Opposing tonically active endogenous opioid systems modulate the mesolimbic dopaminergic pathway

Kappa opioid agonists as targets for pharmacotherapies in cocaine abuse

Kappa opioid receptors derive their name from the prototype benzomorphan, ketocyclazocine (1a) which was found to produce behavioral effects that were distinct from the behavioral effects of morphine but that were antagonized by the opioid antagonist, naltrexone. Recent evidence suggests that agonists and antagonists at kappa opioid receptors may modulate the activity of dopaminergic neurons and alter the neurochemical and behavioral effects of cocaine. Kappa agonists blocked the effects of cocaine in squirrel monkeys in studies of cocaine discrimination and scheduled-controlled responding. Studies in rhesus monkeys suggested that kappa opioids may antagonize the reinforcing effects of cocaine. These studies prompted the synthesis and evaluation of a series of kappa agonists related to the morphinan, L-cyclorphan (3a) and the benzomorphan, L-cyclazocine (2). We describe the synthesis and preliminary evaluation of a series of morphinans, structural analogs of cyclorphan 3a-c, the 10-keto morphinans 4a and b, and the 8-keto benzomorphan 1b, structurally related to ketocyclazocine (1a). In binding experiments L-cyclorphan (3a), the cyclobutyl (3b), the tetrahydrofurfuryl 3c and the 10-keto 4b analogs had high affinity for mu (mu), delta (delta) and kappa (kappa) opioid receptors. Both 3a and 3b were more selective for the kappa receptor than the mu receptor. However, 3b was 18-fold more selective for the kappa receptor in comparison to the delta receptor, while cyclorphan (3a) had only a 4-fold greater affinity for the kappa receptor in comparison to the delta receptor. The cyclobutyl compound 3b was found to have significant mu agonist properties, while 3a was a mu antagonist. All compounds were also examined in the mouse tail flick and writhing assay. Compounds 3a and 3b were kappa agonists. Correlating with the binding results, compound 3a had some delta agonist properties, while 3b was devoid of any activity at the delta receptor. In addition, compounds 3a and 3b had opposing properties at the mu opioid receptor. The cyclobutyl compound 3b was found to have significant mu agonist properties, while 3a was a mu antagonist.

U-69,593 microinjection in the infralimbic cortex reduces anxiety and enhances spontaneous alternation memory in mice

The present report investigated the contributions of the ventromedial prefrontal cortex to the control of spontaneous alternation/working memory and anxiety-related behaviour. In Experiment 1, we examined the effects of microinjections of the selective kappa(1) receptor agonist, U-69,593, in the infralimbic cortex (IL) of CD-1 mice on several ethologically-derived anxiety indices in the elevated plus-maze (EPM) and defensive/withdrawal (D/W) anxiety in the open field, as well as on memory in the EPM transfer-latency (T-L) test and implicit spontaneous alternation memory (SAP) in the Y-maze. In week 1, pretreatment with one injection of vehicle, 1, 10 or 25 nmol/1.0 microliter U-69,593 in the IL dose-dependently prolonged T-L and produced a dose-dependent anxiolytic behavioural profile in the first EPM trial. Following a 24-h delay, the same mice were given a drug-free second trial in the EPM tests of T-L memory and anxiety. Whereas T-L memory was not disturbed, small but detectable carry-over effects were observed in trial-2 EPM behaviour relative to vehicle-treated animals. In week 2, the same groups of mice were again pretreated with one injection of the same doses of U-69,593 in the IL and given a D/W test in an open field, followed immediately by an 8-min SAP trial in the Y-maze. The smallest U-69,593 dose was anxiolytic in the D/W test, and SAP/working memory was dose-dependently enhanced in the Y-maze. In Experiment 2, we evaluated whether 0.5 microliter volume microinjections would produce comparable behavioural and carry-over effects in the IL of three new groups of CD-1 mice, in the event that the 1.0 microl volume injections used in Experiment 1 diffused beyond the IL and therefore may have confounded some effects. Experiment 2 procedures were carried out in the same manner as in Experiment 1, except the animals were tested in reverse order. Thus in week 1, SAP memory was tested in the Y-maze followed by D/W anxiety in the open field for half of the animals in each group, and the other half was tested in reverse order. In week 2, T/L memory and anxiety were tested in the EPM in 2 trials as described in Experiment 1. Pretreatment with one injection of vehicle, 10 or 25 nmol/0.5 microliter U-69,593 in the IL reduced D/W anxiety and enhanced SAP memory regardless of testing order in week 1. In week 2, the same groups of mice were again pretreated with one injection of the same doses of U-69,593 in 0.5 microliter volumes in the IL and tested in the EPM. In a similar fashion to Experiment 1, U-69,593 dose-dependently prolonged T/L and produced an anxiolytic behavioural profile in the first EPM trial. Following a 24-h delay, T/L recall memory was again not significantly influenced, but a robust anxiolytic behavioural profile was observed in the second drug-free anxiety trial in the EPM relative to vehicle-treated animals. Results are discussed relative to a) injection volumes and testing order, b) the possible influence kappa receptors may exert on neurochemical responsivity to anxiety-provoking environments in the IL area of the mPFC, c) the possibility that kappa-mediated anxiolysis from the IL in CD-1 mice results from interactions with neurochemical systems involved in the blunting of incoming anxiety-provoking information, d) evidence that SAP memory may be an implicit subtype of working memory, and e) the possibility that IL implicit working memory processes may modulate the induction and expression of anxiety-related behaviour.

Synthesis and opioid receptor affinity of morphinan and benzomorphan derivatives: mixed kappa agonists and mu agonists/antagonists as potential pharmacotherapeutics for cocaine dependence

This report concerns the synthesis and preliminary pharmacological evaluation of a novel series of kappa agonists related to the morphinan (-)-cyclorphan (3a) and the benzomorphan (-)-cyclazocine (2) as potential agents for the pharmacotherapy of cocaine abuse. Recent evidence suggests that agonists acting at kappa opioid receptors may modulate the activity of dopaminergic neurons and alter the neurochemical and behavioral effects of cocaine. We describe the synthesis and chemical characterization of a series of morphinans 3a-c, structural analogues of cyclorphan [(-)-3-hydroxy-N-cyclopropylmethylmorphinan S(+)-mandelate, 3a], the 10-ketomorphinans 4a,b, and the 8-ketobenzomorphan 1b. Binding experiments demonstrated that the cyclobutyl analogue 3b [(-)-3-hydroxy-N-cyclobutylmethylmorphinan S(+)-mandelate, 3b, MCL-101] of cyclorphan (3a) had a high affinity for mu, delta, and kappa opioid receptors in guinea pig brain membranes. Both 3a,b were approximately 2-fold more selective for the kappa receptor than for the mu receptor. However 3b (the cyclobutyl analogue) was 18-fold more selective for the kappa receptor in comparison to the delta receptor, while cyclorphan (3a) had only 4-fold greater affinity for the kappa receptor in comparison to the delta receptor. These findings were confirmed in the antinociceptive tests (tail-flick and acetic acid writhing) in mice, which demonstrated that cyclorphan (3a) produced antinociception that was mediated by the delta receptor while 3b did not produce agonist or antagonist effects at the delta receptor. Both 3a,b had comparable kappa agonist properties. 3a,b had opposing effects at the mu receptor: 3b was a mu agonist whereas 3a was a mu antagonist.

kappa-Opioid receptors in the substantia nigra pars reticulata mediate the U-50,488-induced locomotor activity of preweanling rats

