# Comparative physiology and morphology of BLA-projecting NBM/SI cholinergic neurons in mouse and macaque

## TL;DR

This study compares the brain cells in mice and macaques that connect to the amygdala, finding differences in how they work and look despite similar functions.

## Contribution

The study reveals distinct physiological and morphological differences between BLA-projecting cholinergic neurons in mice and macaques.

## Key findings

- Macaque BLA-projecting cholinergic neurons are more excitable and less compact than mouse neurons.
- Morpho-electric analysis shows mouse and macaque neurons form two distinct classes despite overlapping on some metrics.
- The findings suggest functional similarities may arise from structurally and physiologically distinct neurons.

## Abstract

Cholinergic projection neurons of the nucleus basalis and substantia innominata (NBM/SI) densely innervate the basolateral amygdala (BLA) and have been shown to contribute to the encoding of fundamental and life-threatening experiences. Given the vital importance of these circuits in the acquisition and retention of memories that are essential for survival in a changing environment, it is not surprising that the basic anatomical organization of the NBM/SI is well conserved across animal classes as diverse as teleost and mammal. What is not known is the extent to which the physiology and morphology of NBM/SI neurons have also been conserved. To address this issue, we made patch-clamp recordings from NBM/SI neurons in ex vivo slices of two widely divergent mammalian species, mouse and rhesus macaque, focusing our efforts on cholinergic neurons that project to the BLA. We then reconstructed most of these recorded neurons post hoc to characterize neuronal morphology. We found that rhesus macaque BLA-projecting cholinergic neurons were both more intrinsically excitable and less morphologically compact than their mouse homologs. Combining measurements of 18 physiological features and 13 morphological features, we illustrate the extent of the separation. Although macaque and mouse neurons both exhibited considerable within-group diversity and overlapped with each other on multiple individual metrics, a combined morpho-electric analysis demonstrates that they form two distinct neuronal classes. Given the shared purpose of the circuits in which these neurons participate, this finding raises questions about (and offers constraints on) how these distinct classes result in similar behavior.