beta-Endorphin in vitro inhibition of striatal dopamine release

Roles of β-Endorphin in Stress, Behavior, Neuroinflammation, and Brain Energy Metabolism

β-Endorphins are peptides that exert a wide variety of effects throughout the body. Produced through the cleavage pro-opiomelanocortin (POMC), β-endorphins are the primarily agonist of mu opioid receptors, which can be found throughout the body, brain, and cells of the immune system that regulate a diverse set of systems. As an agonist of the body's opioid receptors, β-endorphins are most noted for their potent analgesic effects, but they also have their involvement in reward-centric and homeostasis-restoring behaviors, among other effects. These effects have implicated the peptide in psychiatric and neurodegenerative disorders, making it a research target of interest. This review briefly summarizes the basics of endorphin function, goes over the behaviors and regulatory pathways it governs, and examines the variability of β-endorphin levels observed between normal and disease/disorder affected individuals.

Baclofen as a Potential Analgesic Agent

It is proposed that analgesia might be achieved by pharmacological interruption of sensory input to the C.N.S. at the level of the spinal cord. On the basis of early reports that baclofen mimicked presynaptic inhibitory mechanisms in the spinal cord, baclofen was tested for potential analgesic activity in the mouse hot-plate test. Baclofen appeared to have some antinocisponsive activity in its own right and it potentiated the antinocisponsive effect of morphine. This observation has been confirmed and extended by several authors and the antinocisponsive activity appears to reside in the 1-isomer. Both spinal and supraspinal sites have been suggested for the locus of action in the C.N.S., whilst the underlying cellular mechanism may be inhibition of neurotransmitter release (via a bicuculline-refractory GABA receptor) or antagonism of the post-synaptic actions of substance P as a neurotransmitter.

Baclofen is compared with two other antinocisponsive agents (morphine and clonidine) in terms of its effects on release of neurotransmitters. Consideration is given to the possibility that a further characteristic is shared by these compounds, namely that following chronic administration, abrupt cessation of therapy might be accompanied by withdrawal or ‘rebound’ effects.

Multidisciplinary assessment and treatment of self-injurious behavior in autism spectrum disorder and intellectual disability: integration of psychological and biological theory and approach

The objective of this review is to consider the psychological (largely behavioral) and biological [neurochemical, medical (including genetic), and pharmacological] theories and approaches that contribute to current thinking about the etiology and treatment of self-injurious behavior (SIB) in individuals with autism spectrum disorder and/or intellectual disability. Algorithms for the assessment and treatment of SIB in this context, respectively, from a multidisciplinary, integrative perspective are proposed and challenges and opportunities that exist in clinical and research settings are discussed.

Dopaminergic amacrine cells express opioid receptors in the mouse retina

The presence of opioid receptors has been confirmed by a variety of techniques in vertebrate retinas including those of mammals; however, in most reports, the location of these receptors has been limited to retinal regions rather than specific cell types. Concurrently, our knowledge of the physiological functions of opioid signaling in the retina is based on only a handful of studies. To date, the best-documented opioid effect is the modulation of retinal dopamine release, which has been shown in a variety of vertebrate species. Nonetheless, it is not known if opioids can affect dopaminergic amacrine cells (DACs) directly, via opioid receptors expressed by DACs. This study, using immunohistochemical methods, sought to determine whether (1) μ- and δ-opioid receptors (MORs and DORs, respectively) are present in the mouse retina, and if present, (2) are they expressed by DACs. We found that MOR and DOR immunolabeling were associated with multiple cell types in the inner retina, suggesting that opioids might influence visual information processing at multiple sites within the mammalian retinal circuitry. Specifically, colabeling studies with the DAC molecular marker anti-tyrosine hydroxylase antibody showed that both MOR and DOR immunolabeling localize to DACs. These findings predict that opioids can affect DACs in the mouse retina directly, via MOR and DOR signaling, and might modulate dopamine release as reported in other mammalian and nonmammalian retinas.

Naloxone-responsive acute dystonia and parkinsonism following general anaesthesia

Various movement disorders such as dystonia may acutely develop during or at emergence from general anaesthesia in patients with or without pre-existing Parkinson disease. These movements are triggered by a variety of drugs including propofol, sevoflurane, anti-emetics, antipsychotics and opioids. The postulated mechanism involves an imbalance between dopaminergic and cholinergic neurotransmitters in the basal ganglia. We report an acute, severe and generalised dystonic reaction in an otherwise healthy woman at emergence from general anaesthesia, dramatically reversed by the administration of naloxone, pointing to a potential role of the fentanyl and morphine that the patient had received. Recent literature on the mechanisms of abnormal movements induced by opioids are discussed. The severity of the reaction with usual doses of opioids, in a patient with no prior history of parkinsonism, led to further investigation that demonstrated the possibility of an enhanced susceptibility to opioids, involving a genetically determined abnormal function of glycoproteine-P and catechol-O-methyltransferase.

Pro-opiomelanocortin colocalizes with corticotropin- releasing factor in axon terminals of the noradrenergic nucleus locus coeruleus

We previously demonstrated that the opioid peptide enkephalin and corticotropin-releasing factor (CRF) are occasionally colocalized in individual axon terminals but more frequently converge on common dendrites in the locus coeruleus (LC). To further examine potential opioid cotransmitters in CRF afferents we investigated the distribution of pro-opiomelanocortin (POMC), the precursor that yields the potent bioactive peptide beta-endorphin, with respect to CRF immunoreactivity using immunofluorescence and immunoelectron microscopic analyses of the LC. Coronal sections were collected through the dorsal pontine tegmentum of rat brain and processed for immunocytochemical detection of POMC and CRF or tyrosine hydroxylase (TH). POMC-immunoreactive processes exhibited a distinct distribution within the LC as compared to the enkephalin family of opioid peptides. Specifically, POMC fibers were enriched in the ventromedial aspect of the LC with fewer fibers present dorsolaterally. Immunofluorescence microscopy showed frequent coexistence of POMC and CRF in varicose processes that overlapped TH-containing somatodendritic processes in the LC. Ultrastructural analysis showed POMC immunoreactivity in unmyelinated axons and axon terminals. Axon terminals containing POMC were filled with numerous large dense-core vesicles. In sections processed for POMC and TH, approximately 29% of POMC-containing axon terminals (n = 405) targeted dendrites that exhibited immunogold-silver labeling for TH. In contrast, sections processed for POMC and CRF showed that 27% of POMC-labeled axon terminals (n = 657) also exhibited CRF immunoreactivity. Taken together, these data indicate that a subset of CRF afferents targeting the LC contain POMC and may be positioned to dually impact LC activity.

