Non-neural Effects Research Articles

Limitations of ex vivo measurements for in vivo neuroscience

A long history of postmortem studies has provided significant insight into human brain structure and organization. Cadavers have also proven instrumental for the measurement of artifacts and nonneural effects in functional imaging, and more recently, the study of biophysical properties critical to brain stimulation. However, death produces significant changes in the biophysical properties of brain tissues, making an ex vivo to in vivo comparison complex, and even questionable. This study directly compares biophysical properties of electric fields arising from transcranial electric stimulation (TES) in a nonhuman primate brain pre- and postmortem. We show that pre- vs. postmortem, TES-induced intracranial electric fields differ significantly in both strength and frequency response dynamics, even while controlling for confounding factors such as body temperature. Our results clearly indicate that ex vivo cadaver and in vivo measurements are not easily equitable. In vivo examinations remain essential to establishing an adequate understanding of even basic biophysical phenomena in vivo.

Time- and task-dependent non-neural effects of real and sham TMS.

Transcranial magnetic stimulation (TMS) is widely used in experimental brain research to manipulate brain activity in humans. Next to the intended neural effects, every TMS pulse produces a distinct clicking sound and sensation on the head which can also influence task performance. This necessitates careful consideration of control conditions in order to ensure that behavioral effects of interest can be attributed to the neural consequences of TMS and not to non-neural effects of a TMS pulse. Surprisingly, even though these non-neural effects of TMS are largely unknown, they are often assumed to be unspecific, i.e. not dependent on TMS parameters. This assumption is inherent to many control strategies in TMS research but has recently been challenged on empirical grounds. Here, we further develop the empirical basis of control strategies in TMS research. We investigated the time-dependence and task-dependence of the non-neural effects of TMS and compared real and sham TMS over vertex. Critically, we show that non-neural TMS effects depend on a complex interplay of these factors. Although TMS had no direct neural effects, both pre- and post-stimulus TMS time windows modulated task performance on both a sensory detection task and a cognitive angle judgment task. For the most part, these effects were quantitatively similar across tasks but effect sizes were clearly different. Moreover, the effects of real and sham TMS were almost identical with interesting exceptions that shed light on the relative contribution of auditory and somato-sensory aspects of a TMS pulse. Knowledge of such effects is of critical importance for the interpretation of TMS experiments and helps deciding what constitutes an appropriate control condition. Our results broaden the empirical basis of control strategies in TMS research and point at potential pitfalls that should be avoided.

Pre-Stimulus Sham TMS Facilitates Target Detection

Transcranial magnetic stimulation (TMS) allows non-invasive manipulation of brain activity during active task performance. Because every TMS pulse is accompanied by non-neural effects such as a clicking sound and somato-sensation on the head, control conditions are required to ensure that changes in task behavior are indeed due to the induced neural effects. However, the non-neural effects of TMS in the context of a given task performance are largely unknown and, consequently, it is unclear what constitutes a valid control condition. We explored the non-neural effects of TMS on visual target detection. Participants received single pulse sham TMS to each hemisphere at different time points prior to target appearance during a visual target detection task. It was hypothesized that the clicking sound of a sham TMS pulse differentially affects performance depending on the location of the coil and the timing of the pulse.Our results show that, first, sham TMS caused a facilitation of reaction times when preceding the target stimulus by 150, 200, and 250 ms, whereas earlier and later time windows were not effective. Second, positioning the TMS coil ipsilateral instead of contralateral relative to the target stimulus improved reaction times. Third, infrequent noTMS trials that were interleaved with sham TMS trials had oddball-like properties resulting in increased reaction times during noTMS. The clicking sound produced by sham TMS influences task performance in multiple ways. These non-neural effects of TMS need to be controlled for in TMS research and the present findings provide an empirical basis for deciding what constitutes a valid control condition.

INTRODUCTION

Functional subtyping of the receptors responsible for the neural and nonneural effects of substance P (SP) on the canine proximal colon were studied. Selective agonists and antagonists were used with two different in vitro preparations. The mucosa contained the muscularis mucosa and attendant submucosal plexuses, whereas the epithelium was devoid of both. We obtained the following results: \[Sar9,Met(O2)11\]SP (Sar9SP), a neurokinin-1 receptor agonist, stimulated both preparations; senktide, a neurokinin-3 agonist, evoked a response only on the mucosal preparation; \[beta-Ala8\]NKA4-10, a neurokinin-2 agonist, was ineffective on both preparations; tetrodotoxin completely inhibited the mucosal responses to senktide, but the inhibitory effects on Sar9SP were only partial; CP-96,345, a neurokinin-1-selective antagonist, inhibited both epithelial and mucosal responses to Sar9SP and R487 \[Trp7,beta-Ala8\]NKA4-10, a neurokinin-3 antagonist, inhibited mucosal responses to senktide. Thus, the neural effects involved both neurokinin-1 and neurokinin-3 receptors, whereas the nonneural effects were mediated by neurokinin-1 receptors alone. Neurokinin-2 receptors were functionally absent.

Could n-3 polyunsaturated fatty acids reduce pathological pain by direct actions on the nervous system?

The intake of n-3 polyunsaturated fatty acids (PUFAs) in many industrialized countries is relatively low and its increased consumption has protective and modifying effects on such diverse conditions as atherosclerosis, ventricular arrhythmias, multiple sclerosis, major depression and inflammatory and autoimmune diseases. In addition, n-3 PUFAs have been shown to alleviate pain in patients with rheumatoid arthritis, inflammatory bowel disease and in a number of other painful conditions. This has been attributed to the inhibition of pro-inflammatory eicosanoid and cytokine production by peripheral tissues. n-3 PUFAs have also been shown to inhibit eicosanoid production in glial cells, block voltage-gated sodium channels (VGSCs), inhibit neuronal protein kinases and modulate gene expression. They also appear to have mood-stabilizing and sympatholytic effects. The present article explores the possibility that, based on what is known about their neural and non-neural effects, n-3 PUFAs directly attenuate the neuronal and glial processes that underlie neuropathic and inflammatory pain.

