Fos and glutamate AMPA receptor subunit coexpression associated with cue-elicited cocaine-seeking behavior in abstinent rats.

Cocaine-associated cues acquire incentive motivational effects that manifest as craving in humans and cocaine-seeking behavior in rats. We have reported an increase in neuronal activation in rats, measured by Fos protein expression, in various limbic and cortical regions following exposure to cocaine-associated cues. This study examined whether the conditioned neuronal activation involves glutamate AMPA receptors by measuring coexpression of Fos and AMPA glutamate receptor subunits (GluR1, GluR2/3, or GluR4). Rats trained to self-administer cocaine subsequently underwent 22 days of abstinence, during which they were exposed daily to either the self-administration environment with presentations of the light/tone cues previously paired with cocaine infusions (Extinction group) or an alternate environment (No Extinction group). All rats were then tested for cocaine-seeking behavior (i.e. responses without cocaine reinforcement) and Fos and AMPA glutamate receptor subunits were measured postmortem using immunocytochemistry. The No Extinction group exhibited increases in cocaine-seeking behavior and Fos expression in limbic and cortical regions relative to the Extinction group. A large number of Fos immunoreactive cells coexpressed GluR1, GluR2/3, and GluR4, suggesting that an action of glutamate at AMPA receptors may in part drive cue-elicited Fos expression. Importantly, there was an increase in the percentage of cells colabeled with Fos and GluR1 in the anterior cingulate and nucleus accumbens shell and cells colabeled with Fos and GluR4 in the infralimbic cortex, suggesting that within these regions, a greater, and perhaps even different, population of AMPA receptor subunit-expressing neurons is activated in rats engaged in cocaine-seeking behavior.

Paradoxical locomotor activating effects of kappa-opioid receptor stimulation in the preweanling rat: role of the ventromedial thalamus and superior colliculus.

Stimulating kappa-opioid receptors in the substantia nigra pars reticulata robustly increases the locomotor activity of preweanling rats. To determine whether nigrothalamic and nigrotectal connections are necessary for this kappa-opioid-mediated locomotor activity, preweanling rats were given a systemic injection of saline or 5 mg/kg U50,488 (a kappa-opioid receptor agonist) 2 days after receiving sham or bilateral electrolytic lesions of the ventromedial thalamus (VMT) or superior colliculus (SC). Results showed that lesions of the VMT and SC attenuated the U50,488-induced locomotor activity of preweanling rats, indicating that the locomotor activating effects of kappa-opioid receptor stimulation require that nigrothalamic and nigrotectal connections be intact.

Effects of acute and repeated methamphetamine treatment on the ultrasonic vocalizations of postnatal rats.

Repeated exposure to cocaine (COC) has been reported to both increase ultrasonic vocalizations (USVs) of postnatal rats and enhance the crying of human infants. The purpose of the present study was to determine whether acute or repeated treatment with another commonly abused psychostimulant, methamphetamine (MAP), would affect the USV production of postnatal rats. In the first experiment, USVs were measured 30 min after rats were given an acute injection of saline or MAP (1, 2, 4, or 8 mg/kg ip) on postnatal day (PD) 10. In the second experiment, rats were exposed to MAP (0, 1, or 4 mg/kg/day ip) on PD 2-8 or PD 2-9. On PD 10, rats were given an acute injection of saline or MAP (1 or 4 mg/kg ip) 30 min prior to behavioral assessment. Results showed that acute treatment with MAP (4 or 8 mg/kg) decreased the USVs of rats on PD 10, while repeated exposure to MAP did not affect the USV emissions of rats subsequently treated with saline or MAP. The reason why acute MAP treatment decreased USV production is uncertain, but it is possible that MAP alleviates isolation distress by stimulating reward processes. Alternatively, MAP increases heart rate and blood pressure, so acute treatment with this drug may decrease USV emissions through peripheral physiological mechanisms (i.e., by reducing abdominal compression reactions).

Chronic amphetamine exposure during the preweanling period does not affect avoidance learning or novelty-seeking of adult rats.

