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When juvenile interneurons arrive at the cortical environment following tangential migration, they are faced with the task of positioning themselves in cortical space in preparation for local circuit wiring. This includes integration into different cortical layers and cessation of migration at various positions to ensure adequate coverage. Little is known about the signals or mechanisms that initiate a conversion from the migratory phenotype to the arborization phenotype. This study looks at the immediate changes in interneuron morphology after culturing for 24 h in a three-dimensional collagen gel. Immature interneurons taken from different stages of corticogenesis showed increased neurite branching and outgrowth after interneuronal contacts were made. These responses were suppressed in the presence of Slit and brain-derived neurotrophic factor (BDNF) if the interneurons were sourced from early to mid-stages of corticogenesis. However, interneurons taken from the late period of corticogenesis responded to Slit and BDNF by increasing branching and neurite outgrowth. These results suggest an initial interneuronal cell contact as a stimulus for propagating neuronal arborization that may lead to the formation of inhibitory neuronal circuits. In addition, we have identified the late corticogenetic period when interneurons are most sensitive to the neurite promoting effects of Slit and BDNF.