THYROTROPIN RELEASING HORMONE - APPARENT RECEPTOR-BINDING IN RETINA

Thyrotropin releasing hormone (TRH) binds with high affinity to sites on sheep retinal membranes. These sites closely resemble sheep pituitary receptors for TRH, as determined by direct comparison of binding in the two tissues. Specific [3H]TRH binding in sheep pituitary and retina was measured relative to blanks containing 1 μm [3-Me-His2]TRH, an analog more selective and 10-fold more potent than TRH. Incubations were at 0°C; bound radioactivity was separated by filtration. High affinity retinal TRH binding sites resembled pituitary receptors in (1) equilibrium dissociation constant (about 20-40 nm), (2) rate constant for association (about 1-2 μm-1 min-1), (3) rate constant for dissociation (about 0·07 min-1), (4) effectiveness of 16 TRH analogs, ranging over six orders of magnitude in potency, in competing for binding. Major differences were (1) the retinal binding sites were present in only one quarter the concentration of pituitary receptors (about 5 pmol/g wet weight vs. 20 pmol/g wet weight) and (2) the pharmacological properties of retinal binding sites were partially obscured by the simultaneous presence of low affinity binding sites, absent in pituitary, with which 6 analogs appeared to compete more potently than for high affinity sites. These findings support the possibility that TRH or a closely related substance acts as a peptide neurotransmitter or neuromodulator in the retina. © 1979.