Quantitative trait loci influencing morphine antinociception in four mapping populations.

Analgesia (pain reduction, or antinociception) is a classical and clinically important effect of morphine administration, and in rodent models sensitivity to morphine has been shown to be strongly influenced by genotype. For example, several studies have reported marked differences in morphine antinociception between the insensitive C57BL/6 (B6) and sensitive DBA/2 (D2) inbred mouse strains on the hot-plate assay. This prompted the present genome-wide search for quantitative trait loci (QTLs) that are chromosomal sites influencing the magnitude of antinociception, by using four mapping populations derived from the B6 and D2 progenitor inbred strains. These four were the BXD recombinant inbred (RI) strain set, an F2 (B6D2F2) population, short-term selective breeding for antinociception from a B6D2F2 founding population, and incipient or completed congenic strains. In the BXD RI set and in the B6D2F2, a genome-wide search identified 10-12 provisional QTLs at a nominal p <.05. The other populations were subsequently used as confirmation steps to test each of the provisional QTL regions. Based on all available mapping populations, four QTLs emerged as significant (p <.00005) on proximal Chromosome (Chr) 1 (females only), proximal Chr 9 (females only), mid Chr 9, and proximal Chr 10. The Chr 10 QTL comaps to the same region as the micro-opioid receptor gene (Oprm); this receptor is a known mediator of morphine's antinociceptive effects. The Chr 1 QTL was evident only in females and comapped with the kappa-opioid receptor gene, Oprk.

Cocaine-induced seizure thresholds: quantitative trait loci detection and mapping in two populations derived from the C57BL/6 and DBA/2 mouse strains.

Seizures are a well known consequence of human cocaine abuse, and in rodent models, sensitivity to cocaine seizures has been shown to be strongly influenced by genotype. For example, several studies have reported significant differences between the C57BL/6 (B6) and DBA/2 (D2) inbred mouse strains in their sensitivity to cocaine-induced seizures. This prompted our use of the BXD recombinant inbred (RI) strain set and an F(2) population derived from the B6 and D2 progenitor strains for further genetic analyses and for gene mapping efforts in this study. Cocaine was infused into the lateral tail vein, and the doses needed to induce a running bouncing clonic seizure and a tonic hindlimb extensor seizure were recorded for each mouse. In the BXD RI set, a genome-wide search was carried out for QTLs (quantitative trait loci), which are sites on a chromosome containing genes that influence seizure susceptibility. An F(2) population (B6D2F2, n = 408) was subsequently used as a second, confirmation step. Based on both RI and F(2) results, three QTLs emerged as significant (P <.00005): one for clonic seizures on chromosome 9 (distal), and two for tonic seizures on chromosomes 14 (proximal to mid) and 15 (distal). Two additional QTLs emerged as suggestive (P <.0015), both associated with clonic seizures on chromosomes 9 (proximal) and 15 (distal). Both QTLs on chromosome 9 were sex-specific, with much larger effects on the phenotype seen in females than in males.

Pharmacogenetic evidence for the involvement of 5-hydroxytryptamine (Serotonin)-1B receptors in the mediation of morphine antinociceptive sensitivity.

Morphine antinociception has been shown to be influenced significantly by genetic factors, now beginning to be identified in mice. A recent quantitative trait locus analysis revealed a significant statistical association between morphine antinociceptive magnitude and a region of mouse chromosome 9. This region contains the Htr1b gene, which encodes the 5-hydroxytryptamine (serotonin)-1B (5-HT(1B)) receptor subtype. To investigate the possibility that Htr1b represents the quantitative trait locus, C57BL/6 and DBA/2 inbred strains, the progenitors of the original quantitative trait locus mapping populations, were administered a novel 5-HT(1B) receptor antagonist (GR127935) concomitant with morphine. These mice are known to differ in morphine antinociceptive sensitivity on thermal pain assays (DBA/2 high; C57BL/6 low). GR127935 caused a dose-dependent antagonism (both reversal and prevention) of morphine antinociception in DBA/2 mice but had no effect in C57BL/6 mice. However, a 5-hydroxytryptamine-1A subtype (5-HT(1A)) receptor agonist, 8-hydroxydipropylaminotetralin, reversed morphine antinociception equally in the two strains. DBA/2 mice also exhibited significantly greater antinociception than did C57BL/6 mice from the administration of a 5-HT(1B) agonist, CGS12066. These data collectively support a role for 5-HT(1B) receptors in the mediation of morphine antinociception and support the contention that polymorphisms in the Htr1b gene may underlie individual differences in morphine sensitivity.

Heritability of nociception I: responses of 11 inbred mouse strains on 12 measures of nociception.

It is generally acknowledged that humans display highly variable sensitivity to pain, including variable responses to identical injuries or pathologies. The possible contribution of genetic factors has, however, been largely overlooked. An emerging rodent literature documents the importance of genotype in mediating basal nociceptive sensitivity, in establishing a predisposition to neuropathic pain following neural injury, and in determining sensitivity to pharmacological agents and endogenous antinociception. One clear finding from these studies is that the effect of genotype is at least partially specific to the nociceptive assay being considered. In this report we begin to systematically describe and characterize genetic variability of nociception in a mammalian species, Mus musculus. We tested 11 readily-available inbred mouse strains (129/J, A/J, AKR/J, BALB/cJ, C3H/HeJ, C57BL/6J, C58/J, CBA/J, DBA/2J, RIIIS/J and SM/J) using 12 common measures of nociception. These included assays for thermal nociception (hot plate, Hargreaves' test, tail withdrawal), mechanical nociception (von Frey filaments), chemical nociception (abdominal constriction, carrageenan, formalin), and neuropathic pain (autotomy, Chung model peripheral nerve injury). We demonstrate the existence of clear strain differences in each assay, with 1.2 to 54-fold ranges of sensitivity. All nociceptive assays display moderate-to-high heritability (h2 = 0.30-0.76) and mediation by a limited number of apparent genetic loci. Data comparing inbred strains have considerable utility as a tool for understanding the genetics of nociception, and a particular relevance to transgenic studies.

Heritability of nociception II. 'Types' of nociception revealed by genetic correlation analysis.

Clinical pain syndromes, and experimental assays of nociception, are differentially affected by manipulations such as drug administration and exposure to environmental stress. This suggests that there are different 'types' of pain. We exploited genetic differences among inbred strains of mice in an attempt to define these primary 'types'; that is, to identify the fundamental parameters of pain processing. Eleven randomly-chosen inbred mouse strains were tested for their basal sensitivity on 12 common measures of nociception. These measures provided for a range of different nociceptive dimensions including noxious stimulus modality, location, duration and etiology, among others. Since individual members of inbred strains are identical at all genetic loci, the observation of correlated strain means in any given pair of nociceptive assays is an index of genetic correlation between these assays, and hence an indication of common physiological mediation. Obtained correlation matrices were subjected to multivariate analyses to identify constellations of nociceptive assays with common genetic mediation. This analysis revealed three major clusters of nociception: (1) baseline thermal nociception, (2) spontaneously-emitted responses to chemical stimuli, and (3) baseline mechanical sensitivity and cutaneous hypersensitivity. Many other nociceptive parameters that might a priori have been considered closely related proved to be genetically divergent.