Effects of acute "binge" cocaine on preprodynorphin, preproenkephalin, proopiomelanocortin, and corticotropin-releasing hormone receptor mRNA levels in the striatum and hypothalamic-pituitary-adrenal axis of mu-opioid receptor knockout mice

Cocaine administration increases activity at dopamine receptors, increases preprodynorphin (ppDyn) gene expression in the caudate-putamen (CPu), and activates the stress responsive hypothalamic-pituitary-adrenal (HPA) axis. To examine the hypothesis that mu-opioid receptors (MOR) may play roles in these cocaine effects, we tested the effects of acute "binge" pattern cocaine administration in mice with targeted disruption of the MOR gene. Wild-type (+/+) and homozygous MOR-deficient (-/-) mice received three injections of 15 mg/kg cocaine at 1-h intervals. Mice were sacrificed 30 min after the last injection and mRNAs for ppDyn and preproenkephalin (ppEnk) in the CPu and nucleus accumbens (NAc), and for type I corticotropin-releasing hormone receptor (CRH(1) receptor) and pro-opiomelanocortin (POMC) in the hypothalamus and pituitary, were measured by solution hybridization RNase protection assays. Cocaine elevated ppDyn mRNA in the CPu, but not NAc, of both the MOR -/- and wild-type mice. ppEnk mRNA in the CPu, but not NAc, was lower in MOR -/- mice than in wild-type mice following cocaine administration. Hypothalamic CRH(1) receptor and POMC mRNAs were expressed at similar levels in untreated and in cocaine-treated mice of each genotype. However, there were lower basal levels of CRH(1) receptor mRNA in the anterior pituitary of the MOR -/- mice than in wild-type mice and the MOR -/- mice failed to show the cocaine-induced decreases in CRH(1) receptor mRNA found in the wild-type mice. Cocaine activated the HPA axis similarly in MOR -/- and wild-type mice, as reflected in similar increases in plasma corticosterone levels in both genotypes. These results support a specific role for MORs in acute cocaine effects on striatal ppEnk gene expression and fail to support critical roles for these receptors in acute cocaine's effects on either ppDyn gene expression or HPA activation. MOR -/- mice are useful models for studying cocaine effects on ppEnk gene expression that could aid interpretation of the similar postmortem phenomena found in human cocaine addicts.

Involvement of mu-opioid receptors in potentiation of apomorphine-induced climbing behavior by morphine: studies using mu-opioid receptor gene knockout mice

The present study examined the hypothesis that mu-opioid receptors contribute to a behavioral stimulation produced by stimulation of dopamine receptors by comparing responses in mu-opioid receptor knockout and wild type mice. Apomorphine-induced climbing behavior was augmented by 65%, in wild type mice, but not in mu-knockout, following subcutaneous administration of morphine (15 mg/kg). Moreover, pretreatment with either naloxone (an opioid receptor antagonist) or haloperidol (a mixed D(1)/D(2) receptor antagonist) eliminated the enhancement by morphine of climbing behavior in wild type mice. These results indicate that expression of mu-opioid receptors plays an important role in the enhancement of climbing behavior induced by the dopamine receptor agonist, apomorphine. Furthermore, this augmentation is mediated by interaction between dopamine and mu-opioid receptors.

Comparison of the effect of levodopa and bromocriptine on naloxone-precipitated morphine withdrawal symptoms in mice

In this study, the effect of l-dopa and bromocriptine on morphine withdrawal syndrome was compared. Both l-dopa (125, 250 mg/kg, i.p.) and low doses of bromocriptine (0.04, 0.08 mg/kg, i.p.) potentiated naloxone-induced morphine withdrawal symptoms such as jumping, climbing and rearing in mice. Higher doses of bromocriptine (0.16, 0.32 mg/kg, i.p.) attenuated these naloxone-induced symptoms. SKF 83566, D(1) dopamine antagonist (0.4, 0.8 mg/kg, i.p.) and sulpiride, D(2) dopamine antagonist (5, 10 mg/kg, i.p.) when used alone, also produced inhibitory effects on naloxone-induced morphine withdrawal symptoms. Pretreatment with sulpiride (5, 10 mg/kg, i.p.) and SKF 83566 (0.4, 0.8 mg/kg, i.p.) attenuated the potentiating effects of l-dopa on withdrawal symptoms significantly. Pretreatment with sulpiride also decreased the potentiating effect of bromocriptine and reinforced the inhibitory action of it, but SKF 83566 pretreatment just reinforced the effect of higher doses of bromocriptine. Concurrent pretreatment of animals with sulpiride (10 mg/kg, i.p.) and SKF 83566 (0.8 mg/kg, i.p.) markedly decreased the potentiating effects of l-dopa and bromocriptine and reinforced the inhibitory action of bromocriptine on the naloxone-induced morphine withdrawal syndrome. Prazosin, alpha(1) antagonist (1, 2 mg/kg, i.p.) decreased the naloxone-induced morphine withdrawal syndrome significantly. Pretreatment with yohimbine, alpha(2)-antagonist (5 mg/kg, i.p.) reversed the inhibitory effects of bromocriptine (0.16, 0.32 mg/kg, i.p.) on naloxone-induced morphine withdrawal syndrome significantly. In conclusion, our results show that bromocriptine at lower doses (0.04, 0.08 mg/kg, i.p.) acts similar to l-dopa, but at higher doses (0.16, 0.32 mg/kg, i.p.) shows different effects on naloxone-induced morphine withdrawal syndrome which may be due to the interaction of bromocriptine with alpha-adrenoceptors. Copyright 2000 John Wiley & Sons, Ltd.

