Relaxin-3/RXFP3 signalling in mouse hypothalamus: no effect of RXFP3 activation on corticosterone, despite reduced presynaptic excitatory input onto paraventricular CRH neurons in vitro

Relaxin-3/RXFP3 signalling is proposed to be involved in the neuromodulatory control of arousal- and stress-related neural circuits. Furthermore, previous studies in rats have led to the proposal that relaxin-3/RXFP3 signalling is associated with activation of the hypothalamic-pituitary-adrenal axis, but direct evidence for RXFP3-related actions on the activity of hypothalamic corticotropin-releasing hormone (CRH) neurons is lacking. In this study, we investigated characteristics of the relaxin-3/RXFP3 system in mouse hypothalamus. Administration of an RXFP3 agonist (RXFP3-A2) intra-cerebroventricularly or directly into the paraventricular nucleus of hypothalamus (PVN) of C57BL/6J mice did not alter corticosterone levels. Similarly, there were no differences between serum corticosterone levels in Rxfp3 knockout (C57BL/6J^RXFP3TM1) and wild-type mice at baseline and after stress, despite detection of the predicted stress-induced increases in serum corticosterone. We examined the nature of the relaxin-3 innervation of PVN in wild-type mice and in Crh-IRES-Cre;Ai14 mice that co-express the tdTomato fluorophore in CRH neurons, identifying abundant relaxin-3 fibres in the peri-PVN region, but only sparse fibres associated with densely packed CRH neurons. In whole-cell voltage-clamp recordings of tdTomato-positive CRH neurons in these mice, we observed a reduction in sEPSC frequency following local application of RXFP3-A2, consistent with an activation of RXFP3 on presynaptic glutamatergic afferents in the PVN region. These studies clarify the relationship between relaxin-3/RXFP3 inputs and CRH neurons in mouse PVN, with implications for the interpretation of current and previous in vivo studies and future investigations of this stress-related signalling network in normal and transgenic mice, under normal and pathological conditions.

Embedded synaptic feedback in the neuroendocrine stress axis

Neural regulation of blood glucocorticoid levels is critical for defence of homeostasis during physiological or psychoemotional challenges. In mammals, this function is carried out by the neuroendocrine stress axis, coordinated by parvocellular neuroendocrine cells (PNCs) of the paraventricular hypothalamic nucleus. Feedback regulation of PNCs by glucocorticoids provides complex experience-dependent shaping of neuroendocrine responses. We review recent evidence for metaplastic actions of glucocorticoids as 'circuit breakers' at synapses directly regulating PNC excitability and explore how such mechanisms may serve as substrates for stress adaptation.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Metabotropic glutamate receptors: gatekeepers of homeostasis

The capacity to appropriately respond to physiological challenges or perturbations in homeostasis is a requisite for survival. It is becoming increasingly clear that long-lasting alterations in synaptic efficacy are a fundamental mechanism for modifying neuroendocrine and autonomic output. We review recent advances in our understanding of plasticity at glutamate synapses onto magnocellular neurones (MNCs) in the paraventricular and supraoptic nuclei of the hypothalamus, with a focus on the contributions of metabotropic glutamate receptors (mGluRs) to long-lasting modifications in synaptic efficacy. Special attention is paid to the role of presynaptic mGluRs as gatekeepers for metaplasticity and regulation of body fluid homeostasis. The work highlighted here provides insight into the synaptic mechanisms that couple MNC activity to physiological states.

Isolation of various forms of sterol beta-D-glucoside from the seed of Cycas circinalis: neurotoxicity and implications for ALS-parkinsonism dementia complex

The factors responsible for ALS-parkinsonism dementia complex (ALS-PDC), the unique neurological disorder of Guam, remain unresolved, but identification of causal factors could lead to clues for related neurodegenerative disorders elsewhere. Earlier studies focused on the consumption and toxicity of the seed of Cycas circinalis, a traditional staple of the indigenous diet, but found no convincing evidence for toxin-linked neurodegeneration. We have reassessed the issue in a series of in vitro bioassays designed to isolate non-water soluble compounds from washed cycad flour and have identified three sterol beta-d-glucosides as potential neurotoxins. These compounds give depolarizing field potentials in cortical slices, induce alterations in the activity of specific protein kinases, and cause release of glutamate. They are also highly toxic, leading to release of lactate dehydrogenase (LDH). Theaglycone form, however, is non-toxic. NMDA receptor antagonists block the actions of the sterol glucosides, but do not compete for binding to the NMDA receptor. The most probable mechanism leading to cell death may involve glutamate neuro/excitotoxicity. Mice fed cycad seed flour containing the isolated sterol glucosides show behavioral and neuropathological outcomes, including increased TdT-mediated biotin-dUTP nick-end labelling (TUNEL) positivity in various CNS regions. Astrocytes in culture showed increased caspase-3 labeling after exposure to sterol glucosides. The present results support the hypothesis that cycad consumption may be an important factor in the etiology of ALS-PDC and further suggest that some sterol glucosides may be involved in other neurodegenerative disorders.