The purpose of the present study was to determine the neuroanatomical location where kappa-opioid receptor stimulation induces locomotor activity in the preweanling rat. To confirm that the U-50,488-induced locomotor activity of preweanling rats is mediated by kappa-opioid receptors, 18-day-old rats were initially injected with vehicle or the kappa-opioid receptor agonist U-50,488 (5 mg/kg, s.c.) followed, 15 min later, by an injection of the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI; 0, 2, 4, 8, or 12 mg/kg, s.c.). In subsequent experiments, 18-day-old rats were injected with vehicle or U-50,488 (5 mg/kg, s.c.) 15 min prior to bilateral administration (0.25 or 0.5 microl per side) of nor-BNI (0, 5, 10, or 20 microg) into the substantia nigra pars reticulata (SNR) or medial dorsal striatum (MDS). In the final experiment, 18-day-old rats received bilateral administration (0.25 microl per side) of vehicle or U-50,488 (0.0, 0.8, 1.6, or 3.2 microg) into the SNR. Results showed that systemically administered nor-BNI (0-12 mg/kg, s.c.) produced a dose-dependent reduction in the U-50, 488-induced locomotor activity of preweanling rats. The site of action for U-50,488's locomotor-activating effects appeared to be the SNR, because (a) bilateral administration of nor-BNI (5, 10, or 20 microg) into the SNR caused a complete attenuation of U-50, 488-induced locomotion, and (b) bilateral administration of U-50,488 into the SNR caused a dose-dependent increase in the locomotor activity of preweanling rats. Striatal injections of nor-BNI did not affect U-50,488-induced locomotor activity. When these findings are considered together it is apparent that stimulation of kappa-opioid receptors in the SNR is both necessary and sufficient for the occurrence of U-50,488-induced locomotor activity in the preweanling rat.

From opiate pharmacology to opioid peptide physiology

This is a personal account of how studies of the pharmacology of opiates led to the discovery of a family of endogenous opioid peptides, also called endorphins. The unique pharmacological activity profile of opiates has an endogenous counterpart in the enkephalins and beta-endorphin, peptides which also are powerful analgesics and euphorigenic agents. The enkephalins not only act on the classic morphine (mu-) receptor but also on the delta-receptor, which often co-exists with mu-receptors and mediates pain relief. Other members of the opioid peptide family are the dynorphins, acting on the kappa-receptor earlier defined as precipitating unpleasant central nervous system (CNS) side effects in screening for opiate activity, A related peptide, nociceptin is not an opioid and acts on the separate NOR-receptor. Both dynorphins and nociceptin have modulatory effects on several CNS functions, including memory acquisition, stress and movement. In conclusion, a natural product, morphine and a large number of synthetic organic molecules, useful as drugs, have been found to probe a previously unknown physiologic system. This is a unique development not only in the neuropeptide field, but in physiology in general.

Mu and kappa1 opioid-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate binding in cynomolgus monkey brain

Agonist-stimulated [35S]GTPgammaS binding allows the visualization of receptor-activated G-proteins, thus revealing the anatomical localization of functional receptor activity. In the present study, agonist-stimulated [35S]GTPgammaS binding was used to demonstrate mu and kappa1 opioid-stimulated [35S]GTPgammaS binding in tissue sections and membranes from cynomolgus monkey brain using DAMGO and U50,488H, respectively. Concentrations of agonists required to produce maximal stimulation of [35S]GTPgammaS binding were determined in membranes from the frontal poles of the brain. Receptor specificity was verified in both membranes and sections by inhibiting agonist-stimulated [35S]GTPgammaS binding with the appropriate antagonist. Mu opioid-stimulated [35S]GTPgammaS binding was high in areas including the amygdala, ventral striatum, caudate, putamen, medial thalamus and hypothalamus. Dense mu-stimulated [35S]GTPgammaS binding was also found in brainstem nuclei including the interpeduncular nucleus, parabrachial nucleus and nucleus of the solitary tract. Kappa1 opioid-stimulated [35S]GTPgammaS binding was high in limbic and association cortex, ventral striatum, caudate, putamen, globus pallidus, claustrum, amygdala, hypothalamus and substantia nigra. These results demonstrate the applicability of [35S]GTPgammaS autoradiography to examine receptor-activated G-proteins in the primate brain and reveal functional mu and kappa1 opioid receptor activity that may contribute to the reported central nervous system effects of opiates.

Downregulation of kappa opioid receptor mRNA levels by chronic ethanol and repetitive cocaine in rat ventral tegmentum and nucleus accumbens

The combination of ethanol and cocaine is commonly abused by human addicts which has serious clinical consequences. Here, the effects of separately and concurrently administered ethanol and 'binge' cocaine on kappa opioid receptor (KOR) mRNA in the ventral tegmental area (VTA) and nucleus accumbens (NAc) of rats were studied. KOR mRNA was down-regulated in both brain regions during concurrent as well as separate treatment with these drugs. In the VTA, the most pronounced decrease was obtained following combined treatment with ethanol and 'binge' cocaine. In the NAc, the strongest decrease was observed in the 'binge' cocaine group. This profound decrease of KOR mRNA in regions important for brain reward suggests a potential role of the KOR system in the abuse of cocaine and ethanol.

The dopamine hypothesis of reward: past and current status

Mesolimbic dopaminergic neurons are thought to serve as a final common neural pathway for mediating reinforcement processes. However, several recent findings have challenged the view that mesolimbic dopamine has a crucial role in the maintenance of reinforcement processes, or the subjective rewarding actions of natural rewards and drugs of abuse. Instead, there is growing evidence that dopamine is involved in the formation of associations between salient contextual stimuli and internal rewarding or aversive events. This evidence suggests that dopaminergic-neuron activation aids the organism in learning to recognize stimuli associated with such events. Thus, mesolimbic dopaminergic neurons have an important function in the acquisition of behavior reinforced by natural reward and drug stimuli. Furthermore, long-lasting neuroadaptive changes in mesolimbic dopamine-mediated transmission that develop during chronic drug use might contribute to compulsive drug-seeking behavior and relapse.

Long-lasting sensitization towards morphine in motoric and limbic areas as determined by c-fos expression in rat brain

Chronic application of morphine leads to the development of tolerance towards several of its effects, e.g., analgesia or respiratory depression. Simultaneously, however, sensitization arises which becomes apparent in behavioral tests as increased locomotion or increased self-application. A human correlate for the latter may be the increasing craving for opioids in addicts. To identify brain areas involved in these long-lasting processes, we studied the expression of the transcription factor c-fos by in situ hybridization in rat brain as a marker for changes in gene expression after single or repeated morphine applications in the animals. The only c-fos signal that exceeded background after a single dose of morphine (50 mg/kg) was a diffuse expression in the lateral septum. In contrast, repeated dosage twice daily for 10 days and ascending from 10 to 50 mg/kg resulted in a sharply delineated morphine-induced c-fos synthesis in the dorsomedial and lateral striatum, lateral septum, medial mammillary nuclei, anterior thalamus and, in part masked by a high background due to injection stress, in the cingulate cortex. Most of these areas belong to the limbic system or are closely associated with it. The c-fos response was inducible by morphine in pretreated animals for up to 8 weeks after finishing the repeated application scheme. Retrograde tracing studies revealed that the dorsomedial part of the striatum, which was strongly labeled with the c-fos probe, received inputs from limbic as well as from motoric parts of the thalamus and cortex. Therefore, the sensitization of morphine-induced c-fos expression in parts of the striatum seems to correlate with the locomotor effects of repeated morphine application, whereas the observed sensitization in several limbic brain areas might reflect emotional phenomena like increased self-administration in rats or drug craving in humans.