Stimulatory effect of enkephalins on calcium efflux from bovine adrenal chromaffin cells in culture

The effects of leucine- and methionine-enkephalin, opiate peptides, on Ca2+ efflux from cultured bovine adrenal chromaffin cells were examined. These enkephalins stimulated the efflux of 45Ca2+ from cells in a concentration-dependent manner (10(-8) M-10(-6) M). Leucine-enkephalin did not increase the intracellular free Ca2+ level, 45Ca2+ uptake, catecholamine secretion, cAMP level or cGMP level. The peptide-stimulated 45Ca2+ efflux was not inhibited by incubation in Ca2+-free medium, but was inhibited by incubation in Na+-free medium. These results indicate that enkephalins stimulate extracellular Na+-dependent 45Ca2+ efflux from cultured bovine adrenal chromaffin cells, probably by stimulating membrane Na+/Ca2+ exchange.

Pharmacological characterization of morphine-induced in vivo release of cholecystokinin in rat dorsal horn: effects of ion channel blockers

Previous studies indicate that an increased release of cholecystokinin (CCK) in response to morphine administration may counteract opioid-induced analgesia at the spinal level. In the present study we used in vivo microdialysis to demonstrate that systemic administration of antinociceptive doses of morphine (1-5 mg/kg, s.c.) induces a dose-dependent and naloxone-reversible release of CCK-like immunoreactivity (CCK-LI) in the dorsal horn of the spinal cord. A similar response could also be observed following perfusion of the dialysis probe for 60 min with 100 microM but not with 1 microM morphine. The CCK-LI release induced by morphine (5 mg/kg, s.c.) was found to be calcium-dependent and tetrodotoxin-sensitive (1 microM in the perfusion medium). Topical application of either the L-type calcium channel blocker verapamil (50 microg) or the N-type calcium channel blocker omega-conotoxin GVIA (0.4 microg) onto the dorsal spinal cord completely prevented the CCK-LI release induced by morphine (5 mg/kg, s.c.). Our data indicate that activation of L- and N-type calcium channels is of importance for morphine-induced CCK release, even though the precise site of action of morphine in the dorsal horn remains unclear. The present findings also suggest a mechanism for the potentiation of opioid analgesia by L- and N-type calcium channel blocking agents.

Characterization of the decrease of extracellular striatal dopamine induced by intrastriatal morphine administration

The effect of intrastriatally-administered morphine on striatal dopamine (DA) release was studied in freely moving rats. Morphine (1, 10 or 100 microM) was given into the striatum by reversed microdialysis, and concentrations of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were simultaneously measured from the striatal dialysates. Intrastriatally-administered morphine significantly and dose-dependently decreased the extracellular concentration of DA, the concentrations of the acidic DA metabolites were only slightly decreased. The effect of morphine was antagonized by naltrexone (2.25 mg kg(-1), s.c.). Pretreatment with a preferential kappa-opioid receptor antagonist, MR2266 [(-)-5,9 alpha-diethyl-2-(3-furylmethyl)-2'-hydroxy-6,7-benzomorphane; 1 mg kg(-1), s.c.], had no effect on the decrease of extracellular DA evoked by intrastriatal morphine (100 microM). Intrastriatal administration of the selective micro-opioid receptor agonist [D-Ala2,MePhe4,Gly-ol5] enkephalin (DAMGO; 1 microM), significantly decreased the extracellular concentration of DA in the striatum. When the rats were given morphine repeatedly in increasing doses (10-25 mg kg(-1), s.c.) twice daily for 7 days and withdrawn for 48 h, the decrease of extracellular DA induced by morphine (100 microM) was significantly less than that seen in saline-treated controls. Our results show that besides the well-known stimulatory effect there is a local inhibitory component in the action of morphine on striatal DA release in the terminal regions of nigrostriatal DA neurones. Tolerance develops to this inhibitory effect during repeated morphine treatment. Furthermore, our results suggest that the effect of intrastriatally-administered morphine is mediated by the micro-opioid receptors.

Stimulatory effects of opioids on transmitter release and possible cellular mechanisms: overview and original results

Opiates and opioid peptides carry out their regulatory effects mainly by inhibiting neuronal activity. At the cellular level, opioids block voltage-dependent calcium channels, activate potassium channels and inhibit adenylate cyclase, thus reducing neurotransmitter release. An increasing body of evidence indicates an additional opposite, stimulatory activity of opioids. The present review summarizes the potentiating effects of opioids on transmitter release and the possible cellular events underlying this potentiation: elevation of cytosolic calcium level (by either activating Ca2+ influx or mobilizing intracellular stores), blockage of K+ channels and stimulation of adenylate cyclase. Biochemical, pharmacological and molecular biology studies suggest several molecular mechanisms of the bimodal activity of opioids, including the coupling of opioid receptors to various GTP-binding proteins, the involvement of different subunits of these proteins, and the activation of several intracellular signal transduction pathways. Among the many experimental preparations used to study the bimodal opioid activity, the SK-N-SH neuroblastoma cell line is presented here as a suitable model for studying the complete chain of events leading from binding to receptors down to regulation of transmitter release, and for elucidating the molecular mechanism involved in the stimulatory effects of opioid agonists.

