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

Abstract

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.

Figure 1. Characteristic fMRI BOLD response to ghrelin in the nucleus accumbens.

a. Position of the nucleus accumbens (highlighted in red) is shown in a horizontal scheme of the rat brain . b. Ghrelin evoked a negative BOLD response in the nucleus accumbens. Slice thickness is 1 mm.

Figure 2. Mean of time response curves of ghrelin's effect on BOLD responses in the prefrontal cortex (A), nucleus accumbens (B), ventromedial nucleus (C), lateral hypothalamus (D), paraventricular nucleus (E) and suprachiasmatic nucleus (F).

Blue and red colors indicate pretreatment with saline and GHS-R1A antagonist (JMV2959), respectively. Arrows mark ghrelin (GHR) administration, which started at BOLD response number 333 and lasted to 383 (999–1149 s). For quantifying drug effect, mean of the BOLD responses from 901 to 950 was calculated (2703–2850 s). N = 5–7.

Figure 3. Effect of ghrelin on BOLD responses in reward processing centers, hypothalamic nuclei, brainstem and control areas of the rat brain.

The effect of ghrelin after vehicle pretreatment is represented by blue columns and after JMV2959 pretreatment in red columns. Statistically significant differences were found in the prefrontal cortex (PFC), nucleus accumbens (Acb), lateral hypothalamus (LH), paraventricular nucleus (PVN), ventromedial nucleus (VMH) and suprachiasmatic nucleus (SCN). ANOVA and Fisher post hoc test *: p<0.05; **: p<0.01. N = 5–7.

Figure 4. Mean of time response curves of ghrelin's effect on the BOLD responses in the prefrontal cortex (A), nucleus accumbens (B), ventromedial nucleus (C) and paraventricular nucleus (D), in males (blue), ovariectomized females (orange) and ovariectomized females treated with E2 (green).

Arrows mark ghrelin administration, which started at BOLD response 333 and lasted to 383 (999–1149 s). For quantifying drug effect mean of the BOLD responses from 901 to 950 was calculated (2703–2850 s). N = 11–14.

Figure 5. Mean of time response curves of ghrelin's effect on BOLD responses in the nucleus accumbens (A) and septum (B) after vehiculum (blue) or rimonabant pretreatment (red).

Arrows mark ghrelin administration, which started at BOLD response 333 and lasted to 383 (999–1149 s). For quantifying drug effect, mean of the BOLD responses from 901 to 950 was calculated (2703–2850 s). N = 5–7.

Figure 6. Effect of ghrelin on BOLD responses in reward processing centers, hypothalamic nuclei and brainstem areas.

Effect of ghrelin on the BOLD responses was represented by the blue columns after vehicle pretreatment, and red columns after rimonabant pretreatment. Statistically significant differences were found in the nucleus accumbens and septum. ANOVA and Fisher post hoc test. *: p<0.05. N = 5–7.