The purpose of the present study was to determine whether exposure to amphetamine during the preweanling period would impact the learning or reward processes of rats tested in adulthood. In three experiments we examined whether amphetamine treatment (0-10 mg/kg per day) on postnatal days 11-17 altered the subsequent performance of adult Sprague-Dawley rats on a step-down passive avoidance, active avoidance, or novelty-seeking task. There was no evidence that postnatal amphetamine exposure affected performance on any of these tasks. These results suggest that the long-term impact of pre- and postnatal psychostimulant exposure differs, because in utero stimulant treatment is known to produce learning deficits and decrease reinforcement efficacy of rats tested in adulthood.

Long-term effects of postnatal amphetamine treatment on striatal protein kinase A activity, dopamine D(1)-like and D(2)-like binding sites, and dopamine content.

The purpose of the present study was to determine whether exposure to amphetamine during the preweanling period would alter dopaminergic functioning in the dorsal striatum of adult rats. In three experiments, we assessed the effects of repeated amphetamine treatment on striatal protein kinase A (PKA) activity, dopamine (DA) D(1)-like and D(2)-like binding sites, and DA content. Rats were pretreated with saline or amphetamine (2.5 mg/kg, ip) for 7 consecutive days starting on postnatal day (PD) 11. At PD 90, rats were killed and their dorsal striata (i.e., caudate-putamen) were removed and frozen until time of assay. Amphetamine pretreatment produced long-term reductions in both striatal PKA activity and DA content. Early amphetamine exposure also resulted in an upregulation of D(2)-like binding sites, while leaving D(1)-like binding sites unaffected. It is likely that the upregulation of D(2)-like binding sites was stimulated by the persistent decline in striatal DA levels. Although speculative, it is possible that excess striatal D(2)-like receptors were responsible for inhibiting PKA activity through actions on the cAMP signal transduction pathway. The behavioral relevance of these amphetamine-induced neurochemical changes has not yet be determined.

Amphetamine treatment during the preweanling period produces enduring changes in striatal protein kinase A activity.

The purpose of this study was to determine whether chronic exposure to amphetamine during the preweanling period causes enduring changes in behavioral and neuronal functioning. In two experiments rats were injected with saline or amphetamine (2.5 or 5.0 mg/kg) on postnatal days (PD) 11-15. Rats then received a challenge injection of saline or 2.5 mg/kg amphetamine on PD 23 or PD 90 and locomotor activity was measured. After behavioral assessment, rats were killed, and their dorsal striata and nucleus accumbens were dissected and later assayed for protein kinase A (PKA) activity. Interestingly, amphetamine treatment during the preweanling period produced an enduring decline in dorsal striatal and accumbal PKA activity that was still apparent in adulthood. These reductions in PKA activity were not related to the occurrence of locomotor sensitization, because rats did not exhibit a sensitized locomotor response when challenged with amphetamine at PD 23 or PD 90.

Cocaine-induced behavioral sensitization in the young rat.

RATIONALE: Repeated psychostimulant treatment has been shown to sensitize the locomotor activity of young rats, but there is conflicting evidence suggesting that this sensitized response will persist across only a few drug abstinence days. OBJECTIVE: The purpose of the present study was to determine whether: (a) young rats are capable of expressing a sensitized locomotor response after an extended drug abstinence period, and (b) the longevity of the sensitized response is critically affected by either the number of drug pretreatment days or environmental conditioning factors. METHODS: Young rats were pretreated with saline or cocaine (15 mg/kg or 30 mg/kg, i.p.) for either five or ten consecutive days [i.e., on postnatal days (PD) 16-20 or PD 11-20]. After each daily injection, rats were placed in activity chambers, and locomotion was measured for 30 min. To assess environmental conditioning factors, some rats were injected with saline prior to being placed in the activity chambers and then injected with cocaine prior to being returned to the home cage. After one or seven abstinence days (i.e., on PD 22 or PD 28), rats received a challenge injection of saline or cocaine (15 mg/kg) in the activity chamber and locomotion was assessed. RESULTS: Young rats exhibit cocaine-induced locomotor sensitization after either a short (1-day) or long (7-day) drug abstinence period. When a long abstinence period was used, locomotor sensitization was only apparent when cocaine pretreatment lasted for 10 days. Conditioning factors were also important for determining whether locomotor sensitization was expressed, because young rats pretreated with cocaine in the home cage did not show a sensitized locomotor response after seven abstinence days. CONCLUSIONS: Young rats are capable of showing cocaine-induced locomotor sensitization after an extended abstinence period. Both the number of drug pretreatment days and the environmental context in which cocaine was given (i.e., the activity chamber or home cage) influenced the longevity of cocaine-induced locomotor sensitization.

kappa-Opioid receptors in the substantia nigra pars reticulata mediate the U-50,488-induced locomotor activity of preweanling rats.

