Anatomical and molecular features of the amygdalohippocampal transition area and its role in social and emotional behavior processes

Abstract

The amygdalohippocampal transition area (AHi) has emerged as a critical nucleus of sociosexual behaviors such as mating, parenting, and aggression. The AHi has been overlooked in rodent and human amygdala studies until recently. The AHi is hypothesized to play a role in metabolic and cognitive functions as well as social behaviors based on its connectivity and molecular composition. The AHi is small nucleus rich in neuropeptide and hormone receptors and is contiguous with the ventral subiculum of the hippocampus-hence its designation as a "transition area". Literature focused on the AHi can be difficult to interpret because of changing nomenclature and conflation with neighboring nuclei. Here we summarize what is currently known about AHi structure and development, connections throughout the brain, molecular composition, and functional significance. We aim to delineate current knowledge regarding the AHi, identify potential functions with supporting evidence, and ultimately make clear the importance of the AHi in sociosexual function.

Keywords: Amygdala; Amygdalohippocampal transition area; Anatomy; Social behavior.

Figure 1.. Comparative anatomy of human, macaque, and mouse amygdala.

(a.) Diagram of human amygdala and (b.) rhesus macaque, and (c.) mouse amygdala. Abbreviations: AB: accessory basal nucleus of the amygdala, AHA: amygdala-hippocampal transition area; AHiAL: anterolateral AHi; AHiPM: posteromedial AHi, APH: anterior parahippocampal gyrus, BLP: posterior basal lateral amygdala, BLVm: ventromedial-, BLVl: ventrolateral basolateral amygdala, BLl: lateral-, BLD: dorsal basolateral amygdala, BMP: posterior basomedial amygdala, BMv: ventral-, BMd: dorsal basal medial amygdala, CA1U: uncal cornu Ammonis 1, CA1R: rostral cornu Ammonis 1, cp: caudate putamen, CA3: cornu Ammonis 3,ot: optic tract, CoPv: posteroventral-, CoAd: anterodorsal-, CoPm: posteromedial cortical amygdala, COp: posterior cortical nucleus, DG: dentate gyrus, EI: entorhinal cortex intermediate division, ELc: entorhinal cortext lateral division, HATA: hippocampal amygdala transition area, LV: lateral ventricle, MeC: central medial amygdala, ProU: uncal prosubiculum, ProR: rostral prosubiculum, proS: prosubiculum, PLCo: posterolateral-, PMCo: posteromedial cortical amygdala, pu: putamen, SR: rostral subiculum, SU: uncal subiculum, Sub: subiculum, 35: area 35 of the perirhinal cortex, 36c/r: area 36 of the perirhinal cortex (caudal/rostral subregion). Created with BioRender.com

Figure 2.. Summary of AHi connectivity with proposed functionality.

(a.) Projections from the AHi, (b.) inputs to the AHi, (c.) and reciprocal regions that share both inputs and projections with the AHi. The canonical or hypothetical functions of each region are outlined in the key. Abbreviations: Acb (nucleus accumbens), PFC (prefrontal cortex), IL (infralimbic cortex), MDN (medial dorsal nucleus), PMCo (posteromedial cortical amygdaloid nucleus), PLCo (posterolateral cortical amygdaloid nucleus), DRN (dorsal raphe nucleus), PAG (periaqueductal gray), PeFA (perifornical area of hypothalamus), PRC (perirhinal cortex), AIC (anterior insular cortex), La (lateral amygdala), AA (anterior amygdala), CeA (central amygdala), SPF (subparafascicular nucleus), SuMM (supramamillary area), VTA (ventral tegmental area), PVT (paraventricular thalamus), mEnt (medial entorhinal cortex), vEnt (ventral entorhinal cortex), Pir (piriform cortex), Arc (arcuate nucleus), Tu (olfactory tubercle), MMN (medial mamillary nucleus), vCA1 (ventral CA1), vSub (ventral subiculum), PVH (paraventricular hypothalamus), VMHvl (ventrolateral ventromedial hypothalamus), LSv (ventral lateral septum), BMA (basomedial amygdala), SI (substantia innominata), MeA (medial amygdala), BLA (basolateral amygdala), PMV (ventral premamillary nucleus), LHA (lateral hypothalamus), MPOA (medial preoptic area), BNST (bed nucleus of stria terminalis). Created with BioRender.com