Generation of Hrh1-null mice.

A mouse null for the histamine 1 receptor (H₁R) was produced using CRISPR/Cas9. The Hrh1-null allele was identified in a B6D2F1/J founder from a project with the NHLBI Transgenic Core using CRISPR/Cas9 to cut at the Hrh1 stop codon and has the genomic sequence 5′-TCGAACCAAGACCCGTGCTT—1,052 bp deletion—CCACATGGTGGATAGACAAC. This deletion leaves intact only the initial 433 bp of Hrh1 coding sequence and removes the remaining 1,031 bp, including the last four transmembrane domains. The founder was bred to C57BL/6J once, and then heterozygous mice were intercrossed to generate Hrh1^−/− and littermate WT control mice (both sexes were used). PCR genotyping for Hrh1-null (522 bp) vs. WT (369 bp) alleles used multiplexed primers ×619 (forward, 5′-GCCAAGCAGTTGGGTTGTAT), ×660 (forward, 5′-AGATGTGTGAGGGGAACAGG), and ×622 (reverse, 5′-ATGCCTTCACACTGCTCA). On a chow diet at 33 wk of age, there was no body weight difference (males: WT 30.47 ± 0.76 g, Hrh1^−/− 32.16 ± 2.36 g; females: WT 25.80 ± 0.68 g, Hrh1^−/− 25.64 ± 1.35 g; n = 5–7/group). Plasma histamine was assayed by enzyme immunoassay (Beckman Coulter IM2015).