Bombesin receptors as a novel anti-anxiety therapeutic target: BB1 receptor actions on anxiety through alterations of serotonin activity

Abstract

The effects of PD 176252 [3-(1H-indol-3-yl)-N-[1-(5-methoxy-pyridin-2-yl)-cyclohexylmethyl]-2-methyl-2-[3-(nitro-phenyl)ureido]propionamide], a nonpeptide bombesin (BB) BB1/BB2 receptor antagonist, were assessed in rats using several ethologically relevant tests of anxiety. Consistent with a role for the bombesin family of peptides in subserving anxiety behaviors, the antagonist increased social interaction (3.75 and 7.5 mg/kg, i.p.), dose-dependently attenuated the number of vocalizations emitted by guinea pig pups separated from their mother (1-30 mg/kg, i.p.), reduced latency to approach a palatable snack in an anxiogenic (unfamiliar) environment, and reduced the fear-potentiated startle response (5 and 10 mg/kg, i.p., and 100-200 ng per rat, i.c.v.). When administered directly to the dorsal raphé nucleus (DRN), PD 176252 (20-500 ng) increased social interaction under aversive conditions, as did the 5-HT1A receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (50 ng). Furthermore, intra-DRN microinfusion of the peptide antagonist (PD 176252) suppressed, whereas its agonist [neuromedin B (NMB)-30] promoted, the in vivo release of 5-HT in the ventral hippocampus. In parallel, the suppressed social interaction elicited by intra-DRN administration of NMB was attenuated by a systemically administered 5-HT2C (but not 5-HT1A) receptor antagonist. Together, these findings suggest that endogenous BB-like peptides at the DRN evoke the release of 5-HT from the limbic nerve terminals originating from the raphé, specifically at the ventral hippocampus, resulting in anxiogenesis. The finding that this action was attenuated by BB receptor (BB1 and/or BB2) antagonists suggests that these compounds may represent a novel class of anxiolytic agents.

Figure 1.

Effect of PD 176252 on rat social interaction. Each column represents time spent in social interaction during a 5 min test period (mean ± SEM) after injection of vehicle (open column; n = 10), PD 176252 (3.5 mg/kg, i.p.; hatched column; n = 10), or PD 176252 (7.5 mg/kg, i.p.; solid column; n = 10). *p < 0.05 and **p < 0.01 from vehicle condition.

Figure 2.

Effect of PD 176252 on guinea pig pup vocalizations after separation from the dam during a 5 min test period. Each column represents percentage reduction in the number of vocalizations (mean ± SEM) made after PD 176252 (1–30 mg/kg; i.p.; n = 8 per group) or vehicle (n = 9) injection relative to the number of calls (No. calls) made after sham injection (number of vocalizations, 560 ± 42.6). *p < 0.05 and **p < 0.01 from vehicle condition.

Figure 3.

Effect of PD 176252 on snack consumption (grams) and on the latency to approach the snack (seconds) in the home cage and novel cage conditions. Each column depicts snack consumption (mean ± SEM over 15 min) or approach latency (mean ± SEM) under the home cage or the novel cage conditions after PD 176252 (10 mg/kg; i.p.; solid column; n = 8) or vehicle (open column; n = 8) injection. *p < 0.0.5 and **p < 0.01 from respective home cage baselines; ^†p < 0.05 from condition-matched control.

Figure 4.

Effect of PD 176252 on fear-potentiated startle amplitude after intraperitoneal (left) and intracerebroventricular (right) administration. Each column depicts startle amplitude (mean ± SEM) under the noise alone (open columns) and conditioned stimulus plus noise (CS + noise; solid columns) situations after intraperitoneal (left) injections of vehicle (n = 9), PD 176252 (5 mg/kg; n = 9), or PD 176252 (10 mg/kg; n = 9) or intracerebroventricular (right) injections of vehicle (n = 9), PD 176252 (100 ng/3 μl; n = 9), or PD 176252 (200 ng/3 μl; n = 9). *p < 0.05 and **p < 0.01 (CS + noise vs noise alone).

Figure 5.

Representative autoradiographs depicting coronal sections of rat brain taken from rats injected intracerebroventricularly either with 5,7-DHT (n = 7) or vehicle (n = 7) before they were killed. For ease of comparison, sections on the left are from vehicle-treated rats, and sections on the right are from rats treated with 5,7-DHT. DR, Dorsal raphé; MR, median raphé; VH, ventral hippocampal region.

Figure 6.

Effect of systemically administered PD 176252 (10 mg/kg; closed squares; n = 6) or vehicle (open squares; n = 6) on extracellular levels of 5-HT [expressed as a percentage change from baseline (mean ± SEM)] as measured by in vivo microdialysis in the ventral hippocampus of the freely moving rat. *p < 0.05 and **p < 0.01 relative to baseline.

Figure 7.

Effects of intra-DRN infusion of vehicle, PD 176252 (200 ng/0.5 μl; n = 8), or NMB-30 (10 nmol/0.5 μl; n = 6) on extracellular levels of 5-HT [expressed as a percentage change from baseline (mean ± SEM)] as measured in the ventral hippocampus of anesthetized rats using in vivo microdialysis. *p < 0.05 from time-matched sample after vehicle injection.