Hippocampal nociceptive activation in the rat

Abstract

In anaesthetized rat, hind paw injection of formalin, a model of persistent inflammatory pain, evoked a??signal-to-noisea?? processing marked by excitation of a select population of CA1 pyramidal cell against the background of widespread pyramidal cell suppression. These changes were observed in parallel with theta activation, theta being sinusoidal waveform that reflects rhythmic oscillation of CA1 neurons in processing of information. The aim of the present study was to investigate whether injection of formalin in behaving rat recruited the theta functional state of the hippocampus temporally linked to the animal nociceptive experience. Results showed that formalin induced biphasic theta activation that broadly paralleled the aversive behaviors. However, theta activation was not associated with any typical nociceptive behavior per se. Finally, synaptic excitability of CA1 pyramidal cells was attenuated following injection of formalin though the decrease was not biphasic suggesting varied mechanisms influencing theta activation and suppression of CA1 pyramidal cell excitability.