Phase-contrast magnetic resonance imaging of intracranial and extracranial blood flow in carotid near-occlusion

PURPOSE

Compare extracranial internal carotid artery flow rates and intracranial collateral use between conventional ≥ 50% carotid stenosis and carotid near-occlusion, and between symptomatic and asymptomatic carotid near-occlusion.

METHODS

We included patients with ≥ 50% carotid stenosis. Degree of stenosis was diagnosed on CTA. Mean blood flow rates were assessed with four-dimensional phase-contrast MRI.

RESULTS

We included 110 patients of which 83% were symptomatic, and 38% had near-occlusion. Near-occlusions had lower mean internal carotid artery flow (70 ml/min) than conventional ≥ 50% stenoses (203 ml/min, P < .001). Definite use of ≥ 1 collateral was found in 83% (35/42) of near-occlusions and 10% (7/68) of conventional stenoses (P < .001). However, there were no differences in total cerebral blood flow (514 ml/min vs. 519 ml/min, P = .78) or ipsilateral hemispheric blood flow (234 vs. 227 ml/min, P = .52), between near-occlusions and conventional ≥ 50% stenoses, based on phase-contrast MRI flow rates. There were no differences in total cerebral or hemispheric blood flow, or collateral use, between symptomatic and asymptomatic near-occlusions.

CONCLUSION

Near-occlusions have lower internal carotid artery flow rates and more collateral use, but similar total cerebral blood flow and hemispheric blood flow, compared to conventional ≥ 50% carotid stenosis.

Diagnostic separation of conventional ⩾50% carotid stenosis and near-occlusion with phase-contrast MRI

INTRODUCTION

The aim of this study was to assess sensitivity, specificity and interrater reliability of phase-contrast MRI (PC-MRI) for diagnosing carotid near-occlusion.

PATIENTS AND METHODS

Prospective cross-sectional study conducted between 2018 and 2021. We included participants with suspected 50%-100% carotid stenosis on at least one side, all were examined with CT angiography (CTA) and PC-MRI and both ICAs were analyzed. Degree of stenosis on CTA was the reference test. PC-MRI-based blood flow rates in extracranial ICA and intracranial cerebral arteries were assessed. ICA-cerebral blood flow (CBF) ratio was defined as ICA divided by sum of both ICAs and Basilar artery.

RESULTS

We included 136 participants. The ICAs were 102 < 50% stenosis, 88 conventional ⩾50% stenosis (31 with ⩾70%), 49 near-occlusion, 12 occlusions, 20 unclear cause of small distal ICA on CTA and one excluded. For separation of near-occlusion and conventional stenoses, ICA flow rate and ICA-CBF ratio had the highest area under the curve (AUC; 0.98-0.99) for near-occlusion. ICA-CBF ratio ⩽0.225 was 90% (45/49) sensitive and 99% (188/190) specific for near-occlusion. Inter-rater reliability for this threshold was excellent (kappa 0.98). Specificity was 94% (29/31) for cases with ⩾70% stenosis. PC-MRI had modest performance for separating <50% and conventional ⩾50% stenosis (highest AUC 0.74), and eight (16%) of near-occlusions were not distinguishable from occlusion (no visible flow).

CONCLUSION

ICA-CBF ratio ⩽0.225 on PC-MRI is an accurate and reliable method to separate conventional ⩾50% stenosis and near-occlusion that is feasible for routine use. PC-MRI should be considered further as a potential standard method for near-occlusion detection, to be used side-by-side with established modalities as PC-MRI cannot separate other degrees of stenosis.

Prediction of cerebral perfusion pressure during carotid surgery - A computational fluid dynamics approach

BACKGROUND

Maintaining cerebral perfusion pressure in the brain when a carotid artery is closed during vascular surgery is critical for avoiding intraoperative hypoperfusion and risk of ischemic stroke. Here we propose and evaluate a method based on computational fluid dynamics for predicting patient-specific cerebral perfusion pressures at carotid clamping during carotid endarterectomy.

