MEMRI and Hypothalamic in vivo measurements

The paper submitted to NMR in Biomedicine by Yu-Ting Kuo et al 2006, used MEMRI to detect hypothalamic neuronal activity in mice in fasting and non-fasting states. MEMRI is an in vivo technique that uses Mn²⁺ as a Ca²⁺ surrogate to estimate neuronal activity. Mn²⁺ enters excitable cells it has been proven to be a viable contrast agent for MRI causing shortened T₁ relaxation times. The paper focused on the paraventricular nucleus of the hypothalamus is an important brain region responsible for regulation of sympathetic nervous system, hormone secretion, homeostasis and appetite. The current study examined differences in the neuronal activity between fasting and non-fasting states in mice between 16-24 weeks old. Administration of MnCl₂ occurred through the tail vein following implantation of a cannula. A control group of animals had access to ad libitum (n=4) while non-fasting animals had food removed 12-16 hours prior to scanning. All scans began at 9am with three baseline scans before following by continuous slow infusion of MnCl­₂ and sixty-three scans were performed over the course of 2 hour with an average individual scan time of 1 minutes 57 seconds. The results showed approximately 20% signal intensity increase from baseline scans in the PVH, arcuate hypothalamus (Arc), VMH, AP, and fourth ventricle. Each image was normalized to saline phatoms (SI tissue/saline phantom). Overnight fasting lead to significant increases in enhancement in PVH and VHM compared to non-fasted animals (p=0.04). The studies finding indicate that higher neuronal activity is found it the PVN of fasted animals compared to non-fasted animals. Finally, MEMRI is able to determine differences in enhancement in between feeding states and can further our understanding of the PVH based on its various regulatory functions.