Characterization of Tyrosine Hydroxylase‐Positive Neurons Projections from the Paraventricular Nucleus of Hypothalamus to Hindbrain Autonomic Regulatory Centers

Abstract

Hypothalamus is a key brain region for cardiovascular, behavioral, and metabolic regulation that contains complex type of excitatory or inhibitory neurons. Tyrosine hydroxylase (TH)-positive neurons in the hypothalamus play an important role in cardiometabolic regulation; however, the anatomical projections of TH neurons in the paraventricular nucleus hypothalamus (PVN) to key parasympathetic and sympathetic premotor nuclei in preautonomic regions of the brain and their connections to the liver remains undefined. This study aims to characterize the projections from THPVN neurons to ventral preautonomic regulating centers and their connection to liver neuronal projections. To achieve our goal, Cre recombinase-dependent TIT2L-eGFP reporter mice were microinjected with adeno-associated virus encoding the Cre recombinase driven by a rat TH promoter (AAV2-TH-Cre, 6.8 x 108 vg/site) bilaterally into the PVN using a stereotaxic apparatus. A retrograde pseudorabies virus with red fluorescence reporter (PRV-614-RFP) was injected into the liver (3 sites, 3ul/site) to identify liver-related neurons in the brainstem preautonotmic centers. One month after injection, mice were transcardially perfused and brain tissues were processed for confocal imaging. The AAV2-TH-Cre induced a robust expression of GFP mostly in the PVN neuronal cell bodies but not in other brain regions. In the brainstem preautonomic centers, we found dense projections in the nucleus tractus solitarius (NTS) and some projections to the dorsal motor nucleus of the vagus (DMV) and rostral ventrolateral medulla (RVLM). Interestingly, there was also dense projects into the median eminence (ME) that regulates neural hormones release. Importantly, the PRV-RFP retrograded from liver was found throughout the DMV and ventrolateral NTS . Particularly in the NTS, there was dense synaptic contact of the THPVN neuronal projections with the liver related neurons (PRV-RFP+ cell). In addition, we also observed that some PRV-RFP+ cells colocalized with TH-positive neurons in the PVN. In sum, the THPVN neurons project anatomically to the autonomic and neurosecretory regions of brain and may modulate the cardiometabolic activity and liver metabolism through these projections.