永井 拓

We have previously demonstrated that tissue plasminogen activator (tPA) plays an important role through the conversion of plasminogen to plasmin in the release of dopamine in the nucleus accumbens (NAc) evoked by depolarization or the systemic administration of drugs of abuse such as morphine and nicotine. In the present study, we examined the mechanisms by which drugs of abuse increase extracellular tPA activity in the NAc in vivo using in situ zymography. The dopamine D(1) receptor (D(1)R) agonist SKF38393, but not D(2) receptor agonist quinpirole, significantly increased extracellular tPA activity in the NAc. The effect of SKF38393 was blocked by pre-treatment with the dopamine D(1)R antagonist SCH23390. Microinjection of Rp-cAMPs, a protein kinase A inhibitor, into the NAc completely blocked the effect of SKF38393. Systemic administration of morphine and methamphetamine increased extracellular tPA activity in the NAc, and these effects were completely blocked by pre-treatment with SCH23390 and raclopride. The results suggest that activation of post-synaptic dopamine D(1)Rs in the NAc leads to an increase in extracellular tPA activity via protein kinase A signaling. Furthermore, dopamine D(2) receptors are also involved in the release of tPA induced by morphine and methamphetamine.

We have recently found that a combination of ovariectomy (OVX) and chronic restraint stress causes cognitive dysfunction and reduces hippocampal CA3 neurons in female rats and that estrogen replacement suppresses the OVX/stress-induced behavioral and morphological changes. In this study, we examined the effect of Ginkgo biloba extract (EGb 761), a popular herbal supplement, on the cognitive dysfunction and neuromorphological change in OVX/stress-subjected rats. Female Fisher 344 rats were randomly divided into three groups: vehicle-treated OVX, EGb 761 (50 mg/kg)-treated OVX and vehicle-treated sham-operated control groups. Two months after ovariectomy, all animals received restraint stress for 21 days (6 h/day), and were then subjected to a novel object recognition test followed by morphological examination by Nissl staining. EGb 761 was orally administered once daily until the behavioral analysis was done. Treatment with EGb 761 improved memory impairment and neuronal loss of hippocampus in the OVX/stress-subjected group in the same ways as 17 beta-estradiol. On the other hand, EGb 761 did not affect the loss of bone mineral density and increase in body weight after OVX, although 17 beta-estradiol attenuated them. These results have important implications for neuroprotective and cognition enhancing effects of EGb 761 in postmenopausal women and suggest that the effects are mediated by a different mechanism from estrogen. (C) 2007 IBRO. Published by Elsevier Ltd. All rights reserved.

Rationale Although chronic use of methamphetamine (METH) leads to long-lasting cognitive dysfunction in humans, there are few reports about an animal model that reflects METH-induced impairment of working memory. Objective In this study, we investigated the effect of repeated METH treatment on spatial working memory in rats. Materials and methods Rats were repeatedly administered METH (2 mg/kg) once a day for 7 days, and their memory function was assessed with a delayed spatial win-shift task in a radial arm maze. The task consisted of two phases, a training phase and a test phase, separated by a delay. Results METH-treated animals showed an impairment of performance in the test phase when the delay time was increased from 5 to 30 min or longer. The effect of METH persisted for at least 14 days after the drug withdrawal. METH-induced impairment of working memory was reversed by clozapine (3 and 10 mg/kg, for 7 days), but not haloperidol (1 and 2 mg/kg, for 7 days). The improving effect of clozapine diminished 7 days after the withdrawal. Phosphorylated extracellular signal-regulated kinase1/2 (ERK1/2) levels were significantly increased in the hippocampus of saline-treated control rats from 5 to 60 min after the training phase. In contrast, hyperphosphorylation of ERK1/2 was abolished in the hippocampus of rats treated with METH. Conclusions These findings suggest that repeated METH treatment induces impairment of working memory, which is associated with a dysfunctional ERK1/2 pathway in the hippocampus. Furthermore, clozapine may be effective for the treatment of METH-induced cognitive dysfunction.

