The role of dopamine D₂ receptor mechanisms in the development of MDMA sensitisation

<p>Rationale. ±3, 4-methelynedioxymethamphetamine (MDMA) is the primary psychoactive ingredient of the increasingly popular recreational street drug, ecstasy. As with other drugs of abuse, repeated intermitted exposure to MDMA can lead to an increase in the subsequent behavioural effects of the drug. This phenomenon, termed behavioural sensitisation, has been attributed to sensitisation of central DAergic mechanisms considered to underlie several aspects of addiction. Objectives. The purpose of the present research was to investigate the role of DA D₂ receptor mechanisms in the development of MDMA sensitisation and the acquisition of MDMA self-administration in rats. Methods. Rats received daily i.p. injections of the selective D₂ antagonist, eticlopride (0.0, 0.05, 0.3 mg/kg), prior to injections of MDMA (0.0, 10.0 mg/kg) for five days. Two days following the final pre-treatment session, the locomotor activating effects of MDMA (5 mg/kg, i.p.) were determined. Another group of rats were surgically implanted with i.v. jugular catheters before undergoing the same pre-treatment regimen. Two days following the final pre-treatment session, these rats were subsequently tested for acquisition of MDMA self-administration. The locomotor activating effects of MDMA (5 mg/kg i.p.) were determined two days following the last self-administration session. Results. Pre-treatment with MDMA enhanced the locomotor activating effects of MDMA and facilitated the acquisition of MDMA self-administration, as evidenced by an increased likelihood to meet an acquisition criterion. Co-administration of eticlopride during pre-treatment completely blocked the development of sensitisation to MDMA-produced hyperactivity but failed to significantly attenuate the facilitation of MDMA self-administration. Interestingly, pre-treatment with eticlopride alone also facilitated the acquisition of self-administration. MDMA self-administration failed to alter MDMA-produced locomotor hyperactivity. Conclusions. These findings suggest that repeated activation of DA D₂ receptors is required for the development of sensitisation to MDMA-produced hyperactivity but not for the development of sensitisation to MDMA-produced reinforcement. D₂ receptor mechanisms evidently play some role, however, because repeated exposure to eticlopride also facilitated MDMA self-administration. It is suggested that both sensitised DAergic mechanisms and desensitised 5-HTergic mechanisms contribute to the acquisition of MDMA self-administration.</p>

The role of dopamine D₂ receptor mechanisms in the development of MDMA sensitisation

<p>Rationale. ±3, 4-methelynedioxymethamphetamine (MDMA) is the primary psychoactive ingredient of the increasingly popular recreational street drug, ecstasy. As with other drugs of abuse, repeated intermitted exposure to MDMA can lead to an increase in the subsequent behavioural effects of the drug. This phenomenon, termed behavioural sensitisation, has been attributed to sensitisation of central DAergic mechanisms considered to underlie several aspects of addiction. Objectives. The purpose of the present research was to investigate the role of DA D₂ receptor mechanisms in the development of MDMA sensitisation and the acquisition of MDMA self-administration in rats. Methods. Rats received daily i.p. injections of the selective D₂ antagonist, eticlopride (0.0, 0.05, 0.3 mg/kg), prior to injections of MDMA (0.0, 10.0 mg/kg) for five days. Two days following the final pre-treatment session, the locomotor activating effects of MDMA (5 mg/kg, i.p.) were determined. Another group of rats were surgically implanted with i.v. jugular catheters before undergoing the same pre-treatment regimen. Two days following the final pre-treatment session, these rats were subsequently tested for acquisition of MDMA self-administration. The locomotor activating effects of MDMA (5 mg/kg i.p.) were determined two days following the last self-administration session. Results. Pre-treatment with MDMA enhanced the locomotor activating effects of MDMA and facilitated the acquisition of MDMA self-administration, as evidenced by an increased likelihood to meet an acquisition criterion. Co-administration of eticlopride during pre-treatment completely blocked the development of sensitisation to MDMA-produced hyperactivity but failed to significantly attenuate the facilitation of MDMA self-administration. Interestingly, pre-treatment with eticlopride alone also facilitated the acquisition of self-administration. MDMA self-administration failed to alter MDMA-produced locomotor hyperactivity. Conclusions. These findings suggest that repeated activation of DA D₂ receptors is required for the development of sensitisation to MDMA-produced hyperactivity but not for the development of sensitisation to MDMA-produced reinforcement. D₂ receptor mechanisms evidently play some role, however, because repeated exposure to eticlopride also facilitated MDMA self-administration. It is suggested that both sensitised DAergic mechanisms and desensitised 5-HTergic mechanisms contribute to the acquisition of MDMA self-administration.</p>

Genetic depletion of BDNF impairs both context-dependent and context-independent extinction learning of a spatial appetitive task

Brain derived neurotropic factor (BDNF) supports neuronal survival, growth, and differentiation and is involved in forms of hippocampus-dependent learning, as well as hippocampus-dependent learning. Extinction learning (EL) comprises active inhibition of no-longer relevant learned information, in conjunction with a decreased response of a previously learned behavior. It is highly dependent on context, and evidence exists that it requires hippocampal activation. Concordantly, the participation of BDNF in hippocampus-dependent memory is experience-dependent. BDNF has been associated with synaptic plasticity needed for acquisition and extinction learning of fear conditioning. However, little is known about its influence on the extinction and renewal of spatial appetitive extinction learning (EL). In this study, in BDNF+/−-mice we evaluated to what extent BDNF contributes to spatial appetitive EL in the presence (ABA) or absence (AAA) of a context change. Daily training, to reach acquisition criterion in a T-maze, resulted in a similar outcome in BDNF+/−-mice or their wildtype (wt) littermates. EL was delayed in the AAA, and significantly impaired in the ABA-context compared to EL in wt littermates. When renewal was tested in the ABA paradigm we detected a significant response in wt controls, but not in BDNF+/−-mice. Taken together, these results support an important role for BDNF in EL in AAA and ABA context, as well as renewal of a spatial appetitive task, processes that relate to information updating and retrieval.

Crowding and Litter Effects on Retention and Reversal Learning by Young Rats

72 rats both male and female from litters of 4 or 11, placed 1, 2, or 4/cage until age 65 days, acquired a position response under food deprivation. After 24 hr. each rat relearned to the same criterion, which was followed immediately by reversal training. Males acquired the response sooner, but females and litters of four retained more than their counterparts. Males housed 1/cage were slower to reverse than males housed 4/cage. When each group was subdivided into rats that consumed the reward upon the first correct response occurrence (immediate eaters) and those which did not consume the reward immediately (delay eaters), delay eaters required more trials to acquisition criterion, were slower to respond, had higher savings scores, and took longer to reverse. These findings, coupled with significant correlations of lower savings scores with greater increases in latency from acquisition to re-learning, suggested that emotionality interfered with retention by males and litters of 11. Similarly, emotionality that accompanies isolation was assumed to increase among male rats, to retard reversal learning, because the task increased in difficulty.

Effects of Overtraining on Reversal and Nonreversal Discrimination Shifts in a Free Operant Situation

Rats were trained on a single-stimulus, successive discrimination in a free operant situation. An irrelevant stimulus dimension was present at ail times. Following attainment of the acquisition criterion, Ss were shifted immediately or given 4 or 8 days of overtraining before being shifted. Half of Ss were given a reversal shift and half a nonreversal shift. Overtraining did not affect reversal or nonreversal learning. These results were contrasted with those of Mackintosh (1962). Various theoretical issues were discussed in light of the present findings. It was tentatively concluded that the overtraining effect depends upon the role of observing behavior in the formation and overtraining of a discrimination.