Context-driven cocaine-seeking in abstinent rats increases activity-regulated gene expression in the basolateral amygdala and dorsal hippocampus differentially following short and long periods of abstinence

In this study, the expression patterns of zif268 and activity-regulated cytoskeleton-associated gene (arc) were investigated in the basolateral amygdala (BLA) and dorsal hippocampal (dHPC) subregions during context-induced drug-seeking following 22 h or 15 d abstinence from cocaine self-administration. Arc and zif/268 mRNA in BLA and dHPC increased after re-exposure to the cocaine-paired chamber at both timepoints; however, only the BLA increases (with one exception-see below) were differentially affected by the presence or absence of the cocaine-paired lever in the chamber. Following 22 h of abstinence, arc mRNA was significantly increased in the BLA of cocaine-treated rats re-exposed to the chamber only with levers extended, whereas following 15 d of abstinence, arc mRNA in the BLA was increased in cocaine-treated rats returned to the chamber with or without levers extended. In contrast, zif268 mRNA in the BLA was greater in cocaine-treated rats returned to the chamber with levers extended vs. levers retracted only after 15 d of abstinence. In the dentate gyrus (DG) following 22 h of abstinence, zif268 mRNA was greater in rats returned to the chamber where levers were absent regardless of drug treatment whereas arc mRNA was increased in CA1 (cell bodies and dendrites) and CA3 only in cocaine-treated groups. Following 15 d of abstinence, arc mRNA was significantly greater in CA1 and CA3 of both cocaine-treated groups returned to the chamber than in those placed into a familiar, non-salient alternate environment; however, only in CA1 cell bodies the cocaine context-induced increases significantly greater than in yoked-saline controls. In contrast, zif/268 mRNA in all dHPC regions was significantly greater in both cocaine-treated groups returned to the cocaine context than in the cocaine-treated group returned to an alternative environment or saline-treated groups. These data suggest that the temporal dynamics of arc and zif268 gene expression in the BLA and dHPC encode different key elements of drug context-induced cocaine-seeking.     

Patterns of neural activity associated with differential acute locomotor stimulation to cocaine and methamphetamine in adolescent versus adult male C57BL/6J mice

Adolescence is a time period when major changes occur in the brain with long-term consequences for behavior. One ramification is altered responses to drugs of abuse, but the specific brain mechanisms and implications for mental health are poorly understood. Here, we used a mouse model in which adolescents display dramatically reduced sensitivity to the acute locomotor stimulating effects of cocaine and methamphetamine. The goal was to identify key brain regions or circuits involved in the differential behavior. Male adolescent (postnatal day (PN), 30–35) and young adult (PN, 69–74) C57BL/6J mice were administered an i.p. injection of cocaine (0, 15, 30 mg/kg) or methamphetamine (0, 2, 4 mg/kg) and euthanized 90 min later. Locomotor activity was monitored continuously in the home cage by video tracking. Immunohistochemical detection of Fos protein was used to quantify neuronal activation in 16 different brain regions. As expected, adolescents were less sensitive to the locomotor stimulating effects of cocaine and methamphetamine as indicated by a rightward shift in the dose response relationship. After a saline injection, adolescents showed similar levels of Fos as adults in all regions except the dorsal caudate (CPuD) and lateral caudate (CPuL) where levels were lower in adolescents. Cocaine and methamphetamine dose dependently increased Fos in all brain regions sampled in both adolescents and adults, but Fos levels were similar in both age groups for a majority of regions and doses. Locomotor activity was correlated with Fos in several brain areas within adolescent and adult groups, and adolescents had a significantly greater induction of Fos for a given amount of locomotor activity in key brain regions including the caudate where they showed reduced Fos under baseline conditions. Future research will identify the molecular and cellular events that are responsible for the differential psychostimulant-induced patterns of brain activation and behavior observed in adolescent versus adult mice.     

