Stress-induced activation of orexin-A neurons in the lateral hypothalamus is similar in prepubertal and adult male and female rats despite age-dependent differences in hormonal stress reactivity

INTRODUCTION: Pubertal development is associated with changes in hypothalamic-pituitary-adrenal (HPA) axis reactivity, which may contribute to the increase in stress-related vulnerabilities observed during adolescence. In particular, prepubertal rats show significantly protracted stress-induced HPA responses compared to adults. However, the neuroendocrine mechanisms responsible for this developmental change are unclear. In adults, the orexigenic neuropeptide orexin-A has been shown to be a potent modulator of HPA reactivity, and activation of orexin-A neurons aligns with the magnitude of the hormonal stress response. However, it is currently unknown whether pubertal differences in HPA reactivity are associated with changes in orexin-A neurons. METHODS: We examined the hormonal stress response and the number of activated orexin-A neurons in the lateral hypothalamus by co-labeling with c-Fos, a marker of cellular activation, before, during, or after stress exposure in prepubertal (30d) and adult (70d) male and female rats. The number of immunoreactive orexin-A neurons was also quantified in prepubertal (30d), mid-pubertal (45d) and adult (70d) males and females. RESULTS: We found significantly prolonged stress-induced hormonal responses in prepubertal males and females compared to their adult counterparts. However, we found no developmental differences in either the number of orexin-A cells or their stress-induced activation in either sex. CONCLUSIONS: These data suggest minimal association between stress-induced activity of orexin-A neurons and pubertal-related differences in hormonal stress reactivity. However, these data indicate that the number of orexin-A cells is relatively stable throughout adolescent development, and that orexin-A neurons are sensitive to stressors prior to pubertal maturation.