Statistical model relating CA3 burst probability to recovery from burst-induced depression at recurrent collateral synapses

When neuronal excitability is increased in area CA3 of the hippocampus in vitro, the pyramidal cells generate periodic bursts of action potentials that are synchronized across the network. We have previously provided evidence that synaptic depression at the excitatory recurrent collateral synapses in the CA3 network terminates each population burst so that the next burst cannot begin until these synapses have recovered. These findings raise the possibility that burst timing can be described in terms of the probability of recovery of this population of synapses. Here we demonstrate that when neuronal excitability is changed in the CA3 network, the mean and variance of the interburst interval change in a manner that is consistent with a timing mechanism comprised of a pool of exponentially relaxing pacemakers. The relaxation time constant of these pacemakers is the same as the time constant describing the recovery from activity-dependent depression of recurrent collateral synapses. Recovery was estimated from the rate of spontaneous transmitter release versus time elapsed since the last CA3 burst. Pharmacological and long-term alterations of synaptic strength and network excitability affected CA3 burst timing as predicted by the cumulative binomial distribution if the burst pace-maker consists of a pool of recovering recurrent synapses. These findings indicate that the recovery of a pool of synapses from burst-induced depression is a sufficient explanation for burst timing in the in vitro CA3 neuronal network. These findings also demonstrate how information regarding the nature of a pacemaker can be derived from the temporal pattern of synchronous network activity. This information could also be extracted from less accessible networks such as those generating interictal epileptiform discharges in vivo.

Slowly inactivating potassium conductance (I(D)): a potential target for stroke therapy

BACKGROUND AND PURPOSE

Excessive accumulation of extracellular glutamate results in the death of most, but not all, neurons in the central nervous system. Understanding the unique properties of cells that can withstand this excitotoxic challenge may identify specific targets for novel stroke therapies.

METHODS

A combination of in vivo methods for analysis of excitotoxic cell death after activation of N-methyl-D-aspartate (NMDA) receptors and in vitro patch-clamp analysis of specific conductances in hypothalamic slices and dissociated cells has been used to assess the roles of specific potassium conductances in delayed cell death after NMDA receptor activation.

RESULTS

We report that a specific D-type potassium conductance (I(D)), necessary for the rapid repolarization of the membrane after a strong depolarization, serves such a protective purpose in magnocellular neurons of the paraventricular nucleus. Manipulations that inhibit this current (4-aminopyridine or angiotensin II) increase neuronal excitability and augment cell death after NMDA receptor activation. In addition, this protection is not observed in magnocellular neurons of spontaneously hypertensive rats, and intriguingly it can be reestablished by blocking angiotensin II receptors in these animals.

CONCLUSIONS

These observations provide a persuasive experimental explanation for the unexpected finding that therapeutic treatments for hypertension that block central as well as peripheral angiotensin type 1 receptors reduce the severity and occurrence of stroke.

Effect of butylated hydroxyanisole on catalase activity and malondialdehyde content in aging Zaprionus paravittiger (diptera)

Catalase activity and malondialdehyde (MDA) content were measured in whole body and mitochondrial homogenates of the banana fruit fly, Zaprionus paravittiger, fed on control and butylated hydroxyanisole (BHA, 10 mM) mixed diets. Catalase activity increased during the reproductive period and decreased thereafter with age. However, the MDA content increased with advancing age in both sexes. In general, females exhibited higher catalase activity and lower MDA content as compared to their male counterparts. BHA feeding increased catalase activity significantly during all age intervals in both sexes. Mitochondrial fractions had lower catalase activity and lower MDA content than whole body homogenates. However, the pattern of changes was similar in both homogenates with age as well as on antioxidant feeding. These results suggest that BHA strengthens the defense mechanism of the insects by increasing catalase activity and reducing MDA content which may be responsible for increased longevity of insects.

Methionine sulfoximine shows excitotoxic actions in rat cortical slices

Methionine sulfoximine (MSO) is a rare amino acid. It occurs in nature or as a by-product of some forms of food processing. A notable example of the latter was a former method for bleaching wheat flour, using nitrogen trichloride, the "agene process," in use for most of the first 50 years of this century. "Agenized" flour was found to be responsible for various neurological disorders in animals, and MSO was identified as the toxic factor. The agene process was subsequently discontinued in the United States and the United Kingdom circa 1950. MSO inhibits the synthesis of both glutathione and glutamine, and it is possible that its actions on the nervous system arise from alterations in the amount or distribution of these molecules. Structurally, MSO resembles glutamate, an observation that has also raised the possibility that it might have more direct glutamate-like actions on neurons. In the present investigation, we report excitatory and toxic actions of MSO in an in vitro preparation of adult rat cortex. Field potential recordings in this preparation show that MSO application evokes a sustained depolarization, which can be blocked by the N-methyl-D-aspartate (NMDA) antagonist L-(+)-2-amino-5-phosphonovalerate (AP5). However, competition assays using MSO on [3H]CGP-39653 (DL-(E)-2-amino-4-propyl-1-phosphono-3-pentenoate) binding in rat cortical homogenates show only 20% displacement of total binding, suggesting that MSO is acting indirectly, perhaps by releasing glutamate. To investigate this possibility, we measured glutamate release during MSO application. Time course and dose-response experiments with MSO showed significant [3H]glutamate release, which was partially attenuated by AP5. To assess cellular toxicity, we measured lactate dehydrogenase (LDH) release from cortical sections exposed to MSO. MSO treatment led to a rapid increase in LDH activity, which could be blocked by AP5. These data suggest that MSO acts by increasing glutamate release, which then activates NMDA receptors, leading to excitotoxic cell death. These data suggest the possibility that MSO in processed flour had excitotoxic actions that may have been contributing factors to some human neuronal disorders.