Early adolescents show enhanced acute cocaine-induced locomotor activity in comparison to late adolescent and adult rats

Initiation of drug use during adolescence is associated with an increased probability to develop a drug addiction. The present study examined dose-response effects of cocaine (0, 5, 10, or 20 mg/kg, i.p.) on locomotor activity in early adolescent (postnatal day (PND) 35), late adolescent (PND 45), and young adults (PND 60) by measuring total distance moved (TDM) and frequency of start-stops. In response to 20 mg/kg cocaine, early adolescents showed the greatest cocaine-induced increase in TDM in comparison to late adolescent and adult rats. At this same dose, early adolescents showed the greatest cocaine-induced attenuation of start-stops relative to older rats. Results suggest that early adolescents engage in more cocaine-induced locomotor activity and less stationary behavior indicating that early adolescents are more sensitive to locomotor activating effects of high dose cocaine than older rats.     

Local infusion of the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione does not block D1 dopamine receptor-mediated increases in immediate early gene expression in the dopamine-depleted striatum

Administration of selective agonists of D1 dopamine receptors increases immediate early gene expression in striatal neurons, a response which is particularly robust in the dopamine-depleted striatum. Although interactions between dopamine and glutamate receptor-mediated responses in striatal neurons have been demonstrated in a number of experimental paradigms, our previous findings indicate that N-methyl-D-aspartate antagonists do not block D1 receptor-mediated induction of immediate early genes in the dopamine-depleted striatum. In the present study, we therefore examined interactions between D1 dopamine receptors and the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate subtypes of glutamate receptor by determining whether striatal infusion of the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione would block D1 receptor-mediated induction of the immediate early genes c-fos and zif268 in the dopamine-depleted striatum. Striatal infusion of 6-cyano-7-nitroquinoxaline-2,3-dione (1 mM) completely blocked (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate-induced c-fos and zif268 expression. However, 6-cyano-7-nitroquinoxaline-2,3-dione (1 microM-1 mM) did not significantly affect induction of c-fos and zif268 by D1 receptor stimulation (SKF 38393, 2 mg/kg, i.p.) in the dopamine-depleted striatum. To more generally block excitatory input, tetrodotoxin (10 microM) was infused into the striatum of rats receiving a D1 agonist. Local infusion of tetrodotoxin had minimal effect on induction of c-fos and zif268 in the dopamine-depleted striatum. In contrast, tetrodotoxin abolished induction of c-fos and zif268 messenger RNAs by the D2 antagonist eticlopride (0.5 mg/kg, i.p.) in both intact rats and dopamine-depleted rats receiving continuous D2 agonist treatment (quinpirole, 0.5 mg/kg/day). The results indicate that D1 receptor-mediated induction of immediate early genes in the dopamine-depleted striatum occurs by mechanisms that are independent of excitatory input through (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptors.     

Relapse to cocaine-seeking increases activity-regulated gene expression differentially in the striatum and cerebral cortex of rats following short or long periods of abstinence

One of the most insidious features of cocaine addiction is a high rate of relapse even after extended periods of abstinence. A wide variety of drug-associated stimuli, including the context in which a drug is taken, can gain incentive motivational properties that trigger drug desire and relapse to drug-seeking. Both animal and clinical studies suggest that extensive cocaine exposure may induce a transition from cortical to striatal control over decision-making as compulsive drug-seeking emerges. Using an animal model of relapse to cocaine-seeking, the present study investigated the expression patterns of three different activity-related genes (c-fos, zif/268, and arc) in cortical and striatal brain regions implicated in compulsive drug-seeking in order to determine the neuroadaptations that occur during context-induced relapse following brief or prolonged abstinence from cocaine self-administration. Re-exposure to the environment previously associated with cocaine self-administration following 22 h or 15 days of abstinence produced a significant increase in zif/268 and arc, but not c-fos mRNA, in the caudate-putamen and nucleus accumbens. With the exception of arc mRNA levels following 15 days of abstinence, all three genes were increased in the anterior cingulate cortex of animals with a cocaine history when they were re-exposed to the operant chamber. Additionally, c-fos, zif/268, and arc expression was differentially affected in the motor and sensory cortices at both timepoints. Together, these results support convergent evidence that drug-seeking induced by a cocaine-paired context changes the activity of corticostriatal circuits.     