The kappa-opioid agonist, U-69593, decreases acute amphetamine-evoked behaviors and calcium-dependent dialysate levels of dopamine and glutamate in the ventral striatum

The effects of a kappa-opioid receptor agonist on acute amphetamine-induced behavioral activation and dialysate levels of dopamine and glutamate in the ventral striatum were investigated. Amphetamine (2.5 mg/kg i.p.) evoked a substantial increase in rearing, sniffing, and hole-poking behavior as well as dopamine and glutamate levels in the ventral striatum of awake rats. U-69593 (0.32 mg/kg s.c.) significantly decreased the amphetamine-evoked increase in behavior and dopamine and glutamate levels in the ventral striatum. Reverse dialysis of the selective kappa-opioid receptor antagonist, nor-binaltorphimine, into the ventral striatum antagonized the effects of U-69593 on amphetamine-induced behavior and dopamine and glutamate levels. Reverse dialysis of low calcium (0.1 mM) into the ventral striatum decreased basal dopamine, but not glutamate, dialysate levels by 91% 45 min after initiation of perfusion. Strikingly, 0.1 mM calcium perfusion significantly reduced the 2.5 mg/kg amphetamine-evoked increase in dopamine and glutamate levels in the ventral striatum, distinguishing a calcium-dependent and a calcium-independent component of release. U-69593 did not alter the calcium-independent component of amphetamine-evoked dopamine and glutamate levels. These data are consistent with the view that a transsynaptic mechanism augments the increase in dopamine and glutamate levels in the ventral striatum evoked by a moderately high dose of amphetamine and that stimulation of kappa-opioid receptors suppresses the calcium-dependent component of amphetamine's effects.

kappa-Opioid tolerance and dependence in cultures of dopaminergic midbrain neurons

Repeated cocaine exposure upregulates kappa opioids and their receptors in the mesocorticolimbic system; the ensuing kappa-mediated dysphoria appears to contribute to addiction and withdrawal. As a potential rehabilitation strategy to reverse cocaine-induced kappa sensitization, the present study used tritiated dopamine release assays to examine the induction of kappa-opioid tolerance in cultured mesencephalic neurons. Administration of the kappa agonist U69,593 inhibited tetrodotoxin-sensitive, spontaneous (EC(50) = 1.5 nM), and potassium-stimulated (EC(50) = 10 nM) release. These effects were blocked by pertussis toxin and by the kappa antagonist nor-binaltorphimine. The 2 d agonist exposure (1 microM) caused a shift in the U69,593 dose-response curve that was greater in the potassium-stimulated paradigm (140-fold) than in the spontaneous release assay (sixfold). These results were attributable to the attenuation of kappa-receptor signaling mechanisms and to dependence. In the stimulated release assay, attenuation of kappa signaling caused by 4 hr of U69,593 exposure recovered with a half-life of 1.1 hr, whereas attenuation after 144 hr of exposure recovered slowly (t(1/2) = 20 hr). In the spontaneous release assay, attenuation of kappa-opioid signaling occurred slowly (t(1/2) = 22 hr), and resensitization after a 144 hr exposure was rapid (t(1/2) < 1 hr). kappa-Opioid dependence was observed after 144 hr of U69,593 exposure. Thus multiple mechanisms of adaptation to kappa-opioid exposure occur in mesocorticolimbic neurons. These data support the idea that the administration of kappa opioids might facilitate drug rehabilitation.

Fentanyl increases dopamine release in rat nucleus accumbens: involvement of mesolimbic mu- and delta-2-opioid receptors

The effects of the mu-receptor agonist fentanyl on extracellular levels of dopamine in rat nucleus accumbens were studied in awake animals by in vivo brain microdialysis. Fentanyl dose-dependently increased the levels of dopamine when given intravenously (microg/kg) or via a microdialysis probe placed into the ventral tegmental area or the nucleus accumbens (nmol). The effect of fentanyl given into the nucleus accumbens was blocked by systemic administration of the non-selective opioid receptor antagonist naloxone and by accumbens administration of D-Phe-Cys-Tyr-D-Trp-Om-Thr-Phe-Thr-NH2 (nmol), a mu-opioid receptor antagonist, and naltrindole (nmol), a non-selective delta-opioid receptor antagonist, in a dose-dependent manner. The delta2-opioid receptor antagonist, naltriben (nmol), also blocked the effects of fentanyl, whereas the delta1-opioid receptor antagonist, (E)-7-benzylidenenaltrexone (nmol), was ineffective. When marginally effective doses of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH2 and naltriben were given simultaneously, the effect of fentanyl was nearly fully blocked; the pretreatment itself had no effect. Administration of the mu-opioid receptor agonist [D-Ala2, N-Me-Phe4,Gly5-ol]-enkephalin (nmol), the delta1-opioid receptor agonist [D-Pen2,5]-enkephalin (nmol) or the delta2-opioid receptor agonist [D-Ala2,Glu4]-deltorphin (nmol) into the nucleus accumbens enhanced the amount of accumbal dopamine. This study provides evidence that not only activation of delta1- and delta2-opioid receptors, but also activation of mu-opioid receptors in the nucleus accumbens increases the release of accumbal dopamine in freely moving rats. We suggest that the effect of intra-accumbens administration of fentanyl upon accumbal release of dopamine is either due to the simultaneous activation of mu-opioid receptors and delta2-opioid receptors or due to activation of mu-opioid receptors that interact with delta2-opioid receptors in a complex manner.

kappa-Opioid tolerance and dependence in cultures of dopaminergic midbrain neurons

Repeated cocaine exposure upregulates kappa opioids and their receptors in the mesocorticolimbic system; the ensuing kappa-mediated dysphoria appears to contribute to addiction and withdrawal. As a potential rehabilitation strategy to reverse cocaine-induced kappa sensitization, the present study used tritiated dopamine release assays to examine the induction of kappa-opioid tolerance in cultured mesencephalic neurons. Administration of the kappa agonist U69,593 inhibited tetrodotoxin-sensitive, spontaneous (EC(50) = 1.5 nM), and potassium-stimulated (EC(50) = 10 nM) release. These effects were blocked by pertussis toxin and by the kappa antagonist nor-binaltorphimine. The 2 d agonist exposure (1 microM) caused a shift in the U69,593 dose-response curve that was greater in the potassium-stimulated paradigm (140-fold) than in the spontaneous release assay (sixfold). These results were attributable to the attenuation of kappa-receptor signaling mechanisms and to dependence. In the stimulated release assay, attenuation of kappa signaling caused by 4 hr of U69,593 exposure recovered with a half-life of 1.1 hr, whereas attenuation after 144 hr of exposure recovered slowly (t(1/2) = 20 hr). In the spontaneous release assay, attenuation of kappa-opioid signaling occurred slowly (t(1/2) = 22 hr), and resensitization after a 144 hr exposure was rapid (t(1/2) < 1 hr). kappa-Opioid dependence was observed after 144 hr of U69,593 exposure. Thus multiple mechanisms of adaptation to kappa-opioid exposure occur in mesocorticolimbic neurons. These data support the idea that the administration of kappa opioids might facilitate drug rehabilitation.

Orphanin FQ/nociceptin modulation of mesolimbic dopamine transmission determined by microdialysis

Orphanin FQ has been reported to suppress extracellular dopamine levels in the nucleus accumbens after intracerebroventricular administration. This study sought to provide evidence for an intra-ventral tegmental site of action for this effect using a dual-probe microdialysis experimental design. Orphanin FQ was applied to the ventral tegmental area of anesthetized rats by reverse dialysis while extracellular dopamine was sampled with a second dialysis probe in the nucleus accumbens. Orphanin FQ at a probe concentration of 1 mM (but not at 0.1 mM) significantly reduced nucleus accumbens dialysate dopamine levels. The receptor-inactive analogue, des-Phe1-orphanin FQ (1 mM), produced a small but significant increase in nucleus accumbens dialysate dopamine levels. Simultaneous measurement of ventral tegmental area dialysate amino acid content revealed significant increases in both GABA and glutamate during infusion of orphanin FQ (1 mM). To determine if increased GABA overflow mediates the action of orphanin FQ on mesolimbic neurons, orphanin FQ (10 nmol) was microinjected directly into the ventral tegmental area in the presence or absence of the GABA(A) receptor antagonist, bicuculline (1 nmol). Bicuculline transiently blocked the suppressive action of orphanin FQ on accumbens dialysate dopamine levels. These data indicate that orphanin FQ decreases dopamine transmission in the nucleus accumbens by inhibiting dopamine neuronal activity in the ventral tegmental area through a mechanism that may involve an increased overflow of GABA.

