Biomedical subjects

David L Walker

Publications and source records attributed to David L Walker.

16 recordsLinked to original sources

PubMed2005

Glutamate receptor antagonist infusions into the basolateral and medial amygdala reveal differential contributions to olfactory vs. context fear conditioning and expression.

The basolateral amygdala's involvement in fear acquisition and expression to visual and auditory stimuli is well known. The involvement of the basolateral and other amygdala areas in fear acquisition and expression to stimuli of other modalities is less certain. We evaluated the contribution of the basolateral and medial amygdala to olfactory and to context fear and fear conditioning by infusing into these areas the NMDA receptor antagonist AP5, the AMPA/kainate receptor antagonist NBQX, or vehicle prior to either odor-shock pairings or fear-potentiated startle testing. Pre-training AP5 infusions into the basolateral amygdala disrupted fear conditioning to the odor but not the context conditioned stimulus (CS). Pre-test NBQX infusions disrupted fear-potentiated startle to the odor but not context CS. Neither compound blocked fear conditioning when infused into the medial amygdala prior to training, but pre-test NBQX infusions did block fear-potentiated startle. The results confirm and extend recent findings suggesting a role for the basolateral amygdala in olfactory fear and fear conditioning, reveal an unexpected dissociation of the basolateral amygdala's involvement in discrete cue versus context fear and fear conditioning, and implicate for the first time the medial amygdala in fear-potentiated startle.

2-Amino-5-phosphonovalerate↗

PubMed2005

Diminished intrafollicular estradiol levels in in vitro fertilization cycles from women with reduced ovarian response to recombinant human follicle-stimulating hormone.

OBJECTIVE: To correlate serum and intrafollicular E(2) levels in women receiving GnRH agonist and recombinant human (h)FSH for IVF. DESIGN: Prospective study. SETTING: Academic IVF center. PATIENT(S): Twenty-three nonhirsute ovulatory women. INTERVENTION(S): In vitro fertilization cycles were classified as high- or low-normal response, according to whether the amount of recombinant hFSH administered was below or above, respectively, the mean recombinant hFSH given to all women combined. Serum bioactive LH and immunoreactive FSH were measured during pituitary desensitization preceding recombinant hFSH and repeated at oocyte retrieval. The first mature follicle aspirated from either ovary had a mean diameter >15 mm. MAIN OUTCOME MEASURE(S): Serum and follicular fluid (FF) insulin, bioactive LH, immunoreactive FSH, and steroid levels. RESULT(S): Basal serum gonadotropin levels were comparable between female groups. Serum immunoreactive FSH levels, but not bioactive LH levels, at oocyte retrieval were greater in cycles of low-normal vs. high-normal response, whereas maximum serum E2 levels and total oocyte counts were similar between both groups. Despite elevated FF immunoreactive FSH levels, FF E2 was decreased in low-normal response cycles. Intrafollicular T and P also were decreased and increased, respectively, in cycles of low-normal response, although FF bioactive LH levels were comparable in both groups. CONCLUSION(S): Intrafollicular E2 is decreased in cycles of reduced ovarian responsiveness to GnRH agonist and recombinant hFSH and is not predicted by maximal serum E2 or number of oocytes retrieved.

Adult↗

PubMed2005

Use of a novel washing method combining multiple density gradients and trypsin for removing human immunodeficiency virus-1 and hepatitis C virus from semen.

OBJECTIVE: To determine the effectiveness of a novel treatment designed to remove human immunodeficiency virus (HIV) -1 and hepatitis C virus (HCV) from spiked semen and to evaluate sperm motility and viability after treatment. DESIGN: A prospective clinical laboratory-based study. SETTING: The human studies were conducted in academic and national research environments. The bovine study was conducted in an accredited research facility. PATIENT(S): Healthy volunteers provided the semen samples used in the human studies; abattoir-derived material was used for the bovine embryo production study. INTERVENTIONS(S): None. MAIN OUTCOME MEASURE(S): Cytopathic, reverse transcriptase-polymerase chain reaction, and branched DNA assays were used to test the efficacy of the procedure for inactivating or removing viruses from spiked semen; standard semen evaluation criteria were used to assess the effects of the procedures on sperm motility and viability. RESULT(S): Trypsin exposure significantly reduced the infectivity of HIV-1. The triple density gradient treatment, with or without trypsin, had no detrimental affect on fresh or cryopreserved/thawed sperm 2-48 hours after treatment. The treatment of semen spiked with HIV-1 or HCV indicated that the procedure was effective for reducing viral copies to undetectable levels or below levels of clinical relevance. CONCLUSION(S): The procedure was effective for significantly inactivating or reducing HIV-1 and HCV in spiked semen without adversely affecting sperm quality.

