Conditioned place preference to morphine in cannabinoid CB1 receptor knockout mice

Recent reports have suggested an involvement of the brain cannabinoid system in the morphine-reward pathway. To address this question we evaluated whether CB1 receptor knockout mice would show a conditioned place preference to morphine. CB1 receptor knockout mice developed a strong place preference to 4 and 8 mg/kg morphine, similar to that in wild-type Swiss-Webster mice. This data thus does not support a contribution of the brain cannabinoid system to morphine reward.     

Neuroprotective cannabinoid receptor antagonist SR141716A prevents downregulation of excitotoxic NMDA receptors in the ischemic penumbra

Whether cannabinoids act as neuroprotectants or, on the contrary, even worsen neuronal damage after cerebral ischemia is currently under discussion. We have previously shown that treatment with the cannabinoid (CB1) receptor antagonist SR141716A reduces infarct volume by ∼40% after experimental stroke. Since it is suggested that SR141716A may exert neuroprotection besides its cannabinoid receptor-blocking effect, we addressed the question whether SR141716A may act via modulation of postischemic ligand binding to excitatory NMDA and/or α-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA) receptors. For this purpose, rats (n = 12) were treated with either intravenous saline (control) or CB1 receptor antagonist SR141716A (1 mg/kg) 30 min after permanent middle cerebral artery occlusion. Five hours after ischemia, quantitative receptor autoradiography was performed using [³H]CP 55,940, [³H]MK-801, and [³H]AMPA for labeling of CB1, NMDA, and AMPA receptors, respectively. Ligand binding was analyzed within the infarct core, cortical penumbra, and corresponding areas of the contralateral hemisphere and compared to that of sham-operated rats (n = 5). Both in ischemic controls and SR141716A-treated rats [³H]CP 55,940 ligand binding was not specifically regulated in the cortical penumbra or contralateral cortex. Importantly, reduced infarct volumes in SR141716A-treated rats were associated with maintained [³H]MK-801 binding to excitotoxic NMDA receptors in the penumbra, compared to a decrease in the control group. In summary, our data suggest that SR141716A may possess additional intrinsic neuroprotective properties independent of receptor-coupled pathways or due to action as a partial agonist.     

7-Nitroindazole blocks conditioned place preference but not hyperactivity induced by morphine

The effects of 7-nitroindazole (7-NI), a neural nitric oxide synthase (nNOS) inhibitor, on spontaneous locomotor activity, morphine-induced hyperactivity, acquisition of place conditioning and morphine-induced conditioned place preference (CPP) were evaluated in male mice. In experiment 1, animals treated with 7-NI (25, 50 and 100 mg/kg), morphine (40 mg/kg) or morphine (40 mg/kg) plus 7-NI (25, 50 or 100 mg/kg) were placed in an actimeter for 3 h. In experiment 2, animals treated with the same drugs and doses were conditioned following an unbiased procedure. 7-NI did not affect the spontaneous locomotor activity or hyperactivity induced by morphine. However, the moderate and high doses of 7-NI produced conditioned place aversion (CPA) and the lowest dose blocked morphine-induced CPP. Our results suggest that nitric oxide is involved in the rewarding properties of morphine but not in its motor effects.     

Cannabinoid CB1 antagonists possess antiparkinsonian efficacy only in rats with very severe nigral lesion in experimental parkinsonism

We have observed that systemic administration of cannabinoid CB1 antagonists exerts antiparkinsonian effects in rats with very severe nigral lesion (>95% cell loss), but not in rats with less severe lesion (85-95% cell loss). Local injections into denervated striatum and corresponding globus pallidus reduced parkinsonian asymmetry. Infusions into lesioned substantia nigra enhanced motor asymmetries, but this effect was absent after very severe nigral lesion. At the striatal level, CB1 antagonists act enhancing dopamine D1 receptor function and reducing D2 receptor function. Striatal dopaminergic denervation did not affect cannabinoid CB1 receptor coupling to G proteins. These results suggest that (i) systemic administration of CB1 antagonists in rats with severe nigral degeneration is ineffective because striatopallidal-mediated motor effects are antagonized by nigra-mediated activity, and (ii) CB1 antagonists exert antiparkinsonian effects after very severe nigral degeneration because nigra-mediated inhibition disappears. CB1 receptor antagonists that lack psychoactive effects might be of therapeutic value in the control of very advanced stage of Parkinson's disease in humans.     

