Genetic variation in COMT activity impacts learning and dopamine release capacity in the striatum.

A common genetic polymorphism that results in increased activity of the dopamine regulating enzyme COMT (the COMT Val(158) allele) has been found to associate with poorer cognitive performance and increased susceptibility to develop psychiatric disorders. It is generally assumed that this increase in COMT activity influences cognitive function and psychiatric disease risk by increasing dopamine turnover in cortical synapses, though this cannot be directly measured in humans. Here we explore a novel transgenic mouse model of increased COMT activity, equivalent to the relative increase in activity observed with the human COMT Val(158) allele. By performing an extensive battery of behavioral tests, we found that COMT overexpressing mice (COMT-OE mice) exhibit cognitive deficits selectively in the domains that are affected by the COMT Val(158) allele, stimulus-response learning and working memory, functionally validating our model of increased COMT activity. Although we detected no changes in the level of markers for dopamine synthesis and dopamine transport, we found that COMT-OE mice display an increase in dopamine release capacity in the striatum. This result suggests that increased COMT activity may not only affect dopamine signaling by enhancing synaptic clearance in the cortex, but may also cause changes in presynaptic dopamine function in the striatum. These changes may underlie the behavioral deficits observed in the mice and might also play a role in the cognitive deficits and increased psychiatric disease risk associated with genetic variation in COMT activity in humans.

Distinct mechanisms mediating methamphetamine-induced neuronal apoptosis and dopamine terminal damage share the neuropeptide substance p in the striatum of mice.

Methamphetamine (METH) is an addictive psychostimulant that induces damage to the dopamine terminals and the apoptosis of some neurons of the striatum. Our laboratory demonstrated using either a single bolus dose (30 mg/kg) or a binge (10 mg/kg 4x at 2-h intervals) of METH that pharmacological blockade of the substance P receptor (neurokinin-1) attenuates METH-induced damage to both the presynaptic dopamine terminals and the apoptosis of some neurons of the striatum. To determine the phenotype of striatal neuron ablated by METH, we combined TUNEL (Terminal Deoxyncleotidyl Transferase-Mediated dUTP Nick End Labeling) with immunofluorescence for selective markers of projection and interneurons. METH induces the loss of approximately 20% of the projection neurons. The cholinergic and gamma-aminobutyric acid (GABA)-parvalbumin interneurons sustain losses of 30% and 50%, respectively. The somatostatin/neuropeptide Y (NPY)/nitric oxide synthase (NOS) interneurons are not impacted by METH. To investigate the mechanism by which substance P mediates METH-induced damage in this part of the brain, we ablated the striatal interneurons that express the neurokinin-1 receptor (NK-1R) with the selective neurotoxin substance P-SAP. Ablation of the NK-1R-expressing interneurons prevented METH-induced apoptosis in the striatum but was without effect on depletion of dopamine terminal markers. We propose that substance P mediates the apoptosis of some striatal neurons via the intrastriatal activation of nitric oxide synthesis. In contrast, substance P may mediate damage of the dopamine terminals via an extrastriatal mechanism involving the substantia nigra and cortical glutamate release.

Methamphetamine-induced striatal apoptosis in the mouse brain: comparison of a binge to an acute bolus drug administration.

Methamphetamine (METH) is a psychostimulant that induces neural damage in experimental animals and humans. A binge (usually in the 5-10 mg/kg dose range 4 x at 2 h intervals) and the acute bolus drug administration (20-40 mg/kg) of METH have been employed frequently to study neurotoxicity in the brain. In this study we have compared these drug delivery schedules to determine their efficacy to induce striatal apoptosis. Exposure of male mice to a binge of METH at 10mg/kg 4x at 2 h intervals (cumulative dose of 40 mg/kg) was approximately four times less effective in inducing apoptotic cell death (TUNEL staining) 24 h after METH treatment in the striatum than a single bolus administration of 30 mg/kg of METH. The residual TUNEL staining observed three days after METH treatment is proportionately equivalent between a binge and the acute bolus drug administration. Interestingly, a binge of METH induces a hyperthermic response of longer duration. This study demonstrates that an acute bolus drug administration of METH is more effective inducing striatal apoptosis in mice, and therefore, is more suitable for studies assessing the impact of METH on sites post-synaptic to the striatonigral dopamine terminals.

Disparity in the temporal appearance of methamphetamine-induced apoptosis and depletion of dopamine terminal markers in the striatum of mice.