Genetic factors are considered to play an important role in drug dependence/addiction including the development of drug dependence and relapse. With the use of a model of drug self-administration in mutant mice, several specific genes and proteins have been identified as potentially important in the development of drug dependence. In contrast, little is known about the role of specific genes in enduring vulnerability to relapse, a clinical hallmark of drug addiction. Using a mouse model of reinstatement, which models relapse of drug-seeking behavior in addicts, we provide evidence that a partial reduction in the expression of the glial cell line-derived neurotrophic factor (GDNF) potentiates methamphetamine (METH) self-administration, enhances motivation to take METH, increases vulnerability to drug-primed reinstatement, and prolongs cue-induced reinstatement of extinguished METH-seeking behavior. In contrast, there was no significant difference in novelty responses, METH-stimulated hyperlocomotion and locomotor sensitization, food-reinforced operant behavior and motivation, or reinstatement of food-seeking behavior between GDNF heterozygous knockout mice and wild-type littermates. These findings suggest that GDNF may be associated with enduring vulnerability to reinstatement of METH-seeking behavior and a potential target in the development of therapies to control relapse.

Drug addiction places an enormous burden on society through its repercussions on crime rate and healthcare. Repeated exposure to drugs of abuse causes cellular adaptations in specific neuronal populations that ultimately can lead to a state of addiction. In the present study, we have identified a novel molecule "shati" from the nucleus accumbens (NAc) of mice treated with methamphetamine ( METH) using the PCR-select complementary DNA subtraction method. Moreover, we investigated whether shati is involved in METH-induced hyperlocomotion, sensitization, and conditioned place preference (CPP). METH induced expression of shati mRNA dose dependently via dopamine (DA) receptors. We prepared antibodies against shati and, using them, found shati to be expressed in neuronal cells of the mouse brain. Treatment with the shati antisense oligonucleotide (shati-AS), which significantly inhibited the expression of shati mRNA, enhanced the acute METH response, METH-induced behavioral sensitization, and CPP. Blockage of shati mRNA by shati-AS potentiated the METH-induced increase of DA overflow in the NAc and the METH-induced decrease in synaptosomal and vesicular DA uptake in the midbrain. These results suggest that a novel molecule shati is involved in the development of METH-induced hyperlocomotion, sensitization, and CPP. The functional roles of shati in METH-regulated behavioral alternations are likely to be mediated by its inhibitory effects on the METH-induced increase of DA overflow in the NAc and the METH-induced decrease in DA uptake in the midbrain.

Background: Prepulse inhibition (PPI) of the startle response is a measure of the inhibitory function and time-linked information processing by which a weak sensory stimulus (the prepulse) inhibits the startle response caused by a sudden intense stimulus. We attempted to clarify the neuronal circuits underlying the control of PPI of the startle reflex in mice. Methods: c-Fos immunohistochemistry was used to detect neurons activated by startle pulse and/or prepulse trials. Behavioural pharmacology and tracing studies were also conducted. Results: The lateral globus pallidus (LGP) was activated by prepulses. Activation of the caudal pontine reticular nucleus (PnC) evoked by the startle pulses was inhibited under PPI conditions. Double-immunostaining revealed that c-Fos-positive cells in the LGP following prepulse trials were GABAergic neurons. Bilateral microinjections of lidocaine into the LGP resulted in an impairment of PPI. Fluoro-gold infusion into the PnC and the pedunculopontine tegmental nucleus (PPTg) retrogradely labeled neurons in the PPTg and LGP, respectively. Microinjections of phaclofen into the PPTg significantly impaired PPI. Conclusions: These results suggest that GABAergic neurons in the LGP which project to the PPTg play a crucial role through the activation of GABA(B) receptors in the regulation of PPI of the startle reflex in mice.

We have previously demonstrated that tissue plasminogen activator (tPA)-plasmin system participates in the rewarding effect of morphine, by regulating dopamine release in the nucleus accumbens (NAc). However, it is unclear how plasmin increases the morphine-induced release of dopamine and hyperlocomotion. In the present study we investigated whether protease activated receptor-1 (PAR-1) is involved in the regulation of acute morphine-induced dopamine release by the tPA-plasmin system. Morphine significantly but transiently increased extracellular tPA activity in the NAc, which was completely blocked by naloxone. Microinjection of a PAR-1 antagonist, (tyr(-1))-thrombin receptor activating peptide 7, into the NAc significantly reduced morphine-induced dopamine release in the NAc and hyperlocomotion although the treatment had no effect on basal dopamine release and spontaneous locomotor activity. Furthermore, the PAR-1 antagonist blocked the ameliorating effect of plasmin on the defect of morphine-induced dopamine release in the NAc of tPA-deficient mice. In contrast, intracerebroventricular injection of the PAR-1 antagonist had no effect on the antinociceptive effects of morphine in mice. These results suggest that PAR-1 is a target for the tPA-plasmin system in the regulation of acute morphine-induced dopamine release in the NAc.

