Neurophysiology Research Lab
Ischemia Studies, Retinal Physiology, Human Cerebral
Vasculature, and Collaborative Projects

Ischemia Studies Retinal Physiology Human Cerebral Vasculature Collaborative Projects

(a) Middle cerebral artery occlusion (MCAO) stroke (ischemia) model in the rabbit. This model is being used by Dr. Ogilvy to test purified, polymerized hemoglobin from Biopure as a means of achieving neuroprotection by reducing infarct volume. Recently acquired data will be presented at the upcoming annual Society for Neuroscience meeting in November (Soc Neurosci. Abstr. 20, p.180).

(b) The model will also be used to test the strategy that reducing energy requirements of neuronal tissue may be neuroprotective during periods of poor energy supply, e.g. during planned ischemia in some neurosurgical procedures (e.g. clipping aneurysms, arteriovenous malformations), and following ischemic episodes (e.g. stroke).

(c) In order to test this hypothesis, Dr. Ames has developed a "cocktail" of agents which temporarily block energy demanding metabolic processes. In a manuscript recently submitted to the Journal of Cerebral Blood Flow and Metabolism, Drs. Ames and Maynard showed that this "cocktail" was able to functionally (light-evoked compound action potentials from the optic nerve were recorded) protect the in vitro rabbit retina preparation, developed by Dr. Ames, during periods of 3 h of ischemia.

Future efforts involve refinement of the cocktail to include more clinically useful agents, i.e. agents which cross the blood brain barrier, yet have the same effect in both the in vitro and in vivo preparations. Cardiovascular toxicity of such a cocktail is also to be tested in vivo in rats and rabbits.

(d) Drs. Ogilvy and Maynard have recently started a multidepartmental study to develop a baboon model of focal ischemia. Using intra-arterial balloon occlusion of the middle cerebral artery for 3 h, followed by 1 h of reperfusion, this model will combine and compare functional CT scanning to measure the cerebral volume at risk from the 3 h MCAO, and diffusion weighted imaging MRI measurement of the actual infarct volume in the baboon brain ex vivo. Traditional histological staining of the cerebral infarct will also be assessed for microscopic analysis. A recently developed anti-selectin monoclonal antiserum (Boehringer Ingelheim) will be used to assess its potential as a neuroprotective agent in these on-going pilot experiments.

Retinal Physiology

Using the in vitro rabbit retina preparation previously mentioned, Drs. Maynard, Ames and Ogilvy are also examining the involvement of nitric oxide (NO) in retinal (neuronal) function and in ischemia. Staining for both the enzyme required for synthesis of the putative neurotransmitter, NO synthase, and a crucial cofactor, B-Nicotinamide adenine dinucleotide phosphate (NADPH) have been localised in the mammalian retina. In addition, a variety of single cell physiological recording experiments have implicated the involvement of NO in retinal neurotransmission. The preparation used in our laboratory, developed by Dr. Ames, is to our knowledge based on literature searches, the only existing preparation from which both the electroretinogram and compound action potentials from the optic nerve of an intact mammalian retina are physiologically recorded simultaneously. Our initial results were communicated at a satellite symposium on "Nitric oxide in the nervous system", July 1994, Montreal (Neuropharmacology, In Press), and more recent data will be presented at the upcoming annual Society for Neuroscience conference (Soc. Neurosci. Abstr. 20, p.218). A full manuscript has been submitted for publication in Investigative Ophthalmology and Visual Science. These experiments will proceed to examine the physiological consequences of manipulating the activity of NO synthase during ischemia, since the literature to date is unclear as to whether increases or decreases in NO are neuroprotective during stroke.

Human Cerebral Vasculature

Although much is known about the types of perivascular nerves and their origin in the cerebrovasculature through immunohistochemical studies on animals, very little has been reported on human cerebral vessels. Drs. Maynard and Ogilvy have therefore begun investigations to try to characterise the innvervation of the human cerebrovasculature, and to examine the innervation in abnormal cerebral vessels excised during surgical procedures. Recent findings were communicated at the American Heart Association Stroke conference in February 1994 (Stroke 25, 268) and a full paper is accepted for publication in Journal of Neurosurgery. This report identifies the presence and density of innvervation of calcitonin gene-related peptide (sensory), vasoactive intestinal polypeptide (parasympathetic) and neuropeptide Y (sympathetic) in major human cerebral and cerebellar arteries, and their absence in arteriovenous malformations. Future studies will include confirming the assumed characterisation (based on animal studies these peptide are characterised according to the nerve-types in parentheses above), using chemical denervation techniques. In addition, other abnormal vessels excised from surgery will continue to be assessed for the presence/absence of innervation, which may provide some insight as to the control of blood flow in these pathological cases.

Collaborative Projects

Dr. C. S. Ogilvy has a number of clinical research projects ongoing with other members of the Neurosurgical Service. Dr. K. I. Maynard also has ongoing collaborations with Dr. Michael A. Moskowitz of the Neurology Service related to migraine research (see Rebeck, Maynard, Hyman and Moskowitz, 1994, Proc Natl. Acad. Sci. 91, 3666-3669), and also in association with Drs. M. Fisher and P. Huang, investigations examining possible compensatory mechanisms of the cerebrovascular innervation in NO synthase (gene) knock-out mice.

 

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