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Molecular Mechanisms of Neuronal Signaling and Pathologic Changes

D. Gruol, J. Netzeband, T. Nelson, Z. Qiu, C. Trotter, C. Ur, S. Conroy, K. Parsons, D. Sweeney, J. Caguioa, C. Ly, L. Quina, C. Gullette, J. Schneelock

Glutamate is one of the most important signaling molecules in the CNS. In addition to mediating chemical transmission at most excitatory synapses, it plays an important trophic role in CNS development, by promoting neuronal growth and survival and by regulating neuronal cytoarchitecture and circuitry. Disruption of pathways mediated by glutamate contributes to neuronal damage and disease. Thus, the physiologic and pathologic processes that influence the actions of glutamate in the CNS are of considerable interest, and this area has been a major focus of our recent studies involving cytokines, opiates, cannabinoids, and alcohol.

Our previous studies with an in vitro culture model system showed that excitatory synaptic transmission mediated by glutamate is altered when immature rodent CNS neurons are exposed to alcohol during a developmental period that corresponds to the third trimester of fetal development in humans. These results are of interest because of the known detrimental effects of alcohol on the developing CNS. Children of alcoholic mothers are often born with fetal alcohol syndrome or fetal alcohol effects, a subtler form of fetal alcohol syndrome; multifaceted conditions characterized by growth deficiencies; and CNS dysfunction, including mental retardation. The biological bases for these abnormalities have yet to be fully elucidated.

During the past year, we investigated further the effects of alcohol on processes involved in excitatory synaptic transmission mediated by glutamate in developing CNS neurons. The neuronal actions of glutamate involve multiple postsynaptic receptors and transduction mechanisms (Fig. 1). One class of glutamate receptors can be selectively activated by (S)--amino-3-hydroxyl-5-methlisoxazole-4-propionic acid A (AMPA) and are referred to as AMPA receptors. These receptors mediate rapid synaptic events that excite neurons by depolarizing the membrane potential. AMPA receptors also play an important role in the generation of intracellular calcium signals initiated by synaptic transmission.

Calcium signaling is essential to CNS neurons because it determines the level of cytosolic calcium. Cytosolic calcium regulates many cellular functions, such as the activity of enzymes, release of transmitters, expression of genes, and the activity of ion channels. However, excess levels of cytosolic calcium are detrimental to CNS neurons and can cause toxic effects. Thus, alcohol-induced alterations in synaptic responses mediated by AMPA receptors could severely impair many aspects of neuronal function and development and thereby contribute to the CNS dysfunction observed in fetal alcohol syndrome or fetal alcohol effects.

We used a culture model system of rodent CNS neurons, fura-2--based microscopic calcium imaging, and electrophysiologic techniques to investigate the effects of alcohol on neuronal responses mediated by AMPA receptors. We found that calcium signals elicited by stimulation of AMPA receptors were enhanced in the dendritic region of immature CNS neurons treated with intoxicating doses of alcohol during the main period of morphologic development (6--17 days in vitro; Fig. 1). In contrast, calcium signals elicited by stimulation of AMPA receptors in the somatic region of these neurons were relatively insensitive to alcohol treatment. Moreover, membrane depolarization elicited by activation of AMPA receptors retained normal properties, a finding that may reflect tolerance to alcohol resulting from prolonged exposure to alcohol.

The dendrites of CNS neurons are the main site of excitatory transmission mediated by glutamate receptors. Thus, alcohol effects on dendritic function could lead to prominent alterations in synaptic responses, including a disruption of calcium regulation of neuronal biochemistry and excitability, and may lead to neuronal death due to the toxic effects of calcium. Such actions of alcohol could contribute to the known toxic effects of alcohol on the developing nervous system. Moreover, because AMPA receptors mediate most fast synaptic transmission in the brain, many parts of the brain could be affected by this action of alcohol.

PUBLICATIONS

Gruol, D.L. Rat cerebellar Purkinje neurons. In: The Neuron in Tissue Culture. Haynes, H. (Ed.). Wiley, New York, in press.

Gruol, D.L., Nelson, T.E. Physiological and pathological roles of interleukin-6 in the central nervous system. Mol. Neurobiol. 15:307, 1997.

Gruol, D.L., Parsons, K.L., DiJulio, N. Acute alcohol alters calcium signals elicited by glutamate receptor agonists and K⁺ depolarization in cultured cerebellar Purkinje neurons. Brain Res. 773: 82, 1997.

Gruol, D.L., Ryabinin, A.E., Parsons, K.L., Cole, M., Wilson, M.C., Qiu, Z. Granule neurons from neonatal rats exposed to alcohol by vapor inhalation show reduced calcium signals to NMDA. Brain Res., in press.

Gruol, D.L., Sweeney, D.D., Conroy, S.M., Trotter, C., Netzeband, J.G., Qiu, Z. Cannabinoids alter neurotoxicity produced by interleukin-6 in central nervous system neurons. In: AIDS, Drugs of Abuse, and the Neuroimmune Axis. Friedman, H., et al. (Eds.). Plenum Press, New York, in press.

Gruol, D.L., Yu, N., Parsons, K.L., Billaud, J.N., Elder, J.H., Phillips, T.R. Neurotoxic effects of feline immunodeficiency virus, FIV-PPR. J. Neurovirol., in press.

Nelson, T.E., Campbell, I.L., Gruol, D.L. Altered physiology of Purkinje neurons in cerebellar slices from transgenic mice with chronic central nervous system expression of interleukin-6. Neuroscience, in press.