oxytocin, achetylcholine, choline, gamma-aminobutyric acid, glutamate, dopamine, 3,4-dihydroxyphenylacetic acid, homovanilic acid, norepinephrine and epinephrine; (2) in the NRM perfuse liquid, AVP increased the concentrations of L-Ek, M-Ek, beta-Ep, DynA(1-13), 5-HT and 5-HIAA, but did not change the concentrations of oxytocin and the other studied neurotransmitters; (3) AVP antinociception in the NRM was attenuated by cypoheptadine (a 5-HT-receptor antagonist) or naloxone (an opiate receptor antagonist), but was not influenced by the other studied receptor antagonists. The data suggested that AVP antinociception in the NRM might be involved in endogenous opiate peptide and 5-HT system.

RB101-mediated protection of endogenous opioids: potential therapeutic utility?

The endogenous opioids met- and leu-enkephalin are inactivated by peptidases preventing the activation of opioid receptors. Inhibition of enkephalin-degrading enzymes increases endogenous enkephalin levels and stimulates robust behavioral effects. RB101, an inhibitor of enkephalin-degrading enzymes, produces antinociceptive, antidepressant, and anxiolytic effects in rodents, without typical opioid-related negative side effects. Although enkephalins are not selective endogenous ligands, RB101 induces these behaviors through receptor-selective activity. The antinociceptive effects of RB101 are produced through either the mu-opioid receptor alone or through activation of both mu- and delta-opioid receptors; the antidepressant-like and anxiolytic effects of RB101 are mediated only through the delta-opioid receptor. Although little is known about the effects of RB101 on other physiologically and behaviorally relevant peptides, these findings suggest that RB101 and other inhibitors of enkephalin-degrading enzymes may have potential as novel therapeutic compounds for the treatment of pain, depression, and anxiety.

Prenatal exposure to valproic acid disturbs the enkephalinergic system functioning, basal hedonic tone, and emotional responses in an animal model of autism

RATIONALE

It has been suggested that behavioral aberrations observed in autism could be the result of dysfunction of the neuroregulatory role performed by the endogenous opioid peptides. Many of those aberrations have been recently modeled in rats exposed to valproic acid (VPA) on the 12th day of gestation (VPA rats).

OBJECTIVES

The aim of the present study was to elucidate functioning of the enkephalinergic system, one of the endogenous opioid peptide systems strongly involved in emotional responses, in VPA rats using both biochemical and behavioral methods.

MATERIALS AND METHODS

In situ hybridization was used to measure proenkephalin mRNA expression in adult VPA rats' central nucleus of the amygdala, the dorsal striatum, and the nucleus accumbens. Additional groups of animals were examined in a conditioned place aversion to naloxone, the elevated plus maze, and object recognition tests to assess their basal hedonic tone, anxiety, learning and memory, respectively.

RESULTS

Prenatal exposure to VPA decreased proenkephalin mRNA expression in the dorsal striatum and the nucleus accumbens but not in the central nucleus of the amygdala. It also increased anxiety and attenuated conditioned place aversion to naloxone but had no impact on learning and memory.

CONCLUSIONS

The present results suggest that prenatal exposure to VPA may lead to the decreased activity of the striatal enkephalinergic system and in consequence to increased anxiety and disregulated basal hedonic tone observed in VPA rats. Presented results are discussed in light of interactions between enkephalinergic, GABAergic, and dopaminergic systems in the striatum and mesolimbic areas of the brain.