Biochemical and pharmacological studies on a lethal neurotoxic polypeptide from Phoneutria nigriventer spider venom

Fractionation of Phoneutria nigriventer spider venom by gel filtration and HPLC yielded a few fractions that induced different effects when administered intraperitoneally in mice. One of these fractions, PF3, was chemically characterized as a cysteine-rich polypeptide of approximately 8360 MW. Administered at 0.1 mg/kg, i.p., PF3 induced a progressive paralysis and death of mice within 30 minutes. Partial sequence analysis of PF3 revealed certain homologies with other spider toxins already described, particularly omega-AGAIIA (60%) from Agelenopsis aperta. Pharmacological characterization carried out in superfused chopped rat striatal tissues preloaded with [3H]-Dopamine ([3H]-DA) showed that PF3 (0.1 microgram/ml) decreased the [3H]-DA release induced by 20 mM K+ or 100 microM glutamate without changing the basal release. At 1 microgram/ml, PF3 inhibited 33% of the basal release of [3H]-DA; the transmitter release stimulated by K+ or by glutamate was reduced by respectively, 87% and 77% of corresponding control values. PF3 (0.1 micrograms/ml) altered the dose-response curves of glutamate (1 microM-10 mM), by reducing by 36% of its maximal effect. Naloxone (1 microM) did not influence the effect of PF3. The results indicate that PF3 inhibits the [3H]-DA release induced by membrane depolarization or that mediated by NMDA glutamate receptors. These data suggest that the mechanism of action of PF3 may involve a blockade of Ca2+ channels as well as a direct effect on the exocytotic machinery.

Effects of naloxone on the behaviors evoked by amphetamine and apomorphine in adult cats

1. This work was undertaken in order to study whether the opioid system is involved in the modulation of the behaviors induced by two agonists of the dopaminergic system, amphetamine and apomorphine in adult cats. 2. Naloxone, an antagonist of the mu, delta and kappa opioid receptors was administered to twelve female mongrel cats; 0.5, 1.0 and 2.0 mg/kg s.c. were injected in order to analyse its own effect of naloxone. This drug produced NREMs behavior and accordingly the cat showed an overall decrease of its activities. 3. Amphetamine (2.5 mg/kg s.c.) and apomorphine (2.0 mg/kg s.c.) were injected before and after naloxone administration (2.0 mg/kg s.c.), in separate sessions. 4. The behaviors recorded were compared. Some of the behaviors showed modifications both with amphetamine (inappetence was increased and locomotion decreased) and apomorphine (indifference and inappetence increased; locomotion and olfaction decreased). 5. These changes were considered as consequence of the NREMs behavior induced by naloxone and not as a result of a modulation by the opioid system of the activation of the dopaminergic system elicited by amphetamine and apomorphine. Regarding the mechanism of NREMs induced by naloxone probably the dopaminergic, noradrenergic and GABAergic systems may be involved.

Androgenic-anabolic steroids modify beta-endorphin immunoreactivity in the rat brain

Immunocytochemical localization of beta-endorphin in the brains of intact and castrated male rats was conducted after the administration of high levels of androgenic-anabolic steroids (AAS; 14 daily injections of sesame oil or a cocktail of 2 mg/kg testosterone cypionate, 2 mg/kg nandrolone decanoate, and 1 mg/kg boldenone undecylenate) at doses commonly self-administered by athletes who are considered 'heavy abusers'. In normal intact oil-treated males, cytoplasmic immunoreactivity was prevalent throughout the arcuate nucleus while intense fiber tract immunoreactivity was most prevalent in the bed nucleus of the stria terminalis and the paraventricular hypothalamic nucleus. Administration of AAS significantly decreased the number of neurons exhibiting cytoplasmic immunoreactivity only in the rostral region of the arcuate nucleus. AAS treatment had no effect on beta-endorphin immunoreactivity in the middle or caudal aspects of the arcuate nucleus.

The effects of morphine on memory consolidation in mice involve both D1 and D2 dopamine receptors

Post-training administration of morphine (0.25, 0.5, or 1 mg/kg) dose-dependently impairs retention of an inhibitory avoidance response in mice. The effects on retention performance induced by the drug appear to be due to an effect on memory consolidation. In fact, they were observed when drugs were given at short, but not long, periods of time after training, i.e., when the memory trace was susceptible to modulation. Moreover, these effects are not to be ascribed to an aversive or a rewarding or nonspecific action of the drugs on retention performance, because the latencies during the retention test of those mice that had not received a footshock during the training were not affected by post-training drug administration. Pretreatment with either selective D1 or D2 dopamine (DA) receptor antagonists SCH 23390 and (-)-sulpiride administered at per se noneffective doses (0.025 and 6 mg/kg, respectively) potentiated the effects of morphine, while either selective D1 or D2 receptor agonists SKF 38393 and LY 171555 at per se noneffective doses (5 and 0.25 mg/kg, respectively) antagonized the effects of the opiate on memory consolidation. No significant differences were evident between the effects of D1 and D2 receptor active compounds, thus suggesting that D1 and D2 receptor types are similarly involved in the effects of morphine on memory consolidation, in agreement with previously reported results. These results are discussed in terms of a possible inverse relationship of endogenous opioid and DA systems in the brain that are involved in memory processes.

Effect of selective opiate antagonists on striatal acetylcholine and dopamine release

We investigated the effect of selective opiate antagonists on striatal acetylcholine (ACh) and dopamine (DA) release. The mu-receptor antagonist beta-funaltrexamine (beta-FNA), the delta-antagonist naltrindole (NTI), and the kappa-antagonist norbinaltorphimine (nor-BNI) were used to selectively block different subtypes of opiate receptors. The experiments were carried out on isolated superfused striatal slices of rats, loaded with [3H]choline or [3H]dopamine. beta-FNA and NTI significantly enhanced the electrical field stimulation-evoked release of ACh but only if the dopaminergic input had been impaired either by chemical denervation or D2 dopamine receptor blockade. By contrast, neither the selective nor nonselective antagonists had any modulatory effect on the release of dopamine. It is concluded, therefore, that the release of ACh is tonically controlled by endogenous opioid peptide(s) through the stimulation of mu- and delta-opiate receptors located on cholinergic axon terminals, in addition to the tonic control by DA.

