Anatomical evidence for somatotopic maps in the zona incerta of rats.

We explore the patterns of connectivity between the zona incerta (ZI) and major centres of the somatosensory system in rats. Injections of cholera toxin subunit B (CTb) or biotinylated dextran (BD) were made into different somatosensory centres (Par1, FL and HL cortex; principal and spinal trigeminal nuclei; cervical, thoracic and lumbar spinal cord) of rats. Double injections of different coloured fluorescent dextrans (fluororuby and dextran-fluorescein) or BD and CTb (revealed with different chromogens) were also made into adjacent cortical regions (Par1 and FL). After injections into each of these somatosensory centres, a distinctive labelling pattern becomes apparent. Labelled terminals from centres representing the head (Par1, trigeminal nuclei) are found in the medial ZI, while labelled terminals from forelimb (FL, cervical spinal cord), trunk (thoracic cord) and hindlimb (HL, lumbar cord) centres are found in progressively more lateral areas of the nucleus. In general, rich terminal labelling is seen in the ZI after injections into the head and forelimb centres, while very poor labelling is seen after injections into the trunk and hindlimb centres. Our double injections show that adjacent but distinct cortical areas (Par1 and FL) occupy mainly separate territories in the ZI, but with some overlap. After each injection, from the cortex to the spinal cord, only a few labelled cells are seen. Our results indicate anatomical evidence for a somatotopic map within the ZI, with a preferential representation of the head and upper limbs. We suggest that the ZI may function to influence arousal states and orientating movements in relation to particularly active somatosensory inputs.