Enkephalinergic afferents of the centromedial amygdala in the rat

The connectivity of the amygdaloid complex has been extensively explored with both anterograde and retrograde tracers. Even though the afferents of the centromedial amygdala [comprising the central (CEA) and medial (MEA) amygdaloid nuclei] are well established, relatively little is known about the neuropeptide phenotype of these connections. In this study, we first examined the distribution of mu-opioid receptor (MOR) and delta-opioid receptor (DOR) in the amygdala via in situ hybridization and immunohistochemistry. We then investigated the distribution of Met-enkephalin (ENK) and Leu-ENK fibers with immunohistochemistry and examined the distribution of preproenkephalin mRNA in the amygdala by using in situ hybridization. Finally, we examined the ENK projections to the CEA and MEA by using stereotaxic injections of the retrograde tracer cholera toxin subunit B or fluorogold revealed by immunohistochemistry combined with in situ hybridization to identify ENKergic neurons. Our results indicate that the centromedial amygdala receives ENK afferents, as indicated by the presence of MOR, DOR, and ENK fibers in the CEA and MEA, originating primarily from the bed nucleus of the stria terminalis (BST) and from other amygdaloid nuclei. The posterior BST, the basomedial nucleus (BMA), and the cortical nucleus of the amygdala (COA) were found to be the major ENK afferents of the MEA, whereas the anterolateral BST, the COA, the MEA, and the BMA provided the main ENKergic innervation of the CEA. In addition, we found that the ventromedial nucleus of the hypothalamus and the pontine parabrachial nucleus provide a moderate ENK input to the CEA and MEA. The functional implications of these connections in stress, anxiety, and nociception are discussed.

Affective analgesia following the administration of morphine into the amygdala of rats

The amygdala processes stimuli that threaten the individual and organizes the execution of affective behaviors that permit the individual to cope with the threat. The prototypical threat to an individual is exposure to a noxious stimulus. The present study evaluated the contribution of the amygdala in modulating the affective response of rats to noxious stimulation. Vocalization afterdischarges (VADs) are a validated model of the affective response of rats to noxious tailshock. The antinociceptive action of morphine microinjected into the amygdala on VAD thresholds was compared to its effect on the thresholds of other tailshock-elicited responses (vocalizations during shock, VDS and spinal motor reflexes, SMRs). Whereas VADs are organized within the forebrain, VDSs and SMRs are organized at medullary and spinal levels of the neuraxis, respectively. The bilateral administration of morphine into the basolateral complex of the amygdala (BLC) produced dose-dependent increases in VAD and VDS thresholds, although increases in VAD thresholds were significantly greater than increases in VDS thresholds. Administration of morphine into BLC was ineffective in elevating SMR thresholds. Morphine-induced increases in vocalization thresholds were reversed in a dose-dependent manner by microinjection of the opiate receptor antagonist methylnaloxonium into BLC. Microinjection of morphine in the vicinity to the BLC did not alter vocalization thresholds. The present results provide further evidence for the preferential involvement of the amygdala in modulation of the affective component of the pain experience.

Depressant effect on a C-fibre reflex in the rat, of RB101, a dual inhibitor of enkephalin-degrading enzymes

The effect of N-[(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithiol]-1-oxopropyl]-L-phenylalanine benzyl ester (RB101), a dual inhibitor of the enkephalin-degrading enzymes, neutral endopeptidase and aminopeptidase N, was assessed in anaesthetised rats on the C-fibre reflex elicited by electrical stimulation within the sural nerve territory and recorded from the ipsilateral biceps femoris muscle. The temporal evolution of the pharmacological response was monitored by the repeated application of a constant stimulus intensity, namely three times threshold (3 T). In addition, recruitment curves were built by varying the stimulus intensity from 0 to 7 T. RB101 (7.5, 15 and 30 mg kg(-1), i.v.) induced a dose-dependent, naloxone-reversible depression of the reflex, which lasted around 60 min with the highest dose. The ED(50) was calculated as 16.9 mg kg(-1). Analyses of the recruitment curves revealed: (1) a significant increase of threshold; (2) a significant depression of the reflex in the ascending part of the curve; and (3) a lack of major depressive effects on the responses elicited by the strongest stimuli (corresponding to the plateau of the curve). The increase in the nociceptive threshold by enkephalin-degrading enzyme inhibitors, confirms previous data obtained from behavioural tests. In addition, the present study revealed an efficacy of these compounds over a wide range of stimulus intensities, albeit excluding the highest.

