Effect of chronic exposure to an acidified environment on the urophysis of the brook trout, Salvelinus fontinalis.

The activity of the caudal neurosecretory system including its neurohemal organ, the urophysis, was assessed in six populations of brook trout (Salvelinus fontinalis)--three from acidified lakes and three from nonacidified lakes (Laurentides Parc, Québec). The acidified lakes were characterized by low pH (mean annual value around 5.5), poor buffering capacity and low conductivity. The activity of the caudal neurosecretory cells was evaluated by morphometry (cell and nuclear sizes). The urophysial content of the neuropeptide urotensin II was measured by isolated trout rectum bioassay. Fish from acidified lakes showed markedly larger cell and nuclear sizes and a lower content of urotensin II than fish from nonacidified lakes. An acidified environment appears to stimulate the synthetic activity of caudal neurosecretory neurons and promote release of urotensin II from the urophysis. This modified endocrine activity in brook trout may reflect adjustment to adverse external ionic conditions.