It is likely that morphological and functional features unique to a particular subpopulation of gamma-aminobutyric acid–releasing (GABAergic) neurons are mediated by molecules unique to that subpopulation and that the identification of these molecules will ultimately provide a mechanistic basis for these features. This chapter provides evidence to justify this view. In many areas of the mammalian cortex, including the visual cortex, neurons have been characterized extensively by the Golgi method. About seven distinct types of local circuit neurons have been classified based on axonal arborizations and dendritic patterns. Many of these types are now known to accumulate [3H]GABA, or their terminals can be labeled immunocytochemically for GABA or the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD), indicating that they are GABAergic inhibitory neurons. The specific types are generally known by the names given to reflect unique features of their axonal or dendritic branching patterns. Reagents, such as lectins and antibodies, can be a valuable tool in the definition of GABAergic subpopulations of neurons in the visual cortex.
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