All thalamocortical neurones possess a T-type Ca2+ 'window' current that enables the expression of bistability-mediated activities

1. The existence of a non-negligible steady-state ('window') component of the low threshold, T-type Ca2+current (IT) and an appropriately large ratio of IT to ILeak conductance (i.e. gT/gLeak) have been shown to underlie a novel form of intrinsic bistability that is present in about 15 % of thalamocortical (TC) neurones. 2. In the present experiments, the dynamic clamp technique was used to introduce into mammalian TC neurones in vitro either an artificial, i.e. computer-generated, IT in order to enhance endogenous IT, or an artificial inward ILeak to decrease endogenous ILeak. Using this method, we were able to investigate directly whether the majority of TC neurones appear non-bistable because their intrinsic ionic membrane properties are essentially different (i.e. presence of a negligible IT 'window' component), or simply because they possess a gT or gLeak conductance that is insufficiently large or small, respectively. 3. The validity of the dynamic clamp arrangement and the accuracy of artificial IT were confirmed by (i) recreating the low threshold calcium potential (LTCP) with artificial IT following its block by Ni2+ (0.5-1 mM), and (ii) blocking endogenous LTCPs with an artificial outward IT. 4. Augmentation of endogenous IT by an artificial analog or introduction of an artificial inward ILeak transformed all non-bistable TC neurones to bistable cells that expressed the full array of bistability-mediated behaviours, i.e. input signal amplification, slow oscillatory activity and membrane potential bistability. 5. These results demonstrate the existence of a non-negligible IT 'window' component in all TC neurones and suggest that rather than being a novel group of neurones, bistable cells are merely representative of an interesting region of dynamical modes in the (gT, gLeak) parameter space that may be expressed under certain physiological or pathological conditions by all TC neurones and other types of excitable cells that possess an IT 'window' component with similar biophysical properties.

Examination of the metabolic status of rat air pouch inflammatory exudate by high field proton NMR spectroscopy

High field proton (1H) NMR spectroscopy was employed to investigate the metabolic status of rat air pouch inflammatory exudates obtained subsequent to the induction of inflammation with carrageenan, and the 1H NMR profiles of these fluids were compared and contrasted with those of inflammatory human synovial fluid, rat plasma and human serum. The characteristic biochemical features obtained from 1H NMR analysis of these exudates consisted of (1) substantially elevated levels of lactate (11.40+/-1.46x10-3 mol dm-3 for samples collected at a time point of 24 h post induction) with little or no NMR-detectable glucose, data consistent with a hypoxic environment and consequent anaerobic metabolism in the inflamed air pouch, and (2) high levels of the ketone body 3-d-hydroxybutyrate, providing evidence for an increased utilization of fats for energy by lymphocytes, the predominant leucocytes present in this environment. These phenomena represent a pathological extreme of the abnormal metabolic status of inflammatory human synovial fluids.

Recurrent and persistent coagulopathy following pit viper envenomation

BACKGROUND

Coagulation abnormalities following crotaline (pit viper) snakebite have traditionally been considered short-lived, but laboratory studies have rarely been reported beyond the first few days of treatment for envenomation. During the course of an antivenom clinical trial, we observed coagulation defects as late as 2 weeks following envenomation.

OBJECTIVES

To document and characterize the recurrence or persistence of coagulopathy among patients envenomed by pit vipers and treated with a Fab antivenom.

METHODS

Patients with moderate pit viper envenomation were enrolled in a multicenter, prospective clinical trial. A Fab-based antivenom preparation, antivenom polyvalent crotalid (ovine) Fab, was administered in all cases. Platelet count, fibrinogen level, presence of fibrin split products, prothrombin time, and partial thromboplastin time were determined before treatment and at standard intervals during the following 2 weeks.

