Ghrelin modulates the fMRI BOLD response of homeostatic and hedonic brain centers regulating energy balance in the rat.

The orexigenic gut-brain peptide, ghrelin and its G-protein coupled receptor, the growth hormone secretagogue receptor 1a (GHS-R1A) are pivotal regulators of hypothalamic feeding centers and reward processing neuronal circuits of the brain. These systems operate in a cooperative manner and receive a wide array of neuronal hormone/transmitter messages and metabolic signals. Functional magnetic resonance imaging was employed in the current study to map BOLD responses to ghrelin in different brain regions with special reference on homeostatic and hedonic regulatory centers of energy balance. Experimental groups involved male, ovariectomized female and ovariectomized estradiol-replaced rats. Putative modulation of ghrelin signaling by endocannabinoids was also studied. Ghrelin-evoked effects were calculated as mean of the BOLD responses 30 minutes after administration. In the male rat, ghrelin evoked a slowly decreasing BOLD response in all studied regions of interest (ROI) within the limbic system. This effect was antagonized by pretreatment with GHS-R1A antagonist JMV2959. The comparison of ghrelin effects in the presence or absence of JMV2959 in individual ROIs revealed significant changes in the prefrontal cortex, nucleus accumbens of the telencephalon, and also within hypothalamic centers like the lateral hypothalamus, ventromedial nucleus, paraventricular nucleus and suprachiasmatic nucleus. In the female rat, the ghrelin effects were almost identical to those observed in males. Ovariectomy and chronic estradiol replacement had no effect on the BOLD response. Inhibition of the endocannabinoid signaling by rimonabant significantly attenuated the response of the nucleus accumbens and septum. In summary, ghrelin can modulate hypothalamic and mesolimbic structures controlling energy balance in both sexes. The endocannabinoid signaling system contributes to the manifestation of ghrelin's BOLD effect in a region specific manner. In females, the estradiol milieu does not influence the BOLD response to ghrelin.

Distribution of type 1 cannabinoid receptor (CB1)-immunoreactive axons in the mouse hypothalamus.

Type 1 cannabinoid receptor (CB1) is the principal receptor for endocannabinoids in the brain; it mainly occurs in preterminal/terminal axons and mediates retrograde neuronal signaling mechanisms. A large body of physiological and electrophysiological evidence indicates the critical role of CB1 in the regulation of hypothalamic functions. Conversely, the distribution of CB1-containing axons in the hypothalamus is essentially unknown. Therefore, we have analyzed the distribution and the ultrastructural characteristics of the CB1-immunoreactive (IR) axons in the mouse hypothalamus by using an antiserum against the C-terminal 31 amino acids of the mouse CB1. We found that CB1-IR axons innervated densely the majority of hypothalamic nuclei, except for the suprachiasmatic and lateral mammillary nuclei, in which only scattered CB1-IR fibers occurred. CB1-IR innervation of the arcuate, ventromedial, dorsomedial, and paraventricular nuclei and the external zone of the median eminence corroborated the important role of CB1 in the regulation of energy homeostasis and neuroendocrine functions. Ultrastructural studies to characterize the phenotype of CB1-IR fibers established that most CB1 immunoreactivity appeared in the preterminal and terminal portions of axons. The CB1-IR boutons formed axospinous, axodendritic, and axosomatic synapses. Analysis of labeled synapses in the paraventricular and arcuate nuclei detected approximately equal numbers of symmetric and asymmetric specializations. In conclusion, the study revealed the dense and differential CB1-IR innervation of most hypothalamic nuclei and the median eminence of the mouse brain. At the ultrastructural level, CB1-IR axons established communication with hypothalamic neurons via symmetric and asymmetric synapses indicating the occurrence of retrograde signaling by endocannabinoids in hypothalamic neuronal networks.