Changes in NADPH diaphorase activity in forebrain structures of the laminae terminalis after chronic dehydration

Cerebral Vascular Control and Metabolism in Heat Stress

This review provides an in-depth update on the impact of heat stress on cerebrovascular functioning. The regulation of cerebral temperature, blood flow, and metabolism are discussed. We further provide an overview of vascular permeability, the neurocognitive changes, and the key clinical implications and pathologies known to confound cerebral functioning during hyperthermia. A reduction in cerebral blood flow (CBF), derived primarily from a respiratory-induced alkalosis, underscores the cerebrovascular changes to hyperthermia. Arterial pressures may also become compromised because of reduced peripheral resistance secondary to skin vasodilatation. Therefore, when hyperthermia is combined with conditions that increase cardiovascular strain, for example, orthostasis or dehydration, the inability to preserve cerebral perfusion pressure further reduces CBF. A reduced cerebral perfusion pressure is in turn the primary mechanism for impaired tolerance to orthostatic challenges. Any reduction in CBF attenuates the brain's convective heat loss, while the hyperthermic-induced increase in metabolic rate increases the cerebral heat gain. This paradoxical uncoupling of CBF to metabolism increases brain temperature, and potentiates a condition whereby cerebral oxygenation may be compromised. With levels of experimentally viable passive hyperthermia (up to 39.5-40.0 °C core temperature), the associated reduction in CBF (∼ 30%) and increase in cerebral metabolic demand (∼ 10%) is likely compensated by increases in cerebral oxygen extraction. However, severe increases in whole-body and brain temperature may increase blood-brain barrier permeability, potentially leading to cerebral vasogenic edema. The cerebrovascular challenges associated with hyperthermia are of paramount importance for populations with compromised thermoregulatory control--for example, spinal cord injury, elderly, and those with preexisting cardiovascular diseases.

Effect of hyperosmotic stress on the gene expression and activity of neuronal nitric oxide synthase (nNOS) in the preoptic-hypothalamic neurosecretory system of the euryhaline fish Oreochromis mossambicus

We examined the effects of hyperosmotic stress on the gene expression and activity of neuronal nitric oxide synthase (nNOS) in the preoptic/hypothalamic neurosecretory system of the euryhaline tilapia Oreochromis mossambicus (Mozambique tilapia) by means of semiquantitative RT-PCR and NADPHd histochemistry. Expression of nos1 was rapidly and transiently up-regulated in the preoptic region and hypothalamus in response to a salinity change (70% seawater, SW). Expression levels increased 4 h after the salinity change and then returned to basal levels within 8 h of the hyperosmotic challenge. NADPHd histochemistry revealed that positive magnocellular and gigantocellular preoptic neurons increased in number 4 h after the salinity change, while the number of parvocellular preoptic neurons reactive for NADPHd showed no significant change. These results indicate that the nNOS gene expression and NOS activity are stimulated in the preoptic/ hypothalamic neurosecretory system in response to hyperosmotic stress and suggest that NO influences neuronal responses to short-term osmotic stimulation in euryhaline fish.

Collateral axonal projections from rostral ventromedial medullary nitric oxide synthase containing neurons to brainstem autonomic sites

The magnocellular reticular nucleus and adjacent lateral paragigantocellular nucleus have been shown to contain a large population of nitric oxide synthase (NOS) immunoreactive neurons. However, little is known about the projections of these neurons within the central nervous system. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether NOS neurons in this rostral ventromedial medullary (RVMM) region send collateral axonal projections to autonomic sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb). Fluorogold and/or rhodamine labeled latex microspheres were microinjected into the NTS and Amb at sites that elicited bardycardia and/or depressor responses (l-glutamate; 0.25 M; 10 nl). After a survival period of 10-14 days, the rats were sacrificed and tissue sections of the brainstem were processed immunohistochemically for the identification of NOS containing neuronal perikarya. After unilateral injection of the tract-tracers into the NTS and Amb, retrogradely labeled neurons were observed bilaterally throughout the RVMM region. Of the number of RVMM neurons retrogradely labeled from the NTS (684+/-143), 9% were found to be immunoreactive to NOS. Similarly, of those RVMM neurons retrogradely labeled from the Amb (963+/-207), 7% also contained NOS immunoreactivity. Neurons with collateral axonal projections to NTS and Amb (14% and 10%, respectively) were observed predominantly within a region of RVMM that extended co-extensively with approximately the rostrocaudal extent of the facial nucleus. Of these double labeled neurons, 36.4+/-20 (39%) were also found to be immunoreactive to NOS. These data indicate that the RVMM contains at least three population of NOS neurons that send axons to innervate functionally similar cardiovascular responsive sites in the NTS and Amb. Although the function of these NOS containing medullary pathways in cardiovascular control is not known, it is likely that those with collateral axonal projections represent the anatomical substrate by which the RVMM may simultaneously coordinate cardiovascular responses during physiological changes associated with respiration and/or motor movements.

