Electrolytic lesions of the nucleus accumbens enhance locomotor sensitization to nicotine in rats

Acute and chronic methylphenidate administration in intact and VTA-specific and nonspecific lesioned rats

Methylphenidate (MPD) is a psychostimulant used for the treatment of ADHD and works by increasing the bioavailability of dopamine (DA) in the brain. As a major source of DA, the ventral tegmental area (VTA) served as the principal target in this study as we aimed to understand its role in modulating the acute and chronic MPD effect. Forty-eight male Sprague-Dawley rats were divided into control, sham, electrical lesion, and 6-OHDA lesion groups. Given the VTA's implication in the locomotive circuit, three locomotor indices-horizontal activity, number of stereotypy, and total distance-were used to measure the animals' behavioral response to the drug. Baseline recording was obtained on experimental day 1 (ED 1) followed by surgery on ED 2. After recovery, the behavioral recordings were resumed on ED 8. All groups received daily intraperitoneal injections of 2.5 mg/kg MPD for six days after which the animals received no treatment for 3 days. On ED 18, 2.5 mg/kg MPD was re-administered to assess for the chronic effect of the psychostimulant. Except for one index, there was an increase in locomotive activity in all experimental groups after surgery (in comparison to baseline activity), acute MPD exposure, induction with six daily doses, and after MPD re-challenge. Furthermore, the increase was greatest in the electrical VTA lesion group and lowest in the 6-OHDA VTA lesion group. In conclusion, the results of this study suggest that the VTA may not be the primary nucleus of MPD action, and the VTA plays an inhibitory role in the locomotive circuit.

Mechanistic Resolution Required to Mediate Operant Learned Behaviors: Insights from Neuronal Ensemble-Specific Inactivation

Many learned behaviors are directed by complex sets of highly specific stimuli or cues. The neural mechanisms mediating learned associations in these behaviors must be capable of storing complex cue information and distinguishing among different learned associations—we call this general concept “mechanistic resolution”. For many years, our understanding of the circuitry of these learned behaviors has been based primarily on inactivation of specific cell types or whole brain areas regardless of which neurons were activated during the cue-specific behaviors. However, activation of all cells or specific cell types in a brain area do not have enough mechanistic resolution to encode or distinguish high-resolution learned associations in these behaviors. Instead, these learned associations are likely encoded within specific patterns of sparsely distributed neurons called neuronal ensembles that are selectively activated by the cues. This review article focuses on studies of neuronal ensembles in operant learned responding to obtain food or drug rewards. These studies suggest that the circuitry of operant learned behaviors may need to be re-examined using ensemble-specific manipulations that have the requisite level of mechanistic resolution.

Nicotine produces chronic behavioral sensitization with changes in accumbal neurotransmission and increased sensitivity to re-exposure

Tobacco use is often associated with long-term addiction as well as high risk of relapse following cessation. This is suggestive of persistent neural adaptations, but little is known about the long-lasting effects of nicotine on neural circuits. In order to investigate the long-term effects of nicotine exposure, Wistar rats were treated for 3 weeks with nicotine (0.36 mg/kg), and the duration of behavioral and neurophysiological adaptations was evaluated 7 months later. We found that increased drug-induced locomotion persisted 7 months after the initial behavioral sensitization. In vitro analysis of synaptic activity in the core and shell of the nucleus accumbens (nAc) revealed a decrease in input/output function in both regions of nicotine-treated rats as compared to vehicle-treated control rats. In addition, administration of the dopamine D2 receptor agonist quinpirole (5 μM) significantly increased evoked population spike amplitude in the nAc shell of nicotine-treated rats as compared to vehicle-treated control rats. To test whether nicotine exposure creates long-lasting malleable circuits, animals were re-exposed to nicotine 7 months after the initial exposure. This treatment revealed an increased sensitivity to nicotine among animals previously exposed to nicotine, with higher nicotine-induced locomotion responses than observed initially. In vitro electrophysiological recordings in re-exposed rats detected an increased sensitivity to dopamine D2 receptor activation. These results suggest that nicotine produces persistent neural adaptations that make the system sensitive and receptive to future nicotine re-exposure.

Contribution of nucleus accumbens core (AcbC) to behavior control during a learned resting period: introduction of a novel task and lesion experiments

In recent years, the study of resting state neural activity has received much attention. To better understand the roles of different brain regions in the regulation of behavioral activity in an arousing or a resting period, we developed a novel behavioral paradigm (8-arm food-foraging task; 8-arm FFT) using the radial 8-arm maze and examined how AcbC lesions affect behavioral execution and learning. Repetitive training on the 8-arm FFT facilitated motivation of normal rats to run quickly to the arm tips and to the center platform before the last-reward collection. Importantly, just after this point and before confirmation of no reward at the next arm traverse, locomotor activity decreased. This indicates that well-trained rats can predict the absence of the reward at the end of food seeking and then start another behavior, namely planned resting. Lesions of the AcbC after training selectively impaired this reduction of locomotor activity after the last-reward collection without changing activity levels before the last-reward collection. Analysis of arm-selection patterns in the lesioned animals suggests little influence of the lesion in the ability to predict the reward absence. AcbC lesions did not change exploratory locomotor activity in an open-field test in which there were no rewards. This suggests that the AcbC controls the activity level of planned resting behavior shaped by the 8-arm FFT. Rats receiving training after AcbC lesioning showed a reduction in motivation for reward seeking. Thus, the AcbC also plays important roles not only in controlling the activity level after the last-reward collection but also in motivational learning for setting the activity level of reward-seeking behavior.