Ethanol-induced increase in endogenous dopamine release may involve endogenous opiates

The effect of opiate peptides on basal and potassium-stimulated endogenous dopamine (DA) release from striatal slices was studied in vitro. Dual stimulation of the striatal slices gave a reproducible increase in DA release that was calcium dependent. Addition of the delta-opiate receptor agonists Met5-enkephalin, [D-Ala2,D-Leu5]enkephalin (DADLE), and [D-Ser2]Leu-enkephalin-Thr (DSLET), increased the basal DA release without affecting potassium-stimulated release in a dose-dependent manner. The effect of DADLE was antagonized by the addition of naloxone. In contrast, the mu-opioid receptor agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAGO) and the epsilon-opioid agonist beta-endorphin inhibited the stimulated DA release without changing the basal release. The inhibitory effect of DAGO on potassium-stimulated release was antagonized by naloxone. The addition of ethanol (75 mM) to the incubation media produced a delayed increase of both the basal and stimulated DA release. There was no change in stimulated DA release when the change in basal release was subtracted, suggesting that ethanol produced a dose-dependent, selective increase in basal DA release. Naloxone and the selective delta-opiate antagonist ICI 174864 inhibited the ethanol-induced increase in basal DA release. Naloxone and ICI 174864 added alone did not alter either basal or stimulated DA release. We therefore suggest that the ethanol-induced increase in basal DA release is an indirect effect involving an endogenous delta-opiate agonist.

Chronic treatment with MIF-1 prevents the painful stimuli threshold elevation induced by neonatal handling in mice

Chronic postnatal stressful handling results in a hyposensitivity to thermal nociceptive stimuli. This phenomenon is strongly affected by manipulations of the opioid system. In the present experiment, we report that chronic treatment with MIF-1 during the neonatal period prevents the behavioral alterations induced by handling while it is completely ineffective if injected acutely before antinociceptive testing by the tail flick test at 45 days of life.

Naloxone enhances the release of acetylcholine from cholinergic interneurons of the striatum if the dopaminergic input is impaired

Naloxone significantly enhanced the release of radioactive acetylcholine ([3H]ACh) from rat striatal slices loaded with [3H]choline either when the nigrostriatal pathway had been destroyed by 6-hydroxydopamine or when the D2 dopamine receptors had been inhibited by sulpiride. This in vitro study supplies the first neurochemical evidence, that, in addition to D2-receptor-mediated dopaminergic tonic control, there is opiate-receptor mediated presynaptic modulation of striatal ACh release, possibly by endogenous enkephalin released from local neurons. Such modulation occurs under conditions in which the dopaminergic input is impaired.

Alterations in opiate receptor binding in MPTP-induced hemiparkinsonian monkeys

Quantitative autoradiography was used to study [3H]naloxone binding in the striatum of normal monkeys and monkeys made hemiparkinsonian by the unilateral infusion of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The density of [3H]naloxone binding sites was significantly higher in the caudate and putamen on the MPTP-treated side of hemiparkinsonian monkeys, as compared with binding on the untreated side and in the striatum of normal monkeys. A more extensive patchy distribution of binding sites was evident throughout the striatum on the MPTP-treated side than seen in the striatum of the untreated side or in normal striatum.

Focal application of alcohols elevates extracellular dopamine in rat brain: a microdialysis study

Dopaminergic systems are thought to play a major role in the stimulant and reinforcing properties of drugs of abuse, including ethanol. The present study describes the effects of local perfusion with ethanol (and other alcohols) on extracellular dopamine in the striatum and nucleus accumbens. Following the establishment of basal dopamine levels (2-3 h), various concentrations of ethanol in artificial CSF (0.01-10% v/v) were slowly perfused through a microdialysis probe. Each dose of ethanol was found to increase dopamine concentrations in both the striatum and nucleus accumbens. This increase was dose-related in the striatum. The exclusion of calcium and inclusion of 12.5 mM magnesium in the perfusion medium prevented, or greatly attenuated the ethanol-induced dopamine (DA) release. Thus, the release of DA by ethanol is exocytotic in nature and involves calcium-dependent processes. The other alcohols tested, namely methanol and butanol, demonstrated a structure-activity relationship together with ethanol, in their ability to increase extracellular DA. The relative potency was butanol greater than ethanol greater than methanol. The diffusion of ethanol into the brain tissue was investigated following perfusion through the probe. Relatively low concentrations of ethanol were found in striatal tissue during perfusion and they declined rapidly with time, following the removal of ethanol from the perfusate. The concentrations of ethanol achieved in brain tissue following focal application through the microdialysis probe were relevant to human intoxication.

Chronic treatment with chlorpromazine, thioridazine or haloperidol increases striatal enkephalins and their release from rat brain

The aim of this paper is to study the effect of 1, 2 or 3 months' administration of chlorpromazine (CPZ), thioridazine (TDZ) (2 or 6 mg/kg) or haloperidol (HAL) (0.25 or 1 mg/kg) IP on the level of leu- and met-enkephalin (ENK) in striatum. A dose- and time-dependent increase of striatal ENK level was observed after chronic administration of the neuroleptics (NL), but 8 days after withdrawal of chronically administered NL striatal ENK was decreased. Apomorphine pretreatment significantly attenuated the elevation in ENK produced by chronic injections of NL. In perfusion fluid obtained from the lateral ventricle of animals treated 1 month with HAL a dose-dependent increase of ENK levels was observed, which was augmented by potassium ions. It is concluded that: 1) Chronic administration of neuroleptic drugs that block dopamine receptors increases the level and the release of striatal enkephalins; 2) The results support the hypothesis that activation of dopaminergic neurons tonically inhibits the synthesis of enkephalins in the striatum.

