Neurons: different jolts, different notes.

Neurons: Different jolts, different notes

Certain neurons strongly involved in memory formation have two distinct ways of transmitting information, reports Charles Stevens of Yale Medical School in New Haven, Conn. In addition to simply passing along a received signal, these neurons have a "special mode" of signal transmission that is activated only if the cell receives two signals in a row.

"The first signal cocks the gun and the next signal fires it," Stevens says. This special types of signal transmission gives the neuron a different way to process information, he says -- in effect providing the neuron the ability to remember it had been fired shortly before and to act differently when fired the second time, probably by releasing different neurotransmitters.

The cells in question are those with so-called N,DA receptors, intensively studied because the NMDA receptors have recently been recognized as playing a key part in the formation of memories. Blocking the NMDA receptors leads to an inability to form new memories, other scientists have found. Stevens finds that the second mode of signal transmission is made possible by the special ion-transporting properties of the NMDA receptor.

Hallucinogenic drugs such as PCP block the NMDA receptor and therefore the special mode of signal transmission. Some of the behavioral effects of these hallucinogenic drugs may be due to the blocking of this second information processing mode in neurons, Stevens suggests.

In related research, Roger Nicoll of the University of California at San Francisco has shown that although NMADA receptors play an important part in the process of memory formation, they don't do it all by themselves. In order to determine this, Nicoll tried to see if he could give nerve synapses a long-lasting sensitivity to stimulation, a state generally agreed to be the basis of memory formation, by flooding the receptors with NMDA. The NMDA by itself couldn't produce this state, and Nicoll and his colleagues are now looking for other factors, such as other neurotransmitters, that may act with NMDA receptors to form memories.