METHODS

The study consisted of 22 patients with symptomatic carotid stenosis who underwent carotid endarterectomy (73 ± 5 years, 59-80 years, 17 men). The geometry of the circle of Willis was obtained preoperatively from computed tomography angiography and corresponding flow rates from four-dimensional flow magnetic resonance imaging. The patients were also classified as having a present or absent ipsilateral posterior communicating artery based on computed tomography angiography. The predicted mean stump pressures from computational fluid dynamics were compared with intraoperatively measured stump pressures from carotid endarterectomy.

FINDINGS

On group level, there was no difference between the predicted and measured stump pressures (-0.5 ± 13 mmHg, P = 0.86) and the pressures were correlated (r = 0.44, P = 0.039). Omitting two outliers, the correlation increased to r = 0.78 (P < 0.001) (-1.4 ± 8.0 mmHg, P = 0.45). Patients with a present ipsilateral posterior communicating artery (n = 8) had a higher measured stump pressure than those with an absent artery (n = 12) (P < 0.001).

INTERPRETATION

The stump pressure agreement indicates that the computational fluid dynamics approach was promising in predicting cerebral perfusion pressures during carotid clamping, which may prove useful in the preoperative planning of vascular interventions.

Middle cerebral artery pressure laterality in patients with symptomatic ICA stenosis

An internal carotid artery (ICA) stenosis can potentially decrease the perfusion pressure to the brain. In this study, computational fluid dynamics (CFD) was used to study if there was a hemispheric pressure laterality between the contra- and ipsilateral middle cerebral artery (MCA) in patients with a symptomatic ICA stenosis. We further investigated if this MCA pressure laterality (ΔP_MCA) was related to the hemispheric flow laterality (ΔQ) in the anterior circulation, i.e., ICA, proximal MCA and the proximal anterior cerebral artery (ACA). Twenty-eight patients (73±6 years, range 59–80 years, 21 men) with symptomatic ICA stenosis were included. Flow rates were measured using 4D flow MRI data (PC-VIPR) and vessel geometries were obtained from computed tomography angiography. The ΔP_MCA was calculated from CFD, where patient-specific flow rates were applied at all input- and output boundaries. The ΔP_MCA between the contra- and ipsilateral side was 6.4±8.3 mmHg (p<0.001) (median 3.9 mmHg, range -1.3 to 31.9 mmHg). There was a linear correlation between the ΔP_MCA and ΔQ_ICA (r = 0.85, p<0.001) and ΔQ_ACA (r = 0.71, p<0.001), respectively. The correlation to ΔQ_MCA was weaker (r = 0.47, p = 0.011). In conclusion, the MCA pressure laterality obtained with CFD, is a promising physiological biomarker that can grade the hemodynamic disturbance in patients with a symptomatic ICA stenosis.

Characterizing pulsatility in distal cerebral arteries using 4D flow MRI

Recent reports have suggested that age-related arterial stiffening and excessive cerebral arterial pulsatility cause blood-brain barrier breakdown, brain atrophy and cognitive decline. This has spurred interest in developing non-invasive methods to measure pulsatility in distal vessels, closer to the cerebral microcirculation. Here, we report a method based on four-dimensional (4D) flow MRI to estimate a global composite flow waveform of distal cerebral arteries. The method is based on finding and sampling arterial waveforms from thousands of cross sections in numerous small vessels of the brain, originating from cerebral cortical arteries. We demonstrate agreement with internal and external reference methods and show the ability to capture significant increases in distal cerebral arterial pulsatility as a function of age. The proposed approach can be used to advance our understanding regarding excessive arterial pulsatility as a potential trigger of cognitive decline and dementia.

Assessment of Cerebral Blood Flow Pulsatility and Cerebral Arterial Compliance With 4D Flow MRI

BACKGROUND

Four-dimensional flow magnetic resonance imaging (4D flow MRI) enables efficient investigation of cerebral blood flow pulsatility in the cerebral arteries. This is important for exploring hemodynamic mechanisms behind vascular diseases associated with arterial pulsations.

PURPOSE

To investigate the feasibility of pulsatility assessments with 4D flow MRI, its agreement with reference two-dimensional phase-contrast MRI (2D PC-MRI) measurements, and to demonstrate how 4D flow MRI can be used to assess cerebral arterial compliance and cerebrovascular resistance in major cerebral arteries.

