Adenosinergic protection of dopaminergic and GABAergic neurons against mitochondrial inhibition through receptors located in the substantia nigra and striatum, respectively

Abstract

Mitochondrial dysfunction may contribute to dopaminergic (DAergic) cell death in Parkinson's disease and GABAergic cell death in Huntington's disease. In the present work, we tested whether blocking A1 receptors could enhance the damage to DAergic and GABAergic neurons caused by mitochondrial inhibition, and whether blocking A2a receptors could protect against damage in this model. Animals received an intraperitoneal injection of 8-cyclopentyl-1,3-dipropylxanthine (CPX) (A1 antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX) (A2a antagonist) 30 min before intrastriatal infusion of malonate (mitochondrial complex II inhibitor). Damage was assessed 1 week later by measuring striatal dopamine, tyrosine hydroxylase (TH), and GABA content. In mice and rats, malonate-induced depletion of striatal dopamine, TH, or GABA was potentiated by pretreatment with 1 mg/kg CPX and attenuated by pretreatment with 5 mg/kg DMPX. To determine the location of the A1 and A2a receptors mediating these effects, CPX or DMPX was infused directly into the striatum or substantia nigra of rats 30 min before intrastriatal infusion of malonate. When infused into the striatum, CPX (20 ng) potentiated, whereas DMPX (50 ng) prevented malonate-induced GABA loss, but up to 100 ng of CPX or 500 ng of DMPX did not alter malonate-induced striatal dopamine loss. Intranigral infusion of CPX (100 ng) or DMPX (500 ng), however, did exacerbate and protect, respectively, against malonate-induced striatal dopamine loss. Thus, A1 receptor blockade enhances and A2a receptor blockade protects against damage to DAergic and GABAergic neurons caused by mitochondrial inhibition. Interestingly, these effects are mediated by A1 and A2a receptors located in the substantia nigra for DAergic neurons and in the striatum for GABAergic neurons.

Figure 1.

Blocking A₁ receptors potentiates malonate-induced damage to DAergic and GABAergic neurons. A, In mice, malonate-induced (6 μmol) depletion of striatal dopamine and TH levels was potentiated by pretreatment with CPX (1 mg/kg, i.p.). Striatal GABA levels were unaffected by malonate treatment, but mice that received both CPX (1 mg/kg, i.p.) and malonate showed a marked decrease in striatal GABA levels. For dopamine, TH, and GABA analysis, n = 10-13, 8-10, and 4-7 mice, respectively. B, In rats, malonate-induced (3 μmol) depletion of striatal dopamine and GABA levels was potentiated by pretreatment with CPX (1 mg/kg, i.p.). Dorsal striatum, n = 6-10 rats for dopamine and n = 3-4 rats for GABA. CPX alone had no effect on striatal dopamine, TH, or GABA levels in mice or rats. Mean ± SEM; ^ap < 0.05 versus β-CD; ^bp < 0.05 versus malonate. IP, Intraperitoneal.

Figure 2.

Blocking A_2a receptors protects against malonate-induced damage to DAergic and GABAergic neurons. A, In mice, pretreatment with intraperitoneal (IP) DMPX protected against malonate-induced (6 μmol) depletion of striatal dopamine and TH levels (n = 12-13 mice for dopamine; n = 6-9 mice for TH). B, In rats, malonate-induced (3 μmol) depletion of striatal dopamine and GABA levels was attenuated by pretreatment with intraperitoneal DMPX. Dorsal striatum, n = 11-12 rats for dopamine and n = 10 rats for GABA. DMPX alone had no effect on striatal dopamine, TH, or GABA levels in mice or rats. Mean ± SEM; ^ap < 0.05 versus β-CD; ^bp < 0.05 versus malonate.

Figure 3.

A₁- and A_2a-mediated effects on GABAergic but not DAergic neurons occur through receptors located in the striatum. A, Intrastriatal infusion of CPX had no effect on malonate-induced depletion of striatal dopamine levels but potentiated the effect of malonate (3 μmol) on striatal GABA content. Dorsal striatum, n = 5-6 rats for dopamine; medial striatum, n = 7-8 rats for GABA. B, Intrastriatal infusion of DMPX did not protect against malonate-induced dopamine loss but completely blocked GABA loss. Medial striatum, n = 6 rats for dopamine and n = 5-6 rats for GABA. Mean ± SEM; ^ap < 0.05 versus β-CD; ^bp < 0.05 versus malonate. Cntrl, Untreated control.

Figure 4.

A₁- and A_2a-mediated effects on DAergic neurons occur through receptors located in the substantia nigra. A, Intranigral infusion of CPX potentiated malonate-induced (3 μmol) depletion of striatal dopamine. Dorsal striatum, n = 12-15 rats. B, Intranigral infusion of DMPX attenuated malonate-induced depletion of striatal dopamine content. Dorsal striatum, n = 9-11 rats. Mean ± SEM; ^ap < 0.05 versus β-CD; ^bp < 0.05 versus malonate. Cntrl, Untreated control.