Tissue plasminogen activator (tPA) plays a key role in neuroplasticity. We have recently demonstrated that the tPA-plasmin system is involved in the rewarding effects of drugs of abuse by regulating the release of dopamine in the nucleus accumbens. In the present study, we investigated whether tPA is involved in the reinforcing properties of morphine in a paradigm of drug self-administration. Eight-week-old tPA knockout and wild-type control mice were subjected to a single 24-h session of morphine self-administration under a fixed ratio (FIR) 2 or a progressive ratio (PR) schedule of reinforcement after eight daily 30-min sessions of nose-poke training. tPA knockout mice responded significantly more often for morphine self-administration in a dose-dependent manner as compared with wild-type control mice. Under the PR schedule of morphine reinforcement, however, tPA knockout mice showed a lower breaking point than wildtype control mice. There was no significant difference in food-reinforced operant behavior, breaking points to food pellets, and saline self-administration between the two genotypes. The increased responding in tPA knockout mice under the FR2 schedule was significantly attenuated by the dopamine D1 receptor antagonist SCH23390 (0.3 mg/kg), whereas SCH23390, at a dose range of 0.03-2.0 mg/kg, demonstrated biphasic effects on morphine self-administration in wild-type control mice. Our findings suggest that the reinforcing effects of morphine are reduced in tPA knockout mice. Modulation of the tPA system in the brain may be a potential target against drugs of abuse. (c) 2007 IBRO. Published by Elsevier Ltd. All rights reserved.

Several lines of evidence suggest that hormonal changes after menopause may play an important role in the incidence of cognitive dysfunction, and also in the development of Alzheimer's disease. In this study, we investigated the effect of estrogen on cognitive function in rats under different stress environment. Female rats were divided into four groups: two groups were ovariectomized (OVX) and two were sham-operated. One group each of OVX and sham rats was kept in a normal environment, and the other groups were assigned to a daily restraint stress (6 h/day) for 21 days from 2 months after the operation. Following the stress period, subjects were tested for performance in novel object recognition test and then used for morphological and neurochemical analyses. The OVX plus stress (OVX/stress) group showed a significant impairment of recognition of novel objects, compared with the other groups. The OVX/stress group also showed a marked decrease in the number of pyramidal cells of the CA3 region and levels of brain-derived neurotrophic factor mRNA in the hippocampus. We further examined the effect of estrogen against cognitive dysfunction and hippocampal changes of OVX/stress rats. Vehicle or 17 beta-estradiol (E2) at 20 mu g/day was s.c. administered to OVX/stress rats from 2 days before the stress period to the end of behavioral analysis through an implantable osmotic pump. Chronic E2 treatment decreased stress response and improved the cognitive and morphological impairments relative to vehicle group. These data have important implications for cognition enhancing effect of estrogen treatment in postmenopausal women. (c) 2007 IBRO. Published by Elsevier Ltd. All rights reserved.

CD38, a transmembrane glycoprotein with ADP-ribosyl cyclase activity, catalyses the formation of Ca2+ signalling molecules, but its role in the neuroendocrine system is unknown. Here we show that adult CD38 knockout (CD38(-/-)) female and male mice show marked defects in maternal nurturing and social behaviour, respectively, with higher locomotor activity. Consistently, the plasma level of oxytocin (OT), but not vasopressin, was strongly decreased in CD38(-/-) mice. Replacement of OT by subcutaneous injection or lentiviral-vector-mediated delivery of human CD38 in the hypothalamus rescued social memory and maternal care in CD38(-/-) mice. Depolarization-induced OT secretion and Ca2+ elevation in oxytocinergic neurohypophysial axon terminals were disrupted in CD38(-/-) mice; this was mimicked by CD38 metabolite antagonists in CD38(+/+) mice. These results reveal that CD38 has a key role in neuropeptide release, thereby critically regulating maternal and social behaviours, and may be an element in neurodevelopmental disorders.