RESULTS

Of 38 patients completing the study, 20 (53%) had recurrent, persistent, or late coagulopathy 2 to 14 days after envenomation. Thrombocytopenia occurred in patients with prior thrombocytopenia; hypofibrinogenemia occurred only in those with prior hypofibrinogenemia or positive fibrin split products. No patient experienced significant spontaneous bleeding. One patient with coagulopathy developed minor bleeding following minor surgery 12 days after envenomation.

CONCLUSIONS

Prolonged or recurrent coagulopathy may occur after envenomation by North American pit vipers. Patients treated with Fab-based antivenom may benefit from periodic rather than single-bolus dosing. Patients with coagulopathy should undergo close monitoring during the first 2 weeks after snakebite.

Ethanol analysis following consumption of "alcohol-free" beer

Breath alcohol devices are routinely employed by emergency department personnel to reflect blood ethanol levels. No data have been published using these devices following the ingestion of "alcohol-free" beer that are required to contain < 0.5% alcohol. We performed a prospective, randomized, unblinded study on 8 healthy male volunteers that abstained from alcohol for 48 h and fasted for 10 h. Each subject ingested a 6-pack of alcohol-free beer over 1 h; breath alcohol determinations were made at 30, 60 and 120 min. Elevated breath alcohol levels were obtained at 30 and 60 min and decreased in all but 1 subject by 120 min. The elevated breath alcohol levels at 30 and 60 min may represent the previously reported "mouthwash" effect.

The relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow and hypoxia in an animal stroke model

This study examined the relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow (99mTc-hexamethylpropylene amine oxime) and cerebral hypoxia (125I-iodoazomycin arabinoside) in a rat model of stroke. Middle cerebral artery occlusion in the rat was performed using an intraluminal suture approach. Diffusion, hypoxia, and blood flow maps were acquired 2 hr following occlusion, and were compared with T2 images and histology at 7 hr. Two hours following middle cerebral artery occlusion the lesion distributions from the diffusion maps and hypoxic autoradiographs were similar. The blood flow threshold for increased uptake of the hypoxic marker was approximately 34 +/- 7% of the normal flow. The combination of diffusion or hypoxic images with perfusion maps allowed differentiation between four regions: 1) normal tissue; 2) a region of decreased perfusion but normal diffusion and normal uptake of hypoxic marker; 3) a region of decreased perfusion, decreased diffusion and increased uptake of hypoxic marker; 4) a region of decreased perfusion, decreased diffusion and low uptake of hypoxic marker. The areas for increased uptake of hypoxic marker and decreased diffusion are equivalent, indicating similar blood flow thresholds. Regions of oligaemic misery perfusion, ischaemic misery perfusion and lesion core may be delineated with the combination of diffusion or hypoxic images and perfusion maps.

Early changes in water diffusion, perfusion, T1, and T2 during focal cerebral ischemia in the rat studied at 8.5 T

The time evolution of water diffusion, perfusion, T1, and T2 is investigated at high magnetic field (8.5 T) following permanent middle cerebral artery occlusion in the rat. Cerebral blood flow maps were obtained using arterial spin tagging. Although the quantitative perfusion measurements in ischemic tissue still pose difficulties, the combined perfusion and diffusion data nevertheless distinguish between a "moderately affected area," with reduced perfusion but normal diffusion; and a "severely affected area," in which both perfusion and diffusion are significantly reduced. Two novel magnetic resonance imaging observations are reported, namely, a decrease in T2 and an increase in T1, both within the first few minutes of ischemia. The rapid initial decrease in T2 is believed to be associated with an increase in deoxyhemoglobin levels, while the initial increase in T1 may be related to several factors, such as flow effects, an alteration in tissue oxygenation, and changes in water environment.

