Noradrenaline Regulation of Tyrosine Hydroxylase Expression in Arcuate Nucleus Neurons in Young and Adult Rats

Neurons of the arcuate nucleus (AN) produce dopamine, which inhibits prolactin secretion. Tyrosine hydroxylase (TH), the key enzyme of dopamine synthesis, is expressed in AN in dopaminergic neurons and in neurons expressing only TH or both enzymes but lacking the dopamine transporter. These neurons are distributed differently between the ventrolateral and dorsomedial regions of the AN (further-ventrolateral or dorsomedial AN). We hypothesized that noradrenaline released by noradrenergic afferents inhibits TH synthesis in AN neurons postnatally. To test this hypothesis, we assessed: (i) adrenoreceptors gene expression in the ventrolateral and dorsomedial AN of intact rats at postnatal days (P) 5 and 60 and (ii) TH levels in sections of each AN region from rats at P5 and P60 after 6 h incubation in the absence or presence of noradrenaline, as well as noradrenaline with adrenoreceptor antagonists. It was shown that (i) AN neurons express genes for all types of adrenoreceptors in each region of AN on P5 and P60; (ii) neurons in both regions of AN synthesize TH, but to a greater extent at P60 than at P5; and (iii) noradrenaline inhibits TH synthesis, but only in the ventrolateral AN at P60-this action is mediated via α1-adrenoreceptors.