INTRODUCTION:
Epilepsy is a chronic neurological disorder characterized by repeated seizures (> 24 h apart), where one seizure has a strong tendency for recurring or being diagnosed as an epilepsy syndrome. 1 People all over the world are affected with epilepsy with the count being more than 50 million.2
The patients with epilepsy in the general population range between 4-10 per 1000 population, as compared to low and middle income countries where it lies between 7-14 per 1000 population with a higher prevalence. 3 Epilepsy has been treated with traditional antiepileptic drugs like phenytoin, whereas pregabalin and topiramate are relatively new. The effect on learning and memory due to topiramate and pregabalin being less has not been studied.4 The standard method of testing for learning and memory in rats is through using apparatus such as morris water maze and elevated plus maze.
Neurocognitive impairment is one of the adverse drug reactions (ADRs) in the case of drugs used in epilepsy, with almost all drugs having some effect. Loss of memory is among the most common complaint in epilepsy.5 Animal and human volunteer studies however support potential for these drugs to impair memory independently.5 People with epilepsy experiencing problems with cognition are almost 20 to 50%.6 AEDs, that belong to the older generation, or used in high doses, and polytherapy have more overall side effects. 7 Phenytoin (PHT), an effective older generation AED, has shown a decline in cognition in both healthy volunteers and epileptic patients.8,9 Of all anticonvulsive medications, PHT causes maximum side effects related to cognition.10 Phenytoin is a drug with maximum bioavailability and less first-pass metabolism. 11 70%–90% of the drug is absorbed and primarily binds to albumin with a binding of >90% when given orally. 12 A correlation is found between serum drug level and impairment of memory and motor functions with phenytoin.13 Withdrawing PHT improved functions related to learning and memory in patients on antiepileptics.14,15 Topiramate is an antiepileptic drug that works by decreasing abnormal excitement in the brain.16 Topiramate is used as an adjuvant therapy in partial onset seizures or primary generalized tonic-clonic seizures, and as prophylactic therapy in migraine.17 Topiramate, a new antiepileptic has demonstrated its efficacy in the management of intractable seizures in several randomized controlled trials.18,19,20 The FDA has approved topiramate for the prevention of Migraine on August 12, 2004, and the first anti-obesity medications (phentermine plus topiramate) for weight-loss on July 17, 2012.21 Profound adverse effects have been seen in recent studies.22 Cognitive side effects feature prominently among the adverse effects including impaired concentration and memory, slowing of thought process, and difficulties in word finding.19,23,24 PGB has shown good efficiency for the treatment of neuropathic pain and generalized anxiety disorder.25 It has also shown to be efficient in the treatment of partial seizures in clinical trials.26,27,28 In epilepsy it is used as an add-on therapy for partial seizures with or without secondary generalization in adults.29 Pregabalin is also approved for the treatment of diabetic neuropathic pain, postherpetic neuralgia, and fibromyalgia.30,31,32, Pregabalin has anti-convulsant, anxiolytic, analgesic and sleep modulating properties.33 Studies on pregabalin show that it’s negative effects on cognitive functions are mild.34 This study has been done to determine the effect of drugs, phenytoin topiramate and pregabalin respectively on learning and memory in albino rats.
MATERIAL AND METHODS:
This study has been done in the Department of Pharmacology and Therapeutics, Darbhanga Medical College, Darbhanga. Approval of the Institute Animal Ethics Committee (IAEC), DMC, Darbhanga was taken prior to the study.
Inclusion criteria comprised of healthy albino rats in the range of 150-200 gm in weight, feeding properly. A total of 24 rats were used who were first acclimatized for a week. Animals were randomly selected in four groups of six animals each. Each animal was placed in a separate cage. Temperature and humidity were maintained as standard. Feeding was also properly done. Drugs used, that is phenytoin, topiramate and pregabalin were procured from local market. Based on the ratio of surface areas of rat and man (which is 0.018), the dose of the drugs was calculated. The phenytoin group was given a dose of 27mg/kg, topiramate as 8mg/kg and pregabalin as 3mg/kg. Rats in control group were given 0.5ml of normal saline. All the rats in the 4 groups were given their respective drugs for a period of 15 days after which tests were performed on morris water maze and elevated plus maze to estimate the escape latency time and transfer latency time respectively as an assessment for learning. The rats then again received the treatment for another 5 days and assessed for memory using the same parameters.
