Stereological analysis of the rat and monkey amygdala

Abstract

The amygdala is part of a neural network that contributes to the regulation of emotional behaviors. Rodents, especially rats, are used extensively as model organisms to decipher the functions of specific amygdala nuclei, in particular in relation to fear and emotional learning. Analysis of the role of the nonhuman primate amygdala in these functions has lagged work in the rodent but provides evidence for conservation of basic functions across species. Here we provide quantitative information regarding the morphological characteristics of the main amygdala nuclei in rats and monkeys, including neuron and glial cell numbers, neuronal soma size, and individual nuclei volumes. The volumes of the lateral, basal, and accessory basal nuclei were, respectively, 32, 39, and 39 times larger in monkeys than in rats. In contrast, the central and medial nuclei were only 8 and 4 times larger in monkeys than in rats. The numbers of neurons in the lateral, basal, and accessory basal nuclei were 14, 11, and 16 times greater in monkeys than in rats, whereas the numbers of neurons in the central and medial nuclei were only 2.3 and 1.5 times greater in monkeys than in rats. Neuron density was between 2.4 and 3.7 times lower in monkeys than in rats, whereas glial density was only between 1.1 and 1.7 times lower in monkeys than in rats. We compare our data in rats and monkeys with those previously published in humans and discuss the theoretical and functional implications that derive from our quantitative structural findings.

Postnatal development of the amygdala: A stereological study in rats

“…The procedure for Nissl-stained sections followed our standard laboratory protocol described previously (Chareyron et al, 2011;Lavenex et al, 2009). Briefly, sections were taken from the 10% formaldehyde solution, thoroughly washed, mounted on gelatin-coated slides, and air dried overnight at 37 C. Sections were then defatted for 2 Â 1 hour in a mixture of chloroform/ethanol (1:1, vol), partially rehydrated, and air dried overnight at 37 C. Sections were then fully rehydrated and stained for 40 seconds in a 0.25% thionin solution (Fisher Scientific,Waltham,MA;, dehydrated, and coverslipped with DPX (BDH Laboratories, Poole, United Kingdom).…”

“…We used these descriptions to determine the boundaries of the five main nuclei (i.e., lateral, basal, accessory basal, central, and medial) of the rat amygdala ( Fig. 1; Chareyron et al, 2011).…”

“…On average, 23 sections per animal (160 lm apart) were used to measure the volume of the whole amygdala. About 13 sections per animal (160 lm apart) were used for volume estimates of the main amygdala nuclei (for a complete list of amygdala nuclei see Chareyron et al, 2011).…”

“…L, lateral; B, basal; AB, accessory basal; CE, central; M, medial. Nonlabeled areas include the remaining nuclei of the amygdala (see Chareyron et al, 2011). Scale bar ¼ 500 lm.…”

“…This sampling scheme was the same as that used in our previous study of the adult rat amygdala (Chareyron et al, 2011), except for the medial nucleus. Section thickness was measured at every counting site (Table 1).…”