Cocaine, but not morphine, induces conditioned place preference and sensitization to locomotor responses in CB1 knockout mice

The involvement of cannabinoid CB1 receptors in morphine and cocaine motivational effects was investigated using CB1 knockout mice. For this purpose, we evaluated the rewarding effects in the place conditioning paradigm and the sensitization to the locomotor responses induced by these drugs. The hyperlocomotion induced by acute morphine administration (15 mg/kg, s.c.) was preserved, but the sensitization to this locomotor response induced by chronic morphine treatment was abolished in CB1 mutant mice. Morphine (5 mg/kg, s.c.) induced conditioned place preference in wild-type mice but failed to produce any response in knockout mice, indicating the inability of morphine to induce rewarding effects in the absence of CB1 cannabinoid receptors. When the aversive effects of morphine withdrawal were investigated using the place aversion paradigm, no differences between genotypes were observed. Acute cocaine (10 mg/kg, i.p.) induced hyperlocomotor responses in wild-type and knockout mice and a chronic cocaine treatment produced a similar sensitization to this response in both genotypes. In the conditioning place preference paradigm, cocaine (20 mg/kg, i.p.) produced rewarding responses in both wild-type and knockout mice. These results demonstrate that CB1 receptors are essential for adaptive responses produced by chronic morphine but not by chronic cocaine treatment.     

Long-term D1-dopamine receptor sensitization in neonatal 6-OHDA-lesioned rats is blocked by an NMDA antagonist

Repeated administration of the D₁-dopamine agonist SKF-38393 to adult rats having had dopaminergic neurons destroyed early in development results in an increasing enhancement of the behavioral response to SKF-38393 with each dose until a maximum is reached. This increased sensitivity lasts for at least 6 months. In the present study, this long-lasting change in behavioral responsiveness to repeated treatment with SKF-38393, referred to as D₁-dopamine receptor priming, was shown to be dose dependent with smaller doses requiring an increased number of administrations to produce a maximal response when compared to higher doses. In addition, priming occurred equally well when treatment intervals ranged from 1 day to 14 days. These latter data reinforced the view that activation of D₁-dopamine receptors results in a prolonged change in neural function. In subsequent experiments D₁-dopamine receptor priming was blocked by pretreatment with the NMDA-receptor antagonist MK-801. This antagonism of priming could not be attributed to a blockade of D₁-dopamine receptors by MK-801 or to the induction of interfering behaviors. Because an NMDA antagonist interfered with D₁-dopamine priming as it does with other long-term neural messages, a common requirement for these diverse forms of neuronal plasticity appears to involve activation of the NMDA receptor. This functional link between NMDA receptors and dopaminergic function and its relationship to neuronal palsticity could have relevance to the biochemical mechanism involved in learning and to symptons in central disorders during development that worsen over time, particularly those proposed to involved malfunctioning dopaminergic mechanisms.     

The effect of dopamine receptor blockade on the development of sensitization to the locomotor activating effects of amphetamine and morphine

The effect of dopamine (DA) receptor blockade on the development of sensitization to the locomotor activating effects of systemic amphetamine and intra-ventral tegmental area (intra-VTA) morphine was investigated. Rats were pretreated with the D-1 DA receptor antagonist, SCH-23390 (0.04 or 0.2 mg/kg, i.p.) or one of two D-2 DA receptor antagonists, pimozide (0.5 mg/kg, i.p.) and Ro 22-2586 (0.2 mg/kg, i.p.), prior to each of 5 exposures to the sensitizing drug. SCH-23390 blocked the development of sensitization to amphetamine but not to intra-VTA morphine. Pimozide had the opposite effect and Ro 22-2586 had no effect on the development of sensitization to either amphetamine or intra-VTA morphine. All 3 antagonists, at the doses tested, completely blocked the acute locomotor activating effects of these two drugs. Pretreatment in separate animals with low autoreceptor doses of sulpiride (25 mg/kg, i.p. with amphetamine and 10 mg/kg, i.p. with intra-VTA morphine) slightly potentiated the acute locomotor effect and produced a slight enhancement of the sensitized response to amphetamine and intra-VTA morphine. Pretreatment with a higher dose of sulpiride (50 mg/kg, i.p.) blocked the acute locomotor effect of intra-VTA morphine but had no effect on the development of sensitization to this drug. These results suggest that the mechanisms underlying the development of sensitization to the locomotor activating effects of amphetamine and intra-VTA morphine are different even though these may ultimately result in similar changes in the activity of mesencephalic DA neurons. Implications of these findings for the differential involvement of D-1 and D-2 DA receptors and for neural hypotheses of behavioral sensitization are discussed.     

