Biomedical subjects

R S Dyer

Publications and source records attributed to R S Dyer.

At least 19 recordsLinked to original sources

PubMed1999

International Arctic Seas Assessment Project.

The International Atomic Energy Agency responded to the news that the former Soviet Union had dumped radioactive wastes in the shallow waters of the Arctic Seas, by launching the International Arctic Seas Assessment Project in 1993. The project had two objectives: to assess the risks to human health and to the environment associated with the radioactive wastes dumped in the Kara and Barents Seas; and to examine possible remedial actions related to the dumped wastes and to advise on whether they are necessary and justified. The current radiological situation in the Arctic waters was examined to assess whether there is any evidence for releases from the dumped waste. Potential future releases from the dumped wastes were predicted, concentrating on the high-level waste objects containing the major part of the radionuclide inventory of the wastes. Environmental transport of released radionuclides was modelled and the associated radiological impact on humans and the biota was assessed. The feasibility, costs and benefits of possible remedial measures applied to a selected high-level waste object were examined. Releases from identified dumped objects were found to be small and localised to the immediate vicinity of the dumping sites. Projected future annual doses to members of the public in typical local population groups were very small, less than 1 microSv--corresponding to a trivial risk. Projected future doses to a hypothetical group of military personnel patrolling the foreshore of the fjords in which wastes have been dumped were higher, up to 4 mSv/year, which still is of the same order as the average annual natural background dose. Moreover, since any of the proposed remedial actions were estimated to cost several million US$ to implement, remediation was not considered justified on the basis of potentially removing a collective dose of 10 man Sv. Doses calculated to marine fauna were insignificant, orders of magnitude below those at which detrimental effects on fauna populations might be expected to occur. Remediation was thus concluded not to be warranted on radiological grounds.

Animals↗

PubMed1996

What can research contribute to regulatory decisions about the health risks of multiple chemical sensitivity?

Multiple Chemical Sensitivity (MCS), which may not be caused by chemicals at all, is a serious medical problem of unknown origin and uncertain etiology that raises many fundamental science and policy questions. Regulators, for example, are confronted with a dilemma: what, if anything, should be done to protect people from the scientifically uncertain health risks of exposures to extremely low levels of environmental chemicals. Regulatory agencies, such as the Environmental Protection Agency, do not have the luxury of waiting until conclusive scientific evidence is available before making a decision; however, our present lack of scientific understanding about MCS is so acute that it is not possible to ascertain whether the cause of MCS-related symptoms is chemical, biological, physical, psychosocial, or some combination thereof. Nevertheless, many MCS sufferers and advocates for the chemically induced hypothesis are clamoring for regulatory action to reduce putative health risks from very-low-level exposures to chemicals in the environment. Unless steps are taken to improve the quantity and quality of the existing scientific data base, we cannot, with any acceptable degree of certainty, evaluate the extent to which regulatory decisions about MCS are either protective of public health or cost-effective. This article examines how research can strengthen the scientific basis for risk-related decisions about MCS, and proposes a framework for establishing research directions and priorities. It is argued that high-priority research on MCS is distinguishable by four attributes: (1) results are valuable for risk-related decisions; (2) findings significantly advance scientific knowledge and understanding; and the hypothesis being tested is both (3) biologically plausible and (4) readily testable.

Decision Making↗

PubMed1995

Toxicological and chemical evaluation of emissions from carpet samples.