Coexistence of varying combinations of neuropeptides with 5-hydroxytryptamine in neurons of the raphe pallidus et obscurus projecting to the spinal cord

The coexistence of varying combinations of substance P (SP), somatostatin (SOM), thyrotropin-releasing hormone (TRH) and met-enkephalin-Arg-Gly-Leu (ENK) with 5-hydroxytryptamine (5-HT) as semiquantitatively revealed by immunocytochemistry in neuronal perikarya of the raphe pallidus et obscurus in the guinea-pig was analyzed. SOM coexisted most frequently with 5-HT, followed by SP, ENK and TRH. Many 5-HT neurons were immunoreactive to 2 or more peptides such as SP/SOM, SOM/ENK, SP/ENK, SOM/TRH, SP/TRH or SOM/SP/ENK. Most of these neurons were shown to project to the spinal cord by retrograde HRP labeling combined with immunocytochemistry. After hemisection of the cervical spinal cord at the C5 level, ENK and 5-HT immunoreactive nerve terminals in the ipsilateral intermediolateral nucleus of the thoracic spinal cord were decreased in number. The results indicate that neurons in the raphe pallidus et obscurus projecting to the spinal cord can be classified into subpopulations according to which peptides coexist with 5-HT, and may have different functions.

Fentanyl increases catecholamine oxidation current measured by in vivo voltammetry in the rat striatum

A proposed mechanism for fentanyl-induced muscular rigidity is the effect of opioids on dopaminergic transmission in the striatum. The objective of this study was to observe the effect of fentanyl on the rat striatal catechol oxidation current (CA.OC) which reflects extracellular DOPAC (3-4,dihydroxyphenylacetic acid) concentration (a major metabolite of dopamine), as measured by in vivo voltammetry. Male Sprague-Dawley rats, anaesthetized with halothane, were stereotaxically implanted with carbon fibre electrodes in the striatum and after an initial stabilization period of an hour were given a control saline IV injection followed 30 min later by fentanyl 10 micrograms.kg-1 IV over 10 min and at 70 min by the monoamine oxidase inhibitor pargyline 70 mg.kg-1 IP. Fentanyl produced a significant (P less than 0.05 Anova) increase in CA.OC in all animals. This reached a plateau 15 min following the administration of fentanyl and was at a maximum of 148 +/- 10.2 per cent of control 35 min after the administration of fentanyl. Pargyline produced a rapid decline in CA.OC peak height which went from 143 +/- 11.6 to 39 +/- 6.8 per cent of control over 30 min. There were no significant differences between the pH, PaO2 and PaCO2 during the saline and fentanyl injection periods and there was no significant variation of blood pressure throughout the experiment. This study shows that under stable physiological conditions, fentanyl produces a significant increase in CA.OC in the rat striatum.

Stress-induced decrease of 3-methoxytyramine in the nucleus accumbens of the mouse is prevented by naltrexone pretreatment

Pretreatment with naltrexone (2.5 and 5 mg/kg) prevented the decrease of 3-methoxytyramine (3-MT)/dopamine (DA) ratio induced by 2 h immobilization stress in the nucleus accumbens (NAS) of the mouse while it did not affect the stress-induced decrease of 3-MT/DA ratio in caudatus putamen (CP). Naltrexone also produced a slight antagonism of homovanillic acid (HVA)/DA ratio increase produced by stress in the frontal cortex (FC). These results point to an involvement of endogenous opioids in the effects of stress on DA metabolism in the mesolimbic system of the mouse.

Chronic treatment with naltrexone enhances morphine-stimulated dopamine neurotransmission: neurochemical and behavioral evidence

Rats were implanted for 10 days with a slow-release pellet of naltrexone or were given sham surgery. At one of various different intervals during or after implantation of the pellet, the synthesis of dopamine (DA) was assessed in the nigrostriatal and mesolimbic systems. The results indicated that naltrexone alone was without effect on the synthesis of DA. However, one day after removal of the pellet, naltrexone-treated animals displayed an enhanced response to the DA-stimulatory action of morphine (15 mg/kg) in both the nigrostriatal and mesolimbic systems. This change was accompanied by an increase in specific binding of the mu-specific radioligand [3H]DAGO in whole brain and by an increase in the depressant action of morphine on locomotor activity. In contrast, at 10 days after removal of the pellet, naltrexone was without effect on morphine-induced changes in the synthesis of DA and locomotor activity, thus indicating that the supersensitivity to morphine was transient. These results support the idea that opioids modulate DAergic neurotransmission in the nigrostriatal and mesolimbic pathways and that this modulatory role may underlie opiate-induced changes in locomotor behavior.

High-dose naloxone in tardive dyskinesia

Tardive dyskinesia (TD) is thought to result from nigrostriatal dopaminergic supersensitivity secondary to prolonged neuroleptic exposure. Preclinical studies have demonstrated that the opiate antagonist naloxone can acutely reverse a haloperidol-induced hyperdopaminergic state. In a trial of high-dose naloxone, 20 patients with TD received i.v. naloxone (20 mg, 40 mg, and placebo) under double-blind conditions. At baseline and at regular postdrug intervals, patients were evaluated using a battery of motor, clinical, and neuropsychological measures to study effects on neurological, behavioral, and cognitive functions. There was a significant improvement in involuntary movements at 30 min postnaloxone, together with improvement in clinical ratings at that time point, as well as some cognitive changes. The implications of these findings for the putative functional relationship between dopaminergic and enkephalinergic systems in the nigrostriatal area are discussed.