Animals↗

PubMed2005

Cryosystem assessment by glucose uptake of murine blastocysts.

Glucose uptake was used as a measure of metabolic activity and implantation potential to compare vitrification and slow freezing in a prospective randomized trial using murine blastocysts. Frozen 2-cell embryos (n = 132) thawed and cultured for 48 h to the blastocyst stage were randomly divided into four groups: (i) control - not refrozen; (ii) slow freezing using a programmed rate (PR); (iii) vitrification by super-cooled (VSC) liquid nitrogen; and (iv) vitrification in liquid nitrogen (VLN). Upon re-thawing, embryos were cultured individually for 24 h to determine glucose uptake non-invasively. Morphological assessments included total cell counts and inner cell mass (ICM) detection following immunosurgery. Mean glucose uptake was lower for each treatment (PR and VSC, 4.3 pmol/embryo per h; VLN, 4.9 pmol/embryo per h) versus controls (6.8 pmol/embryo per h). PR and VSC embryos had fewer cells (57.4 +/- 24.2 and 64.1 +/- 31.5) versus controls (85.7 +/- 26.2), and fewer embryos containing a detectable ICM (42.9 and 61.8%) compared with controls (88.2%). The only difference between control and VLN embryos was absolute glucose uptake, although in both treatments glucose uptake was increased from embryos with an ICM compared with those without. Glucose uptake appears to be a sensitive, non-invasive method to validate cryopreservation protocols.

Animals↗

PubMed2004

Arsenic exposure exacerbates atherosclerotic plaque formation and increases nitrotyrosine and leukotriene biosynthesis.

A correlation between arsenic and cardiovascular disease (CVD) has been established through epidemiological studies, although the mechanisms are unknown. Using a mouse model that develops atherosclerotic lesions on a normal chow diet, we have confirmed a connection between long-term arsenic intake and CVD. Our results reveal a significant increase in the degree of atherosclerotic plaque stenosis within the innominate artery of ApoE-/-/LDLr-/- mice treated with 10 ppm sodium arsenite (133 microM) in drinking water for 18 weeks compared to controls. Immunohistochemistry shows nitrotyrosine formation, a marker of reactive nitrogen species generation, is significantly higher within the atherosclerotic plaque of arsenic-treated mice. In addition, there is a significant increase in the 5-lipoxygenase (5-LO) product, leukotriene E4 (LTE4), in the serum of arsenic-treated mice. This is supported by induction of the 5-LO protein and subsequent increases in LTE4 synthesis in bovine aortic endothelial cells. This increase in LTE4 is partially inhibited by inhibitors of nitric oxide synthase, suggesting a link between reactive nitrogen species and arsenic-induced inflammation. Furthermore, there is a significant increase in prostacyclin (PGI2) in the serum of arsenic-treated mice. We conclude that changes in specific inflammatory mediators such as LTE4 and PGI2 are related to arsenic-induced atherosclerosis. In addition, amplified synthesis of reactive species such as peroxynitrite results in increased protein nitration in response to arsenic exposure. This finding is consistent with the pathology seen in human atherosclerotic plaques.

Animals↗

PubMed2004

Are fear memories made and maintained by the same NMDA receptor-dependent mechanisms?

A recent finding indicates that inducible knockout of the NR1 NMDA receptor subunit promotes the loss of fear memories formed months earlier. One view is that posttraining NMDA receptor activation protects modified synapses from "synaptic drift." An alternative view is that NMDA receptors help maintain appropriate connectivity in memory-encoding networks.

Animals↗

PubMed2004

Vitrification versus programmable rate freezing of late stage murine embryos: a randomized comparison prior to application in clinical IVF.

