Binding Sites in Rat

The binding of (Y-Ty#)bombesin to rat brain slices was investigated. Radiolabeled (Tyr4)bombesin bound with high affinity (Kd = 4 nM) to a single class of sites (B,,, = 130 fmol/mg of protein); the ratio of specific to nonspecific binding was 6/l. Also, pharmacology studies indicated that the C-terminal of bombesin was important for the high affinity binding activity. Autoradiographic studies indicated that the (12’1Tyr4)bombesin-binding sites were discretely distributed in certain gray but not white matter regions of rat brain. Highest grain densities were present in the olfactory bulb and tubercle, nucleus accumbens, suprachiasmatic and periventricular nuclei of the hypothalamus, central medial thalamic nucleus, medial amygdaloid nucleus, hippocampus, dentate gyrus, subiculum, nucleus of the solitary tract, and substantia gelatinosa. Moderate grain densities were present in the parietal cortex, deep layers of the neocortex, rhinal cortex, caudate putamen, stria terminalis, locus ceruleus, parabrachial nucleus, and facial nucleus. Low grain densities were present in the globus pallidus, lateral thalamus, and midbrain. Negligible grain densities were present in the cerebellum, corpus callosum, and all regions treated with 1 pM unlabeled bombesin. The discrete regional distribution of binding suggests that endogenous bombesin-like peptides may function as important regulatory agents in certain brain loci. Bombesin (BN) represents one class of peptides biologically active in the mammalian brain. BN is a potent satiety agency (Gibbs et al., 1979) and hypothermic agent in cold-exposed rats (Brown et al., 1977a) after central injection. Also, BN is a potent hyperglycemic agent (Brown et al., 1977b) and causes grooming in rats (Pert et al., 1980). In addition, BN increases growth hormone and prolactin secretion from the anterior pituitary after injection into the rat (Rivier et al., 1978). These biological activities may be mediated by the endogenous BNlike peptides which have been detected in discrete brain regions by radioimmunoassay (Brown et al. 1978; Moody and Pert, 1979). Specifically, high levels of immunoreactive BN are present in the substantia gelatinosa and the nucleus of the solitary tract of the hindbrain and throughout the hypothalamus (Moody et al., 1981b). Because immunoreactive BN is released from rat hypothalamic (Moody et al., 1980) and spinal cord slices (Moody et al., 1981a) by depolarizing stimuli, it is probably stored in intracellular vesicles. When BN-like peptides are released from certain CNS neurons, they may activate specific receptors. Central receptors for BN-like peptides have been character1 We thank Dr. R. Quirion for helpful discussions and Ms. Naomi Taylor for technical assistance. This research was supported in part by National Institutes of Health Grants GM 07309, MH 0053, MH 25951,