STUDY TYPE

Prospective.

SUBJECTS

Thirty-five subjects (20 women, 79 ± 5 years, range 70-91 years).

FIELD STRENGTH/SEQUENCE

4D flow MRI (PC-VIPR) and 2D PC-MRI acquired with a 3T scanner.

ASSESSMENT

Time-resolved flow was assessed in nine cerebral arteries. From the pulsatile flow waveform in each artery, amplitude (ΔQ), volume load (ΔV), and pulsatility index (PI) were calculated. To reduce high-frequency noise in the 4D flow MRI data, the flow waveforms were low-pass filtered. From the total cerebral blood flow, total PI (PI_tot ), total volume load (ΔV_tot ), cerebral arterial compliance (C), and cerebrovascular resistance (R) were calculated.

STATISTICAL TESTS

Two-tailed paired t-test, intraclass correlation (ICC).

RESULTS

There was no difference in ΔQ between 4D flow MRI and the reference (0.00 ± 0.022 ml/s, mean ± SEM, P = 0.97, ICC = 0.95, n = 310) with a cutoff frequency of 1.9 Hz and 15 cut plane long arterial segments. For ΔV, the difference was -0.006 ± 0.003 ml (mean ± SEM, P = 0.07, ICC = 0.93, n = 310) without filtering. Total R was 11.4 ± 2.41 mmHg/(ml/s) (mean ± SD) and C was 0.021 ± 0.009 ml/mmHg (mean ± SD). ΔV_tot was 1.21 ± 0.29 ml (mean ± SD) with an ICC of 0.82 compared with the reference. PI_tot was 1.08 ± 0.21 (mean ± SD).

DATA CONCLUSION

We successfully assessed 4D flow MRI cerebral arterial pulsatility, cerebral arterial compliance, and cerebrovascular resistance. Averaging of multiple cut planes and low-pass filtering was necessary to assess accurate peak-to-peak features in the flow rate waveforms.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2020;51:1516-1525.

Accuracy of blood flow assessment in cerebral arteries with 4D flow MRI: Evaluation with three segmentation methods

Background

Accelerated 4D flow MRI allows for high‐resolution velocity measurements with whole‐brain coverage. Such scans are increasingly used to calculate flow rates of individual arteries in the vascular tree, but detailed information about the accuracy and precision in relation to different postprocessing options is lacking.

Purpose

To evaluate and optimize three proposed segmentation methods and determine the accuracy of in vivo 4D flow MRI blood flow rate assessments in major cerebral arteries, with high‐resolution 2D PCMRI as a reference.

Study Type

Prospective.

Subjects

Thirty‐five subjects (20 women, 79 ± 5 years, range 70–91 years).

Field Strength/Sequence

4D flow MRI with PC‐VIPR and 2D PCMRI acquired with a 3 T scanner.

Assessment

We compared blood flow rates measured with 4D flow MRI, to the reference, in nine main cerebral arteries. Lumen segmentation in the 4D flow MRI was performed with k‐means clustering using four different input datasets, and with two types of thresholding methods. The threshold was defined as a percentage of the maximum intensity value in the complex difference image. Local and global thresholding approaches were used, with evaluated thresholds from 6–26%.

Statistical Tests

Paired t‐test, F‐test, linear correlation (P < 0.05 was considered significant) along with intraclass correlation (ICC).

Results

With the thresholding methods, the lowest average flow difference was obtained for 20% local (0.02 ± 15.0 ml/min, ICC = 0.97, n = 310) or 10% global (0.08 ± 17.3 ml/min, ICC = 0.97, n = 310) thresholding with a significant lower standard deviation for local (F‐test, P = 0.01). For all clustering methods, we found a large systematic underestimation of flow compared with 2D PCMRI (16.1–22.3 ml/min).

Data Conclusion

A locally adapted threshold value gives a more stable result compared with a globally fixed threshold. 4D flow with the proposed segmentation method has the potential to become a useful reliable clinical tool for assessment of blood flow in the major cerebral arteries.

