title: 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 (I h) 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(+) (an I h antagonist) to isolate the role of I h 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.
authors.affiliation: Department of Biological Sciences, Wright State University, Dayton, Ohio santin.2@wright.edu.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Santin
authors.affiliation: Department of Biological Sciences, Wright State University, Dayton, Ohio.
authors.forename: Lynn K
authors.initials: LK
authors.lastname: Hartzler
mesh_terms: D000200:Action Potentials; D000818:Animals; D001833:Body Temperature Regulation / Q000187:drug effects; D003080:Cold Temperature; D000242:Cyclic AMP / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D008125:Locus Coeruleus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D008297:Male; D009433:Neural Inhibition / Q000187:drug effects; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D018377:Neurotransmitter Agents / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011892:Rana catesbeiana / Q000502:physiology*; D013997:Time Factors
chemical_list: D018377:Neurotransmitter Agents; D000242:Cyclic AMP
keywords: Q10; hyperpolarization-activated current; neuronal excitability; respiratory control; temperature