Challenge Injection Of Cocaine Research Articles

Cocaine-induced sensitization and glutamate plasticity in the nucleus accumbens core: effects of sex

BackgroundThe development and persistence of addiction is mediated in part by drug-induced alterations in nucleus accumbens (NAc) function. AMPA-type glutamate receptors (AMPARs) provide the main source of excitatory drive to the NAc and enhancements in transmission of calcium-permeable AMPARs (CP-AMPARs) mediate increased cue-triggered drug-seeking following prolonged withdrawal. Cocaine treatment regimens that result in psychomotor sensitization enhance subsequent drug-seeking and drug-taking behaviors. Furthermore, cocaine-induced locomotor sensitization followed by 14 days of withdrawal results in an increase in glutamatergic synaptic transmission. However, very few studies have examined cocaine-induced alterations in synaptic transmission of females or potential effects of experimenter-administered cocaine on NAc CP-AMPAR-mediated transmission in either sex.MethodsMale and female rats were given repeated systemic cocaine injections to induce psychomotor sensitization (15 mg/kg, i.p. 1 injection/day, 8 days). Controls received repeated saline (1 mL/kg, i.p). After 14–16 days of withdrawal brain slices were prepared and whole-cell patch-clamp approaches in the NAc core were used to measure spontaneous excitatory post-synaptic currents (sEPSC), paired pulse ratio, and CP-AMPAR transmission. Additional female rats from this same cohort were also given a challenge injection of cocaine at withdrawal day 14 to assess the expression of sensitization.ResultsRepeated cocaine produced psychomotor sensitization in both sexes. In males this was accompanied by an increase in sEPSC frequency, but not amplitude, and there was no effect on the paired pulse ratio. Males treated with cocaine and saline had similar sensitivity to Naspm. In contrast, in females there were no significant differences between cocaine and saline groups on any measure, despite females showing robust psychomotor sensitization both during the induction and expression phase.ConclusionsOverall, these data reveal striking sex differences in cocaine-induced NAc glutamate plasticity that accompany the induction of psychomotor sensitization. This suggests that the neural adaptations that contribute to sensitization vary by sex.

Glutamate inputs from the laterodorsal tegmental nucleus to the ventral tegmental area are essential for the induction of cocaine sensitization in male mice.

Drug-induced potentiation of ventral tegmental area (VTA) glutamate signaling contributes critically to the induction of sensitization - an enhancement in responding to a drug following exposure which is thought to reflect neural changes underlying drug addiction. The laterodorsal tegmental nucleus (LDTg) provides one of several sources of glutamate input to the VTA. We used optogenetic techniques to test either the role of LDTg glutamate cells or their VTA afferents in the development of cocaine sensitization in male VGluT2::Cre mice. These were inhibited using halorhodopsin during each of five daily cocaine exposure injections. The expression of locomotor sensitization was assessed following a cocaine challenge injection 1-week later. The locomotor sensitization seen in control mice was absent in male mice subjected to inhibition of LDTg-VTA glutamatergic circuitry during cocaine exposure. As sensitization of nucleus accumbens (NAcc) dopamine (DA) overflow is also induced by this drug exposure regimen, we used microdialysis to measure NAcc DA overflow on the test for sensitization. Consistent with the locomotor sensitization results, inhibition of LDTg glutamate afferents to the VTA during cocaine exposure prevented the sensitization of NAcc DA overflow observed in control mice. These data identify the LDTg as the source of VTA glutamate critical for the development of cocaine sensitization in male mice. Accordingly, the LDTg may give rise to the synapses in the VTA at which glutamatergic plasticity, known to contribute to the enhancement of addictive behaviors, occurs.

