Effects of post-training hippocampal injections of midazolam on fear conditioning

Abstract

Benzodiazepines have been useful tools for investigating mechanisms underlying learning and memory. The present set of experiments investigates the role of hippocampal GABA A /benzodiazepine receptors in memory consolidation using Pavlovian fear conditioning. Rats were prepared with cannulae aimed at the dorsal hippocampus and trained with a series of white noise-shock pairings. In the first experiment, animals received intrahippocampal infusion of midazolam or vehicle immediately or 3 h after training. Then, 24 h later, freezing to the training context and the white noise were measured independently. Results show infusion of midazolam immediately, but not 3 h, after training selectively attenuates contextual fear conditioning. In the second experiment, animals received intrahippocampal infusions of an antisense oligodeoxynucleotide (ODN) targeting the ␣5 subunit of the GABA A receptor or a missense control for several days prior to training and testing. Immediately after training, animals received an infusion of either midazolam or vehicle. Western blots conducted after testing showed a significant decrease in ␣5-containing GABA A receptor protein. This reduction did not alter the effectiveness of midazolam immediately after training at impairing context fear memory. Therefore, ␣5-containing GABA A receptors may not contribute to the effects of midazolam on context fear conditioning when given immediately post-training.Pavlovian fear conditioning produces fear through learning that an aversive event is predicted by some neutral stimulus (for review, see LeDoux 2000;Maren 2001;Schafe et al. 2001). Rats quickly learn to associate a conditional stimulus (CS) with a noxious unconditional stimulus (US) and produce conditional responses (CRs) that can be reliably measured. In particular, freezing is a CR defined as the absence of all body movement except that related to respiration (Blanchard and Blanchard 1969;Fanselow 1980). Fear conditioning creates a long-lasting memory of the contextual and discrete cues present at the time of training.Post-training manipulations of the hippocampus have been shown to impair contextual fear conditioning (Kim and Fanselow 1992;Maren et al. 1997;Frankland et al. 1998;Anagnostaras et al. 1999;Barrientos et al. 2002;Dash et al. 2002;Wallenstein et al. 2002;Ji et al. 2003). One current area of research regarding the hippocampus is the analysis of the time course of mechanisms that underlie consolidation. Consolidation refers to the period after acquisition when a new memory transforms from an easily disrupted short-term state to a stable long-term memory (McGaugh 2000). Recent research using inhibitors of protein synthesis and gene expression suggests that memory for inhibitory avoidance has two phases of consolidation, one that occurs around the time of training and another occurring 3 to 6 h after training (Quevedo et al. 1999;Igaz et al. 2002). Similarly, the consolidation of contextual fear memory is also time dependent and may have multiple periods of susceptibility to protein sy...

Disruptive Effect of Midazolam on Fear Memory Reconsolidation: Decisive Influence of Reactivation Time Span and Memory Age

“…Moreover, prior administration of an antagonist of BDZ sites reversed BDZ-induced amnesia (Savic et al, 2005) whereas the administration of the inverse agonist resulted in a promnesic effect, indicating that BDZ sites are implicated in the formation of memory. BDZ's disruptive effect has been observed following administration either pre-or posttraining in diverse learning and memory paradigms (Gafford et al, 2005). Such an effect was also reported after both BDZ intraamygdala (Dickinson-Anson and McGaugh, 1993) and BDZ intradorsal hippocampus (Gafford et al, 2005) infusions.…”

“…BDZ's disruptive effect has been observed following administration either pre-or posttraining in diverse learning and memory paradigms (Gafford et al, 2005). Such an effect was also reported after both BDZ intraamygdala (Dickinson-Anson and McGaugh, 1993) and BDZ intradorsal hippocampus (Gafford et al, 2005) infusions. Consistent with such findings, Bustos et al (2006) have recently demonstrated that midazolam (MDZ), a short-acting BDZ, affects fear memory reconsolidation.…”

Critical role of the 65-kDa isoform of glutamic acid decarboxylase in consolidation and generalization of Pavlovian fear memory

“…The lack of generalization in Gad65 ‫מ/+‬ mice appears to argue against this, but it should be considered that adult heterozygotes display normal GABA content in the amygdala and hippocampus (Stork et al 2000). Second, GAD65 regulation and function may differ between subpopulations of GABA interneurons, which are characterized by particular morphology, physiology, and expression of neurochemical factors (Freund and Buzsáki 1996;Frenois et al 2005;Mascagni and McDonald 2007;Muller et al 2007) and are thought to mediate specific aspects of fear memoryrelated information processing within the amygdala (e.g., Quirk et al 2003;Azad et al 2004;Marowsky et al 2005) and hippocampus (Crestani et al 1999;Gafford et al 2005). Third, if compensatory mechanisms are active in the Gad65 null mutants, then the inability of these animals to change GABAergic function via regulation of GAD65 expression may be important.…”

“…In light of these findings, it is also interesting that spike-timingdependent LTP in the amygdala is tightly controlled by the level of GABAergic inhibition (Shin et al 2006). Concerning the hippocampus, it has been shown that deficits in GABA A receptor clustering increase anxiety in mutant mice heterozygous for the ␥2 subunit of the GABA A receptor (Crestani et al 1999) and that post-training injection of the benzodiazepine midazolam interferes with context memory consolidation (Gafford et al 2005).…”

Selective effects of benzodiazepines on the acquisition of conditioned taste aversion compared to attenuation of neophobia in C57BL/6 mice

“…Fanselow and Helmstetter (1988) found that pre-training midazolam administration attenuated conditioning as shown by subsequent freezing to the context associated with shock. The hippocampus seems to be one site of action for this effect, since immediate post-conditioning application of midazolam to the dorsal hippocampus reduced subsequent contextor cue-induced freezing (Gafford et al 2005). Passive avoidance studies are in substantial agreement in showing that pre-conditioning administration of an anxiolytic benzodiazepine impairs acquisition of the avoidance response in rats and mice (Anglade et al 1994;Broekkamp et al 1984;Cryan et al 2004;Jensen et al 1979;Nagatani and Yamamoto 1991;Oishi et al 1972;Patel et al 1979).…”