Activation state of the hyperpolarization-activated current modulates temperature-sensitivity of firing in locus coeruleus neurons from bullfrogs

Abstract

-Locus coeruleus neurons of anuran amphibians contribute to breathing control and have spontaneous firing frequencies that, paradoxically, increase with cooling. We previously showed that cooling inhibits a depolarizing membrane current, the hyperpolarization-activated current (Ih) in locus coeruleus neurons from bullfrogs, Lithobates catesbeianus (Santin JM, Watters KC, Putnam RW, Hartzler LK. Am J Physiol Regul Integr Comp Physiol 305: R1451-R1464, 2013). This suggests an unlikely role for I h in generating cold activation, but led us to hypothesize that inhibition of I h by cooling functions as a physiological brake to limit the cold-activated response. Using whole cell electrophysiology in brain slices, we employed 2 mM Cs ϩ (an Ih antagonist) to isolate the role of Ih in spontaneous firing and cold activation in neurons recorded with either control or I h agonist (cyclic AMP)-containing artificial intracellular fluid. I h did not contribute to the membrane potential (V m) and spontaneous firing at 20°C. Although voltage-clamp analysis confirmed that cooling inhibits I h, its lack of involvement in setting baseline firing and V m precluded its ability to regulate cold activation as hypothesized. In contrast, neurons dialyzed with cAMP exhibited greater baseline firing frequencies at 20°C due to I h activation. Our hypothesis was supported when the starting level of I h was enhanced by elevating cAMP because cold activation was converted to more ordinary cold inhibition. These findings indicate that situations leading to enhancement of I h facilitate firing at 20°C, yet the hyperpolarization associated with inhibiting a depolarizing cation current by cooling blunts the net V m response to cooling to oppose normal cold-depolarizing factors. This suggests that the influence of I h activation state on neuronal firing varies in the poikilothermic neuronal environment.temperature; Q10; neuronal excitability; hyperpolarization-activated current; respiratory control THE AMERICAN BULLFROG, Lithobates catesbeianus, has a broad geographical distribution within North America and undergoes rapid and variable temperature changes throughout a single day (21, 49). Owing to the temperature sensitivity of neurophysiological mechanisms, changes in temperature pose a challenge to regulating neurally controlled behaviors, like breathing, in the bullfrog and other poikilothermic animals. Despite rate increases of neurally controlled, rhythmic behaviors typically associated with temperature (41), aspects of the respiratory pattern are maintained across temperatures in amphibians and other ectothermic vertebrates. Specifically, breathing frequency has low temperature dependence (Q 10 ϳ 1.7) during changes at higher temperatures (2, 42), and tidal volume (i.e., the volume of air consumed in a breath) is insensitive to variations in temperature (27,51). Consistent with these observations in vivo, respiratory-related cranial nerve activity of the isolated bullfrog brain stem in vitro maintains burst frequency across higher temperatu...

Environmentally induced return to juvenile‐like chemosensitivity in the respiratory control system of adult bullfrog, Lithobates catesbeianus

“…We previously showed that LC neurons from bullfrogs are paradoxically activated by acute exposure to cold temperatures (Santin et al . ; Santin & Hartzler, ). Thus, the next series of experiments was performed to determine whether firing responses to stimulation by another modality are disrupted following aquatic overwintering.…”

“…Neurons were then returned to control aCSF and firing rates recovered to near control values. We have previously reported that chemosensitive LC neurons from bullfrogs are paradoxically activated by cooling Santin & Hartzler, 2015). We used cold-activation to (1) determine if cold-acclimation influences neuronal properties besides chemosensitivity, and (2) to provide a positive control for increases in firing frequency.…”

“…We sought to gain a clearer understanding of whether aquatic overwintering altered the ability for CO 2 to stimulate firing or resulted in a change in LC neuron excitability which might account for reduced firing responses to hypercapnia. We previously showed that LC neurons from bullfrogs are paradoxically activated by acute exposure to cold temperatures Santin & Hartzler, 2015). Thus, the next series of experiments was performed to determine whether firing responses to stimulation by another modality are disrupted following aquatic overwintering.…”

Effect of temperature on chemosensitive locus coeruleus neurons of Savannah monitor lizardsVaranus exanthematicus

“…The number of CO 2 -inhibited neurons was lower (11%) compared with that at 20°C, but, still, the presence of inhibited LC neurons at even this higher temperature is in marked contrast to the absence of CO 2 -inhibited neurons in mammals at a similar temperature (Filosa et al, 2002). Even the effect of temperature on basal firing rate (Q 10 ) was different between savannah monitor lizards (Figs 4 and 6) and bullfrogs (Santin and Hartzler, 2015;. In lizards, warming increased basal firing rate in both chemosensitive and nonchemosensitive LC neurons.…”

Motor inactivity in hibernating frogs: Linking plasticity that stabilizes neuronal function to behavior in the natural environment

“…Specifically, animals that encounter challenging environments in their natural habitats (e.g. , large temperature swings, variable pH) are being used to understand how neural systems can maintain performance over a wide range of perturbations (Chen & von Gersdorff, ; Haley, Hampton, & Marder, ; Marder, Haddad, Goeritz, Rosenbaum, & Kispersky, ; Robertson & Money, ; Roemschied, Eberhard, Schleimer, Ronacher, & Schreiber, ; Santin & Hartzler, ; Vallejo, Santin, & Hartzler, ). Along these lines, I propose that motor systems in hibernating animals make useful models to understand how the nervous system keeps itself stable and how these processes integrate into behaviors in a natural setting.…”