SUMMARY Mitochondrial movements are tightly controlled to maintain energy homeostasis and prevent oxidative stress. Miro is an outer mitochondrial membrane protein that anchors mitochondria to microtubule motors, and is removed to stop mitochondrial motility as an early step in clearance of dysfunctional mitochondria. Here, using human iPSC-derived neurons and other complementary models, we build on a previous connection of Parkinson’s disease (PD)-linked PINK1 and Parkin to Miro, by showing that a third PD-related protein, LRRK2, promotes Miro removal via forming a complex with Miro. Pathogenic LRRK2G2019S disrupts this function, delaying the arrest of damaged mitochondria and consequently slowing the initiation of mitophagy. Remarkably, partial reduction of Miro levels in LRRK2G2019S human neuron and Drosophila PD models rescues neurodegeneration. Miro degradation and mitochondrial motility are also impaired in sporadic PD patients. We reveal that prolonged retention of Miro, and the downstream consequences that ensue, may constitute a central component of PD pathogenesis.

Rationale Inactivating dopamine (DA) receptors in the caudate-putamen (CPu) attenuates basal and DA agonist-induced behaviors of adult rats, while paradoxically increasing the locomotor activity of preweanling rats. Objective The purpose of this study was to determine (a) whether D1 or D2 receptor inactivation is responsible for the elevated locomotion shown by preweanling rats and (b) whether DA receptor inactivation produces a general state in which any locomotor-activating drug will cause a potentiated behavioral response. Methods DMSO or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was bilaterally infused into the CPu on postnatal day (PD) 17. In Experiment 1, DA receptors were selectively protected from EEDQ-induced alkylation by pretreating rats with D1 and/or D2 antagonists. On PD 18, rats received bilateral microinjections of the DA agonist R(–)-propylnorapomorphine into the dorsal CPu and locomotor activity was measured for 40 min. In subsequent experiments, the locomotion of DMSO- and EEDQ-pretreated rats was assessed after intraCPu infusions of the selective DA agonists SKF82958 and quinpirole, the partial agonist terguride, or after systemic administration of nonDAergic compounds. Results Experiment 1 showed that EEDQ's ability to enhance the locomotor activity of preweanling rats was primarily due to the inactivation of D2 receptors. Consistent with this finding, only drugs that directly or indirectly stimulated D2 receptors produced a potentiated locomotor response in EEDQ-treated rats. Conclusions These results show that DA receptor inactivation causes dramatically different behavioral effects in preweanling and adult rats, thus providing additional evidence that the D2 receptor system is not functionally mature by the end of the preweanling period.

Rationale Dopamine (DA) receptor inactivation produces opposing behavioral effects across ontogeny. For example, inactivating DA receptors in the dorsal striatum attenuates DA agonist-induced behaviors of adult rats, while potentiating the locomotor activity of preweanling rats. Objective The purpose of this study was to determine if DA receptor inactivation potentiates the DA agonist-induced locomotor activity of adolescent rats, and whether alterations in D2High receptors are responsible for this effect. Methods In the behavioral experiment, the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) or its vehicle (100% dimethylsulphoxide, DMSO) were bilaterally infused into the dorsal striatum on postnatal day (PD) 39. On PD 40, adolescent rats were given intrastriatal infusions of the DA agonist R(−)-propylnorapomorphine (NPA) or vehicle and locomotor activity was measured for 40 min. In the receptor binding experiment, rats received IP injections of EEDQ or DMSO (1:1 (v/v) in distilled water) on PD 17, PD 39, or PD 84. One day later, striatal samples were taken and subsequently assayed for D2 specific binding and D2High receptors using [3H]-domperidone. Results Unlike what is observed during the preweanling period, EEDQ attenuated the NPA-induced locomotor activity of adolescent rats. EEDQ reduced D2 receptor levels in the dorsal striatum of all age groups, while increasing the proportion of D2High receptors. Regardless of pretreatment condition (i.e., DMSO or EEDQ), preweanling rats had a greater percentage of D2High receptors than adolescent or adult rats. Conclusions DA receptor inactivation affects the behaviors of preweanling and older rats differently. The DA supersensitivity exhibited by EEDQ-treated preweanling rats may result from an excess of D2High receptors.

Rationale The neural mechanisms mediating the ontogeny of behavioral sensitization are poorly understood. Objective The purpose of the present study was to determine the role of the D1 receptor for the induction of dopamine agonist-induced behavioral sensitization during the preweanling period. Methods In the first experiment, the early ontogeny of NPA-induced behavioral sensitization was examined by pretreating male and female rats with saline or NPA (0.5, 1, or 2 mg/kg, IP) before placement in activity chambers on postnatal day (PD) 12, 16, 20, or 24. One day later, rats were tested with lower doses of NPA and the occurrence of locomotor sensitization was determined. In subsequent experiments, rats were injected with saline or the D1 receptor antagonist SCH23390 (0.1, 0.5, 1, or 5 mg/kg, IP) 0, 15, 30, or 60 min before cocaine, methamphetamine (METH), or NPA pretreatment. The next day, rats were tested with the same dopamine agonist again and sensitized responding was assessed. Results NPA produced one-trial behavioral sensitization at all ages tested. In preweanling rats, SCH23390, regardless of dose, was ineffective at preventing the induction of cocaine-, METH-, or NPA-induced one-trial behavioral sensitization. Conclusions The present results are in partial contrast to adult rodent studies, in which SCH23390 blocks the induction of METH- and apomorphine-induced behavioral sensitization, but not cocaine sensitization. When these findings are considered together, it appears that D1 receptor stimulation is not necessary for the induction of behavioral sensitization during the preweanling period, although D1 receptors may play a more important role in adulthood.

Rationale There is suggestive evidence that the neural mechanisms mediating one-trial and multi-trial behavioral sensitization differ, especially when the effects of various classes of dopamine (DA) agonists are examined. Objective The purpose of the present study was to determine the role of the D2 receptor for the induction of one-trial and multi-trial methamphetamine sensitization in preweanling rats. Methods In a series of experiments, rats were injected with saline or raclopride (a selective D2 receptor antagonist), either alone or in combination with SCH23390 (a selective D1 receptor antagonist), 15 min prior to treatment with the indirect DA agonist methamphetamine. Acute control groups were given two injections of saline. This pretreatment regimen occurred on either postnatal days (PD) 13–16 (multi-trial) or PD 16 (one-trial). On PD 17, rats were challenged with methamphetamine and locomotor sensitization was determined. Results Blockade of D2 or D1/D2 receptors reduced or prevented, respectively, the induction of multi-trial methamphetamine sensitization in young rats, while the same manipulations had minimal effects on one-trial behavioral sensitization. Conclusions DA antagonist treatment differentially affected the methamphetamine-induced sensitized responding of preweanling rats depending on whether a one-trial or multi-trial procedure was used. The basis for this effect is uncertain, but there was some evidence that repeated DA antagonist treatment caused nonspecific changes that produced a weakened sensitized response. Importantly, DA antagonist treatment did not prevent the one-trial behavioral sensitization of preweanling rats. The latter result brings into question whether DA receptor stimulation is necessary for the induction of psychostimulant-induced behavioral sensitization during early ontogeny.