Methionine sulfoximine shows excitotoxic actions in rat cortical slices

Methionine sulfoximine (MSO) is a rare amino acid. It occurs in nature or as a by-product of some forms of food processing. A notable example of the latter was a former method for bleaching wheat flour, using nitrogen trichloride, the "agene process," in use for most of the first 50 years of this century. "Agenized" flour was found to be responsible for various neurological disorders in animals, and MSO was identified as the toxic factor. The agene process was subsequently discontinued in the United States and the United Kingdom circa 1950. MSO inhibits the synthesis of both glutathione and glutamine, and it is possible that its actions on the nervous system arise from alterations in the amount or distribution of these molecules. Structurally, MSO resembles glutamate, an observation that has also raised the possibility that it might have more direct glutamate-like actions on neurons. In the present investigation, we report excitatory and toxic actions of MSO in an in vitro preparation of adult rat cortex. Field potential recordings in this preparation show that MSO application evokes a sustained depolarization, which can be blocked by the N-methyl-D-aspartate (NMDA) antagonist L-(+)-2-amino-5-phosphonovalerate (AP5). However, competition assays using MSO on [3H]CGP-39653 (DL-(E)-2-amino-4-propyl-1-phosphono-3-pentenoate) binding in rat cortical homogenates show only 20% displacement of total binding, suggesting that MSO is acting indirectly, perhaps by releasing glutamate. To investigate this possibility, we measured glutamate release during MSO application. Time course and dose-response experiments with MSO showed significant [3H]glutamate release, which was partially attenuated by AP5. To assess cellular toxicity, we measured lactate dehydrogenase (LDH) release from cortical sections exposed to MSO. MSO treatment led to a rapid increase in LDH activity, which could be blocked by AP5. These data suggest that MSO acts by increasing glutamate release, which then activates NMDA receptors, leading to excitotoxic cell death. These data suggest the possibility that MSO in processed flour had excitotoxic actions that may have been contributing factors to some human neuronal disorders.Key words: agene process, glutamate release, lactate dehydrogenase, methionine sulfoximine, N-methyl-D-aspartate (NMDA) receptor, neurological disorders.

Glutathione and signal transduction in the mammalian CNS

The tripeptide glutathione (GSH) has been thoroughly investigated in relation to its role as antioxidant and free radical scavenger. In recent years, novel actions of GSH in the nervous system have also been described, suggesting that GSH may serve additionally both as a neuromodulator and as a neurotransmitter. In the present article, we describe our studies to explore further a potential role of GSH as neuromodulator/neurotransmitter. These studies have used a combination of methods, including radioligand binding, synaptic release and uptake assays, and electrophysiological recording. We report here the characteristics of GSH binding sites, the interrelationship of GSH with the NMDA receptor, and the effects of GSH on neural activity. Our results demonstrate that GSH binds via its gamma-glutamyl moiety to ionotropic glutamate receptors. At micromolar concentrations GSH displaces excitatory agonists, acting to halt their physiological actions on target neurons. At millimolar concentrations, GSH, acting through its free cysteinyl thiol group, modulates the redox site of NMDA receptors. As such modulation has been shown to increase NMDA receptor channel currents, this action may play a significant role in normal and abnormal synaptic activity. In addition, GSH in the nanomolar to micromolar range binds to at least two populations of binding sites that appear to be distinct from all known excitatory amino acid receptor subtypes. GSH bound to these sites is not displaceable by glutamatergic agonists or antagonists. These binding sites, which we believe to be distinct receptor populations, appear to recognize the cysteinyl moiety of the GSH molecule. Like NMDA receptors, the GSH binding sites possess a coagonist site(s) for allosteric modulation. Furthermore, they appear to be linked to sodium ionophores, an interpretation supported by field potential recordings in rat cerebral cortex that reveal a dose-dependent depolarization to applied GSH that is blocked by the absence of sodium but not by lowering calcium or by NMDA or (S)-2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate antagonists. The present data support a reevaluation of the role of GSH in the nervous system in which GSH may be involved both directly and indirectly in synaptic transmission. A full accounting of the actions of GSH may lead to more comprehensive understanding of synaptic function in normal and disease states.

Reciprocal interactions between CA3 network activity and strength of recurrent collateral synapses

In hippocampal slices, synchronous CA3 network activity induced persistent strengthening of active positive-feedback synapses. This altered network operation by increasing probability of future synchronous network activation. Long-term depression of synaptic strength induced by partial blockade of NMDA receptors during synchronous network activity reversed changes in probability of spontaneous network activation. These results suggest that specific network activity patterns selectively alter strength of active synapses. Stable, reversible alterations in network activity can also be effected by corresponding alterations in synaptic strength. These findings confirm the Hebb memory model at the neural-network level and suggest new therapies for pathological patterns of network activity in epilepsy.

Activation of N-methyl-D-aspartate receptors evokes calcium spikes in the dendrites of rat hypothalamic paraventricular nucleus neurons

