Cellular imaging of zif268 expression in the hippocampus and amygdala during contextual and cued fear memory retrieval: selective activation of hippocampal CA1 neurons during the recall of contextual memories

Abstract

The neuroanatomical and molecular basis of fear memory retrieval was studied by analyzing the expression of the plasticity-associated immediate early gene zif268. Cellular quantitative in situ hybridization revealed that zif268 is expressed within specific regions of the hippocampus and amygdala during fear memory retrieval. Within the hippocampus, increased expression of zif268 was observed within CA1 neurons, but not dentate gyrus neurons, during the retrieval of contextual, but not cued, fear associations. In contrast, zif268 expression was increased within neurons of the amygdala (lateral, basal, and central nuclei) during the retrieval of both contextual and cued fear memories. These results demonstrate activation of hippocampal CA1 neurons in contextual fear memory retrieval that was not merely a correlate of the behavioral expression of fear itself, because it was limited to the retrieval of contextual, and not cued, fear memories. Further studies revealed that the selective increase in hippocampal CA1 zif268 expression seen after contextual fear memory retrieval was limited to the retrieval of recent (24 hr) but not older (28 d) memories. These experiments represent the first demonstration that zif268 expression in specific neuronal populations is associated with memory retrieval and suggest that this gene may contribute to plasticity and reconsolidation accompanying the retrieval process.

Fig. 1.

Freezing in response to presentation of discrete or contextually conditioned cues. Freezing (percentage 8 min after cue) in response to presentation of discrete CS (clicker) and contextual cues in rats trained with either clicker–shock pairings (cued;n = 10) or pseudorandom presentations of clicker and shock (context; n = 10). There was greater freezing to the discrete CS in the cued compared with context group and greater freezing to the contextual cues in the context compared with cued group. All results are presented as mean ± SEM values.

Fig. 2.

Expression of zif268 after the retrieval of contextual fear associations. a, Freezing of rats in the box (exposure to context only during training and testing; n = 6), cued (n = 6), and context (n = 6) groups in response to re-exposure to contextual stimuli at the retrieval test. Animals in the control group (n = 6) were given shock and cue presentations on the training day but were not tested later.b–i,Zif268 expression 30 min after re-exposure to contextual stimuli. b, c,Zif268 expression in neurons in the hippocampus: CA1 (b), DG (c). d, e,Photomicrographs (100×) of small dark silver grains associated with CA1 pyramidal cells from an individual rat from the control (d) and context (e) groups.f, i,Zif268 expression in neurons in the amygdala: B (f), LA (g), CeN (h), and AB (i). All results are presented as mean ± SEM values.

Fig. 3.

Expression of zif268 after the retrieval of discrete CS fear associations. a, Freezing of rats in the cued (n = 8) and context (n = 8) groups in response to re-exposure to the discrete cue at retrieval testing. Animals in the control group (n = 8) were given shock and cue presentations on the training day but were not tested later. b–i,Zif268 expression 30 min after re-exposure to the cue stimulus stimuli. b, c,Zif268 expression in neurons in the hippocampus: CA1 (b), DG (c). d, e, Photomicrographs (100×) of small dark silver grains associated with LA pyramidal cells from an individual rat from the control (d) and cued (e) groups. f, i, Zif268expression in neurons in the amygdala: B (f), LA (g), CeN (h), and AB (i). All results are presented as mean ± SEM values.

Fig. 4.

Expression of zif268 after the retrieval of recent versus old contextual–fear associations.a, Freezing of rats in the control and context groups in response to re-exposure to training context 24 hr after training. Expression of zif268 in neurons of the CA1 (b) and DG (c) after memory retrieval at 24 hr. d, Freezing of rats in the control and context groups in response to re-exposure to training context 28 d after training. Expression of zif268 in neurons of the CA1 (e) and DG (f) after memory retrieval at 28 d. All results are presented as mean ± SEM values.