D3 dopamine and kappa opioid receptor alterations in human brain of cocaine-overdose victims

Cocaine is thought to be addictive because chronic use leads to molecular adaptations within the mesolimbic dopamine (DA) circuitry, which affects motivated behavior and emotion. Although the reinforcing effects of cocaine are mediated primarily by blockade of DA uptake, reciprocal signaling between DA and endogenous opioids has important implications for understanding cocaine dependence. We have used in vitro autoradiography and ligand binding to map D3 DA and kappa opioid receptors in the human brains of cocaine-overdose victims. The number of D3 binding sites was increased one-to threefold over the nucleus accumbens and ventromedial sectors of the caudate and putamen from cocaine-overdose victims, as compared to age-matched and drug-free control subjects. D3 receptor/cyclophilin mRNA ratios in the nucleus accumbens were increased sixfold in cocaine-overdose victims over control values, suggesting that cocaine exposure also affects the expression of D3 receptor mRNA. The number of kappa opioid receptors in the nucleus accumbens and other corticolimbic areas from cocaine fatalities was increased twofold as compared to control values. Cocaine-overdose victims exhibiting preterminal excited delirium had a selective upregulation of kappa receptors measured also in the amygdala. Understanding the complex regulatory profiles of DA and opioid synaptic markers that occur with chronic misuse of cocaine may suggest multitarget strategies for treating cocaine dependence.

Regulation of neurotransmitter interactions in the ventral striatum

Transsynaptic activation of neuronal circuits originating in the basal forebrain contributes to psychostimulant-evoked dopamine and glutamate release and consequent changes in medium spiny neuronal gene expression in the ventral striatum. New evidence from microdialysis studies indicates that amphetamine-induced dopamine and glutamate release in vivo is partially calcium dependent. The calcium-dependent component is totally blocked by a kappa opioid receptor agonist, indicating that endogenous opioids may regulate dopamine-glutamate interactions in the ventral striatum. Further, muscarinic receptor blockade increases, and muscarinic receptor stimulation decreases, dialysate glutamate levels in the striatum. Pre- and postsynaptic muscarinic receptors contribute to the ability of the muscarinic antagonist, scopolamine, to augment D1 receptor-stimulated immediate early and neuropeptide gene expression. Moreover, scopolamine prevents a D2 antagonist from blocking D1 agonist-induced gene expression, indicating that activation of cholinergic interneurons contributes to D1/D2 interactions in the striatum. Thus, transsynaptic activity and presynaptic muscarinic and kappa opioid receptors regulate dopamine and glutamate interactions that switch on and off multiple intracellular signaling cascades. Changes in immediate early and neuropeptide gene expression that result from activation of these cascades are mediated by such nuclear transcription factors as phosphorylated cyclase response element-binding protein. In addition, a novel signaling pathway involving the RAR/RXR nuclear hormone receptor complex is implicated in the control of dopamine receptor and neuropeptide gene expression in the striatum.

Locomotor-activity-induced changes in striatal levels of preprotachykinin and preproenkephalin mRNA. Regulation by the dopaminergic and glutamatergic systems

The mechanisms by which dopaminergic and glutamatergic inputs interact to regulate striatal neuropeptide expression during physiological motor activity are poorly understood. In this work, striatal expression of preprotachykinin (PPT) and preproenkephalin (PPE) mRNA was studied by in situ hybridization in rats killed 2 h after treadmill running (36 m/min for 20 min). Treadmill running induced a significant increase in the levels of both PPT (60% increase) and PPE (90% increase) mRNA in the striatum of normal rats. The increase in the level of PPT mRNA was blocked in rats previously subjected to nigrostriatal deafferentation (i.e., 6-hydroxydopamine lesion) or pretreated with D1-receptor antagonist SCH-23390 (0.1 mg/kg), the D2-receptor antagonist eticlopride (0.5 mg/kg), or the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 (0.1 mg/kg). The running-induced increase in the level of PPE mRNA was blocked in rats pretreated with SCH-23390 or MK-801. Rats subjected to nigrostriatal deafferentation or pretreated with eticlopride showed an increase in PPE mRNA levels (around 150% and 40% increase, respectively), that was enhanced by running (around 230% and 160% increase, respectively). These results suggest that locomotor activity increases, in a NMDA receptor dependent fashion, the excitatory influence of the corticostriatal glutamatergic system on the two populations of striatal projection neurons, as reflected by increases in the levels of PPT and PPE mRNA. The results obtained after dopamine depletion or injection of dopamine receptor antagonists suggest that a concomitant increase in dopamine release may enhance PPT mRNA level in striatonigral neurons via D1 receptors, and reduce PPE mRNA level in striatopallidal neurons via D2 receptors. Additionally, levels of dopamine and glutamate may be regulated by other complex indirect mechanisms.

Orphanin FQ and behavioral sensitization to cocaine

The recently identified endogenous ligand for the ORL-1 (opioid receptor-like) receptor, orphanin FQ, has been shown to induce hypolocomotion and to decrease extracellular dopamine levels in the nucleus accumbens (N.Acc) after intraventricular (ICV) administration. This study investigated the effect of intraventral tegmental area (VTA) administration of orphanin FQ on the hyperlocomotor effects of peripheral cocaine administration and on the development of behavioral sensitization to cocaine. The administration of cocaine (40 mg/kg IP) once daily for 3 days to male Sprague-Dawley rats resulted in an enhanced locomotor response to a subsequent challenge of cocaine (10 mg/kg IP) 5 days later. The bilateral administration of orphanin FQ (10 microg/side or 30 microg/side) into the VTA 5-10 min prior to the administration of cocaine (40 mg/kg IP) produced a transient (15-30 min) decrease in the hyperlocomotor response to cocaine on day 1 but not on days 2 and 3 of the sensitization paradigm. Such orphanin FQ pretreatment on days 1-3 had no effect on the development of a sensitized response to cocaine (10 mg/kg IP) 5-7 days after the last orphanin FQ injection. However, repeated intra-VTA administration of orphanin FQ (30 microg/side) alone for 3 days resulted in a sensitized response to a single dose of cocaine (10 mg/kg IP) given 5-7 days later. These results indicate that orphanin FQ decreases the activity of mesolimbic dopamine neurons via an action in the VTA, an effect that is both transient and demonstrates rapid tolerance, and consequently, is insufficient to prevent the development of cocaine sensitization. The ability of the peptide to induce cocaine sensitization when administered alone despite its acute inhibitory effects is unique and requires further study to elucidate the mechanisms responsible.