This study investigated findings that the off-gassing of certain carpets caused sensory and pulmonary irritation, changes in neurobehavioral signs, and death in exposed mice. Two standard test method measures--one for estimating sensory irritancy (ASTM-E981-84), the other for evaluating the neurotoxic potential of chemicals (functional observational battery)--were coupled with a postmortem assessment to ascertain the mechanism of toxicity. The postmortem evaluation included measurements of hemoglobin, serum clinical chemistries, blood and lung lavage white cell counts and differential, organ weights, and a gross necropsy with a microscopic evaluation of all major organs. The study evaluated three treatment groups composed of two preheated carpet emission exposures and one preheated air-control exposure. No toxic effects were associated with exposure to the off-gassing of the two tested carpets. Clinical chemistry and histopathological alterations were observed with exposure to either filter-air or carpet when compared to nonexposed unrestrained control mice, indicating that the exposure procedure caused significant effects unrelated to carpet emissions. A detailed chemical and microbial evaluation of the carpets and carpet emissions showed volatile organic compounds, pesticide residues, and microbiological flora, but at insufficient quantities to result in acute toxicity. Based on this assessment, there was no indication that exposure to emissions from these two carpets poses a serious health risk.

Air Pollution, Indoor↗

PubMed1994

Within-session changes in peak N160 amplitude of flash evoked potentials in rats.

The negative peak occurring approximately 160 ms after stimulation (peak N160) of flash evoked potentials (FEPs) of rats changes with repeated testing. Habituation, sensitization, and arousal have all been invoked to explain these changes, but few studies have directly tested these explanations. We examined within-session changes in peak N160 amplitude with repeated testing, and the modulatory effects of stimulus intensity and auditory white noise. Peak N160 amplitude increased with daily testing (between-session changes), and was larger at greater stimulus intensities. In contrast, peak N160 amplitude underwent within-session increases on early days and within-session decreases on later days. The within-session changes were not affected by stimulus intensity. In rats previously tested in a quiet environment, exposure to acoustic white noise increased motor activity and transiently decreased peak N160 amplitude, which then increased and subsequently decreased with continued photic and acoustic stimulation. Repeated testing in the presence of noise did not alter the within-session changes in peak N160 amplitude. Heart rate showed both within- and between-session decreases, but was unaffected by noise. The data suggest that the within-session changes in peak N160 amplitude may reflect a habituation-like response to the test environment.

Acoustic Stimulation↗

PubMed1993

Acute effects of ethanol on pattern reversal and flash-evoked potentials in rats and the relationship to body temperature.

The effects of acute ethanol treatment on flash and pattern reversal visual evoked potentials (FEPs and PREPs, respectively) were examined in three experiments using Long-Evans rats. The relationships of evoked potential parameters with blood ethanol concentration and body temperature were examined. In Experiment 1, rats were treated i.p. with vehicle or 0.5, 1.0 or 2.0 g ethanol/kg body weight, and tested 30 min later. The 2.0 g/kg group had prolonged latencies of PREP peaks, no changes in PREP peak-to-peak amplitudes, and lower body temperatures than saline-treated controls. The peak latency shifts were significantly correlated with both blood ethanol concentration and body temperature, and were of a magnitude to be expected from similar changes in body temperature alone. Experiment 2 measured both PREPs and paired-flash FEPs in rats 30 min after injection of either 0, 0.5 or 2.0 g/kg ethanol. PREP changes were found following treatment with the high dose which were similar to those of Experiment 1. Some FEP peak latencies were prolonged and peak-to-peak amplitudes were reduced by both doses of ethanol, despite the fact that body temperatures were reduced at only the high dose. At 2.0 g/kg ethanol, the FEP changes in latency, but not amplitude, were in accordance with what would be expected from body temperature changes alone. The third study attempted to investigate the role of reduced body temperature in producing the visual evoked potential changes by testing at room temperatures of 22 or 30 degrees C. Contrary to expectations, the rats receiving 2 g/kg ethanol were approx. 1 degree C cooler than controls at both room temperatures. Evoked potential latencies were greater in ethanol-treated rats than controls at both room temperatures. There were no significant effects of ethanol on FEP amplitudes. Overall, the effects of low doses of ethanol were independent of temperature changes, but the effects of higher doses of ethanol (2.0 g/kg) could not be distinguished from those produced by differences in body temperature alone.

