However, there is no evidence as yet that the ability of memantine to protect against extrasynaptic NMDA receptor-mediated excitotoxicity has a disease-modifying effect in Alzheimers, although this has been suggested in animal models.Memantines antagonism on NMDA receptors has aroused interest in repurposing it for mental illnesses such as bipolar disorder, considering the involvement of the glutamatergic system in the pathophysiology of mood disorders. Serotonin
Memantine acts as a non-competitive antagonist at the 5-HT3 receptor, with a potency similar to that for the NMDA receptor. Many 5-HT3 antagonists function as antiemetics, however the clinical significance of this serotonergic activity in the treatment of Alzheimers disease is unknown. Cholinergic
Memantine acts as a non-competitive antagonist at different neuronal nicotinic acetylcholine receptors (nAChRs) at potencies possibly similar to the NMDA and 5-HT3 receptors, but this is difficult to ascertain with accuracy because of the rapid desensitization of nAChR responses in these experiments. It can be noted that memantine is an antagonist at Alpha-7 nAChR, which may contribute to initial worsening of cognitive function during early memantine treatment. Alpha-7 nAChR upregulates quickly in response to antagonism, which could explain the cognitive-enhancing effects of chronic memantine treatment. It has been shown that the number of nicotinic receptors in the brain are reduced in Alzheimers disease, even in the absence of a general decrease in the number of neurons, and nicotinic receptor agonists are viewed as interesting targets for anti-Alzheimer drugs.
Dopamine
Memantine was shown in a study to act as an agonist at the dopamine D2HIGH receptor with equal or slightly higher affinity than to the NMDA receptors; however, the relevance of this may be negligible, as studies have shown very low affinity for binding to D2 receptors in general. Sigmaergic
Memantine acts as an agonist at the σ1 receptor with a low affinity (Ki 2.6 μM). The consequences of this activity are unclear (as the role of sigma receptors in general is not yet that well understood). Due to this low affinity, therapeutic concentrations of memantine are most likely too low to have any sigmaergic effect as a typical therapeutic dose is 20 mg, however excessive doses of memantine taken for recreational purposes many times greater than prescribed doses may indeed activate this receptor. History
Memantine was first synthesized and patented by Eli Lilly and Company in 1968 as an anti-diabetic agent, but it was ineffective at lowering blood sugar. Later it was discovered to have CNS activity, and was developed by Merz for dementia in Germany; the NMDA activity was discovered after trials had already begun. Memantine was first marketed for dementia in Germany in 1989 under the name Axura.In the US, some CNS activities were discovered at Childrens Hospital of Boston in 1990, and Childrens licensed patents covering uses of memantine outside the field of ophthalmology to Neurobiological Technologies (NTI) in 1995.