Nucleus accumbens lesions modulate the effects of methylphenidate

The psychostimulant methylphenidate (MPD, Ritalin) is the prescribed drug of choice for treatment of ADHD. In recent years, the diagnosis rate of ADHD has increased dramatically, as have the number of MPD prescriptions. Repeated exposure to psychostimulants produces behavioral sensitization in rats, an experimental indicator of a drug's potential liability. In studies on cocaine and amphetamine, this effect has been reported to involve the nucleus accumbens (NAc), one of the nuclei belonging to the motive circuit. The aim of this study was to investigate the role of the NAc on the expression of behavioral sensitization as a response to MPD exposure. In the present study, 20 male Sprague-Dawley rats were divided randomly into three groups: an intact control group, a sham-operated group, and a NAc bilateral electrical lesion group. Locomotor activity was assessed for the first 2h following 2.5mg/kg MPD injection, using open field monitoring systems. Recordings were made during 6 days of continuous MPD administration, and then upon re-challenge with the same dose following 3 days of washout. Acute MPD exposure elicited an increase in locomotor activity in all three groups. However, the NAc lesion group exhibited significantly increased locomotor activity in comparison to sham and control groups. Chronic MPD did not elicit sensitization in the NAc lesion group, while both sham and control groups did exhibit behavioral sensitization to repetitive MPD administration. These findings suggest that the NAc plays a significant role in eliciting locomotor activity as an acute effect of MPD, and in the expression of sensitization due to chronic MPD exposure.

Descending glutamatergic pathways of PFC are involved in acute and chronic action of methylphenidate

Progressive augmentation of behavioral response following repeated psychostimulant administrations is known as behavioral sensitization, and is an indicator of a drug's liability for abuse. It is known that methylphenidate (MPD) (also known as Ritalin), a drug used to treat attention-deficit hyperactivity disorder (ADHD), induces sensitization in animals following repeated injections. It was recently reported that bilateral electric (non-specific) lesion of prefrontal cortex (PFC) prevented MPD elicited behavioral sensitization. Since PFC sends glutamatergic afferents to both ventral tegmental area (VTA) and nucleus accumbens (NAc), sites that are involved in induction and expression of behavioral sensitization respectively and glutamate from PFC is known to modulate dopamine cell activity in VTA and NAc, this study investigated the role of descending glutamate from PFC in MPD elicited behavioral sensitization. Locomotor activity of three groups of rats-control, sham operated and group with specific chemical lesion of glutamate neurons of PFC-was recorded using an open-field assay. On experimental day (ED) 1, the locomotor activity was recorded post a saline injection. The sham and lesion groups underwent respective surgeries on ED 2, and were allowed to recover for 5 days (from ED 3 to ED 7). The post-surgery baseline was recorded on ED 8 following a saline injection. On ED's 9 through 14, 2.5 mg/kg MPD was given, followed by a 4-day washout period (ED 15 -18). All three groups received a rechallenge injection of 2.5 mg/kg on ED 19 and their locomotor activity on various days was analyzed. It was found that ibotenic acid lesion modulated the acute and chronic effects of MPD and hence suggests that PFC glutamatergic afferents are involved in the acute effect of MPD as well as in its chronic effects such as behavioral sensitization to MPD.

Rimonabant blocks the expression but not the development of locomotor sensitization to nicotine in rats

RATIONALE

Cannabinoid, especially CB(1,) receptors have been implicated in the development and expression of a variety of behaviors produced by addictive drugs.

OBJECTIVES

The intent was to determine if coadministration of the selective CB(1) receptor antagonist, rimonabant (SR141716A), would block the development or expression of locomotor sensitization to repeated injections of nicotine.

MATERIALS AND METHODS

Male Long-Evans rats were injected with either 2 mg/kg rimonabant or its vehicle 30 min before an injection of 0.4 mg/kg nicotine or saline and immediately placed in activity chambers for 1 h on each of six sessions on alternating days. Before the two subsequent challenge sessions, all rats were injected with the vehicle and 0.4 mg/kg nicotine combination and then with the 2 mg/kg rimonabant and 0.4 mg/kg nicotine combination, respectively.

RESULTS

Repeated injections of nicotine produced a progressive increase in locomotion that was blocked by coadministration of rimonabant. However, the subsequent nicotine challenge increased locomotion in both nicotine-pretreated groups equally more than in the saline-pretreated groups. Coadministration of rimonabant along with nicotine on the second challenge decreased the locomotion of the nicotine-pretreated rats to equal that of the saline-pretreated rats. Rimonabant had no effect on the saline-pretreated rats.

CONCLUSION

These data suggest that rimonabant blocks the expression but not the development of locomotor sensitization to nicotine.