Rapid T2* mapping using interleaved echo planar imaging

Magnetic resonance imaging methods that are sensitive to T2* are widely used in the study of blood oxygenation changes, most notably in functional studies of the brain. In these studies the signal intensity change in T2*-weighted imaging is related to the coupling of cerebral blood flow and metabolism. Rapid measurement of T2* itself would offer a valuable method to quantify blood oxygenation changes indirectly and monitor their time course. An interleaved echoplanar imaging (EPI) sequence is presented here that allows maps of T2* to be generated in a few seconds. The sequence benefits from reduced geometric distortion and an improved point spread function compared with single-shot EPI. A comparison among a set of T2*-weighted interleaved EPI images, single-shot EPI, and conventional gradient-echo and spin-echo methods is made using a compartmentalized doped water phantom. The interleaved sequence yields accurate T2* values when compared with reference measurements made using the slower gradient-echo technique. Data acquired from the rat brain at 2.35 T prior to and during an anoxic challenge show, with high temporal resolution, the reduction in T2* associated with increased levels of deoxyhemoglobin.

A review of selected seafood poisonings

Seafood poisoning has been recognized as a problem in both coastal and inland populations for millennia. Many types of sea creatures from shellfish to the largest fish have been implicated. Severe cases of many different types of seafood poisonings can result in fatalities. While the pathophysiology of the toxins is well known in some cases, others, like ciguatera, remain somewhat confusing. As a result, the treatment of these conditions remains controversial, although supportive care continues to be the mainstay of therapy. In this manuscript, we review the pathophysiology, clinical presentation, and treatment of some of the most common and toxic varieties of seafood poisoning resulting from toxins.

Soft tissue injection of hydrocarbons: a case report and review of the literature

Hydrocarbon injection, while commonly seen in domestic accidents or attempted suicide, is not extensively addressed in the literature. This article comprises a review of the various complications of intravascular and soft tissue injection of petroleum distillates, and provides recommendations for patient management. An illustrative case involving the subcutaneous injection of dripless oil, a mixture of mineral oil, nonionic detergents, and petroleum naphtha, is presented.

Early postischemic dantrolene-induced amelioration of poly(ADP-ribose) polymerase-related bioenergetic failure in neonatal rat brain slices

In the infant brain, ischemia-induced ionic and enzyme mechanisms may independently lead to cell death by energy depletion: resequestration of calcium mobilized from intracellular stores consumes ATP, and activated poly(ADP-ribose) polymerase (PARP) uses oxidized nicotinamide adenine dinucleotide to form polyADP-ribosyl nuclear proteins associated with DNA damage. Using 31P nuclear magnetic resonance spectroscopy, we have monitored intracellular pH and cellular energy metabolites in ex vivo neonatal rat cerebral cortex before, during, and after substrate and oxygen deprivation. In an insult that exhibited secondary energy failure and apoptosis we identified a relative 25% augmentation of high-energy phosphates at the end of recovery when the ryanodine-receptor antagonist, dantrolene, was introduced in the early (0- to 40-minute) but not late (40- to 120-minute) stage of recovery (P < 0.05). In contrast to the absence of a late dantrolene-sensitive effect, inhibition of PARP with 3-methoxybenzamide was as effective (P < 0.05) as early dantrolene, even when introduced after a 40-minute delay. The dantrolene and 3-methoxybenzamide effects on high-energy phosphates were not additive, rather the early dantrolene-sensitive effect nullified the potential 3-methoxybenzamide effect. Therefore, in this vascular-independent neonatal preparation, postischemic mobilization of calcium from intracellular stores is associated with PARP-related energy depletion. Inhibition of either of these processes confers improved postischemic bioenergetic recovery in the developing brain.

An examination of serial urinalyses in patients with North American crotalid envenomation

STUDY OBJECTIVE

To examine the incidence of abnormal urinalyses after rattlesnake envenomations and its association with bite severity and antivenom administration.

METHODS

A retrospective review of data collected in a prospective manner for an experimental crotalid antivenom trial. Subjects were individuals with minimal to moderate North American crotalid envenomations. Incidence and characterization of abnormal urinalysis after crotalid envenomation is presented. Additionally, the relationship of abnormal urinalysis to bite severity is examined. A preliminary test of antivenom protein urinalysis interference was also conducted.