Comparison of the proopiomelanocortin and proenkephalin opioid peptide systems in brain regions of the alcohol-preferring C57BL/6 and alcohol-avoiding DBA/2 mice

Differences in the activity of distinct components of the endogenous opioid system between ethanol-preferring and ethanol-avoiding animals may be important in controlling their voluntary alcohol consumption. The objective of the present studies was to compare the activity of two opioid peptide systems (enkephalin and beta-endorphin) in distinct regions of the brain, between the C57BL/6 and DBA/2 mice, using sensitive radioimmunoassays, in situ hybridization, and immunohistochemical techniques. The immunohistochemical studies indicated that there was no significant difference in the number of either beta-endorphin or enkephalin immunopositive cells between the C57BL/6 and DBA/2 mice. The in situ hybridization studies demonstrated a 27% higher content of proopiomelanocortin mRNA in the arcuate nucleus of the C57BL/6 than DBA/2 mice, p < 0.02. The content of proenkephalin mRNA was 25% higher in the nucleus accumbens, p < 0.005 and 23% higher in the caudate putamen, p < 0.01, of the C57BL/6 than DBA/2 mice. There was no significant difference in the content of beta-endorphin peptides in the distinct brain regions investigated. The content of met-enkephalin-arg6-phe7 in the nucleus accumbens and caudate was similar between the two strains of mice, while it was significantly lower in the amygdala, hippocampus, ventral tegmental area, and periaqueductal grey of the C57BL/6 than DBA/2 mice. Thus, there are significant differences in the activity of these two endogenous opioid peptide systems in distinct regions of the brain, between ethanol naive C57BL/6 and DBA/2 mice, which may play a role in controlling their alcohol consumption.

Expression of mu, kappa, and delta opioid receptor messenger RNA in the human CNS: a 33P in situ hybridization study

The existence of at least three opioid receptor types, referred to as mu, kappa, and delta, is well established. Complementary DNAs corresponding to the pharmacologically defined mu, kappa, and delta opioid receptors have been isolated in various species including man. The expression patterns of opioid receptor transcripts in human brain has not been established with a cellular resolution, in part because of the low apparent abundance of opioid receptor messenger RNAs in human brain. To visualize opioid receptor messenger RNAs we developed a sensitive in situ hybridization histochemistry method using 33P-labelled RNA probes. In the present study we report the regional and cellular expression of mu, kappa, and delta opioid receptor messenger RNAs in selected areas of the human brain. Hybridization of the different opioid receptor probes resulted in distinct labelling patterns. For the mu and kappa opioid receptor probes, the most intense regional signals were observed in striatum, thalamus, hypothalamus, cerebral cortex, cerebellum and certain brainstem areas as well as the spinal cord. The most intense signals for the delta opioid receptor probe were found in cerebral cortex. Expression of opioid receptor transcripts was restricted to subpopulations of neurons within most regions studied demonstrating differences in the cellular expression patterns of mu, kappa, and delta opioid receptor messenger RNAs in numerous brain regions. The messenger RNA distribution patterns for each opioid receptor corresponded in general to the distribution of opioid receptor binding sites as visualized by receptor autoradiography. However, some mismatches, for instance between mu opioid receptor receptor binding and mu opioid receptor messenger RNA expression in the anterior striatum, were observed. A comparison of the distribution patterns of opioid receptor messenger RNAs in the human brain and that reported for the rat suggests a homologous expression pattern in many regions. However, in the human brain, kappa opioid receptor messenger RNA expression was more widely distributed than in rodents. The differential and region specific expression of opioid receptors may help to identify targets for receptor specific compounds in neuronal circuits involved in a variety of physiological functions including pain perception, neuroendocrine regulation, motor control and reward.

Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

Indirect dopamine agonists augment the locomotor activating effects of the kappa-opioid receptor agonist U-50,488 in preweanling rats

kappa-Opioid receptor agonists (e.g., enadoline or U-50,488) increase the locomotor activity of preweanling rats, while the same drugs depress the locomotor activity of adults. Curiously, direct stimulation of dopamine (DA) D2-like receptors fully attenuates the U-50,488-induced locomotor activity of preweanling rats. The purpose of the present study was to determine whether indirect DA agonists (i.e., cocaine, methylphenidate, and amphetamine) would also attenuate U-50,488's behavioral effects. In two experiments, 17-day-old rats were injected with saline or U-50,488 (5 mg/kg, sc) and locomotor activity and stereotyped sniffing were assessed. After 20 min, the saline- and U-50,488-pretreated rats were injected with saline, cocaine (5, 10, or 20 mg/kg, i.p.), methylphenidate (10 or 20 mg/kg, i.p.), amphetamine (2.5 or 5 mg/kg, i.p.), or the direct D2-like receptor agonist NPA (1 mg/kg, i.p.). As expected, U-50,488 dramatically enhanced the locomotor activity of 17-day-old rats, while attenuating the stereotyped sniffing caused by indirect and direct DA agonists. All three indirect DA agonists augmented U-50,488's locomotor activating effects across the initial 10 min of testing and then activity declined to U-50,488 control values. Direct stimulation of DA receptors produced nearly opposite effects because NPA attenuated U-50,488-induced locomotor activity across the entire testing session. It is uncertain why direct and indirect DA agonists affected U-50,488-induced locomotor activity differently, but the relative amount of DA D1-like receptor activation is probably not responsible.

Differential effects of mu and kappa opioid antagonists on Fos-like immunoreactivity in extended amygdala

It was previously reported that systemic administration of the nonselective opioid antagonist, naltrexone, induces Fos-like immunoreactivity (FLI) within the central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (lateral-dorsal division; BSTLD), nucleus accumbens shell (NACshell) and ventral tegmental area (VTA) of free-feeding rats. These findings suggest that cellular activity in these brain regions is subject to opioid-mediated inhibitory control under basal conditions. Considering the involvement of mesoaccumbens dopamine neurons and components of the 'extended amygdala' in motivated behavior and reward, it was hypothesized that the induction of c-Fos by naltrexone accounts for the motivational-affective consequences of opioid antagonism. In Experiment 1, naltrexone was administered intracerebroventricularly (i.c.v.; 100 microg) to determine whether results obtained in the prior immunohistochemical studies could be attributed to blockade of opioid receptors in brain as opposed to peripheral tissues that convey visceral sensory inputs to the CeA and BSTLD. Naltrexone produced a marked increase in FLI within the CeA and BSTLD, and a moderate increase in NACshell. In Experiment 2, the kappa opioid antagonist, nor-binaltorphimine (Nor-BNI; 20.0 microg, i.c.v.) reproduced the effect of naltrexone in BSTLD and CeA, suggesting that the induction of c-Fos in these two structures is a consequence of kappa receptor blockade. The selective mu antagonist, CTAP (2.0 microg, i.c.v.), reproduced the effect of naltrexone in NACshell, suggesting that the induction of c-Fos in this structure is a consequence of mu receptor blockade. The functional implications of these results are discussed in terms of the known functions of these brain regions and opioid receptor types, and the prior observation that chronic food restriction eliminates the FLI induced by naltrexone in CeA and BSTLD. It is suggested that tonic mu opioid-mediated inhibition in NACshell has a predisposing effect on goal-approach behavior in general while kappa opioid-mediated inhibition in CeA and BSTLD has a predisposing effect on palatability-driven feeding in particular. Finally, a possible relationship between food restriction-induced suppression of the kappa opioid mechanism in CeA/BSTLD, local CRH function, and sensitization of the neural substrate for incentive-motivating effects of abused drugs is discussed.

(+/-)Cyclazocine blocks the dopamine response to nicotine

(+/-)Cyclazocine, synthesized by Archer in 1962, was originally tested as a treatment for heroin addiction. (+/-)Cyclazocine is a mu opioid antagonist and kappa opioid agonist, and because of these actions, would be expected to modulate dopamine release in the nucleus accumbens as well as the reinforcing effects of drugs of abuse. In a recent study (+/-)cyclazocine was reported to decrease cocaine self-administration in rats. The aim of the present study was to determine whether (+/-)cyclazocine would alter the dopaminergic effects of nicotine that are thought to mediate its rewarding effects. Using in vivo microdialysis in awake and freely moving rats, we investigated the effect of (+/-)cyclazocine (0.5 mg/kg, i.p.) on the acute dopamine response to nicotine (0.32 mg/kg, i.v. over a 5 min period, infused 30 min later) in the nucleus accumbens. (+/-)Cyclazocine significantly attenuated the increase in extracellular dopamine levels induced by the nicotine infusion and enhanced nicotine-induced increases in dopamine metabolites. (+/-)Cyclazocine alone did not significantly affect extracellular dopamine levels. However, both the (+) and (-) enantiomers of cyclazocine did alter basal dopamine levels and these effects made it difficult to assess their individual interactions with nicotine. The results suggest that the effects of both enantiomers contribute to the effects of the racemate; (+/-)cyclazocine may decrease the rewarding effect of nicotine and may be the prototype of a potentially novel treatment for smoking.

