Memory is an important cognitive function without which life is not possible. Information is saved through harmonic neural networks in the brain (18). Hippocampus composition plays an important role in the calculation and saving of spatial information. The pyramidal cells of the hippocampus CA1 region are too sensitive and among the first neurons that would be destroyed in pathologic conditions and some brain degenerative diseases (19). There are dominant GABAA subunits receptors of α2 and α5 in the hippocampus that exists in whole regions of CA1 and CA3 (20). GABAA-α2 and GABAA-α5 receptors are high-affinity binding sites for benzodiazepines (4). These two subunits are crucial in learning and memory (21).
The suttle box of the behavioural test has been conducted for the present research to investigate learning and memory, and the results revealed that the group that received solvent showed no significant differences regarding STL amount compared to the intact group. So, the extracting solvent DMSO had no effect on the process of learning and memory in male rats. Meanwhile, the STL amount in the group that received tarragon hydroalcoholic extract with doses of 50 and 100 mg/kg and coumarin with a dose of 3 mg/kg significantly decreased compared to the control+ group. Probably such decrease was due to direct or indirect impact on GABAA receptor which is aligned with the previous studies and researches.
Ding et al. (2014) introduced methyl eugenol as a new agonist of ionotropic GABA receptors. Methyl eugenol has a strong inhibitory activity against nerve stimulation in hippocampal neurons. Methyl eugenol inhibits voltage-gated sodium channels, voltage-gated potassium channels and voltage-gated calcium channels (22). Intraperitoneal injection of methyl eugenol mitigates anxiety behaviours in rats, which is likely to be accompanied by the activation of GABAA receptors. Thus, compounds of tarragon such as estragole, methyl eugenol and coumarin represent a new agonist of GABAA receptors (23).
Kavvadias et al. indicated in a research conducted in 2000 that the tarragon hydroalcoholic extract contains benzodiazepines such as temazepam and lorazepam (100-200 ng/g) (24). Through binding to the GABAA receptors, benzodiazepines produce anticonvulsant effects. Such compounds as the GABAA agonist receptors increases the frequency of opening chlorine channel related to the receptors (25). Moreover, the injection of quercetin to the rats exposed to chemical kindling increases oxidative stress and produces dose-dependent anticonvulsant effects (26). Naringenin passes through the blood-brain barrier and affects the central nervous system via the GABAA receptors (27). The injecting alpha-Pinene to the rats and recording of electroencephalographic curves of the brain also illustrates the binding of the benzodiazepine site of GABAA receptors to alpha-Pinene, which is used as insomnia medication (28). Terpinene induces effects similar to those of pentobarbital and diazepam including strong sedative and anticonvulsant effects (29). Ahmad Tarmizi et al (2014) showed in research that trans-anethole through binding to the GABAA receptor increase its activity and could be used as an anti-epileptic drug (30). Also coumarin injection to the rats produced sedative and anti-epileptic effects that control the brain’s neurons through increasing GABAA release. Coumarin attached to the benzodiazepine site of GABAA receptor in three points and increases the activities of this receptor as an anti-epileptic drug (9).
Jarogniew et al. (2010) used electronic kindling to create convulsion and through injecting intraperitoneal coumarin to rats, they found that convulsion threshold has extremely decreased; and probably such result was due to attachment of coumarin to the GABAA receptor and the increase of its activity (31).
The comparison of the mentioned results with the present research indicates that probably the tarragon hydroalcoholic extract having different substances attached to the benzodiazepine sites in GABAA receptors such as coumarin increases the activities of this receptor and by opening the chloride channel of such receptor fortifies hyperpolarization and finally intensifies the memory disorder in male rats.
