Relationship of the time of origin and death of neurons in rat somatosensory cortex: barrel versus septal cortex and projection versus local circuit neurons.

The birth of a neuron initiates a series of ontogenetic events, e.g., neuronal migration and differentiation. The outcomes of these events are neurons that successfully integrate into the cortical circuitry and neurons that are unsuccessful and ultimately die. The present study determined whether there is a relationship between the generation and death of cortical neurons. The decrease in the density of postmigratory neurons (heavily labeled by a single injection of [3H]thymidine) during normal development was used as an index of neuronal death. The survival indices of neurons varied with their times of origin. Neurons born from gestational day (G) 15 to G18 had the highest rates of survival. In contrast, the earliest and latest generated neurons (i.e., those born on G12-G13 and those born on G19-G21, respectively) had the lowest survival rates. The role of neuronal death in the formation of cortical patterns was determined by assessing the survival of neurons in the barrels and septa of somatosensory cortex. No differences in the survival index were determined for neurons in the C-row barrels and adjacent septa with a particular time of origin. The survival rate of projection and local circuit neurons was determined with a double-labeling technique. One label, [3H]thymidine, was used to determine the time of origin of the neurons. The second label was used to identify the chemical or hodological characteristics of a neuron; projection neurons were labeled either by retrograde transport of horseradish peroxidase or by glutamate immunohistochemistry, and local circuit neurons were immunohistochemically identified with an antibody directed against gamma-aminobutyric acid (GABA) antibody.(ABSTRACT TRUNCATED AT 250 WORDS)