Fos, RVLM-projecting neurons, and spinally projecting neurons in the PVN following hypertonic saline infusion

Hypertonic saline (HTS; 1.7 M) infused intravenously into conscious rats increases the production of Fos, a marker of cell activation, in the hypothalamic paraventricular nucleus (PVN). The parvocellular PVN contains subpopulations of neurons. However, which subpopulations are activated by HTS is unknown. We determined whether PVN neurons that innervate the rostral ventrolateral medulla (RVLM) or the spinal cord (important autonomic sites) expressed Fos following HTS. Experiments were performed 24-96 h after chronic implantation of an intravenous cannula. HTS significantly increased the number of Fos-positive cells. In the parvocellular PVN, the maximum number of Fos-positive cells occurred rostral of the anterior-posterior level at which the number of neurons that projected to the medulla or spinal cord peaked. Compared with controls, HTS did not significantly increase the number of double-labeled neurons. These findings demonstrate that an elevation in plasma osmolality activates PVN neurons but not the subgroups of PVN neurons with projections to the RVLM or to the spinal cord

nNOS-containing neurons in the hypothalamus and medulla project to the RVLM

Nitric oxide (NO) within the brain is known to have an important influence on sympathetic nerve activity (SNA). NO is found in the paraventricular nucleus (PVN), caudal ventrolateral medulla (CVLM) and the nucleus tractus solitarius (NTS), regions that project to the rostral ventrolateral medulla (RVLM), an area that is critical in the regulation of SNA. The aim of the present study was to determine whether neurons in the PVN, NTS and CVLM that project to the RVLM contain the neuronal isoform of nitric oxide synthase (nNOS) and are, therefore, capable of producing NO. Under pentobarbitone general anaesthesia, the retrogradely-transported tracer, rhodamine-tagged microspheres, were microinjected into the RVLM of rats (n = 6). Two weeks later, the animals were re-anaesthetised, perfused with para-formaldehyde and the brains were removed. Hypothalamic and medullary sections were processed for nNOS immunohistochemistry and the RVLM-projecting neurons were identified using fluorescence microscopy. We found nNOS-containing neurons were present throughout the PVN, CVLM and NTS and that these were intermingled with neurons that projected to the RVLM. Of the neurons in the PVN and CVLM that projected to the RVLM, approximately 12 +/- 1% and 8 +/- 3%, respectively, contained nNOS. In the NTS only 1 +/- 1% of the neurons were double-labeled. This study highlights anatomical pathways emanating from the PVN and CVLM, in particular, which may contribute to the effects on SNA elicited by NO within the brain

Activation of NADPH-Diaphorase-Positive Projections to the Rostral Ventrolateral Medulla Following Cardiac Mechanoreceptor Stimulation in the Conscious Rat

Stimulation of cardiac mechanoreceptors during volume expansion elicits reflex compensatory changes in sympathetic nerve activity (SNA). The hypothalamic paraventricular nucleus (PVN) and nucleus of the tractus solitarius (NTS) are autonomic regions known to contribute to this reflex. Both of these nuclei project to the rostral ventrolateral medulla (RVLM), critical in the tonic generation of SNA. Recent reports from our laboratory show that these pathways 1) are activated following cardiac mechanoreceptor stimulation, and 2) produce nitric oxide, known to influence SNA. The aims of the present study were to determine whether 1) the activated neurons within the PVN and NTS were nitrergic and 2) these neurons projected to the RVLM. Animals were prepared, under general anesthesia, by microinjection of a retrogradely transported tracer into the pressor region of the RVLM and the placement of a balloon at the right venoatrial junction. In conscious rats, the balloon was inflated to stimulate the cardiac mechanoreceptors or was left uninflated. Balloon inflation elicited a significant increase in Fos-positive neurons in the parvocellular PVN (sevenfold) and NTS (fivefold). In the PVN, 51% of nitrergic neurons and 61% of RVLM-projecting nitrergic neurons were activated. In the NTS, these proportions were 8 and 18%, respectively. The data suggest that nitrergic neurons within the PVN and, to a lesser extent, in the NTS, some of which project to the RVLM, may contribute to the central pathways influencing SNA elicited by cardiac mechanoreceptor stimulation

Paraventricular Nucleus Modulates Excitatory Cardiovascular Reflexes during Electroacupuncture

The paraventricular nucleus (PVN) regulates sympathetic outflow and blood pressure. Somatic afferent stimulation activates neurons in the hypothalamic PVN. Parvocellular PVN neurons project to sympathoexcitatory cardiovascular regions of the rostral ventrolateral medulla (rVLM). Electroacupuncture (EA) stimulates the median nerve (P5-P6) to modulate sympathoexcitatory responses. We hypothesized that the PVN and its projections to the rVLM participate in the EA-modulation of sympathoexcitatory cardiovascular responses. Cats were anesthetized and ventilated. Heart rate and mean blood pressure were monitored. Application of bradykinin every 10-min on the gallbladder induced consistent pressor reflex responses. Thirty-min of bilateral EA stimulation at acupoints P5-P6 reduced the pressor responses for at least 60-min. Inhibition of the PVN with naloxone reversed the EA-inhibition. Responses of cardiovascular barosensitive rVLM neurons evoked by splanchnic nerve stimulation were reduced by EA and then restored with opioid receptor blockade in the PVN. EA at P5-P6 decreased splanchnic evoked activity of cardiovascular barosensitive PVN neurons that also project directly to the rVLM. PVN neurons labeled with retrograde tracer from rVLM were co-labeled with μ-opioid receptors and juxtaposed to endorphinergic fibers. Thus, the PVN and its projection to rVLM are important in processing acupuncture modulation of elevated blood pressure responses through a PVN opioid mechanism