Opioid influence on some aspects of stereotyped behavior induced by repeated amphetamine treatment

Rats were administered repeated IP injections of dl-amphetamine (AMPH) according to a chronic escalating dose schedule (three doses per 24 hr, for four days, two days or one day). Animals treated for four days exhibited a diminished oral stereotypy in response to a challenge of 12 mg/kg AMPH or 2 mg/kg SC apomorphine (APO), 72 hr after withdrawal. Pretreatment with 2 mg/kg IP naloxone (NAL) during the period of chronic AMPH administration prevented the reduction in oral stereotypy induced by AMPH or APO. No differences were detected among the mean of stereotypy scores from the different treatments in response to a challenge dose of 6 mg/kg AMPH. Neurochemical data showed that NAL pretreatment reversed the depletion of striatal dopamine content induced by chronic AMPH. When repeated injections of AMPH were given only one day, the diminished stereotypy response to AMPH or APO was not observed. Animals treated simultaneously with 1 mg/kg IP morphine or 5 micrograms/kg IP beta-endorphin and repeated AMPH injections for one day, showed a reduced stereotyped response to AMPH or APO. These results suggest that opioid peptides are involved in the mechanisms underlying the decrease in oral behaviors following AMPH treatment.

Interaction between enkephalin and dopamine in the avian retina

Biochemical and pharmacological techniques were utilized to investigate the interaction between the enkephalinergic and dopaminergic systems in the chicken retina. Exogenously applied enkephalin and its analogues were observed to inhibit the release of preloaded dopamine from the retina. This inhibition was concentration-dependent and was suppressed by the opiate antagonist, naloxone. The relationship between enkephalinergic and dopaminergic amacrine cells was studied in retinas which were subjected to 6-hydroxydopamine (6-OHDA) treatments. 6-OHDA degenerated approximately 80-90% of those cells which exhibit high affinity uptake of [3H]dopamine. In 6-OHDA-treated retinas, the capacity of 3H-labelled [D-Ala2]methionine enkephalinamide to bind specifically to opiate receptors was substantially reduced (only 70-75% of the control). Scatchard analyses and ligand displacement studies indicated that this decrease in binding was due to a reduction in the number of opiate receptors. Taken together, these observations strongly indicate that a fraction of the opiate receptors in the chicken retina (25-30%) are closely associated with the population of dopaminergic amacrine cells.

Glutamate-evoked release of endogenous brain dopamine: inhibition by an excitatory amino acid antagonist and an enkephalin analogue

The present study examined the effect of a selective delta-opioid receptor agonist [D-Ala2-D-Leu5] enkephalin (DADL) on the spontaneous and the L-glutamic acid (L-Glu)-evoked release of endogenous dopamine from superfused slices of rat caudate-putamen. The amount of dopamine in slice superfusates was measured by a sensitive method employing high-performance liquid chromatography with electrochemical detection (h.p.l.c.-e.d.) after a two-step separation procedure. The spontaneous release of endogenous dopamine was partially dependent on Ca2+, enhanced in Mg2+-free superfusion medium, partially reduced by tetrodotoxin (TTX, 0.3 microM), partially reduced by the putative excitatory amino acid receptor antagonist DL-2-amino-7-phosphonoheptanoic acid (DL-APH, 1 mM), and increased 10 fold by the dopamine uptake blocker, nomifensine (10 microM). DADL (5 and 50 nM) did not significantly affect spontaneous dopamine release. L-Glu (0.1-10 mM) produced a concentration-dependent release of endogenous dopamine from slices of caudate-putamen. This effect was Ca2+-dependent, strongly inhibited by 1.2 mM Mg2+, attenuated by DL-APH (1 mM), attenuated by TTX (0.3 microM), and enhanced by nomifensine (10 microM). In the presence of nomifensine DADL (50 nM) reduced significantly the L-Glu-evoked release of endogenous dopamine by 20%. The inhibitory effect of DADL was blocked by 10 microM naloxone. These results indicate that L-Glu stimulates the Ca2+-dependent release of endogenous dopamine in the caudate-putamen by activation of N-methy-D-aspartate-type of excitatory amino acid receptors. This release can be selectively modified by the delta-opioid agonist DADL in a naloxone-sensitive manner.

A functional separation of behavioral stereotypy based on naloxone-reversible effects of seryl enkephalinamide: comparison with morphine

Amphetamine's stereotypic behavioral actions, produced by the stimulant at a moderate dose, were inhibited by the systemic administration of seryl enkephalinamide, D-Ser2-D-Ser5-enkephalinamide, (Wy 42,896). The classical sequelae of stimulatory behavioral events: sniffing, head bobbing, rearing and locomotor activity, were significantly inhibited by the seryl enkephalinamide. Subsequently, pretreatment with the opiate receptor antagonist, naloxone, significantly blocked the inhibitory effects of the seryl enkephalinamide on the stereotypic and locomotor components. Concomitantly, the behavioral stereotypic component, licking, a behavior usually produced by opiates and only high doses of amphetamine, was significantly induced by the seryl enkephalinamide. Pretreatment with naloxone on the stimulatory behavioral effect of licking, produced a significant inhibitory effect. The combination treatment, consisting of both the seryl enkephalinamide and the stimulant amphetamine, caused a naloxone-reversible synergistic effect. These data show that the seryl enkephalinamide, produced concomitant, naloxone-reversible, inhibitory and stimulatory behavioral stereotypic effects. These data are discussed within the context of current neuronal theories which might underly the observed dose-related continuum of behavioral stereotypies produced by morphine and amphetamine.

