The L-type calcium channel blocker nifedipine impairs extinction, but not reduced contingency effects, in mice

Abstract

We recently reported that fear extinction, a form of inhibitory learning, is selectively blocked by systemic administration of L-type voltage-gated calcium channel (LVGCC) antagonists, including nifedipine, in mice. We here replicate this finding and examine three reduced contingency effects after vehicle or nifedipine (40 mg/kg) administration. In the first experiment, contingency reduction was achieved by adding USs to the training protocol (degraded contingency), a phenomenon thought to be independent of behavioral inhibition. In the second experiment, contingency reduction was achieved by varying the percentage of CS-US pairing, a phenomenon thought to be weakly dependent on behavioral inhibition. In the third and fourth experiments, contingency reduction was achieved by adding CSs to the training protocol (partial reinforcement), a phenomenon thought to be completely dependent on behavioral inhibition. We found that none of these reduced contingency effects was impaired by nifedipine. In a final experiment, we found that extinction conducted 1 or 3 h post-acquisition, but not immediately, was LVGCC-dependent. Taken together, the results suggest that reduced contingency effects and extinction depend on different molecular mechanisms and that LVGCC dependence of behavioral inhibition develops with time after associative CS-US learning.Pavlovian fear conditioning in rodents has been widely used in the laboratory setting to model human fear and anxiety, as well as excitatory learning and memory processes (Fendt and Fanselow 1999;Maren 2001;Ohman and Mineka 2001;Fanselow and Gale 2003;LeDoux 2003). Fear conditioning is achieved by temporally pairing an initially neutral conditional stimulus (CS), such as a tone, with an aversive unconditional stimulus (US), usually a mild footshock. Rats show robust fear responding to a CS with as little as one CS-US pairing (Fanselow 1990). Extinction of conditional fear, the progressive weakening of the fear response by repeated presentations of the CS alone, has been an important model in the development of behavior therapy for human anxiety disorders (Wolpe 1969;Wolpe and Rowan 1988;Craske 1999;Myers and Davis 2002) and is the prototypical procedure for inducing inhibitory learning (Davis et al. 2003;Delamater 2004).While the molecular and cellular mechanisms of fear acquisition have been intensely studied for decades, the mechanisms of fear extinction are only beginning to be unraveled. For instance, lesion and local infusion studies suggest that the amygdala (Falls et al. 1992;Quirk et al. 1997;Lu et al. 2001;Marsicano et al. 2002;Royer and Pare 2002;Walker et al. 2002;Davis et al. 2003;Ledgerwood et al. 2003; Lin et al. 2003a,b,c) and prefrontal cortex (Morgan et al. 1993;Quirk et al. 2000;Herry and Garcia 2002;Milad and Quirk 2002;Santini et al. 2004;Shah et al. 2004) participate in the learning and retention of fear extinction. Additionally, both local infusion and systemic behavioral pharmacology studies have identified important roles in extinction for the NMDA-type ...

Mechanisms of fear extinction

“…As recovery effects have served as the impetus for new learning accounts of extinction, the lack of recovery in the short interval group would seem to be explained most parsimoniously in terms of erasure of conditioned fear and/or prevention of consolidation of fear memory. Perhaps consistent with this, evidence is emerging for a neurobiological difference between short and long interval extinction as well: Cain et al 64 reported that immediate extinction is not affected by the L-type voltage-gated calcium channel (L-VGCC) inhibitor nifedipine, whereas delayed extinction is impaired; and Mao et al 70 found that fear extinction initiated 1 h after fear acquisition reversed a fear conditioning-induced change in a particular glutamate receptor (the GluR1 subunit of the 2-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) receptor) within the amygdala, whereas this reversal did not occur when extinction was initiated 24 h after acquisition.…”

“…In many cases extinction at the shortest time point was less complete than that at the longest time point (cf. Cain et al, 64 Cammarota et al 68 and Maren and Chang 69 ), but this did not seem to explain the lack of recovery in the 10-min group. As recovery effects have served as the impetus for new learning accounts of extinction, the lack of recovery in the short interval group would seem to be explained most parsimoniously in terms of erasure of conditioned fear and/or prevention of consolidation of fear memory.…”

“…As yet there is little evidence for or against this idea. In the same set of experiments in which they found that delayed but not immediate extinction is blocked by the L-VGCC inhibitor nifedipine, Cain et al 64 observed no effect of nifedipine on the acquisition of fear conditioned under a partial reinforcement schedule, suggesting that perhaps immediate extinction and partial reinforcement are similar mechanistically. However, this same group also reported that fear acquisition conditioned under a continuous (100%) reinforcement schedule was not affected by nifedipine, 64 making it difficult to interpret the lack of effect under the partial reinforcement schedule.…”

“…Hull, 47 for example, proposed that a nonassociative 'reactive inhibition' (I R ) developed during extinction together with an associative form of inhibition ( S I R ), and that the dissipation of reactive inhibition over time contributed to effects such as spontaneous recovery. The concept of reactive inhibition or response fatigue finds some support in observations that massed extinction training proceeds more readily than does spaced extinction training 4,63,64 as would be expected if it is assumed that massed response elicitations cause response fatigue to accumulate more rapidly than do spaced response elicitations, although the literature on trial spacing in extinction is rife with negative effects as well as reports of spaced trials being more efficacious than massed trials (for discussion see Mackintosh 46 ). On the other hand, evidence that extinction occurs even when the conditioned response is physically prevented 65,66 are difficult for a response fatigue account to explain.…”

“…This difference in the consolidation state of fear memory at the two time points may offer a different substrate for extinction to act upon, and thus may be a critical factor in determining the mechanism of extinction. 64,72 Alternatively it seems possible that extinction training occurring in close temporal proximity to CS-US pairings may be perceived by the organism as a continuation of a single training episode rather than a distinct phase of training. In other words, acquisition plus short interval extinction may be analogous to partial reinforcement.…”