A prospective randomized trial was performed to compare post-thaw development of murine blastocysts following programmable rate freezing and two methods of vitrification. Frozen 2-cell murine embryos (n = 429) thawed and cultured for 48 h, were randomly allocated by stage of development into four groups: control (not refrozen), programmable rate freezing (PR) in 0.25 ml straws, vitrification in flexible micropipettes by immersion in super-cooled (VSC) liquid nitrogen (LN2), and vitrification in flexible micropipettes by immersion in LN2 (VLN). Survival, developmental stage progression, presence or absence of an inner cell mass (ICM), and cell counts were recorded 24 h post-thaw. All measured outcomes were different between embryos from the control group and all freezing methods. Controlled-rate freezing resulted in the lowest total cell counts and fewest embryos with a distinct ICM. A higher percentage of embryos survived 24 h post-thaw, progressed to more advanced developmental stages and had higher total cell counts after VLN compared with PR. Moreover, fewer embryos, frozen by either PR or VSC, contained a detectable ICM compared with VLN. These data demonstrate that vitrification may be a better method for freezing murine blastocysts than PR, and may prove to be a superior method for freezing human blastocysts.

Animals↗

PubMed2004

Insulin and messenger ribonucleic acid expression of insulin receptor isoforms in ovarian follicles from nonhirsute ovulatory women and polycystic ovary syndrome patients.

Insulin action is mediated by two insulin receptor (IR) isoforms, differing in mitogenic and metabolic function. IR isoform expression might occur in human granulosa cells and could be altered in polycystic ovary syndrome (PCOS) from hyperinsulinemia. To determine the relationship between granulosa cell IR isoform expression and follicular fluid insulin concentration in individual follicles, 18 normal women and seven PCOS patients receiving gonadotropins for in vitro fertilization were studied. Glucose tolerance testing was performed before pituitary desensitization, and fasting serum insulin was measured at oocyte retrieval. Granulosa cells and fluid aspirated from the first follicle were used to determine IR isoform mRNA expression and insulin concentration, respectively. IR isoform A mRNA expression was greater than that of IR isoform B expression in normal mural granulosa and cumulus cells, without a cell type effect. Intrafollicular insulin levels increased with adiposity and serum insulin levels at oocyte-retrieval but did not predict IR mRNA expression. Total IR mRNA expression, but not intrafollicular insulin levels, was elevated in PCOS patients, whereas intrafollicular insulin levels were increased in women with impaired glucose tolerance. Granulosa cell IR heterogeneity, together with adiposity-dependent intrafollicular insulin availability, introduces a novel mechanism by which insulin may affect granulosa cell function within the follicle.

Adult↗

PubMed2003

Role of the bed nucleus of the stria terminalis versus the amygdala in fear, stress, and anxiety.

The bed nucleus of the stria terminalis is a limbic forebrain structure that receives heavy projections from, among other areas, the basolateral amygdala, and projects in turn to hypothalamic and brainstem target areas that mediate many of the autonomic and behavioral responses to aversive or threatening stimuli. Despite its strategic anatomical position, initial attempts to implicate the bed nucleus of the stria terminalis in conditioned fear were largely unsuccessful. Recent studies have shown, however, that the bed nucleus of the stria terminalis does participate in certain types of anxiety and stress responses. In this work, we review these findings and suggest from the emerging pattern of evidence that, although the bed nucleus of the stria terminalis may not be necessary for rapid-onset, short-duration behaviors which occur in response to specific threats, the bed nucleus of the stria terminalis may mediate slower-onset, longer-lasting responses that frequently accompany sustained threats, and that may persist even after threat termination.

Amygdala↗

PubMed2003

Equivalent blastocyst rates after freezing murine embryos in Cryo Bio System high security or standard instruments-medicine-veterinarian straws.

OBJECTIVE: To validate the Cryo Bio System (CBS) straw in our current cryopreservation system before using it in clinical practice. DESIGN: A prospective comparison of blastocyst development rates in 278 murine embryos after refreezing and thawing at the two-cell stage against the standard Instruments-Medicine-Veterinarian (IMV) straw used in our cryopreservation program. SETTING: Private IVF laboratory. PATIENT(S): No human subjects or material was used in this study. INTERVENTION(S): Frozen two-cell murine embryos were thawed and randomized into three treatments [1] refreezing in the CBS straws, [2] refreezing in IMV 0.25-mL straws, and [3] control embryos remaining in culture without refreezing. Embryos were refrozen using identical cryoprotectants and identical programmed controlled-rate freezers. After cryopreservation, straws were held in liquid nitrogen for a brief period before thawing and continued culture. MAIN OUTCOME MEASURE(S): Postthaw murine blastocyst development rate. RESULT(S): When the manufacturer's filling and loading protocol was used for the CBS straw there was no significant difference in the blastocyst development rate between CBS (75.0%) and IMV (76.4%) straws. CONCLUSION(S): The CBS straw may be a viable and potentially safer alternative for cryopreservation of human embryos, particularly for patients with known infections.

