Effects of SR 48692, a selective non-peptide neurotensin receptor antagonist,on two dopamine-dependent behavioural responses in mice and rats

One major mechanism underlying the central action of neurotensin is an interaction with the function of dopamine (DA)-containing neurons. In addition, direct or indirect DA agonists have been reported to promote neurotensin release. We have found that SR 48692, a non-peptide neurotensin receptor antagonist (0.04 – 0.64 mg/kg orally), antagonizes (50–65%) yawning induced by apomorphine (0.07 mg/kg SC) or bromocriptine (2 mg/kg IP) in rats, and turning behaviour induced by intrastriatal injection of apomorphine (0.25 µg), (+) SKF 38393 (0.1 µg), bromocriptine (0.01 ng) or (+) amphetamine (10 µg) in mice. Other apomorphine-induced effects in mice and rats such as climbing, hypothermia, hypo- and hyper-locomotion, penile erections and stereotypies were not significantly modified by SR 48692. Taken together, these data suggest that neurotensin may play a permissive role in the expression of some but not all behavioural responses to DA receptor stimulation.     

The nonpeptide neurotensin antagonist, SR 48692, used as a tool to reveal putative neurotensin receptor subtypes.

The nonpeptide neurotensin (NT) antagonist, SR 48692, was recently shown to inhibit NT binding to the cloned rat and human NT receptor and to antagonize NT effects in a variety of in vitro and in vivo assays. Here, we show that, in contrast to its antagonistic action on NT-induced hypomotility in the rat, SR 48692 failed to antagonize NT-induced hypothermia and analgesia in the mouse and rat. We suggest that these effects might be mediated through a subtype of SR 48692-insensitive NT receptor.     

SR 48692, a specific neurotensin receptor antagonist, has no effect on oesophageal motility in humans

BACKGROUND: The administration of exogenous neurotensin can reduce the lower oesophageal sphincter pressure, but it is unclear whether this effect is pharmacological or physiological. AIM: A specific neurotensin receptor antagonist (SR 48692) was used to assess the effect of endogenous neurotensin on lower oesophageal sphincter function. METHODS: Twenty-four healthy male subjects were included in a double-blind, placebo-controlled, randomized, cross-over study designed to determine the effects of two single doses (90 and 300 mg, preceded by a loading dose) of SR 48692 on the resting lower oesophageal sphincter pressure, transient lower oesophageal sphincter relaxations, primary oesophageal peristalsis and oesophageal acid exposure. Oesophageal pH and motility recordings were performed during 1 h of fasting and 3 h post-prandially. Plasma neurotensin-like immunoreactivity release was determined by radioimmunoassay. RESULTS: During fasting, the lower oesophageal sphincter pressure, transient lower oesophageal sphincter relaxation rate and reflux episodes were similar with the two doses of SR 48692 and placebo. Meal ingestion induced a rise in plasma neurotensin-like immunoreactivity, a decrease in lower oesophageal sphincter pressure and an increase in both the transient lower oesophageal sphincter relaxation rate and the number of reflux episodes, which were not significantly modified by SR 48692. SR 48692 did not affect oesophageal primary peristalsis. CONCLUSION: This study shows that SR 48692, a specific neurotensin 1 receptor antagonist, has no effect on oesophageal motility in humans.     

Blockade of mast cell histamine secretion in response to neurotensin by SR 48692, a nonpeptide antagonist of the neurotensin brain receptor.

1. Pretreatment of rat isolated mast cells with SR 48692, a nonpeptide antagonist of the neurotensin (NT) receptor, prevented histamine secretion in response to NT. 2. This inhibition was rapid in onset (approximately 1 min) and dependent upon the concentration of SR 48692 (IC50 approximately 1-10 nM). 3. SR 48692 (1-1000 nM) did not inhibit histamine secretion elicited by substance P, bradykinin or compound 48/80, or by anti-IgE stimulation of sensitized mast cells. 4. When SR 48692 was injected intradermally (5 pmol in 50 microliters) into anaesthetized rats, 15 min before the intradermal injection of NT, it reduced the effect of NT on vascular permeability. 5. When injected intravenously, SR 48692 attenuated the effects of NT on haematocrit and blood stasis. 6. These results demonstrate that SR 48692 selectively antagonizes the actions of NT on rat isolated mast cells as well as mast cells in vivo. Given the demonstrated specific interaction of SR 48692 with receptors for NT in brain, our results suggest the presence of specific NT receptors on mast cells.     

