Immature excitatory neurons in the amygdala come of age during puberty

Abstract

The human amygdala is critical for emotional learning, valence coding, and complex social interactions, all of which mature throughout childhood, puberty, and adolescence. Across these ages, the amygdala paralaminar nucleus (PL) undergoes significant structural changes including increased numbers of mature neurons. The PL contains a large population of immature excitatory neurons at birth, some of which may continue to be born from local progenitors. These progenitors disappear rapidly in infancy, but the immature neurons persist throughout childhood and adolescent ages, indicating that they develop on a protracted timeline. Many of these late-maturing neurons settle locally within the PL, though a small subset appear to migrate into neighboring amygdala subnuclei. Despite its prominent growth during postnatal life and possible contributions to multiple amygdala circuits, the function of the PL remains unknown. PL maturation occurs predominately during late childhood and into puberty when sex hormone levels change. Sex hormones can promote developmental processes such as neuron migration, dendritic outgrowth, and synaptic plasticity, which appear to be ongoing in late-maturing PL neurons. Collectively, we describe how the growth of late-maturing neurons occurs in the right time and place to be relevant for amygdala functions and neuropsychiatric conditions.

Keywords: Amygdala; Development; Migration; Neurogenesis; Paralaminar nucleus; Primates.

Graphical abstract

Fig. 1

Adult human PL anatomy. Serial coronal sections of the adult, 27 year old human amygdala. Sections indicate the location of late-maturing neurons in the amygdala paralaminar nucleus and surrounding regions and are spaced anterior to posterior across 2 mm intervals. Light gray shading indicates damaged tissue areas. AB: accessory basal nucleus, BA: basolateral amygdala, BLVM: basolateral ventromedial division, EC: entorhinal cortex, LA: lateral amygdala, LPL: lateral paralaminar nucleus, MPL: medial paralaminar nucleus, PL: paralaminar nucleus, tLV: temporal lateral ventricle, unf: uncinate fasciculus, wm: white matter.

Fig. 2

Cell types in the PL and local environment. A cluster of immature PL neurons is depicted adjacent to the dorsolateral wall of the temporal lobe lateral ventricle, at an anterior-posterior level corresponding to approximately (iv) in Fig. 1. The arrangement of immature PL neurons and the prevalence of other cell types varies depending on anatomical location. The type of dense cluster depicted here is consistent with those present in the LPL and is representative of what might be observed during puberty. Neurons of intermediate maturity (co-expressing DCX, PSA-NCAM, and NEUN) are localized facing the basal or lateral amygdala (top right of field of view). tLV: temporal lobe lateral ventricle.

Fig. 3

Age related changes in PL cell types. A, 13 year old human amygdala medial paralaminar region immunostained for DCX, PSA-NCAM, and NEUN. (B–D), DAPI⁺ nuclei in the MPL and LPL quantified from birth to 77 years, plotted as a density with 95% confidence intervals (B), as a rate of change (C), and example histology at 13 years (D) from insets 1 and 2 in panel (A). (E–G) DCX⁺PSA-NCAM⁺ cells quantified from birth to 77 years, plotted as a percentage of DAPI⁺ cells with 95% confidence intervals (E), as a rate of change (F), and example histology at 13 years (G) from insets 1 and 2 in panel (A). (H–J) NEUN⁺ cells quantified from birth to 77 years, plotted as a percentage of DAPI⁺ cells with 95% confidence intervals (H), as a rate of change (I), and example histology at 13 years (J) from insets 1 and 2 in panel (A). The shaded region of the graphs represent the age range of puberty across girls and boys (8–15 years, mean age of 11.5). Example of a mature NEUN⁺ cell that does not express DCX or PSA-NCAM (arrow) and a putative intermediate maturity cell that is DCX⁺ and NEUN⁺ (dashed arrow), and a small immature neuron that is DCX⁺PSA-NCAM⁺ (arrowhead). Scale bars: 100 µm (A); 20 µm (D,G,J). Data re-analyzed from Sorrells et al., 2019.

Fig. 4

Age related changes in PL cells in the cell cycle. A,B, Ki-67⁺ cells quantified from birth to 24 years in the MPL, LPL, and BLA, plotted as a density (A), as a rate of change (B). C,D, Ki-67⁺SOX2⁺ cells quantified from birth to 24 years in the MPL, LPL, and BLA, plotted as a density (C), as a rate of change (D). E,F, The percentage of Ki-67⁺ cells also expressing SOX2 between birth and 24 years in the MPL, LPL, and BLA, plotted as a percent (E), as a rate of change (F). The shaded region of the graphs represent the age range of puberty across girls and boys (8–15 years, mean age of 11.5). G, 6 year old and H, 24 year old human amygdala medial paralaminar region immunostained for Ki-67, SOX2, and DCX. Insets show single channels of Ki-67⁺SOX2⁺ cells (arrow) near DCX⁺ immature neurons (arrowhead). Scale bars: 20 µm. Data re-analyzed from Sorrells et al., 2019.

Fig. 5

PL maturation processes. A, Depiction of the growth of the PL between birth and adult ages. At birth the PL clusters are very dense and primarily composed of immature neurons. By late childhood ages, many of these immature neurons remain, although some mature neurons are present. During teenage years and into adult ages, the development of the PL plateaus. B, At birth, the PL is composed of dense clusters of immature neurons. As the PL cells mature during postnatal life, multiple processes associated with their maturation may be ongoing. Although a higher proportion of Ki-67⁺SOX2⁺ cells are present in the PL compared to the rest of the BLA at birth, it is not known if these are indeed neural precursor cells (NPCs) and whether these cells persist into postnatal ages. Cells with migratory morphology are present in the PL across postnatal ages, but it is not known if these cells are truly migrating, if they are born directly from NPCs, or if the immature neurons in the region lacking migratory morphology are able to re-activate their ability to migrate. As the neurons in the PL mature, activity of inputs to these regions and these cells likely sculpts their maturation, but the mechanisms of this remain to be discovered. In the mature PL it is not known whether late-maturing neurons adopt a similar identity to neurons that developed on a normal timeline, or whether there are a variety of subtypes of neurons that mature late.