Cross-sensitization between testosterone and cocaine in adolescent and adult rats

Cocaine and anabolic–androgenic steroids are substances commonly co-abused. The use of anabolic steroids and cocaine has increased among adolescents. However, few studies investigated the consequences of the interaction between anabolic–androgenic steroids in animals’ model of adolescence. We examined the effects of acute and repeated testosterone administration on cocaine-induced locomotor activity in adult and adolescent rats. Rats received ten once-daily subcutaneous (s.c.) injections of testosterone (10mg/kg) or vehicle. Three days after the last testosterone or vehicle injections rats received an intraperitoneal (i.p.) challenge injection of either saline or cocaine (10mg/kg). A different subset of rats was treated with a single injection of testosterone (10mg/kg) or vehicle and three days later was challenged with cocaine (10mg/kg, i.p.) or saline. Immediately after cocaine or saline injections the locomotor activity was recorded during forty minutes. Our results demonstrated that repeated testosterone induced locomotor sensitization to cocaine in adolescent but not adult rats.

Tolerance to cocaine in brain stimulation reward following continuous cocaine infusions

This study examined tolerance to cocaine's threshold-lowering effect in brain stimulation reward (BSR) following continuous cocaine infusions and secondly, used the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) to determine NO's involvement in the development of cocaine tolerance. Animals were continuously infused with saline or cocaine (30mg/kg per day) via osmotic minipump for 14days and injected daily with saline or L-NAME (30mg/kg, i.p.) following BSR testing. Saline-treated animals continuously infused with saline showed stable BSR thresholds across the 14-day infusion period. Saline-treated animals continuously infused with cocaine showed markedly lowered BSR thresholds on Day 1 followed by a progressive increase in BSR thresholds across the infusion period – indicating the development of tolerance. L-NAME-treated animals continuously infused with cocaine showed stimulation thresholds that were not significantly different from saline-treated animals continuously infused with cocaine.A cocaine challenge injection (10mg/kg, i.p.) administered 3 and again at 10days following minipump removal revealed that saline-treated animals continuously infused with saline showed lowered BSR thresholds. Saline-treated animals continuously infused with cocaine displayed lowered BSR thresholds that were not significantly different from saline-infused animals. L-NAME treated animals continuously infused with cocaine showed higher BSR thresholds to a challenge 3days following pump removal. However, stimulation thresholds for this group failed to reach statistical significance on both days (i.e., Days 3 and 10) following pump removal.Results showed that animals continuously infused with cocaine develop robust tolerance to cocaine's threshold-lowering effect during the 14-day infusion period. Tolerance to cocaine's threshold-lowering effect was short-lived and dissipated soon after minipump removal. L-NAME treatment failed to significantly alter the development of tolerance to cocaine's threshold-lowering suggesting that NO does not have a primary role in the development of cocaine tolerance.

Enhancement of Behavioral Sensitization, Anxiety-Like Behavior, and Hippocampal and Frontal Cortical CREB Levels Following Cocaine Abstinence in Mice Exposed to Cocaine during Adolescence

Adolescence has been linked to greater risk-taking and novelty-seeking behavior and a higher prevalence of drug abuse and risk of relapse. Decreases in cyclic adenosine monophosphate response element binding protein (CREB) and phosphorylated CREB (pCREB) have been reported after repeated cocaine administration in animal models. We compared the behavioral effects of cocaine and abstinence in adolescent and adult mice and investigated possible age-related differences in CREB and pCREB levels. Adolescent and adult male Swiss mice received one daily injection of saline or cocaine (10 mg/kg, i.p.) for 8 days. On day 9, the mice received a saline injection to evaluate possible environmental conditioning. After 9 days of withdrawal, the mice were tested in the elevated plus maze to evaluate anxiety-like behavior. Twelve days after the last saline/cocaine injection, the mice received a challenge injection of either cocaine or saline, and locomotor activity was assessed. One hour after the last injection, the brains were extracted, and CREB and pCREB levels were evaluated using Western blot in the prefrontal cortex (PFC) and hippocampus. The cocaine-pretreated mice during adolescence exhibited a greater magnitude of the expression of behavioral sensitization and greater cocaine withdrawal-induced anxiety-like behavior compared with the control group. Significant increases in CREB levels in the PFC and hippocampus and pCREB in the hippocampus were observed in cocaine-abstinent animals compared with the animals treated with cocaine in adulthood. Interestingly, significant negative correlations were observed between cocaine sensitization and CREB levels in both regions. These results suggest that the behavioral and neurochemical consequences of psychoactive substances in a still-developing nervous system can be more severe than in an already mature nervous system.

