Increase in medial frontal cortex ERK activation following the induction of apomorphine sensitization

Morphine and dopamine: Low dose apomorphine can prevent both the induction and expression of morphine locomotor sensitization and conditioning

The disinhibition of dopamine neurons in the VTA by morphine is considered an important contributor to the reward potency of morphine. In this report, three experiments were conducted in which a low dose of apomorphine (0.05mg/kg) was used as a pretreatment to reduce dopamine activity. Locomotor hyperactivity was used as the behavioral response to morphine (10.0mg/kg). In the first experiment, five treatments with morphine induced the development of locomotor and conditioned hyperactivity that were prevented by apomorphine given 10min prior to morphine. Apomorphine before either vehicle or morphine induced equivalent reductions in locomotion. In the second experiment, the apomorphine pretreatment was initiated after induction of a conditioned hyperactivity and the apomorphine prevented the expression of the conditioning. To assess the effects of the apomorphine on VTA and the nucleus accumbens, ERK measurements were carried out after the induction of locomotor and conditioned hyperactivity. Increased ERK activation was found and these effects were prevented by the apomorphine in both experiments. A third experiment was conducted to assess the effects of acute morphine on ERK before locomotor stimulation was induced by morphine. Acute morphine did not increase locomotion, but a robust ERK response was produced indicating that the morphine induced ERK activation was not secondary to locomotor stimulation. The ERK activation was again prevented by the apomorphine pretreatment. We suggest that contiguity between the ongoing behavioral activity and the morphine activation of the dopamine reward system incentivizes and potentiates the ongoing behavior generating equivalent behavioral sensitization and conditioned effects.

Context evoked morphine conditioned effects can be equivalent to morphine induced drug effects in terms of behavioral response and ERK activation in reward associated subcortical brain structures

Conditioned drug cues can evoke brief drug-like responses. In this report we show that using brief test sessions, contextual cues can induce conditioned hyperlocomotion and ERK responses equivalent to morphine induced responses. To assess acute unconditioned effects, rats that received morphine (MOR-1) or vehicle (VEH-1), were immediately placed onto an arena for a 5-min locomotion recording session after which ERK was measured in the ventral tegmental area (VTA) and nucleus accumbens (NAc). There were no differences in locomotion between the groups. However, the MOR-1 group had strong ERK activation in VTA and NAc. To assess MOR-conditioned effects, a chronic phase was carried out according to a Pavlovian conditioning protocol. There were two MOR paired groups (MORP), one MOR unpaired (MOR-UP) group and two VEH groups. The treatments were administered over 5 daily five minute test sessions. The final conditioning test was on day 6, in which one of the MOR-P groups and one of the VEH groups received VEH (MOR-P/VEH-6 and VEH/VEH-6, respectively). The other MOR-P group and VEH group received MOR (MOR-P/MOR; VEH/MOR-6, respectively). The MOR-UP group received VEH (MOR-UP/VEH-6). Rats received the treatments immediately prior to a 5-min arena test, and after the session ERK was measured. No morphine induced locomotor stimulation was observed on day 1 but on days 2 to 5, hyperlocomotion in both MOR-P groups occurred. On test day 6, the MOR-P/VEH-6 and the MOR-P/MOR-6 groups had comparable locomotor stimulant responses and similar ERK activity in the VTA and NAc. The MOR-UP group did not differ from the VEH group. We suggest that ERK activation evoked by acute morphine served as a Pavlovian unconditioned stimulus to enable the contextual cues to acquire morphine conditioned stimulus properties and increase the incentive value of the contextual cues.

Morphine reward effects and morphine behavioral sensitization: The adventitious association of morphine activation of brain reward effects with ongoing spontaneous activity

The development of sensitization is one of the hallmarks of addictive drugs such as morphine. We administered morphine (10 mg/kg; MOR) to induce locomotor sensitization and ERK activation in the VTA and NAc. In the first experiment, four groups of rats received five daily 30 min sessions in an open-field, and locomotion was measured. For the first four sessions, one group received MOR pre-test (MOR-P); a second group received vehicle pre-test (MOR-UP) and MOR 30 min post-test; the remaining 2 groups received vehicle (VEH) pre-test. On the fifth session, the MOR-P, MOR-UP, and one VEH group received MOR pre-test and the remaining VEH group received VEH. Sensitization emerged in the first 5 min and progressed over to the second and third 5 min blocks only in the MOR-P group. For the second experiment, 4 groups received MOR and 4 groups VEH, and were then returned to their home cage and after 5, 15, 30 or 60 min post-injection, were euthanized for ERK measurements in VTA and NAc. ERK activation increased and peaked at 5 min post injection in the MOR group and then declined to VEH levels by 30 min. Another two groups received either MOR or VEH immediately before a 5 min arena test and ERK was measured immediately post-test. MOR had no effect on locomotion but increased ERK in the VTA and NAc. The peak ERK activation in VTA reflected activation of reward systems by morphine that reinforced locomotor behavior and with repeated treatments, induced a sensitization effect.

Reciprocal activation/inactivation of ERK in the amygdala and frontal cortex is correlated with the degree of novelty of an open-field environment

RATIONALE

Phosphorylated extracellular signal-regulated kinase (ERK) has been used to identify brain areas activated by exogenous stimuli including psychostimulant drugs.

OBJECTIVE

Assess the role of the amygdala in emotional responses.

METHODS

Experimental manipulations were performed in which environmental familiarity was the variable. To provide the maximal degree of familiarity, ERK was measured after removal from the home cage and re-placement back into the same cage. To maximize exposure to an unfamiliar environment, ERK was measured following placement into a novel open field. To assess whether familiarity was the critical variable in the ERK response to the novel open field, ERK was also measured after either four or eight placements into the same environment. ERK quantification was carried out in the amygdala, frontal cortex, and the nucleus accumbens.

RESULTS

After home cage re-placement, ERK activation was found in the frontal cortex and nucleus accumbens but was absent in the amygdala. Following placement in a novel environment, ERK activation was more prominent in the amygdala than the frontal cortex or nucleus accumbens. In contrast, with habituation to the novel environment, ERK phosphors declined markedly in the amygdala but increased in the frontal cortex and nucleus accumbens to the level observed following home cage re-placement.

CONCLUSIONS

The differential responsiveness of the amygdala versus the frontal cortex and the nucleus accumbens to a novel versus a habituated environment is consistent with a reciprocal interaction between these neural systems and points to their important role in the mediation of behavioral activation to novelty and behavioral inactivation with habituation.