Immunohistochemical localization of nNOS in the head kidney of larval and juvenile rainbow trout, Oncorhynchus mykiss.

The aim of this investigation was to assess whether in teleosts, as in mammals, nitric oxide (NO) is involved in the regulation of cellular activity in the adrenal homolog. Larval and juvenile stages of the rainbow trout, Oncorhynchus mykiss, were used, in which the adrenal homolog consists of chromaffin adrenergic and interrenal steroidogenic cells localized mainly in the head kidney where there are also ganglion cells and nerve fibres that innervate the gland. In 12-month-old juveniles, the immunohistochemical reaction for neuronal nitric oxide synthase (nNOS), which catalyzes the synthesis of NO, revealed the presence of this enzyme in some nerve fibres and ganglion cells and only rarely in chromaffin cells. The latter are identified by the immunohistochemical reaction for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT). In larvae at 27 days postfertilization, numerous cells dispersed in the head kidney are nNOS positive, whereas the TH and PNMT positive cells are very rare. At hatching (31 days postfertilization), the positivity for nNOS in the cells of the head kidney disappears and reappears at 60 days posthatching in some nerve cells and fibres. These results suggest an involvement of NO in the regulation of adrenal function as in mammals and the nature of nNOS positive cells present in the head kidney of larvae of 27 days is discussed. Copyright 2001 Academic Press.