Brain Function

Robertson, J

Abstract

This project is addressed to the general problem of the nature of the cellular basis of learning and memory (L&M) using the brain of Octopus vulgaris as a test object because it is the most highly developed invertebrate brain and it is assumed that the basic principles found to operate in it will be applicable to all brains. The general principles discovered should help in understanding ways to improve learning and memory ability in humans as well as find ways to alleviate diseases such as Alzheimer's in which memory is greatly impaired. The basic approach is to study the tactile learning part of the octopus brain by electron microscopy. The study will be focused on certain small interneurons called amacrines that are believed to participate in L&M. The tactile learning neuropils of the brain will be studied by transmission and scanning electron microscopy (EM), comparing brains of trained and untrained animals. The trained animals will be taught a touch learning paradigm which requires that the particular neuropil studied be intact. It is assumed that cellular changes occur with L&M and that these can be detected by EM techniques. We have found that the tactile learning neuropils in octopus contain moderate numbers of filopodia like those seen in growth cones in neuronal cell cultures and that the volume and surface area fractions of filopodia in the tactile learning neuropils increase significantly during training to a tactile paradigm as measured by morphometric techniques. We intend to investigate this phenomenon further. It has been found by others in studying tissue cultures that the drug cytochalasin B has a remarkable effect on growing nerve fibers. Nerve fibers grow by extending minute finger-like processes called filopodia which contain no organelles except actin filaments. Cytochalasins B and D cause filopodia to collapse reversibly in growing axon tips in tissue culture. This is thought to depend on inhibition of the polymerization of actin, which is necessary for filopodial growth. In embryonic brain and in tissue culture, when synapses between neurons are formed, the neurons characteristically extend filopodia toward one another that establish the initial contacts. We postulate that something like this might occur in the adult learning neuropil during learning, accounting for our preliminary observations. If this is correct injections of cytochalasin into such a neuropil should block learning by breaking down filopodia. This should be reversible and should not affect established synapses on which previous memories are based. We have found, in preliminary experiments that this appears to be true at least for cytochalasin B. The project is designed to test this proposition further. One long range goal, if the hypothesis proves true, is to see if injections of nerve growth factor and other agents, which strengthen filopodia in tissue culture, improve learning ability.