Whisker stimulation metabolically activates thalamus following cortical transplantation but not following cortical ablation

Abstract

Local cerebral glucose utilization was assessed during whisker stimulation by 2-deoxyglucose autoradiography. Whisker stimulation increased local cerebral glucose utilization in brainstem, thalamus and whisker sensory cortex in normal rats. Whereas whisker stimulation increased glucose metabolism in brainstem, whisker stimulation failed to increase glucose metabolism in thalamus of rats that had whisker sensory cortex ablated 5 h to five weeks previously. The failure of whisker stimulation to activate thalamus after cortical ablations was probably not due to decreased cortical input to thalamus because whisker stimulation activated thalamus after large cortical tetrodotoxin injections. Failure of whisker stimulation to activate thalamus at early times (5 h and one day) after cortical ablations was not due to thalamic neuronal death, since it takes days to weeks for axotomized thalamic neurons to die. The failure of whisker stimulation to activate thalamus at early times after cortical ablations was likely due to the failure of trigeminal brainstem neurons that project to thalamus to activate axotomized thalamic neurons. This might occur because of synaptic retraction, glial stripping or inhibition of trigeminal brainstem synapses onto thalamic neurons. The thalamic neuronal death that occurs over the days and weeks following cortical ablations was associated with thalamic hypometabolism. This is consistent with the idea that the thalamic neurons die because of the absence of a cortically derived trophic factor, since the excitotoxic thalamic cell death that occurs following cortical kainate injections is associated with thalamic hypermetabolism. The glucose metabolism of parts of the host thalamus was higher and the glucose metabolism in surrounding nuclei lower than the normal side of thalamus in rats that sat quietly and had fetal cortex transplants placed into cavities in whisker sensory cortex five to 16 weeks previously. Whisker stimulation in these subjects activated the contralateral host thalamus and fetal cortical transplants. This was accomplished using a double-label 2-deoxyglucose method to assess brain glucose metabolism in the same rat while it was resting and during whisker stimulation. The high glucose metabolism of parts of host thalamus ipsilateral to the fetal cortical transplants is consistent with prolonged survival of some axotomized thalamic neurons. The finding that whisker stimulation activates portions of host thalamus further suggests that the cortical transplants maintained survival of the host thalamic neurons and that synaptic connections between whisker brainstem and thalamic neurons were functional.