The Electrophysiological and Neuropathological Perspective

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1 Neuroscience: From Fundamentals to Pharma East Hanover, NJ May 6, 2011 The Electrophysiological and Neuropathological Perspective Joseph Arezzo, Ph.D. Professor Neuroscience and Neurology Albert Einstein College of Medicine

2 Expansion of Interest in Neuropathy Vulnerability of elements of nervous system (length of axon) - neurotoxic effects are often dose limiting Recognition of different types of neuropathy unique characteristics and recovery profiles New drug development targeting CNS Growing awareness that exposure to toxins may contribute to development of neurologic diseases (e.g. Parkinson s, dementias, autism)

3 Neuroscience Mechanisms (axonal transport, glia, mitochondria, excitotoxicity,etc.) Differential targets and vulnerabilities Different degree of recovery (CNS vs PNS) Dissociation of function and structure

4 Broadly Defined Types of Neuropathy Axonopathy (e.g. taxol) Neuronopathy (e.g. mercury) Channelopathy (e.g. spider venom) Peripheral Myelinopathy (e.g. ddc) Central Myelinopathy (e.g. vigabatrin) Neuromuscular Block Imbalance in Excitation and Inhibition Deficits in Energy Utilization

5 Unique Challenges of Assessing Neurotoxic Insult Highly complex and differentiated structures - multiple sites of insult Blood-brain and blood-nerve barrier Continued insult after cessation of exposure Little or no recovery following some types of damage Poor accessibility (skull) - distal-to-proximal gradient (PNS) Many of the more common forms of neurotoxic insult may have little or no structural correlates (e.g., some types of seizure)

6 Blood-brain and blood-nerve barrier is a critical factor in all forms of induced neurotoxicity Endothelial cell targets potential implications for neurotoxicity Courtesy of Dr. Cedric Raine

7 Available Methods for Monitoring Neuropathy Histology Electrophysiology Neurochemistry Quantitative Behavioral Measures Neuroimaging (diffusion spectrum imaging)

8 Histology (granddaddy of measures) Strengths: Highly validated Unquestionable evidence of deficits Can be very sensitive to induced change Limitations: Often only sensitive to an active process axonal debris Can simply miss changes in focal regions (e.g. mammillary bodies) Can be insensitive to distal-proximal issues Often, the absence of active histopathologic findings is incorrectly considered to represent recovery

9 Non-Invasive Electrophysiology Stimulation

10 Electrophysiology Strengths: Non-invasive longitudinal assessment Objective, reliable, focused Quantitative Can evaluate distal-to-proximal changes Identical procedures in the clinic Limitations: Reflects activity in only a subset of neurons Insensitive to deficits in some areas (fornix)

11 Early Stage Slight shift in onset and peak latency; NCV m/sec Amplitude decreased by approximately 10% Area normal Mechanism Redistribution of ion channels Nodal changes in myelin Diminished axonal diameter Intermediate Stages Greater slowing; NCV m/sec Amplitude decreased by approximately 50% Area minimally affected duration prolonged Mechanisms Continuation of ion changes Onset of demyelination Mild axonal atrophy Late Stage Dramatic slowing; NCV < 30 m/sec Amplitude decreased by more than 50% or absent Area diminished due to fiber loss Mechanism Frank demyelination Severe axonal atrophy Reduction in fiber density Conduction block normal PN

12 Treatment-related slowing of sural NCV in a single monkey (approximately 15 m/sec) Segment 2, 1:46:11 PM Segment 1, 1:46:11 PM Delayed onset latency 6-weeks post treatment Sural [Baseline.acq, CH1] Onset baseline uvolts milliseconds

13 Chemotherapy Agent Linked to a Monoclonal Antibody - devastating 70 Nerve Conduction Velocities Baseline versus 6-weeks mg/kg Group 60 p = p = p = p = mv base 6 week median -motor peroneal sural median-sensory

14 PNS Histopathology Toxic Neuropathy

15 VEP in the Rabbit: Intraocular Challenge with IFN-γ Intravascular non-occluding thrombi Intravascular adhesion of inflammatory cells

16 Neuropathy Associated with Microtubule Inhibitors (taxanes, Ixabepilone) linked to reduction in axonal transport, secondary to altered microtubules progressive sensorimotor loss; numbness, paresthesias, gait abnormalities

17 Simple Diagram of Factors Associated with Excitotoxicity

18 Two Forms of Excitotoxicity in Cortex and Hippocampus Both Involving Glutamate Rapid 3-5 minutes NMDA mediated Direct entry of Ca 2+ Slow Several hours AMPA/Kainate receptor mediated indirect entry of Ca 2+

