Specific [3H]spiperone binding sites in the pituitary of rainbow trout (Oncorhynchus mykiss) and goldfish (Carassius auratus)

Abstract

Dopamine, a catecholamine neurohormone, modulates pituitary hormone release in teleost fishes and other vertebrates. The existence and binding parameters of a pituitary dopamine–neuroleptic receptor from trout were examined and compared with those from goldfish. Pituitary homogenate was incubated with [3H]spiperone (D2 antagonist) under several experimental paradigms; incubations were terminated by filtration and bound 3H radioactivity was assessed by liquid scintillation spectroscopy. Specific binding of [3H]spiperone was tissue dependent. Equilibrium displacement analyses using domperidone (D2 antagonist) indicated a single class of binding site (LIGAND) with Kd = 2.49 ± 0.89 μM and a capacity of 3.10 ± 0.45 nmol/mg protein; the goldfish Kd and capacity were both significantly (p < 0.05) larger: Kd = 4.63 ± 0.30 μM and capacity = 20.66 ± 2.03 nmol/mg protein. The Kd and capacity for the trout pars distalis (2.45 ± 0.33 μM and 3.27 ± 0.24 nmol/mg protein, respectively) did not differ significantly (p < 0.05) from that of the neurointermediate lobe (2.50 ± 0.08 μM and 3.58 ± 0.56 nmol/mg protein, respectively). Dopamine D2 receptor ligands differentially displaced [3H]spiperone from the trout pituitary, while D1 ligands, a D4 ligand, and a 5-hydroxytryptamine (5HT2) receptor antagonist had only small nonspecific effects. Comparison of the trout and goldfish pituitary dopamine–neuroleptic receptor indicates conservation of receptor affinity (Kd); however, differences in receptor numbers and in the distribution of receptors between the pars distalis and neurointermediate lobe in the two species may be due in part to species or developmental differences, and may reflect differences in the role(s) and degrees of influence of dopamine in these fishes.Key words: pituitary, dopamine, receptor, rainbow trout, goldfish.