RESULTS

Forty-three percent of the urinalyses reported prior to antivenom treatment had abnormalities. Thirty-three of 41 subjects (80%) had an abnormal urinalysis, defined as the presence of cells, blood, glucose, or protein, at some time during the 2-week period following envenomation. All but 3 of these subjects had urinalyses which returned to normal by 2 weeks postenvenomation. Fifteen of 22 subjects (68%) with minimal envenomations had an abnormal urinalysis at some time following envenomation, while 18 of 19 subjects (95%) with moderate envenomations had abnormal urinalyses (p < 0.05). In addition, high concentrations of antivenom added to urine were found to produce a positive urine dipstick test for protein.

CONCLUSION

In our study of patients with minimal to moderate North American crotalid envenomations, there was a high incidence of abnormal urinalyses. The urine abnormalities tended to be more common with increased bite severity and more frequent during the first few hours following envenomation. Antivenom appearance in the urine could be responsible for some of our findings.

The dynamics of synchronized neurotransmitter release determined from compound spontaneous IPSCs in rat dentate granule neurones in vitro

1. The properties of GABAA receptor-mediated spontaneous IPSCs generated in hippocampal dentate granule neurones were analysed using whole-cell voltage-clamp techniques in order to explore the functional consequences of the low number (6-12) and close proximity of synaptic contacts made by single GABAergic interneurones. 2. Spontaneous IPSCs (sIPSCs) occurred with a frequency of 14.0 +/- 9.1 Hz (n = 31) and revealed a multi-modal positively skewed amplitude distribution (39.0 +/- 19.8 pA, median values). 3. The variance of 10-90% rise times and decay kinetics between IPSCs decreased with increasing peak amplitude. Larger amplitude events had significantly faster rise times, consistent with their site of generation being proximal to the soma. The decay kinetics of sIPSCs did not significantly change with amplitude. 4. Large amplitude sIPSCs occurred singularly or in discrete bursts, repeated regularly at low frequency. The rising phase of such sIPSCs were multi-phasic, composed of clear step-like inflections that were not a product of noise. The variability between the rising phase of individual sIPSCs was quantified by calculating their standard deviation, which produced fast rising (0.22 +/- 0.05 ms time to peak, n = 16) functions with half-widths of 0.38 +/- 0.10 ms, which declined to plateaux. 5. Computer simulations demonstrated that IPSCs with properties similar to those recorded experimentally could be generated by the linear summation of groups of temporally dispersed component events. Standard deviation functions of the rising phase of simulated IPSCs accurately described distributions of the temporal dispersion of unitary components. 6. The GABA uptake inhibitor (R)-N[4,4-bis(3-methyl-2-thienyl)but-3-enl-yl] nipecotic acid (tiagabine) (10 microM, n = 12) significantly prolonged the decay of mIPSCs (6.5 +/- 0.8 to 8.7 +/- 1.0 ms, median values) and sIPSCs (6.2 +/- 0.4 to 7.3 +/- 1.2 ms, median values), but failed to alter the frequency of occurrence, 10-90% rise times or peak amplitude of events. The application of flurazepam (30 microM, n = 7; 50 microM, n = 4) prolonged the decay of sIPSCs regardless of their amplitude. 7. These data indicate that sIPSCs are formed by the summation of unitary components that occur asynchronously and that GABA released from multiple sites has independent post-synaptic actions.