Effects of 14-methoxymetopon, a potent opioid agonist, on the responses to the tail electric stimulation test and plus-maze activity in male rats: neuroendocrine correlates

We have studied the effects of 14-methoxymetopon (HS 198), a potent opioid agonist, on the responses to the tail electric stimulation test and plus-maze activity of adult male rats. The prototype mu agonist morphine was used as the drug of reference. Besides we addressed the effects of HS 198 on the serum corticosterone levels and on serotonergic systems of discrete brain regions. Both drugs were administered subcutaneously. Morphine (5 mg/kg) and HS 198 (30 microg/kg) induced a similar effect on the nociceptive test, with both drugs significantly increasing the threshold for the vocalization afterdischarge, which is related to the emotional component of pain. In the plus-maze, morphine (5 mg/kg) and HS 198 (20 and 30 microg/kg) induced similar increases in the percentages of entries and time in the open arms, two parameters related to the anxiety state of the animals. The results indicate that HS 198 is far more potent than morphine in reducing the emotive/affective component of pain and in inducing an anxiolytic effect. HS 198 (30 microg/kg) also induced parallel increases in the serum corticosterone levels and the hypothalamic serotonin content. A possible correlation between the anxiolytic action of the drug and its effect on the hypothalamic serotonergic system is suggested.

Processing of nociceptive mechanical and thermal information in central amygdala neurons with knee-joint input

Pain has a strong emotional dimension, and the amygdala plays a key role in emotionality. The processing of nociceptive mechanical and thermal information was studied in individual neurons of the central nucleus of the amygdala, the target of the spino-parabrachio-amygdaloid pain pathway and a major output nucleus of the amygdala. This study is the first to characterize nociceptive amygdala neurons with input from deep tissue, particularly the knee joint. In 46 anesthetized rats, extracellular single-unit recordings were made from 119 central amygdala neurons that were activated orthodromically by electrical stimulation in the lateral pontine parabrachial area and were tested for receptive fields in the knee joints. Responses to brief mechanical stimulation of joints, muscles, and skin and to cutaneous thermal stimuli were recorded. Receptive-field sizes and thresholds were mapped and stimulus-response functions constructed. Neurons in the central nucleus of the amygdala with excitatory input from the knee joint (n = 62) typically had large symmetrical receptive fields in both hindlimbs or in all four extremities and responded exclusively or preferentially to noxious mechanical stimulation of deep tissue (n = 58). Noxious mechanical stimulation of the skin excited 30 of these neurons; noxious heat activated 21 neurons. Stimulus-response data were best fitted by a sigmoid nonlinear regression model rather than by a monotonically increasing linear function. Another 15 neurons were inhibited by noxious mechanical stimulation of the knee joint and other deep tissue. Fifteen neurons had no receptive field in the knee but responded to noxious stimulation of other body areas; 27 nonresponsive neurons were not activated by natural somesthetic stimulation. Our data suggest that excitation is the predominant effect of brief painful stimulation of somatic tissue on the population of central amygdala neurons with knee joint input. Their large symmetrical receptive fields and sigmoid rather than monotonically increasing linear stimulus-response functions suggest a role of nociceptive central amygdala neurons in other than sensory-discriminative aspects of pain.

Distribution and development of FMRFamide-like immunoreactive neuronal systems in the brain of the brown trout, Salmo trutta fario

The distribution of Phe-Met-Arg-Phe-amide (FMRFamide) peptide-immunoreactive (FMRF-ir) cells and fibers in the terminal nerve and central nervous system was investigated in developing stages and adults of the brown trout, Salmo trutta fario. The first FMRF-ir neurons appeared in the terminal nerve system of 8-mm embryos in and below the olfactory placode. In the brain, FMRF-ir neurons were first observed in the rostral hypothalamus, primordial hypothalamic lobe, mesencephalic laminar nucleus, and locus coeruleus of 12- to 13 -m embryos. After hatching, FMRF-ir cells appeared in the lateral part of the ventral telencephalic area and the anterior tuberal nucleus. In adult trout, FMRF-ir cells were observed in all these areas. The number of FMRF-ir neurons increased markedly in some of these populations during development. Dense innervation by FMRF-ir fibers was observed in the dorsal and lateral parts of the dorsal telencephalic area, and in the ventral telencephalic area, the lateral preoptic area, the medial hypothalamic and posterior tubercle regions, midbrain tegmentum and rhombencephalic reticular areas, the central gray, the superior raphe nucleus, the secondary visceral nucleus, the vagal nuclei, and the area postrema. Fairly rich FMRF-ir innervation was also observed in the optic tectum and some parts of the torus semicircularis. The saccus vasculosus and hypophysis received a moderate amount of FMRF-ir fibers. Innervation of most of these regions appeared either in late alevins or fry, although FMRF-ir fibers in the preoptic area, hypothalamus, and reticular areas appeared in embryos. Comparative analysis of the complex innervation pattern observed in the brain of trout suggests that FMRF is involved in a variety of functions, like the FMRF family of peptides in mammals.

