Electrophysiological Effects of Pressure‐Ejected Bombesin‐Like Peptides on Hamster Suprachiasmatic Nucleus Neurons in vitro

The suprachiasmatic nuclei (SCN) of the rodent hypothalamus function as a light entrainable circadian pacemaker. The SCN contain moderate to high concentrations of a number of neuropeptides including peptides showing structural homology with the amphibian derived tetradecapeptide, bombesin (BN), called bombesin‐like peptides (BNLPs). BNLPs include the 27 amino acid peptide, gastrin releasing peptide (GRP,_1–27), a smaller decapeptide (GRP_18–27) and another decapeptide with less structural homology, neuromedin B (NmB). Immunoreactivity for BN and receptors for BNLPs have been demonstrated in the region of the rat SCN receiving photic input. We studied the effects of local pressure ejections of BNLPs dissolved in saline/1% bovine serum albumin (BSA) vehicle on the extracellularly recorded firing rates of Syrian hamster SCN neurons in a hypothalamic slice preparation. In one study, an ejecting electrode containing BN (10⁻⁸ to 10⁻⁴ M) was positioned 20 to 60 μm from a recording electrode. Of 74 cells tested with BN, 50 (67.6%) showed significant increases in firing rate, while 3 of 19 cells (15.8%) tested with vehicle ejections were activated. In a second study, a single electrode was used for both recordings and pressure ejections. Of 48 cells tested, BN (10⁻⁶ to 10⁻⁴ M) activated 30 (62.5%) and suppressed firing in 4 (8.3%). Of 208 cells tested with GRP_1–27 (10⁻⁹ to 10⁻⁴ M), 105 (50.5%) were activated and 2 (1.0%) were suppressed. The percentage of cells responding increased with the concentration of GRP_1–27 used in the electrode. No circadian variation in responsiveness to GRP_1–27 was detected. GRP_18–27 (5 ± 10⁻⁵ to 10⁻⁴ M) activated 10 out of 18 cells tested (55.6%), while NmB (10⁻⁴ M) activated 2 out of 30 cells tested (6.7%) and vehicle ejections activated 1 out 36 cells tested (2.8%). GRP_1–27, GRP_18–27 and BN, the BNLPs showing the greatest degree of structural homology, activate ∼50% of SCN cells, apparently via the BN/GRP‐preferring receptor subtype, and may play a role in photic entrainment.