Free 3-nitrotyrosine causes striatal neurodegeneration in vivo.

Peroxynitrite formation has been demonstrated in several neurodegenerative disorders; thus far, protein nitration and consequent alterations in protein function are implicated as mechanistic events. Free 3-nitrotyrosine (free-3NT) is also elevated in these settings; a neurotoxic role for this modified amino acid has not been investigated. We tested the hypothesis that free-3NT is neurotoxic in vivo, using a mouse model of striatal degeneration. The neurodegenerative effects of the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA) (unilateral intrastriatal injection, 64 nmol) were compared with free-3NT (32 nmol) or free-tyrosine (free-TYR) (32 nmol). 6-OHDA-treated mice exhibited significant ipsilateral turning behavior after d-amphetamine challenge, indicative of unilateral striatal injury (ipsilateral-contralateral turning differential, 21.1 +/- 6.8). Significant turning behavior was also observed in free-3NT-treated mice but not in free-tyrosine-treated mice (free-3NT, 16.0 +/- 3.9; free-TYR, 1 +/- 2.7; p < 0.01). Immunohistochemistry was used to evaluate striatal tyrosine hydroxylase (TH) content. 6-OHDA or free-3NT treatment caused severe reductions in TH immunoreactivity in injected striata compared with the contralateral hemisphere (injected/contralateral immunoreactivity ratio: 6-OHDA, 0.23 +/- 0.07; free-3NT, 0.49 +/- 0.02). Free-tyrosine treatment had no effect (1.03 +/- 0.09). Turning behavior was correlated with striatal TH ratio (p < 0.01). Furthermore, we observed a striking unilateral reduction in TH-positive cell body counts in the substantia nigra pars compacta of 6-OHDA- and free-3NT-treated mice (injected/contralateral cell count ratio: 6-OHDA, 0.40 +/- 0.04; free-3NT, 0.59 +/- 0.02). Free-tyrosine treatment had no effect (1.05 +/- 0.04). No evidence for increased striatal protein incorporation of 3NT was observed in any treatment group. These data represent the first evidence that free-3NT can elicit neurodegenerative effects in vivo; free-3NT may have a causal role in neurodegenerative conditions.

NMDA antagonists block expression of sensitization of amphetamine- and apomorphine-induced stereotypy.

We have been studying sensitization of psychostimulant-induced stereotyped behavior in mice using both single and multiple pretreatment paradigms. In the present study, we tested whether NMDA receptor antagonists and an inhibitor of nitric oxide synthesis inhibit expression of sensitization in either of these models. Male CF-1 mice were pretreated with a single dose or with three daily doses of amphetamine (14 mg/kg) or apomorphine (40 mg/kg). Two days following these pretreatments, mice were injected with ((+/-)3-(2-carboxypiperazine-4yl)-propyl-1-phosphonic acid (CPP, 20 mg/kg), dizocilpine maleate (MK-801, 0.1 mg/kg), 7-nitroindazole (25 mg/kg), or vehicle 30 min before receiving amphetamine (7 mg/kg) or apomorphine (3 mg/kg). The stereotyped behavioral response was enhanced in mice pretreated with amphetamine or apomorphine, indicating that sensitization had developed. CPP, MK-801, and 7-nitroindazole prevented the expression of the sensitized stereotyped response induced by either amphetamine or apomorphine in both paradigms. These drugs did not attenuate the stereotypy elicited by amphetamine and apomorphine in drug-naïve mice. The effect of 7-nitroindazole was reversed by pretreatment with 500 mg/kg of L-arginine but not by D-arginine. These results suggest that glutamatergic transmission and subsequent NMDA receptor activation and the production of nitric oxide play a critical role in the expression of the sensitized stereotyped behavioral response elicited by amphetamine or apomorphine.

NMDA glutamate receptor role in the development of context-dependent and independent sensitization of the induction of stereotypy by amphetamine or apomorphine.

