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A calbindin immunoreactive 'deep pain' recipient thalamic nucleus in the rat

The ventrolateral periaqueductal gray (vlPAG) is the only brain region known to receive convergent deep noxious inputs and to mediate the reactions characteristic of deep pain. Injections of biotinylated dextran into the vlPAG of the rat revealed a strong projection to a discrete, calbindin terminal-immunoreactive region of the caudal ventromedial nucleus (VMc) of the thalamus. This nucleus appears homologous to the calbindin-positive, pain-and temperature-specific posterior ventromedial thalamic region of primates. We suggest that the vlPAG to VMc projection represents an important new route via which deep noxious inputs reach thalamus. As the rat is the species of choice in most experimental studies of pain, the functional-anatomical definition of this projection should further investigation of the thalamic representation of deep pain.