The purpose of the present study was to determine the neuroanatomical location where kappa-opioid receptor stimulation induces locomotor activity in the preweanling rat. To confirm that the U-50,488-induced locomotor activity of preweanling rats is mediated by kappa-opioid receptors, 18-day-old rats were initially injected with vehicle or the kappa-opioid receptor agonist U-50,488 (5 mg/kg, s.c.) followed, 15 min later, by an injection of the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI; 0, 2, 4, 8, or 12 mg/kg, s.c.). In subsequent experiments, 18-day-old rats were injected with vehicle or U-50,488 (5 mg/kg, s.c.) 15 min prior to bilateral administration (0.25 or 0.5 microl per side) of nor-BNI (0, 5, 10, or 20 microg) into the substantia nigra pars reticulata (SNR) or medial dorsal striatum (MDS). In the final experiment, 18-day-old rats received bilateral administration (0.25 microl per side) of vehicle or U-50,488 (0.0, 0.8, 1.6, or 3.2 microg) into the SNR. Results showed that systemically administered nor-BNI (0-12 mg/kg, s.c.) produced a dose-dependent reduction in the U-50, 488-induced locomotor activity of preweanling rats. The site of action for U-50,488's locomotor-activating effects appeared to be the SNR, because (a) bilateral administration of nor-BNI (5, 10, or 20 microg) into the SNR caused a complete attenuation of U-50, 488-induced locomotion, and (b) bilateral administration of U-50,488 into the SNR caused a dose-dependent increase in the locomotor activity of preweanling rats. Striatal injections of nor-BNI did not affect U-50,488-induced locomotor activity. When these findings are considered together it is apparent that stimulation of kappa-opioid receptors in the SNR is both necessary and sufficient for the occurrence of U-50,488-induced locomotor activity in the preweanling rat.

Kappa opioid-mediated behavioral sensitization in the preweanling rat: relationship to Fos immunoreactivity.

When given acutely, drugs that stimulate kappa opioid receptors (e.g., U-50,488) enhance the locomotor activity of preweanling rats and induce regional increases in Fos immunoreactivity (IR). In contrast, the effects of chronic treatment with kappa opioid agonists are unknown. The purpose of the present study was two-fold: first, to determine whether repeated treatment with a kappa opioid agonist would sensitize the locomotor activity of preweanling rats and, second, to determine whether changes in Fos IR correspond with the occurrence of locomotor sensitization. To test these hypotheses, rats were injected with U-50,488 (5 mg/kg, s.c.) or saline on either postnatal days (PD) 5-9 or PD 11-15. For rats pretreated on PD 5-9, a test day injection of U-50,488 or saline was given after either 1 or 7 abstinence days (i.e., at PD 11 or PD 17). For rats pretreated on PD 11-15, a test day injection of U-50,488 or saline was given after 1 abstinence day (i.e., at PD 17). In two additional experiments, the acute and chronic effects of U-50,488 treatment were assessed in adult rats. As expected, repeated treatment with U-50,488 produced locomotor sensitization at both PD 11 and PD 17, but only when the test day occurred 1, and not 7, days after cessation of drug pretreatment. Thus, the persistence of the sensitized response was very brief. Test day treatment with U-50,488 stimulated Fos IR in various brain regions of the preweanling rat, including the medial striatum, nucleus accumbens, lateral habenula, and septal area. Chronic treatment with U-50,488 depressed Fos expression in a number of brain regions (relative to acutely treated rats); however, these changes in Fos IR did not necessarily coincide with the occurrence of behavioral sensitization. Repeated treatment with U-50,488 did not produce locomotor sensitization in adult rats, so Fos IR was not assessed in this age group. Therefore, while acute treatment with U-50,488 both increased locomotor activity and stimulated Fos IR in preweanling rats, chronic U-50,488 treatment produced behavioral changes that did not correspond with Fos expression.