Ontogeny of somatostatin receptors in the rat somatosensory cortex

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Abstract
The distribution and density of SRIF receptors (SRIF‐R) were studied during development in the rat somatosensory cortex by in vitro autoradiography with monoiodinated [Tyr0‐DTrp8]S14. In 16‐day‐old fetuses (E16), intense labeling was evident in the intermediate zone of the cortex while low concentrations of SRIF‐R were detected in the marginal and ventricular zones. The highest density of SRIF‐R was measured in the intermediate zone at E18. At this stage, labeling was also intense in the internal part of the developing cortical plate; in contrast, the concentration of binding sites associated with the marginal and ventricular zones remained relatively low. Profound modifications in the distribution of SRIF‐R appeared at birth. In particular, a transient reduction of receptor density occurred in the cortical plate. During the first postnatal week, the density of receptors measured in the intermediate zone decreased gradually; conversely, high levels of SRIF‐R were observed in the developing cortical layers (II to VI). At postpartum day 13 (P13), a stage which just precedes completion of cell migration in the parietal cortex, the most intensely labeled regions were layers V–VI and future layers II–III. From P13 to adulthood, the concentrations of SRIF‐R decreased in all cortical layers (I to VI) and the pattern of distribution of receptors at P21 was similar to that observed in the adults.Detailed analysis of the results revealed (i) a close association between the distribution of SRIF‐R and the areas containing migrating neuroblasts and (ii) a temporal correlation between the presence of SRIF‐R and the transient expression of the neuropeptide SRIF in the developing somatosensory cortex. Taken together, our data suggest the existence of two populations of SRIF‐R: a transient population, which could be implicated in the development of the parietal neocortex from E16 to P13, and a population of SRIF‐R only expressed after the first postnatal week, which is likely to be involved in the process of neurotransmission.