Development of axonal arbors of layer 4 spiny neurons in cat striate cortex

Abstract

Spiny neurons in layer 4 of cat striate cortex are the primary recipients of geniculocortical afferents and provide crucial links to other cortical layers for processing visual information. Using intracellular staining, we examined the development of the local axonal projections of these neurons to determine (1) whether the laminar specificity of their projections emerged specifically or was sculpted from transient exuberant projections and (2) whether the emergence of excitatory connections from layer 4 to layer 2/3 could contribute to the activity-dependent development of clustered horizontal connections of layer 2/3 pyramidal neurons. Differences in the extent of projections to infragranular (layers 5 and 6, which receive sparse projections) versus superficial layers (layers 2/3 and 4, which receive extensive projections) developed specifically from the outset. By postnatal day 15 (P15) projections to infragranular layers matured and were indistinguishable from those in the oldest animal studied (P33). In contrast, projections to superficial layers continued to increase in complexity after P15. Projections within layer 4, which were the most elaborate at all ages studied, reached maturity at about P20, while those to layer 2/3 continued to increase in complexity through P33. No evidence for exuberant projections to any of these cortical layers was observed. At very early postnatal ages (P5) projections to the subplate region were evident. These disappeared by P8-P11, suggesting the presence of transient connections from layer 4 spiny neurons to subplate neurons. Binocular deprivation did not prevent the emergence of projections from layer 4 spiny neurons into layer 2/3 or development of normal laminar differences in projection density. Connections from layer 4 to layer 2/3 emerged after horizontal connections in layer 2/3 were crudely clustered, but in synchrony with the later refinement of clusters. Collaterals from layer 4 cells first crossed into layer 3 at P11, but were extremely short (extending only 50-200 μm beyond the laminar boundary) and uncommon (only 4 of 19 cells). Since by P8 horizontal projections of layer 2/3 pyramidal neurons are already crudely clustered, the emergence of crude clustering is probably independent of layer 4 to layer 2/3 excitatory projections. The proportion of cells projecting to layer 2/3 and the complexity of their arbors both increased in the subsequent weeks, closely matching the timing of both the refinement of crudely clustered horizontal connections and the emergence of visual responsiveness in layer 2/3. These results suggest that projections from layer 4 to layer 2/3 are necessary for activation of layer 2/3 cells by visual stimuli and that this activation may in turn allow layer 2/3 neurons to extract relevant information from the animal's environment to direct the refinement of clustered horizontal connections.