On the action of the anti-absence drug ethosuximide in the rat and cat thalamus

The action of ethosuximide (ETX) on Na+, K+, and Ca2+ currents and on tonic and burst-firing patterns was investigated in rat and cat thalamic neurons in vitro by using patch and sharp microelectrode recordings. In thalamocortical (TC) neurons of the rat dorsal lateral geniculate nucleus (LGN), ETX (0.75-1 mM) decreased the noninactivating Na+ current, INaP, by 60% but had no effect on the transient Na+ current. In TC neurons of the rat and cat LGN, the whole-cell transient outward current was not affected by ETX (up to 1 mM), but the sustained outward current was decreased by 39% at 20 mV in the presence of ETX (0.25-0.5 mM): this reduction was not observed in a low Ca2+ (0.5 mM) and high Mg2+ (8 mM) medium or in the presence of Ni2+ (1 mM) and Cd2+ (100 microM). In addition, ETX (up to 1 mM) had no effect on the low-threshold Ca2+ current, IT, of TC neurons of the rat ventrobasal (VB) thalamus and LGN and in neurons of the rat nucleus reticularis thalami nor on the high-threshold Ca2+ current in TC neurons of the rat LGN. Sharp microelectrode recordings in TC neurons of the rat and cat LGN and VB showed that ETX did not change the resting membrane potential but increased the apparent input resistance at potentials greater than -60 mV, resulting in an increase in tonic firing. In contrast, ETX decreased the number of action potentials in the burst evoked by a low-threshold Ca2+ potential. The frequency of the remaining action potentials in a burst also was decreased, whereas the latency of the first action potential was increased. Similar effects were observed on the burst firing evoked during intrinsic delta oscillations. These results indicate an action of ETX on INaP and on the Ca2+-activated K+ current, which explains the decrease in burst firing and the increase in tonic firing, and, together with the lack of action on low- and high-threshold Ca2+ currents, the results cast doubts on the hypothesis that a reduction of IT in thalamic neurons underlies the therapeutic action of this anti-absence medicine.

A second-generation screen of the human genome for susceptibility to insulin-dependent diabetes mellitus

During the past decade, the genetics of type 1 (insulin-dependent) diabetes mellitus (IDDM) has been studied extensively and the disorder has become a paradigm for genetically complex diseases. Previous genome screens and studies focused on candidate genes have provided evidence for genetic linkage between polymorphic DNA markers and 15 putative IDDM susceptibility loci, designated IDDM1-IDDM15. We have carried out a second-generation screen of the genome for linkage and analysed the data by multipoint linkage methods. An initial panel of 212 affected sibpairs (ASPs) was genotyped for 438 markers spanning all autosomes, and an additional 467 ASPs were used for follow-up genotyping. Other than the well-established linkage with the HLA region at chromosome 6p21.3, there was only one region, located on chromosome 1q and not previously reported, where the log likelihood ratio (lod) was greater than 3. Lods between 1.0 and 1.8 were found in six other regions, three of which have been reported in other studies. Another reported region, on chromosome 6q and loosely linked to HLA, also had an elevated lod. Little or no support was found for most reported IDDM loci (lods were less than 1), despite larger sample sizes in the present study.

Resource-use analysis of a medical toxicology consultation service

STUDY OBJECTIVE

To evaluate the impact of a medical toxicology consulting service (MTCS) on resource use and efficiency of care in patients hospitalized with a diagnosis of tricyclic antidepressant (TCA) poisoning.

METHODS

We conducted a retrospective, case-controlled medical-records review at two urban tertiary care teaching hospitals. The study population comprised patients who presented to the emergency department with a diagnosis of TCA poisoning, in two phases. The first phase was longitudinal; we evaluated cases over 4 years before and after inception of an MTCS at one institution. In the second phase we compared consecutive cases of TCA poisoning treated at two urban teaching hospitals located within a mile of each other with similar patient populations, one with and one without MTCS backup. Inclusion criteria consisted of complete medical records, ingestion of first- and second-generation TCAs, and age greater than 16 years. Patients were excluded if history, physical examination, or laboratory analysis suggested that multiple substances had been ingested. Extensive demographic data were collected in each case. Clinical information obtained from each patient included admission vital signs, pupil size, QRS and corrected QT duration, and the presence of markers of severe TCA toxicity such as hypotension, seizures, pulmonary edema, respiratory insufficiency necessitating intubation, and the occurrence of antimuscarinic signs and symptoms. Information regarding treatment was also collected from each case, including laboratory tests, decontamination procedures, administration of sodium bicarbonate, and use of other medications and therapies.

