Acute Depletion of D2 Receptors from the Rat Substantia Nigra Alters Dopamine Kinetics in the Dorsal Striatum and Drug Responsivity

Abstract

Recent studies have used conditional knockout mice to selectively delete the D2 autoreceptor; however, these approaches result in global deletion of D2 autoreceptors early in development. The present study takes a different approach using RNA interference (RNAi) to knockdown the expression of the D2 receptors (D2R) in the substantia nigra (SN), including dopaminergic neurons, which project primarily to the dorsal striatum (dStr) in adult rats. This approach restricts the knockdown primarily to nigrostriatal pathways, leaving mesolimbic D2 autoreceptors intact. Analyses of dopamine (DA) kinetics in the dStr reveal a decrease in DA transporter (DAT) function in the knockdown rats, an effect not observed in D2 autoreceptor knockout mouse models. SN D2 knockdown rats exhibit a behavioral phenotype characterized by persistent enhancement of locomotor activity in a familiar open field, reduced locomotor responsiveness to high doses of cocaine and the ability to overcome haloperidol-induced immobility on the bar test. Together these results demonstrate that presynaptic D2R can be depleted from specific neuronal populations and implicates nigrostriatal D2R in different behavioral responses to psychotropic drugs.

Increased vulnerability of nigral dopamine neurons after expansion of their axonal arborization size through D2 dopamine receptor conditional knockout

“…We found a general decrease in DA release that was partially rescued in the presence of a DAT antagonist. Our finding of a decrease in evoked DA overflow, although somewhat counter-intuitive when considering the autoreceptor function of the D2 receptor, is in line with previous observations of constitutive or conditional D2R KO mice [43–47] (but see [48]). While we found here that this reduced DA release could be rescued by nomifensine, in a previous study, the use of a DAT antagonist was not sufficient to return DA levels to normal in the engrailed1-based D2-cKO [44].…”

“…We also did not detect a significant change in DAT surface levels using a DAT surface biotinylation assay. However, a reduction in Vmax has been observed in a previous study in which the D2 receptor was knocked down acutely using siRNA [47], although reuptake kinetics (tau) were not reported. The difference with our data could be explained by the acute nature of the deletion in this previous study.…”

Ventral midbrain astrocytes display unique physiological features and sensitivity to dopamine D2 receptor signaling

“…Although there is no question that D2 autoreceptor activity critically regulates DA neuron activity, a downregulation of astrocytic D2Rs by chronic drug exposure may also be partly responsible for drug-induced plasticity in the ventral midbrain. Similarly, enhancing or dampening astrocytic D2Rs may partly account for the effects of D2R-targeted drugs or genetic manipulation of D2R expession on drug-seeking behavior [67–70]. With the development of new, selective astrocyte mouse lines [71], future studies will be able to define the relative contribution of astrocytic and DA neuron D2Rs to drug-related plasticity and behaviors.…”

Susceptibility to traumatic stress sensitizes the dopaminergic response to cocaine and increases motivation for cocaine

“…Nevertheless, studies that examined DA uptake following targeted genetic manipulation of D2 autoreceptors in DA neurons have produced conflicting results (Anzalone et al, 2012; Bello et al, 2011; Budygin et al, 2016). Consequently, it is not surprising that in the current studies heightened D2 autoreceptor sensitivity observed in resilient subjects did not correspond with alterations in DA uptake.…”