Level of Evidence: 2

Technical Efficacy: Stage 2

J. Magn. Reson. Imaging 2019;50:511–518.

A 6-hour working day--effects on health and well-being

The effect of the total amount of work hours and the benefits of a shortening is frequently debated, but very little data is available. The present study compared a group (N = 41) that obtained a 9 h reduction of the working week (to a 6 h day) with a comparison group (N = 22) that retained normal work hours. Both groups were constituted of mainly female health care and day care nursery personnel. The experimental group retained full pay and extra personnel were employed to compensate for loss of hours. Questionnaire data were obtained before and 1 year after the change. The data were analyzed using a two-factor ANOVA with the interaction term year*group as the main focus. The results showed a significant interaction of year*group for social factors, sleep quality, mental fatigue, and heart/respiratory complaints, and attitude to work hours. In all cases the experimental group improved whereas the control group did not change. It was concluded that shortened work hours have clear social effects and moderate effects on well-being.

El Niño effects on the dynamics of terrestrial ecosystems

New studies are showing that the El Niño Southern Oscillation (ENSO) has major implications for the functioning of different ecosystems, ranging from deserts to tropical rain forests. ENSO-induced pulses of enhanced plant productivity can cascade upward through the food web invoking unforeseen feedbacks, and can cause open dryland ecosystems to shift to permanent woodlands. These insights suggest that the predicted change in extreme climatic events resulting from global warming could profoundly alter biodiversity and ecosystem functioning in many regions of the world. Our increasing ability to predict El Niño effects can be used to enhance management strategies for the restoration of degraded ecosystems.

Three distinct and sequential steps in the release of sodium ions by the Na+/K+-ATPase

The Na+/K+ pump, a P-type ion-motive ATPase, exports three sodium ions and then imports two potassium ions in each transport cycle. Ions on one side of the membrane bind to sites within the protein and become temporarily occluded (trapped within the protein) before being released to the other side, but details of these occlusion and de-occlusion transitions remain obscure for all P-type ATPases. If it is deprived of potassium ions, the Na+/K+ pump is restricted to sodium translocation steps, at least one involving charge movement through the membrane's electric fields. Changes in membrane potential alter the rate of such electrogenic reactions and so shift the distribution of enzyme conformations. Here we use high-speed voltage jumps to initiate this redistribution and show that the resulting pre-steady-state charge movements relax in three identifiable phases, apparently reflecting de-occlusion and release of the three sodium ions. Reciprocal relationships among the sizes of these three charge components show that the three sodium ions are de-occluded and released to the extracellular solution one at a time, in a strict order.

Blocker protection in the pore of a voltage-gated K+ channel and its structural implications

The structure of the bacterial potassium channel KcsA has provided a framework for understanding the related voltage-gated potassium channels (Kv channels) that are used for signalling in neurons. Opening and closing of these Kv channels (gating) occurs at the intracellular entrance to the pore, and this is also the site at which many open channel blockers affect Kv channels. To learn more about the sites of blocker binding and about the structure of the open Kv channel, we investigated here the ability of blockers to protect against chemical modification of cysteines introduced at sites in transmembrane segment S6, which contributes to the intracellular entrance. Within the intracellular half of S6 we found an abrupt cessation of protection for both large and small blockers that is inconsistent with the narrow 'inner pore' seen in the KcsA structure. These and other results are most readily explained by supposing that the structure of Kv channels differs from that of the non-voltage-gated bacterial channel by the introduction of a sharp bend in the inner (S6) helices. This bend would occur at a Pro-X-Pro sequence that is highly conserved in Kv channels, near the site of activation gating.