19 Possible Mechanisms for Acute Neuropathy Partial list includes:: Intracellular edema Altered channel function (hyper or hypo) Excitotoxicity Re-distribution of ion channels Inflammatory processes

20 Possible Mechanisms for Chronic Neuropathy Partial list includes: Disrupted axonal transport Axo-glial disjunction Frank demyelination Axonal atrophy Cell death and Wallerian degeneration

21 EM neurons in the medulla of a dog abundant cytoplasmic vacuoles that contain electron-dense laminated whorls consistent with phospholipidosis

22 H&E stained sections of a mesenteric lymph node in a dog - swollen macrophages and abundant eosinophilic, foamy to clear vacuoles consistent with phospholipidosis Cartwright et al., 2009

24 Schaumburg et al., 2006 A-delta and C-fibers Schaumburg et al., 2010

25 September 22-25, 2008 Madison, Wisconsin USA - 8 th Annual Meeting of the Safety Pharmacology Society

26 Trace Recovery of Function Msec baseline T 2 T 4 T 6 T 8 T 10 T 12 T 14 R 1 Treatment/Recovery Weeks R 3 R 5 R 7 R 9 R 11 control GVG

27 Electrophysiologic Procedures Peripheral Nervous System Measure Principal Sensitivity Principal Correlation with Histopathology Effects with Little or No Correlation with Histopathology Maximal Nerve Conduction velocity (NCV) Speed of conduction in large-diameter myelinated axons - measured in m/sec - often limited to activity within specific portions of the peripheral nerve Alteration in myelin, change in mean axon cross-sectional axon diameter, axonal atrophy, Wallerian degeneration Slowing due to changes at the molecular level (e.g., altered transmembrane currents), structural deficits limited to the nodes of Ranvier, physiologic conduction block clear changes in small-diameter neurons will have little or no affect on NCV Compound Muscle Action Potential Size of the muscle contraction when the associated motor nerve is stimulated at maximal intensity measured in mv Deficit in conduction within large-diameter motor axons, loss of motor axons, structural damage to the neuromuscular junction correlates with weakness Altered muscle contraction due to molecular changes at the neuromuscular junction (e.g., botulism), altered synchrony in otherwise intact fibers Sensory Nerve Action Potential Size of the cmpound sensory signal, often directly measured from only a single location along the stimulated nerve measured in µv Deficits in conduction within large-diameter sensory axons, loss of sensory axons, pathology at the dorsal root ganglia (DRG) - correlate with sensory signs and symptoms (e.g., numbness, paresthesia) In combination with segmental NCV can be used to identify deficits in the ventral root, spinal alpha motor neurons, proximal portion of the lower motor neuron, or subtle deficits distributed along the entire pathway Deficits limited to the distal portion of the sensory axons (e.g., length-dependant distal axonopathy) often dissociated from histopathology findings at more proximal sites F-wave Conduction along the entire length of the motor nerve, including proximal segments measured in msec Structural changes can be very subtle (e.g., alterations in the g-ratio) and not apparent to typical histopathologic procedures; changes can reflect the excitability of ventral horn interneurons rather than motor axons H-Reflex Orthodromic conduction along the sensory nerve from the point of stimulation, the spinal reflex arc, and conduction along the entire motor nerve, measured in msec A wide variety of deficits along the sensorymotor reflex pathway underlying the stretch reflex - correlated with general markers of peripheral sensory-motor neuropathy Structural changes can be very subtle, and initial deficits in this measure can often antecede frank evidence of structural pathology Slow NCV Speed of conduction in unmyelinated axons (ie., C fibers) - measured in m/sec (often < 5 m/sec) Alterations in small-diameter and unmyelinated axons previously not assessed by standard NCV procedures correlated with pain or thermal insensitivity Demyelination may be present in histopathology, but have little or no effect on slow NCV values EMG The timing and pattern of activity in isolated motor units Muscle atrophy, unusual muscle activity at rest (e.g., fasciculations) or uncoordinated motor unit firing related to a motor neuropathy or a myopathy correlated with weakness or spasticity Abnormal patters of EMG activity may reflect hyperexcitation or altered neuromuscular patterns with sparse evidence of structural deficits Arezzo, Litwak, Zotova, Toxic Pathology, 2010

