Pubertal growth of the medial amygdala delayed by short photoperiods in the Siberian hamster, Phodopus sungorus

Abstract

We investigated whether puberty influences the morphology of the medial nucleus of the amygdala (MeA) by comparing Siberian hamsters (Phodopus sungorus) that had been raised from birth in either long day (LD; 16:8 h light:dark) or short day (SD; 8:16) photoperiods. Hamsters were sacrificed at 42-49 days of age, at which point all LD hamsters were reproductively mature, as evidenced by adult-like testes weights (mean: 657 mg). In contrast, the testes weights of the SD hamsters were low (mean: 31 mg), indicating that the SD photoperiod had delayed puberty. The regional volume and mean soma size of the four MeA subnuclei was estimated bilaterally by stereological procedures. In the posterior dorsal and ventral MeA subnuclei, regional volume was 22-25% larger, and mean soma size 18% larger, in LD males than SD males. Unbiased cell counts in the posterior dorsal MeA showed that LD and SD hamsters have equivalent neuron numbers. In the anterior MeA subnuclei, regional volumes and soma sizes from LD and SD hamsters were equivalent. Additionally, the regional volume of the posteroventral subnucleus was larger in the right hemisphere than the left, but this laterality did not respond to photoperiod manipulation. These results suggest that the extant neurons within the posterior MeA, a steroid-sensitive nucleus implicated in socio-sexual behavior, grow in response to the elevated levels of circulating androgen accompanying puberty, and that photoperiodic regulation of puberty affects morphological maturation of this nucleus.

Figure 1

Photomicrographs of the MeA in P. sungorus showing the typical appearance and boundaries of the subnuclei. A–B. The rostral MeA contains the MeAad. C. A section containing the MeApd, the MeApv, and the MeAav. D. Caudal portion containing the MeApd and MeApv. Bar = 0.5 mm

Figure 2

Mean regional volume of the four MeA subnuclei in the left and right hemispheres. In both hemispheres, rearing in long day (LD) photoperiods associated with gonadal maturation resulted in greater regional volumes only in the posterior nuclei (MeApd, C, and MeApv, D) compared to rearing in short days (SD; both ps < 0.01). Photoperiod had no effect on the anterior subnuclei. The right MeApv was significantly larger than the left in both groups of males (p < 0.01). E. MeApd from a SD hamster representing the mean regional volume for that group. F. MeApd from an LD hamster representing the mean regional volume for that group. Bar = 0.5 mm

Figure 3

A. Mean bilateral neuronal soma sizes in the four MeA subnuclei. Male hamsters reared in LD had larger neuronal somata in the MeApd and MeApv subnuclei than males raised in SD (both ps ≤ 0.01). The anterior subnuclei were not affected by photoperiod. B. Photomicrograph of MeApd somata from an SD hamster. C. Photomicrograph of MeApd somata from an LD hamster. Bar = 5 μm.