The activation gate of a voltage-gated K+ channel can be trapped in the open state by an intersubunit metal bridge

Voltage-activated K+ channels are integral membrane proteins containing a potassium-selective transmembrane pore gated by changes in the membrane potential. This activation gating (opening) occurs in milliseconds and involves a gate at the cytoplasmic side of the pore. We found that substituting cysteine at a particular position in the last transmembrane region (S6) of the homotetrameric Shaker K+ channel creates metal binding sites at which Cd2+ ions can bind with high affinity. The bound Cd2+ ions form a bridge between the introduced cysteine in one channel subunit and a native histidine in another subunit, and the bridge traps the gate in the open state. These results suggest that gating involves a rearrangement of the intersubunit contacts at the intracellular end of S6. The recently solved structure of a bacterial K+ channel shows that the S6 homologs cross in a bundle, leaving an aperture at the bundle crossing. In the context of this structure, the metal ions form a bridge between a cysteine above the bundle crossing and a histidine below the bundle crossing in a neighboring subunit. Our results suggest that gating occurs at the bundle crossing, possibly through a change in the conformation of the bundle itself.

Antibody-coated sheep erythrocytes suppress the ability of polyclonal B-cell activators to induce plaque-forming cells against sheep erythrocytes

The primary immune response to untreated sheep erythrocytes (SRBC) in vitro was suppressed by the addition of antibody-coated SRBC. A mixture of SRBC and antibody-coated SRBC also suppressed the induction of anti-SRBC plaque-forming cells by the polyclonal B cell activators lipopolysaccharide, purified protein derivative of tuberculin, and native dextran. Injection of a mixture of SRBC and antibody-coated SRBC into mice led to an increased response to SRBC. It seems plausible from the in vitro findings that the Fc part of antibodies complexed to an antigen can exert a negative signal on antigen-specific B cells that cannot be overcome by positive signals delivered by polyclonal B cell activators.

Gated access to the pore of a voltage-dependent K+ channel

Voltage-activated K+ channels are integral membrane proteins that open or close a K(+)-selective pore in response to changes in transmembrane voltage. Although the S4 region of these channels has been implicated as the voltage sensor, little is known about how opening and closing of the pore is accomplished. We explored the gating process by introducing cysteines at various positions thought to lie in or near the pore of the Shaker K+ channel, and by testing their ability to be chemically modified. We found a series of positions in the S6 transmembrane region that react rapidly with water-soluble thiol reagents in the open state but not the closed state. An open-channel blocker can protect several of these cysteines, showing that they lie in the ion-conducting pore. At two of these sites, Cd2+ ions bind to the cysteines without affecting the energetics of gating; at a third site, Cd2+ binding holds the channel open. The results suggest that these channels open and close by the movement of an intracellular gate, distinct from the selectivity filter, that regulates access to the pore.

Trapping of organic blockers by closing of voltage-dependent K+ channels: evidence for a trap door mechanism of activation gating

Small organic molecules, like quaternary ammonium compounds, have long been used to probe both the permeation and gating of voltage-dependent K+ channels. For most K+ channels, intracellularly applied quaternary ammonium (QA) compounds such as tetraethylammonium (TEA) and decyltriethylammonium (C10) behave primarily as open channel blockers: they can enter the channel only when it is open, and they must dissociate before the channel can close. In some cases, it is possible to force the channel to close with a QA blocker still bound, with the result that the blocker is "trapped." Armstrong (J. Gen. Physiol. 58:413-437) found that at very negative voltages, squid axon K+ channels exhibited a slow phase of recovery from QA blockade consistent with such trapping. In our studies on the cloned Shaker channel, we find that wild-type channels can trap neither TEA nor C10, but channels with a point mutation in S6 can trap either compound very efficiently. The trapping occurs with very little change in the energetics of channel gating, suggesting that in these channels the gate may function as a trap door or hinged lid that occludes access from the intracellular solution to the blocker site and to the narrow ion-selective pore.

N-type inactivation and the S4-S5 region of the Shaker K+ channel

The intracellular segment of the Shaker K+ channel between transmembrane domains S4 and S5 has been proposed to form at least part of the receptor for the tethered N-type inactivation "ball." We used the approach of cysteine substitution mutagenesis and chemical modification to test the importance of this region in N-type inactivation. We studied N-type inactivation or the block by a soluble inactivation peptide ("ball peptide") before and after chemical modification by methanethiosulfonate reagents. Particularly at position 391, chemical modification altered specifically the kinetics of ball peptide binding without altering other biophysical properties of the channel. Results with reagents that attach different charged groups at 391 C suggested that there are both electrostatic and steric interactions between this site and the ball peptide. These findings identify this site to be in or near the receptor site for the inactivation ball. At many of the other positions studied, modification noticeably inhibited channel current. The accessible cysteines varied in the state-dependence of their modification, with five- to tenfold changes in reactions rate depending on the gating state of the channel.

