Functional properties of two bombesin-like peptide receptors revealed by the analysis of mice lacking neuromedin B receptor

Abstract

The neuromedin B-preferring receptor (NMB-R) is one of the members of the bombesin (BN)-like peptide receptor subfamily in mammals. Previously, we have generated and characterized mice with targeted disruption of the two other BN-like peptide receptors, bombesin receptor subtype-3 (BRS-3) and gastrin-releasing peptide-preferring receptor (GRP-R). Here we describe the generation and analysis of NMB-R-deficient mice to investigate how NMB-R differs from BRS-3 and GRP-R. Compensation for NMB-R deficiency by overexpression of GRP-R and/or BRS-3 was not detected. Although the hypothermic effect of NMB was reduced by 50% in NMB-R-deficient mice, the effect of GRP infusion was comparable to the wild-type mice. In contrast, fundic smooth muscle contraction on stimulation with NMB or GRP was normal in NMB-R-deficient mice. Administration of GRP but not NMB suppressed glucose intake in both normal and NMB-R-deficient mice. These results suggest that the NMB-R has an essential role in thermoregulation, but not for smooth muscle contraction of the fundus or for the suppression of feeding behavior. In addition, the behavioral phenotypes of GRP-R-deficient mice were not observed in NMB-R-deficient mice. These data show that the functions of NMB-R and GRP-R are distinct, with only partial overlap.

Fig. 1.

Targeted disruption of the NMB-R gene.A, Structure of the wild-type allele, gene-targeting vector (NMBRTV), and mutated allele. Predicted lengths of restriction fragments for diagnosis are shown.a, b, and c indicate the position of PCR primers used in genotyping. 5′pr, 5′ probe; 3′pr, 3′ probe; BII,BglII; Sp, SpeI;RI, EcoRI; A,AccI; P, PstI;Sm, SmaI; pBS, pBluescript. B, Southern blot analysis of DNA prepared from a homologous recombinant ES clone.

Fig. 2.

Expression of BN-like peptide receptors in NMB-R-deficient (−/−) and control (+/+) mouse brain.A, ¹²⁵I-GRP binding to the brain section. Binding of ¹²⁵I-GRP to the anterior olfactory nucleus, where NMB-R is expressed abundantly, is evident in the control mouse brain section, but ¹²⁵I-GRP fails to bind to the same nucleus in the NMB-R-deficient mouse. B, Expression levels of BN-like peptide receptors in NMB-R-deficient mice compared with the wild-type-mice revealed by semiquantitative RT-PCR.Numbers at the top of the panels indicate the cycle numbers of PCR. PCR products for β-actin gene show that the quantity of cDNA template for both genotypes was equivalent. NMB-R-deficient mice express only truncated forms of NMB-R gene and lack normal NMB-R-expression, and the expression levels of GRP-R and BRS-3 are not changed compared with the control mice.

Fig. 3.

Regulation of body temperature in NMB-R-deficient mice. A, Thermoregulatory response at 4°C. NMB-R-deficient mice did not show any deficiency in thermoregulation during cold exposure. B, Effect of ventricular infusion of NMB or GRP in thermoregulation. In both male and female mutant mice, the hypothermic response to NMB was reduced, although the response to GRP was unchanged compared with the wild-type mice. Values are mean ± SEM. **p < 0.01.

Fig. 4.

Smooth muscle contraction in the gastric fundus. Concentration–response curves for NMB (A) or GRP (B) show that the contractile responses to both NMB and GRP were not affected in NMB-R-deficient mice. Data are expressed as percentage of peak tension induced by 60 mmKCl. Values are mean ± SEM.

Fig. 5.

NMB-R and GRP-R gene expressions in gastric fundus. A, Comparison of amplification efficiencies of NMB-R and GRP-R primers. Plasmids containing mouse NMB-R or GRP-R cDNA were used as templates (indicated at the top of the panels). Concentration of templates was adjusted using PCR with M13 forward and reverse primers. Products were analyzed at the indicated cycles. The bottom panel shows the results of PCR with NMB-R primers (for NMB-R cDNA template) and GRP-R primers (for GRP-R cDNA template), indicating that the PCR efficiencies were comparable for NMB-R and GRP-R primers. B, Expression of NMB-R and GRP-R in the gastric fundus. In wild-type mice, GRP-R expression is predominant compared with the NMB-R. Using the same amount of cDNA from wild-type and NMB-R-deficient mice (determined by PCR with β-actin primers), PCR with GRP-R primers reveals that the expression of the GRP-R gene is not upregulated in NMB-R-deficient mice.

Fig. 6.

Effect of NMB or GRP administration on glucose intake. Intake of glucose solution after intraperitoneal injection of saline, NMB, or GRP was measured. GRP induced a significant suppression of glucose intake in both NMB-R-deficient and wild-type-mice, but NMB failed to produce this effect in both mice. Values are mean ± SEM. *p < 0.05; **p < 0.01.

Fig. 7.

Behavioral measurements of NMB-R-deficient mice. Long-term (24 hr) activity measured in their home cage (A) and short-term activity (every 10 min for 60 min) under novel environment (B) were not affected in NMB-R-deficient mice. Social interaction was evaluated by counting the scores of nonaggressive (sniffing, following, mounting, and approaching the intruder) (C) and overt aggressive social behaviors (biting, fighting, and vocalization against the intruder) (D). Social interaction of NMB-R-deficient mice was comparable to the wild-type mice. Values are mean ± SEM.