In the past decade we have witnessed the discovery of large families and super-families of neural genes whose related but different sequences provide great insight if we can comprehend their functions and exploit their diversity. There can be no doubt that a major challenge facing modern neurobiology is the understanding and manipulation of gene function, both known and unknown. Institutions concerned with the critical issues of mental health and retardation must look beyond cloning and into the structure and function of the proteins encoded by these newly discovered sequences and the roles these proteins play in the development and behavior of the individual.

While the techniques for gene discovery and cloning have become less expensive and more accessible, the most powerful techniques for the study of function have become quite the opposite – they are now more expensive and more technically demanding. It is increasingly difficult for any single investigator to be able to fully explore the myriad dimensions of gene function: the structure of a gene and its regulation, the structure of the protein it encodes, the localization of the protein in the animal, the role of the protein in the normal animal, and the consequences of the absence or alteration of that protein in disease. However, the successes of  a decade of gene discovery now demand each of these activities, in part simply to set priorities for further studies. It is the purpose of the MRDDRC cores to provide access to techniques and assays that will allow the investigator to make maximum progress, without undue duplication of effort at the institutional level.

The Mouse Physiology Core is designed to provide MRDDRC investigators with a battery of functional assays that will provide initial insight into the neurophysiologic consequences of a specific mutation. This core is considered a significant component of the MRDDRC because it will help address the most common question following the creation of a new mouse mutant: “What is wrong with my mouse?” The MRDDRC will provide its investigators access to a battery of electrophysiologic assays that  are designed to help answer this question and direct the investigator’s attention to experiments that may directly address the role of a particular gene in generating a mental retardation or developmental disability phenotype.

The objectives of the Mouse Physiology Core are two-fold. Our first goal is to provide  MRDDRC members with the resources to determine  whether their mutant mice display derangements in neuronal synaptic transmission and synaptic plasticity, using primary cultures as well as the hippocampal slice preparation in vitro, to obtain a detailed analysis of synaptic function. Our second goal is to enable MRDDRC investigators to evaluate and correlate the development of cortical excitability and brain function with behavioral activity using continuous video/EEG monitoring in behaving mutant mice. Thus the Physiology Core will assess neuronal physiology in mouse models of human mental retardation across a broad functional range – from baseline synaptic function, to short- and long-term forms of plasticity, to the behavior of the neuron when imbedded in its native circuit in vivo. These functional assessments specifically examine the basic cellular processes that are likely to be  prominently associated with MR, including long-term alterations in synaptic function and epileptic seizures.

The Mouse Physiology Core is divided into two components. The Synaptic Physiology component of the Core allows investigators to determine if the basic attributes of synaptic transmission and synaptic plasticity are intact in their animal models. The analysis of synaptic properties from autaptic cultured neurons from newborn mice is a highly reliable method to screen for basic dysfunctions in synaptic transmission. Here the analysis focuses on the behavior of individual synapses. Putative malfunction in any one of many steps that underlie the complex process of synaptic transmission can be separated and quantitatively characterized. We employ neuronal cultures from hippocampus and the striatum to look not only at brain areas highly relevant for mental retardation, but also as they allow for separation of two major classes of neurons: the inhibitory GABAergic neurons and the excitatory glutamatergic neurons. The experimental approaches screen for changes in amplitudes of evoked responses, readily releasable vesicle pool sizes, spontaneous release, vesicle filling with neurotransmitters, vesicle priming and Ca2+ triggering dynamics, postsynaptic glutamate and GABA receptor density and function. In addition, quantitative morphological analysis of morphology can be applied to identify dysfunctions in synapse formation and maintenance, as well as in imbalanced dendritic outgrowth. To investigate imbalances in excitability in neuronal networks and putative effects in synaptic plasticity we further offer hippocampal slice network physiology, given the well-documented role of the hippocampus in learning and memory. The established procedures will allow the assessment of several parameters related to normal synaptic physiology, as well as several short- and long-term forms of synaptic plasticity including paired-pulse facilitation, post-tetanic potentiation, and long-term potentiation (LTP). All of these procedures utilize extracellular recording in the hippocampal slice preparation, using standard protocols already used here on an ongoing basis.

The Video/Electroencephalography component of the Core will enable MRDDRC investigators to evaluate the development of cortical excitability and brain function over prolonged periods in behaving animal models of mental retardation produced by genetic engineering techniques. Depressed excitability or abnormal patterns of brain rhythms are among the earliest objective phenotypes of genetic human mental retardation syndromes. A high incidence of epilepsy is also associated with MR, and our facility specializes in state of the art seizure detection techniques and assessment of seizure threshold. The core has pioneered the use of chronic recordings of behaving mice utilizing 10 electrode EEG arrays, permitting mapping of focal seizures and regional rhythmic activity disturbances. The ability to correlate spontaneous EEG activity with behavioral analysis by use of synchronized video/EEG monitoring is critical to the interpretation of the mutant nervous system phenotypes studied by the MRDDRC. The core also has an extensive archive of normative data to facilitate analysis and comparisons with other epileptic mouse mutant phenotypes at different developmental ages.

© 2003 Baylor College of Medicine - Mental Retardation and Developmental Disabilities Research Center
Phone: (713) 798-7353    Fax: (713) 798-8704    Houston, Texas 77030    E-mail: mrddrc@bcm.tmc.edu
Modified November 25, 2003