Expression of somatostatin and neuropeptide Y in the embryonic, postnatal, and adult mouse amygdalar complex.

Recent developmental studies indicate that distinct neuronal subpopulations in the amygdala, including somatostatin (SOM)-containing neurons, originate from progenitor domains in the anterior entopeduncular area, thus suggesting a different origin from subpallial territories for amygdalar versus cortical SOM-expressing interneurons, the latter derived from the dorsal part of the medial ganglionic eminence. In this context, we carried out an immunohistochemical study analyzing spatiotemporal expression patterns for SOM- and neuropeptide Y (NPY)-containing neurons in the embryonic, postnatal, and adult mouse amygdala. Our results indicate that SOM- and NPY-immunoreactive cells are present in the amygdalar complex from embryonic day (E)12.5, and that these peptidergic cells seem to arise from the anterior entopeduncular area progenitor domain. From E12.5 on there was a notable increase in the number and immunoreactivity of cells containing these peptides in distinct territories of the amygdalar complex, reaching a peak around birth. The distribution pattern for NPY neurons was very similar to that of SOM neurons in most nuclei of the amygdala, although the number of NPY neurons was always lower than that of SOM. At postnatal ages a reduction in the number of immunoreactive cells is observed in most amygdalar nuclei, remaining then similar from P14 to the adult. We interpret this reduction of the number of immunoreactive neurons in relation to the increased immunoreactivity for axons that occurs postnatally. We also suggest that the anterior entopeduncular area-derived SOM- and NPY-containing neurons in pallial and subpallial amygdaloid nuclei become local interneurons and projection neurons, respectively.