We have been studying sensitization of psychostimulant-induced stereotyped behavior in mice using both a context-dependent and a context-independent paradigm. In the present study, we tested whether N-methyl-D-aspartate (NMDA) receptor antagonists prevent development of sensitization in either of these models. Male CF-1 mice were pretreated with 20 mg/kg (+)3-(2-carboxypiperazine-4yl)-propyl-1-phosphonic acid (CPP), 0.1 mg/kg (+)5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohopten-5, 10-imine maleate (MK-801, dizocilpine maleate), or 25 mg/kg 7-nitroindazole 30 min before a single dose (context-dependent paradigm) or each of three daily doses (context-independent paradigm) of 14 mg/kg amphetamine or 40 mg/kg apomorphine. Two days following this pretreatment, mice were injected with 7 mg/kg amphetamine or 3 mg/kg apomorphine. The stereotyped behavioral response was enhanced in mice pretreated with amphetamine or apomorphine alone, indicating that sensitization had developed. Both CPP and MK-801 prevented the development of sensitization in the context-dependent model but not in the context-independent paradigm. 7-Nitroindazole did not attenuate development of sensitization in either model. The results suggest that activation of glutamatergic receptors is important in some sensitization paradigms but not others, indicating that glutamate can be important but is not always required for the development of sensitization.

Importance of environmental context in the development of amphetamine- or apomorphine-induced stereotyped behavior after single and multiple doses.

The present study was designed to determine whether single and repeated pretreatment regimens with amphetamine or apomorphine differ in the context dependency of sensitization of stereotyped behavior. Male CF-1 mice that were pretreated with a single high dose of amphetamine (14 mg/kg, IP) or apomorphine (40 mg/kg, SC) only became sensitized to a lower test dose of amphetamine (7 mg/kg, IP) or apomorphine (3 mg/kg, SC) when placed in an environment that was the same as the pretreatment environment. However, animals pretreated with three high doses (24 h apart) of amphetamine (14 mg/kg, IP) or apomorphine (40 mg/kg, SC) did demonstrate sensitization to a lower test dose of amphetamine (7 mg/kg, IP) or apomorphine (3 mg/kg, SC) when placed in an environment that was different from the pretreatment environment. Context-dependent sensitization, but not context-independent sensitization, was extinguished by pairing the test environment with saline injections instead of drug injections. In addition, it was determined that neither sensitization model could be related to pharmacokinetic factors. Therefore, the results indicate that repeated exposure to amphetamine or apomorphine overcomes the context-dependent component of sensitization of amphetamine- or apomorphine-induced stereotyped behavior.

Importance of environmental context in the development of amphetamine- or apomorphine-induced stereotyped behavior after single and multiple doses.

The present study was designed to determine whether single and repeated pretreatment regimens with amphetamine or apomorphine differ in the context-dependency of sensitization of stereotyped behavior. Male CF-1 mice that were pretreated with a single high dose of amphetamine (14 mg/kg intraperitoneally [IP]) or apomorphine (40 mg/kg subcutaneously [SC]) only became sensitized to a lower test dose of amphetamine (7 mg/kg IP) or apomorphine (3 mg/kg SC) when placed in an environment that was the same as the pretreatment environment. However, animals pretreated with 3 high doses (24-h apart) of amphetamine (14 mg/kg IP) or apomorphine (40 mg/kg SC) did demonstrate sensitization to a lower test dose of amphetamine (7 mg/kg IP) or apomorphine (3 mg/kg SC) when placed in an environment that was different from the pretreatment environment. Context-dependent sensitization, but not context-independent sensitization, was extinguished by pairing the test environment with saline injections instead of drug injections. In addition, it was determined that neither sensitization model could be related to pharmacokinetic factors. Therefore, the results indicate that repeated exposure to amphetamine or apomorphine overcomes the context-dependent component of sensitization of amphetamine- or apomorphine-induced stereotyped behavior.

Sensitization of apomorphine-induced stereotyped behavior in mice is context dependent.

RATIONALE: The role of the environment in the sensitization of the stereotyped behavioral effects of apomorphine is unclear, since sensitization of this drug effect has either been difficult to demonstrate or has been shown to occur with a low but not a higher dose of apomorphine. OBJECTIVES: The present study was designed to determine whether sensitization of the stereotyped behavioral effects induced by a single dose of apomorphine is dependent on environmental context. METHODS: CF-1 mice were pretreated with apomorphine or vehicle under different environmental conditions and tested for stereotyped behavior after apomorphine challenge. Animals were scored positively for stereotyped behavior if they remained stationary and exhibited repetitive head and/or fore-limb movements, and data are reported as the percentage of mice rated as positive for stereotyped behavior. RESULTS: When mice were pretreated with 40 mg/kg apomorphine and later tested in the same environment, the dose-response curve for stereotyped behavior elicited by apomorphine was shifted threefold to the left 48 h after pretreatment, and this sensitization persisted for at least 28 days after pretreatment. Mice pretreated with apomorphine did not have higher brain levels of apomorphine after administration of the test dose of apomorphine. When the pretreatment environment was different from the test environment, mice did not exhibit sensitization to apomorphine. CONCLUSIONS: These results show that pre-exposure to a single high dose of apomorphine induces a long-lasting sensitization of apomorphine-induced stereotyped behavior that is context dependent. Since apomorphine directly activates dopamine receptors, these observations suggest that a mechanism located postsynaptic to dopamine neurons may be responsible for sensitization of stereotyped behavior.

