Preservation of dendritic D2 receptor transmission in substantia nigra dopamine neurons with age.

Substantia nigra pars compacta (SNc) dopamine neurons are required for voluntary movement and reward learning, and advanced age is associated with motor and cognitive decline. In the midbrain, D2-type dopamine receptors located at dendrodendritic synapses between dopamine neurons control cell firing through G protein-activated potassium (GIRK) channels. We previously showed that aging disrupts dopamine neuron pacemaker firing in mice, but only in males. Here we show that the amplitude of D2-receptor inhibitory postsynaptic currents (D2-IPSCs) are moderately reduced in aged male mice. Local application of dopamine revealed a reduction in the amplitude of the D2-receptor currents in old males compared to young, pointing to a postsynaptic mechanism. Further experiments indicated that reduced D2 receptor signaling was not due to a general reduction in GIRK channel currents or degeneration of the dendritic arbor. Kinetic analysis showed no differences in D2-IPSC shape in old versus young mice or between sexes. Potentiation of D2-IPSCs by corticotropin releasing factor (CRF) was also not affected by age, indicating preservation of one mechanism of plasticity. These findings have implications for understanding dopamine transmission in aging, and reduced D2 receptor inhibition could contribute to increased susceptibility of males to SNc dopamine neuron degeneration in Parkinson's disease.