Antagonism by SR 48692 of mechanical responses to neurotensin in rat intestine.

1. The effects of SR 48692 on neurotensin (NT)-induced mechanical responses were investigated in rat duodenum and proximal colon by use of isometric, isovolumic preparations. 2. SR 48692 inhibited the relaxant responses to NT in duodenal circular and longitudinal muscle. It also antagonized the NT-induced contractile effects in duodenal circular muscle and in proximal colon (both muscular layers). 3. From Schild analysis and pA2 value for SR 48692 was 8.2 in tissues where NT induced relaxant effects and 7.5 in tissues where NT induced contractile effects and the slope of the regression line was not significantly different from unity, indicating competitive antagonism. 4. SR 48692 did not antagonize the duodenal relaxant effect induced by noradrenaline and the contractile response to carbachol or substance P in duodenum and colon. 5. Our results demonstrate that SR 48692 selectively antagonizes the mechanical actions of NT in rat intestine and confirm the existence of specific NT receptors. Receptors that subserve a relaxant effect seem to be related, but not identical, to those that mediate contractile effects.     

Effects of the selective neurotensin antagonist SR 142948A on 3,4-methylenedioxymethamphetamine-induced behaviours in mice

Neurotensin is one of the genes previously found up-regulated in mice striatum after acute injection of MDMA (9 mg/kg). In order to examine the pharmacological significance of this effect, the involvement of the neurotensinergic system in MDMA-induced behaviors was explored in mice using the neurotensin receptor antagonist SR142948A (1 mg/kg). We found that acute administration of the antagonist inhibited the MDMA-elicited locomotor activity. SR142948A pre-treatment had no effect on the acquisition of conditioned place preference (CPP) to MDMA but abolished the expression of this behavior. We also studied the effects of acute and repeated exposure to MDMA on the mRNA level of neurotensin in mice striatum. Kinetic analysis of the regulation 1, 2, 6 and 12 h after acute injection of MDMA showed that the drug transiently up-regulates neurotensin mRNA in this structure. The time course of the modulation suggests that the effects observed with SR142948A are attributable to the release of a preexisting endogenous pool rather than the newly synthesized peptide. Repeated exposure to MDMA following the same injection pattern used in the CPP paradigm revealed an increase in mRNA level of neurotensin in mice striatum. These results indicate that endogenous neurotensin plays a role in both the acute locomotor activity and the expression of CPP induced by MDMA.     

The CB(1) cannabinoid receptor antagonist AM251 attenuates amphetamine-induced behavioural sensitization while causing monoamine changes in nucleus accumbens and hippocampus

Endogenous cannabinoids modulate the activity of dopamine reward pathways and may play a role in the development of behavioural sensitization to psychostimulants. Here, we investigated the effects of the CB(1) cannabinoid receptor antagonist AM251 on amphetamine-induced locomotor sensitization in mice. Furthermore, we measured post-mortem monoamine concentrations in nucleus accumbens and hippocampus after termination of the behavioural tests. The results can be summarized as follows: Mice pre-treated with AM251 (3 mg/kg; i.p.) showed less sensitivity to the psychomotor stimulant as well as locomotor sensitizing effects of amphetamine (2 mg/kg; i.p.) resembling previous results obtained with CB(1) receptor-deficient animals. Furthermore, the behavioural effects of AM251 were paralleled by increased dopamine concentration in nucleus accumbens and increased serotonin concentration/turnover rate in hippocampus, respectively. The present data indicate that under normal conditions activation of the CB(1) receptor facilitates those adaptive responses elicited by repeated psychostimulant administration and resulting in sensitization, possibly by reducing dopamine biosynthesis and serotonin turnover in the nucleus accumbens and hippocampus.     

Promotion by SR 48692 of gastric emptying and defaecation in rats suggesting a role of endogenous neurotensin.

We investigated the influence of the nonpeptide neurotensin receptor antagonist, SR 48692, administered orally, on gastric emptying and on acute defaecation. SR 48692 dose-dependently (ED50 approximately 0.7 microgram kg-1) increased gastric emptying of a food suspension, but it had no effect on that of a non-caloric meal. SR 48692 also dose-dependently promoted defaecation and increased faecal water content. We suggest that antagonism of endogenous neurotensin accounts for the gastric emptying and defaecation promoting action of SR 48692.     