Involvement of the cerebellar adenosine A1 receptor in cannabinoid-induced motor incoordination in the acute and tolerant state in mice

Cannabinoids are known to impair motor function in humans and laboratory animals. We have demonstrated an accentuation of cannabinoid (CP55,940)-induced motor incoordination in mice by the adenosine A₁ receptor-selective agonist N⁶-cyclohexyladenosine (CHA) (4 ng) using an intracerebellar (ICB) microinjection method. This effect was mediated by the A₁ receptor because pre-treatment with ICB 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (100 ng), an adenosine A₁ receptor selective antagonist, completely abolished the accentuation. Furthermore, ICB pre-treatment with DPCPX (100 ng) before ICB CP55,940 (15 μg) attenuated the motor incoordination suggesting a modulation by an endogenous adenosine A₁ system. ICB microinjection of CHA or DPCPX prior to ICB vehicle had no effect on normal motor coordination. ICB microinjection of dipyridamole (25 μg), an adenosine transport inhibitor, significantly accentuated the motor incoordination by ICB CP55,940 (15 μg), providing further support for the involvement of endogenous adenosine in the action of CP55,940. Tolerance to the motor incoordinating effect of ICB CP55,940 was demonstrated following 3 days of i.p. CP55,940 (0.1, 1 or 2 mg/kg every 12 or 24 h; total of six or three injections, respectively). Interestingly, animals which exhibited tolerance to ICB CP55,940 also demonstrated tolerance to the accentuating effect of ICB CHA suggesting cross-tolerance between adenosine agonists and cannabinoids. Cross-tolerance was also demonstrated following 3 days of i.p. CHA (0.25 or 1 mg/kg every 24 h; total of three injections) as further evidence of the modulatory role of the cerebellar adenosine system in the acute manifestation of CP55,940-induced motor incoordination. The involvement of cerebellar adenosine and the A₁ receptor in cannabinoid actions is circumstantially supported by previous evidence that CB₁ receptors and A₁ receptors are both localized on cerebellar granule cell parallel fiber terminals and basket cell neurons where they serve to inhibit the release of neurotransmitters.     

Effects of Δ-THC on VIP-induced prolactin secretion in anterior pituitary cultures: evidence for the presence of functional cannabinoid CB1 receptors in pituitary cells

Peripheral administration of cannabinoid CB₁ receptor agonists to laboratory rats induce a brief rise in plasma prolactin (PRL) levels followed by a prolonged decrease in PRL secretion from the pituitary. While the inhibitory component of this biphasic response depends on the cannabinoid-induced activation of dopamine release from hypothalamic terminals located in the median eminence, the neurobiological mechanisms underlying the activation phase of PRL release remains to be explained. In the present study the possible direct effect of the cannabinoid receptor agonist Δ⁹-Tetrahydrocannabinol (THC) on prolactin secretion and cAMP accumulation was examined in anterior pituitary cultures. THC (0.1 and 1 μM) increased cAMP levels, and induced PRL release (1 and 10 μ). THC did not affect vasoactive intestinal peptide (VIP, 0.5 μM) induced cAMP accumulation in pituitary cultures, showing additive effects at THC 1 μM concentration. However, THC did prevent VIP-dependent increases in prolactin secretion. These results indicate that THC, through a direct pituitary action, activates both the synthesis of cAMP and PRL release and interferes with intracellular mechanisms involved in PRL secretion by VIP. These actions could be mediated through cannabinoid CB₁ receptors which were found to be present in anterior pituitary cells, including lactotrophs, as revealed by immunocytochemistry with a specific polyclonal antibody raised against the CB₁ receptor protein.     