Different locomotor sensitization responses to repeated cocaine injections are associated with differential phosphorylation of GluA1 in the dorsomedial striatum of adult rats

Behavioral sensitization to psychostimulants reflects neural adaptation, which might share a common mechanism with drug addiction. Outbred male rats show different locomotor sensitization responses to cocaine, and cocaine also produces varied addictive progress in humans. We investigated whether differences in the induction of sensitization would affect the long-term persistence of sensitized locomotor activity, and we sought to determine the molecular basis for the variability in sensitization. Male Sprague-Dawley rats that showed sensitized locomotor responses over 5 consecutive daily cocaine injections (SENS) had significantly lower initial locomotor responses to the 1st cocaine exposure than did rats that did not show locomotor sensitization (NONS). Furthermore, rats that underwent 1 month of cocaine withdrawal after 5 repeated cocaine injections also exhibited sensitized or non-sensitized locomotor responses to a challenge injection of cocaine (SENS-C or NONS-C, respectively). This variability was also related to the initial responsiveness to cocaine. We examined the level of phosphorylation of the GluA1 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropioniate receptor (AMPAR) in the dorsal striatum and found that there were significant differences between the sensitized rats and the non-sensitized rats. pGluA1-Ser831 was increased in the SENS rats during the induction of locomotor sensitization, and pGluA1-Ser845 was increased in the SENS-C rats during the expression of locomotor sensitization. These phosphorylation changes were observed in the dorsomedial striatum (DMS) of adult rats but not in the dorsolateral striatum (DLS) of adults. Our findings suggest that differential phosphorylation of AMPAR might be an important mechanism that contributes to the development of locomotor sensitization to cocaine in adult rats.

Differential regulation of locomotor activity to acute and chronic cocaine administration by acid-sensing ion channel 1a and 2 in adult mice

Acid-sensing ion channels (ASICs) are densely expressed in the brain with ASIC1a and ASIC2 channels being the predominant subtypes. These channels are enriched at synaptic sites and are central for the regulation of normal synaptic transmission. Moreover, increasing evidence links ASICs to the pathogenesis of various neurological and neuropsychiatric disorders. In this study, we explore the putative role of ASIC1a and ASIC2 in the regulation of behavioral sensitivity to the psychostimulant cocaine by utilizing ASIC1a or ASIC2 knockout mice. Acute cocaine injection induced a typical dose-dependent increase in locomotor activities in wild-type (WT) mice. However, in ASIC1a and ASIC2 mutant mice, different motor responses to cocaine were observed. In ASIC1a−/− mice, cocaine induced a significantly less motor response at all doses (5, 10, 20, and 30mg/kg), while in ASIC2−/− mice, cocaine (5–20mg/kg) stimulated locomotor activity to an extent comparable to WT mice. Only at 30mg/kg, the cocaine-stimulated motor activity was reduced in ASIC2−/− mice. In a chronic cocaine administration model (20mg/kg, once daily for 5days), a challenge injection of cocaine (10mg/kg, after 2-week withdrawal) caused an evident behavioral sensitization in the cocaine-pretreated WT mice. This behavioral sensitization to challenge cocaine was also displayed in ASIC1a−/− and ASIC2−/− mice. However, ASIC2−/− mice showed less sensitization to challenge cocaine when compared to WT and ASIC1a−/− mice. Our results demonstrate the important role of ASIC1a and ASIC2 channels in the modulation of behavioral sensitivity to cocaine. The two synapse-enriched ASIC subtypes are believed to play distinguishable roles in the regulation of behavioral responses to acute and chronic cocaine administration.