SR 141716A, a cannabinoid receptor antagonist, reverses the behavioural effects of anandamide-treated rats

We employed the CB1 cannabinoid receptor antagonist SR 141716A (3 mg/kg, i.p.) to investigate whether behavioural effects induced in rats by anandamide, an endogenous cannabinoid (20 mg/kg, i.p.), were mediated by the cannabinoid CB1 receptor. Anandamide reduced ambulatory (67%) and non-ambulatory activities (rearing and grooming, 84% and 90% respectively), with a strong cataleptic effect, produced hypothermia (about -1 degree C) and hindlimb splaying, and reduced defecation (79%). It did not significantly increase either the tail-flick or hot-plate latencies. Except for the decreased defecation, these responses were all blocked by SR 141716A. Although only single doses of the agonist and antagonist were used, the findings indicate that these behavioural effects are probably mediated by an interaction with cannabinoid CB1 receptors.     

The neurotensin receptor antagonist SR 142948A blocks the efflux of dopamine evoked in nucleus accumbens by neurotensin ejection into the ventral tegmental area

The neuropeptide neurotensin (NT) exerts a wide range of central and peripheral effects. In particular, ejection of NT (10(-7) M, 65 nl) into the ventral tegmental area (VTA) in anaesthetised rats pre-treated with pargyline increases the dopamine (DA) efflux within the nucleus accumbens (NAcc) as measured by differential pulse amperometry (DPA) combined with carbon fibre electrodes. However, this effect is not blocked by systemic pre-treatment with the potent and selective non-peptide NT receptor antagonists SR 48692 and SR 142948A, at any dose studied. The present study was designed to determine the ability of these NT receptor antagonists to block the increase in DA efflux evoked within the NAcc when they are locally applied with the peptide into the VTA. The competitive N-methyl- D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP-5), applied into the VTA 1 min before NMDA, blocked the effect of NMDA on DA efflux concentration and volume dependently, thus demonstrating the suitability of our experimental procedure for characterizing both an agonist and an antagonist specific for receptors present on mesencephalic dopaminergic neurons and involved in the regulation of DA efflux within the NAcc. Intra-VTA application of SR 142948A blocked the NT-evoked increase in DA efflux within the NAcc dose dependently whereas SR 48692, at the concentration used, was inactive. These results suggest that NT regulates mesencephalic dopaminergic activity through NT receptors sensitive to SR 142948A, but possibly not to SR 48692.     

Characterization of the effect of SR48692 on inositol monophosphate, cyclic GMP and cyclic AMP responses linked to neurotensin receptor activation in neuronal and non-neuronal cells.

1. Neurotensin stimulated inositol monophosphate (IP1) formation in both human colonic carcinoma HT29 cells and in mouse neuroblastoma N1E115 cells with EC50 values of 3.5 +/- 0.5 nM (n = 4) and 0.46 +/- 0.02 nM (n = 3), respectively. Neurotensin also stimulated cyclic GMP production with an EC50 of 0.47 +/- 1.2 nM and inhibited cyclic AMP accumulation induced by forskolin (0.5 microM) with an IC50 of 1.33 +/- 1.5 nM (n = 3) on the N1E115 cell line. 2. The competitive antagonism by the non-peptide neurotensin receptor antagonist, SR48692 of neurotensin-induced IP1 formation revealed pA2 values of 8.7 +/- 0.2 (n = 3) for HT29 and 10.1 +/- 0.2 (n = 3) for N1E115 cells. SR48692 also antagonized the cyclic GMP and cyclic AMP responses induced by neurotensin in the N1E115 cell line with pA2 values of 10.7 +/- 0.7 (n = 3) and 9.8 +/- 0.3 (n = 3), respectively. 3. In CHO cells transfected with the rat neurotensin receptor, neurotensin stimulated IP1 and cyclic AMP formation with EC50 values of 3.0 +/- 0.5 nM (n = 3) and 72.2 +/- 20.7 nM (n = 3), respectively. Both effects were antagonized by SR48692, giving pA2 values of 8.4 +/- 0.1 (n = 3) for IP1 and 7.2 +/- 0.4 (n = 3) for cyclic AMP responses. 4. Radioligand binding experiments, performed with [125I]-neurotensin (0.2 nM), yielded IC50 values of 15.3 nM (n = 2) and 20.4 nM (n = 2) for SR48692 versus neurotensin receptor binding sites labelled in HT29 and N1E115 cells, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