First discrete autoradiographic distribution of aminopeptidase N in various structures of rat brain and spinal cord using the selective iodinated inhibitor [125I]RB 129

The selective and potent aminopeptidase N inhibitor [125I]RB 129 has been used for the radioautographic localization of this enzyme in rat brain, spinal cord and intestine. Brain microvessels and intestine brush-border cells were shown to present a high concentration of aminopeptidase N. Moreover, a labeling of various brain structures was observed. A very high level of binding occurred in the meninges, choroid plexus, pineal gland, paraventricular nucleus and pituitary gland. Moderate to high labeling was also observed in the cortex, caudate-putamen, subthalamic nucleus, central periaqueductal gray, thalamus, as well as in the dorsal and ventral horn of the spinal cord, which are known to contain a high concentration of enkephalins, opioid receptors and neutral endopeptidase. This co-localization confirms the physiological implication of aminopeptidase N in the inactivation of enkephalins accounting for the requirement of dual inhibition of neutral endopeptidase and aminopeptidase N to observe highly significant morphine-like effects induced by the protected endogenous opioid peptides. Aminopeptidase N was also visualized in moderate to high levels in other brain structures such as the hippocampus, nucleus accumbens, substantia nigra, hypothalamus (dorsomedial and ventromedial nuclei), raphe nucleus, pontine nucleus, inferior olive, and in high concentration in the granular layer of cerebellum. In summary, aminopeptidase N has been visualized for the first time in numerous brain areas using the selective inhibitor [125I]RB 129. This iodinated probe could allow the ex vivo and in vivo localization of aminopeptidase N in various tissues to be investigated and may also be used to evaluate quantitative changes in aminopeptidase N expression in pathological situations. Aminopeptidase N, which preferably removes NH2-terminal neutral amino acids from peptides, has probably a host of substrates. Nevertheless, a certain in vivo selectivity could be achieved by the presence of the enzyme in structures where the peptide effector and its receptors are also co-localized.

Circuitry underlying antiopioid actions of cholecystokinin within the rostral ventromedial medulla

It is now well established that the analgesic actions of opioids can be modified by "anti-analgesic" or "antiopioid" peptides, among them cholecystokinin (CCK). Although the focus of much recent work concerned with CCK-opioid interactions has been at the level of the spinal cord, CCK also acts within the brain to modify opioid analgesia. The aim of the present study was to characterize the actions of CCK in a brain region in which the circuitry mediating the analgesic actions of opioids is relatively well understood, the rostral ventromedial medulla (RVM). Single-cell recording was combined with local infusion of CCK in the RVM and systemic administration of morphine in lightly anesthetized rats. The tail-flick reflex was used as a behavioral index of nociceptive responsiveness. Two classes of RVM neurons with distinct responses to opioids have been identified. OFF cells are activated, indirectly, by morphine and mu-opioid agonists, and there is strong evidence that this activation is crucial to opioid antinociception. ON cells, thought to facilitate nociception, are directly inhibited by opioids. Cells of a third class, NEUTRAL cells, do not respond to opioids, and whether they have any role in nociceptive modulation is unknown. CCK microinjected into the RVM by itself had no effect on tail flick latency or the firing of any cell class but significantly attenuated opioid activation of OFF cells and inhibition of the tail flick. Opioid suppression of ON-cell firing was not significantly altered by CCK. Thus CCK acting within the RVM attenuates the analgesic effect of systemically administered morphine by preventing activation of the putative pain inhibiting output neurons of the RVM, the OFF cells. CCK thus differs from another antiopioid peptide, orphanin FQ/nociceptin, which interferes with opioid analgesia by potently suppressing all OFF-cell firing.