Stimulation of adenosine A2A receptors elicits zif/268 and NMDA epsilon2 subunit mRNA expression in cortex and striatum of the "weaver" mutant mouse, a genetic model of nigrostriatal dopamine deficiency

Interaction between basal ganglia and cerebral cortex is critical for normal goal-directed behavior. In the present study we have used the immediate early gene zif/268, as functional marker to investigate how the stimulation of adenosine A2A receptors, i.e. of the "indirect" striatal output pathway, affects striatal and cortical function in "weaver" mouse, a genetic model of dopamine deficiency. Furthermore, we have examined the effect of A2A receptor stimulation on glutamate receptor expression in the "weaver" brain. A single injection of CGS21680 (A2A receptor agonist), induced strong expression of zif/268 mRNA, detected by in situ hybridization, not only in striatum but also in the motor cortex of the "weaver" mutant. This cortical response seems to be elicited through the basal-ganglia-thalamo-cortical circuit, rather than through a direct cortical effect, since A2A receptors are not detectable in cortex according to our autoradiographic study. Co-administration of CGS21680 and quinpirole (D2 receptor agonist) attenuated the expression of zif/268 mRNA in dorsal striatum but not in motor cortex, indicating that the cortical response is dopamine-D2-receptor-independent. However, this co-administration induced an increase in zif/268 mRNA expression in somatosensory cortex, which could rely on disinhibition of the thalamo-cortical pathway. The motor cortical response could be of clinical interest, as it would further stimulate the "indirect" striatal pathway in a feed forward circuit, thus worsening the parkinsonian symptoms. Furthermore, the up-regulation of epsilon2 subunit mRNA of the NMDA receptor, induced by CGS21680 administration, seen in striatum and cortex of the "weaver" mouse, would lead to overactivity of these receptors worsening dyskinesias. These results suggest adenosine to play a significant role in regulating striatal and cortical neurochemistry in a dopamine-depleted mouse. Blockade of these receptors by specific A2A antagonists could ameliorate parkinsonian symptoms.     

Reduction of zif268 messenger RNA expression during prolonged withdrawal following "binge" cocaine self-administration in rats

Cocaine self-administration increases dopamine efflux and neuronal activity in the mesocorticolimbic dopamine system compared with experimenter-administered cocaine. Following a prolonged cocaine self-administration binge, dopamine efflux in the nucleus accumbens is attenuated and behaviors emerge that are indicative of anhedonia and anxiety. The neuronal correlates of these behavioral and neurochemical effects of a cocaine binge were assessed using in situ hybridization histochemistry to detect changes in zif268 messenger RNA expression. Rats were fitted with intravenous catheters; one group was trained to self-administer cocaine (0.5mg/injection), then allowed continuous access to cocaine during a 16h binge, while yoked animals received either saline or cocaine according to the same schedule. Measurement of tactile startle responses and ultrasonic distress calls either immediately after termination of cocaine access or one or 14 days later confirmed peak withdrawal at 24h after the binge. The level of zif268 messenger RNA was lower upon termination of cocaine self-administration than in both yoked treatment groups in the ventral tegmental area and hippocampus. In contrast, zif268 messenger RNA expression increased in the periaqueductal gray matter one day after termination of passive cocaine treatment, coincident with enhanced expression of ultrasonic vocalizations. Zif268 messenger RNA expression decreased over time in the nucleus accumbens core and infralimbic cortex, with reduced expression observed in the nucleus accumbens core, caudatoputamen, hippocampus and amygdala 14 days after termination of cocaine self-administration.The results suggest that withdrawal following a cocaine self-administration binge produces a long-lasting reduction of constitutive zif268 messenger RNA expression in mesocorticolimbic brain regions related to the nucleus accumbens. The relatively greater effect in animals that self-administered cocaine implies a relationship of certain regional responses to behavioral conditioning.     