Methamphetamine (METH) causes damage in the striatum at pre- and post-synaptic sites. Exposure to METH induces long-term depletions of dopamine (DA) terminal markers such as tyrosine hydroxylase (TH) and DA transporters (DAT). METH also induces neuronal apoptosis in some striatal neurons. The purpose of this study is to demonstrate which occurs first, apoptosis of some striatal neurons or DA terminal toxicity in mice. This is important because the death of striatal neurons leaves the terminals in a state of deafferentation. A bolus injection (i.p.) of METH (30 mg/kg) induces apoptosis (TUNEL staining) in approximately 25% of neurons in the striatum at 24 h after METH. However, in contrast to apoptosis, depletion of TH (Western blotting) begins to appear at 24 h after METH in dorsal striatum while the ventral striatum is unaffected. The peak of TH depletion (approximately 80% decrease relative to control) occurs at 48 h after METH. Autoradiographic analysis of DAT sites showed that depletion begins to appear 24 h after METH and peaks at 2 days (approximately 60% depletion relative to control). Histological analysis of the induction of glial fibrillary acidic protein (GFAP) by METH in striatal astrocytes revealed an increase at 48 h after METH that peaked at 3 days. These data demonstrate that striatal apoptosis precedes the depletion (toxicity) of DA terminal markers in the striatum of mice, suggesting that the ensuing state of deafferentation of the DA terminals may contribute to their degeneration.

Substance P and cholecystokinin regulate neurochemical responses to cocaine and methamphetamine in the striatum.

The mechanism of action of drugs of abuse like cocaine and amphetamines has been studied extensively in the dopamine terminal field areas of the caudate-putamen (CPu) and the nucleus accumbens (NAc) of the rodent brain. These brain regions contain several neuropeptides that must play important roles in the normal physiological functions of these brain regions. The study of neuropeptide physiology in the context of the neurobiological responses to drugs of abuse may shed some light on the intrinsic mechanism of action of neuropeptides of the CPu and the NAc. The neuropeptides substance P (SP) and cholecystokinin (CCK) are present in the striatum where they could play an important role regulating the effects of psychostimulants like cocaine and amphetamines (methamphetamine [METH] is a long acting derivative of d-amphetamine). These highly addictive agents induce the release of dopamine (DA) (and other catecholamines) from dopaminergic terminals of the striatum. The excessive release of DA in the striatum and the NAc has been implicated in the habit-forming properties of these drugs. In order to study the contribution of SP and CCK in the striatum during psychostimulant treatment, we employed selective non-peptide neurokinin-1 (NK-1) and cholecystokinin-2 (CCK-2) receptor antagonists that readily cross the blood brain barrier. We infused the neurokinin-1 receptor (NK-1R) antagonist, L-733,060, into the striatum of freely moving rats via a microdialysis probe in order to assess the effects of SP on cocaine-induced DA overflow in the striatum. Infusion of the NK-1R antagonist prior to a systemic injection of cocaine (10 mg/kg i.p.) significantly attenuated DA overflow in the striatum. Conversely, infusion of a CCK-2 receptor (CCK-2R) antagonist, L-369,293, through the microdialysis probe evoked DA overflow in the striatum in the absence of cocaine and potentiated DA overflow after a single injection of cocaine (10 mg/kg i.p.). Exposure to METH (10 mg/kg 4x at two-hour intervals) produced deficits of dopamine transporters (DAT) in mice striatum that are detectable three days after the treatment and are long lasting. Pre-treatment (i.p. injections) with the NK-1R antagonist, WIN-51,708 30 minutes before the 1st and 4th injections of METH prevented the loss of DAT in the striatum. Moreover, pre-treatment with the NK-1R antagonist prevents METH-induced cell death. Taken together, these results demonstrate that the NK-1R and the CCK-2R are important modulators of the actions of the psychostimulants cocaine and METH. Neuropeptide receptors represent an important control point mediating the effects of the neurotransmitter DA in the striatum of the rodent brain.

Induction of striatal pre- and postsynaptic damage by methamphetamine requires the dopamine receptors.

Methamphetamine (METH) is a psychostimulant that induces excessive release of dopamine (DA) in the striatum. In this study we have assessed the role of DA D1 and D2 receptors (D1R and D2R) on striatal METH-induced apoptosis and depletion of DA-terminal markers. Male mice were given one i.p. injection of METH (30 mg/kg). Apoptosis was assessed at 24 h, and DA-terminal marker depletion 3 days, after METH. A single toxic dose of METH induced apoptosis in approximately 10-13% of striatal neurons. This was completely prevented by pretreatment (30 min before METH) with either the D1R antagonist SCH-23390 (0.1 mg/kg) or the D2R antagonist raclopride (1 mg/kg). The same dose of METH induced depletion of DA transporter sites up to 61, 56, 71, and 69% in dorsal-medial, ventral-medial, dorsal-lateral, and ventral-lateral striatum, respectively, relative to vehicle-injected controls. Similarly, METH induced depletion of TH protein levels up to 80, 72, 87, and 90% in those respective quadrants. METH induced the expression of glial fibrillary acidic protein throughout the striatum. All these neurochemical changes were significantly attenuated by pretreatment with SCH-23390 (0.1 mg/kg) or raclopride (1 mg/kg). However, pretreatment with either raclopride or SCH-23390 did not prevent METH-induced hyperthermia in mice. These data demonstrate that the induction by METH of both striatal apoptosis and DA-terminal damage requires the activity of the postsynaptic DA receptors in the mouse brain. Moreover, since blockade of either receptor subtype protected from METH, the activity of both DA receptor subtypes is required for the induction of toxicity by METH in the striatum.