Activation of dendritic voltage-dependent calcium (Ca2+) conductances in neuroendocrine cells of the hypothalamus may underlie previously documented Ca2+ spikes in these cells. The present study, in which whole-cell recordings were obtained from paraventricular nucleus neurons in a hypothalamic slice preparation, addresses this issue by directly activating dendritic N-methyl-D-aspartate receptors in the presence of tetrodotoxin. Application of tetrodotoxin abolished spontaneous action potentials in all paraventricular nucleus neurons tested (n = 27). Following tetrodotoxin, spikes were evoked by depolarizing current pulses, in an all-or-none fashion in the majority of cells (n = 20). Removal of extracellular Ca2+ (n = 6) or addition of 500 microM CdCl2 (n = 4) abolished the spikes in response to pulses. Repetitive spiking activity (in tetrodotoxin) was also observed following N-methyl-D-aspartate agonist application in 75% of the cells tested (n = 15). The spikes, underscored by a slow membrane depolarization, were abolished by the administration of CdCl2 (n = 4). N-Methyl-D-aspartate agonist elicited a slow inward current in cells voltage-clamped at -60 mV (n = 5). Additionally, larger amplitude, transient inward currents were observed near the onset of the response. The activation threshold to elicit spikes following N-methyl-D-aspartate agonist application was significantly more negative (-54.6+/-3.6 mV) than the potential at which spikes were initiated as a result of depolarizing current injection (-32.3+/-1.8 mV; Student's t-test: P < 0.0001). In contrast to this, Na+ spikes in control solution had an invariable threshold (-49.6+/-0.7 mV vs -51.5+/-1.2 mV; P > 0.05), regardless of the stimulus used to initiate the spikes. These observations suggest that direct activation of N-methyl-D-aspartate receptors located on the dendrites of paraventricular nucleus neurons triggers Ca2+ spikes. Although the precise function of these spikes is unclear, previous data reporting dendritic neuropeptide release in the paraventricular nucleus raise the possibility that dendritically initiated spikes may serve as a local signal to trigger such release.

Did consumption of flour bleached by the agene process contribute to the incidence of neurological disease?

The present report proposes the hypothesis that increased levels of neurodegenerative disorders in humans may have arisen due to inclusion in the diet of methionine sulfoximine (MSO), a byproduct of the bleaching of flour by nitrogen trichloride. This method of bleaching, the 'agene process' was in use from early in the century and continued until at least 1949/1950. Estimates indicate that, at least in the UK, as much as 80% of all flour during this period was produced by this process. MSO acts directly to inhibit the production of two crucial molecules, glutathione (GSH) and glutamine. Decreases in GSH, a key antioxidant and free radical scavenger, diminish the body's antioxidant defenses and may lead to increased oxidative stress. Decreases in glutamine synthesis may act to increase free glutamate and give rise to increased levels of ammonia. Cells in the nervous system are particularly sensitive to a decline in either GSH or glutamine. The combined effects of decreases in these molecules, particularly with long-term exposure to MSO in bleached flour, may have had quite drastic effects on neuronal health and survival. The present hypothesis may provide clues to the etiology of neurological disorders such as Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), suggesting that such disorders may arise in part due to toxic actions of some compounds in processed human foods.

Presynaptic modulation of CA3 network activity

The simultaneous discharge of hippocampal CA3 pyramidal cells is a widely studied in vitro model of physiological and pathological network synchronization. This network is rapidly activated because of extensive positive feedback mediated by recurrent axon collaterals. Here we show that population-burst duration is limited by depletion of the releasable glutamate pool at these recurrent synapses. Postsynaptic inhibitory conductances further limit burst duration but are not necessary for burst termination. The interval between bursts in vitro depends on the rate of replenishment of releasable glutamate vesicles and the probability of release of those vesicles at recurrent synapses. Therefore presynaptic factors controlling glutamate release at recurrent synapses regulate the probability and duration of synchronous discharges of the CA3 network.

Leptin depolarizes rat hypothalamic paraventricular nucleus neurons

Leptin, the protein product of the ob/ob gene, is thought to have a central site of action, presumably within the hypothalamus, through which it regulates feeding behavior. THe paraventricular nucleus (PVN) is one structure that has been implicated in regulating feeding behavior. Using patch-clamp recording techniques, this study examines the direct membrane effects of leptin on neurons in a coronal PVN slice. Bath application of the physiologically active leptin fragment (amino acids 22-56) elicited dose-related depolarizations in 82% of the type I cells tested (n = 17) and 67% of the type II cells tested (n = 9). By contrast, the physiologically inactive leptin fragment (amino acids 57-92) had no discernible effect on membrane potential (n = 7). The effects of this peptide were unaffected following synaptic isolation of the cells by bath application of the sodium channel blocker tetrodotoxin (n = 5). Voltage clamp recordings in six cells demonstrated that leptin increased a nonspecific cation conductance with a reversal potential near -30 mV. These findings suggest that neurons in PVN may play an important role in the central neuronal circuitry involved in the physiological response to leptin.

Hyperpolarizing after-potentials regulate generation of long-duration plateau depolarizations in rat paraventricular nucleus neurons

Activation of N-methyl-D-aspartate (NMDA) receptors in a population of neurons of the paraventricular nucleus (PVN) results in long-duration plateau depolarizations during which the membrane rapidly depolarizes, reaching a stable plateau near -20 mV. These responses were observed in 29% of the Type II PVN neurons tested with 1 microM NMDA agonist (n = 84). The stable plateau phase is characterized by an increase in ionic conductance, from 1.19+/-0.11 nS to 5.24+/-2.17 nS (n = 5). Bath application of tetrodotoxin (n = 4) or alternatively inclusion of QX-314 in the pipette solution (n = 3) prevented the generation of these events. The remaining cells tested (n = 56) also depolarized in response to NMDA agonist, but long duration plateau depolarizations were not observed. Previous evidence from hypothalamic cultures has demonstrated synaptically driven plateau potentials following the blockade of repolarizing conductances. Pharmacological blockade of the post-spike hyperpolarizing afterpotential with 4-aminopyridine (200 microM), in cells that did not generate plateaux, resulted in the observance of long duration plateau depolarizations in response to a subsequent application of NMDA agonist (n = 4). Our results demonstrate that this 4-aminopyridine-sensitive ionic conductance plays a critical role in determining whether a cell will depolarize for a prolonged duration in response to NMDA receptor activation. As a prolonged depolarization of the postsynaptic membrane and accompanying membrane permeability changes are essential for neurotoxicity, these findings provide evidence for a potential protective mechanism that depends solely on the ability of the cell, through its ionic conductances, to control imposed changes in membrane potential.

