DISTRIBUTION AND CHANGES IN MU-OPIATE AND KAPPA-OPIATE RECEPTORS DURING THE MIDLIFE NEURODEVELOPMENTAL PERIOD OF COHO SALMON, ONCORHYNCHUS-KISUTCH

Parr-smolt transformation (PST) in coho salmon is associated with a pl asma thyroid hormone (PT4) surge and a critical period of neural devel opment that includes axonal sprouting, neurogenesis, and surges of sel ect neurotransmitters. Here we provide a description of the selectivit y, distribution, and the changes in the density of mu- and kappa-opiat e receptors during PST, as revealed by quantitative in vitro autoradio graphy of [H-3]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol ([H-3]DAMGO) and [H-3]ethy lketocyclazocine ([H-3]EKC), respectively. The concentration of mu-rec eptors increased significantly in select cell groups in the early stag es of parr-smelt transformation, until a peak was reached at the time coinciding with the peak of the PT4 surge. In other cell groups, the p eaks occurred 1 or 2 weeks later. With one exception, this increase wa s followed by a decrease in concentration. The brain areas showing the highest concentrations are the dorsal nucleus of the ventral telencep halic area, the glomerular region, the granular layer of the valvula c erebelli, the nucleus diffuses of the inferior lobe, and the nucleus d iffusus of the torus lateralis. Other regions with distinctly elevated mu-receptor concentrations are the stratum griseum centrale of the op tic tectum and the preoptic area. The distribution of kappa-receptors is more diffuse, and the densities are considerably lower. The overlap in distribution of mu- and kappa-receptors is considerable, but signi ficant exceptions are noted. For example, the dorsomedial nucleus of t he dorsal telencephalic area, the habenular nucleus, and the dorsomedi al nucleus of the thalamus exhibit a surge in density of kappa-recepto rs at the time of the PT4 surge, while the density of mu-receptors in these nuclei remain very low throughout parr-smelt transformation. The kappa-receptor containing cell groups are not identifiable until 3 we eks before the PT4 surge because of low densities. The most prominentl y labeled kappa-receptor regions are the ventral and dorsal nuclei of the ventral telencephalic area, the medial and dorsal zones of the dor sal telencephalic area, the optic tectum (all layers), the dorsomedial nucleus of the thalamus, the torus lateralis of the ventral hypothala mus, and the preoptic area. An increase of mu- and kappa-opiate recept or densities in specific brain regions may reflect roles in the altera tion of brain organization, olfactory imprinting, neuroendocrine activ ity or other physiological activities. The overall distribution of the se receptors are relatively more extensive in salmon than in other ver tebrates so far studied. (C) 1996 Wiley-Liss, Inc.