Analysis of Variance↗

PubMed1992

Alterations in rat flash and pattern reversal evoked potentials after acute or repeated administration of carbon disulfide (CS2).

Because solvents may selectively alter portions of visual evoked potentials, we examined the effects of carbon disulfide (CS2) on flash (FEPs) and pattern reversal (PREPs) evoked potentials. Long-Evans rats were administered ip carbon disulfide either acutely or for 30 days. FEPs or PREPs were recorded prior to and 1, 2, 4, 8, or 24 hr after a single dose of CS2 (0, 100, 200, 400, or 500 mg/kg). Flash evoked potentials were also recorded 1, 2, 6, and 24 hr after the last of 30 doses of 200 mg CS2/kg/day. Acute exposure to CS2 consistently decreased the amplitude of FEP peak N160 at 1 hr, depressed peak N30 amplitude over 2-4 hr, and increased the latency of peaks P21, N30, P46, N56, and N160 for up to 4 hr after treatment. Carbon disulfide decreased the amplitude of PREP peaks P65, N83, P88, and N122 4 hr after treatment. Colonic temperature was depressed up to 8 hr after treatment. Administration of 200 mg CS2/kg/day decreased the amplitude of FEP peak N30 and increased the latencies of peaks P21, N30, P46, N56, and N160 up to 24 hr after the last dose. The differential effects of CS2 on portions of FEPs indicate that FEP peaks can be independently modulated. Changes in PREPs were temporally correlated with alterations in early FEP peaks, but FEP peak N160 was depressed at an earlier time point. Repeated CS2 exposure affected FEPs at lower doses and for a longer time than an acute exposure, similar to the reported greater severity of neurological disturbances following repeated CS2 exposures in humans.

Animals↗

PubMed1991

Rat flash-evoked potential peak N160 amplitude: modulation by relative flash intensity.

The flash-evoked potential (FEP) of rats has a large negative peak (N160) approximately 160 ms following stimulation. This peak has been reported to be modulated by the subject's state of behavioral arousal and influenced by several test parameters. These experiments examined the influences of repeated testing, the number of stimuli/session, interactions of ambient illumination and flash intensity, and the effect of pupillary dilation on the development and amplitude of peak N160. The amplitude of peak N160 increased with daily testing and reached an asymptotic amplitude by about day 10. This amplitude was affected by the intensity of the flash stimulus relative to the ambient illumination (RFI) and appeared to reach a "ceiling" amplitude at greater than 50 dB RFI. The number of stimuli/session and dilation of the subject's pupils did not have a large influence on the growth or asymptotic level of peak N160 amplitude. The data are consistent with the hypothesis that the growth of peak N160 may represent a sensitization-like phenomenon.

Animals↗

PubMed1989

Peak N160 of rat flash evoked potential: does it reflect habituation or sensitization?

Flash evoked potentials recorded from awake rats contain a negative peak occurring about 160 msec after the flash (N160). This peak has been associated with a specific level of arousal, and/or habituation by various authors. The current studies attempted to determine whether changes in N160 amplitude which accompany repeated testing reflect processes associated with sensitization or habituation. This paper describes experiments in Long-Evans hooded rats which demonstrate the effects of repeated testing, varying stimulus intensity, varying stimulus frequency, and discharging an alarm bell before and during a test session. Repeated testing produced increases in N160 amplitude which were greater at high than low stimulus intensities. Repeated exposure to the test chamber without flashing did not alter N160 amplitude, nor did altering stimulus rate within the range of 0.5 to 4.0 Hz. Discharging an alarm increased N160 amplitude. Taken together, the data suggest that amplitude of N160 more closely reflects sensitization to the stimulus than habituation to either the stimulus or any feature of the test situation.

Animals↗

PubMed1988

Convulsant properties of cyclotrimethylenetrinitramine (RDX): spontaneous audiogenic, and amygdaloid kindled seizure activity.

