BRAIN RESEARCH LABORATORY

W (Liam) Burke

RESEARCH REPORT


PERSONNEL in 1995 and 1996

Dr W (Liam) Burke Emeritus Professor 1956-


Research in this Laboratory in 1995 was in two separate areas: (i) central visual pathways, and (ii) the myoclonic effects of gallamine and curare.

PROJECTS in 1995

Convergence of Y and non-Y information channels on single neurones in the PMLS area of cat visual cortex

W Burke

(collaborators: C Wang, KR Huxlin, B Dreher, Dept of Anatomy & Histology)

Several lines of evidence suggest that in the retinogeniculo-cortical pathway of mammals with well developed visual systems, information is processed in several parallel channels (in the cat called X, Y and W channels). Numerous studies of one of the extrastriate visual cortical areas of the cat, the posteromedial lateral suprasylvian area (PMLS), have revealed its important role in motion analysis. In 1995 a study was made of the receptive field properties of PMLS neurons after selectively blocking conduction in the large-fibre (Y-type) channel in one optic nerve. The magnitudes of the responses of PMLS neurons to photic stimuli presented via the Y-blocked eye were significantly smaller than those to the photic stimuli presented via the normal eye. Nevertheless, as in normal animals, over 90% of PMLS neurons could be photically activated through either eye (binocular cells). Furthermore, a number of receptive field properties such as receptive field sizes, receptive field organization, direction and orientation selectivities appeared to be unaffected by the Y-block. On the other hand, the velocity response profiles of PMLS neurons when stimulated via the Y-blocked eye were shifted to a significantly lower velocity range. The results support the findings from many other laboratories that the Y channel constitutes an important input to the PMLS area. Nevetherless, retention of binocularity by the great majority of PMLS neurons in cats with one optic nerve Y-blocked, indicates that the great majority of PMLS neurons also receives excitatory non-Y-input. This study suggests that the Y-input from the ipsilateral eye is crucial for the detection of high velocity motion by PMLS neurons when stimulated via the ipsilateral eye. Y-type input from the contralateral eye is less important in detection of the high velocity motion. Other aspects of motion detection such as direction and velocity discrimination do not seem to depend on the presence of the Y-input. Furthermore, the Y-input makes a strong contribution to the peak discharge rate of PMLS neurons. Finally, in view of the fact that most neurons in primate middle-temporal (MT) motion area receive their principal excitatory input from the large fibre information channel (magnocellular channel or Y-like channel) and a weaker input from the parvocellular (X-like channel), the results are consistent with putative homology between area PMLS of the cat and area MT of the primates.

Myoclonus due to gallamine and curare

W Burke

(collaborator: I Ramzan, Dept of Pharmacy)

Gallamine and curare, neuromuscular blocking agents, do not normally cross the blood-brain barrier. If they are allowed to enter the central nervous system (CNS) they produce widespread synchronized twitching of muscles (myoclonus) which eventually becomes very rhythmical at about 8-10 Hz. Slow waves appear in the inferior olive, synchronized with the twitches. Both the twitches and the olivary slow waves persist for a day or more after the gallamine or curare can no longer be detected in the cerebrospinal fluid (CSF). However, an unidentified substance replaces them in the CSF. This substance is not a metabolite of either gallamine or curare. It was thought that it might be corticotropin-releasing factor (CRF) since this is present in the inferior olivary neurones and can also cause convulsions. Injection of CRF into the CNS results in the appearance of slow waves in the inferior olive; no twitches were elicited but this may be because the dose of CRF was too small and/or the animal was too deeply anaesthetized. Mass spectrometry has failed to detect CRF in the CSF at a time when the unidentified substance is present or at earlier times after injection of curare or gallamine.


RESEARCH PLANNED for 1996

The following will be investigated: myoclonic effects of gallamine and curare; elucidation of the nature of the unidentified substance referred to above; demyelination and remyelination in pressure-blocked optic nerves; properties of axonal conduction in demyelinated optic nerve fibres; role of Y system in the plasticity of visual cortical neurones.

COLLABORATIONS

The Y system and central visual pathways in the cat: Dr Bogdan Dreher, Dept of Anatomy & Histology (1988-present).

Gallamine and myoclonus: Dr Iqbal Ramzan, Dept of Pharmacy (1988-present).

Demyelination and remyelination in cat optic nerve:Dr Barrie Harrison, Dept of Biological Sciences, Univ of Technology, Sydney (1989-present) and Dr Kris Turlejski, Nencki Institute, Warsaw (1992-present).

Conduction properties of demyelinated optic nerve fibres:Prof WR (Bill) Levick, Neuroscience Div, John Curtin School of Medical Research, ANU (1990-present).

FACILITIES

The Laboratory occupies room 241 of the Anderson Stuart Building.

Electrophysiological equipment comprises probes, amplifiers, oscilloscopes, stimulators, tape recorders, pen recorder, MacLab and facility for recording in the conscious animal. Stereotaxic equipment comprises headholders, coarse- and fine-manipulators and a lesion maker. For visual stimulation there is a photo-stimulator, tangent screen, projectors, and wands. There are surgical instruments, an autoclave and oven for surgery and sterilization; and facility for making electrodes: stainless steel, enamelled and platinum/iridium-in-glass. Prof Burke's office is room 365.

5-Year Research Publications


FUNDING in 1995 and 1996

NHMRC Autoradiography of high affinity Balcar V 1993

uptake of L-glutamate in the CNS Dreher B 1994 $42,410

(Lab share = 0%) Burke W 1995

NHMRC The role of intrinsic and feedback Dreher B 1995 $78,560

connections in cat visual cortex Burke W 1996 $61,769

(Lab share = 50%) 1997

ARC Interactions between the information Dreher B 1994

channels in the visual forebrain of Burke W 1995 $45,000 the cat 1996 $46,000

(Lab share = 50%)

Total for 1995: $61,780

Total for 1996: $53,884


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