Effect of memantine on lithium chloride induced head twitches in rats.

Sir,

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss, altered behavior and signs of cortical disconnection including apraxia, aphasia and agnosia.[1] Although AD was once thought to result from cholinergic deficit alone, researchers now believe that this view is simplistic.[23] Studies using human neocortical tissue have shown that multiple neurotransmitters, such as dopamine, noradrenaline, serotonin and glutamate, are decreased or dysregulated in AD. The interactive regulation of neurotransmitters complicates our understanding of the processes that work together, but may eventually permit the development of more specific drug therapies for AD.[45]

Memantine, an N-methyl-D-aspartate (NMDA) receptor antagonist, produces use-dependent blockade of NMDA receptors. In patients with moderate to severe AD, use of memantine is associated with a reduced rate of clinical deterioration. Whether this is due to a true disease modifying effect (reduced excitotoxicity) or symptomatic effect of the drug remains unclear. Many “trapping channel blockers” of NMDA receptors have been described, e.g. ketamine.[6] Memantine and ketamine are the two drugs that appear to have strikingly similar channel blocking properties, with dissimilar clinical effects. Despite their similarities, the pharmacological actions of these drugs are strongly divergent. Ketamine causes memory deficit, reproduces the symptoms of schizophrenia and is widely abused.[7] Memantine, on the other hand, is well tolerated and the incidence of side effects is remarkably low.[8] Memantine improves memory in AD patients, and in some studies in animals it does not appear to have abuse potential, but rather has shown promise in treating addiction.

The beneficial effects of memantine and the neurotoxic effects of ketamine, in spite of having large similarities in their influences on NMDA receptors, do not offer a clear explanation for the clinical effectiveness of memantine. Early evidences suggest that memantine antagonizes serotonin (10 mM)-induced inward currents in the N1E-115 cell line with a similar potency to that reported for NMDA receptors[9] and it is possible that the clinical effects reported for memantine are mediated, at least in part, by inhibiting serotonergic transmission. In the light of the above data, it was considered worthwhile to use electroconvulsive shock (ECS) in 5-hydroxytryptamine (5-HT) receptor specific behavioral model, i.e. lithium chloride induced head twitches, to study the effect of memantine on serotonergic transmission in rats.

Experimentally naοve, male Sprague-Dawley albino rats weighing 80–110 g at the onset of experimentation were used. The rats were housed six per cage in 40 × 28 × 14 cm high polypropylene, partially transparent cages, with a floor area of approximately 112 cm2 per rat for 6 days before use. Apart from once-daily replenishment of food (standard pellet diet, Pranav Agro Foods, Pune, India) and water, the rats were left undisturbed during this period and were exposed to a 12-h light–dark cycle. Housing and laboratory temperatures were maintained at 20–24°C. The study was approved by Institutional Animal Ethics Committee.

Electroconvulsiometer was purchased from ST1 Instruments Private Ltd., Pune, India. Memantine hydrochloride and lithium chloride were purchased from Sigma (Mumbai, India) and Thomas Baker Ltd. (Mumbai, India), respectively. Distilled water was used as vehicle.

Lithium-Induced Head Twitches (5-HT-Mediated Behavior)[10]

This model was used to study the interaction of memantine with 5-HT. The rats were grouped as follows: Group I ( n = 6) served as control. Distilled water was administered orally. Group II (n = 6): Each rat was given ECS (150 V, 50 Hz sinusoidal with intensity of 210 mA for 0.5 s through crocodile clip ear electrodes) as a single daily dose for 8 days. Group III (n = 6): Each rat was given ECS (150 V, 50 Hz sinusoidal with intensity of 210 mA for 0.5 s) and memantine (5 mg/kg p.o.) as a single daily dose for 8 days. Group IV (n = 6): Each rat was given ECS (150 V, 50 Hz sinusoidal with intensity of 210 mA for 0.5 s) and memantine (10 mg/kg p.o.) as a single daily dose for 8 days. Group V (n = 6): Each rat was given ECS (150 V, 50 Hz sinusoidal with intensity of 210 mA for 0.5 s) and memantine (20 mg/kg p.o.) as a single daily dose for 8 days. In groups III, IV and V, memantine was administered 1 hour after the administration of ECS. On day 9, the number of head twitches induced by lithium chloride (150 mg/kg i.p.) was counted in intervals of 10 min, starting immediately from the time of injection up to a period of 90 min.

The results of the lithium-induced head twitches were analyzed by analysis of variance (ANOVA) followed by Turkey's honestly significant difference (HSD) test. P < 0.05 was considered as significant.

The maximum number of head twitches in different groups was seen between 31 and 40 min. So, head twitches at this interval were compared in different groups. The number of lithium-induced head twitches was 0.67 ± 0.2 (mean ± SD) in group I. Following a course of ECS, it increased to 58.33 ± 2.1 in group II. It decreased to 14.83 ± 1.2, 7.65 ± 0.9, 2.45 ± 0.6 in groups III, IV and V, respectively. Memantine significantly decreased ECS-induced enhancement of lithium chloride induced head twitches (P < 0.001) in a dose-dependent manner. The results of this study are shown in Figure 1.

Figure 1

Number of lithium chloride induced head twitches in different groups (P< 0.001 – groups I, III, IV and V vs. group II)

For many years, improving memory has been part of the realm of imagination and fiction. Currently, our knowledge pertaining to the neural construct of intelligence and memory is in its infancy. Several neurotransmitter systems that have different anatomical locations are involved in various aspects of memory.[11] Serotonergic system has also been implicated in this. Evidences suggest that serotonergic transmission inhibits the working memory performances. Fenfluramine, a serotonin reuptake inhibitor and releasing agent which effectively increases serotonin levels, appeared to impair the working memory. It is also suggested that blocking serotonergic transmission in brain is a possible mechanism to enhance the working memory performances.[1213]

Head twitches induced by lithium chloride in rats constitute a useful model for quantifying the 5-HT activity in the brain and for the screening of potential antagonists at 5-HT receptors.[13] In the present study, statistically significant increase was seen in the number of lithium chloride induced head twitches in group II (ECS pretreated group) as compared to group I (control group) (0.67 ± 0.2 vs. 58.33 ± 2.1). Animal studies suggest that after ECS administration, there is enhancement of serotonin-mediated behavioral responses and increased density of 5-HT2 receptors.[1314] This explains the enhancement in lithium-induced head twitches following ECS. Our study shows that memantine significantly decreases ECS-induced enhancement of lithium chloride induced head twitches in a dose-dependent manner. It has been reported that blocking serotonergic transmission in the brain is a mechanism to enhance the memory.[1213] Our results are in agreement with this. Thus, it can be suggested that the clinical effects of memantine, at least in part, are due to inhibition of serotonergic transmission.