Dose-effect and time course relationships were determined for the effects of the explosive cyclotrimethylenetrinitramine (RDX) on seizure susceptibility. Male Long Evans rats treated with 0-60 mg/kg RDX po were monitored for spontaneous seizures during an 8-hr interval between dosing and audiogenic (AG) seizure testing. Blood samples for analyzing plasma RDX concentrations were obtained immediately thereafter. Spontaneous and AG seizures were observed at dosages as low as 10-12.5 mg/kg, with significant seizure incidence induced by dosages of 25.0 mg/kg (5.34 micrograms RDX/ml plasma) and 50.0 mg/kg (8.28 micrograms RDX/ml plasma), respectively. Spontaneous seizure incidence peaked at 2 hr for all RDX treatment groups, then decreased (12.5 and 25.0 mg/kg) or remained elevated (50.0 mg/kg) for the remaining 6 hr. In contrast, AG seizures (37.5 mg/kg) could be elicited only at 8 and 16 hr, despite significant elevation of plasma RDX concentrations at 2 and 4 hr. Because limbic system involvement was suggested by spontaneous seizure characteristics, the rate of amygdaloid kindling was measured following daily treatment with 6.0 mg/kg. This dosage significantly accelerated kindling development without inducing spontaneous seizures or producing an accumulation of RDX in plasma. These data provide preliminary evidence that limbic structures may participate in RDX-induced seizure susceptibility.

Acoustic Stimulation↗

PubMed1988

Temperature-dependent changes in visual evoked potentials of rats.

The effects of alterations in body temperature on flash and pattern reversal evoked potentials (FEPs and PREPs) were examined in hooded rats whose thermoregulatory capacity was compromised with lesions of the preoptic/anterior hypothalamic area and/or cold restraint. Body temperature, measured with a rectal thermometer, was manipulated via exposure to different ambient temperatures. To describe the data, a model was used in which both linear and quadratic relationships could be estimated. PREP amplitudes were not significantly influenced by body temperature over the range of 27-42 degrees C, although in one experiment FEP amplitudes did show a linear decline as temperatures fell below approximately 30 degrees C. Both FEP and PREP latencies were strongly influenced by temperature and became progressively longer as body temperature was lowered. The non-linear component affecting latencies became more prominent as body temperature decreased. These data demonstrate the temperature dependence of FEP and PREP latencies independent of anesthetic or other drugs.

Animals↗

PubMed1988

Ketamine alters rat flash evoked potentials.

Discovering the neurotransmitters involved in the generation of flash evoked potentials (FEPs) would enhance the use of FEPs in screening for and assessment of neurological damage. Recent evidence suggests that the excitatory amino acids, glutamate and aspartate, may be transmitters in the visual system. Ketamine selectively antagonizes the actions of excitatory amino acids on n-methyl-d-aspartate receptors and may be administered systemically. Two experiments were designed to test the effects of ketamine on rat FEPs. First, the effects of ketamine (37, 75, 150 mg/kg) on FEPs recorded in light and dark backgrounds were investigated at a single (10 min) posttreatment interval. Ketamine administration resulted in dose-dependent alterations in FEP peak amplitudes and latencies. Peak P1 amplitude increased by a factor of 4, in a dose-dependent manner. Peak N1 virtually disappeared at 150 mg/kg. Peak P2 amplitude increased by 50%, but only in the light background, and only at 150 mg/kg. Second, ketamine (150 mg/kg) effects on FEPs were investigated 5 min and 30 min following administration. The decrease in peak N1 amplitude was maximal 5 min after administration and the amplitude was recovering at 30 min. The effects on peak P1 and peak N3 amplitudes were maximal 5 min after ketamine administration, but were not recovering 30 min postinjection. The various peak latencies were also affected differently. The possible role of glutamate or aspartate in the generation of rat FEPs is discussed.

Animals↗

PubMed1987

Time-dependent neurobiological effects of colchicine administered directly into the hippocampus of rats.

