Anterior Cingulate Cortex Metabotropic Glutamate Receptor Type-2 Expressing Neurons Regulate Pain Behavior: Dependence on Sex and Estrous Cycle

Abstract

Our lab previously determined that metabotropic glutamate receptor type-2 (mGluR2) containing pyramidal neurons of the anterior cingulate cortex (ACC) become hyperexcitable following persistent pain in mice. The consequence of pain induced hyperexcitability of mGluR2 expressing neurons of the ACC is unknown, but can be informed by examining the function of ACC mGluR2 expressing neurons in naive mice. We aim to determine the function of ACC mGluR2 neurons in naive mice using a multidimensional behavioral approach which includes pain-related affective, motivational, and sensory assays. Sex and ovarian cycle influence the multidimensional experience of pain, and we included sex and estrous stage as a factor in analyses. Determining the neural substrates that mediate sex and ovarian cycle dependent differences in pain-related affect, motivation, and sensory processes may lead to better pain therapies. We utilized transgenic mice that express cre recombinase specifically in mGluR2 expressing cells. We delivered a viral vector containing a cre-dependent caspase to ablate ACC mGluR2 neurons. We crossed our mGluR2-cre transgenic line with a cre-dependent channelrhodopsin transgenic line to optogenetically activate ACC mGluR2 neurons. Mice underwent a thermal pain tolerance assay and a complimentary array of pain threshold and affect related tests, including hot plate, Hargreaves, open field, and conditioned place preference. We found that estrous cycle interacts with thermal pain tolerance. Females in proestrus, a putatively high estradiol state, have higher pain tolerance for a sucrose reward. Ablation of ACC mGluR2 neurons prevents the increase in pain tolerance seen in proestrus females. Independent of sex, optogenetic activation of ACC mGluR2 neurons results in conditioned place aversion and produces antinociception. Together, these data show that ACC mGluR2 neurons participate in motivational responses to noxious stimuli, and may contribute to emotional and sensory dysregulation seen in pathological pain conditions.