Nucleus lateralis tuberis-pars lateralis and background adaptation in the rainbow trout, Salmo gairdneri

In the rainbow trout, perikarya of the lateral part of the nucleus lateralis tuberis (NLT-pl), located in the ventral hypothalamus, are connected via their axons with the neurointermediate lobe of the pituitary. To investigate the functional significance of this morphological relationship experiments testing adaptation to background colour were carried out. The NLT-pl cells of trout adapted for 5 days to either a white or black background were studied by immunostaining (PAP technique) with an antiserum against 27-S-methylglucagon. The intensity of the immuno-labelling and the number of reactive NLT-pl cells were strongly increased in fish adapted to the black background. Trout varying in their adaptive response to background within an experimental group were recognized by the corresponding effect of the NLT-pl cells. Moreover, in addition to the fibres running toward the pars intermedia, immunoreactive varicose fibres were observed in other parts of the hypothalamus, thalamus and mesencephalon. These fibres were more frequently detected in black-adapted fish.