The Journal of Neuroscience, February 15, 2003, 23(4):1535
Single Neurons in CA1 Hippocampus Encode Trace Interval Duration
during Trace Heart Rate (Fear) Conditioning in Rabbit
Matthew D.
McEchron,
Wilbur
Tseng, and
John F.
Disterhoft
Department of Physiology and Institute for Neuroscience,
Northwestern University Medical School, Chicago, Illinois 60611
This study sought to determine whether CA1 hippocampal neurons
encode the duration of the trace interval during trace fear conditioning. Single neurons were recorded extracellularly in the CA1
of rabbits during and after a single trace fear classical conditioning
session. Trace fear conditioning trials consisted of an auditory
conditioned stimulus (CS; 3 sec) and a fear-producing shock
unconditioned stimulus (US; 0.5 sec) separated by a silent trace
interval. One group of rabbits was trained using a 10 sec trace
interval (n = 5), and another group was trained
using a 20 sec trace interval (n = 4). These groups
were compared with pseudoconditioning control rabbits
(n = 5 and n = 4, respectively) that received unpaired CSs and USs. One day after trace and pseudo fear
conditioning rabbits received a CS-alone retention session in which no
USs were presented. The trace conditioned groups showed larger
bradycardiac-fear responses on CS-alone trials compared with the
pseudoconditioning groups. A significant percentage of CA1 neurons from
the 10 and 20 sec trace groups (24 and 28%, respectively) showed
maximal firing on CS-alone retention trials timed to 10 sec (±1.5 sec)
and 20 sec (±2.0 sec) after CS offset, respectively. These latencies
were similar to the duration of the trace interval used on previous
CS-trace-US trials. Timed CA1 firing was not seen in
pseudoconditioning control animals, suggesting that a subset of CA1
neurons encoded the trace interval duration. The percentage of neurons
encoding trace duration was largest when rabbits exhibited significant
fear responses to the CS, suggesting that trace encoding was related to
the strength of the CS and US association.
Key words:
learning; trace; temporal; conditioning; heart
rate; CA1; hippocampus; single neuron
Copyright © 2003 Society for Neuroscience 0270-6474/03/2341535-13$05.00/0
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