Nicotine self-administration in animals as a dependence model

Various animal models of nicotine dependence now exist. To study the positive reinforcing effects of nicotine, there are choices of animal species, strains, and operant paradigms to use. This manuscript describes the use of one particular paradigm, a model in which work is done by laboratory animals to obtain intravenous infusions of nicotine. This model is particularly useful for examining the mechanisms in the brain that are responsible for the maintenance of drug-taking behavior. Two examples of ongoing studies of the mechanisms of dependence are discussed: the role of cholinergic projections to midbrain dopamine cells, and the influence of opioid receptors in the vicinity of these same dopamine cells.

Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

Acute subjective effects of dynorphin A(1-13) infusion in normal healthy subjects

Twelve healthy subjects with no history of substance abuse participated in a placebo-controlled single-blinded study of subjective response to acute i.v. administration of placebo and two doses of the natural shortened peptide sequence of the kappa-opioid agonist, dynorphin A(1-13) (low dose 120 micrograms/kg, high dose 500 micrograms/kg). Visual analog scales showed small but significant negative mood and positive drug effect 10 min post infusion in the high dose dynorphin compared to placebo infusion. The differences were no longer apparent at 30 min. These results show that dynorphin A(1-13), shown previously to have both neuroendocrine and modest analgesic effects, was well tolerated and produced modest transient subjective responses.

Quantitative autoradiography of mu-,delta- and kappa1 opioid receptors in kappa-opioid receptor knockout mice

Mice deficient in the kappa-opioid receptor (KOR) gene have recently been developed by the technique of homologous recombination and shown to lack behavioural responses to the selective kappa1-receptor agonist U-50,488H. We have carried out quantitative autoradiography of mu-, delta- and kappa1 receptors in the brains of wild-type (+/+), heterozygous (+/-) and homozygous (-/-) KOR knockout mice to determine if there is any compensatory expression of mu- and delta-receptor subtypes in mutant animals. Adjacent coronal sections were cut from the brains of +/+, +/- and -/- mice for the determination of binding of [3H]CI-977, [3H]DAMGO (D-Ala2-MePhe4-Gly-ol5 enkephalin) or [3H]DELT-I (D-Ala2 deltorphin I) to kappa1-, mu- and delta-receptors, respectively. In +/- mice there was a decrease in [3H]CI-977 binding of approximately 50% whilst no kappa1-receptors could be detected in any brain region of homozygous animals confirming the successful disruption of the KOR gene. There were no major changes in the number or distribution of mu- or delta-receptors in any brain region of mutant mice. There were, however some non-cortical regions where a small up-regulation of delta-receptors was observed in contrast to an opposing down-regulation of delta-receptors evident in mu-knockout brains. This effect was most notable in the nucleus accumbens and the vertical limb of the diagonal band, and suggests there may be functional interactions between mu- and delta-receptors and kappa1- and delta-receptors in mouse brain.

Chronic morphine exposure and spontaneous withdrawal are associated with modifications of dopamine receptor and neuropeptide gene expression in the rat striatum

The influence of chronic morphine and spontaneous withdrawal on the expression of dopamine receptors and neuropeptide genes in the rat striatum was investigated. Morphine dependence was induced by subcutaneous implantation of two morphine pellets for 6 days. Rats were made abstinent by removal of the pellets 1, 2 or 3 days before they were killed. The mRNA levels coding for D1- and D2-dopamine receptors, dynorphin, preproenkephalin A and substance P were determined by quantitative in situ hybridization. The caudate putamen and the nucleus accumbens showed equivalent modifications in dopamine receptor and neuropeptide gene expression. After 6 days of morphine, a decrease in D2-dopamine receptor and neuropeptide mRNA levels was observed (-30%), but there was no change in D1-dopamine receptor mRNA. In abstinent rats, both D1- and D2-dopamine receptor mRNA levels were decreased 1 day after withdrawal (-30% compared with chronic morphine). In contrast, neuropeptide mRNA levels were unaffected when compared with those observed after 6 days of morphine. During the second and third day of withdrawal, there was a gradual return to the levels seen in the placebo-treated group, for both dopamine receptor and neuropeptide mRNAs. Phenotypical characterization of striatal neurons expressing mu and kappa opioid receptor mRNAs showed that, in striatonigral neurons, both mRNAs were colocalized with D1-receptor and Dyn mRNAs. Our results suggest that during morphine dependence, dopamine and morphine exert opposite effects on striatonigral neurons, and that effects occurring on striatopallidal neurons are under dopaminergic control. We also show that withdrawal is associated with a down regulation of the postsynaptic D1 and D2 receptors.

What peptides these deltorphins be

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

Naltrexone affects cocaine self-administration in naïve rats through the ventral tegmental area rather than dopaminergic target regions

Behavioural studies have shown an involvement of central endogenous opioid systems in experimental cocaine addiction. Seeking to further localize the attenuating effect of opioid blockade on the reinforcing effects of cocaine, naltrexone was administered locally to different regions of the mesocorticolimbic system, which are thought to be critically involved in cocaine self-administration behaviour. Both cell body and nerve terminal regions of this system were targeted. Using a model for the initiation of cocaine self-administration behaviour, no effect of naltrexone was found in caudate, amygdaloid or accumbens nuclei, nor in the medial prefrontal cortex. However, blockade of endogenous opioid receptors in the ventral tegmental area region attenuated cocaine self-administration. With the initiation model, this finding reflects an attenuating effect on the reinforcing effects of cocaine. The attenuation of self-administration was dependent on the naltrexone dose. The present findings suggest that endogenous opioid systems in the ventral tegmental area modulate the reinforcing efficacy of cocaine.

Increased responsiveness of mesolimbic and mesostriatal dopamine neurons to cocaine following repeated administration of a selective kappa-opioid receptor agonist

Previous data have shown that the repeated administration of kappa-opioid receptor agonists attenuates the acute behavioral effects of cocaine. The site and mechanism by which kappa-agonists interact with this psychostimulant, however, are unknown. Accordingly, the present microdialysis study characterized the effects of prior, repeated administration of the selective kappa-opioid receptor agonist U69593 on basal and cocaine-evoked DA levels within the nucleus accumbens (NAC) and caudate putamen (CPU). The influence of U69593 treatment on the locomotor-activating effects of an acute cocaine challenge was also assessed. Rats received once daily injections of U69593 (0.16-0.32 mg/kg/day) or vehicle (1.0 ml/kg/day) for 3 days. The behavioral and neurochemical effects produced by an acute cocaine challenge (20 mg/kg i.p.) were assessed 2 days following treatment cessation. Administration of cocaine to control animals increased locomotor activity. This effect was attenuated in animals which had previously received U69593 (0.32 mg/kg/day x 3 days). Prior administration of U69593 failed to modify basal DA levels in either the NAC or CPU. Thus, 2 days following the cessation of U69593 treatment, dialysate DA levels did not differ from that of controls. Administration of cocaine to vehicle-treated animals increased dialysate levels of DA in both brain regions. However, in animals previously exposed to U69593 (0.32 mg/kg/day x 3 days), a significant enhancement in the response of DA neurons to cocaine was seen. These data demonstrate that prior, repeated administration of a selective kappa-opioid receptor agonist attenuates the locomotor-activating effects of cocaine and increases cocaine-evoked DA overflow in terminal projection areas of mesostriatal and mesolimbic DA neurons. These findings indicate that the behavioral interactions of kappa-agonists with cocaine observed in this and previous studies cannot be attributed to a presynaptic inhibition of DA release. Rather, they suggest that postsynaptic or non-DA mechanisms mediate the interaction of these agents with cocaine.

