Distribution of α(1a) adrenergic receptor mRNA in the rat brain visualized by in situ hybridization

Norepinephrine has been implicated in a number of physiological, behavioral, and cellular modulatory processes in the brain, and many of these modulatory effects are attributable to α₁ adrenergic receptors. At least three α₁ receptor subtypes have been identified by molecular criteria, designated α(1A), α(1B), and α(1D). The distributions of α(1B) and α(1D) receptor mRNA expression in rat brain have been described previously, but the cDNA for the rat α(1A) receptor has only recently been cloned and characterized. In the present study, we used a radiolabelled riboprobe derived from the rat α(1A) receptor cDNA to describe the distribution of α(1A) message expression in the rat brain. The highest levels of α(1A) adrenergic receptor mRNA expression were seen in the olfactory bulb, tenia tectae, horizontal diagonal band/magnocellular preoptic area, zona incerta, ventromedial hypothalamus, lateral mammillary nuclei, ventral dentate gyrus, piriform cortex, medial and cortical amygdala, magnecellular red nuclei, pontine nuclei, superior and lateral vestibular nuclei, brainstem reticular nuclei, and several cranial nerve motor nuclei. Dual in situ hybridization combining a radioactive riboprobe for choline acetyltransferase mRNA with a digoxigenin-labeled α(lA) riboprobe in the fifth and seventh cranial nerve motor nuclei showed that the α(1A) mRNA is expressed in cholinergic motor neurons. Prominent α(1A) hybridization signal was also seen in the neocortex, claustrum, lateral amygdala, ventral cochlear nucleus, raphe magnus, and in the ventral horn of thoracic spinal cord. This overall pattern of expression, considered in comparison with that previously described for the other α₁ adrenergic receptor subtypes, may shed light on the different roles of the α₁ receptors in mediating the neuromodulatory effects of norepinephrine in processes such as arousal, neuroendocrine control, sensorimotor regulation, and the stress response.