Neurodegenerative disorders in humans: the role of glutathione in oxidative stress-mediated neuronal death

Oxidative stress has been implicated in both normal aging and in various neurodegenerative disorders and may be a common mechanism underlying various forms of cell death including necrosis, apoptosis, and excitotoxicity. In this review, we develop the hypothesis that oxidative stress-mediated neuronal loss may be initiated by a decline in the antioxidant molecule glutathione (GSH). GSH plays multiple roles in the nervous system including free radical scavenger, redox modulator of ionotropic receptor activity, and possible neurotransmitter. GSH depletion can enhance oxidative stress and may also increase the levels of excitotoxic molecules; both types of action can initiate cell death in distinct neuronal populations. Evidence for a role of oxidative stress and diminished GSH status is presented for Lou Gehrig's disease (ALS), Parkinson's disease, and Alzheimer's disease. Potential links to the Guamanian variant of these diseases (ALS-PD complex) are discussed. In context to the above, we provide a GSH-depletion model of neurodegenerative disorders, suggest experimental verifications of this model, and propose potential therapeutic approaches for preventing or halting these diseases.

Long duration pressor responses following activation of subfornical organ neurons in rats are the result of increased circulating vasopressin

Electrical stimulation in the subfornical organ (SFO) of male Sprague-Dawley rats resulted in biphasic increases in blood pressure (BP) without a change in heart rate. The initial short duration (0-10 s) increase in BP lasted throughout the 10 s stimulation period (area under the curve (AUC) = 104.3+/-15.26 mmHg/s, (mean+/-SEM) P < 0.001). Upon termination of the electrical stimulus, the BP remained elevated for approximately 55 s (long duration response, AUC = 327.5+/-48.22 mmHg/s, P < 0.001). This long duration BP response was determined to be the result of an increase in circulating vasopressin (VP) as administration of a V1 receptor antagonist abolished this response (AUC = -210.7 +/- 42.38 mmHg/s, P < 0.01). The results of the present study demonstrate that the long duration component of the biphasic increase in BP observed on response to electrical stimulation of the SFO is the result of increased concentrations of circulating VP.

Reduced NMDA receptor sensitivity may underlie the resistance of subpopulations of PVN neurons to excitotoxicity

Magnocellular neurons in the paraventricular nucleus are resistant to excitotoxic cell damage. We tested the hypothesis that a modified post-synaptic response following NMDA receptor activation may underlie this resistance. Whole-cell recordings from hypothalamic slices showed that NMDA receptor activation caused dose-dependent depolarizations in both Type I (putative magnocellular) and Type II (putative parvocellular) neurons. Type II cells, however, were an order of magnitude more sensitive (10 nM) than Type I neurons (100 nM). The depolarizations recorded in Type II cells were also significantly greater (> 35% resulting in sodium channel inactivation) than those recorded in Type I neurons. This differential sensitivity of neurons to NMDA receptor activation may explain the selective resistance of magnocellular PVN neurons to excitatory neurotoxins.

Nitric oxide depolarizes type II paraventricular nucleus neurons in vitro

Nitric oxide is a labile gas which has been implicated in neuronal signalling. The enzyme responsible for the production of this molecule is present in the paraventricular nucleus of the hypothalamus, yet a specific role for nitric oxide in neurotransmission within this nucleus remains unclear. Using whole-cell patch-clamp recordings from paraventricular nucleus neurons in a coronal hypothalamic slice, we have assessed the acute effects of nitric oxide on membrane potential and ionic conductance. Recordings were obtained from 78 neurons with a mean resting membrane potential of -57.8 +/- 0.6 mV and a mean input resistance of 972 +/- 146 M omega. Cells were electrophysiologically classified into Type I or Type II according to previously established criteria. Bath application of nitric oxide (delivered either as a gas dissolved in solution, or liberated from the donor compound, N-acetyl-S-nitroso-D-penicillamine) elicited reversible membrane depolarizations (3 mV) in 14 of the 19 Type II cells tested. These cells also exhibited a decrease in input resistance following nitric oxide application. Similar effects were observed in response to bath application of L-arginine, with 11 of 14 cells displaying depolarizations and accompanying decreases in input resistance. Inhibition of nitric oxide synthase abolished the responses to L-arginine (n=2). The nitric oxide effects persisted when voltage-activated Na+ channels were blocked by tetrodotoxin (n=6). The depolarizations observed in Type II cells were mimicked by bath application of a membrane permeable cyclic GMP analogue (8-bromo-cyclic GMP) (n=8). Furthermore, nitric oxide depolarizations were abolished by pre-treatment of the slice with the guanylate cyclase inhibitor, LY83583 (n=4). Type I cells did not depolarize in response to nitric oxide (n=11). It is concluded that nitric oxide specifically depolarizes parvocellular neurons within the paraventricular nucleus via a mechanism that requires activation of guanylate cyclase and subsequent production of cyclic GMP. These findings provide the first insight into the cellular mechanisms underlying the acute effects of nitric oxide on neurons in the paraventricular nucleus.