Mice deficient for delta- and mu-opioid receptors exhibit opposing alterations of emotional responses

The role of the opioid system in controlling pain, reward and addiction is well established, but its role in regulating other emotional responses is poorly documented in pharmacology. The mu-, delta- and kappa- opioid receptors (encoded by Oprm, Oprd1 and Oprk1, respectively) mediate the biological activity of opioids. We have generated Oprd1-deficient mice and compared the behavioural responses of mice lacking Oprd1, Oprm (ref. 6) and Oprk1 (ref. 7) in several models of anxiety and depression. Our data show no detectable phenotype in Oprk1-/- mutants, suggesting that kappa-receptors do not have a role in this aspect of opioid function; opposing phenotypes in Oprm-/- and Oprd1-/- mutants which contrasts with the classical notion of similar activities of mu- and delta-receptors; and consistent anxiogenic- and depressive-like responses in Oprd1-/- mice, indicating that delta-receptor activity contributes to improvement of mood states. We conclude that the Oprd1-encoded receptor, which has been proposed to be a promising target for the clinical management of pain, should also be considered in the treatment of drug addiction and other mood-related disorders.

Amygdaloid-thalamic interactions mediate the antinociceptive action of morphine microinjected into the periaqueductal gray

The bilateral administration of the serotonin receptor antagonist methysergide (2.5 microg, 5 microg, and 10 microg) into either the central nucleus of the amygdala (ACe) or nucleus parafascicularis thalami (nPf) produced dose-dependent inhibition of the antinociceptive action of ventrolateral periaqueductal gray (vPAG)-administered morphine. Unilateral administration of these doses of methysergide into either the ACe or nPf had no effect on morphine-induced antinociception. However, the combined unilateral administration of these doses of methysergide into the ACe and nPf produced dose-dependent inhibition of morphine antinociception that was identical to that observed after its bilateral administration into either site. This latter finding is interpreted as evidence that a functional interaction between the ACe and nPf supports the antinociceptive action of morphine administered into the vPAG.

Immunoprecipitation of high-affinity, guanine nucleotide-sensitive, solubilized mu-opioid receptors from rat brain: coimmunoprecipitation of the G proteins G(alpha o), G(alpha i1), and G(alpha i3)

Antibodies directed against the C-terminal and the N-terminal regions of the mu-opioid receptor were generated to identify the G proteins that coimmunoprecipitate with the mu receptor. Two fusion proteins were constructed: One contained the 50 C-terminal amino acids of the mu receptor, and the other contained 61 amino acids near the N terminus of the receptor. Antisera directed against both fusion proteins were capable of immunoprecipitating approximately 70% of solubilized rat brain mu receptors as determined by [3H][D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin ([3H]DAMGO) saturation binding. The material immunoprecipitated with both of the antisera was recognized as a broad band with a molecular mass between 60 and 75 kDa when screened in a western blot. Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) had an EC50 of 0.4 nM in diminishing [3H]DAMGO binding to the immunoprecipitated pellet. The ratio of G proteins to mu receptors in the immunoprecipitated material was 1:1. When the material immunoprecipitated with affinity-purified antibody was screened for the presence of G protein a subunits, it was determined that G(alpha)o, G(alpha)i1, G(alpha)i3, and to a lesser extent G(alpha)i2, but not G(alpha)s or G(alpha)q11, were coimmunoprecipitated with the mu receptor. Inclusion of GTPgammaS during the immunoprecipitation process abolished the coimmunoprecipitation of G proteins.