RESULTS

We identified a total of 88 patients in both phases of the study. Comparison groups were similar with respect to age, sex, presenting vital signs, presenting QRS and corrected QT duration, and incidence of recorded antimuscarinic signs, hypotension, seizures, and respiratory insufficiency requiring intubation. Total length of stay in a monitored hospital bed was also similar between groups. One fatality was recorded, but all other patients were discharged home or to a psychiatric facility. Patients seen by the MTCS consumed fewer health care resources in the form of less decontamination and fewer laboratory tests.

CONCLUSION

The MTCS may provide a resource-efficient means of treating patients with TCA poisoning. A larger, multicenter study of a variety of poisoned patients should be undertaken to further investigate this issue.

Clarithromycin induced digoxin toxicity

A case of digoxin poisoning following the co-administration of digoxin and clarithromycin in a 28 year old male is described. Since the aetiology of chronic digoxin poisoning is often unclear, clinicians should be aware of the potential drug-drug interaction between digoxin and clarithromycin.

Time course and magnitude of movement-related gating of tactile detection in humans. I. Importance of stimulus location

The time course and spatial extent of movement-related suppression of the detection of weak electrical stimuli (intensity, 90% detected at rest) was determined in 118 experiments carried out in 47 human subjects. Subjects were trained to perform a rapid abduction of the right index finger (D2) in response to a visual cue. Stimulus timing was calculated relative to the onset of movement and the onset of electromyographic (EMG) activity. Electrical stimulation was delivered to 10 different sites on the body, including sites on the limb performing the movement (D2, D5, hand, forearm and arm) as well as several distant sites (contralateral arm, ipsilateral leg). Detection of stimuli applied to the moving digit diminished significantly and in a time-dependent manner, with the first significant decrease occurring 120 ms before movement onset and 70 ms before the onset of EMG activity. Movement-related and time-dependent effects were obtained at all stimulation sites on the homolateral arm as well as the adjacent trunk. A pronounced spatiotemporal gradient was observed: the magnitude of the movement-related decrease in detectability was greatest and earliest at sites closest to the moving finger and progressively weaker and later at more proximal sites. When stimuli were applied to the distant sites, only a small (approximately 10%), non-time-dependent decrease was observed during movement trials. A simple model of perceptual performance adequately described the results, providing insight into the distribution of movement-related inhibitory controls within the CNS.

The 'window' component of the low threshold Ca2+ current produces input signal amplification and bistability in cat and rat thalamocortical neurones