28 Non-invasive Electrophysiologic Procedures Central Nervous System Measure End Points Principal Sensitivity Visual Evoked Potentials Somatosensory Evoked Potentials Brainstem Auditory Evoked Potentials Laser Evoked Potentials Onset and peak latency at cortex to flash or checkerboard pattern - amplitude of subcortical waves and specific cortical components Onset latency at cerebral cortex to electrical or mechanical stimulation of peripheral nerve - amplitude of subcortical waves and specific cortical components Peak latency of components I-V and calculation of transmission through the brainstem (ie., I-V interval) - change in latency and amplitude as a function of stimulus intensity in dbs Onset and peak latency of cortical response to rapid thermal pulses at specific dermatomes Photoreceptors retinal ganglia cells optic nerve optic tract lateral geniculate nucleus thalamocortical radiations primary visual cortex (Brodmann s areas 17 and 18) Large- diameter myelinated fibers in the peripheral nerve - dorsal columns of the spinal cord dorsal column nuclei in the medulla medial lemniscus ventral thalamus thalamocortical radiations primary somatosensory cortex Cochlea auditory nerve auditory stria lateral lemniscus olivary body caudal (inferior) colliculus Small-diameter fibers carrying thermal signals in the peripheral nerve dorsal horn ventrolateral tracts of the spinal cord ventral thalamus thalamocortical radiations primary and secondary somatosensory cortex I Cognitive Evoked Potentials Latency of late responses (e.g., mismatch negativity, P300) that are evoked not by the stimulus itself, but rather by novelty or context Subcortical regions associated with attention primary and secondary regions of sensory cortex portions of association cortex Magnetic Stimulation of Descending Motor Pathways Latency of a muscle contraction following activation of contralateral motor cortex using a pulsed magnet external to the skull Motor cortex cortical spinal tract internal capsule medullary pyramids descending motor tracts of the spinal cord ventral horn of the spinal cord Electroencephalography (EEG) Presence of EEG signals known to be associated with increased seizure risk (e.g., organized repetitive bursts of sharp waves or excessive synchrony) Presence of focal points of seizure generation (i.e., tumor) - alteration in the balance between excitation and inhibition EEG (Spectral Content) Change in the relative power of EEG energy in the different frequency bands (i.e., delta, beta, alpha, theta, gamma) Change in arousal vs sedation at the subcortical and/or cortical level damage to specific structures (e.g., hippocampus, reticular formation) Arezzo, Litwak, Zotova, Toxic Pathology, 2010

29 Application of techniques originally designed for cortex to assess slow conduction in peripheral nerve

30 Compound-induced seizures Mechanism is often simply unknown Can be related to metabolites Can be related to indirect causes e.g., hypoglycemia Can have a structural correlate (tumor, AV malformation), but often is simply related to an imbalance in excitatory and inhibitory circuits Structural pathology may be a consequence of the seizure rather than a cause of the seizure

Classification of Epileptic Seizures Partial (focal, local) seizures: Simple: motor, somatosensory, autonomic Complex: Impaired consciousness at outset Partial seizures evolving to generalized

31 Classification of Epileptic Seizures Partial (focal, local) seizures: Simple: motor, somatosensory, autonomic Complex: Impaired consciousness at outset Partial seizures evolving to generalized tonic-clonic (GTC) Generalized seizures (convulsive or non-convulsive): Absence seizures Myoclonic Clonic Tonic Tonic-clonic Atonic Simple Partial 21% Complex Partial 39% Tonic-clonic 25% Myoclonic 2% Other 9 % Other generalized 4%

Temporal Cz -T2 50.0000 0.00000-50.0000 50.0000 0.00000-50.0000-100.

32 Harvard Geodesics, Preclinical studies of EEG in a monkey Frontal Fz-Cz Temporal Cz -T uvolts uvolts Occipital O1-O uvolts seconds

33 Parallel studies in humans and experimental animals

34 Isolated sharp wave Fz-Cz uv Cz-T uvolts uvolts seconds rat uvolts seconds

35 Burst of linked sharp waves - Increase and then decrease in amplitude; Clear Evidence of Pre-Seizure Activity

36 Bursts of Repetitive, Organized Sharp Waves 3:31:50 PM Fz-Cz Cz-T uv uvolts seconds :37:12 PM uvolts Fz-Cz Cz-T uv uvolts seconds uvolts

37 Status Epilepticus PANTING HEAVILY FZ - CZ uvolts Cz - T uvolts O1 - O uvolts seconds

TOXIC AND NUTRITIONAL DISORDER MODULE

TOXIC AND NUTRITIONAL DISORDER MODULE Objectives: For each of the following entities the student should be able to: 1. Describe the etiology/pathogenesis and/or pathophysiology, gross and microscopic morphology