Mild Hypothermia Suppresses Calcium-Sensing Receptor (CaSR) Induction Following Forebrain Ischemia While Increasing...

Hypothermia improves neurological outcome from cardiac arrest. The mechanisms of protection are multifold, but identifying some may be useful in exploring potential therapeutic targets. The extracellular calcium-sensing receptor (CaSR) was originally found in parathyroid cells in which the receptor senses minute changes in extracellular [Ca(2+)] and promotes Ca(2+) influx and intracellular Ca(2+) release. The CaSR is broadly expressed in the CNS and colocalized with the inhibitory γ-aminobutyric acid-B receptor 1 (GABA-B-R1). In hippocampal neurons, GABA-B-R1 heterodimerizes with CaSR and suppresses CaSR expression. To study the interplay between these two receptors in the development of ischemic cell death and neuroprotection by hypothermia, we subjected C57/BL6 mice to global cerebral ischemia by performing bilateral carotid artery occlusion (10 min) followed by reperfusion for 1-3 days with or without therapeutic hypothermia (33°C for 3 h at the onset of reperfusion). Terminal deoxynucleotidyl transferase dUTP nick end labeling staining and immunohistochemistry showed that forebrain ischemia increased CaSR expression, decreased GABA-B-R1 expression, and promoted cell death. These changes were particularly evident in hippocampal neurons and could be reversed by mild hypothermia. The induction of CaSR, along with reciprocal decreases in GABA-B-R1 expression, may together potentiate ischemic neuronal death, suggesting a new therapeutic target for treatment of ischemic brain injury.