Sensitization of stereotyped behavior to amphetamine is context and response dependent.

The present study was designed to determine whether the environmental context in which amphetamine is administered plays a role in the development of sensitization to the stereotyped behavioral effects of amphetamine in mice. In male CF-1 mice, the dose-response curve for stereotyped behavior elicited by amphetamine was shifted 1.9-fold to the left 48 h after pretreatment with 14 mg/kg amphetamine. Behavioral sensitization only developed in mice that were pretreated in the same or a similar environment as that of the test environment. In addition, when mice were placed in an environment that attenuated the acute expression of stereotyped behavior elicited by the pretreatment dose of amphetamine, sensitization never developed. A further experiment showed that 96% of the mice that expressed stereotypy after the ED50 pretreatment dose of 10 mg/kg amphetamine showed a stereotyped behavioral response to the lesser dose of 7 mg/kg 48 h later, indicating sensitization. In contrast, mice that did not express stereotypy after the ED50 dose of amphetamine failed to show a significant stereotyped behavioral response to amphetamine challenge compared to vehicle-pretreated controls. Therefore, the results indicate that preexposure to a single high dose of amphetamine produces context- and response-dependent sensitization to amphetamine-induced stereotyped behavior.

Dopaminergic agonists administered into the nucleus accumbens: effects on extracellular glutamate and on locomotor activity.

The hypothesis to be tested was that increased dopaminergic transmission induced by amphetamine in the nucleus accumbens results in increased glutamatergic neurotransmission in this brain area and that the increase in level of this neurotransmitter contributes to behavioral effects of psychostimulant drugs. Amphetamine (1 mg/kg, i. p.) increased the amount of extracellular glutamate in the accumbens, as measured by in vivo dialysis, and stimulated locomotor activity. Amphetamine (10 mM) infused into the accumbens by reverse dialysis through the probe produced a similar stimulation of locomotor activity as systemic amphetamine but a greater increase in extracellular glutamate levels. Both of these responses to amphetamine were attenuated by either the selective D1 antagonist SCH23390 or the selective D2 antagonist eticlopride. The combination of a D1 and D2 agonist, SKF38393 (20 mM) and quinpirole (50 mM), administered into the accumbens by reverse dialysis also increased extracellular glutamate and stimulated locomotor activity. Administration of a glutamate uptake inhibitor, threo-beta-hydroxy-aspartate (50 mM), increased extracellular glutamate but did not stimulate locomotor activity. Systemic administration of caffeine (15 mg/kg, i.p.) increased locomotor activity but did not increase extracellular levels of glutamate. These data suggest that activation of dopaminergic receptors in the nucleus accumbens results in stimulation of locomotor activity and in activation of glutamatergic transmission in this brain region. However, an increase in glutamate levels in the nucleus accumbens is neither sufficient nor necessary to produce a stimulation of locomotor activity.

Design and synthesis of enantiomers of 3,5-dinitro-o-tyrosine: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA) receptor antagonists.

The R- and S-isomers of 3,5-dinitro-o-tyrosine (6a,b) have been synthesized through the use of chemoenzymatic synthesis and shown to bind differentially with the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA, 3) receptors. The phenolic functional group of these o-tyrosine analogues was designed to act as a bioisostere of the gamma-carboxyl group of glutamate. The S-isomer of 3,5-dinitro-o-tyrosine (6b) was 6.5 times more potent than the R-isomer (6a) in inhibiting [3H]AMPA binding with IC50 values of 13 +/- 7 and 84 +/- 26 microM, respectively. The phenolic group was important for binding affinity since the methoxy compound 7 was less potent than the phenolic compound 6 in inhibiting the binding of AMPA. The free amino group was also shown to be important since the N-acetyl analogue 15 and the N-t-BOC compounds 16 and 17 exhibited very low affinity for the AMPA receptors. AMPA receptor functional tests showed that the o-tyrosine analogues are antagonists and that the S-isomer 6b (IC50 = 630 +/- 140 microM) was more potent than the racemate 6 (IC50 = 730 +/- 88 microM) while the R-isomer 6a was inactive up to 1 mM concentration, which is consistent with the S-isomer having higher binding affinity than the R-isomer.