1. The mechanism underlying a novel form of input signal amplification and bistability was investigated by intracellular recording in rat and cat thalamocortical (TC) neurones maintained in slices and by computer simulation with a biophysical model of these neurones. 2. In a narrow membrane potential range centred around -60 mV, TC neurones challenged with small (10-50 pA), short (50-200 ms) current steps produced a stereotyped, large amplitude hyperpolarization (> 20 mV) terminated by the burst firing of action potentials, leading to amplification of the duration and amplitude of the input signal, that is hereafter referred to as input signal amplification. 3. In the same voltage range centred around -60 mV, single evoked EPSPs and IPSPs also produced input signal amplification, indicating that this behaviour can be triggered by physiologically relevant stimuli. In addition, a novel, intrinsic, low frequency oscillation, characterized by a peculiar voltage dependence of its frequency and by the presence of plateau potentials on the falling phase of low threshold Ca2+ potentials, was recorded. 4. Blockade of pure Na+ and K+ currents by tetrodotoxin (1 microM) and Ba2+ (0.1-2.0 mM), respectively, did not affect input signal amplification, neither did the presence of excitatory or inhibitory amino acid receptor antagonists in the perfusion medium. 5. A decrease in [Ca2+]o (from 2 to 1 mM) and an increase in [Mg2+]o (from 2 to 10 mM), or the addition of Ni2+ (2-3 mM), abolished input signal amplification, while an increase in [Ca2+]o (from 2 to 8 mM) generated this behaviour in neurones where it was absent in control conditions. These results indicate the involvement of the low threshold Ca2+ current (IT) in input signal amplification, since the other Ca2+ currents of TC neurones are activated at potentials more positive than -40 mV. 6. Blockade of the slow inward mixed cationic current (Ih) by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium++ + chloride (ZD 7288)(100-300 microM) did not affect the expression of the large amplitude hyperpolarization, but abolished the subsequent repolarization to the original membrane potential. In this condition, therefore, input signal amplification was replaced by bistable membrane behaviour, where two stable membrane potentials separated by 15-30 mV could be switched between by small current steps. 7. Computer simulation with a model of a TC neurone, which contained only IT, Ih, K+ leak current (ILeak) and those currents responsible for action potentials, accurately reproduced the qualitative and quantitative properties of input signal amplification, bistability and low frequency oscillation, and indicated that these phenomena will occur at some value of the injected DC if, and only if, the 'window' component of IT (IT,Window) and the leak conductance (gLeak) satisfy the relation (dIT,Window/dV)max > gLeak. 8. The physiological implications of these findings for the electroresponsiveness of TC neurones are discussed, and, as IT is widely expressed in the central nervous system, we suggest that 'window' IT will markedly affect the integrative properties of many neurones.

Morphology and membrane properties of neurones in the cat ventrobasal thalamus in vitro

1. The morphological (n = 66) and electrophysiological (n = 41) properties of eighty-six thalamocortical (TC) neurones and those of one interneurone in the cat ventrobasal (VB) thalamus were examined using an in vitro slice preparation. The resting membrane potential for thirty-seven TC neurones was -61.9 +/- 0.7 mV, with thirteen neurones exhibiting delta oscillation with and without DC injection. 2. The voltage-current relationships of TC neurones were highly non-linear, with a mean peak input resistance of 254.4 M omega and a mean steady-state input resistance of 80.6 M omega between -60 and -75 mV. At potentials more positive than -60 mV, outward rectification led to a mean steady-state input resistance of 13.3 M omega. At potentials more negative than -75 mV, there was inward rectification, consisting of a fast component leading to a mean peak input resistance of 14.5 M omega, and a slow time-dependent component leading to a mean steady-state input resistance of 10.6 M omega. 3. Above -60 mV, three types of firing were exhibited by TC neurones. The first was an accelerating pattern associated with little spike broadening and a late component in the spike after-hyperpolarization. The second was an accommodating or intermittent pattern associated with spike broadening, while the third was a burst-suppressed pattern of firing also associated with spike broadening, but with broader spikes of a smaller amplitude. All TC neurones evoked high frequency (310-520 Hz) burst firing mediated by a low threshold Ca2+ potential. 4. Morphologically TC neurones were divided into two groups: Type I (n = 31 neurones) which had larger soma, dendritic arbors that occupied more space, thicker primary dendrites and daughter dendrites that followed a more direct course than Type II (n = 35). The only electrophysiological differences were that Type I neurones (n = 16) had smaller peak input and outward rectification resistance and spike after-hyperpolarization, but greater peak inward rectification resistance, and exhibited delta oscillation less often than Type II (n = 13). 5. The morphologically identified interneurone exhibited no outward rectification, only moderate inward rectification, and no high frequency firing associated with the offset of negative current steps below -55 mV. This interneurone had a regular accommodating firing pattern, but the spike after-hyperpolarization had a late component, unlike the accommodating firing in TC neurones. 6. Therefore, the differentiation of TC neuronal types in the cat VB thalamus based on their morphology was reflected by differences in peak input resistance, outward rectification and spike after-hyperpolarization, which could be accounted for by their difference in soma size. More importantly, the firing pattern of the majority of TC neurones in the cat VB thalamus were different from those of TC neurones in other sensory thalamic nuclei. 7. Thalamocortical neurones in the cat VB thalamus were also clearly distinguishable from the interneurone based on the presence of their prominent outward rectification, peak inward rectification and robust low threshold Ca2+ potentials.