Impaired cognitive function and mental performance in mild dehydration

Dehydration is a reliable predictor of impaired cognitive status. Objective data, using tests of cortical function, support the deterioration of mental performance in mildly dehydrated younger adults. Dehydration frequently results in delirium as a manifestation of cognitive dysfunction. Although, the occurrence of delirium suggests transient acute global cerebral dysfunction, cognitive impairment may not be completely reversible. Animal studies have identified neuronal mitochondrial damage and glutamate hypertransmission in dehydrated rats. Additional studies have identified an increase in cerebral nicotinamide adenine dinucleotide phosphate-diaphorase activity (nitric oxide synthase, NOS) with dehydration. Available evidence also implicates NOS as a neurotransmitter in long-term potentiation, rendering this a critical enzyme in facilitating learning and memory. With ageing, a reduction of NOS activity has been identified in the cortex and striatum of rats. The reduction of NOs synthase activity that occurs with ageing may blunt the rise that occurs with dehydration, and possibly interfere with memory processing and cognitive function. Dehydration has been shown to be a reliable predictor of increasing frailty, deteriorating mental performance and poor quality of life. Intervention models directed toward improving outcomes in dehydration must incorporate strategies to enhance prompt recognition of cognitive dysfunction.

Modifications to central neural circuitry during heart failure

AIM

During heart failure (HF), excess sodium retention is triggered by increased plasma renin-angiotensin-aldosterone activity and increased basal sympathetic nerve discharge (SND). Enhanced basal SND in the renal nerves plays a role in sodium retention. Therefore, as a hypothetical model for the central sympathetic control pathways that are dysregulated as a consequence of HF, the central neural pathways regulating the sympathetic motor output to the kidney are reviewed in the context of their role during HF.

CONCLUSION

From these findings, a model of the neuroanatomical circuitry that may be affected during HF is constructed.

Central action of nitric oxide in the saltwater-acclimated duck: modulation of extrarenal sodium excretion and vasotocin release

Hypothalamic nuclei close to the third ventricle (VIII) represent key structures in avian osmoregulation concerned with the control of salt gland activity and release of the antidiuretic hormone [Arg8]vasotocin (AVT). Nitric oxide (NO) acting as a paracrine transmitter in the hypothalamus has been shown to contribute to the maintenance of salt and fluid balance in mammals. The saltwater-acclimated duck was used in the present study as a well-characterized osmoregulatory model to investigate the role of central NO in hypothalamic perception or integration of osmoregulatory signals in marine birds. During osmotically induced steady-state salt gland secretion, the VIII of conscious ducks was microperfused with artificial cerebrospinal fluid (aCSF) alone, aCSF containing the NO-donor SNAP or the peptide [Val5]angiotensin II (ANGII) and alterations in salt gland activity, arterial pressure and the release of AVT were continuously monitored. No changes occurred during intracerebroventricular microperfusion with aCSF. Central application of ANGII, a known inhibitory hypothalamic transmitter in the control of salt gland function, markedly blocked salt gland osmolal excretion. Central stimulation with the NO-donor SNAP significantly reduced osmolal excretion from 0.41+/-0.02 to 0. 22+/-0.04 mosmol/min. Both ANGII and SNAP caused a rise in plasma AVT at either slightly elevated (ANGII) or constant (SNAP) arterial pressure. Employing NADPH-diaphorase histochemistry in the duck hypothalamus to localize sites of NO synthesis, periventricular neurons, nerve fibers in close association to the VIII and also parvocellular neurons of the paraventricular nucleus could be labeled. These data suggest a modulatory role for hypothalamic NO within the central osmoregulatory circuitry controlling salt gland function and AVT release in marine birds.