Structure--activity studies for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid receptors: acidic hydroxyphenylalanines.

Antagonists of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA) receptors may have therapeutic potential as psychotropic agents. A series of mononitro- and dinitro-2- and 3-hydroxyphenylalanines was prepared, and their activity compared with willardiine, 5-nitrowillardiine, AMPA, and 2,4,5-trihydroxyphenylalanine (6-hydroxydopa) as inhibitors of specific [3H]AMPA and [3H]kainate binding in rat brain homogenates. The most active compounds were highly acidic (pKa 3-4), namely, 2-hydroxy-3,5-dinitro-DL-phenylalanine (13; [3H]AMPA IC50 approximately equal to 25 microM) and 3-hydroxy-2,4-dinitro-DL-phenylalanine (19; [3H]AMPA IC50 approximately equal to 5 microM). Two other dinitro-3-hydroxyphenylalanines, and 3,5-dinitro-DL-tyrosine, were considerably less active. Various mononitrohydroxyphenylalanines, which are less acidic, were also less active or inactive, and 2- and 3-hydroxyphenylalanine (o- and m-tyrosine) were inactive. Compounds 13 and 19, DL-willardiine (pKa 9.3, [3H]AMPA IC50 = 2 microM), and 5-nitro-DL-willardiine (pKa 6.4, [3H]AMPA IC50 = 0.2 microM) displayed AMPA >> kainate selectivity in binding studies. Compound 19 was an AMPA-like agonist, but 13 was an antagonist in an AMPA-evoked norepinephrine release assay in rat hippocampal nerve endings. Also, compound 13 injected into the rat ventral pallidum antagonized the locomotor activity elicited by systemic amphetamine.

Differential effect of sensitizing regimen on induction of Fos-protein and locomotor activity elicited by apomorphine in rats.

At doses that elicit behavioral activation, apomorphine does not induce Fos-protein in the nucleus accumbens of control animals but does under conditions of denervation supersensitivity. The purpose of this study was to determine whether apomorphine induces Fos-protein in animals sensitized by repeated treatment with apomorphine and whether the magnitude of such induction parallels the magnitude of behavioral response observed after different sensitizing paradigms. Apomorphine did induce Fos-protein expression in animals pretreated with apomorphine; however, the rats showing highest levels of induction were not those showing the largest behavioral responses.

Comparison of sensitization elicited by amphetamine and pertussis toxin: characterization of locomotor behavior and limbic dopamine release.

1. Male, Sprague-Dawley rats were pretreated with one of several regimens of repeated, intermittent amphetamine or with a single-dose of intra-VTA pertussis toxin (PTX). 2. An amphetamine challenge dose (0.5 mg/kg, i.p.) produced increased locomotor activity in both amphetamine and pertussis toxin-pretreated rats. 3. The magnitude of activity in PTX pretreated rats exceeded 5-fold that of the amphetamine-pretreated rats. 4. There were no significant differences in the levels of sensitized behavior elicited by 4 distinct amphetamine pretreatment protocols. 5. Neither of the drug pretreatments caused significant changes in the ability of 10 microM amphetamine to promote dopamine efflux from nucleus accumbens or striatal tissue in vitro. 6. The sensitized behaviour cannot be explained by in vitro alterations in pre-synaptic dopamine release, which may suggest an up-regulation of post-synaptic activity.

Effects of repeated cocaine administration on sensory inhibition in rats: preliminary data.

We have recently reported that acute administration of cocaine to rats alters their sensory inhibitory capacity as tested in a paired click paradigm (S1/S2). Whether such acutely induced changes are persistent, is not known. In order to shed some light on the degree of spontaneous reversibility of cocaine-induced decreased sensory inhibition, rats were tested immediately after cocaine administration and 9 days after cessation of cocaine exposure. Six rats received cocaine HCl 20 mg/kg intraperitoneally and six rats received normal saline for 5 consecutive days. The amplitudes of the S1 responses were significantly decreased in the cocaine animals during the injection days only, but not 9 days later. Two measures of sensory inhibition were employed, S2/S1 x 100 amplitude ratio and S1-S2 amplitude difference. The ratio measure indicated a significant decrease in inhibitory capacity in the cocaine group during the injection days, and remained significantly decreased 9 days after cessation of cocaine administration. The data suggest that repeated cocaine administration can induce persistent deficit in the ability of the rat's brain to inhibit incoming irrelevant sensory stimuli.

