An antinociceptive role for substance P in acid-induced chronic muscle pain.

Release of substance P (SP) from nociceptive nerve fibers and activation of its receptor neurokinin 1 (NK1) are important effectors in the transmission of pain signals. Nonetheless, the role of SP in muscle pain remains unknown. Here we show that a single i.m. acid injection in mice lacking SP signaling by deletion of the tachykinin precursor 1 (Tac1) gene or coadministration of NK1 receptor antagonists produces long-lasting hyperalgesia rather than the transient hyperalgesia seen in control animals. The inhibitory effect of SP was found exclusively in neurons expressing acid-sensing ion channel 3, where SP enhances M-channel-like potassium currents through the NK1 receptor in a G protein-independent but tyrosine kinase-dependent manner. Furthermore, the SP signaling could alter action potential thresholds and modulate the expression of TTX-resistant sodium currents in medium-sized muscle nociceptors. Thus, i.m. SP mediates an unconventional NK1 receptor signal pathway to inhibit acid activation in muscle nociceptors, resulting in an unexpected antinociceptive effect against chronic mechanical hyperalgesia, here induced by repeated i.m. acid injection.

Use of mutant mouse lines to investigate origin of gonadotropin-releasing hormone-1 neurons: lineage independent of the adenohypophysis.

Mutant mouse lines have been used to study the development of specific neuronal populations and brain structures as well as behaviors. In this report, single- and double-mutant mice were used to examine the lineage of GnRH-1 cells. GnRH is essential for vertebrate reproduction, with either GnRH-1 or GnRH-3 controlling release of gonadotropins from the anterior pituitary, depending on the species. It is clear that the neuroendocrine GnRH cells migrate from extracentral nervous system locations into the forebrain. However, the embryonic origin of GnRH-1 and GnRH-3 cells is controversial and has been suggested to be nasal placode, adenohypophyseal (anterior pituitary) placode, or neural crest, again dependent on the species examined. We found that mutant mice with either missing or disrupted anterior pituitaries (Gli2(-/-), Gli1(-/-)Gli2(-/-), and Lhx3(-/-)) exhibit a normal GnRH-1 neuronal population and that these cells are still found associated with the developing vomeronasal organ. These results indicate that in mice, GnRH-1 cells develop independent of the adenohypophyseal placode and are associated early with the formation of the nasal placode.

Convergent analysis of cDNA and short oligomer microarrays, mouse null mutants and bioinformatics resources to study complex traits.

Gene expression data sets have recently been exploited to study genetic factors that modulate complex traits. However, it has been challenging to establish a direct link between variation in patterns of gene expression and variation in higher order traits such as neuropharmacological responses and patterns of behavior. Here we illustrate an approach that combines gene expression data with new bioinformatics resources to discover genes that potentially modulate behavior. We have exploited three complementary genetic models to obtain convergent evidence that differential expression of a subset of genes and molecular pathways influences ethanol-induced conditioned taste aversion (CTA). As a first step, cDNA microarrays were used to compare gene expression profiles of two null mutant mouse lines with difference in ethanol-induced aversion. Mice lacking a functional copy of G protein-gated potassium channel subunit 2 (Girk2) show a decrease in the aversive effects of ethanol, whereas preproenkephalin (Penk) null mutant mice show the opposite response. We hypothesize that these behavioral differences are generated in part by alterations in expression downstream of the null alleles. We then exploited the WebQTL databases to examine the genetic covariance between mRNA expression levels and measurements of ethanol-induced CTA in BXD recombinant inbred (RI) strains. Finally, we identified a subset of genes and functional groups associated with ethanol-induced CTA in both null mutant lines and BXD RI strains. Collectively, these approaches highlight the phosphatidylinositol signaling pathway and identify several genes including protein kinase C beta isoform and preproenkephalin in regulation of ethanol- induced conditioned taste aversion. Our results point to the increasing potential of the convergent approach and biological databases to investigate genetic mechanisms of complex traits.

Functional characterization of PCCA mutations causing propionic acidemia.

Propionic acidemia (PA, MIM 232000 and 232050) is caused by a deficiency of mitochondrial biotin-dependent propionyl-CoA carboxylase (PCC, EC 6.4.1.3), a heteropolymeric enzyme composed of alpha and beta subunits, which are encoded by the PCCA and PCCB genes, respectively. The PCCA protein (alpha subunit) is responsible for the formation of carboxybiotin upon hydrolysis of ATP and contains a C-terminal biotin-binding domain and a biotin carboxylase domain, defined by homology with other biotin-dependent carboxylases, some of them characterized structurally. More than 24 mutations have been found in the PCCA gene in patients with PA, among them 14 missense mutations and one in-frame deletion, for which the precise molecular effect is unknown. In this study, we have established the pathogenicity of 11 PCCA mutations (10 missense and an in-frame deletion) by expression studies in deficient fibroblasts and in a cell-free in vitro system, and analyzed the effect of each mutation on PCC activity, protein stability and domain structure. The results show that most mutant proteins show an increased turnover and are functionally deficient, suggesting that the structural alterations they cause are incompatible with normal assembly to produce a stable, functional PCC oligomer. These results are discussed in the context of the genotype-phenotype correlations in PCCA-deficient PA patients.

Narcolepsy in orexin knockout mice: molecular genetics of sleep regulation.

Neurons containing the neuropeptide orexin (hypocretin) are located exclusively in the lateral hypothalamus and send axons to numerous regions throughout the central nervous system, including the major nuclei implicated in sleep regulation. Here, we report that, by behavioral and electroencephalographic criteria, orexin knockout mice exhibit a phenotype strikingly similar to human narcolepsy patients, as well as canarc-1 mutant dogs, the only known monogenic model of narcolepsy. Moreover, modafinil, an anti-narcoleptic drug with ill-defined mechanisms of action, activates orexin-containing neurons. We propose that orexin regulates sleep/wakefulness states, and that orexin knockout mice are a model of human narcolepsy, a disorder characterized primarily by rapid eye movement (REM) sleep dysregulation.