ESTROGEN-RECEPTOR EXPRESSION IN BRAIN-STEM NORADRENERGIC NEURONS OF THE SHEEP

Noradrenergic neurons are implicated in the estrogen-dependent neural regulation of luteinizing hormone secretion in a variety of mammalian species. The current study has used immunocytochemical methods to dete rmine whether estrogen receptors (ER) are expressed within the brainst em of the ewe and to establish their relationship to noradrenergic neu rons. Using a monoclonal mouse antiserum directed against the N-termin al of ER alpha, four distinct populations of ER alpha-immunoreactive c ells were identified in ovine medulla and pens. The largest population was found in the superficial laminae of the spinal nucleus of the tri geminal nerve, followed by the nucleus tractus solitarius, lateral are a postrema, and ventrolateral medulla. Double-labelling immunocytochem istry using antisera directed against the ERa and dopamine-beta-hydrox ylase revealed that noradrenergic neurons expressing ER immunoreactivi ty were only found in ventrolateral medulla (A1 cell group) and nucleu s tractus solitarius (A2 cell group). No double-labelled cells were id entified in the A5, A6, or A7 noradrenergic cell groups. ERs were expr essed with a clear rostrocaudal topography within the A1 and A2 popula tions, with 80-90% of noradrenergic neurons expressing ERa in the caud almost medulla as compared with less than 5% rostral to the obex. Our findings demonstrate that, as in the rat, the ovine Al and A2 neurons express ERs in a defined topographical manner, while, dissimilar to th e rat, ERa is not synthesized by noradrenergic neurons in the other ce ll groups. These observations indicate that A1 and A2 noradrenergic ne urons in the ovine brainstem are likely to be influenced by circulatin g estrogens and lay the neuroanatomical foundations for investigating the functional role of these cell populations within the gonadotropin- releasing hormone neuron network of the sheep.