Enhancement of the locomotor response to apomorphine in pertussis toxin-treated animals depends on the site of pertussis toxin injection into the ventral tegmental area.

The locomotor stimulant response to either systemic or intra-accumbens amphetamine was enhanced 1-2 weeks after pre-treatment with pertussis toxin (PTX) into any of 3 different sites in the ventral tegmental area (VTA). An enhanced response to either systemic or intra-accumbens apomorphine occurred only after pre-treatment into the central VTA. Thus, enhanced sensitivity to drugs of either pre- or post-synaptic elements in the nucleus accumbens may develop depending on the site of the PTX pre-treatment within the VTA.

Effect of pertussis toxin injected into the ventral tegmental area on amphetamine-induced Fos protein in the nucleus accumbens.

Amphetamine produces an enhanced locomotor stimulatory response in animals injected with pertussis toxin into the ventral tegmental area. This response is dependent on the activation of D1 receptors in the nucleus accumbens. The immediate early gene, c-Fos, has been used as a cellular marker for increases in dopamine neurotransmission in the nucleus accumbens. The purpose of the present study was to determine whether the administration of pertussis toxin into the ventral tegmental area results in an increased ability of amphetamine to induce Fos-positive immunoreactivity in the nucleus accumbens. Amphetamine (1 mg/kg and 2 mg/kg IP) produced a greater number of Fos-positive cells in the nucleus accumbens of pertussis toxin-treated animals as compared to vehicle-treated controls. However, the increase in Fos immunoreactivity at the higher amphetamine dose was not associated with a corresponding increase in locomotor activity. These data suggest that amphetamine produces an enhanced increase in dopamine neurotransmission in the nucleus accumbens of pertussis toxin-treated animals, resulting in an increased induction of Fos-related antigens.

Synthesis of chiral 1-(2'-amino-2'-carboxyethyl)-1,4-dihydro-6,7-quinoxaline-2,3-diones: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor agonists and antagonists.

Recently discovered 6,7-disubstituted quinoxaline-2,3-diones, 1, have been found to antagonize specific binding and functional responses to both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainic acid. Although a variety of studies have analyzed the activity of quinoxaline-2,3-diones with various substitutions at positions 6 and 7, there is little information regarding the effects of N-substitution. A racemic mixture of 1-(2'-amino-2'-carboxyethyl)-1,4-dihydroquinoxaline-2, 3-dione (QXAA, 2, R1 = R2 = H) has been synthesized from 1 (R1 = R2 = H). This compound inhibited specific [3H]AMPA binding but not [3H]kainate binding. IC50 values for QXAA, AMPA, and DNQX were 0.69, 0.012, and 0.74 microM, respectively. The R- and S-enantiomers were prepared by asymmetric synthesis. The S-isomer (2b) was 160-fold more potent in binding assays than the R-isomer (2d), with IC50 values of 0.23 and 38 microM, respectively. Both enantiomers were agonists in a functional assay, with the S-isomer having an EC50 value of 3 microM while that for the R-isomer was greater than 1 mM. Methyl substitutions at positions 6 and 7 (2a and 2c) resulted in antagonist compounds characterized by the S- and R-isomers being nearly equipotent, with IC50 values of 51 and 22 microM in the binding assay and EC50 values of 290 and 300 microM in the functional assay. AMPA had an EC50 value of 11 microM and DNQX an EC50 value of 30 microM in the functional assay. Analogs of quinoxalinediones with side chains other than an amino acid moiety on the nitrogen did not show good binding activities.

Induction of locomotor activity by the glutamate antagonist DNQX injected into the ventral tegmental area.

DNQX, an antagonist of AMPA/kainate receptors, was injected into the ventral tegmental area (VTA) to test the hypothesis that AMPA/kainate receptors in this brain region might be involved in regulation of locomotor activity. Bilateral injection of 1 microgram DNQX into the VTA increased locomotor activity. In addition, unilateral injection of DNQX into this site produced contraversive turning, which was potentiated by coadministration of amphetamine (1 mg/kg, i.p.). These results suggest that a glutamatergic afferent to the VTA is tonically active in inhibiting locomotor activity. The locomotor stimulation produced by DNQX was not associated with a change in DOPAC/DA level in the nucleus accumbens or the striatum. However, the locomotor stimulation produced by DNQX was markedly attenuated following blockade of dopaminergic receptors by haloperidol (0.5 mg/kg, s.c.) or following dopamine depletion induced by reserpine plus alpha-methyl-para-tyrosine pretreatment. These results suggest that a basal activation of dopaminergic receptors is required for expression of the locomotor activity elicited by DNQX.

