Lack of D2 receptor mediated regulation of dopamine synthesis in A₁₁ diencephalospinal neurons in male and female mice

Dopamine (DA) neurons comprising the A₁₁ diencephalospinal system represent the major source of DA innervation to the spinal cord. These neurons project axons throughout the rostrocaudal extent of the spinal cord, terminating predominantly in the dorsal horn. Loss of DA-mediated sensorimotor function in the lumbar segment of spinal cord is implicated in the etiology of Restless Legs Syndrome (RLS), which is more prevalent in females as compared with males. The purpose of the present study was to compare the density (DA concentrations) and catabolic activity (3,4-dihydroxyphenylacetic acid; DOPAC) of A₁₁ DA neurons innervating the lumbar spinal cord of male and female C57/BL6 mice, and to determine if there is a sexual difference in the regulation of these neurons by D2 autoreceptor-mediated mechanisms. DA concentrations in the lumbar spinal cord were higher in males, suggesting a greater A₁₁ DA innervation as compared with females, whereas there was no sexual difference in the activity (DOPAC/DA ratio) of these DA neurons under basal conditions. Blockade of D2 receptors with raclopride caused a significant increase in the DOPAC/DA ratio in the striatum and nucleus accumbens in both males and females, but had no effect in the spinal cord. Blockade of neuronal impulse flow and DA release with γ-butyrolactone (GBL) increased DA concentrations in the spinal cord, but this increase was not prevented by pretreatment with the D2 agonist quinelorane. These results are consistent with the conclusion that A₁₁ diencephalospinal DA neurons in both males and females lack presynaptic synthesis modulating D2 autoreceptors.