Further evidence that a single dose of an opiate can increase dopamine receptor sensitivity in mice

Previous findings in our laboratory indicated that a single administration of morphine or levorphanol to mice could induce the development of supersensitive dopamine receptors. To further study this phenomenon, the ability of haloperidol to inhibit dopamine agonist-induced climbing was determined in mice 3 h following morphine (10 mg/kg i.p.) or levorphanol (2.0 mg/kg i.p.) pretreatment. The dose of haloperidol required to inhibit climbing behavior induced by 2.4 mg/kg (i.p.) of apomorphine or (-) N-n-propylnorapomorphine was increased significantly in the opiate-pretreated mice. Morphine pretreatment did not significantly affect the apparent pA2 of apomorphinehaloperidol for climbing behavior suggesting that the affinity of the dopamine receptor was not altered. This observation was supported by a lack of difference in the Kd of haloperidol binding sites between saline- and morphine-pretreated mice. There was, however, a significant increase in the Bmax in the morphine-pretreated animals. This increase was blocked when 5 mEq/kg of lithium (i.p.) or 5 mg/kg naloxone (i.p., administered twice) was administered concurrently with the morphine. Concurrent lithium or naloxone administration also attenuated the morphine-induced increase in dose of haloperidol required to inhibit dopamine agonist-induced climbing behavior. These results suggest that a single administration of an opiate can cause the development of dopamine receptor supersensitivity which is due to an increase in dopamine receptor density.

Neuropeptide modulation of apomorphine-induced stereotyped behavior

The nucleus accumbens contains many neuropeptides whose functions are presently unknown. The purpose of this study was to determine the extent to which these neuropeptides act in conjunction with the mesolimbic dopamine system. Microinjections of cholecystokinin, neurotensin, met-enkephalin, somatostatin, bombesin, as well as glutamate and muscimol, were made into the medial nucleus accumbens after systemic injection of apomorphine. Cholecystokinin and neurotensin, in nanogram doses, potentiated apomorphine-induced stereotypy. Met-enkephalin reduced, while somatostatin and bombesin were without effect on, apomorphine-induced stereotypy. In addition, both glutamate and muscimol potentiated this effect. These results suggest that several neuropeptides and amino acids act in the nucleus accumbens to modulate apomorphine-induced stereotyped behaviors.

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.

Coexistence of neuropeptides in sympathetic preganglionic neurons of the cat

Coexistence of four neuropeptides in sympathetic preganglionic neurons (SPN) was investigated immunohistochemically in cats after intrathecal administration of colchicine. Neurons were studied for the coexistence of all combinations of enkephalin-, neurotensin-, somatostatin-, and substance P-like immunoreactivity (ENK, NT, SS, and SP, respectively) in the intermediolateral cell column (IML), nucleus intercalatus (IC), and central autonomic area (CA). The results indicate that SP coexists with all three other peptides, SS coexists with NT and SP, and ENK coexists only with SP. In all cases, SPN which contained two peptides were found in the IML in almost all levels of the thoraco-lumbar cord. Much smaller numbers of SPN which contained two peptides (in the same combinations as above) were found in the IC and not all segments contained such neurons. In the CA, only one neuron was found which contained two peptides (SP/SS). The distribution of SPN containing two peptides suggests that these neurons may participate in more general functions of the autonomic nervous system and that they are not likely involved in the innervation of specific visceral organs.

Persisting subsensitivity of the striatal dopamine system after fetal exposure to beta-endorphin

Fetal exposure of rats to beta-endorphin during the third trimester, either alone or with alpha-MSH, resulted in mild developmental delay and significant decreases in striatal dopamine receptor density (subsensitivity) persisting through maturity. The apparent paradoxical down-regulation of dopamine receptors in the presence of beta-endorphin was consistent with fetal exposure to dopamine receptor antagonists and synthesis inhibitors. These findings suggest biophysical properties of receptors which are unique to fetal development including loss of plasticity after exposure to antagonists. Permanent, down-regulation of the striatal dopamine system may be one mechanism underlying delayed development after fetal exposure to beta-endorphin which may accompany hypoxia. Even though there were no statistically significant differences between males and females in density of the dopamine receptor, the behavioral profile after peptide treatment was sexually demorphic. Behaviorally, female rats appeared sensitized to perinatal alpha-MSH and males to alpha-endorphin.

Influence of naloxone upon apomorphine-induced rotational behavior in rats with electrolytic versus chemolytical lesions of the substantia nigra

In rats with unilateral electrolytic lesions of the substantia nigra, apomorphine induced ipsilateral rotational behavior that was significantly potentiated by naloxone. Yet, in rats with unilateral chemolytic (6-hydroxydopamine) lesions of the substantia nigra, apomorphine induced dose-dependent contralateral turning that was not influenced by pretreatment with even high doses of naloxone. High performance liquid chromatographic analysis of corpora striata from these rats revealed electrolytically-lesioned animals to have a 60-70% reduction in dopamine content while chemolytically-lesioned animals were virtually completely depleted of dopamine. These results suggest that striatal dopamine may be required for naloxone potentiation of apomorphine-induced effects in the rat rotational behavior paradigm.

Antagonism of footshock stress-induced inhibition of intracranial self-stimulation by naloxone or methamphetamine

Rats were trained to lever-press for intracranial self-stimulation (ICSS) with electrodes implanted in the ventral tegmental area (VTA). The effect of inescapable footshock on response rates to ICSS was examined in the present study. Markedly decreased response rates to ICSS were observed 15 min to 24 h following inescapable footshock. Naloxone (10.0 mg/kg) itself was without effect on response rates to ICSS, but completely antagonized the decreased response rates by the stressor treatment. A relatively low dose of methamphetamine (0.5 mg/kg), which showed no effect on ICSS rates in naive rats, also antagonized the decreased response rates to ICSS. The present results suggest that inescapable footshock may release endorphin in the mesolimbic or mesocortical area; the released endorphin may act on dopaminergic nerve endings and interrupt dopaminergic transmission. The decreased activity of dopaminergic neurons may cause the decreased response rates to ICSS.