Rationale: There are controversial pieces of evidence whether combination therapies using memantine and cholinesterase inhibitors are beneficial over their monotreatments. However, results of preclinical studies are promising when memantine is combined with agonists and allosteric modulators of the alpha7 nicotinic acetylcholine receptor (nAChR). Objectives: Here, we tested the hypothesis that cognitive enhancer effects of memantine can be potentiated through modulating alpha7 nAChRs in a scopolamine-induced amnesia model. Methods: Monotreatments, as well as co-administrations of selective alpha7 nicotinic acetylcholine receptor agonist PHA-543613 and memantine were tested in the Morris water maze task in rats. The efficacy of the co-administration treatment was observed on different domains of spatial episodic memory. Results: Low dose of memantine (0.1 mg/kg) and PHA-543613 (0.3 mg/kg) successfully reversed scopolamine-induced short-term memory deficits both in monotreatments and in co-administration. When recall of information from long-term memory was tested, pharmacological effects caused by co-administration of subeffective doses of memantine and PHA-543613 exceeded that of their monotreatments. Conclusion: Our results further support the evidence of beneficial interactions between memantine and alpha7 nAChR ligands and suggest a prominent role of alpha7 nAChRs in the procognitive effects of memantine.

Songorine (SON) is a diterpenoid alkaloid from Aconitum plants. Preparations of Aconitum roots have been employed in traditional oriental herbal medicine, however, their mechanisms of action are still unclear. Since GABA-receptors are possible brain targets of SON, we investigated which subtypes of GABA-receptors contribute to the effects of SON, and how SON affects anxiety-like trait behavior and psychomotor cognitive performance of rats. First, we investigated the effects of microiontophoretically applied SON alone and combined with GABA-receptor agents picrotoxin and saclofen on neuronal firing activity in various brain areas. Next, putative anxiolytic effects of SON (1.0–3.0 mg/kg) were tested against the GABA-receptor positive allosteric modulator reference compound diazepam (1.0–5.0 mg/kg) in the elevated zero maze (EOM). Furthermore, basic cognitive effects were assessed in a rodent version of the psychomotor vigilance task (PVT). Local application of SON predominantly inhibited the firing activity of neurons. This inhibitory effect of SON was successfully blocked by GABA(A)-receptor antagonist picrotoxin but not by GABA(B)-receptor antagonist saclofen. Similar to GABA(A)-receptor positive allosteric modulator diazepam, SON increased the time spent by animals in the open quadrants of the EOM without any signs of adverse psychomotor and cognitive effects observed in the PVT. We showed that, under in vivo conditions, SON acts as a potent GABA(A)-receptor agonist and effectively decreases anxiety without observable side effects. The present findings facilitate the deeper understanding of the mechanism of action and the widespread pharmacological use of diterpene alkaloids in various CNS indications.

Promising new pharmacological strategies for the enhancement of cognition target either nicotinic acetylcholine receptors (nAChR) or N-methyl-D-aspartate receptors (NMDAR). There is also an increasing interest in low-dose combination therapies co-targeting the above neurotransmitter systems to reach greater efficacy over the monotreatments and to reduce possible side effects of high-dose monotreatments. In the present study, we assessed modulatory effects of the α7 nAChR-selective agonist PHA-543613 (PHA), a novel α7 nAChR positive allosteric modulator compound (CompoundX) and the NMDAR antagonist memantine on the in vivo firing activity of CA1 pyramidal neurons in the rat hippocampus. Three different test conditions were applied: spontaneous firing activity, NMDA-evoked firing activity and ACh-evoked firing activity. Results showed that high but not low doses of memantine decreased NMDA-evoked firing activity, and low doses increased the spontaneous and ACh-evoked firing activity. Systemically applied PHA robustly potentiated ACh-evoked firing activity with having no effect on NMDA-evoked activity. In addition, CompoundX increased both NMDA- and ACh-evoked firing activity, having no effects on spontaneous firing of the neurons. A combination of low doses of memantine and PHA increased firing activity in all test conditions and similar effects were observed with memantine and CompoundX but without spontaneous firing activity increasing effects. Our present results demonstrate that α7 nAChR agents beneficially interact with Alzheimer's disease medication memantine. Moreover, positive allosteric modulators potentiate memantine effects on the right time and the right place without affecting spontaneous firing activity. All these data confirm previous behavioral evidence for the viability of combination therapies for cognitive enhancement.

