Brain surface temperature under a craniotomy

Abstract

Many neuroscientists access surface brain structures via a small cranial window, opened in the bone above the brain region of interest. Unfortunately this methodology has the potential to perturb the structure and function of the underlying brain tissue. One potential perturbation is heat loss from the brain surface, which may result in local dysregulation of brain temperature. Here, we demonstrate that heat loss is a significant problem in a cranial window preparation in common use for electrical recording and imaging studies in mice. In the absence of corrective measures, the exposed surface of the neocortex was at ∼28°C, ∼10°C below core body temperature, and a standing temperature gradient existed, with tissue below the core temperature even several millimeters into the brain. Cooling affected cellular and network function in neocortex and resulted principally from increased heat loss due to convection and radiation through the skull and cranial window. We demonstrate that constant perfusion of solution, warmed to 37°C, over the brain surface readily corrects the brain temperature, resulting in a stable temperature of 36-38°C at all depths. Our results indicate that temperature dysregulation may be common in cranial window preparations that are in widespread use in neuroscience, underlining the need to take measures to maintain the brain temperature in many physiology experiments.

Fig. 1.

Rapid cooling of the brain surface in an in vivo mouse preparation. A: schematic representation of a cranial window during recording of temperature and single-cell activity in the anesthetized mouse. The main potential routes of heat transfer are indicated. B: brain surface temperature measured with the thermocouple during replacement of the artificial cerebrospinal fluid (ACSF) with fresh ACSF warmed to 38°C. ACSF was replaced twice, indicated by the arrowheads.

Fig. 2.

Example of effects of surface temperature on ongoing synaptic activity. A: 2-photon maximum intensity fluorescence side-projection of a layer 2/3 pyramidal neuron in barrel cortex during somatic whole-cell recording. The neuron was filled with indicator (Alexa 594) through the recording pipette, which is visible to the right of the image. The pial surface of the cortex is visible as punctate staining near the top of the image. B: surface temperature (temp.) and intracellular membrane potential (Vm.) of the neuron in A. The brain surface was warmed by perfusion with ACSF at 38°C. The brain surface was initially at ∼28°C with no perfusion. The perfusion began after almost 10 min of recording. After the surface temperature warmed to ∼36°C, the heater was switched off, and the brain surface cooled once more to ∼29°C. Heating the perfusate a 2nd time had a similar effect. The membrane potential of the neuron was recorded throughout these manipulations. C: membrane potential recordings from 5 periods at ∼28°C (blue) and ∼36°C (red) at the times indicated in B. Dashed horizontal lines mark −70 mV. D: membrane potential frequency histograms for the traces in C.

Fig. 3.

Effects of temperature on cortical Up and Down states. The effects of temperature on the mean membrane potentials of Up and Down states, percentage of time spent in each state, and the duration of Up and Down states are shown. Each pair of points (connected by a line) shows the mean ± SE values for a single neuron. n = 7 Recordings from 6 mice. Temperature was manipulated by perfusion of warmed ACSF across the brain surface. Down state membrane potential, percentage of time in Up and Down states, and Down state duration each changed significantly (P < 0.05, paired t-test) with temperature.

Fig. 4.

Effects of temperature on low frequency power. A: example of the effect of temperature on mean power in the 0- to 1.5-Hz band for a voltage recording from layer 2/3. Trials classified as warm and cold are denoted by red and blue symbols, respectively. B: mean spectra for the recording in A at colder (blue; 9 trials) and warmer (red; 9 trials) temperatures. Shaded areas represent SE. Inset: plot of mean spectral power at 0–1.5 Hz as a function of temperature for the 18 trials shown in A and B. C: mean ± SE 0- to 1.5-Hz power at warm and cold temperatures for 8 recordings from 7 mice.

Fig. 5.

Effects of temperature on membrane properties of cortical pyramidal neurons. A: voltage-current (V-I) curves, sorted by state and temperature, for an example neuron. Temperature was manipulated by perfusion of warmed ACSF across the brain surface. B: effects of temperature on input resistance (R_N), the coefficient of anomalous rectification (c_AR), and rheobase. n = 5 Recordings from 5 mice for input resistance and anomalous rectification. n = 9 Recordings from 7 mice for rheobase. Asterisks denote significant differences (P < 0.05, paired t-test).

Fig. 6.

Warming the microscope objective has no effect on brain surface temperature. Examples of the lack of effect on brain surface temperature of warming of the microscope objective to 37°C (A) and 50°C (B). In each case, the objective warmer was switched on at t = 1 min and remained on thereafter.

Fig. 7.

Brain surface temperature in the absence of conduction. A: schematic showing the likely routes of heat loss for a preparation without dental acrylic, steel plate, or microscope objective. B: temperature change on replacing the ACSF with ACSF warmed to 38°C in the preparation shown in A. The ACSF was replaced twice, as indicated by the arrowheads. C: overlaid temperature measurements from addition of warmed ACSF with (gray) and without (black) a head plate, illustrating the negligible effect of the head plate on heat loss.

Fig. 8.

Brain surface temperature during an increase in rectal temperature. A: example of the effect of raising core temperature on brain surface temperature. Heat was supplied to the mouse via a heating blanket with a rectal temperature probe and feedback control circuit. The command temperature was switched between 38 and 42°C. B: results from a similar experiment in which the thermocouple was inserted ∼5 mm into the brain.

Fig. 9.

Brain temperature as a function of depth and effect of perfusion with warmed ACSF. A: repeated measurement of brain temperature as a function of depth into the brain with no perfusion (blue lines) and during perfusion of the brain surface with warmed ACSF (at 37.4 ± 1.3°C; red). Four depth profiles were measured sequentially by pushing the thermocouple into the brain, with no perfusion, perfusion with warmed ACSF, no perfusion, and perfusion with warmed ACSF. Room temperature was 23–24°C. B: brain temperature as a function of depth, shown schematically, in the absence and presence of surface perfusion with warmed ACSF.