The Morris water maze is a method of testing spatial learning found by Richard Morris. 35 It requires rodents to move from a start point of a swimming area to locate an escape platform, submerged under water. 36 Repeated trials need to be performed. It comprises of a water tank with diameter of 1.8-2.0 m and height of 0.4-0.5m. It is made opaque by adding colour which is non-toxic. The water tank was divided into four fixed quadrants and a platform with heavy base is placed in the center of any one for rats to escape on. The platform is made invisible by adding water.
The
Elevated Plus-Maze (EPM) was used as a test of anxiety in rats (Pellow et al.,
1985) and mice (Lister,1987) in the initial period and thereafter it was used
for testing memory (Itoh et al., 1990; Sharma and Kulkarni, 1992).37
Apparatus consists of two open
(50×10cm) and two enclosed arms (50
10×40cm). The height of maze is elevated to
50 cm. Rats were placed in the water at a fixed location to start from. The
time taken by rats to find the platform from the initial point was called as
escape latency whereas transfer latency
was the time taken by the animal to enter the enclosed zone with all its four
legs having crossed from the start point. Each rat was tested for four trials/day on morris
water maze and two trials/day on elevated plus maze. A resting period of two minute
was kept between the trials.
Exclusion criteria comprised of rats not falling in the weight range of 50 to 200 gm and taking more than 120 seconds to reach the platform. Data entry was done on MS excel and ‘SPSS version 17’ software was used for data analysis. Mann - Whitney test was used for comparing the results. The first step of test was to mix all the data of the two groups which had to be compared and arrange them in ascending order. Next step was to assign a rank in increasing order. For further analysis this rank was used instead of the raw value. So, with the help of these ranks a rank mean was calculated for both groups. The next step was to find whether the difference in this mean rank is significant or not. The test statistic for this (Mann- Whitney Test) is U.
U is given by: U=R1 – n1(n1+1)/2 Where n1 is the sample size for sample 1, and R1 is the sum of the ranks in sample 1.
RESULTS:
The data obtained after administering respective drugs for 15 days are shown in graph1 and graph2.Graph1 represents the overall graph representing learning curve on morris water maze and Graph2 represents the overall graph representing learning curve on elevated plus maze.
Graph 1: Overall graph representing learning curve on morris water maze
Graph 2: Overall graph representing learning curve on elevated plus maze
Day wise p-values of various drug groups when escape latency on morris water maze was compared were found as shown in table1. Day wise p-values of various drug groups when transfer latency on elevated plus maze was compared were found as shown in table2.
Significance levels on day 21 were assessed using escape latency time on morris water maze to see the effect on memory and the values were as shown in table3. Significance levels on day 21 were assessed using transfer latency time on elevated plus maze to see the effect on memory and the values were as shown in table4.
Table 1: Day wise p-values of drug groups with respect to escape latency on morris water maze
|
p-value |
Day1 |
Day2 |
Day3 |
Day4 |
Day5 |
|
Phenytoin vs control |
0.093 |
0.041 |
0.002 |
0.132 |
0.002 |
|
Phenytoin vs topiramate |
0.485 |
0.818 |
1.000 |
0.937 |
0.937 |
|
Phenytoin vs pregabalin |
0.699 |
0.041 |
0.002 |
0.093 |
0.002 |
|
Pregabalin vs control |
0.485 |
1.000 |
0.093 |
0.699 |
0.937 |
|
Pregabalin vs topiramate |
0.394 |
0.065 |
0.002 |
0.093 |
0.002 |
|
Topiramate vs control |
0.093 |
0.065 |
0.065 |
0.132 |
0.002 |
Table 2: Day wise p-values of drug groups with respect to transfer latency on elevated plus maze
|
p-value |
Day1 |
Day2 |
Day3 |
Day4 |
Day5 |
|
Phenytoin vs control |
0.240 |
0.589 |
0.180 |
0.004 |
0.015 |
|
Phenytoin vs topiramate |
0.937 |
0.699 |
0.093 |
0.394 |
0.041 |
|
Phenytoin vs pregabalin |
0.494 |
0.348 |
0.133 |
0.955 |
0.301 |
|
Pregabalin vs control |
0.818 |
0.180 |
0.065 |
0.041 |
0.485 |
|
Pregabalin vs topiramate |
0.310 |
0.394 |
0.699 |
0.818 |
0.485 |
|
Topiramate vs control |
0.394 |
0.132 |
0.065 |
0.015 |
0.937 |
Table 3: Significance levels assessed using escape latency time on morris water maze to see effect on memory
|
p-value |
Phenytoin vs control |
Pregabalin vs control |
Topiramate vs control |
|
Day21 |
0.004 |
0.132 |
0.093 |
Table 4: Significance levels assessed using transfer latency time on elevated plus maze to see effect on memory
|
p-value |
Phenytoin vs control |
Pregabalin vs control |
Topiramate vs control |
|
Day21 |
0.485 |
0.937 |
0.818 |
DISCUSSION:
In the present study, antiepileptics i.e. Phenytoin(PHT), Topiramate(TPM) and Pregabalin(PG) were compared for their effect on learning and memory. Since memory and learning cannot be entirely separated from each other this interpretation can also be stated in terms of observed behavioral changes rather than learning and memory processes.