Morphine-induced place preference in the CXBK mouse: characteristics of μ opioid receptor subtypes

The role of μ opioid receptor subtypes, μ₁ and μ₂, in morphine-conditioned place preference was examined using ddY and μ₁ opioid receptor-deficient CXBK mice. In ddY mice, the μ receptor agonist morphine caused a dose-related preference for the drug-associated place, but the κ agonist U-50,488H produced a dose-related place aversion. These results demonstrated that the mouse is available for place preference conditioning using opioids. Under this condition, the influence of pretreatment with selective μ₁ opioid receptor antagonist naloxonazine morphine-induced place preference was investigated in ddY mice. Although pretreatment with the selective μ₁ antagonist naloxonazine (35 mg/kg, s.c.) did not modify the morphine-induced place preference, pretreatment with the selective μ antagonist β-funaltrexamine (β-FNA 10 mg/kg, s.c.) eliminated the appetitive effect of morphine. Furthermore, morphine (1–5 mg/kg, s.c.) produced a dose-related preference for the drug-associated place in CXBK mice. These findings suggest that the morphine-induced conditioned place preference may be mediated by naloxonazine-insensitive sites (μ₂ opioid receptors). In addition, chronic infusion of the dopamine D₁ antagonist SCH23390 (1.0 mg/kg/day) during the conditioning sessions eliminated the morphine-induced place preference in CXBK mice. Similarly, morphine combined with naloxonazine failed to produce the place preference in ddY mice chronically treated with SCH23390. The blocking effect of SCH23390 on the morphine-conditioned place preference suggests that μ₂ receptors may regulate the dopaminergic system, especially dopamine D₁ receptors, and are also involved in the reinforcing effects of morphine.     

Prenatal cannabis exposure increases heroin seeking with allostatic changes in limbic enkephalin systems in adulthood.

BACKGROUND: Prenatal cannabis exposure is a growing concern with little known about the long-term consequences on behavior and neural systems relevant for reward and emotional processing. METHODS: We used an animal model to study the effects of prenatal exposure to Delta(9)-tetrahydrocannabinol (THC) on heroin self-administration behavior and opioid neural systems in adult males (postnatal day 62). Rats were exposed to THC (.15 mg/kg) or vehicle from gestational day 5 to postnatal day 2. RESULTS: Both pretreatment groups showed similar heroin intake, but THC-exposed rats exhibited shorter latency to the first active lever press, responded more for low heroin doses, and had higher heroin-seeking during mild stress and drug extinction. THC exposure reduced preproenkephalin (PENK) mRNA expression in the nucleus accumbens during early development, but was elevated in adulthood; no adult striatal changes on preprodynorphin mRNA or PENK in caudate-putamen. PENK mRNA was also increased in the central and medial amygdala in adult THC-exposed animals. THC animals had reduced heroin-induced locomotor activity and nucleus accumbens mu opioid receptor coupling. CONCLUSIONS: This study demonstrates enduring effects of prenatal THC exposure into adulthood that is evident on heroin-seeking behavior during extinction and allostatic changes in mesocorticolimbic PENK systems relevant to drug motivation/reward and stress response.     

Involvement of dopamine D1 and D2 receptors in the ethanol-associated place preference in rats exposed to conditioned fear stress

The present study was designed to investigate: (1) the involvement of dopamine D₁ and D₂ receptors, and (2) the roles of these receptors and endogenous opioid systems (endorphinergic and enkephalinergic systems) in the ethanol-induced place preference in rats exposed to conditioned fear stress using the conditioned place preference paradigm. The administration of ethanol (300 mg/kg, i.p.) induced a significant place preference. The selective D₁ receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H3-benzazepine)hydrochloride (SCH23390; 0.01 and 0.03 mg/kg, s.c.) and the selective D₂ receptor antagonist S(−)-5-(aminosulfonyl)-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride; 20 and 40 mg/kg, s.c.) significantly attenuated the ethanol-induced place preference. The administration of ethanol (75 mg/kg, i.p.) tended to produce a place preference, but this effect was not significant. SCH23390 (0.03 mg/kg, s.c.) and sulpiride (40 mg/kg, s.c.) significantly attenuated the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by the μ-opioid receptor agonist morphine (0.1 mg/kg, s.c.). In addition, SCH23390 (0.03 mg/kg, s.c.) also significantly attenuated the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by the selective δ-opioid receptor agonist 2-methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydroquinolino[2,3,3,-g]isoquinoline (TAN-67; 20 mg/kg, s.c.). On the other hand, sulpiride (40 mg/kg) had no significant effect on the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by TAN-67. These results suggest that D₁ and D₂ receptors may be involved in the rewarding mechanism of ethanol under psychological stress. In addition, D₁ receptors may participate in the rewarding effect of ethanol modulated by the activation of μ- and δ-opioid receptors, whereas D₂ receptors may participate in the rewarding effect of ethanol modulated by the activation of μ-opioid receptors, but not in that modulated by the activation of δ-opioid receptors.     

