Somatostatin receptors: identification and characterization in rat brain membranes

We have identified and characterized specific receptors for tetradecapeptide somatostatin (SRIF; somatotropin release-inhibiting factor) in rat brain using [125I]Tyr11]SRIF as the radioligand. These receptors are present in membranes obtained from a subfraction of synaptosomes. Membranes derived from cerebral cortex bind SRIF with high affinity (Ka = 1.25 X 10(10) M-1) and have a maximum binding capacity (Bmax) of 0.155 X 10(-12) mol/mg. Neither opiates nor other neuropeptides appear to influence the binding of SRIF to brain membranes. Synthetic analogs with greater biological potency than SRIF--[D-Trp8]SRIF, [D-Cys14]SRIF, and [D-Trp8, D-Cys14]SRIF--bind to the receptors with greater avidity than SRIF, whereas inactive analogs [(2H)Ala3]SRIF and [Ala6]SRIF exhibit low binding. The ratio of receptor density to endogenous somatostatin is high in the cortex, thalamus, and striatum, low in the hypothalamus, and extremely low in the brain stem and cerebellum. Thus, SRIF receptors in the brain appear to be a distinct, new class of receptors with a regional distribution different from that of endogenous somatostatin.