Retrograde transport of fluorescent tracers reveals extensive ipsi- and contralateral claustrocortical connections in the rat.

The projections from the claustrum to the cerebral cortex in the rat were examined by means of retrogradely transported fluorescent tracers Fast Blue (FB) and Diamidino Yellow dihydrochloride (DY), injected in the prefrontal, motor, somatosensory, auditory, and visual fields. In all cases, substantial numbers of retrogradely labeled neurons were observed in the ipsilateral and moderate to scant numbers in the contralateral claustrum insulare. Symmetrical bilateral injections of FB and DY as well as simultaneous injections of the tracers in the motor and visual cortex of the same hemisphere revealed no double-labeled neurons in the claustrum. The following conclusions may be drawn: The claustral projections to the motor, somatosensory, and visual cortex are prominent. The projection to the prefrontal cortex is less substantial and that to the auditory cortex is relatively modest. The claustrocortical connections lack the clear-cut topographic pattern of the thalamic nuclei but are, to some degree, preferentially arranged, albeit with considerable overlapping of the subpopulations of corticopetal neurons, a coarse anteroposterior topographic distribution appears to exist also in rodents. Neurons contributing to the claustrocortical connection project either ipsilaterally or contralaterally but not bilaterally. Projections to different cortical fields of one hemisphere also originate from separate claustral neurons.