Identification of the source of the bilateral projection system from cortex to somatosensory neostriatum and an exploration of its physiological actions

Microinjections of cholera toxin B subunit were made into the area of the n eostriatum that receives input from the primary somatosensory barrel cortex (SI) in the rat. Studies of the cortices then allowed retrograde identific ation of the cortical cells supplying the striatal input. When injections w ere restricted to the neostriatum, retrograde labelling was found in layer V of both SI cortices. Ipsilateral to the injection, cells were retrogradel y filled with toxin in all parts of the barrel field, in adjacent parietal cortex, in the motor cortex and in prefrontal areas. A similar distribution across cortical areas was seen contralaterally; however, the stained cells in the SI were between rather than within barrel columns. An earlier anter ograde study suggested two inputs from the SI to the neostriatum. The prese nt results indicate that one input to the somatosensory area of the neostri atum arises bilaterally from neurons between the barrels of the SI, while t he topographic pathway from below the barrels is present only ipsilaterally . These anatomical results indicate that separate stimulation of the two co rticostriatal pathways from the barrel cortex is possible. Electrical stimu lation of the contralateral cortex will activate the bilateral pathway, whi le electrical stimulation of the whisker pads activates the barrels and hen ce the topographic pathway. Neurons in the somatosensory region of the stri atum responded to stimuli in the contralateral cortex and in the contralate ral whisker pad. In spite of very different path lengths, stimuli via the t wo routes gave rise to excitatory postsynaptic potentials in the striatal c ells with similar latencies. The excitatory postsynaptic potentials to whis ker pad stimulation had a rapid rise time and usually resulted in at least one action potential. Responses to stimulation of the contralateral cortex rose to a peak more slowly and were more variable in latency, but also gave rise to an action potential in the majority of cases. All the neurons had the physiological characteristics of medium-sized densely spiny cells and a fter intracellular filling with biocytin had the appropriate morphology. In summary, we propose that two corticostriatal pathways arise from layer V cells in the barrel area of the somatosensory cortex; one is bilateral and arises from cells mainly below the septa, while a topographical pathway ar ises from cells below the barrels. Both pathways can raise the spiny output cells of the striatum to firing threshold. The latencies from the contrala teral cortex imply slowly conducting fibres with considerably more temporal dispersion than the pathway from below the barrels, which we excited from the contralateral periphery. (C) 2001 IBRO. Published by Elsevier Science L td. All rights reserved.