Local connectivity and glutamic acid decarboxylase immunoreactivity in the rat barrel cortex

Intracortical circuitry is thought to have a central role in shaping receptive field properties of neurons in sensory cortical areas. It is likely that cortical processing of sensory inputs relies on highly specified connectivity among neurons in various corticol laminae, and involves excitatory and inhibitory components. Numerous anatomical and physiological studies have suggested that functionally defined groups of vertically oriented neurons forming cortical columns receive incoming thalamic information, transform it and then distribute it other cortical columns and regions. However, the location and nature of thalamocortical transformation, relative roles of excitatory and inhibitory circuitry, the sequence of cortical processing and the degree of interaction between cortical columns are unresolved issues. The present study was designed to examine the overall intracortical connectivity and inhibitory components of the face region of the rat somatosensory barrel cortex. Lamina IV of the face region in rats contains anatomically discrete groups of interconnected neurons, called barrels, that are related in a one-to-one fashion to individual facial vibrissae. We used immunocytochemistry to identify the normal anatomical distribution of GABAergic inhibitory elements in barrel cortex of adult rats. Simple whisker trimming was then used to produce sensory deprivation in adult and newborn rats, to examine the development and maintenance of inhibitory neurons and terminals in barrel cortex. The local connectivity of barrel neurons was studied using HRP transport, with a particular emphasis on connections between barrels and barrel-related columns, in an attempt to understand the anatomical substrates for intracortical processing of sensory inputs. Our main findings were that there are no direct barrel-to-barrel connections in lamina IV, and that GABAergic inhibitory elements are concentrated in the lamina IV barrels and responsive to changes in afferent input. These findings fit well with a model of serial processing within each barrel column beginning with transformation of thalamic inputs within a lamina IV barrel by local circuitry and inhibition. In the behaving animal, this would result in multiwhisker inputs being processed across barrel cortex in parallel fashion within individual barrels and barrel-related columns.