Repeated treatment with the selective kappa opioid agonist U-69593 produces a marked depletion of dopamine D2 receptors

U-69593, the selective K-opioid agonist, was repeatedly administered in single daily injections (0.32 mg/kg) to male, Sprague-Dawley rats. Two or ten days later, the rats were euthanized and dopamine D1 and D2 receptors were measured using (3H]SCH 23390 or [3H]sulpiride, respectively, in caudate putamen and nucleus accumbens. Two days after the last of three injections, dopamine D2 receptors in the caudate putamen were decreased by approximately 40%, with no change in D1 receptors. Dopamine D2 receptor number had returned to normal by 10 days posttreatment. In contrast, in the nucleus accumbens there was a small, nonsignificant decrease in dopamine D2 receptors 2 days after treatment, but a large increase (65%) after 10 days. In agreement with the changes in D2 receptors, there was a significant downward shift in the locomotor activity curve for the D2 agonist quinpirole after a 2-day withdrawal. There were no differences in either the total amount of dopamine taken up or in the IC50 for cocaine to inhibit dopamine uptake following this treatment, suggesting that the dopamine transporter and presynaptic terminals were intact. The results of these studies demonstrate that repeated administration of a selective K-opioid agonist induces long-term alterations in dopamine D2 receptors. Furthermore, the finding that these changes in receptor number require both repeated injections and a withdrawal time greater than 1 day suggests that these alterations are compensatory in nature.

Kappa-opioid receptor blockade with nor-binaltorphimine modulates cocaine self-administration in drug-naive rats

The modulation of the reinforcing effects of cocaine by the kappa-opioid receptor antagonist, nor-binaltorphimine was studied by using the initiation of intravenous self-administration in drug-naive Wistar rats. Treatment with nor-binaltorphimine (3.0 mg/kg s.c.) 48 h before the start of the first of five daily self-administration sessions significantly decreased the intake of cocaine when offered in a threshold unit dose (30 micrograms per infusion), but had no effect on cocaine intake when it was offered in a higher unit dose (60 micrograms per infusion). It is concluded that blockade of the kappa-opioid receptor by nor-binaltorphimine may produce a rightward shift of the unit dose-response relationship of cocaine reward, thus decreasing the sensitivity to cocaine reward. These data suggest an involvement of endogenous kappa-opioid systems in the mechanisms underlying the initiation of cocaine self-administration behaviour.

Effects of cyclazocine on cocaine self-administration in rats

Cyclazocine is a kappa-opioid receptor agonist and mu-opioid receptor antagonist that was studied in the 1960s as a potential treatment for heroin addicts. Based on the evidence that opioid mechanisms modulate the reinforcing effects of cocaine, it has been suggested that cyclazocine be reconsidered for use in treating cocaine dependence. In the present study, the effects of orally administered (+/-)-cyclazocine, (+)-cyclazocine and (-)-cyclazocine on intravenous cocaine self-administration were assessed in rats. (+/-)-Cyclazocine produced a dose-related (2-8 mg/kg) decrease in cocaine intake without affecting bar-press responding for water. Neither enantiomer significantly altered responding for either cocaine or water. The efficacy of orally administered (+/-)-cyclazocine on cocaine self-administration was comparable to that previously observed using the intraperitoneal route. Distinct actions of the enantiomers of cyclazocine that might contribute to the unique efficacy of the racemate are discussed. Although the mechanistic basis for the results are not entirely understood, the data suggest that (+/-)-cyclazocine should be considered as a potential treatment for cocaine dependence.

Opioid receptor gene expression in the rat brain during ontogeny, with special reference to the mesostriatal system: an in situ hybridization study

The three main types of opioid receptors micro, delta and kappa are found in the central nervous system and periphery. In situ hybridization study was undertaken to determine the expression of mu, delta, kappa-opioid receptors mRNAs in the brain during pre- and postnatal development, especially in the mesostriatal system. By G13, mu and kappa-opioid receptor mRNA were detectable in the telencephalon; mu-opioid receptor mRNA was found in the striatal neuroepithelium and cortical plate and kappa-opioid receptor mRNA in the corroidal fissure. By G15, kappa-opioid receptor mRNA was detectable in the nucleus accumbens and dorsal striatum, and in the substantia nigra and ventral tegmental area, suggesting an early expression of the corresponding receptor on dopaminergic terminal fibers. For the mu-opioid receptor mRNA in the striatum, patches appeared at G20. Delta-opioid receptor mRNA was first detected at G21, in many areas including the accumbens nucleus and the dorsal striatum. At P8, delta-opioid receptor mRNA was detected in large-sized cells of the striatum, possibly cholinergic, suggesting a possible modulation by opioids of the striatal cholinergic neurons. Our results demonstrate the early appearance of mu and kappa-opioid receptor mRNA (G13) and the relatively late development of delta-opioid receptor mRNA (G21) in the brain. We also show a distinct pattern of expression for mu, delta and kappa-opioid receptor mRNAs in the mesostriatal system during the development.

Rational treatment of addiction

Treatment of alcohol and drug addictions, which has been neglected medically for a long time, is currently sparked with optimism. Craving for alcohol can be treated with two newly registered drugs: naltrexone and acamprosate. New approaches to symptom relief during detoxification or during maintenance therapies are rationally based on experimental and clinical work. It is now clear that addictive drugs are surrogates of natural substances involved in the 'reward system'.

Mu and kappa opioid receptors in periaqueductal gray and rostral ventromedial medulla

The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are important brain stem pain modulating regions. Recent evidence suggests that kappa opioids antagonize the effects of mu opioids in the RVM. However, the anatomical relationship between mu and kappa opioid receptors in PAG and RVM is not well characterized. This study examined relationships between mu and kappa opioid receptor immunoreactivity (IR) and mRNA in PAG and RVM. Brain slices were processed for either immunocytochemistry or in situ hybridization. We found considerable anatomical overlap of mu and kappa opioid IR and mRNA in the RVM and PAG. These results provide an anatomical basis for recent behavioral and electrophysiological findings in RVM, and suggest modulatory interactions between mu and kappa opioids in PAG.

Differences in the kappa opioid receptor mRNA content in distinct brain regions of two inbred mice strains

The inbred C57BL/6 mouse strain is known to prefer an ethanol solution in a two-bottle choice procedure, whereas the inbred DBA/2 mouse strain avoids drinking ethanol in this paradigm. The genetic basis of this behavior is still unclear but the endogenous opioid system is one of the factors thought to be involved. Therefore, we were interested to see if there are basal differences between the two lines of mice in the kappa opioid receptor (kappa OR) mRNA content in different brain regions. Because of the low expression level of this gene and the limited amount of tissue we developed a sensitive competitive reverse transcription-polymerase chain reaction (RT-PCR) assay for evaluation of levels of kappa OR mRNA in brain tissue. In septum and hypothalamus the DBA/2 mice showed a significantly higher basal level of kappa OR mRNA than did C57BL/6 mice. It is suggested that a difference in basal amount of kappa OR mRNA among these strains could lead to differences in kappa OR activity and subsequently to variations between the strains in distinct behaviors such as in ethanol preference.

