The responses to vestibular stimulation of brain stem neurons that regulate sympathetic outflow and blood flow have been studied extensively in decerebrate preparations, but not in conscious animals. In the present study, we compared the responses of neurons in the rostral ventrolateral medulla (RVLM), a principal region of the brain stem involved in the regulation of blood pressure, to whole body rotations of conscious and decerebrate cats. In both preparations, RVLM neurons exhibited similar levels of spontaneous activity (median of ∼17 spikes/s). The firing of about half of the RVLM neurons recorded in decerebrate cats was modulated by rotations; these cells were activated by vertical tilts in a variety of directions, with response characteristics suggesting that their labyrinthine inputs originated in otolith organs. The activity of over one-third of RVLM neurons in decerebrate animals was altered by stimulation of baroreceptors; RVLM units with and without baroreceptor signals had similar responses to rotations. In contrast, only 6% of RVLM neurons studied in conscious cats exhibited cardiac-related activity, and the firing of just 1% of the cells was modulated by rotations. These data suggest that the brain stem circuitry mediating vestibulosympathetic reflexes is highly sensitive to changes in body position in space but that the responses to vestibular stimuli of neurons in the pathway are suppressed by higher brain centers in conscious animals. The findings also raise the possibility that autonomic responses to a variety of inputs, including those from the inner ear, could be gated according to behavioral context and attenuated when they are not necessary.

September 30, 2009; doi:10.1152 doi:10. /ajpregu.00551.2009 show that the vestibular system contributes to blood pressure regulation. Prior studies reported that lesions that eliminate inputs from the inner ears attenuate the vasoconstriction that ordinarily occurs in the hindlimbs of conscious cats during head-up rotations. These data led to the hypothesis that labyrinthine-deficient animals would experience considerable lower body blood pooling during head-up postural alterations. The present study tested this hypothesis by comparing blood flow though the femoral artery and vein of conscious cats during 20 -60°head-up tilts from the prone position before and after removal of vestibular inputs. In vestibular-intact animals, venous return from the hindlimb dropped considerably at the onset of head-up tilts and, at 5 s after the initiation of 60°rotations, was 66% lower than when the animals were prone. However, after the animals were maintained in the head-up position for another 15 s, venous return was just 33% lower than before the tilt commenced. At the same time point, arterial inflow to the limb had decreased 32% from baseline, such that the decrease in blood flow out of the limb due to the force of gravity was precisely matched by a reduction in blood reaching the limb. After vestibular lesions, the decline in femoral artery blood flow that ordinarily occurs during head-up tilts was attenuated, such that more blood flowed into the leg. Contrary to expectations, in most animals, venous return was facilitated, such that no more blood accumulated in the hindlimb than when labyrinthine signals were present. These data show that peripheral blood pooling is unlikely to account for the fluctuations in blood pressure that can occur during postural changes of animals lacking inputs from the inner ear. Instead, alterations in total peripheral resistance following vestibular dysfunction could affect the regulation of blood pressure.blood flow patterning; venous return; cardiac output; orthostatic hypotension HEAD-UP BODY ROTATIONS in humans or animals typically result in some pooling of blood in the periphery and a resulting reduction in return of blood to the heart. Because cardiac output is directly related to cardiac preload (Starling's law of the heart) (27), cardiac output tends to decrease during head-up movements (25). Furthermore, cardiac output and peripheral vascular resistance determine systemic blood pressure, such that the sympathetic nervous system must produce a rapid net increase in peripheral resistance by inducing vasoconstriction at the onset of head-up body rotations to maintain stable blood pressure (7). Arterial baroreceptor mechanisms play an important role in regulating peripheral vasoconstriction during postural alterations (23). In addition, there is considerable evidence from studies in animals (6,8,10,11,21,22,32) and humans (1,4,12,23,26,28,30) that the vestibular system also participates in triggering increases in vasomotor activity during movements that promote peripheral blood pooling...

