Hippocampal CA1 Neuronal and Sympathetic Nerve Activity in Response to Repeated Fear Conditioning in Rats

Abstract

The aim of the present study was to assess the responses of hippocampal and sympathetic nerve activity (SNA) to fear conditioning in rats. Wistar rats were chronically instrumented with a combined probe comprising multiple electrodes (100‐μm stainless steel wires) for measurement of hippocampal CA1 neuronal activity, a glass fiber for measurement of hippocampal cerebral blood flow (CBF), electrodes for measurement of renal SNA (RSNA) and lumbar SNA (LSNA), and electroencephalogram, electromyogram, and electrocardiogram, and with a catheter for measurement of systemic arterial pressure (AP). Standard fear‐conditioning trials, in which a tone was paired with a brief electrical shock (5 mA, 1s), were given to rats twice per day over 3 days. Hippocampal CA1 neuronal activity and CBF in the CA1 region increased progressively over the 3‐day fear conditioning trial, as did AP and RSNA. By contrast, heart rate (HR) and LSNA did not change on day 2, but increased on day 3. These data suggest that the repeated fear conditioning trial causes long‐term potentiation of hippocampal CA1 neuronal activity in a progressive and additive manner. Further, this potentiation was correlated quantitatively with hippocampal CA1 CBF and RSNA, but not with LSNA or HR. Thus, CA1 activation induced by repeated fear conditioning may influence RSNA, LSNA, AP, and HR in a region‐specific manner. JSPS Grant.