Several lines of evidence suggest that extracellular signal-regulated kinase1/2 (ERK1/2) and dopaminergic system is involved in learning and memory. However, it remains to be determined if the dopaminergic system and ERK1/2 pathway contribute to cognitive function in the prefrontal cortex (PFC). The amount of phosphorylated ERK1/2 was increased in the PFC immediately after exposure to novel objects in the training session of the novel object recognition test. An inhibitor of ERK kinase impaired long-term recognition memory 24 h after the training although short-term memory tested 1 h after the training was not affected by the treatment. The dopamine D1 receptor agonist increased ERK1/2 phosphorylation in the PFC in vivo as well as in cortical neurons in vitro. Microinjection of the dopamine D1 receptor antagonist into the PFC impaired long-term recognition memory whereas the D2 receptor antagonist had no effect. Immunohistochemistry revealed that exposure to novel objects resulted in an increase in c-Fos expression in the PFC. Microinjection of the protein synthesis inhibitor anisomycin into the PFC impaired the long-term recognition memory. These results suggest that the activation of ERK1/2 following the stimulation of dopamine D1 receptors is necessary for the protein synthesis-dependent long-term retention of recognition memory in the PFC.

It has recently been suggested that neurogenesis in the dentate gyrus is decreased in schizophrenia and this phenomenon may contribute to the pathogenesis of the disorder. Since repeated administration of psychostimulants such as phencyclidine (PCP), MK-801, and methamphetamine (METH) induces schizophrenia-like behavioral changes in animals, we investigated whether repeated administration of these psychostimulants affects neurogenesis in the dentate gyrus of mice. Newborn cells were labeled by bromodeoxyuridine (BrdU) and detected by immunohistochemistry. Repeated administration of PCP and MK-801, but not METH, resulted in a decrease in the number of BrdU-labeled cells in the dentate gyrus. PCP-induced decrease in the number of BrdU-labeled cells was negated by co-administration of clozapine, but not haloperidol, although repeated antipsychotics treatment by themselves had no effect. Furthermore, co-administration of D-serine and glycine, but not L-serine, inhibited the PCP-induced decrease in the number of BrdU-Iabeled cells. These results suggest that chronic dysfunction of NMDA receptors causes a decrease in neurogenesis in the dentate gyrus.

In this study, we examined the combination effects of L-DOPA and adenosine receptor antagonists on rotational behaviors in a hemi-Parkinsonian mouse model induced by unilateral 6-hydroxydopamine (6-OHDA) injection. The adenosine A(2A) antagonist SCH-58261, but not the A(1)-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine or A(2B)-receptor antagonist alloxazine, synergistically potentiated the L-DOPA-induced rotational behaviors in the 6-OHDA-lesioned mice. In addtion, the 6-OHDA-induced lesions of the dopaminergic system did not affect the in vivo binding of an adenosine A(2A)-receptor tracer [C-11]SCH-442416 in the striatatum. These findings suggest that adenosine A(2A) antagonists are extremely useful for pharmacotherapy of L-DOPA in Parkinson's disease patients.

The effect of ZSET1446 (spiro[imidazo[1,2-a]pyridine-3,2-indan]-2(3H)-one) on cognitive impairment in mice, previously treated with methamphetamine (METH) at a dose of 1 mg/kg for 7 days, was investigated. ZSET1446 showed a significant ameliorating effect on METH-induced impairment of recognition memory, although it had no effect on exploratory behavior. ZSET1446 (1 mu g/kg) recovered the defect of the novelty-induced activation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the prefrontal cortex (PFC) of METH-treated mice. The compound increased phosphorylated ERK1/2 levels in the hippocampus but not PFC of naive mice without affecting the total ERK1/2 levels. The ameliorating effect of ZSET1446 on recognition memory in METH-treated mice was negated by pretreatment with a mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor, SL327 (alpha-[amino-4-aminophenylthio) methylene]-2-(trifluoromethyl) phenylacetonitrile). Furthermore, the dopamine D1 receptor antagonist, SCH23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5- tetrahydro-1H-3-benzazepine], and N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801 [5H-dibenzo[a, d] cyclohepten-5,10-imine (dizocilpine maleate)], blocked the ameliorating effect of ZSET1446 on METH-induced memory impairment, whereas the D2 receptor antagonist, raclopride, had no effect. These results suggest that the ameliorative effect of ZSET1446 on METH-induced memory impairment is associated with indirect activation of ERK1/2 following stimulation with dopamine D1 and NMDA receptors of the PFC. ZSET1446 would be a potential candidate for further preclinical study aimed at the treatment of cognitive deficits in Alzheimer's disease and schizophrenia, as well as METH psychosis.