Intraaccumbens injections of substance P, morphine and amphetamine: effects on conditioned place preference and behavioral activity

The nucleus accumbens of the rat plays a critical role in behavioral activation and appetitive motivation. Within the nucleus accumbens, the shell subarea may be especially relevant, since this site is anatomically related to other brain areas that are considered to play a critical role in the processing of motivation. We investigated the behavioral effects of local drug treatments aimed at the shell of the nucleus accumbens and tested the indirect dopamine agonist d-amphetamine, the opiate agonist morphine, and the neurokinin substance P. These substances are known to exert positive reinforcing effects, and can affect behavioral activity; effects that are physiologically closely related to the nucleus accumbens and its inputs and outputs. Our results show that unilateral microinjections of amphetamine (1.0 microg, 10.0 microg) into the shell of the nucleus accumbens dose-dependently stimulated behavioral activity (locomotion, rears, sniffing), and led to conditioned place preference. Furthermore, the effect of amphetamine on place preference was negatively related to the psychomotor stimulant action on rears. Morphine injections (5.0 microg) also stimulated behavioral activity and elicited contraversive turning, but were ineffective with respect to place preference. Finally, the neuropeptide substance P, injected in a dose range of 0.1-10.0 ng, had no significant behavioral effects. These findings are discussed with respect to the role of dopaminergic, peptidergic and cholinergic mechanisms in the nucleus accumbens. It is suggested that dopamine, opiates, and neurokinins in the shell of the nucleus accumbens are differentially involved in mediating behavioral activity and appetitive motivation.

Comparison of mesocorticolimbic neuronal responses during cocaine and heroin self-administration in freely moving rats

To compare neuronal activity within the mesocorticolimbic circuit during the self-administration of cocaine and heroin, multiple-channel single-unit recordings of spike activity within the rat medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) were obtained during the consecutive self-administration of cocaine and heroin within the same session. The variety of neuronal responses observed before the lever press are termed anticipatory responses, and those observed after the lever press are called post-drug infusion responses. For the total of the 110 mPFC and 111 NAc neurons recorded, 30-50% of neurons, depending on the individual sessions, had no alteration in spike activity in relation to either cocaine or heroin self-administration. Among the neurons exhibiting significant neuronal responses during a self-administration session, only a small portion (16-25%) of neurons responded similarly under both reinforcement conditions; the majority of neurons (75-84%) responded differently to cocaine and heroin self-administration as revealed by variations in both anticipatory and/or post-drug infusion responses. A detailed video analysis of specific movements to obtain the self-administration of both drugs provided evidence against the possibility that locomotive differences contributed to the observed differences in anticipatory responses. The overall mean activity of neurons recorded in mPFC and NAc measured across the duration of the session segment for either cocaine or heroin self-administration also was different for some neurons under the two reinforcement conditions. This study provides direct evidence that, in mPFC and NAc, heterogeneous neuronal circuits mediate cocaine and heroin self-administration and that distinct, but overlapping, subpopulations of neurons in these areas become active during operant responding for different reinforcers.

Comparison of mesocorticolimbic neuronal responses during cocaine and heroin self-administration in freely moving rats

To compare neuronal activity within the mesocorticolimbic circuit during the self-administration of cocaine and heroin, multiple-channel single-unit recordings of spike activity within the rat medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) were obtained during the consecutive self-administration of cocaine and heroin within the same session. The variety of neuronal responses observed before the lever press are termed anticipatory responses, and those observed after the lever press are called post-drug infusion responses. For the total of the 110 mPFC and 111 NAc neurons recorded, 30-50% of neurons, depending on the individual sessions, had no alteration in spike activity in relation to either cocaine or heroin self-administration. Among the neurons exhibiting significant neuronal responses during a self-administration session, only a small portion (16-25%) of neurons responded similarly under both reinforcement conditions; the majority of neurons (75-84%) responded differently to cocaine and heroin self-administration as revealed by variations in both anticipatory and/or post-drug infusion responses. A detailed video analysis of specific movements to obtain the self-administration of both drugs provided evidence against the possibility that locomotive differences contributed to the observed differences in anticipatory responses. The overall mean activity of neurons recorded in mPFC and NAc measured across the duration of the session segment for either cocaine or heroin self-administration also was different for some neurons under the two reinforcement conditions. This study provides direct evidence that, in mPFC and NAc, heterogeneous neuronal circuits mediate cocaine and heroin self-administration and that distinct, but overlapping, subpopulations of neurons in these areas become active during operant responding for different reinforcers.

Sensitization to the conditioned rewarding effects of morphine and cocaine: differential effects of the kappa-opioid receptor agonist U69593

The ability of the kappa-opioid receptor agonist U69593 to attenuate the sensitization and cross-sensitization which develops to the conditioned rewarding effects of morphine and cocaine was examined using an unbiased place-preference conditioning procedure. The influence of U69593 treatment upon sensitization and cross-sensitization to cocaine was also assessed. Doses of morphine (1.0-5.0 mg kg(-1)) which failed to produce a conditioned response in drug-naive rats produced marked preferences for the drug-paired place in animals which had previously received once daily injections of morphine (5.0 mg kg(-1); s.c.) or cocaine (10.0 mg kg(-1); i.p.) for 5 days. Morphine-induced place preferences also occurred in animals which had received morphine in combination with U69593 (0.04-0.32 mg kg(-1); s.c.) on either days 3-5 or 1-5 of the morphine treatment regimen. In contrast, morphine failed to produce significant conditioning in animals which had received U69593 with cocaine for 5 days. Doses of cocaine (1.0-5.0 mg kg(-1)) which did not produce a conditioned response in naive rats produced preferences for the drug-paired place in animals which had received once daily injections of cocaine (10.0 mg kg(-1) day(-1) x 5 days; i.p.) or morphine (5.0 mg kg(-1) day(-1) x 5 days; s.c.). No enhancement of cocaine-induced conditioning occurred in animals which had received U69593 on days 3-5 or on days 1-5 of the five-day cocaine treatment. In animals, however, which had received U69593 with morphine for 5 days, an enhanced response to cocaine was still seen. These findings confirm that sensitization and cross-sensitization develop to the conditioned rewarding effects of cocaine and morphine. They also indicate that the ability of a kappa-opioid receptor agonist to prevent the development of these sensitized responses depends on the sensitizing agent employed. U69593 prevents sensitization and cross-sensitization induced by cocaine, but does not modify morphine-induced sensitization or the cross-sensitization which develops to cocaine after morphine administration.

Opioid reward mechanisms: a key role in drug abuse?

There is increasing evidence to implicate the mesolimbic dopamine system in the rewarding effects of drugs of abuse such as opioids, psychostimulants, and alcohol, and in addition endogenous opioids may play a key role in the underlying adaptive mechanisms. Opioid agonists with affinity for µ and delta opioid receptors are rewarding, whereas opioid agonists with affinity for kappa receptors are aversive. These opposing motivational effects are paralleled by an increase and decrease, respectively, of dopamine release in the nucleus accumbens. Opposite effects are induced in response to selective antagonists for these different receptor types, pointing to tonically active endogenous opioid reward mechanisms. Withdrawal from chronic morphine results in sensitization for opioid reward; an effect that is counteracted by kappa opioid agonists. The rewarding effects of psychostimulants such as cocaine and amphetamine, mediated by the mesolimbic dopamine pathway, are modulated by opioid mechanisms in both directions: sensitization by morphine pretreatment, inhibition by kappa receptor agonists. A modulatory role of endogenous opioids is also suggested from biochemical data, showing increased dynorphin and kappa receptor expression after chronic cocaine treatment. Alcohol reward involves the mesolimbic reward system also, and opioids modulate this behaviour. Naltrexone as well as selective µ and delta opioid receptor antagonists decrease alcohol consumption in operant conditioning models. Biochemical approaches point to a functional deficit of endogenous opioids in genetic models exhibiting high prevalence for alcohol intake. The therapeutic implications of these data are discussed.Key words: reward mechanisms, endogenouos opioid systems, psychostimulants, alcohol.