Mark Fry

The sensory circumventricular organs: brain targets for circulating signals controlling ingestive behavior.

Sensory circumventricular organs (CVOs) are specialized areas of the brain that lack a normal blood-brain barrier, and therefore are in constant contact with signaling molecules circulating in the bloodstream. Neurons of the CVOs are well endowed with a wide spectrum of receptors for hormones and other signaling molecules, and they have strong connections to hypothalamic and brainstem nuclei. Therefore, lying at the blood-brain interface, the sensory CVOs are in a unique position of being able to detect and integrate humoral and neural information and relay the resulting signals to autonomic control centers of the hypothalamus and medulla. This review focuses primarily on the roles played by the sensory CVOs in fluid balance and energy metabolism.

Area postrema neurons are modulated by the adipocyte hormone adiponectin.

Adiponectin is an adipocyte-derived peptide hormone involved in energy homeostasis and the pathogenesis of obesity, including hypertension. Area postrema (AP) lacks a blood–brain barrier and is a critical homeostatic integration center for humoral and neural signals. Here we investigate the role of AP in adiponectin signaling. We show that rat AP expresses AdipoR1 and AdipoR2 adiponectin receptor mRNA. We used current-clamp electrophysiology to investigate whether adiponectin influenced membrane properties of AP neurons and found that ∼60% of rat AP neurons tested were sensitive to adiponectin. Additional electrophysiology experiments coupled with single-cell reverse transcription-PCR indicated that all neurons that expressed both subtypes of receptor were sensitive to adiponectin, whereas neurons expressing only one subtype were predominantly insensitive. Last, microinjection of adiponectin into AP caused significant increases in arterial blood pressure, with no change in heart rate, suggesting that adiponectin acts at AP to provide a possible link between control of energy homeostasis and cardiovascular function.

Making Sense of It: Roles of the Sensory Circumventricular Organs in Feeding and Regulation of Energy Homeostasis

Obesity is associated with significant health risks including stroke and heart disease. The prevalence of obesity has dramatically increased over the past 20 years. Although the development of obesity is clearly related to changing lifestyles, the central nervous system plays a key role in regulation of energy balance. To develop effective strategies for treating obesity, we must gain a clearer understanding of the neuro-circuitry and signaling mechanisms involved. Toward this end, recent progress has been made in the understanding of the roles played by the sensory circumventricular organs (CVOs) of the brain. These areas lack the normal blood-brain barrier and thus act as transducers of signals between the blood, other centers in the brain, and the cerebrospinal fluid. This review focuses on the roles played by the sensory CVOs in detecting and responding to a number of signals that carry information regarding nutritional status, including cholecystokinin, amylin, ghrelin, peptide YY, pancreatic polypeptide, leptin, adiponectin, and glucose.

Adiponectin selectively inhibits oxytocin neurons of the paraventricular nucleus of the hypothalamus

Adiponectin is an adipocyte derived hormone which acts in the brain to modulate energy homeostasis and autonomic function. The paraventricular nucleus of the hypothalamus (PVN) which plays a key role in controlling pituitary hormone secretion has been suggested to be a central target for adiponectin actions. A number of hormones produced by PVN neurons have been implicated in the regulation of energy homeostasis including oxytocin, corticotropin releasing hormone and thyrotropin releasing hormone. In the present study we investigated the role of adiponectin in controlling the excitability of magnocellular (MNC – oxytocin or vasopressin secreting) neurons within the PVN. Using RT‐PCR techniques we have shown expression of both adiponectin receptors in the PVN. Patch clamp recordings from MNC neurons in hypothalamic slices have also identified mixed (27% hyperpolarization, 42% depolarization) effects of adiponectin in modulating the excitability of the majority of MNC neurons tested. These effects are maintained when cells are placed in synaptic isolation using tetrodotoxin. Additionally we combined electrophysiological recordings with single cell RT‐PCR to examine the actions of adiponectin on MNC neurons which expressed oxytocin only, vasopressin only, or both oxytocin and vasopressin mRNA and assess the profile of receptor expression in these subgroups. Adiponectin was found to hyperpolarize 100% of oxytocin neurons tested (n= 6), while vasopressin cells, while all affected (n= 6), showed mixed responses. Further analysis indicates oxytocin neurons express both receptors (6/7) while vasopressin neurons express either both receptors (3/8) or one receptor (5/8). In contrast 6/6 oxytocin/vasopressin neurons were unaffected by adiponectin. Co‐expressing oxytocin and vasopressin neurons express neither receptor (4/6). The results presented in this study suggest that adiponectin plays specific roles in controlling the excitability oxytocin secreting neurons, actions which correlate with the current literature showing increased oxytocin secretion in the obese population.

