No nerve growth factor response to treatment with memantine in adult rats.

Nerve growth factor (NGF) is the most widely examined neurotrophin in the experimental models of Alzheimer's disease (AD) and has been shown to prevent the retrograde degeneration of cholinergic neurons. In this study we examined NGF and cholineacetyltransferase (ChAT) changes in several rat brain regions after excitotoxic lesion of the entorhinal cortex with quinolinic acid and tested the effect of memantine on spatial learning in the radial maze after lesion. We observed a significant increase (+26%, p=0.02) of NGF concentrations in the hippocampus of the lesioned rats when compared to sham-lesioned rats. Chronic treatment with memantine showed no significant effect on the NGF increase in the hippocampus (p=0.72). The ChAT activity was significantly increased in the lesioned rats when compared to controls (+16%, p<0.05) and did not depend on treatment with memantine. In spite of this, memantine improved performance of the radial maze. This indicates that memory improving effects of memantine observed in experimental animals and in clinical studies are probably not related to changes in brain NGF content, whereas the observed NGF increase in the denervated hippocampus is probably trauma-related reflecting impaired retrograde transport of hippocampal NGF.

The muscarinic receptor agonist xanomeline has an antipsychotic-like profile in the rat.

The muscarinic receptor agonist xanomeline was examined and compared with the antipsychotics clozapine and/or haloperidol in the following in vivo rat models: apomorphine-induced disruption of prepulse inhibition (PPI), amphetamine-induced hyperlocomotion, and the conditioned emotional response (CER) test. The effects of xanomeline were also assessed ex vivo on dopamine turnover in the rat medial prefrontal cortex. Under conditions of varying dose and prepulse intensity, xanomeline, like haloperidol, had no effect on PPI. In contrast, the muscarinic receptor antagonist scopolamine and the muscarinic receptor agonist pilocarpine both induced significant dose-dependent deficits in PPI. Haloperidol and xanomeline, but not pilocarpine, dose dependently reversed apomorphine-induced disruption of PPI. Thus, xanomeline induced a clear antipsychotic-like effect in PPI, whereas pilocarpine appeared to induce a psychotomimetic-like effect. Xanomeline attenuated amphetamine-induced hyperactivity at doses that had no effect on spontaneous activity, possibly indicating a separation between attenuation of limbic hyperdopaminergic function and the induction of hypolocomotion. Haloperidol and clozapine also reversed amphetamine-induced hyperlocomotion, but at similar doses to those that reduced spontaneous locomotion. Clozapine, but not haloperidol had an anxiolytic-like effect in the CER test. The effects of xanomeline in the CER test were similar to those of clozapine, although at the anxiolytic dose it tended to disrupt baseline levels of lever pressing. Finally, haloperidol, clozapine, pilocarpine, and xanomeline, all induced an increase in dopamine turnover in medial prefrontal cortex. The antipsychotic-like effects of xanomeline in the animal models used here suggest that it may be a useful treatment for psychosis.

Dopamine release in the prefrontal cortex in response to memantine following sub-chronic NMDA receptor blockade with memantine: a microdialysis study in rats.

Memantine is an uncompetitive N-methyl-D-aspartate receptor antagonist which blocks the NMDA receptor with moderate-affinity in a use- and voltage dependent manner. In clinical practice it is used chronically in the treatment of dementia and does not induce psychotomimetic effects as, high affinity, uncompetitive antagonists. Thus, it was of interest to determine dopamine (DA) and metabolite (DOPAC - dihydroxyphenylacetic acid and HVA - homovanillic acid) concentrations in the prefrontal cortex (PFC) in response to 14 days administration of memantine (20 mg/kg/day). It was previously determined that in rats this treatment induces sensitization to the locomotor effect and tolerance to the learning impairing properties of high doses of memantine. Acute administration of memantine (20 mg/kg, i.p.) did not affect dopamine levels in the PFC. It did however increase DA metabolite (DOPAC and HVA) concentrations. Administration of memantine (20 mg/kg/day) for 14 days before the acute challenge only slightly changed memantine's effect on PFC neurochemistry even though pharmacokinetic tolerance was observed. When memantine was administered to the sham group, which had been repeatedly treated with Hypnorm (including neuroleptic), an increase in PFC dopamine and metabolite content was seen. In accordance with the fact that memantine does not possess psychotomimetic activity at therapeutically relevant doses, these experiments showed that it does not affect the prefrontal cortex dopamine levels.

