Recent evidence from our laboratory have shown that discontinuation from chronic ethanol administration results in a clear enhancement of contextual fear conditioning. These results were observed 4 days after the last ethanol administration when withdrawal-induced anxiety was no longer evident. Although there is a better comprehension about the characteristics and neural basis of fear conditioning much less is known about the neural basis of acquisition, retention and extinction of the conditioned fear response following drug withdrawal.
Therefore, we examine in ethanol withdrawn animals: 1) the acquisition and retention of the extinction conditioned freezing response, 2) the effect of D-cycloserine (DCS, a partial NMDA agonist) on the extinction of conditioned freezing and 3) the reinstatement of fear acquisition in rats treated with DCS at the time of the extinction training.
Male Wistar rats were allowed to consume an ethanol-containing liquid diet (6% v/v) as a sole source of nutrients with water available ad libitum for 14 days. Control animals were pair-fed the same diet with dextrose substituted isocalorically for the ethanol. Three days after removal of ethanol containing diet (ethanol withdrawal), rats were exposed to 3 foot-shocks (0.7mA, duration 3s and intershock interval 30s). Test trials were performed 24, 48, 72 and 96 h later exposing the animals to the context without shock delivery and freezing behavior was registered during each trial for a 10 min period (extinction training). Immediately after the first session of extinction training (24 h), half of the animals were injected with DCS 5 mg/kg i.p- a dose that does not influence the extinction in control rats- and the other half with saline. Seven days after the last session of extinction training animals were re-exposed to the conditioned context to evaluate the spontaneous recovery. Twenty four h later, animals received a single 0.7-mA, 3 s shock in the same context. The following day, the freezing response was again evaluated in for 10 min.
Ethanol withdrawn animals exhibited an enhanced freezing response and a marked resistance to extinguish during the first test trial. A facilitatory effect of DCS on the extinction process was observed in ethanol withdrawn animals. Control and ethanol withdrawn rats showed the same level of freezing on day 4 of the extinction training. A comparable low level of fear behavior was detected in all animals evaluated 7 days after the last session of extinction indicating a retention of extinction memory. Following the reminder shock cue, ethanol withdrawn animals displayed higher levels of freezing than controls, this increase was prevented by DCS pretreatment. The enhanced sensitivity to the facilitatory effect of DCS on both extinction and reversal of fear reinstatement in ethanol withdrawn animals may be mediated by changes in NMDA receptor induced by chronic ethanol treatment.
Supported by Agencia CórdobaCiencia, SECYT-UNC, CONICET and Agencia Nacional de Promoción Científica y Tecnológica (Argentina).