-Although it is well established that bulbospinal neurons located in the rostral ventrolateral medulla (RVLM) play a pivotal role in regulating sympathetic nerve activity and blood pressure, virtually all neurophysiological studies of this region have been conducted in anesthetized or decerebrate animals. In the present study, we used time-and frequency-domain analyses to characterize the naturally occurring discharges of RVLM neurons in conscious cats. Specifically, we compared their activity to fluctuations in carotid artery blood flow to identify neurons with cardiac-related (CR) activity; we then considered whether neurons with CR activity also had a higher-frequency rhythmic firing pattern. In addition, we ascertained whether the surgical removal of vestibular inputs altered the rhythmic discharge properties of RVLM neurons. Less than 10% of RVLM neurons expressed CR activity, although the likelihood of observing a neuron with CR activity in the RVLM varied between recording sessions, even when tracking occurred in a very limited area and was higher after vestibular inputs were surgically removed. Either a 10-Hz or a 20-to 30-Hz rhythmic discharge pattern coexisted with the CR discharges in some of the RVLM neurons. Additionally, the firing rate of RVLM neurons, including those with CR activity, decreased after vestibular lesions. These findings raise the prospect that RVLM neurons may or may not express rhythmic firing patterns at a particular time due to a variety of influences, including descending projections from higher brain centers and sensory inputs, such as those from the vestibular system. vestibular system; sympathetic nervous system; baroreceptors; 10-Hz rhythm; cardiac-related activity THERE IS A CONSENSUS THAT bulbospinal neurons located in the rostral ventrolateral medulla (RVLM) play a pivotal role in regulating the activity of sympathetic preganglionic neurons in the spinal cord that control peripheral vascular resistance and blood pressure (13,15,16). Neurophysiological studies conducted in decerebrate or anesthetized animals have characterized the firing patterns and responses to a variety of stimuli of RVLM neurons (e.g., 3, 5-7, 18, 23). A hallmark of sympathetic nerve activity (SNA) and of the activity in brain stem neurons that regulate cardiovascular function is the appearance of a cardiac-related (CR) rhythm, reflecting the influence of the baroreceptor reflex (14,20,23,25,27). In addition to the CR rhythm, several groups have noted the appearance of a 10-Hz rhythm in SNA of cats (4, 9, 10, 14, 24).We recently provided the first examples of recordings of RVLM neuronal activity in conscious animals (18). In that study, we compared in decerebrate and awake cats the responses of RVLM neurons to whole-body rotations that activate vestibular receptors. We demonstrated that RVLM neuronal responses to vestibular inputs are ordinarily suppressed in conscious animals and hypothesized that autonomic responses to a variety of inputs, including those from the inner ear, are gated by higher brai...

Vestibular influences on outflow from the spinal cord are largely mediated via spinal interneurons, although few studies have recorded interneuronal activity during labyrinthine stimulation. The present study determined the responses of upper thoracic interneurons of decerebrate cats to electrical stimulation of the vestibular nerve or natural stimulation of otolith organs and the anterior and posterior semicircular canals using rotations in vertical planes. A majority of thoracic interneurons (74/102) responded to vestibular nerve stimulation at median latencies of 6.5 msec (minimum of ~3 msec), suggesting that labyrinthine inputs were relayed to these neurons through trisynaptic and longer pathways. Thoracic interneuronal responses to vertical rotations were similar to those of graviceptors such as otolith organs, and a wide array of tilt directions preferentially activated different cells. Such responses were distinct from those of cells in the cervical and lumbar enlargements, which are mainly elicited by ear-down tilts and are synchronous with stimulus position when low rotational frequencies are delivered, but tend to be in phase with stimulus velocity when high frequencies are employed. The dynamic properties of thoracic interneuronal responses to tilts were instead similar to those of thoracic motoneurons and sympathetic preganglionic neurons. However, the preferred tilt directions of the interneurons were more heterogeneous than thoracic spinal outputs, showing that the outputs do not simply reflect an addition of local interneuronal activity.

The objective of this study was to assess the water collection, treatment, and contamination in Upper and Lower Belén neighborhoods of Iquitos, Peru to address the longstanding prevalence of waterborne disease. Fifty household surveys were performed and 45 water samples were analyzed for fecal coliforms (FC). Most drinking water was untreated at home and negative for FC. Of the FC-positive waters (11.1%), the source was Sedaloreto tap water, the municipally owned water company and treatment facility. No FC-positive samples resulted from residences reporting home treatment. This suggests sufficient sterilization at the point of treatment but contamination within the distribution system or individual homes. Therefore, it is critical to emphasize the importance of home-based treatment until the distribution system can be reliable. Although the prevalence of FC-positive samples were similar in Upper and Lower Belén, residents in Lower Belén remain at increased risk for waterborne disease due to seasonal environmental conditions such as flooding and the lack of home sanitation facilities.