Role of dopaminergic mechanisms in the stimulatory effects of MK-801 injected into the ventral tegmental area and the nucleus accumbens.

The bilateral administration of 10 micrograms of (+)MK-801, but not (-)MK-801, into either the VTA or the N.Ac. stimulated locomotor activity. The stimulation induced by (+)MK-801 at both sites was inhibited by reserpine (5 mg/kg, SC) and the D1 antagonist, SCH 23390 (0.1 mg/kg, SC). Eticlopride (0.03 mg/kg, SC), a D2 antagonist, inhibited the stimulation produced by MK-801 in the VTA but not in the N.Ac. Baclofen (32 ng), a GABAB receptor agonist, injected into the VTA inhibited the stimulatory response to MK-801 injected systemically, into the VTA, or into the N.Ac., but did not significantly inhibit spontaneous locomotion or the stimulatory response to apomorphine (5 mg/kg, SC). These observations suggest that the stimulatory effects of MK-801 in the VTA and the N.Ac. are dependent on endogenous dopamine. In addition, the effects produced by MK-801 injected into the VTA closely resemble those produced by the systemic administration of low doses of MK-801, suggesting that this is the primary site of action of MK-801.

Spontaneous and drug-stimulated locomotor activity after the administration of pertussis toxin into the ventral tegmental area.

Pertussis toxin (PTX) injected into the ventral tegmental area (VTA) produces an enhanced locomotor response to amphetamine. In the present study, we have evaluated the role of dopamine receptors on spontaneous locomotor activity and the enhanced locomotor response to dopaminergic agonists after the administration of PTX into the VTA. PTX injected into the VTA of rats produced a delayed increase in spontaneous locomotor activity with a latency of 4 d. This activity was markedly increased by day 6 and remained elevated for at least 28 d after PTX treatment. This increased spontaneous locomotor activity of PTX-treated animals was antagonized by the administration of the D1 receptor antagonist SCH23390 (0.03 and 0.1 mg/kg sc), but not by the D2 receptor antagonist eticlopride (0.1 and 0.3 mg/kg sc). After adaptation to the locomotor cages, the animals showed a markedly enhanced motor response to amphetamine (0.5 mg/kg ip) and apomorphine (5 mg/kg sc). The heightened locomotor responses to these dopaminergic agonists could be elicited for at least 2 mo after PTX administration. The enhanced response to amphetamine was antagonized by the administration of SCH23390 (0.03 and 0.1 mg/kg sc), but not by eticlopride (0.1 mg/kg). The increased response to apomorphine in PTX-treated animals was inhibited by SCH23390 (0.1 mg/kg sc) and partially inhibited by eticlopride (0.1 mg/kg sc). Both of these antagonists inhibited the spontaneous and the drug-induced locomotor responses in vehicle-treated control animals. These results suggest that the administration of PTX into the VTA leads to an increase in spontaneous and drug-induced locomotor activity in which D1 receptors seem to play an important role.

DNQX in the nucleus accumbens inhibits cocaine-induced conditioned place preference.

Several lines of evidence suggest that activation of both AMPA/kainate receptors and dopaminergic receptors in the nucleus accumbens may be required for psychostimulant drug induced reward. However, it has been reported that dopaminergic antagonists fail to block acquisition of conditioned place preference to cocaine. The goal of these experiments was to determine whether AMPA receptor antagonist injected into the nucleus accumbens could block conditioned place preference elicited by cocaine under conditions where dopaminergic antagonists do not inhibit acquisition of place preference. DNQX (1 microgram/0.5 microliter), injected into the nucleus accumbens just before systemic injections of cocaine (20 mg/kg i.p.) during the training sessions, attenuated the acquisition of place preference. This suggests that AMPA receptors are involved in acquisition of place preference to cocaine. By contrast, fluphenazine (2.5 micrograms/0.5 microliter), injected into the nucleus accumbens during training, did not alter cocaine-induced place preference. Analysis of locomotor activity showed that the ability of flyphenazine to inhibit cocaine-induced hyperactivity progressively decreased with each training session. These observations suggest that the failure of dopaminergic antagonists to block cocaine-induced place preference may be related to adaptations occurring following repeated exposure to these drugs. Both DNQX and fluphenazine blocked the expression of conditioned place preference in rats that had been previously trained with cocaine alone. This result suggests that both AMPA and dopaminergic receptors are involved in the expression of a conditioned place preference to cocaine.