Repetitive mild traumatic brain injuries (TBI) impair cognitive abilities and increase risk of neurodegenerative disorders in humans. We developed two repetitive mild TBI models in rats with different time intervals between successive weight-drop injuries, and assessed cognitive performance and biomarker profiles. Rats were subjected to repetitive Sham (no injury), single mild (mTBI), repetitive mild (rmTBI – 5 hits, 24 h apart), rapid repetitive mild (rapTBI – 5 hits, 5 min apart) and single severe (sTBI) TBI. We assessed cognitive performance 2 and 8 weeks after TBI in the novel object recognition test (NOR), and 6-7 weeks after TBI in the water maze (MWM). Acute immunohistochemical markers were checked 24 h after TBI, and blood biomarkers were measured with ELISA 8 weeks after TBI. In the NOR, both rmTBI and rapTBI showed poor performance at 2 weeks post-injury. At 8 weeks post-injury, the rmTBI group still performed worse than the Sham and mTBI groups, while the rapTBI group recovered. In the MWM, the rapTBI group performed worse than Sham and mTBI. Acute APP and RMO-14 immunohistochemistry showed axonal injury at the pontomedullary junction in the sTBI, but not in other groups. ELISA showed increased serum GFAP levels 8 weeks after sTBI, while no differences were found between the injury groups in the levels of phosphorylated-tau and S100β. Results suggest that the rmTBI protocol is the most suitable model for testing cognitive impairment after mild repetitive head injuries. The lack of common biomarkers suggests novel unknown underlying mechanisms of rmTBI.

Alpha7 nicotinic acetylcholine receptors (nAChRs) are promising novel targets for the treatment of neurocognitive disorders. Although the cognitive enhancer potential of alpha7 nAChR agonists and positive allosteric modulators (PAMs) has been confirmed in several preclinical animal models, there are only sparse in vivo electrophysiological data on their effects on the firing activity and excitability of neurons. The present study investigated and compared local effects of alpha7 nAChR agonist PHA-543613 and PAMs PNU-120596 and NS-1738 on the spontaneous and N-methyl-D-aspartate-evoked (NMDA-evoked) firing rate of rat CA1 hippocampal pyramidal cells, in vivo . Furthermore, effects of alpha7 nAChR antagonist methyllycaconitine (MLA) and GABA were also tested. Results showed substantially different effects of the alpha7 nAChR agonist and PAMs. While PNU-120596 and NS-1738 predominantly and significantly increased both spontaneous and NMDA-evoked firing rate of the neurons, application of PHA-543613 resulted in almost equal distribution of facilitatory and inhibitory effects. The increase of the NMDA-evoked firing rate exerted by NS-1738 was superadditive over the sum of the single effects of NMDA and NS-1738. The simultaneous application of alpha7 nAChR agonist PHA-543613 and PAM NS-1738 resulted in additive increase of both spontaneous and NMDA-evoked firing rate. However, NS-1738 counteracted inhibitory effects of PHA-543613 in 5 out of 6 neurons, resulting in a synergistic potentiation of their firing responses to NMDA. Our results suggest that alpha7 nAChR PAMs increase neuronal excitability more potently than agonists, while the remarkable occurrence of inhibitory effects of PHA-543613 (possibly originating from receptor desensitization) implies that agonists may exert neuroprotective effects.

Combination treatments based on pharmacological interactions at α7 nAChRs are promising therapeutic approaches for neurocognitive disorders (NCDs). Memantine, an already approved medication in some NCDs, may not only act as an antagonist of the glutamatergic NMDA receptors (NMDAR) but also serve as an antagonist of the cholinergic α7 nAChRs. Here we set out to utilize a combination treatment regime with an α7 nAChR-selective agonist (PHA-543613) and a novel proprietary α7 nAChR ligand with marked positive allosteric modulator (PAM) activity (CompoundX). The cognitive efficacy of combination treatments was tested in a naturally aged rat model. Naturally aged rats showed marked cognitive decline in the novel object recognition (NOR) test, and they displayed pathological changes at the molecular level in terms of various inflammatory markers. In addition, aged rats also exhibited cholinergic changes such as mRNA upregulation of α7 nAChRs. Memantine-PHA-543613 and memantine-CompoundX combination treatments successfully alleviated the age-related decline of recognition memory of rats by exceeding the null-effects of the corresponding subtherapeutic levels of monotreatments. Results indicate a positive interaction between memantine and α7-nAChR agonists and PAMs, which reflects a prominent role of α7 nAChRs in the cognitive enhancement of combination treatments especially in age-related cognitive decline. Moreover, the putative direct action of memantine on α7 nAChRs may also contribute to the observed synergistic interaction between memantine and other selective α7 nAChR compounds.