Serotonin (2C) receptor regulation of cocaine-induced conditioned place preference and locomotor sensitization

Previous studies have identified an inhibitory regulatory role of the 5-HT2C receptor in serotonin and dopamine neurotransmission. As cocaine is known to enhance serotonin and dopamine transmission, the ability of 5-HT2C receptors to modulate cocaine-induced behaviors was investigated. Alterations in cocaine reward behavior were assessed in the conditioned place preference (CPP) paradigm. Mice were injected with a selective 5-HT2C receptor agonist, Ro 60-0175 (0, 1, 3, 10mg/kg, i.p.) prior to cocaine administration (10mg/kg, i.p.) on cocaine-conditioning days. Administration of Ro 60-0175(10mg/kg) prior to cocaine attenuated the development of cocaine place preference. To assess the potential of the 5-HT2C receptor to influence cocaine-induced behavioral sensitization, mice were pretreated with either saline or Ro 60-0175 (10mg/kg, i.p.) and 30min later, administered cocaine (20mg/kg, i.p.) or saline once daily for 5 days. Locomotor activity was measured daily following cocaine administration. After a 10-day drug-free period, locomotor activity was measured on day 16 following a challenge injection of cocaine (20mg/kg, i.p.). Pharmacological activation of 5-HT2C receptors withRo 60-0175 attenuated acute cocaine-induced activity on days 1–5, as well as the development of long-term cocaine-induced locomotor sensitization. Thus, activation of 5-HT2C receptors attenuated the rewarding and locomotor-stimulating effects of cocaine, as well as inhibited the development of sensitization. The current study shows that 5-HT2C receptor activity exerts an inhibitory influence on the short-term and long-term behavioral responses to cocaine.

Cocaine-Induced Adaptations in Cellular Redox Balance Contributes to Enduring Behavioral Plasticity

Impaired glutamate homeostasis in the nucleus accumbens has been linked to cocaine relapse in animal models, and results in part from cocaine-induced downregulation of the cystine-glutamate exchanger. In addition to regulating extracellular glutamate, the uptake of cystine by the exchanger is a rate-limiting step in the synthesis of glutathione (GSH). GSH is critical for balancing cellular redox in response to oxidative stress. Cocaine administration induces oxidative stress, and we first determined if downregulated cystine-glutamate exchange alters redox homeostasis in rats withdrawn from daily cocaine injections and then challenged with acute cocaine. Among the daily cocaine-induced changes in redox homeostasis were an increase in protein S-glutathionylation and a decrease in expression of GSH-S-transferase pi (GSTpi). To mimic reduced GSTpi, a genetic mouse model of GSTpi deletion or pharmacological inhibition of GSTpi by administering ketoprofen during daily cocaine administration was used. The capacity of cocaine to induce conditioned place preference or locomotor sensitization was augmented, indicating that reducing GSTpi may contribute to cocaine-induced behavioral neuroplasticity. Conversely, an acute cocaine challenge after withdrawal from daily cocaine elicited a marked increase in accumbens GSTpi, and the expression of behavioral sensitization to a cocaine challenge injection was inhibited by ketoprofen pretreatment; supporting a protective effect by the acute cocaine-induced rise in GSTpi. Together, these data indicate that cocaine-induced oxidative stress induces changes in GSTpi that contribute to cocaine-induced behavioral plasticity.

Genotype-dependent effects of adolescent nicotine exposure on dopamine functional dynamics in the nucleus accumbens shell in male and female mice: a potential mechanism underlying the gateway effect of nicotine

The tendency to use cocaine is determined by genetic and environmental effects across the lifespan. One critical environmental effect is early drug exposure, which is both driven by and interacts with genetic background. The mesoaccumbens dopamine system, which is critically involved in the rewarding properties of drugs of abuse, undergoes significant development during adolescence, and thus may be at particular risk to repeated nicotine exposure during this period, thereby establishing vulnerability for subsequent adult psychostimulant use. We tested the hypotheses that adolescent nicotine exposure results in attenuation of the enhancing effects of cocaine on medial forebrain bundle (MFB) electrical stimulation-evoked dopamine release in the nucleus accumbens shell (AcbSh) in adulthood and that this effect is significantly influenced by genotype. Mice from the progenitor strains C57BL/6J and DBA/2J and those from the BXD20/TyJ and BXD86/RwwJ recombinant inbred lines were exposed to nicotine via osmotic minipumps from postnatal day (P) 28 to P56. When mice reached P70, dopamine functional dynamics in AcbSh was evaluated by means of in vivo fixed potential amperometry in combination with electrical stimulation of mesoaccumbens dopaminergic axons in the MFB. Adolescent exposure to nicotine in all strains dose-dependently reduced the ability of a fixed-dose challenge injection of cocaine (10mg/kg, i.p.) to enhance MFB electrical stimulation-evoked dopamine release in AcbSh in adults. The magnitude of this effect was genotype-dependent. These results suggest a genotype-dependent mechanism by which nicotine exposure during adolescence causes persistent changes in the sensitivity to "hard" stimulants such as cocaine.

