Different types of bombesin receptors on neurons in the dorsal raphe nucleus and the rostral hypothalamus in rat brain slices in vitro

Abstract

The actions of the peptides bombesin (BN), gastrin releasing peptide (GRP), neuromedin C (NMC), litorin and neuromedin B (NMB) were studied on neurons in slices of rat brain maintained in vitro to determine the BN receptor type present in different brain areas. Intracellular and extracellular recordings were made from hypothalamic neurons on the border of the periventricular nucleus (PVN) and suprachiasmatic nucleus (SCN) and from mesencephalic 5-HT sensitive neurons in the dorsal raphe nucleus. In the region of the brain containing the SCN and PVN, BN and the BN-related peptides excited 31 out of 77 neurons on which they were tested. There was little difference in the potency of the BN-related peptides as excitants of neurons, the EC 50 being about 10 nM. The response to the peptides usually lasted between 5 and 15 min with little sign of desensitization. Using NMC, GRP and NMB as agonists, the equilibrium constant for the GRP receptor antagonist [ d-Phe 6]-BN-(6–13)-ethylamide was ∼ 10 nM. The response to the peptides fully recovered on washout of the antagonist. The CCK B/gastrin receptor antagonist CI-988 (1 μM) had no effect on either GRP- or NMC-mediated excitation. In the dorsal raphe nucleus 40 of 75 neurons were sensitive to the BN-related peptides. BN, [Tyr 4]-BN, NMB and litorin, were 10–20 times more potent than GRP and NMC. The responses to the BN-related peptides were not blocked by the selective GRP receptor antagonists [ d-Phe 6]-BN-(6–13)-methylester, [DF5Phe 6][ d-Ala 11]-BN-(6–13)-methylester and [ d-Phe 6]-BN-(6–13)-ethylamide. These results suggest that the PVN and SCN contain a population of neurons expressing GRP receptors whilst in the dorsal raphe the majority of BN receptors expressed are of the NMB subtype.