On the use of thiol-modifying agents to determine channel topology

A powerful tool in the study of cloned ion channels is the combined use of site-directed mutagenesis and chemical modification. Site-directed mutagenesis is used to introduce new cysteine residues at specific positions in a channel protein, and chemical modification by thiol-specific reagents is then used to assess the exposure of the introduced cysteins. This method has been used to assess secondary structure, membrane topology and conformational changes. We report that one commonly used, charged reagent (MTSEA; aminoethyl methanethiosulfonate) can cross the membrane quite readily. We also find that other reagents that are quite membrane-impermeant can cross the membrane when patches are electrically leaky. Both of these undesired effects can be controlled by the use of a thiol scavenger. These findings argue for caution in the use of modifying reagents to determine the membrane topology of channels and other membrane proteins.

Pre-steady-state transient currents mediated by the Na/K pump in internally perfused Xenopus oocytes

Pre-steady-state transient currents have been investigated in the vegetal pole of Xenopus oocytes using the open-oocyte vaseline-gap technique of Taglialatela, Toro, and Stefani (Biophysical Journal. 61:78-82, 1992). Voltage pulses 40 ms in duration were made from a holding potential of -40 mV to command potentials over the range -160 to +60 mV in increments of 20 mV. Current records (averaged 20X; sampled every 200 microseconds) in the presence of dihydroouabain (DHO) or absence of external Na+ (Nao) were subtracted from current records obtained under Na/Na exchange conditions, i.e. internally perfused with 50 mM Na+, 5 mM ATP, and 5 mM ADP (K(+)-free) and externally superfused with 100 mM Na+,K(+)-free solution. Transient currents were dependent on intracellular Na+ and nucleotides, and diminished by activation of forward pumping; they were also reduced by 10 micrograms ml-1 of oligomycin B applied to the external solution. These properties of the pre-steady state currents are consistent with the Na/K pump operating in its electroneutral Na/Na exchange mode. The voltage dependence of the DHO- and Nao-sensitive transient currents was analyzed using a pseudo two-state model in which only the rate coefficient for Nao-binding/reocclusion is voltage-dependent (Rakowski, R. F. 1993. J. Gen. Physiol. 101:117-144). The apparent valence of the charge moved during the on (zq-on) and off (zq-off) of the pulse were 0.96 +/- 0.05 and 0.95 +/- 0.05 for Nao-sensitive, and 1.10 +/- 0.07 and 0.85 +/- 0.06 for DHO-sensitive transient currents, respectively. The total amount of charge moved (Qtot) and the mid-point voltage of the charge distribution (Vq) were 230 +/- 15 pC and -56.2 +/- 5.1 mV, and 268 +/- 34 pC and -67.0 +/- 7.6 mV for Nao- and DHO-sensitive transient currents, respectively. The apparent valence (zk) and the voltage at which the forward and backward rates are equal (Vk) obtained from the relaxation rates were 0.80 +/- 0.05 and -129.3 +/- 10.0 mV, and 0.86 +/- 0.10 and -135.1 +/- 9.0 mV for the Nao- and DHO-sensitive pre-steady state currents, respectively. The values of the parameters were not statistically significantly different between the Nao- and DHO-sensitive transient currents. Excluding the first 600 microseconds after the onset of a voltage step which was not temporally resolved, transient currents showed no indication of a rising phase. These results support the idea that charge translocation occurs within an external access channel at a rate that is governed by a voltage-dependent binding/reocclusion process and a voltage-independent deocclusion/unbinding process.

