PVH and Neuron Projections

It is widely accepted that the hypothalamus has neurons that are responsible for the regulation of arterial pressure. The PVH has known projects to the RVLM, which is a major brain stem region responsible for the regulation of sympathetic nerve activity. Furthermore, the PVH also has neuronal projections to the NTS which is paramount is receiving afferent inputs from the cardiovascular system and relaying vasomotor effects on the PVH and ultimately the RVLM. The NTS also has projections to presympathetic ganglionic neurons within the spinal cord that influence SNA and regulation cardiovascular function. The RVLM and NTS have been shown to have separate hypothalamic inputs however; no attempts have shown hypothalamic inputs to both structures. In this experiment by Badoer, they attempted to identify hypothalamic neurons having projections to functionally identified areas of the RVLM and NTS. Male Sprague Dawley rats were anesthetized and injected with tracers DY (diamidino yellow) and FB (fast blue). The animals were allowed to recover for 3-5 days for transport. Animals were sacrificed and the brains were sliced and studied using fluorescence microscopy. The results showed that numerus labeled neurons were shown to have projections to along the rostral-caudal axis of the hypothalamus. Most interestingly, the neurons in the hypothalamus had project to the RVLM and NTS were overlapping in the hypothalamus. Despite this finding, Badoer concludes that double labeled neurons were rare which would indicate that neurons from the hypothalamus to the RVLM and NTS have direct projections and influence to one or the other, not both. It does seem likely thought that both overlapping neurons that project to the RVLM or NTS may influence each other through synaptic contacts. Badoer also located neurons with the PVH that project to the NTS are found more ventral in the hypothalamus whereas projections to the RVLM were found more throughout the hypothalamus. Overall, this experiment by Badoer is critical in understanding neuronal projections to and from the hypothalamus to keep regions responsible for cardiovascular function.

Brief PVH review

The PVN is anatomically composed of two types of neurons, magnocellular and parvocellular neurons. Both types of neurons are further subdivided into three magnocelluar and five parvocellular neurons. The magnocellular subdivisions are known as anterior, posterior, and medial subnuclei that project to the neurohypophysis and are responsible for the production of posterior pituitary hormones. The parvocellular neurons are subdivided into dorsal, lateral, medial, periventricular, and anterior subnuclei. These regions project to the autonomic nuclei in the brain stem as well as the spinal cord and are responsible for cardiovascular regulation through activation of sympathetic nervous system. The major regulator of the sympathetic nervous system is the RVLM which has numerous projections from the PVN that influence its regulation of arterial pressure.

Information regarding cardiovascular regulation reaches the PVH through a hindlimb brain region known as the NTS. The NTS is the main site of terminating fibers from various cardiovascular receptors such as the baroreceptors, chemoreceptors, and cardiopulmonary receptors. Axons from the caudal portion of the NTS have been found to terminate in the parvocellular and dorsal cap regions of the PVH however, the final target is not known yet.

PVH neurons are continuously active and subject to tonic inhibition arising from GABA and nitric oxide. Administration of NO causes as a decrease in sympathetic nerve activity and it has been found that the majority of the NO is the magnocellular neurons and it is hypothesized that magnocellular neurons may contribute to the autonomic regulation of SNA.  A functional experiment was performed which reported that administration of sodium nitroprusside into the PVH decreased rSNA, AP, and heart rate. Furthermore, administration of NO antagonist blocked the inhibitory effect of the NO on the SNA indicating that NO is inhibitory to sympathetic outflow.

Overall, the paper reviews several ways to examine the PVH and concludes that regulating synaptic activity of the PVH at the level of the parvocellular neurons contributes to sympathetic control and setting basal activity levels. In setting this basal tone, NO, GABA, glutamate and vasopressin are all contributors to tonic activity of the PVH. Therefore, disturbances in these pathways can lead to various cardiovascular disease states.

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.