Histamine H3 receptor antagonist E177 attenuates amnesia induced by dizocilpine without modulation of anxiety-like behaviors in rats

Abstract

Background: Alzheimer disease (AD) is the main cause of dementia in elderly people. The potential of histamine H3 receptor (H3R) antagonists as a pharmacological treatment of several neuropsychiatric diseases is well established.

Methods: The novel non-imidazole-based H3R antagonist E177 was screened for its pro-cognitive effects on the inhibitory avoidance paradigm (IAP) and novel object recognition (NOR) task in a dizocilpine (DIZ)-induced model of amnesia in male Wistar rats. Donepezil, an acetylcholine esterase inhibitor, was used as the reference drug.

Results: Acute systemic treatment with E177 (1.25, 2.5, 5, and 10 mg/kg intraperitoneally [i.p.]) significantly attenuated the cognitive impairments induced by DIZ in the IAP (all P-values <0.05, n=7), and the protective effect of the most promising dose of E177 (5 mg/kg) was abrogated when H3R agonist R-(α)-methylhistamine (RAMH; 10 mg/kg i.p.) was co-administered (P=0.281 for DIZ-amnesia group vs DIZ + E177 + RAMH group, n=7). The discrimination index calculated for E177 (5 mg/kg, i.p.) showed a significant memory-enhancing effect on DIZ-induced short-term memory impairment in the NOR task (P<0.05, n=6), with the enhancement nullified when animals were co-administered RAMH (10 mg/kg). Moreover, the results revealed that E177 (5 and 10 mg/kg, i.p.) did not alter the anxiety levels and locomotor activity of animals naïve to the open-field test (all P-values >0.05, n=8) or the elevated plus maze test (all P-values >0.05, n=6-8), which indicated that the E177-induced enhancement of memory performance in the IAP or NOR task was unrelated to changes in emotional response or in spontaneous locomotor activity.

Conclusion: The observed results suggested a possible contribution of H3Rs in the alteration of brain neurotransmitters that accompany neurodegenerative diseases, such as AD.

Keywords: Histamine H3 receptors; antagonist; anxiety; dizocilpine-induced amnesia; elevated plus maze; inhibitory avoidance paradigm; memory; novel object recognition; open field test.

Figure 1

Structure and in vitro affinities of the H3R antagonist E177, 1-(6- (naphthalen-2-yloxy)hexyl)azepane, on histamine receptor subtypes. Notes: ^a[³H]N^α-methylhistamine binding assay was performed using a cell membrane preparation of HEK-hH3 cells stably expressing the human H3R (n=3). ^b[³H]Histamine binding assay was performed using a cell membrane preparation of Sf9 cells transiently expressing the human histamine H4R and co-expressed with G_αi2 and β1γ2 subunits (n=2). ^c[³H]pyrilamine binding assay was performed with the cell membrane preparation of CHO-hH1R cells stably expressing the human H1R (n=2).

Figure 2

E177 attenuated DIZ-induced memory deficits in the IAP. Notes: Average STL time measured on the first training day before the delivery of foot shock (white columns), and average STL time measured on the test day (black columns). Acute systemic administration of E177 at different doses (1.25, 2.5, 5, and 10 mg/kg i.p.) or DOZ (1 mg/kg i.p.) 30–45 minutes before the test session. #P<0.0001 for average STL time compared with that in the saline-treated group. *P<0.0001 for average STLs compared with the DIZ-treated group. ^&P<0.0001 compared with the DOZ-treated group. ^$P<0.05 compared with the DOZ-treated group. The data are expressed as the mean ± SEM (n=7). Abbreviations: DIZ, dizocilpine; DOZ, donepezil; IAP, inhibitory avoidance paradigm; i.p., intraperitoneally; SAL, saline; SEM, standard error of the mean; STL, step-through latency.

Figure 3

RAMH reversed E177-induced memory enhancement of the DIZ-induced deficit in the IAP. Notes: Average of STL time measured on the first training day before the delivery of foot shock (white columns), and average of the STL time measured on the test day (black columns). Acute systemic administration of E177 (5 mg/kg, i.p.), RAMH, and E177 (5 mg/kg) + RAMH (10 mg/kg) for 15–20 minutes (for RAMH) or 30–45 minutes (for H3R antagonist E177) before the test session. ^#P<0.001 average STLs compared with the saline-treated group. *P<0.001 for the average STL time compared with that in the DIZ-treated group. The data are expressed as the mean ± SEM (n=7). Abbreviations: DIZ, dizocilpine; IAP, inhibitory avoidance paradigm; i.p., intraperitoneally; RAMH, R-(α)-methyl-histamine dihydrochloride; SAL, saline; SEM, standard error of the mean; STL, step-through latency.

Figure 4

E177 mitigated DIZ-induced short-term deficits in NOR paradigm. Notes: E177 (2.5, 5, or 10 mg/kg, i.p.) or DOZ (1 mg/kg, i.p.) was injected after T1 (training session) with DIZ (0.1 mg/kg, i.p.). The T2 (test session) was conducted 120 minutes (STM, A) or 24 hours (LTM, B) after T1 (training session). The results are expressed as DIs for the time spent exploring both objects (familiar and novel). ^#P<0.0001 compared with the SAL-treated group. *P<0.05 compared with the DIZ-treated group. ***P<0.0001 compared with the DIZ-treated group. ^$P<0.05 compared with the E177 (5 mg)-treated group. The data are expressed as the mean ± SEM (n=6). Abbreviations: DIs, discrimination indices; DIZ, dizocilpine; DOZ, donepezil; i.p., intraperitoneally; LTM, long-term memory; NOR, novel object recognition; RAMH, R-(α)-methyl-histamine dihydrochloride; SAL, saline; SEM, standard error of the mean; STM, short-term memory.

Figure 5

E177 failed to alter anxiety-like behavior or locomotion in the EPM test. Notes: Effect of E177 (5 or 10 mg/kg, i.p.) and DZP (10 mg/kg, i.p.) injections on the percentage of time spent in open arms (A), total number of open arm entries (B), and total number of closed arms entries (C). *P<0.05 for the value of the DZP-treated group compared with the saline-, E177 (5 mg)-, or E177 (10 mg)-treated groups. The data are expressed as the mean ± SEM (n=6). Abbreviations: DZP, diazepam; EPM, elevated plus maze; i.p., intraperitoneally; SAL, saline; SEM, standard error of the mean; OA, open arms; CA, closed arms.