External Ca effect on water permeability, regulatory volume decrease, and extracellular space in barnacle muscle cells

The effect of extracellular Ca2+ (Cao) on sarcolemmal hydraulic water permeability (L'p), regulatory volume decrease (RVD), and extracellular space (ECS) was studied in barnacle muscle cells. Absence or presence of Cao had no effect on L'p [0 Cao = 2.762 +/- 0.098 x 10(-5), and 11 mM Cao = 2.720 +/- 0.222 x 10(-5) cm.kg.s-1 x osmol x 1-kgH2O-1]. Likewise, cells exposed to anisosmotic media (for < 30 min) behaved as osmometers in 0 and 11 mM Cao, showing similar slopes and intercepts in van't Hoff plots. At longer incubation times, however, hyposmotic conditions promoted a Cao-dependent RVD. The relationship between Cao and the percentage of cells responding with RVD to a hyposmotic challenge was sigmoidal (half-maximal Cao = 4.83 mM). The mean rate of RVD (40 nl/min) was independent of the level of swelling in response to hyposmotic challenges. However, the magnitude of RVD increased with larger hyposmotic challenges. Both the presence of Cao and hypotonicity reduced the "apparent" ECS by 47 +/- 6 and 39 +/- 6%, respectively. Three-dimensional reconstruction of autoradiographs of the cells was made to interpret these results.

Serial dependency in the discharge pattern of dorsal spinocerebellar tract neurons: a computer simulation analysis

We have developed a model in order to analyze the factors eventually responsible for the strong negative serial dependency between successive interspike intervals in the discharge of the Dorsal Spinocerebellar Tract (DSCT) neurons. This dependency is reflected, phenomenologically, by short intervals followed by long ones and, quantitatively, by the first order correlation coefficient (R1-2); which can be lower than -.6 (Jansen, Nicolaysen & Rudjord, 1966; Kröller and Grüsser, 1982). We have found that the lowest values of R1-2 are always related with model parameter values which were very similar to those obtained experimentally. It was observed that EPSP amplitude distribution plays an important role in the discharge patterns of the DSCT neurons. There is one fiber that elicits EPSPs greater than 6 mV, which is responsible for the genesis of the short intervals in the discharge. Long intervals are determined basically by a suprathreshold depolarization and the afterhyperpolarization processes.

Characterization of the antinociceptive effects of some adenosine analogues in the rat

The antinociceptive effects of the stable adenosine analogues N6-phenylisopropyladenosine (L-PIA), N6-cyclohexyladenosine (CHA) and 5'-N-ethylcarboxamidoadenosine (NECA) were investigated in conscious rats using cutaneous thermal tests (hot plate and tail flick). Subcutaneous administration of the adenosine analogues induced a dose-dependent antinociceptive response for all agents. However, NECA was approximately 15 times more potent than PIA and CHA. Approximately the same potency order and response was seen when the adenosine analogues were administered intrathecally at the lumbar level. By this route of administration, the adenosine analogues were approximately 10-20 times more potent than after S.C. administration. Intracerebroventricular administration (lateral ventricles), however, induced a variable response, in most cases a slight hyperalgesia. The nonspecific adenosine antagonist theophylline (S.C.) rapidly reduced the antinociceptive effect induced by PIA (S.C.) but enprofylline, a bronchodilating xanthine with low ability to antagonize adenosine did not influence PIA-induced antinociception. It is concluded that stable adenosine analogues and presumably adenosine itself have potent antinociceptive effects via specific adenosine receptors in the rat. The effects seem to be mediated mainly by a spinal mechanism of action.

Adrenal catecholamines during and following hypoxia in neonatal rats

The synthesis and content of adrenal catecholamines during hypoxia (6% O2) and the following recovery period were investigated in 4-day old rats. Adrenal catecholamine synthesis, as indicated by the dopamine content of the glands, increased significantly during hypoxia. During hypoxia a significant decrease in the adrenal noradrenaline but not adrenaline content was noted. In the recovery period following 30 min of hypoxia the noradrenaline level was rapidly restored to the control value. The results show that hypoxia causes increased catecholamine synthesis in the immature rat adrenal gland, presumably mediated by an increased tyrosine hydroxylase activity induced via the splanchnic nerves. In the neonate rat hypoxia seems to stimulate the release of a larger proportion of noradrenaline than adrenaline from the adrenal glands when compared to the adult animal.