Animals↗

PubMed2003

Role of the amygdala in fear extinction measured with potentiated startle.

Although much is now known about the neural basis of excitatory fear conditioning, much less is known about the neural basis of inhibitory conditioning. One type of inhibitory conditioning is extinction, a process in which stimuli that elicit fear by virtue of previous associations with aversive stimuli such as shock (excitatory fear conditioning) are now presented in the absence of the aversive stimuli (extinction training). As a result, the ability of the conditioned stimulus to elicit fear gradually diminishes. Extinction is different from forgetting and does not reflect an erasure of the original fear memory. Instead, extinction is an active form of inhibitory learning that competes with excitatory fear conditioning. Infusions into the amygdala (a brain area essential for excitatory fear conditioning) of either NMDA receptor antagonists or inhibitors of the NMDA-receptor-linked mitogen-activated protein kinase cascade block extinction learning. Conversely, the NMDA receptor agonist D-cycloserine facilitates extinction after either systemic administration or intra-amygdala infusion. Because therapeutic interventions based on extinction procedures are commonly used to treat fear disorders, and because D-cycloserine is a widely available and safe compound, D-cycloserine or similar agents might be usefully combined with traditional extinction-based approaches in the treatment of clinical fear.

Amygdala↗

PubMed2002

Quantifying fear potentiated startle using absolute versus proportional increase scoring methods: implications for the neurocircuitry of fear and anxiety.

RATIONALE: The fear-potentiated startle paradigm [increased startle in the presence of a conditioned fear stimulus (CS)] has become increasingly popular as a tool for evaluating the potential efficacy of putative anxiolytic compounds. However, when the tested compounds also influence baseline startle, it is unclear how comparisons with control groups can best be made. OBJECTIVE: To evaluate the validity of absolute difference (startle amplitude on CS minus non-CS test trials) vs. proportional increase (the absolute difference score divided by startle amplitude on non-CS test trials) scoring methods. METHODS: The effect on proportional increase and absolute difference scores of baseline shifts that occur with or without concomitant increases in fear was evaluated in rats. A reliable measure should yield similar scores across shifting baselines, provided that fear levels remain constant. RESULTS: Preexisting baseline differences, and those brought about by different startle-eliciting noise burst intensities, by strychnine injections, or by CRH infusions, each increased absolute difference scores without markedly influencing proportional change scores. These baseline differences were not associated with different fear levels. Increases in baseline startle brought about by unsignaled footshocks or by a second CS - increases which are associated with increased fear - partially occluded additional CS-induced increases using either measure. CONCLUSIONS: Across different baselines, CS-elicited fear is most accurately reflected in proportional change scores. Under certain conditions saturation effects may interfere with an accurate assessment using either measure. However, these same saturation effects may provide opportunities to explore the neural circuitry of fear and anxiety in novel ways.

Acoustic Stimulation↗

PubMed2002

Facilitation of conditioned fear extinction by systemic administration or intra-amygdala infusions of D-cycloserine as assessed with fear-potentiated startle in rats.

NMDA receptor antagonists block conditioned fear extinction when injected systemically and also when infused directly into the amygdala. Here we evaluate the ability of D-cycloserine (DCS), a partial agonist at the strychnine-insensitive glycine-recognition site on the NMDA receptor complex, to facilitate conditioned fear extinction after systemic administration or intra-amygdala infusions. Rats received 10 pairings of a 3.7 sec light and a 0.4 mA footshock (fear conditioning). Fear-potentiated startle (increased startle in the presence vs the absence of the light) was subsequently measured before and after 30, 60, or 90 presentations of the light without shock (extinction training). Thirty non-reinforced light presentations produced modest extinction, and 60 or 90 presentations produced nearly complete extinction (experiment 1). DCS injections (3.25, 15, or 30 mg/kg) before 30 non-reinforced light exposures dose-dependently enhanced extinction (experiment 2) but did not influence fear-potentiated startle in rats that did not receive extinction training (experiment 3). These effects were blocked by HA-966, an antagonist at the glycine-recognition site (experiment 4). Neither DCS nor HA-966 altered fear-potentiated startle when injected before testing (experiment 5). The effect of systemic administration was mimicked by intra-amygdala DCS (10 microg/side) infusions (experiment 6). These results indicate that treatments that promote NMDA receptor activity after either systemic or intra-amygdala administration promote the extinction of conditioned fear.

