Distribution and changes in μ- and κ-opiate receptors during the midlife neurodevelopmental period of coho salmon, Oncorhynchus kisutch

Parr-smolt transformation (PST) in coho salmon is associated with a plasma thyroid hormone (PT₄) surge and a critical period of neural development that includes axonal sprouting, neurogenesis, and surges of select neurotransmitters. Here we provide a description of the selectivity, distribution, and the changes in the density of μ- and κ-opiate receptors during PST, as revealed by quantitative in vitro autoradiography of [³H]Tyr-D-Ala-Gly-NMe-Phe-Gly-ol ([³H [DAMGO) and [³H]ethylketocyclazocine ([³H]EKC), respectively. The concentration of μ- receptors increased significantly in select cell groups in the early stages of parr-smolt transformation, until a peak was reached at the time coinciding with the peak of the PT₄ surge. In other cell groups, the peaks occurred 1 or 2 weeks later. With one exception, this increase was followed by a decrease in concentration. The brain areas showing the highest concentrations are the dorsal nucleus of the ventral telencephalic area, the glomerular region, the granular layer of the valvula cerebelli, the nucleus diffuses of the inferior lobe, and the nucleus diffuses of the torus lateralis. Other regions with distinctly elevated μ-receptor concentrations are the stratum griseum centrale of the optic tectum and the preoptic area. The distribution of κ-receptors is more diffuse, and the densities are considerably lower. The overlap in distribution of μ- and κ-receptors is considerable, but significant exceptions are noted. For example, the dorsomedial nucleus of the dorsal telencephalic area, the habenular nucleus, and the dorsomedial nucleus of the thalamus exhibit a surge in density of κ-receptors at the time of the PT₄ surge, while the density of μ- receptors in these nuclei remain very low throughout parr-smolt transformation. The κ-receptor containing cell groups are not identifiable until 3 weeks before the PT₄ surge because of low densities. The most prominently labeled κ-receptor regions are the ventral and dorsal nuclei of the ventral telencephalic area, the medial and dorsal zones of the dorsal telencephalic area, the optic tectum (all layers), the dorsomedial nucleus of the thalamus, the torus lateralis of the ventral hypothalamus, and the preoptic area. An increase of μ- and κ-opiate receptor densities in specific brain regions may reflect roles in the alteration of brain organization, olfactory imprinting, neuroendocrine activity or other physiological activities. The overall distribution of these receptors are relatively more extensive in salmon than in other vertebrates so far studied.