Enhanced dopamine transporter activity in middle-aged Gdnf heterozygous mice

Glial cell line-derived neurotrophic factor (GDNF) supports the viability of midbrain dopamine (DA) neurons that degenerate in Parkinson's disease. Middle-aged, 12 month old, Gdnf heterozygous (Gdnf^+/-) mice have diminished spontaneous locomotor activity and enhanced synaptosomal DA uptake compared with wild type mice. In this study, dopamine transporter (DAT) function in middle-aged, 12 month old Gdnf^+/- mice was more thoroughly investigated using in vivo electrochemistry. Gdnf^+/- mice injected with the DAT inhibitor, nomifensine, exhibited significantly more locomotor activity than wild type mice. In vivo electrochemistry with carbon fiber microelectrodes demonstrated enhanced clearance of DA in the striatum of Gdnf^+/- mice, suggesting greater surface expression of DAT than in wild type littermates. Additionally, 12 month old Gdnf^+/- mice expressed greater D₂ receptor mRNA and protein in the striatum than wild type mice. Neurochemical analyses of striatal tissue samples indicated significant reductions in DA and a faster DA metabolic rate in Gdnf^+/- mice than in wild type mice. Altogether, these data support an important role for GDNF in the regulation of uptake, synthesis, and metabolism of DA during aging.     

Blocking systemic nitric oxide production alters neuronal activation in brain structures involved in cardiovascular regulation during polymicrobial sepsis

In a previous study, we concluded that overproduction of nitric oxide (NO) by inducible nitric oxide synthase (iNOS) in the late phase of sepsis prevents hypothalamic activation, blunts vasopressin secretion and contributes to hypotension, irreversible shock and death. The aim of this follow-up study was to evaluate if the same neuronal activation pattern happens in brain structures related to cardiovascular functions. Male Wistar rats received intraperitoneal injections of aminoguanidine, an iNOS inhibitor, or saline 30 min before cecal ligation and puncture (CLP) or sham surgeries. The animals were perfused 6 or 24 h after the surgeries and the brains were removed and processed for Fos immunocytochemistry. We observed an increase (P < 0.001) in c-fos expression 6 h after CLP in the area postrema (AP), nucleus of the tractus solitarius (NTS), ventral lateral medulla (VLM), locus coeruleus (LC) and parabrachial nucleus (PB). At 24 h after CLP, however, c-fos expression was strongly decreased in all these nuclei (P < 0.05), except for the VLM. Aminoguanidine reduced c-fos expression in the AP and NTS at 6 h after CLP, but showed an opposite effect at 24 h, with an increase in the AP, NTS, and also in the VLM. No such effect was observed in the LC and PB at 6 or 24 h. In all control animals, c-fos expression was minimal or absent. We conclude that in the early phase of sepsis iNOS-derived NO may be partially responsible for the activation of brain structures related to cardiovascular regulation. During the late phase, however, this activation is reduced or abolished.     

Differential regional zif268 messenger RNA expression in an escalating dose/binge model of amphetamine-induced psychosis

Amphetamine-induced psychosis is most often associated with a high-dose multiple binge pattern of stimulant abuse. To simulate these conditions in rats, we used an escalating dose/binge administration paradigm. Animals were pretreated with escalating doses of amphetamine (1.0-8.0mg/kg) over four days, then exposed to nine daily binges (8.0mg/kg every 2h; four injections/day). Other animals received either multiple injections of saline, saline followed by acute amphetamine (8.0mg/kg) or single daily injections of amphetamine (8.0mg/kg) in parallel with the escalating dose/binge treatment. One hour after the last injection, all animals were decapitated and regional brain activation patterns were assessed using in situ hybridization with antisense probes for zif268. Acute amphetamine resulted in a significant elevation of zif268 messenger RNA in both the nucleus accumbens and dorsal striatum. However, whereas after single daily amphetamine treatment this index was no longer elevated above control levels in the dorsal striatum, multiple binge exposures were required for the nucleus accumbens to return to baseline. Agranular insular cortex and medial olfactory tubercle zif268 messenger RNA expression was also markedly increased after acute amphetamine treatment but, unlike the nucleus accumbens and dorsal striatum, this increase was not significantly attenuated by either single daily injection or multiple binge treatment. Zif268 messenger RNA expression in the lateral nucleus of the amygdala also remained elevated above baseline after binge treatment.The possible relationships of these changes in zif268 messenger RNA regional expression patterns to the development of psychosis in high-dose stimulant abusers are discussed.     

