Morphological plasticity in dendritic spines of cultured hippocampal neurons

Rat hippocampal neurons, grown in dissociated culture for about 18 days, were exposed for 6 h to three days to stimuli which cause either an increase (GABAA antagonists, bicuculline or picrotoxin), or decrease (tetrodotoxin) in spontaneous neuronal activity. Individual neurons were stained with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate and visualized with a confocal laser scanning microscope. GABA antagonists caused a marked, up to 60%, increase in spine density on secondary dendrites of cultured hippocampal neurons. This was associated with a small decrease in spine length. The rise in spine density was partially prevented by treatment with the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetra-acetate, or by blockade of protein synthesis with cycloheximide. Tetrodotoxin caused a marked elongation of dendritic spines (but did not cause a decrease in spine density comparable to the increase caused by picrotoxin). This effect was seen primarily but not exclusively in spines with no distinct head. Both treatments were most effective within 24 h of exposure. There were no other systematic effects of the drugs on the morphology of the dendritic spines. These results indicate that dendritic spines in cultured neurons depend on ongoing synaptic activity to maintain their shape, and that neurons respond to an increase in synaptic demand by an increase in spine density. Thus, dendritic spines are likely to have a role in short-term synaptic interaction rather than to constitute a long-term memory storage device.