Nicotine, a primary component of tobacco, is one of the most abused drugs worldwide. Approximately four million people die each year because of diseases associated with tobacco smoking. Mesolimbic dopaminergic neurons mediate the rewarding effects of abused drugs, including nicotine. Here we show that the tissue plasminogen activator (tPA)-plasmin system regulates nicotine-induced reward and dopamine release by activating protease activated receptor-1 (PAR1). In vivo microdialysis revealed that microinjection of either tPA or plasmin into the nucleus accumbens (NAc) significantly potentiated whereas plasminogen activator inhibitor-1 reduced the nicotine-induced dopamine release in the NAc in a dose-dependent manner. Nicotine-induced dopamine release was markedly diminished in tPA-deficient (tPA(-/-)) mice, and the defect of dopamine release in tPA(-)/(-) mice was restored by microinjection of either exogenous tPA or plasmin into the NAc. Nicotine increased tPA protein levels and promoted the release of tPA into the extracellular space in the NAc. Immunohistochemistry revealed that PAR1 immunoreactivity was localized to the nerve terminals positive for tyrosine hydroxylase in the NAc. Furthermore, we demonstrated that plasmin activated PAR1 and that nicotine-induced place preference and dopamine release were diminished in PAR1-deficient (PAR1(-)/(-)) mice. Targeting the tPA-plasmin-PAR1 system would provide new therapeutic approaches to the treatment of nicotine dependence.

Tissue plasminogen activator ( tPA), a serine protease, catalyzes the conversion of plasminogen to plasmin. In the present study, we investigated the role of the tPA-plasmin system in depolarization-evoked dopamine ( DA) and acetylcholine ( ACh) release in the nucleus accumbens ( NAc) and hippocampus, respectively, of mice, by using in vivo microdialysis. Microinjection of either tPA or plasmin significantly potentiated 40 mM KCl-induced DA release without affecting basal DA levels. In contrast, plasminogen activator inhibitor-1 dose-dependently reduced 60 mM KCl-induced DA release. The 60 mM KCl-evoked DA release in the NAc was markedly diminished in tPA-deficient ( tPA-/-) mice compared with wild-type mice, although basal DA levels did not differ between the two groups. Microinjections of either exogenous tPA ( 100 ng) or plasmin ( 100 ng) into the NAc of tPA-/- mice restored 60 mM KCl-induced DA release, as observed in wild-type mice. In contrast, there was no difference in either basal or 60 mM KCl-induced ACh release in the hippocampus between wild-type and tPA-/- mice. Our findings suggest that the tPA-plasmin system is involved in the regulation of depolarization-evoked DA release in the NAc.

The role of the phosphorylation of hippocampal extracellular signal-regulated kinase 1/2 in spatial working memory in rats was assessed with a delayed spatial win-shift task in a radial arm maze. The task consisted of two phases, a training phase and a test phase, separated by a delay. Phosphorylated extracellular signal-regulated kinase 1/2 levels were significantly and transiently increased in the hippocampus by 60 min, and then returned to the control levels 120 min after the training phase. Bilateral microinjections of the PD98059, an inhibitor of the extracellular signal-regulated kinase 1/2 kinase MEK, into the hippocampus impaired performance in the test phase of the delayed spatial win-shift task at 5-min delay. These results suggest that extracellular signal-regulated kinase 1/2 activation in the hippocampus plays a crucial role in spatial working memory.