Differential role of N-methyl-d-aspartate receptor subunits 2A and 2B in mediating phencyclidine-induced perinatal neuronal apoptosis and behavioral deficits

The mechanism underlying phencyclidine (PCP)-induced apoptosis in perinatal rats and the development of schizophrenia-like behaviors is incompletely understood. We used antagonists for N-methyl-d-aspartate (NMDA) receptor subunit NR2A- and NR2B-containing NMDA receptor to test the hypothesis that the behavioral and apoptotic effects of PCP are mediated by blockade of NR1/NR2A-containing receptors, rather than NR1/NR2B-containing receptors. Sprague-Dawley rats were treated on PN7, PN9, and PN11 with PCP (10 mg/kg), PEAQX (NR2A-preferring antagonist; 10, 20, or 40 mg/kg), or ifenprodil (selective NR2B antagonist; 1, 5, or 10 mg/kg) and sacrificed for measurement of caspase-3 activity (an index of apoptosis) or allowed to age and tested for locomotor sensitization to PCP challenge on PN28-PN35. PCP or PEAQX on PN7, PN9, and PN11 markedly elevated caspase-3 activity in the cortex; ifenprodil showed no effect. Striatal apoptosis was evident only after subchronic treatment with a high dose of PEAQX (20 mg/kg). Animals treated with PCP or PEAQX on PN7, PN9, and PN11 showed a sensitized locomotor response to PCP challenge on PN28-PN35. Ifenprodil treatment had no effect on either measure. Therefore, PCP blockade of cortical NR1/NR2A, rather than NR1/NR2B, appears to be responsible for PCP-induced apoptosis and the development of long-lasting behavioral deficits.     

Single administration of 1-benzyl-1,2,3,4-tetrahydroisoquinoline increases the extracellular concentration of dopamine in rat striatum

We performed a combined neurochemical and behavioral study to determine the effects of 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1-BnTIQ) on the extracellular dopamine concentrations in the striatum. Single dose administration of 1-BnTIQ (20, 40, and 80 mg/kg i.p.) increased striatal dopamine extracellular levels in a dose-dependent manner when an in vivo microdialysis technique was used to assess dopamine levels in the striatum of rats. Enhancement of striatal dopamine levels by systemic administration of a single dose of 1-BnTIQ was suppressed by perfusion of tetrodotoxin and a calcium ion-free solution into the striatum. This 1-BnTIQ-induced increase in extracellular dopamine concentration was also inhibited by pre-treatment with a dopamine uptake inhibitor, GBR12909 (1-(2-[bis(4-Fluorophenyl)-4-(3-phenylpropyl)piperazine dihydrochloride). Local application of 1-BnTIQ into the striatum via a dialysis probe failed to enhance the extracellular concentration of dopamine. However, microinjection of 1-BnTIQ into the substantia nigra pars compacta increased the extracellular dopamine levels in the striatum. Locomotor activity was increased by systemic administration of a single dose of 1-BnTIQ in a dose-dependent manner. This 1-BnTIQ-induced locomotor activity was attenuated by pre-treatment with SCH23390 (R(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochlodride) and raclopride, D₁ and D₂ dopaminergic receptor antagonists, respectively. Moreover, 1-BnTIQ induced ipsilateral rotational behavior in 6-hydroxydopamine–lesioned rats. These results suggest that systemic administration of a single dose of 1-BnTIQ increases striatal extracellular dopamine concentration through activation of dopaminergic nigra striatal neurons via the dopamine transporter.     

