Unique responses of midbrain CART neurons in macaques to ovarian steroids

CART (cocaine and amphetamine regulated transcript) is a neuropeptide involved in the control of several physiological processes, such as response to psychostimulants, food intake, depressive diseases and neuroprotection. It is robustly expressed in the brain, mainly in regions that control emotional and stress responses and it is regulated by estrogen in the hypothalamus. There is a distinct population of CART neurons located in the vicinity of the Edinger-Westphal nucleus of the midbrain that also colocalize urocortin-1. The aims of this study were 1) to determine the distribution of CART immunoreactive neurons in the monkey midbrain, 2) to examine the effects of estrogen (E) and progesterone (P) on midbrain CART mRNA and peptide expression and 3) to determine whether midbrain CART neurons contain steroid receptors. Adult female rhesus monkeys (Macaca mulatta) were spayed and either treated with placebo (OVX), estrogen alone (E), progesterone alone (P) or E+P. Animals were prepared (a) for RNA extraction followed by microarray analysis and quantitative (q) RT-PCR (n=3/group); (b) for immunohistochemical analysis of CART and CART+tryptophan hydroxylase (TPH), CART+estrogen receptors (ER) or CART+progesterone receptors (n=5/group) and (c) for Western blots (n=3/group). Both E- and E+P-administration decreased CART gene expression on the microarray and with qRT-PCR. Stereological analysis of CART immunostaining at five levels of the Edinger-Westphal nucleus indicated little effect of E or E+P administration on the area of CART immunostaining. However, P administration increased CART-immunopositive area in comparison to the OVX control group with Student's t-test, but not with ANOVA. CART 55-102 detection on Western blot was unchanged by hormone administration. ERbeta and PR were detected in CART neurons and CART fibers appeared to innervate TPH-positive serotonin neurons in the dorsal raphe. In summary, E decreased CART mRNA, but this effect did not translate to the protein level. Moreover, P administration alone had a variable effect on CART mRNA, but it caused an increase in CART immunostaining. Together, the data suggest that CART neurons in the midbrain have a unique steroid response, which may be mediated by nuclear receptors, neuroactive steroids or interneurons.