Modulation of long-term memory for object recognition via HDAC inhibition

Abstract

Histone acetylation is a chromatin modification critically involved in gene regulation during many neural processes. The enzymes that regulate levels of histone acetylation are histone acetyltransferases (HATs), which activate gene expression and histone deacetylases (HDACs), that repress gene expression. Acetylation together with other histone and DNA modifications regulate transcription profiles for specific cellular functions. Our previous research has demonstrated a pivotal role for cyclicAMP response element binding protein (CREB)-binding protein (CBP), a histone acetyltransferase, in long-term memory for novel object recognition (NOR). In fact, every genetically modifiedCbp mutant mouse characterized thus far exhibits impaired long-term memory for NOR. These results suggest that long-term memory for NOR is especially sensitive to alterations in CBP activity. Thus, in the current study, we examined the role of HDACs in memory for NOR. We found that inducing a histone hyperacetylated state via HDAC inhibition transforms a learning event that would not normally result in long-term memory into an event that is now remembered long-term. We have also found that HDAC inhibition generates a type of long-term memory that persists beyond a point at which normal memory for NOR fails. This result is particularly interesting because one alluring aspect of examining the role of chromatin modifications in modulating transcription required for long-term memory processes is that these modifications may provide potentially stable epigenetic markers in the service of activating and/or maintaining transcriptional processes.

Fig. 1.

Effect of training period (3 min versus 10 min) and habituation to context on long-term memory for the familiar object. (A) Two groups of mice were habituated and then exposed to the objects for either 10 min or 3 min. A third group received no habituation followed by a 10-min training period. All 3 groups exhibited equal exploration times for each object. (B) During the 24-h retention test, animals receiving 10 min of training after 3 days of habituation (n = 10) displayed a significant preference for the novel object, whereas those that received 3 min of training (n = 10) or 10 min of training with no habituation (n = 10) showed no significant preference (P > 0.05). (C) During a 90-min retention test, animals receiving 10 min of training (n = 10) displayed a significant preference for the novel object, whereas those that received 3 min of training (n = 10) showed no significant preference.

Fig. 2.

HDAC inhibition facilitates long-term memory, measured at 24 h, after 3 min of object recognition training. (A) During a 24-h retention test, mice that received 3 min of training followed by an i.p. administration of NaBut (n = 8) displayed significantly enhanced preference for the novel object compared with vehicle-treated mice (n = 8). (B) During a 90-min retention test, mice that received NaBut (n = 8) immediately after 3 min of training showed no preference for the novel object compared with vehicle-treated mice (n = 8). (C) Mice that did not receive habituation to the context before a 10-min training period were unable to form a long-term memory for the familiar object. On the training day, mice were exposed to the experimental apparatus with 2 identical objects for 10 min. Both groups showed similar exploration times for each object. During the 24-h retention test, mice treated with NaBut (n = 8) or vehicle (n = 8) did not display a significant preference for the novel object.

Fig. 3.

HDAC inhibition generates a type of long-term memory that persists at least 7 days. (A) During the 7-day retention trial, animals receiving 3 min of training followed by an i.p. administration of NaBut (n = 9) displayed a significant preference for the novel object compared with vehicle-treated controls (n = 8). (B) To determine the effect of NaBut on retrieval, animals receiving a 10-min training period were injected with i.p. NaBut 1 h before a 7-day retention test. We examined 2 different doses of NaBut (normal dose: 1.2 g/kg, n = 9; low dose: 0.6 g/kg, n = 11; vehicle, n = 9). Neither dose of NaBut affected behavior of mice in the retrieval test compared with vehicle-treated mice. (C) To demonstrate that NaBut was active during the retention test in Fig. 3B, we retested the same mice 24 h later using a different novel object paired with the former novel object in the 7-day retention test. NaBut-treated mice (low dose, n = 11; normal dose, n = 9) exhibited significantly enhanced preference for the novel object compared with vehicle-treated mice (n = 9).

Fig. 4.

HDAC inhibition can induce a form of memory for object recognition that persists beyond the point at which normal memory fails. (A) CBP^KIX/KIX mice that received 10 min of training immediately followed by an i.p. injection of NaBut (n = 7) displayed a significant preference for the novel object compared with vehicle-treated CBP^KIX/KIX mice (n = 7) during a 7-day retention test. (B) Wild-type CBP^+/+ mice that received 10 min of training immediately followed by an i.p. injection of NaBut (n = 9) displayed a significant preference for the novel object compared with vehicle-treated wild-type CBP^+/+ mice (n = 7) during a 7-day retention test.