Blunted response to cocaine in the Flinders hypercholinergic animal model of depression

The Flinders sensitive line (FSL) rat is a proposed genetic hypercholinergic animal model of human depression. Considering the strong comorbidity between depression and cocaine dependence we investigated the well-documented behavioral and molecular effects of cocaine in the FSL and their control Flinders resistant line (FRL) rats. First, we found no difference between the two lines to establish cocaine self-administration; both lines reached stable responding within 10 days of training at a fixed ratio-1 schedule of reinforcement (1.5 mg/kg/injection). However, the FSL rats exhibited reduced cocaine intake at a dose of 0.09 mg/kg/injection in a within-session dose-response curve (0.02, 0.09, 0.38, 1.5 mg/kg/injection). Second, we examined the effects of repeated cocaine administration on locomotor activity, dopamine overflow and striatal prodynorphin mRNA expression. We found the FSL rats to be low responders to novelty and to exhibit less locomotor activation after repeated cocaine administration (30 mg/kg, i.p., daily injections for 10 days) than their controls. Microdialysis sampling from the nucleus accumbens shell revealed no significant difference in the dopamine overflow between the rat lines, neither during baseline nor after cocaine stimulation. Postmortem analyses of striatal prodynorphin mRNA expression (using in situ hybridization histochemistry) revealed a differentiated response to the cocaine exposure. In contrast to control FRL rats, the FSL rats showed no typical cocaine-evoked elevation of prodynorphin mRNA levels in rostral subregions of the striatum whereas both strains expressed increased prodynorphin mRNA levels in the caudal striatum after cocaine administration. In conclusion, the FSL animal model of depression demonstrates marked blunting of the locomotor and dynorphin neuroadaptative responses to cocaine in accordance with its enhanced cholinergic sensitivity.     

Striatal patch compartment lesions alter methamphetamine-induced behavior and immediate early gene expression in the striatum,substantia nigra and frontal cortex

Methamphetamine (METH) induces stereotypy, which is characterized as inflexible, repetitive behavior. Enhanced activation of the patch compartment of the striatum has been correlated with stereotypy, suggesting that stereotypy may be related to preferential activation of this region. However, the specific contribution of the patch compartment to METH-induced stereotypy is not clear. To elucidate the involvement of the patch compartment to the development of METH-induced stereotypy, we determined if destruction of this sub-region altered METH-induced behaviors. Animals were bilaterally infused in the striatum with the neurotoxin dermorphin–saporin (DERM–SAP; 17 ng/μl) to specifically ablate the neurons of the patch compartment. Eight days later, animals were treated with METH (7.5 mg/kg), placed in activity chambers, observed for 2 h and killed. DERM–SAP pretreatment significantly reduced the number and total area of mu-labeled patches in the striatum. DERM–SAP pretreatment significantly reduced the intensity of METH-induced stereotypy and the spatial immobility typically observed with METH-induced stereotypy. In support of this observation, DERM–SAP pretreatment also significantly increased locomotor activity in METH-treated animals. In the striatum, DERM–SAP pretreatment attenuated METH-induced c-Fos expression in the patch compartment, while enhancing METH-induced c-Fos expression in the matrix compartment. DERM–SAP pretreatment followed by METH administration augmented c-Fos expression in the SNpc and reduced METH-induced c-Fos expression in the SNpr. In the medial prefrontal, but not sensorimotor cortex, c-Fos and zif/268 expression was increased following METH treatment in animals pre-treated with DERM–SAP. These data indicate that the patch compartment is necessary for the expression of repetitive behaviors and suggests that alterations in activity in the basal ganglia may contribute to this phenomenon.     

Hippocampal cells encode places by forming small anatomical clusters

The hippocampus has been hypothesized to function as a “spatial” or “cognitive” map, however, the functional cellular organization of the spatial map remains a mystery. The majority of electrophysiological studies, thus far, have supported the view of a random-type organization in the hippocampus. However, using immediate early genes (IEGs) as an indicator of neuronal activity, we recently observed a cluster-type organization of hippocampal principal cells, whereby a small number (∼4) of nearby cells were activated in rats exposed to a restricted part of an environment. To determine the fine structure of these clusters and to provide a 3D image of active hippocampal cells that encode for different parts of an environment, we established a functional mapping of IEGs zif268 and Homer1a, using in situ hybridization and 3D-reconstruction imaging methods. We found that, in rats exposed to the same location twice, there were significantly more double IEG-expressing cells, and the clusters of nearby cells were more “tightly” formed, in comparison to rats exposed to two different locations. We propose that spatial encoding recruits specific cell ensembles in the hippocampus and that with repeated exposure to the same place the ensembles become better organized to more accurately represent the “spatial map.”     

