Physiology & behavior
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Physiology & behavior · Apr 2015
Involvement of trigeminal astrocyte activation in masseter hyperalgesia under stress.
It is commonly accepted that psychological stress contributes to the development of temporomandibular joint disorders, in which chronic orofacial pain is the main symptom. However, the central mechanism underlying the development of these disorders has remained unclear. The current study was performed to determine the involvement of the glia in the trigeminal spinal subnucleus caudalis in stress-induced increases in masseter muscle hyperalgesia in rats. ⋯ In addition, the expression of interleukin-1β (IL-1β) and phosphorylated N-methyl-d-aspartic acid receptor 1 (p-NR1) in the Vc was significantly increased after chronic restraint stress, whereas LAA dramatically inhibited the overexpression of IL-1β and p-NR1. Taken together, these results suggest that activated astrocytes in the Vc may be one of the most important factors in the pathophysiology of masseter hyperalgesia induced by restraint stress and the following overexpression of IL-1β and excessive NMDAR phosphorylation may ultimately contribute to masseter hyperalgesia. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for the treatment of orofacial pain induced by stress.
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Physiology & behavior · Mar 2015
Characterization of attenuated food motivation in high-fat diet-induced obesity: Critical roles for time on diet and reinforcer familiarity.
Prior work using animal models to study the effects of obesogenic diets on food motivation have generated inconsistent results, with some reporting increases and others reporting decreases in responding on food-reinforced tasks. Here, we identified two specific variables that may account for these discrepant outcomes - the length of time on the obesigenic diet and the familiarity of the food reinforcer - and examined the independent roles of these factors. Time on diet was found to be inversely related to food motivation, as rats consuming a 40% high-fat diet (HFD) for only 3weeks did not differ from chow-fed rats when responding for a sucrose reinforcer on a progressive ratio (PR) schedule, but responding was suppressed after 6weeks of ad lib HFD consumption. ⋯ Finally, after 8weeks on a HFD, rats did not express a conditioned place preference for sucrose, indicating a decrement in reward value independent of motivation. These findings are consistent with prior literature showing an increase in food motivation for rats with a shorter time consuming the obesigenic diet, and for those with more prior experience with the reinforcer. This account also helps reconcile these findings with increased food motivation in obese humans due to extensive experience with palatable food and suggests that researchers engaging in non-human animal studies of obesity would better model the conditions under which human obesity develops by using a varied, cafeteria-style diet to increase the breadth of food experiences.
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Physiology & behavior · Mar 2015
Randomized Controlled TrialWhen pain is not only pain: inserting needles into the body evokes distinct reward-related brain responses in the context of a treatment.
The aim of this study was to compare behavioral and functional brain responses to the act of inserting needles into the body in two different contexts, treatment and stimulation, and to determine whether the behavioral and functional brain responses to a subsequent pain stimulus were also context dependent. Twenty-four participants were randomly divided into two groups: an acupuncture treatment (AT) group and an acupuncture stimulation (AS) group. Each participant received three different types of stimuli, consisting of tactile, acupuncture, and pain stimuli, and was given behavioral assessments during fMRI scanning. ⋯ Brain activation in response to pain stimuli was significantly attenuated in the bilateral secondary somatosensory cortex and the right dorsolateral prefrontal cortex after prior acupuncture needle stimulation in the AT group but not in the AS group. Inserting needles into the body in the context of treatment activated reward circuitries in the brain and modulated pain responses in the pain matrix. Our findings suggest that pain induced by therapeutic tools in the context of a treatment is modulated differently in the brain, demonstrating the power of context in medical practice.
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Physiology & behavior · Mar 2015
Intermittent access to liquid sucrose differentially modulates energy intake and related central pathways in control or high-fat fed mice.
Intake of sodas has been shown to increase energy intake and to contribute to obesity in humans and in animal models, although the magnitude and importance of these effects are still debated. Moreover, intake of sugar sweetened beverages is often associated with high-fat food consumption in humans. We studied two different accesses to a sucrose-sweetened water (SSW, 12.3%, a concentration similar to that usually found in sugar sweetened beverages) in C57BL/6 mice fed a normal-fat (NF) or a high-fat (HF) diet in a scheduled access (7.5h). ⋯ In HF-fed mice, 2h-intermittent access to SSW induced a greater body weight gain than mice drinking water, and led to hyperphagia on the HF diet when SSW was accessible compared to days without SSW 2h-access (leading to greater overall caloric intake), possibly through inactivation of the anorexigenic neuropeptide POMC in the hypothalamus. This was not observed in NF-fed mice, but 2h-intermittent access to SSW stimulated the expression of dopamine, opioid and endocannabinoid receptors in the nucleus accumbens compared to water-access. In conclusion, in mice, a sucrose solution provided 2h-intermittently and a high-fat diet have combined effects on peripheral and central homeostatic systems involved in food intake regulation, a finding which has significant implications for human obesity.
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Physiology & behavior · Feb 2015
Cocaine-induced suppression of saccharin intake and morphine modulation of Ca²⁺ channel currents in sensory neurons of OPRM1 A118G mice.
Several studies have shown that human carriers of the single nucleotide polymorphism of the μ-opioid receptor, OPRM1 A118G, exhibit greater drug and alcohol use, increased sensitivity to pain, and reduced sensitivity to the antinociceptive effects of opiates. In the present study, we employed a 'humanized' mouse model containing the wild-type (118AA) or variant (118GG) allele to examine behavior in our model of drug-induced suppression of a natural reward cue and to compare the morphine pharmacological profile in acutely isolated sensory neurons. ⋯ However, repeated cocaine exposure in 118GG mice led to a leftward shift of the morphine concentration-response relationship when compared with 118GG control mice, while a rightward shift was observed in 118AA mice. These results suggest that cocaine exposure of mice carrying the 118G allele leads to a heightened sensitivity of the reward system and a blunted modulation of Ca(2+) channels by morphine in sensory neurons.