The 5-HT6 serotonin receptor antagonist SB-271046 attenuates the development and expression of nicotine-induced locomotor sensitisation in Wistar rats

5-HT₆ receptors are almost exclusively expressed in the central nervous system, particularly in areas relevant for addictive behaviour. Based on this, together with other data, this receptor may be a viable target for the control of drug abuse.The present study tested the ability of the 5-HT₆ receptor antagonist SB-271046 to attenuate the development and expression of nicotine-induced behavioural sensitisation.Rats were habituated to the test apparatus prior to experimentation (day 0) and locomotor activity recorded. On days 1 and 5, animals were placed in locomotor test apparatus and after 30 min injected with SB-271046 (1, 3, and 6 mg/kg, intraperitoneally IP) or vehicle. Thirty minutes later, nicotine (0.4 mg/kg, subcutaneously SC) or saline were administered and activity recorded for 60 min. On days 2, 3 and 4 treatments were performed in the home cage. After 17 days of withdrawal (day 23), a challenge test was performed with nicotine (0.4 mg/kg SC) or saline. In a separate experiment of similar design the effects of SB-271046 (1, 3, and 6 mg/kg IP) was tested for its ability to reduce the expression of behavioural sensitisation (day 23).SB-271046 dose dependently reduced the development and expression of nicotine sensitisation vs respective controls.In conclusion, the 5-HT₆ receptor antagonist SB-271046 reduced both the development and expression of nicotine sensitisation, suggesting that the 5-HT₆ receptor may be a viable target for the control of nicotine abuse. Further studies are warranted to substantiate this conclusion and further understand the role of 5-HT₆ receptors in addiction.     

Negative modulation of nitric oxide production by neurotensin as a putative mechanism of the diuretic action of SR 48692 in rats

1. We investigated the effect of the non-peptide neurotensin (NT) antagonist SR 48692 on renal function in rats and the involvement of nitric oxide (NO) in the diuretic action of this compound.
2. In fed animals, SR 48692 dose-dependently (0.5 to 12.5 mg kg⁻¹, p.o., 0.03 to 1 mg kg⁻¹, i.p. and 0.1 to 1 μg/rat, i.c.v.) increased urine output and urinary excretion of Na⁺, K⁺ and Cl⁻ and reduced urine osmolality. The diuretic activity was also evident in water-deprived, fasted animals and in fasted, water-loaded rats.
3. NT (0.1 μg/rat, i.c.v.) had no effect on urine output in fed rats, but reduced the diuretic action of SR 48692 (1 μg/rat, i.c.v.). The opposite result was obtained in fasted, water-loaded animals: NT dose-dependently (0.01 and 0.1 μg/rat, i.c.v.) inhibited diuresis and this effect was significantly inhibited by i.c.v. SR 48692. In this experimental condition, SR 48692 did not further increase the on-going diuresis.
4. The NO synthesis inhibitor N^ω-nitro-L-arginine methyl ester (L-NAME; 30 mg kg⁻¹, i.p.) alone had no effect on urine output in fed rats but prevented the diuretic action of i.c.v. or i.p. SR 48692; L-arginine (1 g kg⁻¹, i.p.) but not D-arginine (1 g kg⁻¹, i.p.) restored the SR 48692-dependent increase in diuresis. L-NAME had no effect on furosemide-stimulated diuresis.
5. Systemically administered L-NAME or i.c.v. NT in fasted, water-loaded rats significantly reduced water diuresis but this effect was no longer seen in animals given i.p. L-arginine. Rats receiving i.c.v. NT, whose diuresis was significantly reduced, also excreted less nitrates and nitrites in urine.
6. Increased diuresis after central or systemic administration of SR 48692 to fed rats was paralleled by increased urinary excretion of nitrates and nitrites, this being consistent with peripheral enhancement of NO production after NT-receptor blockade by SR 48692. The increase in diuresis after furosemide also involved an increase of nitrates and nitrites in urine, but this effect was about half that attained with an equipotent diuretic dose of SR 48692.
7. In fed rats, the NO donor isosorbide-dinitrate, reduced systolic blood pressure (unlike SR 48692 which did not affect blood pressure) but also dose-dependently (1 and 5 mg kg⁻¹, i.p.) stimulated urine output.
8. The overall effects of SR 48692 strongly support a link between the actions of endogenous NT, AVP and peripheral NO production in the modulation of renal excretion of water, Na⁺, K⁺ and Cl⁻.

