Chemogenetic activation of histamine neurons promotes retrieval of apparently lost memories

Abstract

Memory retrieval can become difficult over time, but it is important to note that memories that appear to be forgotten might still be stored in the brain, as shown by their occasional spontaneous retrieval. Histamine in the central nervous system is a promising target for facilitating the recovery of memory retrieval. Our previous study demonstrated that histamine H3 receptor (H3R) inverse agonists/antagonists, activating histamine synthesis and release, enhance activity in the perirhinal cortex and help in retrieving forgotten long-term object recognition memories. However, it is unclear whether enhancing histaminergic activity alone is enough for the recovery of memory retrieval, considering that H3Rs are also located in other neuron types and affect the release of multiple neurotransmitters. In this study, we employed a chemogenetic method to determine whether specifically activating histamine neurons in the tuberomammillary nucleus facilitates memory retrieval. In the novel object recognition test, control mice did not show a preference for objects based on memory 1 week after training, but chemogenetic activation of histamine neurons before testing improved memory retrieval. This selective activation did not affect the locomotor activity or anxiety-related behavior. Administering an H2R antagonist directly into the perirhinal cortex inhibited the recovery of memory retrieval induced by the activation of histamine neurons. Furthermore, we utilized the Barnes maze test to investigate whether chemogenetic activation of histamine neurons influences the retrieval of forgotten spatial memories. Control mice explored all the holes in the maze equally 1 week after training, whereas mice with chemogenetically activated histamine neurons spent more time around the target hole. These findings indicate that chemogenetic activation of histamine neurons in the tuberomammillary nucleus can promote retrieval of seemingly forgotten object recognition and spatial memories.

Keywords: Chemogenetics; Histamine; Memory; Perirhinal cortex; Retrieval; Tuberomammillary nucleus.

Fig. 1

Enhanced retrieval of object recognition memories via chemogenetic activation of histamine neurons. A Illustration depicting the injection of AAV8-hSyn-DIO-hM3Dq-mCherry or AAV8- hSyn-DIO-mCherry into the TMN of HDC-IRES-Cre mice. B Confocal microscopy image showing hM3Dq-mCherry expression in the TMN. C Representative images showing HDC expression in hM3Dq-mCherry positive neurons. D The proportion of HDC positive neurons among hM3Dq-mCherry positive neurons. N = 6 mice. E Representative images showing c-Fos expression in hM3Dq-mCherry positive neurons. F Administration of CNO increased the proportion of c-Fos positive neurons among hM3Dq-mCherry positive neurons (**P = 0.0014, unpaired t-test). hM3Dq-Saline: N = 4 mice, hM3Dq-CNO: N = 4 mice. G Novel object recognition memory was assessed 1-week post-training. H Mice with hM3Dq in histamine neurons treated with CNO showed a preference for exploring the novel object (**P < 0.0001, Sidak’s test after two-way repeated measures ANOVA (interaction, F(3, 34) = 8.60, P = 0.0002)). I The discrimination score, a measure of distinguishing novel objects from familiar objects, was greater in the hM3Dq-CNO group compared to the control groups (**P < 0.01, Tukey’s test after two-way ANOVA (interaction, F(1, 34) = 6.35, P = 0.0166)). J The distance traveled during the test session was comparable across groups. mCherry-Saline: N = 9 mice, mCherry-CNO: N = 9 mice, hM3Dq-Saline: N = 10 mice, hM3Dq-CNO: N = 10 mice. Values are reported as mean ± SEM

Fig. 2

No impact of chemogenetic activation of histamine neurons on anxiety-related behavior. A Time spent in open arms of the elevated plus maze was consistent across the 4 behavioral groups. B The frequency of visits to open arms was similar across all groups. mCherry-Saline: N = 11 mice, mCherry-CNO: N = 10 mice, hM3Dq-Saline: N = 12 mice, hM3Dq-CNO: N = 13 mice. Values are reported as mean ± SEM

Fig. 3

Activation of histamine H2 receptors in the PRh is necessary for the retrieval recovery. A Illustration depicting the placement of a guide cannula above the PRh. B A representative image showing the trajectory of the guide cannula, positioned 1 mm above the PRh. C Mice with hM3Dq expressed in histamine neurons were subjected to an object recognition memory test 1 week after their training session. Thirty minutes before the test, they received either ranitidine or saline directly into the PRh, followed by administration of CNO. D Mice treated with ranitidine showed a reduced discrimination score in comparison to those treated with saline. *P = 0.0153, unpaired t-test. N = 10 mice. Values are reported as mean ± SEM

Fig. 4

Chemogenetic activation of histamine neurons promotes retrieval of forgotten spatial memories. A Mice underwent 4 days of training in the Barnes maze. Memory testing occurred either 1 day or 1 week after the last training day. The mice showed a preference for the target and adjacent holes in the 1-day test, but no hole preference was observed in the 1-week test (**P < 0.001, Sidak’s test after two-way repeated measures ANOVA (interaction, F(19, 114) = 4.53, P < 0.0001)). N = 4 mice. B Mice with hM3Dq underwent the same training and a probe test 1 week later. The mice with CNO spent more time around the target hole compared to those treated with saline (**P < 0.0001, Sidak’s test after two-way repeated measures ANOVA (interaction, F(19, 285) = 2.87, P < 0.0001)). Saline: N = 8 mice, CNO: N = 9 mice. Values are reported as mean ± SEM