Cannabinoid-induced Fos expression within A10 dopaminergic neurons

Mesocorticolimbic dopaminergic systems subserve cognitive processes, motivated behavior, the central stress response, and the reinforcing properties of drugs of abuse. Hyperdopaminergic states have been suggested to contribute to the psychotropic effects of the cannabinoids; however, the mechanisms by which cannabinoids activate mesocorticolimbic dopaminergic systems are not well understood. We have examined the role of noradrenergic neurotransmission in the mediation of cannabinoid-induced activation of A10 dopaminergic neurons using Fos as a marker of neuronal activation in mice. Administration of the CB(1) receptor agonist CP55940 differentially increased the number of Fos-like immunoreactive (Fos-li) A10 dopaminergic cells within three anatomically distinct regions (parabrachial pigmented, paranigral, and caudal linear nuclei) compared to vehicle-treated mice. Similar results were obtained using the CB(1) receptor agonist Win 55212-2; and pretreatment with the CB(1) receptor antagonist SR141716 significantly inhibited CP55940-induced Fos expression. Pretreatment with the alpha(1)-adrenergic receptor antagonist, prazosin, and the alpha(2)-adrenergic receptor agonist, clonidine, reduced the number of Fos-li dopaminergic neurons induced by CP55940 in a subregion-specific manner. CP55940 and Win 55212-2 increased the number of Fos-li neurons within the locus coeruleus. Finally, CB(1) receptor immunoreactivity was detected on fibers within the CL but not in either PBP or PN. Our data demonstrate that cannabinoids induce Fos expression within A10 dopaminergic neurons in a heterogeneous anatomical pattern, and suggest that enhanced noradrenergic neurotransmission contributes to cannabinoid-induced activation of A10 dopaminergic neurons in vivo.     

Involvement of GABAB receptor systems in action of antidepressants: baclofen but not bicuculline attenuates the effects of antidepressants on the forced swim test in rats

Involvement of GABAergic systems in action of antidepressants was examined in the forced swim test in rats. Rats were forced to swim in a cylinder for 15 min on day 1 and for 5 min on day 2. Desipramine, mianserin and buspirone, administered after the 15-min swim session on day 1 and before the 5-min swim test on day 2, dose-dependently decreased the duration of immobility in the swim test on day 2. Baclofen attenuated the decreased duration of immobility induced by desipramine, mianserin and buspirone in the swim test, although baclofen did not affect the duration of immobility when it was injected alone. Muscimol dose-dependently decreased the duration of immobility in the swim test on day 2. Bicuculline antagonized the decreased duration of immobility induced by muscimol. However, bicuculline failed to antagonize the decreased duration of immobility induced by desipramine, mianserin and buspirone. These results suggest that GABA_B but not GABA_A receptor systems may be involved in action of antidepressants.     

Loss of cannabinoid CB1 receptor expression in the 6-hydroxydopamine-induced nigrostriatal terminal lesion model of Parkinson's disease in the rat

The endocannabinoid system is emerging as a potential alternative to the dopaminergic system for the treatment of Parkinson's disease. Like all emerging targets, validation of this system's potential for treating human Parkinsonism necessitates testing in animal models of the condition. However, if components of the endocannabinoid system are altered by the induction of a Parkinsonian state in animal models, this could have an impact on the interpretation of such preclinical experiments. This study sought to determine if expression of the CB₁ subtype of cannabinoid receptor is altered in the two most commonly used rat models of Parkinson's disease. Parkinsonian lesions were induced by stereotaxic injection of 6-hydroxydopamine into the axons (medial forebrain bundle) or terminals (striatum) of the nigrostriatal pathway. On days 1, 3, 7, 14 and 28 post-lesion, rats were sacrificed and brains were processed for tyrosine hydroxylase and CB₁ receptor immunohistochemistry. The CB₁ receptor was expressed strongly in the substantia nigra pars reticulata, minimally overlapping with tyrosine hydroxylase immunoreactivity in the pars compacta. Interestingly, while there was little change in CB₁ receptor expression following axonal lesion, expression of the receptor was significantly reduced following terminal lesion. Loss of CB₁ receptor expression in the pars reticulata correlated significantly with the loss of striatal and nigral volume after terminal lesion indicating this may have been due to 6-hydroxydopamine-induced non-specific damage of striatonigral neurons which are known to express CB₁ receptors. Thus, this result has implications for the choice of model and interpretation of studies used to investigate potential cannabinoid-based therapies for Parkinson's disease as well as striatonigral diseases such as Huntington's disease and Multiple Systems Atrophy.     

