Regional Temperature Changes in the Brain during Somatosensory Stimulation

Time-dependent variations in the brain temperature ( T t ) are likely to be caused by fluctuations of cerebral blood flow (CBF) and cerebral metabolic rate of oxidative consumption (CMR O 2 ), both of which are seemingly coupled to alterations in neuronal activity. We combined magnetic resonance, optical imaging, temperature sensing, and electrophysiologic methods in α-chloralose anesthetized rats to obtain multimodal measurements during forepaw stimulation. Localized changes in neuronal activity were colocalized with regional increases in T t (by ∼0.2%), CBF (by ∼95%), and CMR O 2 (by ∼73%). The time-to-peak for T t (42 ± 11 secs) was significantly longer than those for CBF and CMR O 2 (5 ± 2 and 18 ± 4 secs, respectively) with a 2-min stimulation. Net heat in the region of interest (ROI) was modeled as being dependent on the sum of heats attributed to changes in CMR O 2 ( Q m ) and CBF ( Q f ) as well as conductive heat loss from the ROI to neighboring regions ( Q c ) and to the environment ( Q e ). Although tissue cooling because of Q f and Q c can occur and are enhanced during activation, the net increase in T t corresponded to a large rise in Q m , whereas effects of Q e can be ignored. The results show that T t increases slowly (by ∼0.1°C) during physiologic stimulation in α-chloralose anesthetized rats. Because the potential cooling effect of CBF depends on the temperature of blood entering the brain, T t is mainly affected by CMR O 2 during functional challenges. Implications of these findings for functional studies in awake humans and temperature regulation are discussed.