Rats were given bilateral injections of colchicine into the dorsal and ventral hippocampus. Behavioral, neurochemical and histopathological measurements were taken, up to 12 weeks after surgery. Colchicine produced a consistent increase in spontaneous motor activity, enhanced acoustic startle reactivity, and accelerated acquisition of two-way shuttle box avoidance, but did not affect reactivity to a noxious thermal stimulus. Measurement of dynorphin in the hippocampus indicated that colchicine rapidly depleted this neuropeptide, which is thought to be contained preferentially in the mossy fibers of granule cells of the hippocampus. Colchicine also decreased Met-enkephalin in the hippocampus, but the magnitude of the change (22%) was less than that (89% depletion) observed for hippocampal dynorphin. Examination of hippocampal morphology using light microscopic techniques indicated that colchicine caused approximately 60% degeneration of granule cells in the hippocampus. Although the length of the pyramidal cells was decreased (12-16%), the width of the CA1 and CA3 region of the hippocampus was not affected. These data underscore the importance of the granule cells in the mediation of behavioral processes such as motor activity, startle reactivity and performance of shuttle box avoidance.

Acoustic Stimulation↗

PubMed1987

Possible role of the brainstem in the mediation of prepulse inhibition in the rat.

Bilateral stimulation of electrodes aimed at the cuneiform nucleus produced significant inhibition of the startle response produced by presentation of an 8-kHz, 110-dB tone. Stimulation of electrodes aimed at the deep mesencephalic nucleus also reduced the magnitude of the startle response, but the effect was less than that following stimulation sites near the cuneiform nucleus. Histological reconstruction of the electrode tip locations revealed a significant negative correlation between the maximum magnitude reduction of the acoustic startle response following an electrical prepulse stimulus and the distance from the cuneiform nucleus. Histological examination also indicated that some electrodes aimed at the cuneiform nucleus were located in or near the inferior colliculus or parabrachial nucleus, all of which are thought to be part of an inhibitory circuit parallel to the acoustic startle reflex arc. These experiments support the view that the prepulse inhibition of the acoustic startle reflex originates in the brainstem.

Acoustic Stimulation↗

PubMed1987

Focal lesions of visual cortex--effects on visual evoked potentials in rats.

Focal lesions were placed in the visual cortex of Long-Evans hooded rats, immediately below skull screw recording electrodes. Lesions were produced by heat, and extended an average depth of about 0.9 mm below the cortical surface. Evoked potentials recorded from the electrode overlying the cortical lesion were compared with simultaneously recorded potentials from a contralateral homotopic site. The effects of the lesion were selective. Flash-evoked potential peaks P1, P2, and N2 were depressed by the lesion, and peaks N1 and P3 were augmented; peak N3 was unaffected. Pattern reversal evoked potential peak N3 was depressed by the lesion, and peaks N1 and P2 were made more distinct. The results emphasized that different peaks have different generators, and suggest in particular that flash-evoked potential peaks P1 and N2, and peak N3 of the pattern reversal-evoked potential require the superficial layers of the cortex.

Animals↗

PubMed1987

Differential effects of caffeine, picrotoxin, and pentylenetetrazol on hippocampal afterdischarge activity and wet dog shakes.

