Specific thermal responsiveness of ventromedial hypothalamic neurons to localized scrotal heating and cooling in rats

1. The specificity of thermoresponsive ventromedial hypothalamic (VMH) neurons to localized, incremental scrotal thermal cooling and heating of urethane-anaesthetized male Sprague-Dawley rats (maintained at 37 degrees C colonically) was investigated. 2. Ventromedial hypothalamic extracellular neuronal activity and surface (scalp) electroencephalogram (EEG) activity from the parietal region were recorded. Intrascapular brown adipose tissue (TIBAT), surface tail (Tt) and scrotal (Tsc) temperatures, where thermal stimulation was evoked, were also monitored. 3. One hundred and twenty-five VMH neurons were recorded, with forty (32%) VMH neurons classified as warm-responsive neurons (WRNs), twenty-three (18%) as coldresponsive neurons (CRNs) and sixty-two (50%) as thermal non-responsive neurons (TNRNs) based on their thermal coefficients. Of VMH WRNs, 60% (i.e. 24) were classified as having biphasic neuronal activity responses, as were 60% (i.e. 14 of 23) of the CRNs. Forty per cent of WRNs and CRNs were classified as having monophasic changes in neuronal activity. 4. Scrotal heating or cooling from 5 to 40 degrees C resulted in specific firing rate changes of VMH WRNs and CRNs without any associated change in EEG activity (i.e. no significant change in EEG frequency or amplitude from initial baseline EEG activity when Tsc was 20 degrees C). EEG desynchronization (increased EEG frequency, decreased amplitude) was only observed when scrotal temperatures were at 45 degrees C or after each tail pinch (noxious stimulation) but not with scrotal brushing (mechanical stimulation). 5. With core temperature maintained at 37 degrees C, localized, scrotal heating and cooling of rats did not induce IBAT temperature changes indicative of brown adipose tissue activation, but delayed changes in tail temperature, indicative of vasoactive effector responses, did occur.