Acoustic Stimulation↗

PubMed2002

The role of amygdala glutamate receptors in fear learning, fear-potentiated startle, and extinction.

Using a paradigm known as fear-potentiated startle, we have examined the neurobiological substrates of Pavlovian fear conditioning. In these experiments, rats are trained to fear an initially neutral stimulus by pairing that stimulus with shock. The amount of fear elicited by the stimulus [i.e., now a conditioned stimulus (CS)] is later assessed by presenting startle-eliciting noise bursts both in the presence and also the absence of the CS. After training, startle responses are typically greater in the presence of the CS. Findings reviewed here suggest that amygdala N-methyl-D-aspartate (NMDA) receptors play a key role in triggering the neural changes that support fear learning and also the loss of fear that accompanies extinction training. Amygdala (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors also participate in fear learning. However, unlike NMDA receptor antagonists, AMPA receptor antagonists also block fear-potentiated startle when infused prior to testing. Very recent data indicate that glutamate metabotropic Group II receptor agonists also block fear learning when infused into the amygdala prior to training, and block fear-potentiated startle when infused prior to testing. A fuller understanding of the role of amygdala glutamate systems in fear and fear learning may suggest novel pharmacological approaches to the treatment of clinical anxiety disorders.

Acoustic Stimulation↗

PubMed2002

Group II metabotropic glutamate receptors within the amygdala regulate fear as assessed with potentiated startle in rats.

The contribution to fear and fear learning of amygdala Group II metabotropic glutamate receptors was examined in rats. Pretest intra-amygdala infusions of the Group II receptor agonist LY354740 (0.3 or 1.0 microg/side) significantly disrupted fear-potentiated startle. The same rats were unimpaired when later tested without drug. The Group II receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (3.0 microg/side) mimicked the effect of LY354740, and coadministration of the Group II receptor antagonist LY341495 (0.3 microg/side) prevented it. Pretraining LY354740 (0.3 microg/side) infusions also blocked learning. The effects on learning and performance were significantly less pronounced in rats with misplaced cannulas. Thus, Group II metabotropic receptors within or very near the amygdala regulate fear and fear learning and are a potential target for anxiolytic compounds.

Amygdala↗

PubMed2001

Light-enhanced startle: further pharmacological and behavioral characterization.

RATIONALE: Previous findings indicate that the acoustic startle response is elevated when rats are tested in bright light. The phenomenon is disrupted by the 5HT1A partial agonist and the D2 receptor antagonist buspirone, a compound that also blocks the effect on startle of conditioned fear, and it was suggested that light-enhanced startle reflects an anxious state produced by bright light. It was also suggested that pre-test handling may be necessary for light-enhanced startle. To characterize this phenomenon further, we evaluate here the sensitivity of light-enhanced startle to the anxiolytic compound chlordiazepoxide, to the noradrenergic beta-receptor antagonist propranolol, and to pre-test handling. METHODS: Startle was measured for 20 min in the dark (phase I), followed 5 min later by a second test (phase II) either in the dark or the light. Immediately prior to testing, rats received IP injections of chlordiazepoxide (5, 10, or 20 mg/kg; experiment 1), propranolol (10 or 20 mg/kg; experiment 2) or saline. Using the minimally effective doses from the light-enhanced startle experiments, conditioned fear to a shock-paired cue was also evaluated. In a third experiment, rats were (a) removed from the test cages and briefly handled between phases I and II, (b) were not handled during this interval, or (c) were tested without the interposed delay. RESULTS: Propranolol (10 and 20 mg/kg) and chlordiazepoxide (10 mg/kg) disrupted light-enhanced startle at doses comparable to those required to disrupt fear-potentiated startle to a shock-paired cue. There was no effect of handling. CONCLUSION: These results further characterize the pharmacology of light-enhanced startle, provide additional support for the view that the effects of light on startle reflect an influence of anxiety, and offer additional information concerning the procedural variables that influence this behavior.

Adrenergic beta-Antagonists↗