Cooling blocks rat hippocampal neurotransmission by a presynaptic mechanism: observations using 2‐photon microscopy

Abstract

Over the past decade there has been great interest in the therapeutic potential of brain cooling for epilepsy, stroke, asphyxia and other neurological diseases. However, there is still no consensus regarding the neurophysiological effect(s) of brain cooling. We employed standard physiological techniques and 2-photon microscopy to directly examine the effect of temperature on evoked neurotransmitter release in rat hippocampal slices. We observed a monotonic decline in extracellular synaptic potentials and their initial slope over the temperature range 33-20• C, when the slices were cooled to a new set point in less than 5 s. Imaging the fluorescent synaptic marker FM1-43 with 2-photon microscopy showed that the same cooling protocol dramatically reduced transmitter release between 33 and 20• C. Cooling also reduced the terminal FM1-43 destaining that was induced by direct depolarization with elevated K + , indicating that axonal conduction block cannot account for our observations. The temperature dependence of FM1-43 destaining correlated well with the effect of temperature on field potential slope, compatible with a presynaptic explanation for our electrophysiological observations. Optical measurement of FM1-43 dissociation from cell membranes was not affected by temperature, and rapid cooling of slices loaded with FM1-43 did not increase their fluorescence. Our experiments provide visible evidence that a major neurophysiological effect of cooling in the mammalian brain is a reduction in the efficacy of neurotransmitter release. This presynaptic effect may account for some of the therapeutic benefits of cooling in epilepsy and possibly stroke.

Synaptic Vesicle Mobility in Mouse Motor Nerve Terminals with and without Synapsin

“…In summary, the overall exocytic rate was approximately doubled by increasing temperature from 23°C to 37°C, in good agreement with estimates using cultured hippocampal synapses (Micheva and Smith, 2005), hippocampal slices (Yang et al, 2005), and the calyx of Held (Kushmerick et al, 2006).…”

“…Several recent studies of murine synapses demonstrated reduced exocytic and recycling rates at room temperature compared with physiological temperature (FernandezAlfonso and Ryan, 2004;Micheva and Smith, 2005;Yang et al, 2005;Kushmerick et al, 2006). We set out to determine whether synaptic vesicle mobility was also affected by temperature using FRAP of FM 1-43-labeled vesicles.…”

The Therapeutic Potential of Focal Cooling for Neocortical Epilepsy

“…Our own recent observations indicate that hypothermia rapidly reduces transmitter release from presynaptic vesicles and suggest that this may be a dominant effect of rapid cooling on central neuronal excitability. 14 The potential clinical utility of cooling for neurological disease has been discussed for half a century. Fay 15 began an extensive investigation of brain cooling in 1938.…”

Application of Focal Cerebral Cooling for the Treatment of Intractable Epilepsy

“…1,10,13,24,29) A 2-photon microscopy technique demonstrated that a major neurophysiological effect of cooling in the brain is reduction in presynaptic neurotransmitter release. 37) Furthermore, examination of rat hippocampal slices demonstrated that moderate cooling (219 C) induced a reversible block in the network synchrony, which was required to generate EDs, without any blockage of the synaptic transmissions. 15) …”