A fundamental and unresolved issue in the development of the mammalian cerebral cortex is how neurons with similar receptive field properties become organized into the well-known vertical columns first described by Mountcastle in somatosensory cortex and by Hubel and Wiesel in visual cortex (Mountcastle, 1978; Hubel and Wiesel, 1977). Elucidating the cellular and molecular mechanisms responsible for the development of this neuronal arrangement, so ubiquitous in neocortex, will very likely shed light on some of the most fundamental aspects of the rules governing the formation of neural circuits in this part of the brain. This, in turn, will ultimately simplify what is one of the challenging problems in contemporary cortical neurobiology, namely, uncovering the basic rules of cortical information processing and defining the neuronal circuits that implement these computational rules. The work proposed in this application seeks to combine anatomical, electrophysiological, immunocytochemical and in situ hybridization techniques to characterize the modulation and biophysical properties of gap junctions formed between neurons in the developing rat cerebral cortex. Specifically, the technique of intracellular tracer injections will be combined with voltage clamp using patch electrodes in the whole- cell recording mode to study gap junction coupling in the cortical brain slice preparation, where the neuronal environment and connectivity is representative of the situation in the intact brain. this approach, along with methods for localizing connexin protein and mRNA will be applied to the developing neocortex to: (a) investigate the extent to which this form of intercellular signaling can be modulated over time by relevant physiological stimuli, (b) characterize the biophysical properties of neuronal coupling, (c) identify specific connexins expressed in neurons, and (d) define further the degree of specificity that governs junction formation between neurons. While the results derived from this work are expected to be applicable in large measure to other regions of the nervous system, the cerebral cortex has been chosen for these studies because the role of gap junctions in this region of the mammalian brain is of great interest, both theoretically and for understanding specific neurological disorders in humans. From a theoretical point of view, it would be extremely useful to understand the role of gap junctions in the way mature patterns of connections are established between neurons, and more specifically gap junction involvement in the formation of functional columns in cortex, a role suggested by recent findings (Yuste et al., 1992; Peinado et al., in press). Likewise, a more thorough knowledge of the causes and consequences of abnormal gap junction communication could shed light on the nature of such disorders of the nervous system as epilepsy (where elevated levels of connexin mRNA have been demonstrated in neocortex (Naus et al., 1991)) and various developmental brain abnormalities of devastating consequences for millions of affected human beings.