Microarray Analysis of the Transcriptome of the Subfornical Organ in the Rat: Regulation by Fluid and Food Deprivation

We have employed microarray technology using Affymetrix 230 2.0 genome chips to initially catalog the transcriptome of the subfornical organ (SFO) under control conditions and to also evaluate the changes (common and differential) in gene expression induced by the challenges of fluid and food deprivation. We have identified a total of 17,293 genes tagged as present in one of our three experimental conditions, transcripts, which were then used as the basis for further filtering and statistical analysis. In total, the expression of 46 genes was changed in the SFO following dehydration compared with control animals (22 upregulated and 24 downregulated), with the largest change being the greater than fivefold increase in brain-derived neurotrophic factor (BDNF) expression, while significant changes in the expression of the calcium-sensing (upregulated) and apelin (downregulated) receptors were also reported. In contrast, food deprivation caused greater than twofold changes in a total of 687 transcripts (222 upregulated and 465 downregulated), including significant reductions in vasopressin, oxytocin, promelanin concentrating hormone, cocaine amphetamine-related transcript (CART), and the endothelin type B receptor, as well as increases in the expression of the GABA(B) receptor. Of these regulated transcripts, we identified 37 that are commonly regulated by fasting and dehydration, nine that were uniquely regulated by dehydration, and 650 that are uniquely regulated by fasting. We also found five transcripts that were differentially regulated by fasting and dehydration including BDNF and CART. In these studies we have for the first time described the transcriptome of the rat SFO and have in addition identified genes, the expression of which is significantly modified by either water or food deprivation.

Ghrelin modulates electrical activity of area postrema neurons

Ghrelin, a peptide hormone secreted from the stomach, is known to have a potent appetite-stimulating activity. Recently, it has been shown that area postrema (AP), a caudal brain stem center that lacks a blood-brain barrier, is a key site of activity for ghrelin in stimulating appetite and regulating pancreatic protein secretion. In this study, we have examined the ability of ghrelin to regulate the electrical activity of area postrema neurons using patch-clamp electrophysiology. Using current-clamp configuration, we found that at a concentration of 10 nM, ghrelin caused inhibition in 19% of neurons tested, while a further 19% were excited by similar application of ghrelin. The remaining 62% of AP neurons were insensitive to ghrelin. These effects were concentration dependent, with an apparent EC(50) of 1.9 nM. Voltage-clamp recordings revealed that ghrelin caused a potentiation of voltage-gated K(+) currents in neurons that exhibited a hyperpolarization and a potentiation of a depolarizing nonspecific cation current (NSCC) in those neurons that exhibited a depolarization of membrane potential. These are the first data showing that ghrelin exerts a direct effect on electrical activity of AP neurons and supports the notion that ghrelin can act via the AP to regulate energy homeostasis.