Brain distribution of an uncompetitive NMDA receptor antagonist; comparison to its in vitro potency in electrophysiological studies.

Although the concentration of drugs in brain homogenates is relatively easy to determine, such data are sometimes misleading due to accumulation in intracellular compartments. This is apparent for uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists where concentrations assessed in this manner are much higher than those sufficient to block the NMDA channel from the extracellular space. The aim of the present study was to determine whether free brain concentrations (extracellular fluid - ECF) of a new uncompetitive NMDA receptor antagonist MRZ 2/579 (1-amino-1,3,3,5,5-pentamethyl-cyclohexane hydrochloride) following administration of doses effective in animal models are sufficient to block NMDA receptors based on its potency in vitro. This issue was addressed using brain microdialysis corrected for in vivo recovery and patch clamp experiments.MRZ 2/579 blocked steady-state inward current responses of cultured hippocampal neurones to NMDA with an IC50 of 1.11 microM at -70 mV. Much higher concentrations of MRZ 2/579 blocked voltage-activated Ca2+ channels with an IC50 of 340 microM. MRZ 2/579 (10 microM) reduced peak inward current responses of neuronal nicotinic receptors only to 72.3% of control. MRZ 2/579 (10-100 microM) had little or no effect at AMPA and GABA(A) receptors. Following chronic s.c. infusion of MRZ 2/579 (40 mg/kg day for 7 days) brain ECF (2.15 microM) and cerebro-spinal fluid (CSF) levels (2.16 microM) were twofold lower than free plasma levels (4.3 microM). MRZ 2/579 showed pronounced accumulation in brain tissue compared to free plasma (28-fold) and ECF (58-fold). After acute i.p. administration (5, 10 and 20 mg/kg) peak concentrations in ECF were 0.70, 0.96 and 2.53 microM, respectively. In conclusion, MRZ 2/579 is indeed strongly accumulated in brain tissue compared to brain ECF, CSF and plasma. However, the brain ECF levels attained following administration of behaviourally effective doses are sufficient for selective NMDA receptor blockade.

Adaptations of NMDA and dopamine D2, but not of muscarinic receptors following 14 days administration of uncompetitive NMDA receptor antagonists.

Behavioral changes have previously been reported following administrations of uncompetitive NMDA receptor antagonists memantine, amantadine and MK-801 for 14 days, at the doses that produce plasma levels comparable to those seen in patients (20, 100 and 0.31 mg/kg/day respectively). Using the same doses, the effect on receptor binding (autoradiography) was studied in rats. [3H]MK-801 binding was increased in the dentate gyrus and CA3 region of the hippocampus (35.2 and 24.3% respectively) following 3 days S.C. infusion of memantine by ALZET minipumps. One daily injection of memantine for 14 days, increased [3H]MK-801 binding in the frontal cortex by 40.3%. The same treatment with amantadine did increase [3H]raclopride binding to dopamine D2 receptors by 13.5%. None of these treatments changed the expression of muscarinic receptors. It is concluded that subchronic blockade of the NMDA receptor by uncompetitive antagonists at moderate (therapeutically-relevant) doses induced only minor changes in NMDA and dopamine D2 receptor expression.

The role of probenecid-sensitive organic acid transport in the pharmacokinetics of N-methyl-D-aspartate receptor antagonists acting at the glycine(B)-site: microdialysis and maximum electroshock seizures studies.

The purpose of the present study was to determine whether the probenecid-sensitive organic acid transporter is responsible for the short duration of action of a new group of N-methyl-D-aspartate receptor glycine(B)-site antagonists, MRZ 2/570, 2/571, and 2/576. A prolongation of their anticonvulsant activity from 60 to 180 to 240 min, was found in mice after pretreatment with probenecid (200 mg/kg i.p.). Microdialysis studies in rats showed that this is likely due to a change in central nervous system concentrations of these drugs because cotreatment with probenecid caused an increase in the brain extracellular fluid half-life (0.5- to 4-fold) and the brain area under the curve (1.8- to 3.6-fold). In serum the half-life of MRZ 2/576 (30 mg/kg) was also increased by coadministration of probenecid from 15.6 +/- 1.3 to 40.6 +/- 6.0 min. At steady state (MRZ 2/576, 20 mg/kg/h i.v.), brain extracellular fluid concentration was elevated 2.5-fold by concomitant administration of probenecid. These results clearly show that these glycine(B)-site antagonists are rapidly cleared from the systemic circulation and the central nervous system by the probenecid-sensitive organic acid transport system. Moreover, the present data show that MRZ 2/570, 2/571, and 2/576 reach the brain in concentrations (1.34-2.32 microM) above the range of their in vitro potencies at the glycine site of the N-methyl-D-aspartate receptor (0.1-1.0 microM).

