A subset of neurons in the paraventricular nucleus of the hypothalamus directly project to liver-related premotor neurons in the ventrolateral medulla.

Sympathetic circuits including pre-sympathetic neurons in the ventrolateral medulla (VLM) and in the paraventricular nucleus (PVN) of the hypothalamus play an important role in the regulation of hepatic glucose metabolism. Despite the importance of central regulatory pathways, specific information regarding the circuits of liver-related neurons is limited. Here, we tested the hypothesis that PVN neurons are directly connected to spinally-projecting liver-related neurons in the VLM of mice. Pseudorabies virus (PRV) was used to identify liver-related neurons and time-dependent analyses revealed the location and distribution of neurons in the PVN and ventral brainstem. Four days following PRV injection, most liver-related neurons were found in the VLM and consist of both catecholaminergic (CA) and non-CA neurons. Furthermore, in addition to PRV inoculation, a monosynaptic viral tracer was used to identify VLM-projecting PVN neurons to specifically dissect PVN-VLM connections within the liver pathway. Five days following PRV inoculation, our anatomical findings revealed that a small population of liver-related PVN neurons projected to the VLM. In addition, photo-stimulation of axonal projections from SIM1-expressing PVN neurons resulted in evoked excitatory postsynaptic currents in a subset of spinally projecting liver-related neurons in the VLM. In summary, our data demonstrate the existence of monosynaptic, glutamatergic connections between PVN neurons and pre-sympathetic liver-related neurons in the VLM. These new findings regarding the central circuits involved in the sympathetic regulation of the liver provide further information necessary for developing new strategies to improve glucose homeostasis via modulation of the autonomic nerves.