Increased longevity, reduced fecundity, and delayed development in fruitfly (Zaprionus paravittiger) fed on butylated hydroxy anisole

Oxygen free radicals are generated as a by-product during normal metabolism, and they cause damage to proteins, lipids, and DNA in organisms. The defense system of the body counteracts these highly reactive chemical moieties and neutralizes them. However, a small fraction of free radicals escapes, which causes lipid peroxidation and hence aging of the organism. It has been hypothesized that, if the free radicals are arrested/reduced, then aging can be delayed or life span could be enhanced. To test the above hypothesis, we fed butylated hydroxy anisole (BHA) to a drosophilid insect, Zaprionus paravittiger, and observed its effect on life span, fecundity, and developmental period. The insects were reared and maintained on standard corn meal agar (CMA) medium at 26 degrees +/- 2 degrees C. Various concentrations (1, 5, 10, 25, 50, and 100 mM) of BHA were mixed with CMA medium, and the cultures were reared and maintained on these mixtures to study the life span of insects. Survivor curves showed that lower concentrations (5, 10, 25 mM) of BHA increased the life span, while higher concentrations (50, 100 mM), which were rather toxic, decreased life span. The most suitable concentration was 10 mM, which increased median (LT50) (27% and 15% in male and female) and maximum (LT100) (18% and 27% in male and female) life spans of insects maximally. Females exhibited longer life spans compared with males. The cultures were fed on the optimal concentration (10 mM) of BHA to study its effect on developmental period and egg-laying rate. The total developmental period was delayed by 7.2%, and egg-laying rate was reduced by 19.7% on BHA feeding. The extension in developmental period and reduction in egg-laying capacity could be the contributory factors to the favorable effect of BHA on life span.

Nitric oxide regulates NMDA-driven GABAergic inputs to type I neurones of the rat paraventricular nucleus

1. Whole-cell recordings were obtained from type I paraventricular nucleus (PVN) neurones in coronal slices of rat hypothalamus to study the involvement of nitric oxide (NO) in the modulation of inhibitory transmission resulting from the activation of N-methyl-D-aspartate (NMDA) receptors by the high affinity receptor agonist D,L-tetrazol-5-ylglycine. 2. A brief pulse of NMDA agonist (0.1-10 microM) faithfully elicited increases in action potential firing frequency in all type I cells tested (n = 55). In cells with membrane potentials positive to -75 mV, this excitation was accompanied by an underlying depolarization (> 2 mV) in the majority of cases (n = 45). At membrane potentials negative to -75 mV, NMDA agonist application elicited an initial monotonie depolarization, which was auxiliary to profound, rhythmic oscillations of the membrane potential, resulting in the emergence of burst-like activity in these cells (n = 8). 3. In addition to depolarizing the neurones, the NMDA agonist also elicited inhibitory postsynaptic potentials (IPSPs) in 40% (n = 22) of the cells tested. The IPSPs were inhibited by the GABAA receptor antagonist bicuculline methiodide (BMI). 4. Microdialysis of NO into the PVN has been shown to increase local levels of inhibitory neurotransmitters, including GABA. The possibility that NO-induced increases in GABA lead to an increase in inhibitory synaptic activity in PVN was investigated by administering NO by three different methods. Bath application of the donor compound, S-nitroso-N-acetyl-penicillamine (SNAP; n = 7), bubbled NO solution (n = 5), or the NO precursor L-arginine (n = 6) all elicited increases in IPSP frequency. 5. Production of NO in other brain centres has been linked to the activation of the NMDA receptor. In order to determine whether the increase in IPSPs following NMDA was the result of activation of NO, the production of NO was blocked with the NO synthase inhibitor N omega-nitro-L-arginine methylester (L-NAME). Subsequent NMDA receptor activation elicited more pronounced depolarizations, but there was no accompanying increase in IPSP frequency (n = 5). 6. This study demonstrates that GABAergic inhibition resulting from NMDA receptor activation can be regulated profoundly by NO. By increasing inhibitory transmission within a nucleus, NO may serve as an important intermediary in the regulation of neuronal excitability in the central nervous system.

Actions of angiotensin in the subfornical organ and area postrema: implications for long term control of autonomic output

1. Considerable physiological and anatomical evidence indicates that circulating angiotensin II (AngII), plays important roles in the long-term regulation of autonomic output as a result of actions in two circumventricular structures, the subfornical organ (SFO) and area postrema (AP). 2. Extracellular recordings have demonstrated excitatory actions of AngII on neurons from both of these structures which are AT1 receptor mediated, maintained when cells are placed in synaptic isolation, and are dose dependent. Interestingly SFO neurons appear to be an order of magnitude more sensitive to AngII than those in AP. 3. Recent calcium imaging studies have demonstrated that AngII induces increases in intracellular calcium in both SFO and AP neurons. Whole cell patch recordings have also begun to provide important information suggesting that AngII actions may modulate voltage activated ion channels in these two structures to elicit its observed actions on circumventricular organs (CVO) neurons at the blood-brain interface. 4. Through these actions circulating AngII is thus able to influence efferent projections from these CVO which in turn influence the output of hypothalamic cells projecting to the posterior pituitary (vasopressin secretion), nucleus tractus solitarius (NTS), and intermediolateral cell column of the spinal cord (to influence sympathetic preganglionics), and medullary neurons in the NTS.