In the present study, a histology test has been performed using Nissle colouring for measuring the density of the pyramidal healthy neurons cells of the hippocampus. The results show that the decrease of pyramidal healthy cells in the treated group with 50 and 100 mg/kg doses of tarragon hydroalcoholic extract, 3 and 5 mg/kg coumarin indicated the destruction of hippocampus pyramidal cells compared to the control+ group probably through substances in the tarragon hydroalcoholic extract which confirms the results of Guo et al. researches conducted in 2020. The findings of the research showed that the intraventricular injection of methyl eugenol to rats ended in the increase of tension in rats. Meanwhile, methyl eugenol extremely caused hippocampus functional disorder at behavioural, cellular and molecular levels and this indicated potential dangers of methyl eugenol to the central nervous system (32). Moreover, Pawel Kowalczyk et al (2020) researches showed the anti-cancer and antimicrobial effects of coumarin; yet, excessive consumption of coumarin might negatively affect the live cell and even destroy lung and liver’s cells. In addition, the toxic effect of coumarin destroys the live cells including bacteria (33, 34). Thus, it seems the toxic feature of coumarin depending on its dose has affected the hippocampus pyramidal cells.
A real-time PCR test was used in the present study to express the gene. The results of the RT_PCR test indicated that GABAA-α2 and GABAA-a5 gene expression in the control+ group showed no significant difference compared to the intact group. So, the solvent had probably no effect on the two genes expression. Yet, the GABAA-α5 gene expression in the group treated with 50 and 100 mg/kg dose of tarragon hydroalcoholic extract significantly increased in comparison to the control+ group. The two subunits of α2 and α5 of GABAA receptors play important roles in memory (35). GABAA receptors have also binding sites for componds of benzodiazepine, barbital and steroids; these receptors at the presence of GABA and in some cases directly, induce GABAA and activate the receptors’ channel (36). Since the high density of GABAA receptors in the hippocampus indicate that activating the GABAA receptors can control the process of learning and memory (37), it is probable that the memory disorder reported in this group is due to the increase of GABAA-α5 gene expression, the increase of GABAA inhibitory synapses, the increase of time duration of such synapses and finally because of the more sensitivity of such receptors to the GABA. The present research result shows the overlapping of histology and RT_PCR tests, which is consistent with previous findings.
Zhao et al. found in research that injecting a low dose of isoflurane (anaesthesia substance) can decrease the GABAA-α5 gene mRNA expression in hippocampus nerve cells in old ages, yet it is increased in young animals. These findings showed that L-655708 injection (agonist GABAA-α5) inhibits spatial learning, memory disorder and spatial memory due to isoflorane in Wistar old rats (38).
Magnin et al. found in research that inhibiting GABAA-α5 in rats controlled can also improve spatial learning compared to the rats having the inactive vasoactive intestinal peptide in the CA1 region (39).
Etherington et al. carried out a study that S44819 is a tricyclic peptide that can act as the chosen agonist of GABAA-α5 Receptor, thus, S44819 causes the hippocampus synopsis flexibility and increases pre-recognition effectiveness. Regarding such features, S44819 might improve the recognition functions in neurological disorders and make it easier to recover after a stroke (40).
Vinnakota et al. (2020) conducted research indicated that α5IA injection, a reversed agonist of GABAA-α5 receptors to Alzheimer induced rats could prevent cell destruction due to (Aβ1– 42) in a laboratory condition that might hold neuroprotective features (41).
Magnin et al. (2018) considered a relation between memory disorder and the increase of GABAA-α5 receptor gene in hippocampus CA1 pyramidal cells (40). Moreover, GABAA-α2 gene expression in the group treated with 25 mg/kg tarragon hydroalcoholic extract showed a significant increase compared to the control+ group. Since the high density of GABA receptors in the hippocampus indicates that activating GABA receptors controls the process of learning and memory, (2) it is probable that memory disorder in this group was due to the increase of GABAA-α2 gene expression, the increase of GABA inhibitory synopsis, the increase of the synopsis time duration and the more the sensitivity of such receptors to GABA. These findings are consistent with the previous results.
Julian I. Hofmann et al. showed in research that prescribing diazepam to wild rats with GABAA-α2 mutant gene resulted in changes in the automatic electronic activity patterns in brain cortex through using benzodiazepine so that the low-frequency increase of produced γ by diazepam might reflect the increased inside cortex harmony. By increasing the high-frequency band, diazepam decreases the frequency power through GABAA-α2 receptors (42).
Toshihiro Nomura et al. conducted research and found that enhancing subunits α2 holding GABAA receptors in Dravet (severe Myoclonic epilepsy in infants) syndrome model can protect rats against convulsions (43).