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

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.

RGS14 modulates locomotor behavior and ERK signaling induced by environmental novelty and cocaine within discrete limbic structures

RATIONALE

In rodents, exposure to novel environments or psychostimulants promotes locomotion. Indeed, locomotor reactivity to novelty strongly predicts behavioral responses to psychostimulants in animal models of addiction. RGS14 is a plasticity-restricting protein with unique functional domains that enable it to suppress ERK-dependent signaling as well as regulate G protein activity. Although recent studies show that RGS14 is expressed in multiple limbic regions implicated in psychostimulant- and novelty-induced hyperlocomotion, its function has been examined mostly in the context of hippocampal physiology and memory.

OBJECTIVE

We investigated whether RGS14 modulates novelty- and cocaine-induced locomotion (NIL and CIL, respectively) and neuronal activity.

METHODS

We assessed Rgs14 knockout (RGS14 KO) mice and wild-type (WT) littermate controls using NIL and CIL behavioral tests, followed by quantification of c-fos and phosphorylated ERK (pERK) induction in limbic regions that normally express RGS14.

RESULTS

RGS14 KO mice were less active than WT controls in the NIL test, driven by avoidance of the center of the novel environment. By contrast, RGS14 KO mice demonstrated augmented peripheral locomotion in the CIL test conducted in either a familiar or novel environment. RGS14 KO mice exhibited increased thigmotaxis, as well as greater c-fos and pERK induction in the central amygdala and dorsal hippocampus, when cocaine and novelty were paired.

CONCLUSIONS

RGS14 KO mice exhibited anti-correlated locomotor responses to novelty and cocaine, but displayed increased thigmotaxis in response to either stimuli which was augmented by their combination. Our findings also suggest RGS14 may reduce neuronal activity in limbic subregions by inhibiting ERK-dependent signaling.

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.

GPR88 in D1R-Type and D2R-Type Medium Spiny Neurons Differentially Regulates Affective and Motor Behavior

The orphan receptor GPR88 is highly expressed in D1 receptor (D1R)- and D2R-medium spiny neurons (MSNs) and has been associated to striatum-dependent functions in rodents. The total deletion of Gpr88 in mice was shown to decrease anxiety-like behaviors, increase stereotypies and locomotion, and impair motor coordination and motor learning. Knowing the opposing role of D1R- and D2R-MSNs, we here investigated the respective roles of GPR88 in the two MSN subtypes for these behaviors. To do so, we compared effects of a conditional Gpr88 gene knock-out (KO) in D1R-MSNs (D1R-Gpr88 mice) or D2R-MSNs (A_2AR-Gpr88 mice) with effects of the total Gpr88 KO (CMV-Gpr88 mice). Overall, most phenotypes of CMV-Gpr88 mice were recapitulated in A_2AR-Gpr88 mice, including reduced marble burying, increased social interactions, increased locomotor activity and stereotypies in the open field, and reduced motor coordination in the rotarod. Exceptions were the reduced habituation to the open field and reduced motor skill learning, which were observed in CMV-Gpr88 and D1R-Gpr88 mice, but not in A_2AR-Gpr88 mice. D1R-Gpr88 mice otherwise showed no other phenotype in this study. Our data together show that GPR88 modulates the function of both D1R- and D2R-MSNs, and that GPR88 activity in these two neuron populations has very different and dissociable impacts on behavior. We suggest that GPR88 in D2R-MSNs shapes defensive and social behavior and contributes in maintaining the inhibition of basal ganglia outputs to control locomotion, stereotypies and motor coordination, while GPR88 in D1R-MSNs promotes novelty habituation and motor learning.

SCOP/PHLPP1β in the basolateral amygdala regulates circadian expression of mouse anxiety-like behavior

While disruption of the circadian clock triggers a spectrum of affective abnormalities, how the clock regulates mammalian emotionality remains unclear. Here, we characterized the time-of-day-dependent regulation of mouse anxiety-like behaviors. We show that anxiety-like behaviors are expressed in a circadian manner in mice and demonstrate that the clock machineries in the dorsal telencephalon (dTel) are required for the time-of-day-dependent regulation of anxiety-like behaviors. We identify suprachiasmatic nucleus circadian oscillatory protein (SCOP/PHLPP1β) as an essential intracellular signaling molecule mediating this temporal regulation downstream of the clock. Using viral-mediated, basolateral amygdala (BLA)-specific knockout of Scop, we demonstrate that deletion of SCOP in the BLA exerts anxiolytic effects on the elevated plus maze at early subjective night, thereby blunting the circadian variation in the anxiety-like behavior. We conclude that the circadian expression of SCOP in the BLA plays a key role in generating circadian rhythmicity in the anxiety-like behavior. Our results demonstrate SCOP as a regulator of anxiety-like behaviors and reveal its key roles in the anxiogenic functions of the BLA.