Cellular Mechanisms of Desynchronizing Effects of Hypothermia in an In Vitro Epilepsy Model

Abstract

Hypothermia can terminate epileptiform discharges in vitro and in vivo epilepsy models. Hypothermia is becoming a standard treatment for brain injury in infants with perinatal hypoxic ischemic encephalopathy, and it is gaining ground as a potential treatment in patients with drug resistant epilepsy. However, the exact mechanism of action of cooling the brain tissue is unclear. We have studied the 4-aminopyridine model of epilepsy in mice using single-and dual-patch clamp and perforated multielectrode array recordings from the hippocampus and cortex. Cooling consistently terminated 4-aminopyridine induced epileptiform-like discharges in hippocampal neurons and increased input resistance that was not mimicked by transient receptor potential channel antagonists. Dual-patch clamp recordings showed significant synchrony between distant CA1 and CA3 pyramidal neurons, but less so between the pyramidal neurons and interneurons. In CA1 and CA3 neurons, hypothermia blocked rhythmic action potential discharges and disrupted their synchrony; however, in interneurons, hypothermia blocked rhythmic discharges without abolishing action potentials. In parallel, multi-electrode array recordings showed that synchronized discharges were disrupted by hypothermia, whereas multi-unit activity was unaffected. The differential effect of cooling on transmitting or secreting γ-aminobutyric acid interneurons might disrupt normal network synchrony, aborting the epileptiform discharges. Moreover, the persistence of action potential firing in interneurons would have additional antiepileptic effects through tonic γ-aminobutyric acid release.

Therapeutic brain hypothermia, its mechanisms of action, and its prospects as a treatment for epilepsy

“…This preferential effect of hypothermia on synchrony between CA1 and CA3 pyramidal neurons but not on action potential firing in interneurons (Fig. ) could be due to different synaptic afferents to pyramidal cells and interneurons, different intrinsic action potential firing mechanisms in these neurons, or different thresholds in voltage‐gated channels (Motamedi et al., ). Hippocampal interneurons and pyramidal neurons express two different subunits of voltage‐gated Na 2+ channels, that is, NaV1.1 and NaV1.6, respectively.…”

“…Temperature recording is shown below the cell recording traces; 4‐aminopyridine (4‐ AP ) 50 μ m , Green GAD gfp mice at postnatal day 15; hippocampal slice; CA3; dual current clamp recordings. From (Motamedi et al., ).…”

“…Intracellular current clamp in hippocampal pyramidal neurons during hypothermia has shown blockade of action potentials in hippocampal neurons, either through a tetrodotoxin (TTX)–sensitive depolarization or hyperpolarization, both associated with action potential firing block and increased membrane input resistance (Payton et al., ; Thompson et al., ; Volgushev et al., ; Motamedi et al., ). These findings suggest closure of a tonically active inward current that would normally be activated by warmer temperatures possibly involving transient receptor potential (TRP) channels such as TRPV4, which is expressed in hippocampal neurons and is activated by temperatures >30°C (Shibasaki et al., ).…”

“…These findings suggest closure of a tonically active inward current that would normally be activated by warmer temperatures possibly involving transient receptor potential (TRP) channels such as TRPV4, which is expressed in hippocampal neurons and is activated by temperatures >30°C (Shibasaki et al., ). However, a selective TRPV4 agonist and nonspecific agonists and antagonists of several other TRP channels failed to induce currents during hypothermia, making involvement of TRP channels less likely (Motamedi et al., ).…”

Potential mechanisms and clinical applications of mild hypothermia and electroconvulsive therapy on refractory status epilepticus

“…These researchers found that hypothermia caused changes in voltage-gated sodium channels that led to a loss of functioning and affected neuronal depolarization, which can suspend epileptiform discharges [23,24]. Subsequently, Motamedi et al [25] found that continuous low temperatures terminated epileptiform discharges in hippocampal and cortical neurons using single or double patch clamps and multi-electrode array technology in the 4-aminopyridine (4-AP) model. A simultaneous recording of dual patch clamps in CA1 and CA3 pyramidal cells showed that low temperatures blocked action potentials in these regions and interfered with neuronal firing rhythms.…”