Gender specific alterations in antioxidant status of aging Zaprionus paravittiger fed on propyl gallate

Zaprionus paravittiger fed on propyl gallate (PG) supplemented diet (2.5, 25 and 250 micrograms/ml) showed an increase in life span. Further increase in concentration (2500, 5000 and 7500 micrograms/ml) accelerated the mortality rate. Females exhibited longer life span as compared to males. Antioxidant enzymes (catalase, peroxidase and glutathione reductase) were measured in control and optimum concentration of PG (25 micrograms/ml) fed flies at various age intervals. Antioxidant enzyme activities showed an increase during reproductive phase. Catalase and glutathione reductase activities decreased with age however no significant change was observed in peroxidase activity. The females exhibited higher enzyme activities as compared to males in control and PG fed group at most of the age intervals. PG feeding caused a significant increase in catalase and glutathione reductase activities in both the sexes. These findings suggest the PG has dose dependent and sex specific influence on longevity of Z. paravittiger and support the view that longevity and activity of antioxidant enzymes are positively linked.

Electrophysiology of the circumventricular organs

Since the first anatomical description of the circumventricular organs (CVOs) as a structurally distinct group of regions in the central nervous system (CNS), considerable information has implicated these structures as physiologically significant autonomic control centers located at the blood-brain interface. Specialized features of these structures, such as their extensive vasculature, lack of the normal blood-brain barrier (BBB) (i.e., capillaries have a fenestrated endothelium), and dense aggregations of a variety of peptidergic receptors, support an involvement of the CVOs in communication between the circulation and the CNS. The two best understood examples of CVOs with the ability to sense circulating substances impermeable to the BBB are the subfornical organ (SFO) and the area postrema (AP). Specifically, the ability of numerous peptides to influence CNS function, as the result of actions on the neural substrate of these structures has been especially well documented. Considerable anatomical, biochemical, pharmacological, and physiological evidence has implicated these structures as CNS sites at which angiotensin (ANG), atrial natriuretic peptide (ANP), vasopressin (VP), and endothelin (ET) act to influence neuroendocrine and other more classical autonomic functions. In the following sections, we review neurophysiological studies which have provided new and exciting insights regarding the specific neural pathways and cellular mechanisms through which CVO neurons are able to exert their profound influences over central autonomic control.

Effect of vitamin E on life span, malondialdehyde content and antioxidant enzymes in aging Zaprionus paravittiger

Zaprionus paravittiger fed with vitamin E supplemented diet (1, 5 and 10 micrograms/ml) showed an increase in median and maximum life spans. Further increase in concentration accelerated the mortality rate. Females exhibited longer life span as compared to males. Malondialdehyde (MDA) content and antioxidant enzymes (catalase and peroxidase) were measured in control and optimum concentration of vitamin E (5 micrograms/ml)-fed flies at various age intervals. MDA content showed an increase with age in control and vitamin E-fed group whereas catalase and proxidase activities showed a decrease with age. The females exhibited lower MDA content and higher activities of catalase and peroxidase as compared to males in control and vitamin E group. Vitamin E feeding caused a significant decrease in MDA and increase in catalase and peroxidase activities. These findings suggest that vitamin E has dose-dependent and sex-specific influence on longevity of Z. paravittiger and support the view that longevity and activity of antioxidant enzymes are positively linked.

Axonal atrophy in aging is associated with a decline in neurofilament gene expression

Neurofilaments (Nfs) are major determinants of axonal caliber. Nf transcript levels increase during development and maturation, and are associated with an increase in Nf protein, Nf numbers, and caliber of axons. With aging there is axonal atrophy. In this study we asked whether the axonal atrophy of aging was associated with a decline in Nf transcript expression, Nf protein levels, and Nf numbers. Expression of transcripts for the three Nf subunits was evaluated in dorsal root ganglia (DRG) of Fischer-344 rats aged 3-32 months by Northern and in situ hybridization. There was an approximately 50% decrease in Nf subunit mRNA levels in DRG of aged (> 23 months) as compared to young and mature (3 and 12 months) rats, whereas expression of another neuronal mRNA, GAP-43, showed no decline. Western analysis showed a corresponding decrease in Nf subunit proteins and no decline in GAP-43. Morphometric analysis showed a 50% decrease in Nf numbers within axons. The decrease in Nf gene expression and Nf numbers was accompanied by a decrease in cross-sectional area and circularity of all myelinated fibers, with the largest fibers showing the most marked changes, and a shrinkage in the perikaryal area of large neurons. Furthermore, we found a concomitant decrease in the expression of transcripts for the nerve growth factor receptors trkA and p75 with aging. Although the mechanisms leading to the decrease in Nf gene expression with aging are not known, a decrease in the availability of growth factors, or the neuron's ability to respond to them, may play a role in this process.

Paraventricular nucleus neurons projecting to the spinal cord receive excitatory input from the subfornical organ

The present study utilized electrophysiological techniques to determine the effects of subfornical organ (SFO) stimulation on the activity of neurons in the paraventricular nucleus (PVN) projecting to the spinal cord. Single-unit recordings were obtained from 79 PVN neurons antidromically identified as projecting to the intermediolateral cell column (IML). Antidromically evoked action potentials showed a mean latency of 94.6 +/- 5.3 ms and a mean threshold for activation of 1.58 +/- 0.11 mA. Electrical stimulation of SFO (100 microA-1.5 mA, 0.1 ms) resulted in excitatory responses in 18 of the 27 neurons tested (67%). Peristimulus histogram analysis of such effects demonstrated a duration of < 50 ms in 14 of the 18 cells so influenced (78%), whereas the remaining 4 cells (22%) showed excitatory responses with a longer duration. Systemic administration of the nonpeptidergic angiotensin II (ANG) type 1 (AT1) receptor antagonist losartan (3 mg/kg) blocked the long-duration excitatory responses in 100% (3 of 3) of the cells tested but was without effect on the short-duration excitations (0 of 5). Twenty-two identified PVN neurons were also tested for their responses to systemic ANG (20-500 ng), which had no observable effect on the activity of any of these cells. These data demonstrate that neurons in SFO provide excitatory input to PVN cells that project to the IML. One of the neurotransmitters responsible for communication in this pathway is ANG.