We identified changes in hippocampal afterdischarge activity that follow administration of subcon vulsant doses (one-half the convulsant dose) of analeptic agents with known pharmacological action. Long-Evans rats (N = 104) with chronic bipolar electrodes implanted in the dorsal hippocampus, were injected i.p. with saline, caffeine (75 mg/kg), picrotoxin (2 mg/kg), or pentylenetatrazol (20 mg/kg) in 1 ml/kg volume 15 min before testing. Body temperature was measured at the beginning of the session to determine if significant change was associated with any of the treatments. Beginning at 10 microA, current (2-s train of 50-Hz biphasic pulses) was applied to hippocampal electrodes and intensity was increased in 10-microA steps until the afterdischarge sequence was elicited. Afterdischarge threshold, wet dog shake frequency, and the duration of the primary afterdischarge, the postprimary depression, and the rebound afterdischarge were measured. Caffeine administration produced a dramatic prolongation of the rebound afterdischarge, without affecting the duration of the primary afterdischarge. All other afterdischarge variables were unchanged by the caffeine treatment. Because caffeine blocks adenosine receptors at physiologic concentrations, adenosine action is implicated in the termination of the second, but not the first, spike train. Picrotoxin and pentylenetetrazol had no influence on the EEG, despite evidence of slight (1 degrees C) hypothermia. A decrease in wet dog shake frequency, however, was associated with picrotoxin administration. As picrotoxin and pentylenetetrazol are known gamma-aminobutyric acid (GABA) antagonists, the results suggest that GABA is involved minimally, if at all, in the hippocampal afterdischarge sequence.

Animals↗

PubMed1987

Urethane affects the rat visual system at subanesthetic doses.

Urethane is an anesthetic which is commonly used in neurophysiological studies because it is presumed to have minimal effects upon neuronal activity. This study investigated the influence of urethane anesthesia upon flash evoked potentials (FEPs) recorded from hooded rats. Subanesthetic dosages (25 g/kg and 0.5 g/kg) and an anesthetic dosage (1.0 g/kg) were administered, and subsequently recorded FEPs were compared to vehicle-injected controls. Urethane produced profound qualitative and quantitative effects upon the FEP. At 0.5 g/kg, the P1 (normal latency = 20 msec) and N1 (normal latency = 30 msec) peaks became unrecognizable. Peak N1 disappeared and peak P1 merged with P2 (normal latency = 45 msec). Peak P2 increased in amplitude by about 100%. The results indicate that in the visual system, urethane has a significant influence upon neuronal activity. Caution should be used in interpreting data obtained from urethane-anesthetized rats.

Anesthesia↗

PubMed1987

Surface distribution of flash-evoked and pattern reversal-evoked potentials in hooded rats.

Simultaneous recording from 21 electrode sites in a 4 X 4 mm area over the posterior cortex was used to determine the surface distribution of all major peaks which constitute flash-evoked potentials (FEPs) and pattern reversal evoked-potentials (PREPs) in hooded rats. Topographical maps were constructed with respect to Bregma and midline reference points. The data indicate that not all of the peaks which constitute either evoked potential have their greatest amplitude within the classically defined primary visual cortex. Further, since the FEPs were produced by uniform stimulation, the data suggest that surface regions of the rat visual cortex differ in ways other than simply the portion of the visual field from which information is received.

Animals↗

PubMed1987

Ontogeny of flash-evoked potentials in unanesthetized rats.

The effects of age and stimulation frequency (0.2/sec, 1.0/sec, 2.0/sec, or 4.0/sec) on flash-evoked potentials (FEPs) were investigated in awake, unsedated, unrestrained rats. Animals were tested daily from postnatal day (PND) 8 to PND 20, and every 3 or 4 days thereafter until PND 41. On PND 9, a single negative wave (N1a) was observed following 0.2/sec flash presentation. Animals tested on PND 10 exhibited a positive wave (P2) following the return of peak N1a to baseline. On PND 13 another negative wave (N1) appeared on the leading shoulder of peak N1a. Peak N1 became the dominant negative wave on PND 14. Peak N1a merged into N1 and had disappeared by PND 19. Peak N3 was first observed as a negative shift following peak P2 on PND 15. Peaks N2 and P3 were not observed in the group average waveforms until PND 34. Peak latencies decreased through the fifth postnatal week. Peak amplitudes increased with age until after eye opening (PND 15), but were variable thereafter. No FEPs were observed following higher than 0.2/sec flash presentation until PND 13. Increasing stimulation frequency decreased N1 and P2 peak amplitudes, but had no effect on peak latencies.

Aging↗