Phenytoin versus others:
Phenytoin caused deranged learning in comparison to the control as well as pregabalin group on morris water maze (day2, day3, day5), but when compared to topiramate group, no significant difference was found. Results on elevated plus maze was significant for phenytoin compared to control (day4, day5) and for phenytoin compared to topiramate on day5. Memory was also seen to be deranged significantly on morris water maze. According to previous studies, antiepileptic drugs like phenytoin and sodium valproate worsened learning and memory in epileptic patients. 38 Phenytoin was found to be worst in cognitive performance when compared to other drugs used in epilepsy (Thompson et al. 1981). 39 Use of phenytoin in healthy people and epilepsy patients is causative in impairing cognitive functions, like memory, concentration and motor speed. 40 However, subtle differential cognitive effects of AEDs, was found in studies recently which included phenytoin. 41
Topiramate versus Others:
On both morris water maze and elevated plus maze, in comparison to control as well as pregabalin group in morris water maze significant derangement in learning was seen in case of topiramate, but no such significant impairment was seen in elevated plus maze. There was no difference between the impairment caused by topiramate group compared to phenytoin group except on day 5 in elevated plus maze. This implies that topiramate causes no less impairment of learning function as compared to phenytoin group. But topiramate does not have significant derangement in memory as compared to control on both morris water maze as well as elevated plus maze. In both partial and generalized epilepsy, topiramte has proved to be effective as monotherapy. 42, 43 Cognitive impairment, ranging from 11 to 20%, was seen in patients with refractory epilepsy under polytherapy owing to topiramate. 44 This attribute is the reason for discontinuing the drugs even though it helps in improving seizure frequency. 44 Topiramate caused derangement of learning which was significant in comparison to the control. Topiramate and phenytoin do not have a significant difference. This implies that topiramate is causing impairment no less than phenytoin. This similarity between them might be due to similar mechanism of action.
Pregabalin versus Others:
No difference was found between pregabalin and control group on morris water maze. Learning on day 4 was impaired on elevated plus maze. Phenytoin and topiramate showed significant derangement in comparison to pregabalin on learning in morris water maze but fail to show any significance in elevated plus maze. Moreover no such difference was seen in performance on morris water maze and elevated plus maze at the end of day 21 for pregabalin group compared to the control group. This shows that there is no significant difference in memory among the two groups. Pregabalin has been used in the treatment of focal seizures and various indications, such as neuropathic pain, restless leg syndrome, and anxiety disorders. Not much is known about its effects on cognition. Pregabalin did not modify the performance related to cognition or affect short-term memory in mice according to some animal studies. 45 Studies demonstrate that pregabalin used in full therapeutic doses for 12 weeks has negative effects on cognition as measured by standard cognitive test. 46 Pregabalin might mildly affect cognition in healthy volunteers according to other studies. 44 The finding in various studies about pregabalin are therefore not conclusive.
CONCLUSION:
The results for phenytoin and pregabalin in this study are similar to previous studies, but finding in case of topiramate causing impairment of learning are different from previous studies. since both these drugs i.e. phenytoin and topiramate mainly acts by blocking repetitive firing by sodium channels the result in this study seems reasonable unless there is some other mechanism involved with topiramate. Cognition being an important parameter for discontinuing a drug, needs to be studied in greater detail with respect to all drugs used in epilepsy and hence more research in this field is crucial.
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Received on 15.11.2021 Modified on 23.10.2022
Accepted on 30.05.2023 © RJPT All right reserved
Research J. Pharm. and Tech 2023; 16(7):3451-3455.
DOI: 10.52711/0974-360X.2023.00570