Repeated N-Acetyl Cysteine Reduces Cocaine Seeking in Rodents and Craving in Cocaine-Dependent Humans

Addiction is a chronic relapsing disorder hypothesized to be produced by drug-induced plasticity that renders individuals vulnerable to craving-inducing stimuli such as re-exposure to the drug of abuse. Drug-induced plasticity that may result in the addiction phenotype includes increased excitatory signaling within corticostriatal pathways that correlates with craving in humans and is necessary for reinstatement in rodents. Reduced cystine-glutamate exchange by system x(c)- appears to contribute to heightened excitatory signaling within the striatum, thereby posing this as a novel target in the treatment of addiction. In the present report, we examined the impact of repeated N-acetyl cysteine, which is commonly used to activate cystine-glutamate exchange, on reinstatement in rodents in a preclinical study and on craving in cocaine-dependent humans in a preliminary, proof-of-concept clinical experiment. Interestingly, repeated administration (7 days) of N-acetyl cysteine (60 mg/kg, IP) produced a significant reduction in cocaine (10 mg/kg, IP)-induced reinstatement, even though rats (N=10-12/group) were tested 24 h after the last administration of N-acetyl cysteine. The reduction in behavior despite the absence of the N-acetyl cysteine indicates that repeated N-acetyl cysteine may have altered drug-induced plasticity that underlies drug-seeking behavior. In parallel, our preliminary clinical data indicate that repeated administration (4 days) of N-acetyl cysteine (1200-2400 mg/day) to cocaine-dependent human subjects (N=4 per group) produced a significant reduction in craving following an experimenter-delivered IV injection of cocaine (20 mg/70 kg/60 s). Collectively, these data demonstrate that N-acetyl cysteine diminishes the motivational qualities of a cocaine challenge injection possibly by altering pathogenic drug-induced plasticity.

Blockade of nucleus accumbens 5-HT2A and 5-HT2C receptors prevents the expression of cocaine-induced behavioral and neurochemical sensitization in rats.

RationaleThe serotonin 5-HT2A and 5-HT2C receptors regulate the capacity of acute cocaine to augment behavior and monoamine levels within the nucleus accumbens (NAC), a brain region involved in cocaine’s addictive and psychotogenic properties.ObjectivesIn the present study, we tested the hypothesis that NAC 5-HT2A and 5-HT2C receptor activation is involved in the expression of cocaine-induced neuroplasticity following protracted withdrawal from a sensitizing repeated cocaine regimen (days 1 and 7, 15 mg/kg; days 2–6, 30 mg/kg, i.p.).MethodsThe effects of intra-NAC infusions of the 5-HT2A antagonist R-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine methanol (MDL 100907; 0, 50, 100, 500 nM) or the 5-HT2C antagonist [6-chloro-5-methyl-1-(6-(2-methylpiridin-3-yloxy)pyridine-3-yl carbamoyl] inodoline dihydrochloride (SB 242084; 0, 50, 100, 500 nM) were first assessed upon the expression of locomotor activity elicited by a 15-mg/kg cocaine challenge injection administered at 3-week withdrawal. A follow-up in vivo microdialysis experiment then compared the effects of the local perfusion of 0, 50, or 100 nM of each antagonist upon cocaine-induced dopamine and glutamate sensitization in the NAC.ResultsAlthough neither MDL 100907 nor SB 242084 altered acute cocaine-induced locomotion, SB 242084 reduced acute cocaine-elevated NAC dopamine and glutamate levels. Intra-NAC perfusion with either compound blocked the expression of cocaine-induced locomotor and glutamate sensitization, but only MDL 100907 pretreatment prevented the expression of cocaine-induced dopamine sensitization.ConclusionsThese data provide the first evidence that NAC 5-HT2A and 5-HT2C receptors are critical for the expression of cocaine-induced neuroplasticity following protracted withdrawal, which has relevance for their therapeutic utility in the treatment of addiction.

