Rat Cerebellar Grafts Have I125‐Vasoactive Intestinal Peptide Binding Sites

Neuropeptides and their receptors may play a role in the growth and/or differentiation of fetal brain transplants. Previously, we found that gastrin releasing peptide (GRP) receptors develop on fetal cortex grafts transplanted into the adult rat cortex, spinal cord or 4th ventricle/1,2/. Here we investigated the presence of vasoactive intestinal peptide (VIP) receptors in rat fetal brain transplants. Rat fetal cerebellum was transplanted into the cisterna magnum as described previously/1/. After 1 month the rat was sacrificed and the brain removed. Sections (12/m) were derived from fresh frozen rat brain. The slices were incubated in incubation buffer comprised of 130 mM NaC1, 5 mM MgClz, 5 mM KC1, 1 mM EGTA, 1 mg/ml of bacitracin and 1% bovine serum albumin in 10 mM HEPES.NaOH plus 0.2 nM xzSI-VIP (1100 Ci.mmol) at 22"C in the presence or absence of competitor for 3 h. Free xz5I-VIP was then removed by 3 washes in incubation buffer for 15 min each at 22C. The sections which contained bound peptide were placed on Ultrofilm for 2 weeks at -80C and the film developed. Previously, using in vitro autoradiographic techniques, we found that lz5I-VIP binding sites were present in the adult rat cortex, striatum, hippocampus, pineal, nucleus tractus solitarius and cerebellum/3/. Also, when the E16 rat cortex was transplanted into the 4th ventricle, some z5I-VIP binding sites were present in the transplant after 4 weeks whereas a high density of GRP receptors developed /1/. Here xz5I-VIP binding sites were present in the E18 rat cortex and cerebellum. When E18 rat cerebellum was transplanted into the rat, the transplant developed between the rat cerebellum and hindbrain. A high density of lzSI-VIP binding sites was present on the transplant and host cerebellum and nucleus tractus solitarius. In contrast, high grain densities for (zSI-Tyr4) bombesin (GRP receptors) were present on the nucleus tractus solitarius but not the transplant, and high grain densities for lZ5I-physalaemin (substance P receptors) were present on the nucleus tractus solitarius and cerebellar lobule 10 but not the transplant. These data indicate that VIP but not GRP or substance P receptors develop on rat cerebellar transplants. The distribution of 25I-VIP grains in the transplant was irregular being highest in areas derived from the cerebellar cortex. Densitometry analysis indicated that specific 25I-VIP binding to the transplant was inhibited with high affinity by VIP (ICs0 5 nM). Similar data were obtained using the host cerebellum and nucleus tractus solitarius. These data suggest that VIP receptors develop in fetal cerebellar transplants and that the pharmacological properties of the transplant receptors are similar to those observed in the adult rat brain. Because VIP is a potent vasodilator, the transplant VIP receptors may regulate blood flow to the graft.