Quantification of intensity variations in functional MR images using rotated principal components

In functional MRI (fMRI), the changes in cerebral haemodynamics related to stimulated neural brain activity are measured using standard clinical MR equipment. Small intensity variations in fMRI data have to be detected and distinguished from non-neural effects by careful image analysis. Based on multivariate statistics we describe an algorithm involving oblique rotation of the most significant principal components for an estimation of the temporal and spatial distribution of the stimulated neural activity over the whole image matrix. This algorithm takes advantage of strong local signal variations. A mathematical phantom was designed to generate simulated data for the evaluation of the method. In simulation experiments, the potential of the method to quantify small intensity changes, especially when processing data sets containing multiple sources of signal variations, was demonstrated. In vivo fMRI data collected in both visual and motor stimulation experiments were analysed, showing a proper location of the activated cortical regions within well known neural centres and an accurate extraction of the activation time profile. The suggested method yields accurate absolute quantification of in vivo brain activity without the need of extensive prior knowledge and user interaction.

Brevetoxin-3 (PbTx-3) inhibits oxygen consumption and increases Na + content in mouse liver slices through a tetrodotoxin-sensitive pathway

F. A. Rodriguez, N. Escobales and C. Maldonado. Brevetoxin-3 (PbTx-3) inhibits oxygen consumption and increases Na + content in mouse liver slices through a tetrodotoxin-sensitive pathway. Toxicon 32, 1385–1395, 1994.—To have insights into possible non-neural effects of PbTx-3 (0.07 μg/ml), its effects on various parameters of hepatic metabolism were evaluated. PbTx-3 inhibits oxygen consumption (QO 2) in liver slices by 25 ± 1%, and increases hepatocyte Na +/K + ratio by 72 ± 4%. Ouabain, a Na +-K +-pump inhibitor, also reduced QO 2 by 19 ± 2% and raised the Na +/K + ratio by 77 ± 4%. The effects of PbTx-3 and ouabain on QO 2 and Na +/K + ratio were not additive, suggesting a role for the Na +-K +-pump in these actions of PbTx-3. However, Na +-K +-pump activity determinations, using 86Rb as a K + tracer, did not reveal inhibitory effects of this toxin on the transport system. This indicates that the pump is not a target of PbTx-3. The effect of PbTx-3 on liver slices Na + content was abolished by the sodium channel blocker tetrodotoxin (0.1 μM). Tetrodotoxin also antagonized the inhibition of oxygen consumption by PbTx-3. These results suggest that in the liver PbTx-3 can induce effects that appear to be similar to those observed in excitable tissue.

Peripheral-type benzodiazepine receptors

Since their first description as anomalous high affinity diazepam binding sites in rat peripheral tissues, the peripheral-type benzodiazepine receptor (PBR) has been increasingly studied to better understand nonneural effects of the benzodiazepines. The mammalian PBR is ubiquitously distributed with high concentrations in the outer mitochondrial membrane of secretory tissues. In regions of the brain, the density of PBR can equal or exceed the density of central-type benzodiazepine receptors. High affinity PK 11195 binding is diagnostic for the receptor while the affinity for benzodiazepines is species dependent. Recent cDNA cloning of a PBR component, the isoquinoline binding protein (IBP), shows no apparent sequence homology with any GABA A receptor subunits known to comprise central benzodiazepine receptor subtypes. The PBR seems at best only distantly related to CBRs. Recent advances in the pharmacology, biochemistry and molecular biology of the PBR are reviewed.

Temporal analysis of dithiobiuret neurotoxicity in rats and assessment of potential nonneural causes

To evaluate the hypothesis that depressed neuromuscular transmission causes dithiobiuret (DTB)-induced muscle weakness in rats, the temporal development of impaired treadmill performance and deficits in the nerve-elicited muscle contractions were compared during daily treatment with the toxicant (DTB, 1 mg/kg/day × 6 days). Diminished treadmill test performance after 4 days of treatment marked the initial detection of impaired motor function. At this time fading (loss of tension during tetanus) of gastrocnemius contractions elicited in response to 100-Hz sciatic nerve stimulation occurred in DTB-treated rats but not in controls. After 5 and 6 days of treatment, treadmill failure became complete, tetanic fade worsened dramatically, and peak contractile tension measured during trains of nerve stimulation (10–250 Hz) decreased progressively. Appearing by Day 6 were marked body weight loss, dehydration, hypothermia, and a depression in serum concentrations of thyroid hormones. Total oxygen content of the blood was not reduced at any time during treatment, and serum concentrations of glucose, sodium, potassium, calcium, chloride, and phosphorus in DTB-treated rats on Day 6 were similar to those of control animals. Therefore, hypoxia, hypoglycemia, or a serum electrolyte imbalance do not initiate or modulate the neuromuscular toxicity. Light microscopic evaluation of liver, kidney, lung, thyroid, and other organs in intoxicated rats was unremarkable and in skeletal muscles and selected sites of brain, spinal cord, and sciatic nerve no morphologically significant lesions were observed. Even when DTB-intoxicated rats were maintained in a state of flaccid muscle weakness for 5 continuous days, peripheral nerve lesions proximal to the intramuscular nerves were not detected. Thus, depressed neuromuscular transmission appears to be the primary cause of the flaccid muscle weakness and no evidence was obtained that nonneural effects of DTB initiate or modulate this effect.