Social isolation during perinatal development alters the behavioral response to cocaine in juvenile rats

Stress is a risk factor in drug addiction, enhancing development and increasing relapse. However its role during development has received little attention. This study will investigate if social isolation during early development alters the behavioral response to cocaine. Pregnant Sprague Dawley rats were housed 1 (prenatal isolation group) or 2 (control group) rats per cage. From postnatal day 2‐9, pups from the neonatal isolation group were isolated for 60 min in a separated cage. At day 21 (weaning), pups from the postnatal isolation group were housed individually. Another group was isolated during both prenatal and postnatal period. Control rats were maintained with dam and siblings and at weaning, housed 4‐5/cage of the same sex. At day 22, and for 5 consecutive days, pups were injected daily with saline or cocaine (15 mg/kg/i.p.) followed by 7 drug free days. A challenge injection of saline or cocaine (15 mg/kg,i.p.) was given on day 13. Locomotor activity was recorded on days 1, 5, and 13. Only rats that were isolated during both prenatal and postnatal period showed an increase in cocaine‐induced behavioral sensitization as compared to control rats. These data indicate that isolation stress during perinatal development enhances the behavioral response to drugs of adolescent rats. [Support contributed by NIH grants: U54NS39405 (SNRP); SO6‐GM08224 (MBRS/SCORE)]

Integrin expression is altered after acute and chronic cocaine

Cocaine addiction is associated with an increase in actin cycling and alterations in dendritic spines in the nucleus accumbens. Both actin polymerization and spine morphology are regulated in part by beta-(β) integrins. Mice were administered acute or daily injections of cocaine or saline for 7 days. After 3 weeks of withdrawal, the level of β-integrins in the postsynaptic density enriched subfraction from nucleus accumbens tissue was quantified by immunoblotting at 0, 30 or 120 min following an a cocaine challenge injection. After chronic treatment and withdrawal the basal level of β1-integrin was increased while β3-integrin was unaltered. However, following a cocaine challenge in chronic cocaine, but not saline-treated animals, β3-integrin was transiently up-regulated while β1-integrin was transiently downregulated. These data demonstrate a bidirectional regulation of β-integrins by chronic cocaine treatment that may contribute to cocaine-induced changes in actin cycling and dendrite morphology.

Cocaine-induced behavioral sensitization in adolescent rats endures until adulthood: Lack of association with GluR1 and NR1 glutamate receptor subunits and tyrosine hydroxylase

Exposure to repeated cocaine induces enduring behavioral sensitization, which has been implicated in the psychostimulant-induced craving and psychosis. Adaptations in dopamine and glutamate neurotransmission in the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) seem to mediate psychostimulant-induced behavioral sensitization. The abuse of drugs often begins during adolescence; however few studies have been devoted to study the effects of drugs of abuse at this age. The aim of our study was to examine whether repeated cocaine during adolescence could induce behavioral sensitization that endures into adulthood. Moreover, the protein levels of Tyrosine Hydroxylase (TH) and the glutamate receptor subunits GluR1 and NR1 in the NAc and mPFC were measured following the behavioral tests. Adolescent rats were treated with cocaine from postnatal day (PND) 30 to PND34 and behavioral sensitization was verified recording locomotor activity after cocaine challenge injection to adolescent (PND37) or adult (PND64 or 94) rats in separate groups at each time point. TH, GluR1, and NR1 protein levels were measured by Western blotting. Rats exposed to cocaine during adolescence expressed behavioral sensitization when tested on PND37 and PND64. In cocaine sensitized rats GluR1 protein was increased in the mPFC on PND37 but not in other ages. Thus, cocaine-induced behavioral sensitization during adolescence endures into early adulthood. However, cocaine pretreatment during adolescence induced a transient increase of GluR1 in the mPFC only when animals were challenged in the same age.