Habituation reduces novelty-induced FOS expression in the striatum and cingulate cortex

Exposure to novel environments is known to induce c-fos expression in the cingulate cortex and dorsomedial striatum. This study examined the effects of 1, 3, and 7 days of exposure to sequential exposure to three novel environments. It was hypothesized that the ability of novel environment shuttling (NES) would induce c-fos expression as indicated by cells displaying FOS-like immunoreactivity (FLI) in the striatum and cingulate cortex which would decrease with repeated exposure. NES elicited FLI in both the cingulate cortex and striatum when compared to home-cage controls and this effect was attenuated with repeated exposure. Behavioral measures of activity decreased with repeated exposure suggesting that the decrease in FLI may be due to either a reduction in motor activity or increasing familiarity with the environments.     

Effect of a functional impairment of corticostriatal transmission on cortically evoked expression of c-Fos and zif 268 in the rat basal ganglia.

The activity-dependent induction of immediate-early genes is commonly used to map activated neuronal networks. In a previous analysis of the cortico-basal ganglia circuits, we have shown that a cortical stimulation produces Fos protein expression in the striatum and the subthalamic nucleus, with a pattern which conforms to the anatomical organization of cortical projections [Sgambato V. et al. (1996) Neuroscience 81, 93-112]. In the present study, we examined the effects of a unilateral blockade of the corticostriatal transmission on c-fos and zif 268 messenger RNA expression evoked in the substantia nigra pars reticulata and the subthalamic nucleus following stimulation of the ipsilateral motor cortex. The blockade of the corticostriatal pathway was performed either by an excitotoxic striatal lesion or by an application of the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione within the striatum. After application of the glutamate receptor antagonist, which prevented the cortical stimulation activating the GABAergic striatonigral pathway, the induction of both c-fos and zif 268 messenger RNAs was facilitated in the ipsilateral substantia nigra pars reticulata. In the subthalamic nucleus ipsilateral to the application of 6-cyano7-nitroquinoxaline-2,3-dione, the cellular discharges evoked by stimulation of the cortex were considerably shortened as a result of the blockade of the disinhibitory striato-pallido-subthalamic circuit. However, a strong expression of immediate-early genes was still induced by the cortical stimulation. By contrast, after unilateral kainate lesion of the striatum, the cortical stimulation was no longer able to induce c-fos and zif 268 messenger RNA expression in the ipsilateral subthalamic nucleus and in the substantia nigra pars reticulata bilaterally. The lack of immediate-early gene induction strongly contrasted with the neuronal discharges evoked in these nuclei by the cortical stimulation. Comparison between the cortically evoked neuronal activities and the pattern of immediate-early gene expression suggests that the induction of immediate-early genes in the basal ganglia mainly reflects the level of synaptic activity rather than the frequency of discharge of the postsynaptic neurons. Moreover, the results stress that modifications of immediate-early gene expression observed in the basal ganglia after an acute or a chronic interruption of the corticostriatal transmission are not superimposable.     

Handling alters cocaine-induced activity in adolescent but not adult male rats

The developmental period of adolescence is one that is characterized by increased levels of stress and vulnerability to drugs. Pre-test handling is an experimental manipulation that is used to acclimate animals prior to behavioral testing and exposure to a novel environment. Therefore, the present study was conducted in order to address the issue of pre-test handling of adolescent and adult male rats on subsequent cocaine-induced locomotor activity upon presentation to a novel environment. On days one through four, postnatal day (PND) 41-44 or PND 56-59, respectively, animals were handled b.i.d. for three minutes. On the fifth day, PND 45 or PND 60, animals were administered 30 mg/kg/ip cocaine or saline and immediately placed in a novel environment where locomotor activity was measured for 30 minutes. Cocaine increased locomotor activity similarly in all non-handled animals, regardless of age. Interestingly, adolescent animals expressed a differential effect when handled prior to an acute cocaine administration. Specifically, handling increased cocaine-induced locomotor activity in adolescent but not adult animals. These findings indicate that adolescent males that have been acclimated to the handling procedure experience significantly more behavioral reactivity than do adults to a high dose of cocaine upon exposure to a novel environment.