Brain penetration and in vivo recovery of NMDA receptor antagonists amantadine and memantine: a quantitative microdialysis study.

PURPOSE: To determine free brain concentrations of the clinically used uncompetitive NMDA antagonists memantine and amantadine using microdialysis corrected for in vivo recovery in relations to serum, CSF and brain tissue levels and their in vitro potency at NMDA receptors. METHODS: Microdialysis corrected for in vivo recovery was used to determine brain ECF concentrations after steady-state administration of either memantine or amantadine. Additionally CSF, serum, and brain tissue were analyzed. RESULTS: Following 7 days of infusion of memantine or amantadine (20 and 100 mg/kg/day respectively) whole brain concentrations were 44-and 16-fold higher than free concentrations in serum respectively. The free brain ECF concentration of memantine (0.83 +/- 0.05 microM) was comparable to free serum and CSF concentrations. In case of amantadine, it was lower. A higher in vivo than in vitro recovery was found for memantine. CONCLUSIONS: At clinically relevant doses memantine reaches a brain ECF concentration in range of its affinity for the NMDA receptor and close to its free serum concentration. This is not the case for amantadine and different mechanisms of action may be operational.

Modifications of the behavioral profile of non-competitive NMDA receptor antagonists, memantine, amantadine and (+)MK-801 after chronic administration.

Non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists show antiparkinsonian-like activity in animal models, and possess neuroprotective properties. However they also induce a number of behavioral side effects in rodents at higher doses; these include learning impairment, hyperlocomotion, and ataxia. The present study focused on the possible development of tolerance, or sensitization, to any of these effects after sustained administration, either by repeated injection or continuous infusion. When memantine or (+)MK-801 (20 and 0.31 mg/kg/day respectively) were either infused or repeatedly injected for 14 days, tolerance was observed to their learning impairing effect at high doses, in a passive avoidance test. Tolerance to their ataxic effect developed after repeated administration ((+)MK-801 and memantine), or after infusion (memantine). Sensitization to the locomotor stimulation was seen following repetitive injections of memantine for 14 days, but not seen with (+)MK-801. In animals with an unilateral 6-OHDA lesion of the nigrostriatal system, acute administration of memantine caused ipsilateral rotations, which were augmented following 14 days of infusion. The potency of amantadine to antagonize neuroleptic-induced catalepsy was unchanged following either infusion or repeated injections. The various acute effects of non-competitive NMDA receptor antagonists were modified differently by sustained treatment (i.e. tolerance to learning impairment and ataxia; sensitization to memantine's locomotor stimulation). The anti-cataleptic activity of amantadine remained unaltered. However, differences between drugs and the two treatment regimens (i.e. repetitive versus continuous treatments) were apparent.

The use of intracerebral microdialysis to determine changes in blood-brain barrier transport characteristics.

The aim of this study was to determine whether changes in the transport of drugs into the brain could be determined by in vivo intracerebral microdialysis. Atenolol was used as a model drug to determine blood-brain barrier (BBB) transport characteristics. In rats, unilateral opening of the blood-brain barrier was achieved by infusion of hyperosmolar mannitol (25%, w/v) into the left internal carotid artery. BBB transport, expressed as the ratio of the area under the curve (AUC) of atenolol in brain extracellular fluid over plasma, was three times higher for the mannitol treated hemisphere as compared with the contralateral brain or after infusion of saline, being (mean +/- SEM) 0.094 +/- 0.024 (n = 16), 0.029 +/- 0.007 (n = 12) and 0.030 +/- 0.009 (n = 12) respectively. Further evaluation of the data indicated that for experiments performed in the morning the mannitol infusion had little effect on the extent of transport of atenolol into the brain, while in the afternoon BBB transport was about 10-fold higher than in the contralateral and saline group. The mean "afternoon" ratios +/- SEM were 0.155 +/- 0.038 (n = 8), 0.012 +/- 0.003 (n = 6) and 0.018 +/- 0.006 (n = 6) respectively. It is concluded that intracerebral microdialysis is capable of revealing changes in BBB transport and regional and time-dependent differences in drug levels can be demonstrated with the use of this technique.