Somatostatin binding sites in the white matter of the developing human brainstem: inverse relationship with the myelination process

Postnatal Development of NPY and Somatostatin-28 Peptidergic Populations in the Human Angular Bundle

The angular bundle is a white matter fiber fascicle, which runs longitudinally along the parahippocampal gyrus. It is best known for carrying fibers from the entorhinal cortex (EC) to the hippocampus through the perforant and alvear pathways, as well as for carrying hippocampal output to the neocortex, and distributing fibers to polysensory cortex. The angular bundle is already present prenatally at the beginning of the fetal period. Connections between the EC and the hippocampus are established by the 20th gestational week (gw). In the postnatal period, it shows increasing myelination. The angular bundle, as well as other white matter portions of gyral surfaces in the brain, presents interstitial neurons, a remnant of subplate neurons. Those interstitial neurons show neurochemical phenotypes both prenatally and postnatally, among which, neuropeptide Y (NPY) and Somatostatin-28 (SOM-28) peptidergic populations are noticeable, and accompany the fiber connections in the maturation of the hippocampal formation. We sought to investigate the topography of the postnatal distribution and relative density of neurons immunoreactive for NPY or SOM in the angular bundle along the rostrocaudal axis of the hippocampus. The study was carried out in 15 cases, ranging from 35 gws, up to 14 year old. All cases showed positive neurons showing a polygonal or spindle shaped morphology for both peptides, scattered throughout the angular bundle. The highest number of positive neurons appeared around birth and the ensuing weeks. Up to one and a half years, the density of both peptidergic populations decreased slightly. However, cases older than 2 years of age showed a substantial decrease in density of immunolabeled neurons, density that did not showed a minor decrease in density of positive neurons in cases older than 2 years. In addition, a topography from caudal to rostral levels of the angular bundle was detected at all ages. The functional significance of interstitial cells is unknown, but the existence of SOM and NPY peptidergic neurons, presumably inhibitory, in the white matter of the angular bundle, could contribute to the basic wiring of the hippocampal formation, through which autobiographical and spatial memories can begin to be stored in the infant brain.

Identification of multiple somatostatin receptors in the rat somatosensory cortex during development

The present study was aimed at identifying somatostatin receptor subtypes on the basis of their ligand-binding properties in the rat somatosensory cortex during fetal and postnatal development. Characterization of somatostatin-binding sites was performed in individual cortical layers by using three radioligands and eight competitors with known selectivities for the five somatostatin receptor subtypes. Binding sites sensitive to sst2-selective ligands were detected with high densities in the intermediate zone of the fetal cortex. From embryonic day 21 to 21 days postnatal (P21), mixed populations of receptors were detected in the cortical plate and emerging layers I-VI. Putative sst2 receptors were detected throughout the entire period but displayed different affinities for somatostatin and analogs, and a different sensitivity to GTP, depending on the developmental stage and the cortical layer considered. High densities of binding sites exhibiting characteristics of the sst1, sst3/5, and sst4 receptor subtypes were observed from P4 to P7, P7 to P14, and P7 to P21, respectively. In addition, each type of site exhibited a particular distribution pattern across the cortical layers that varied during the development. In the adult cortex, binding sites with sst1 and sst2 receptor characteristics were predominant. This study provides evidences of developmental expression windows of four sst receptor subtypes in selected areas of the rat cerebral cortex.