Estrous cyclicity and the locomotor response to acute and repeated cocaine administration.

Several studies have shown that gonadal hormones modulate the behavioral response to cocaine. In this study we examined if endogenous changes in sex steroids throughout the estrous cycle affect the locomotor response to repeated cocaine administration. Adult female Sprague Dawley rats were cycled for 21 days. Rats then received a daily injection of saline or cocaine (15 mg/kg,i.p.) for 5 days, followed by 7 drug free days. A challenge injection of saline or cocaine (15 mg/kg,i.p.) was given on day 13. Behavioral activity was recorded on days 1, 5 and 13. Vaginal lavages were performed daily, and on days 1, 5 and 13, after behavioral testing.Cocaine increased horizontal activity and total distance traveled of drug naive female rats independent of the stage of the estrous cycle. A similar pattern was observed on day 5 of cocaine treatment. However, after 1 week of withdrawal, rats that were in estrous showed higher cocaine‐induced hyperactivity compared to females in proestrous or diestrous. If we compare cocaine‐induced hyperactivity of females at different stages of the estrous cycle we observe that only females in estrous show an increase from day 1 to day 13, indicating that females in estrous become sensitized to cocaine. These results are similar to those obtained by other groups. An unexpected finding was the shortening of the estrous cycle. In summary, we observed that the stage of the estrous cycle affects the locomotor response to repeated cocaine administration. These data may have implications in treatment strategies for female cocaine addicts.

A BDNF infusion into the medial prefrontal cortex suppresses cocaine seeking in rats

The medial prefrontal cortex (mPFC) is critical for reinstatement of cocaine seeking and is the main source of brain-derived neurotrophic factor (BDNF) to striatal regions of the brain relapse circuitry. To test the hypothesis that BDNF in the mPFC regulates cocaine-seeking behavior, rats were trained to press a lever for cocaine infusions (0.2 mg/inf, 2 h/day) paired with light+tone conditioned stimulus (CS) presentations on 10 consecutive days. After the last self-administration session, rats received a single infusion of BDNF (0.75 microg/0.5 microL/side) into the mPFC; this manipulation produced protracted effects on cocaine-seeking behavior (non-reinforced lever pressing). BDNF pretreatment administered after the last session attenuated cocaine seeking 22 h later and, remarkably, it also blocked cocaine-induced suppression of phospho-extracellular-regulated kinase and elevated BDNF immunoreactivity in the nucleus accumbens. The same pretreatment also suppressed cocaine-seeking behavior elicited by response-contingent CS presentations after 6 days of forced abstinence or extinction training, as well as a cocaine challenge injection (10 mg/kg, i.p.) after extinction training. However, BDNF infused into the mPFC had no effect on food-seeking behavior. Furthermore, BDNF infused on the sixth day of abstinence failed to alter responding, suggesting that the regulatory influence of BDNF is time limited. The suppressive effects of BDNF infused into the mPFC on cocaine seeking indicate that BDNF regulates cortical pathways implicated in relapse to drug seeking and that corticostriatal BDNF adaptations during early abstinence diminish compulsive drug seeking.

Electrolytic lesions of a discrete area within the nucleus accumbens shell attenuate the long-term expression, but not early phase, of sensitization to cocaine

Repeated exposure to cocaine leads to behavioral sensitization, which is the augmentation of the locomotor response to a subsequent exposure to the drug. The nucleus accumbens (NAc), a major termination site of dopaminergic neurons, is believed to be involved in behavioral sensitization and studies have demonstrated that the NAc shell can be split into five zones of analysis; the vertex, arch, cone, intermediate and ventrolateral zones [Todtenkopf MS, Stellar JR. Assessment of tyrosine hydroxylase immunoreactive innervation in five subregions of the nucleus accumbens shell in rats treated with repeated cocaine. Synapse 2000;38:261–70]. Several reports show cocaine-induced c-fos expression particularly in the intermediate zone after 14, but not 2, drug-free days following repeated cocaine administration, suggesting that this region may be involved in sensitization and particularly in the later phase of expression, versus the earlier phase of sensitization. Bilateral electrolytic lesions of the intermediate zone were made in two groups of rats, which were then repeatedly exposed to cocaine (15 mg/kg, twice/day for 5 days). One group was subsequently given a single cocaine challenge injection (15 mg/kg) after 14 drug-free days, while the other group was challenged after only 2 drug-free days. Two sham surgery groups in which an electrode was lowered but no current was passed served as controls. Results show that lesioned animals as well as sham controls exhibited behavioral sensitization to the drug. However, following a 14-day drug-free period, the lesioned animals showed significant reduction in sensitization, compared to sham controls. Together these findings suggest that the intermediate zone of the NAc shell is indeed involved in the expression phase of behavioral sensitization to cocaine.