Angiotensin II neurotransmitter actions in paraventricular nucleus are potentiated by a nitric oxide synthase inhibitor

There is increasing evidence that nitric oxide (NO) plays a role within the central nervous system as a novel messenger. Neuronal culture work suggests NO to be involved specifically in mediating actions of angiotensin II (ANG). The present study examined the potential role of NO within the paraventricular nucleus (PVN), a structure involved in mediating the cardiovascular changes initiated by activation of the subfornical organ (SFO). The pressor response to stimulation of SFO, which can be divided into a short (SD) and long duration (LD) component was enhanced following administration of an NO synthase inhibitor (L-NAME) (SD control: 101 +/- 4 vs. post L-NAME: 145 +/- 10 mmHg.s (P < 0.05); LD control: 387 +/- 167 vs. post L-NAME: 1737 +/- 617 mmHg.s (P < 0.05)). This effect was specific to activation of SFO efferents as the blood pressure responses to either, stimulation of PVN, or systemic administration of vasopressin were not potentiated by administration of L-NAME. These findings suggest that NO may be acting within PVN to inhibit further release of ANG, thereby attenuating the cardiovascular response to stimulation of SFO.

Angiotensin II actions in paraventricular nucleus: functional evidence for neurotransmitter role in efferents originating in subfornical organ

Angiotensin II (ANG) has been suggested to be the neurotransmitter utilised by subfornical organ (SFO) efferents projecting to the paraventricular nucleus (PVN). The PVN has been shown to be involved in mediating the cardiovascular response elicited by electrical stimulation of SFO. The possible role of ANG as a neurotransmitter in these pathways has been examined in the present study. The cardiovascular effects of ANG microinjection into the PVN were examined in urethane anaesthetized, male Sprague-Dawley rats. Microinjection of 20 ng or 50 ng ANG into PVN resulted in mean increases in blood pressure of 12.8 +/- 0.6 mmHg (P < 0.0005), and 16.2 +/- 1.4 mmHg (P < 0.0001) respectively, without effect on heart rate. These responses were significantly attenuated following systemic administration of losartan, an ANG type 1 receptor (AT1) antagonist (Control, +12.8 +/- 0.6 mmHg; post-losartan, +5.6 +/- 1.7 mmHg), but were unaffected by the AT2 receptor antagonist, PD123319 (Control, +10.8 +/- 1.6 mmHg; post-PD123319, +11.6 +/- 2.4 mmHg). Initial and later components of the biphasic pressor response elicited by electrical stimulation of SFO (200 microA, 10 Hz, 1 ms pulse width, 10 s) were also significantly attenuated by losartan, but unaffected by PD123319. The short latency increase in mean arterial pressure was 16.6 +/- 2.3 mmHg in comparison to a post-losartan increase of 9.3 +/- 1.6 mmHg (P < 0.001). Similarly, the secondary response consisted of a control increase of 9.6 +/- 1.3 mmHg and a post-losartan increase of 3.4 +/- 0.9 mmHg (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Functional evidence that the angiotensin antagonist losartan crosses the blood-brain barrier in the rat

Losartan is a novel nonpeptidergic antagonist of angiotensin (ANG) II subtype 1 (AT1) receptors, which effectively lowers blood pressure in high-renin hypertensive rat and blocks the pressor response to systemic ANG II. It is well known that high densities of ANG II receptors exist in the hypothalamic paraventricular nucleus (PVN). In addition, activation of putative angiotensinergic afferents to the PVN originating in subfornical organ (SFO) elevates blood pressure and facilitates the activity of PVN neurons. We report here that systemic administration of losartan (3 mg/kg) significantly attenuates the pressor response to electrical stimulation of SFO. The excitatory responses of PVN neurons to SFO stimulation or local pressure microinjection of ANG II were also significantly inhibited in 58.8% and 88.9% of PVN cells, respectively, by intravenous administration of losartan. These pharmacological effects were rapid and reversible, and were accompanied by little change of basal arterial blood pressure or spontaneous neuronal activity. These observations suggest that systemic losartan crosses the blood-brain barrier (BBB) and acts at AT1 receptors within the PVN.

Effect of propyl gallate feeding on glutathione content in ageing Zaprionus paravittiger (Diptera)

Glutathione content increased during the reproductive period and decreased thereafter up to the 43rd day of age in whole body as well as mitochondrial homogenates of ageing Zaprionus paravittiger. However, females exhibited higher levels of glutathione as compared to males. In whole body homogenates, propyl gallate (PG; 25 micrograms/ml) increased the glutathione content significantly up to the 22nd day of life whereas in mitochondria it increased during all age intervals in the two sexes with the exception of the 36th and 43rd day of survival in males. In conclusion, the higher level of glutathione on PG feeding might be one of the factors leading to the prolonged life span of flies.