The maturation of the somatostatin systems in the rat visual cortex

The development of somatostatin-immunoreactive (SS) neurons and processes in the rat visual cortex (VC) was studied in animals from embryonic day 20 (E20) to postnatal day 21 (D21). Three distinct patterns of immunoreactivity were seen. From E20 to birth (D0), VC was characterized by a small number of mainly bipolar SS neurons throughout the cortical plate. In the perinatal period, from D1 to D6, there were large numbers of immature immunoreactive neurons which were confined to layer VI and the subplate zone, a few bipolar neurons in the cortical plate and an extremely dense plexus of SS processes throughout the neuropil. The third phase, from D8 to weaning, was characterized by the absence of immature SS neurons, an increase in the number of multipolar SS neurons and a decrease in the density of SS fibers. By D15, the time of eye-opening, the number and distribution of SS neurons and processes was close to that seen in the adult. These results indicate that the SS system of neurons and fibers is among the earliest of the transmitter systems to be established in VC and suggests a role for the peptide in cortical organization as well as visual processing.