Effect of neonatal handling on brain enkephalin-degrading peptidase activities

Neonatal handling decreases neutral endopeptidase 24.11 activity in the amygdala. However, this procedure does not affect aminopeptidase activities in any of the brain areas studied. Neonatal handling has been one of the most commonly used strategies to study the plasticity of the nervous system. The crucial role of the opioids in the control of different aspects of behaviour and development has been well documented. Regarding this subject, the endogenous opioid system might mediate some of the effects induced by neonatal handling. In this work, we have studied the effects of neonatal handling on several enkephalin-degrading peptidases, including soluble and membrane-bound aminopeptidases (puromycin-sensitive and -insensitive) and neutral endopeptidase 24.11 in different rat brain areas. Tyrosine aminopeptidase activities were measured fluorimetrically using tyrosine-beta-naphthylamide as substrate and puromycin as selective inhibitor of one of the membrane-enzymes. Dansyl-D-Ala-Gly-Phe(pNO2)-Gly was the fluorogenic substrate for neutral endopeptidase. The reduced neutral endopeptidase 24.11 activity in the amygdala of neonatal handled rats could reduce enkephalin catabolism in this area and it could be responsible for some of the effects induced by neonatal handling.

Chronic naltrindole administration does not modify the inhibitory effect of morphine on vocalization responses in the tail electric stimulation test in rats

To address the existence of possible functional interactions between delta- and mu- receptors in relation to the affective component of pain, we have studied the effects of functional blockade of delta-receptors by a chronic treatment with naltrindole (1 mg/kg, 8 consecutive days) on antinociceptive responses to morphine (2 and 5 mg/kg) in the tail electric stimulation test, in adult male rats. The thresholds for the motor response (tail withdrawal), vocalization during stimulus and vocalization afterdischarge were assessed. These responses are considered to be integrated at spinal, medulla oblongata and diencephalon-rhinencephalon levels, respectively. The results show that the vocalization during stimulus and the vocalization afterdischarge were significantly affected by morphine in a dose dependent manner, the latter response being the most sensitive to the effects of the mu-opioid agonist. However, no significant effect was observed on motor responses at the doses used in this study. Chronic naltrindole treatment did not modify the inhibitory effect of morphine on the vocalization responses. Since the vocalization afterdischarge is related to the affective component of pain, the data suggest that the delta-opioid receptor is not involved in the supraspinal mechanisms at which these responses are organized and that there is not a mu-delta interaction in the modulation of the affective responses to noxious electrical stimulation.

Aspects on tail-flick, hot-plate and electrical stimulation tests for morphine antinociception

The objective of this study was to compare the results of three nociceptive tests, tail-flick, hot-plate and electrical stimulation vocalisation, reflecting the responses from different sites in the CNS. A subcutaneous morphine dose (5 mg/kg) was administered to three parallel groups of rats in which the nociceptive response was measured by one of the three methods. The baseline decreased during the period of measurement for the hot-plate test, but remained stable for the other methods. The spinally mediated tail-flick response was more sensitive to the morphine effects as compared to the supraspinally mediated hot-plate and electrical stimulation vocalisation responses. The electrical stimulation vocalisation-test demonstrated more even effect-time profiles and less variability among the rats than did the tail-flick and the hot-plate methods. In the tail-flick group, 59% of the observations attained the cut-off latency at this morphine dose, leading to underestimation of the peak effect, the area under the effect curve (AUEC), and the variability among the rats. In the hot-plate group, 13% of the observations were at the cut-off latency, and 2% in the electrical stimulation vocalisation group. Different ways of presenting the data are discussed. In conclusion, the test selected for measuring the nociceptive response will influence the effect-time profile and subsequently any pharmacodynamic parameters describing it.

Herpes virus-mediated preproenkephalin gene transfer to the amygdala is antinociceptive

To evaluate the role of the amygdala in pain modulation and opioid-mediated antinociception, a recombinant, replication-defective herpes virus carrying the human preproenkephalin cDNA was injected bilaterally into the rat amygdala. Four days after gene delivery nociceptive behavior was assessed by the formalin test. Rats infected with the virus expressing preproenkephalin showed a selective, naloxone-reversible abolition of phase 2 flinching behavior compared to rats infected with a control virus. The results implicate the amygdala in the control of pain and in opioid analgesia and demonstrate the use of recombinant herpes viruses as tools for studying gene function in specific neural pathways of the central nervous system.