Ghrelin: Central Nervous System Sites of Action in Regulation of Energy Balance

Ghrelin, a peptide hormone secreted by the stomach, has been shown to regulate energy homeostasis by modulating electrical activity of neurons in the central nervous system (CNS). Like many circulating satiety signals, ghrelin is a peptide hormone and is unable to cross the blood-brain barrier without a transport mechanism. In this review, we address the notion that the arcuate nucleus of the hypothalamus is the only site in the CNS that detects circulating ghrelin to trigger orexigenic responses. We consider the roles of a specialized group of CNS structures called the sensory circumventricular organs (CVOs), which are not protected by the blood-brain barrier. These areas include the subfornical organ and the area postrema and are already well known to be key areas for detection of other circulating hormones such as angiotensin II, cholecystokinin, and amylin. A growing body of evidence indicates a key role for the sensory CVOs in the regulation of energy homeostasis.

Developmental expression of Na+ currents in mouse Purkinje neurons

As Purkinje neurons mature during postnatal development, they change from electrically quiescent to active and exhibit high frequency spontaneous action potentials. This change in electrical activity is determined by both alteration in ion channel expression and the acquisition of synaptic input. To gain a better understanding of the development the intrinsic electrical properties of these neurons, acutely isolated Purkinje neurons from mice aged postnatal day 4 (P4) to P18 were examined. This included recording action potential frequency, threshold, height and slope, and input resistance and capacitance. Changes in a number of these properties were observed, suggesting significant changes in voltage‐gated Na+ currents. Because voltage‐gated Na+ currents, including the transient, resurgent and persistent currents, are known to play important roles in generating spontaneous action potentials, the developmental changes in these currents were examined. A large increase in the density of transient current, resurgent current and persistent current was observed at times corresponding with changes in action potential properties. Interestingly, the developmental up‐regulation of the persistent current and resurgent current occurred at rate which was faster than the up‐regulation of the transient current. Moreover, the relative amplitudes of the persistent and resurgent currents increased in parallel, suggesting that they share a common basis. The data indicate that developmental up‐regulation of Na+ currents plays a key role in the acquisition of Purkinje neuron excitability.

Switching control of sympathetic activity from forebrain to hindbrain in chronic dehydration

Non‐technical summary  Dehydration, a life‐threatening condition, occurs when the body does not replace adequate water lost through urination, sweating or when ill with diarrhoea. This presents the body with a major challenge of maintaining blood pressure – essential for consciousness that is dependent on the degree of body hydration, which dictates blood volume. We know that a major control mechanism involves a brain region called the hypothalamus that automatically maintains blood pressure. Our study has described the gene networks in key brain regions involved in the response to dehydration. We reveal a new structure in the brain that regulates blood pressure in dehydration and a unique genetic mechanism that exists within it. Moreover, our study unearths a remarkable form of flexibility within the brain during dehydration that involves switching control of blood pressure between two spatially distinct structures. We have provided new mechanistic insight to explain how the brain maintains body stability in face of the significant challenge of low water content.

The transcriptome of the medullary area postrema: the thirsty rat, the hungry rat and the hypertensive rat

The area postrema (AP) is a sensory circumventricular organ characterized by extensive fenestrated vasculature and neurons which are capable of detecting circulating signals of osmotic, cardiovascular, immune and metabolic status. The AP can communicate these messages via efferent projections to brainstem and hypothalamic structures that are able to orchestrate an appropriate response. We have used microarrays to profile the transcriptome of the AP in the Sprague–Dawley (SD) and Wistar–Kyoto rat and present here a comprehensive catalogue of gene expression, focusing specifically on the population of ion channels, receptors and G protein‐coupled receptors expressed in this sensory tissue; of the G protein‐coupled receptors expressed in the rat AP, we identified ∼36% that are orphans, having no established ligand. We have also looked at the ways in which the AP transcriptome responds to the physiological stressors of 72 h dehydration (DSD) and 48 h fasting (FSD) and have performed microarrays in these conditions. Comparison between the DSD and SD or between FSD and SD revealed only a modest number of AP genes that are regulated by these homeostatic challenges. The expression levels of a much larger number of genes are altered in the spontaneously hypertensive rat AP compared with the normotensive Wistar–Kyoto control rat, however. Finally, analysis of these ‘hypertension‐related’ elements revealed genes that are involved in the regulation of both blood pressure and immune function and as such are excellent targets for further study.