Transhemispheric cortical reorganization in rat SmI and involvement of central noradrenergic system

Responses of single units in the hindpaw representational area of the left primary somatosensory cortex to electrical stimulation of both hindpaws and the right forepaw were recorded under urethane anaesthesia in three groups of adult male rats: a control group and two groups in which the right hindpaw representational area had been ablated 3-4 weeks previously, immediately after intraperitoneal injection of saline vehicle or DSP4, to destroy cortical noradrenergic terminals arising from the locus coeruleus. The lesion increased the overall number of neurones responding within 500 ms after the stimulation of the contralateral hindpaw (from 64 to 91%), and the proportion exhibiting short-latency response increased from 41 to 61%. Interestingly, the proportion of neurones with bilateral representation increased from 3 to 10% after the cortical lesioning. The changes were prevented by injection of DSP4 prior to lesioning and therefore depended on an intact central noradrenergic system. The increase in bilateral representation could not have been due to direct interhemispheric connections between corresponding representational areas because it occurred after lesioning of the homologous area in the contralateral hemisphere. The phenomenon was termed 'transhemispheric reorganization' and because it was somatotopically oriented (e.g. to either hindpaw); its function may be to ensure that when a sensory cortical area is damaged, its basic sensory functions are 'taken over' by the corresponding contralateral area.