     

The novel small-molecule antagonist of PAC1 receptor attenuates formalin-induced inflammatory pain behaviors in mice

We recently developed PA-8, a novel small-molecule antagonist of PACAP type 1 (PAC1) receptor. In the present study, we examined whether PA-8 was effective against formalin-induced inflammatory pain in mice. Both intrathecal and oral administration of PA-8 resulted in the dose-dependent attenuation of the second phase of formalin-induced nociceptive responses. PA-8 also inhibited c-fos upregulation in the ipsilateral dorsal horn of the spinal cord. The results suggested that PACAP-PAC1 receptor signaling system in the spinal cord were primarily involved in the transmission of inflammatory pain, and PA-8 could be useful for the development of novel analgesics for treating inflammatory pain.     

The mGlu5 receptor antagonist MTEP attenuates opiate self-administration and cue-induced opiate-seeking behaviour in mice

The mGlu5 receptor (mGluR5) has been implicated in the rewarding effect of various drugs of abuse and drug-seeking behaviour. In the present study we investigated the impact of antagonism of mGluR5 with the selective negative allosteric, modulator 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) on operant self-administration of morphine as well as cue-induced drug-seeking in adult CD1 mice. Administration of MTEP (20 mg/kg, i.p.) attenuated operant responding for morphine (0.1 mg/kg/infusion) and cue-induced morphine-seeking after a period of forced abstinence. Collectively, these data implicate mGluR5 in the reinforcing effects of opiates and support the proposition that mGluR5 is a potential therapeutic target for treatment of drug addiction.     

Effects of benzodiazepine receptor antagonist, flumazenil, on antinociceptive and behavioural responses to the elevated plus-maze in mice

Brief exposure to an elevated plus-maze has been shown to induce antinociception in male mice, a reaction that is not attenuated by manipulations of opiate receptors but which is fully blocked by diazepam. The present study examined the effects of the benzodiazepine receptor antagonist, flumazenil (5–20 mg/kg), on behavioural and antinociceptive responses to the elevated plus-maze in male DBA/2 mice. The results showed that, in the absence of an effect on total arm entries or rearing, flumazenil increased the time spent on the closed arms of the maze (an anxiogenic profile) and significantly enhanced antinociception induced by the elevated plus-maze. Data are discussed in relation to an “endogenous ligand theory” and it is concluded that the present findings are consistent with the proposed involvement of anxiety in at least certain forms of adaptive inhibition of pain.     

The cannabinoid CB1 receptor antagonist SR141716A attenuates the memory impairment produced by Δ9-tetrahydrocannabinol or anandamide

 The administration of Δ⁹-tetrahydrocannabinol (THC), the principle psychoactive ingredient in marijuana, or the endogenous cannabinoid anandamide, has been shown to impair recent memory. The purpose of the present investigation was to determine if the cannabinoid CB₁ receptor antagonist SR141716A could attenuate THC- or anandamide-induced memory impairment, and to assess the effects on memory of SR141716A alone. Memory was assessed in rats well-trained in a two-component instrumental discrimination task, consisting of a conditional discrimination, and a non-match-to-position to assess recent or working memory. SR141716A (0.0–2.0 mg/kg) had no effect on either the conditional discrimination or the non-match-to-position. However, SR141716A (0.0–2.0 mg/kg) attenuated the memory impairment produced by THC (2.0 or 4.0 mg/kg) as indexed by an enhancement of performance in the non-match-to-position. When administered to rats pretreated with anandamide (2.0 mg/kg), SR141716A (0.0–2.5 mg/kg) impaired performance in the conditional discrimination at the highest dose. This was interpreted as a deficit in some capacity unrelated to memory (e.g., motor impairment). However, lower doses of SR141716A (0.1 and 0.5 mg/kg) attenuated the anandamide-induced impairment of performance in the non-match-to-position without affecting the conditional discrimination. This is the first report that the memory impairment produced by anandamide can be attenuated by a cannabinoid antagonist; results suggest that anandamide-induced memory disruption is mediated by CB₁ receptors. Received: 25 June 1997 / Final version: 7 February 1998     

Minocycline attenuates hyperlocomotion and prepulse inhibition deficits in mice after administration of the NMDA receptor antagonist dizocilpine.