The selective D3 receptor antagonist SB‐277011A attenuates morphine‐triggered reactivation of expression of cocaine‐induced conditioned place preference

We examined the effect of acute administration of the selective D₃ receptor antagonist SB‐277011A on morphine‐triggered reactivation of cocaine‐induced conditioned place preference (CPP) in adult male Sprague‐Dawley rats. Repeated pairing of animals with 15 mg/kg i.p. of cocaine HCl or vehicle to cue‐specific CPP chambers produced a significant CPP response compared to animals paired only with vehicle in both chambers. Expression of the CPP response to cocaine was then extinguished by repeatedly giving the animals vehicle injections in the cocaine‐paired chambers. The magnitude of the CPP response after extinction was not significantly different from that of animals paired only with vehicle. Expression of the extinguished CPP response was reactivated by acute administration of 5 mg/kg i.p. of morphine but not by vehicle. Acute administration of 6 or 12 mg/kg i.p. (but not 3 mg/kg) of SB‐277011A significantly attenuated morphine‐triggered reactivation of the cocaine‐induced CPP. SB‐277011A itself (12 mg/kg i.p.) did not reactivate the extinguished CPP response. Overall, SB‐277011A decreases the incentive motivational actions of morphine. The present findings suggest that central D₃ dopamine receptors are involved in relapse to cocaine‐seeking behavior, that a final common neural mechanism exists to mediate the incentive motivational effects of psychostimulants and opiates, and that selective dopamine D₃ receptor antagonists constitute promising compounds for treating addiction. Synapse 67:469–475, 2013. © 2013 Wiley Periodicals, Inc.     

Increase of cannabinoid CB1 receptor density in the hippocampus of streptozotocin-induced diabetic rats

In the hippocampus, impaired neurophysiology, compromised neurogenesis, and eventually apoptosis accompany cognitive deficits in insulinopenic (type-1) diabetes (T1D). The underlying pathological mechanisms remain to be defined. The hippocampus has a high density of the cannabinoid type 1 receptor (CB(1)R), which has been shown to control several brain functions affected by diabetes, such as synaptic plasticity, glucose utilisation, memory consolidation and cognition, and maturation and survival of hippocampal neurons. However, the role of this receptor has not been investigated yet in diabetic encephalopathy. We report now that in the streptozotocin animal model of T1D, the hippocampal densities of CB(1)R protein and of specific CB(1)R binding sites are significantly increased both in the nerve terminals and in total membranes (changes between 13% and 38%), whereas CB(1)R mRNA expression is decreased by 25%, suggesting that CB(1)Rs might play a role in diabetic encephalopathy.     

Sensitization occurs to the locomotor effects of morphine and the specific μ opioid receptor agonist, DAGO, administered repeatedly to the ventral tegmental area but not to the nucleus accumbens

Several recent reports have demonstrated that opiate action in both the ventral tegmental area (VTA) and the nucleus accumbens (N.Acc.) produces an increase in locomotor activity. In the present experiments, the effect of repeated bilateral injections into these sites of either morphine or the mu opioid receptor agonist Tyr-d-Ala-Gly-NMe-Phe-Gly-ol (DAGO) was investigated. As previously reported with morphine and other opioids, repeated injections of either morphine or DAGO into the VTA produced a progressive enhancement or sensitization of their locomotor activating effects. On the other hand, although both substances injected into the N.Acc. elicited increased locomotion, repeated injections did not lead to sensitization. It has been suggested that the increased locomotor activity produced by opiate injection into the VTA is dopamine-dependent while that produced by intra-N.Acc. injections is not¹⁵. The present findings provide neuroanatomical support for the view thatsensitization to the locomotor activating effects of opiates and opioids brought about by repeated drug exposure involves the mesolimbic dopamine system.