COCAINE MODULATION OF D2/D3 RECEPTOR FUNCTION IN THE LATERAL HYPOTHALAMUS AND THALAMUS OF MALE RATS

The mediodorsal thalamus (MD) and lateral hypothalamus (LH) are brain areas involved in motivational and emotional aspects of pleasurable behaviors such as feeding and drug addiction. Previous studies have shown that sex steroids modulate both behaviors, and although these areas contain sex steroid receptors, their role in mediating sex differences in addictive behavior has not been explored. We use functional autoradiography to investigate cocaine modulation of D2/D3 receptors in gonadectomized Sprague-Dawley male rats with (GDX-T) and without (GDX) testosterone replacement. Adult rats received a daily injection of saline or cocaine (15 mg/kg,i.p.) for 5 days, followed by 7 drug free days. A challenge injection of saline or cocaine (15 mg/kg,i.p.) was given on day 13. To determine D2/D3 receptor function, quinpirole-stimulated [35S] GTP γS binding was assessed in the LH and MD. Cocaine decreased D2/D3 signal transduction in the LH and MD of GDX-T males. However, quinpirole-stimulated [35S] GTPγS binding was increased in the LH of GDX males. No effect of cocaine was observed in the MD of GDX males. The same GDX-T males did not show cocaine-induced behavioral sensitization, suggesting that in these brain areas, D2/D3 receptors may enhance cocaine sensitization. Support Contributed By: NIH grants U54 NS3905 04 (NINDS/SNRP); SO6-GM08224 (MBRS/SCORE), RR03051 (NCRR/RCMI); and R25 GM61838 (MBRS/RISE).

C-Fos and ΔFosB expression are differentially altered in distinct subregions of the nucleus accumbens shell in cocaine-sensitized rats

Repeated cocaine administration in rats can lead to sensitization as evidenced by an increased locomotor response to a subsequent exposure (challenge) dose of cocaine even after a drug-free period. Expression of the immediate early gene product, c-Fos, differs among distinct subregions of the nucleus accumbens shell. This would suggest that these subregions may be differentially involved in sensitization. The present study quantified c-Fos- and ΔFosB-immunoreactive nuclei in subterritories of the nucleus accumbens in animals behaviorally sensitized to cocaine. Rats received a sensitization-inducing regimen of cocaine (twice-daily injections of 15mg/kg i.p. for five consecutive days). Fourteen days following the last injection, rats were given a challenge injection of cocaine (15mg/kg i.p.), and killed 2 h later. Sections through the nucleus accumbens were processed for tyrosine hydroxylase and either c-Fos or ΔFosB. The number of immunoreactive nuclei was quantified in five subregions of the nucleus accumbens shell: the vertex, arch, cone, intermediate zone and ventrolateral zone, which can be identified by differential histological staining for tyrosine hydroxylase. Repeated cocaine administration resulted in robust sensitization that was associated with more ΔFosB in the vertex, arch, and cone compared with saline-treated controls. As previously reported, c-Fos immunoreactivity was increased in the intermediate zone in cocaine-sensitized rats. ΔFosB was significantly elevated in rats that did not receive a cocaine challenge, attesting to the long half-life of this transcription factor. These results provide further evidence suggesting distinct anatomical neuroadaptations within the nucleus accumbens shell that may play a functional role in psychomotor-stimulant sensitization.