The present study was undertaken to examine whether the second generation antibiotic drug minocycline attenuates behavioral changes (eg, acute hyperlocomotion and prepulse inhibition (PPI) deficits) in mice after the administration of the N-methyl-D-aspartate (NMDA) receptor antagonist (+)-MK-801 (dizocilpine). Dizocilpine (0.1 mg/kg)-induced hyperlocomotion was significantly attenuated by pretreatment with minocycline (40 mg/kg). Furthermore, the PPI deficits after a single administration of dizocilpine (0.1 mg/kg) were attenuated by pretreatment with minocycline (10, 20, or 40 mg/kg), in a dose-dependent manner. Moreover, in vivo microdialysis study in the free-moving mice revealed that pretreatment with minocycline (40 mg/kg, i.p.) significantly attenuated the increase of extracellular dopamine (DA) levels in the frontal cortex and striatum after administration of dizocilpine (0.1 mg/kg), suggesting that the inhibition of dizocilpine-induced DA release by minocycline may, at least in part, be implicated in the mechanism of action of minocycline with respect to dizocilpine-induced behavioral changes in mice. These findings suggest that minocycline could attenuate behavioral changes in mice after the administration of the NMDA receptor antagonist dizocilpine. Therefore, it is possible that minocycline would be a potential therapeutic drug for schizophrenia.     

The actions of the cannabinoid receptor antagonist,SR 141716A,in the rat isolated mesenteric artery

1. The actions of the cannabinoid receptor antagonist, SR 141716A, were examined in rat isolated mesenteric arteries. At concentrations greater than 3 μM, it caused concentration-dependent, but endothelium-independent, relaxations of both methoxamine- and 60 mM KCl-precontracted vessels.
2. SR 141716A (at 10 μM, but not at 1 μM) inhibited contractions to Ca²⁺ in methoxamine-stimulated mesenteric arteries previously depleted of intracellular Ca²⁺ stores. Neither concentration affected the phasic contractions induced by methoxamine in the absence of extracellular Ca²⁺.
3. SR 141716A (10 μM) caused a 130 fold rightward shift in the concentration-response curve to levcromakalim, a K⁺ channel activator, but had no effect at 1 μM.
4. SR 141716A (10 μM) attenuated relaxations to NS 1619 (which activates large conductance, Ca²⁺-activated K⁺ channels; BK_Ca). The inhibitory effect of SR 141716A on NS 1619 was not significantly different from, and was not additive with, that caused by a selective BK_Ca inhibitor, iberiotoxin (100 nM). SR 141716A (1 μM) did not effect NS 1619 relaxation.
5. SR 141716A (10 μM) had no effect on relaxations to the nitric oxide donor S-nitroso-N-acetylpenicillamine, or relaxations to carbachol in the presence of 25 mM KCl.
6. The results show that, at concentrations of 10 μM and above, SR 141716A causes endothelium-independent vasorelaxation by inhibition of Ca²⁺ entry. It also inhibits relaxations mediated by K⁺ channel activation. This suggests that such concentrations of SR 141716A are not appropriate for investigation of cannabinoid receptor-dependent processes.

     

Naloxonazine, a specific mu-opioid receptor antagonist, attenuates the increment of locomotor activity induced by acute methamphetamine in mice

Our previous studies indicated that the endogenous opioid system, particularly the mu-opioid receptor, may be involved in the modulation of methamphetamine (METH)-induced increases in locomotor behavior in mice. This study investigates the effects of naloxonazine, a specific mu-opioid receptor antagonist, on the locomotor behavioral response and phosphorylation pattern of dopamine and cAMP-regulated phosphoprotein of Mr32 (DARPP-32) in striatal dopaminergic transmissions induced by acute administration of METH to mice. Mice were injected with a single dose of naloxonazine (20 mg/kg, i.p.) 60 min before injecting (i.p.) either saline or 1 mg/kg of METH. After a 2-h behavioral test, striatal tissues were collected to determine the protein levels of DARPP-32 and the phosphorylated form of DARPP-32 at the Thr34 and Thr75 sites. Results show that pretreatment with naloxonazine significantly attenuated the acute METH-induced increase in locomotor activity and phosphor-Thr75 DARPP-32 levels. Our data indicate that the mu-opioid receptor blockade reduces the acute METH-induced increase in locomotor activity. This effect may be related to the inhibition of DARPP-32 phosphorylation at the Thr75 site in the striatum of the mice.