Background Recent clinical studies have suggested that chronic use of methamphetamine (METH) induces long-term cognitive deficits. To clarify the mechanism of METH-induced cognitive impairment, we investigated the effect of METH on cognitive function in mice. Methods: Mice were repeatedly administered METH for 7 days, and their cognitive function was assessed using a novel-object recognition task. Therapeutic effects of clozapine and haloperidol on METH-induced cognitive impairment were investigated. Western blotting and specific inhibitors were employed to determine the role of extracellular signal-regulated kinase 1/2 (ERK1/2). Results: Repeated METH treatment induced an impairment of recognition of novel objects and behavioral sensitization. These effects persisted for at least 28 days after the drug withdrawal. Clozapine, but not haloperidol, reduced METH-induced cognitive impairment. Hyperphosphorylation of ERK1/2 was found in the prefrontal cortex of mice exposed to the novel objects, but was abolished in mice treated with METH. Inhibition of ERK1/2 by the microinjection of PD98059 into the prefrontal cortex resulted in cognitive impairment. Conclusions: These results suggest that repeated METH treatment induces cognitive impairment, which is associated with the dysfunction of the ERK1/2 pathway in the prefrontal cortex.

Animals under stress take adaptive actions that may lead to various types of behavioral disinhibition. Such behavioral disinhibition, when expressed excessively and impulsively, can result in harm in individuals and cause a problem in our society. We now show that, under social or environmental stress, mice deficient in prostaglandin E receptor subtype EP1 (Ptger1(-/-)) manifest behavioral disinhibition, including impulsive aggression with defective social interaction, impaired cliff avoidance, and an exaggerated acoustic startle response. This phenotype was reproduced in wild-type mice by administration of an EP1-selective antagonist, whereas administration of an EP1-selective agonist suppressed electric-shock-induced impulsive aggression. Dopamine turnover in the frontal cortex and striatum was increased in Ptger1(-/-) mice, and administration of dopaminergic antagonists corrected their behavioral phenotype. These results suggest that prostaglandin E-2 acts through EP1 to control impulsive behavior under stress, a finding potentially exploitable for development of drugs that attenuate impulsive behavior in humans.

We investigated the effects of memantine and donepezil on amyloid beta (A beta)-induced memory impairment in rats, which was assessed by a delayed-matching to position (DMPT) paradigm in three-lever operant chambers. Aggregated A beta 1-40 was microinjected bilaterally (1 nmol/side) into both CA1 and CA3 subfields of the hippocampus in rats that had previously performed the DMTP task. Memantine (20 mg/(kg day), s.c.) was continuously infused by an osmotic minipump for 4 weeks from 3 days before the microinjection of A beta. Donepezil (2.5 mg/kg, p.o.) was administered 60 min before the DMTP test session. Bilateral microinjections of A beta 1-40 into the hippocampus resulted in a delayed, but persistent impairment of DMTP performance, which appeared more than 50 days after the injection. Memantine prevented the development of A beta-induced memory impairment, while donepezil symptomatically alleviated the deficits. Because of a ceiling effect, the combination of donepezil with memantine failed to produce any additive or synergic effects. These results support the clinical data showing that memantine and donepezil are effective for the treatment of Alzheimer's disease. Moreover, it is suggested that memantine is effective for preventing A beta-induced short-term memory impairment. (c) 2005 Elsevier B.V. All rights reserved.

The extracellular serine protease tissue plasminogen activator (tPA) that converts plasminogen into plasmin is abundantly expressed throughout the central nervous system. We have recently demonstrated that the tPA-plasmin system participates in the rewarding and locomotor-stimulating effects of morphine by acutely regulating morphine-induced dopamine release in the nucleus accumbens (NAc). In the present study, we examined the effects of microinjections of plasminogen activator inhibitor-1 (PAI-1), tPA or plasmin into the NAc on morphine-induced dopamine release, hyperlocomotion and anti-nociceptive effects in ICR mice. A single morphine treatment resulted in an increase in protein levels of PAI-1 in the NAc. Microinjection of PAI-1 into the NAc dose-dependently reduced morphine-induced dopamine release and hyperlocomotion. In contrast, microinjection of tPA into the NAc significantly potentiated morphine-induced dopamine release and hyperlocomotion without affecting basal levels. Furthermore, microinjection of plasmin enhanced morphine-induced dopamine release, but did not modify the hyperlocomotion induced by morphine. The intracerebroventricular injection of PAI-1, tPA and plasmin at high doses had no effect on the anti-nociceptive effects of morphine. These results suggest that the tPA-plasmin system is involved in the regulation of morphine-induced dopamine release and dopamine-dependent behaviors but not the anti-nociceptive effects of morphine.