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Archive for 13/01/2011

Gap junctions

There are (at least) two types of junctions between neurons: chemical synapses and electrical synapses. In chemical synapses there is a tiny space between the membranes of the two neurons, molecules of neurotransmitter are bled into this space by one neuron and taken in by the other. In this way a signal is sent from the pre-synaptic neuron to the post-synaptic one. The signaling occurs when the pre-synaptic cell is firing and the post-synaptic cell is influenced to fire more easily, or less easily in inhibitory synapses. But there is a slightly simpler junction found both in the brain and in other tissues. It is a cluster of tiny channels in one cell membrane that lines up with a similar set of channels through another cell’s membrane so that small chemicals or electrical currents can pass directly between the two cells. The tiny channels can be opened and closed. We think of the brain as using only chemical synapses when actually the gap junctions exist in many areas of the brain, especially between glial cells. They are noted to pass electrical currents in the form of charged ions but may also signal using calcium ions and other chemicals. A group of cells connected by gap junctions have, in effect, a continuous cytoplasm as far as small molecules and electrical charge are concerned. The activity of cells using gap junctions is another whole level of activity in the brain.

In a recent paper reported in ScienceDaily (here) M. Fanselow and his group have looked at gap junction activity in the hippocampus especially during the memory of fearful situations.

Fanselow said, “We hypothesized that these gap junctions may be very important. Because the gap junctions cause the inhibitory neurons to fire together, they may cause these inhibitory neurons to act as a pacemaker for the excitatory neurons, making them fire at the same time so they are better able to make fear memories. Neuronal gap junctions form where inhibitory neurons touch one another. They are like an opening between nerve cells, a gap in the membranes separating the cells from one another; they let the electrical activity in one neuron affect the neuron it touches.

Gap junction may have (or are likely to have, really) a role in maintaining synchrony in the brain and synchrony appears to be a the heart of many process including consciousness.

Abstract: The role of electrical synapses in synchronizing neuronal assemblies in the adult mammalian brain is well documented. However, their role in learning and memory processes remains unclear. By combining Pavlovian fear conditioning, activity-dependent immediate early gene expression, and in vivo electrophysiology, we discovered that blocking neuronal gap junctions within the dorsal hippocampus impaired context-dependent fear learning, memory, and extinction. Theta rhythms in freely moving rats were also disrupted. Our results show that gap junction–mediated neuronal transmission is a prominent feature underlying emotional memories.

I could not reach the original paper but the citation is:

Stephanie Bissiere, Moriel Zelikowsky, Ravikumar Ponnusamy, Nathan S. Jacobs, Hugh T. Blair, Michael S. Fanselow. Electrical Synapses Control Hippocampal Contributions to Fear Learning and Memory. Science, 2011; 331 (6013): 87-91 DOI: 10.1126/science.1193785