Background Carbon dioxide (CO2) inhalation, a biological challenge and pathological marker in Panic Disorder, evokes intense fear and panic attacks in susceptible individuals. The molecular identity and anatomical location of CO2-sensing systems that translate CO2-evoked fear remains unclear. We investigated contributions of microglial acid sensor T cell death associated gene-8 (TDAG8) and microglial pro-inflammatory responses in CO2-evoked behavioral and physiological responses. Methods CO2-evoked freezing, autonomic and respiratory responses were assessed in TDAG8-deficient (−/−) and wildtype (+/+) mice. Involvement of TDAG8-dependent microglial activation and pro-inflammatory cytokine IL-1β with CO2-evoked responses was investigated using microglial blocker, minocycline and IL-1β antagonist, IL- 1RA. CO2-chemosensitive firing responses using single-cell patch clamping were measured in TDAG8−/− and +/+ mice to gain functional insights. Results; TDAG8 expression was localized in microglia enriched within the sensory circumventricular organs (CVOs). TDAG8−/− mice displayed attenuated CO2-evoked freezing and sympathetic responses. TDAG8 deficiency was associated with reduced microglial activation and pro-inflammatory cytokine, IL-1β within the subfornical organ (SFO). Central infusion of microglial activation blocker, minocycline and IL-1β antagonist, IL-1RA attenuated CO2-evoked freezing. Finally, CO2-evoked neuronal firing in patch clamped SFO neurons was dependent on acid sensor TDAG8 and IL-1β. Conclusions Our data identify TDAG8-dependent microglial acid-sensing as a unique chemosensor for detecting and translating hypercapnia to fear-associated behavioral and physiological responses, providing a novel mechanism for homeostatic threat detection of relevance to psychiatric conditions such as panic disorder.

Neural systems use homeostatic plasticity to maintain normal brain functions and to prevent abnormal activity. Surprisingly, homeostatic mechanisms that regulate circuit output have mainly been demonstrated during artificial and/or pathological perturbations. Natural, physiological scenarios that activate these stabilizing mechanisms in neural networks of mature animals remain elusive. To establish the extent to which a naturally inactive circuit engages mechanisms of homeostatic plasticity, we utilized the respiratory motor circuit in bullfrogs that normally remains inactive for several months during the winter. We found that inactive respiratory motoneurons exhibit a classic form of homeostatic plasticity, up-scaling of AMPA-glutamate receptors. Up-scaling increased the synaptic strength of respiratory motoneurons and acted to boost motor amplitude from the respiratory network following months of inactivity. Our results show that synaptic scaling sustains strength of the respiratory motor output following months of inactivity, thereby supporting a major neuroscience hypothesis in a normal context for an adult animal.

Highlights d The expression ratios among ion channels determine neuronal activity d The signals that regulate these patterns remain unclear d Ongoing membrane voltage is a major coordinator of ion channel relationships d Membrane activity can maintain or suppress correlated ion channel mRNA levels

Santin JM, Watters KC, Putnam RW, Hartzler LK. Temperature influences neuronal activity and CO 2/pH sensitivity of locus coeruleus neurons in the bullfrog, Lithobates catesbeianus. Am J Physiol Regul Integr Comp Physiol 305: R1451-R1464, 2013. First published October 9, 2013 doi:10.1152/ajpregu.00348.2013.-The locus coeruleus (LC) is a chemoreceptive brain stem region in anuran amphibians and contains neurons sensitive to physiological changes in CO 2/pH. The ventilatory and central sensitivity to CO2/pH is proportional to the temperature in amphibians, i.e., sensitivity increases with increasing temperature. We hypothesized that LC neurons from bullfrogs, Lithobates catesbeianus, would increase CO 2/pH sensitivity with increasing temperature and decrease CO 2/pH sensitivity with decreasing temperature. Further, we hypothesized that cooling would decrease, while warming would increase, normocapnic firing rates of LC neurons. To test these hypotheses, we used whole cell patch-clamp electrophysiology to measure firing rate, membrane potential (Vm), and input resistance (Rin) in LC neurons in brain stem slices from adult bullfrogs over a physiological range of temperatures during normocapnia and hypercapnia. We found that cooling reduced chemosensitive responses of LC neurons as temperature decreased until elimination of CO 2/pH sensitivity at 10°C. Chemosensitive responses increased at elevated temperatures. Surprisingly, chemosensitive LC neurons increased normocapnic firing rate and underwent membrane depolarization when cooled and decreased normocapnic firing rate and underwent membrane hyperpolarization when warmed. These responses to temperature were not observed in nonchemosensitive LC neurons or neurons in a brain stem slice 500 m rostral to the LC. Our results indicate that modulation of cellular chemosensitivity within the LC during temperature changes may influence temperature-dependent respiratory drive during acid-base disturbances in amphibians. Additionally, cold-activated/warm-inhibited LC neurons introduce paradoxical temperature sensitivity in respiratory control neurons of amphibians. chemosensitivity; temperature sensitivity; bullfrog; locus coeruleus; respiratory control THE BULLFROG, LITHOBATES CATESBEIANUS (formerly Rana catesbiena), inhabits much of North America and experiences a broad range of daily and seasonal changes in temperature. Activity during high and low temperatures can cause substantial disturbances to acid-base status in amphibians (1, 52). Unlike the endothermic mammals and birds, which regulate constant arterial pH (pH a ), amphibians and other ectothermic vertebrates preserve acid-base balance by maintaining temperature-dependent pH a . This acid-base regulatory strategy is achieved by allowing pH a to vary inversely with body temperature (T b ; for recent review, see Ref. 11). Although extensive work has attempted to illuminate the precise physiological role of an inverse pH-temperature relationship, it is still unclear as to which intracellular or extracellular variable...