On the nature of anomalous rectification in thalamocortical neurones of the cat ventrobasal thalamus in vitro

1. Intracellular sharp electrode current clamp and discontinuous single electrode voltage clamp recordings were made from thalamocortical neurones (n = 57) of the cat ventrobasal thalamus in order to investigate the mechanism underlying anomalous rectification. 2. Under current clamp conditions, voltage-current (V-I) relationships in a potential range of -55 to -110 mV demonstrated anomalous rectification with two components: fast rectification, which controlled the peak of negative voltage deviations, and time-dependent rectification. Time-dependent rectification was apparent as a depolarizing sag generated during the course of negative voltage deviations, was first formed at potentials in the range -60 to -70 mV, and was sensitive to 3 mM Cs+ (n = 6). Similarly, under voltage clamp conditions, instantaneous and steady-state I-V relationships demonstrated anomalous rectification. A slowly activating inward current with an activation threshold in the range of -65 to -70 mV formed time-dependent rectification. This current was sensitive to Cs+ (3 mM) (n = 3) and had properties similar to the slow inward mixed cationic current (Ih). 3. 4-(N-Ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium++ + chloride (ZD 7288) (100-300 microM) irreversibly blocked time-dependent rectification mediated by Ih (n = 23 of 25 neurones), and led to a hyperpolarization of the resting membrane potential (6.8 +/- 0.5 mV). In the presence of ZD 7288, V-I and I-V relationships, exhibited fast anomalous rectification, first activated from potential more negative than -80 mV. 4. Ba2+ (100 microM) (n = 8), in the continuous presence of ZD 7288, reversibly linearized peak V-I and instantaneous I-V relationships over a potential range of -70 to -120 mV, and led to a membrane depolarization (13.3 +/- 4.2 mV) or tonic inward current (192 +/- 36 pA). 5. The co-application of ZD 7288 and Ba2+ revealed a depolarizing sag in negative voltage deviations under current clamp conditions, or a large inward current with kinetics two to three times slower than those of Ih under voltage clamp conditions. This novel form of time-dependent rectification was first apparent at potentials more negative than about -85 mV, was sensitive to 5 mM Cs+ (n = 4), and is termed Ih,slow. Ih,slow tail currents reversed between -65.3 and -56.6 mV (with potassium acetate electrodes, n = 3) or -57.6 and -50.3 mV (with KCl electrodes, n = 3). 6. Computer simulations confirmed that the pattern of anomalous rectification in thalamocortical neurones of the cat ventrobasal thalamus is mediated by the concerted action of Ih and a Ba(2+)-sensitive current with properties similar to an inwardly rectifying K+ current (IKIR).

Effects of diffusion anisotropy on lesion delineation in a rat model of cerebral ischemia

The effects of white and gray matter diffusion anisotropy on ischemic lesion delineation have been studied in the rat model of middle cerebral artery occlusion. Apparent diffusion coefficient (ADC) maps obtained by conventional pulsed gradient spin echo diffusion-weighted imaging (PGSE-DWI) were compared with maps of the trace of the diffusion tensor in both normal and occluded animals. Diffusion tensor trace maps were derived from the average of the ADC maps from three separate experiments with diffusion weighting along three orthogonal axes, and also from a single-scan method. A marked degree of diffusion anisotropy was observed in both cortical gray matter and white matter from ADC maps of the control animals. In the occluded animals, the systematic effects of anisotropy on ADC and lesion area influenced the delineation of the ischemic territory in the PGSE-DWI ADC maps. However, the two trace methods eliminated these effects and gave consistent ischemic lesion depiction, despite the use of differing diffusion times in the two measurements.