Seasonal regulation of membrane and intracellular corticosteroid receptorsin the house sparrow brain

A number of studies have demonstrated seasonal regulation of the adrenocort ical response to stress, or of corticosteroid binding globulins, but very f ew studies have examined seasonal regulation of corticosteroid receptor lev els. As a result, there have been few attempts to produce an integrated pic ture of seasonal plasticity of the stress response, We measured baseline an d stress-induced corticosterone (CORT), corticosteroid binding globulin and neuronal cytosolic and membrane corticosteroid receptor levels in male and female, wild-caught house sparrows (Passer domesticus) during three differ ent seasons over the annual cycle (nesting, molting and winter). We identif ied three neuronal corticosteroid receptors in the house sparrow brain: two intracellular receptors and one membrane-associated receptor. Little is kn own about corticosteroid receptors in neuronal membranes of avian and mamma lian species, but we found that the levels of membrane corticosteroid recep tors varied seasonally, being lowest during the nesting season. Cytosolic c orticosteroid receptor numbers (both low and high affinity receptors) also varied seasonally, In contrast to the membrane bound receptors, however, th e numbers of low and high affinity cytosolic receptors were lowest during w inter. In addition, mean levels of total basal and stress-induced CORT in t he plasma varied seasonally. Both basal and stress-induced levels of total CORT were significantly higher during nesting than during winter or molt. F inally, corticosteroid binding globulin levels in plasma were also seasonal ly regulated, in a pattern similar to total CORT, so that estimated free CO RT levels did not vary between seasons, These data indicate that multiple c omponents of the stress response are seasonally regulated in birds obtained from wild populations. Interactions between these regulated components pro vide a basis for seasonal differences in behavioural and physiological resp onses to stress.