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.

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.

Homozygous transgenic knockout mice without -opioid receptors lack morphine-induced antinociception

The perceived intensity of a painful stimulus is determined in part by the stimulus intensity and environmental conditions. The purpose of this study was to determine the influence of genetic factors in nociception and its contribution to the potency of morphine to produce antinociception. Eight inbred strains of mice were tested across a range of stimulus intensities in thermal (hot plate) and chemical irritant (acetic acid) nociceptive tests. Stimulus intensities in the thermal test included hot plate temperatures of 51, 53, 55, 57 and 59 degrees C. Stimulus intensities in the chemical irritant test included acetic acid concentrations of 0.1, 0.3 and 0.6%. Linear interpolation of stimulus-effect curves revealed large genotype-dependent differences in the effective temperature resulting in a 10 s latency on the hot-plate (ET10") and the acetic acid concentration resulting in the same number of writhes as determined by the area under the curve (AUC50). There was no genetic correlation between sensitivity to thermal versus chemical stimuli. Morphine dose response curves were then determined at a fixed stimulus intensity in each test (55 degrees C and 0.6% acetic acid) to determine analgesic ED50 doses for each inbred strain. A significant effect of genotype on relative sensitivity to morphine-induced analgesia in both the thermal and chemical irritant tests was found, however there was no genetic correlation between the potency of morphine in each test. There was an inverse genetic correlation between sensitivity to thermal and chemical stimuli and morphine ED50 values in each respective test. In both tests, strains less sensitive to the nociceptive stimuli were more sensitive to the antinociceptive effects of morphine. Confirmation studies in a separate genetic population confirmed the inverse relationship between hot-plate sensitivity and antinociceptive potency. In summary, this study demonstrated (i) a large degree of genetically-determined variability in sensitivity to painful stimuli, (ii) sensitivity to thermal stimuli (hot-plate) is genetically unrelated to sensitivity to chemical (acetic acid) stimuli, (iii) the mechanism by which morphine produces its antinociceptive effects against thermal stimuli is largely genetically independent of the mechanism by which morphine produces its antinociceptive effects against chemical stimuli, and (iv) inherent differences in sensitivity to painful stimuli may be responsible, in part, for individual differences in the potency of morphine's antinociceptive effects.