A novel alternated dual task (ADT) was designed to clarify whether increasing novelty and alternation factors in a task will increase or decrease the short term and long term memory in rats. Rats were made to learn T-maze (TM) spontaneous alternation task and radial arm maze (RAM) task alternatively. Administration of anticholinergic drug scopolamine (SC) and a cholinesterase inhibitor, rivastigmine (RVM) was used to see whether adopting ADT procedure could withstand the deterioration in spatial memory caused by anticholinergic drug and is it comparable to a cognitive enhancement caused by cholinesterase inhibitor. The results showed that ADT help to learn a complex task faster than learning it in isolation from other tasks. An anticholinergic drug, SC showed similar decrement in both TM and RAM tasks. But this decrement can be decreased by adopting ADT procedure, especially for complex task (RAM). The influence of ADT on SC treated rats was similar to the effects of rivastigmine. But this similarity was restricted to RAM task and TM task was less influenced by ADT procedure. The improvement in spatial learning and memory due to ADT are discussed on the basis alternation and novelty drive hypothesis.
Animals who are moving in space may compute their current position by path integration, that is, by detecting movement-generated or idiothetic cues; or they may use allothetic cues generated by combinations of environmental land marks.1 It has been suggested that allocentric spatial impairments reflect the role of the hippocampus system in detecting and controlling the animal’s movements through space1. Also it was shown that hippocampal – system lesions typically disrupt allocentric (defined with respect to external land marks) spatial learning but leave egocentric (defined with respect to rat’s body axis) learning intact.1 Spatial memory is the part of memory responsible for recording information about rat's environment and its spatial orientation. Spatial working memory in this article specifically refers to the memory for intermediate results that must be held during performance of a spatial task by the rat.
Allocentric spatial learning can be assessed using T-maze spontaneous alternation (TM) task and also using Radial arm maze (RAM) task.2, 3 In general there is a tendency to go to the part of the environment that has been least recently explored.4 In its simplest form, spontaneous alternation behavior was first described nearly 85 years ago5, the phenomenon has been ascribed to the operation of a variety of mechanisms including Hullian reactive inhibition6, stimulus satiation7, action decrement 8, curiosity 9, habituation to novelty 10, foraging strategies 11 and spatial working memory.12
A more complex dual alternated task was designed to clarify whether increasing novelty and alternation factors in a task will increase or decrease the short term and long term memory in rats. In this study both T-maze spontaneous alternation task and RAM task was used. The evaluation of allocentric spatial learning was done by making rats to learn both the task alternatively as well as learning them separately. The influence of one behavioural task on another one depending on its complexity was investigated. A simple task like T-maze task was made to learn by the rats in alternation to a complex task like RAM task. We named this particular task as ‘alternated dual task’ (ADT). In the present study the aim was to determine whether learning a simple task (T-maze) along with a complex task (RAM), elongates or reduces the acquisition time for either of the task, and whether any influence on retention (memory) can be observed.
It has been suggested that allocentric spatial impairments reflect the role of the hippocampus system in detecting and controlling the animal’s movements through space.13,14 Also it has been shown that hippocampal – system lesions typically disrupt allocentric (defined with respect to external land marks) spatial learning but leave egocentric (defined with respect to rat’s body axis) learning intact.15,16,17 A number of cholinesterase inhibitors have been shown to improve cognitive functions.18-21
On the other hand, anticholinergic drugs, like scopolamine (SC), can disrupt short-term or working memory in humans and animals.22,23,24 Peripheral injections of anticholinergic drugs like scopolamine and hippocampal lesions produce similar effects in rats in a number of behavioral tasks. 25- 29
For assessing the involvement of hippocampus in ‘alternated dual task’ the administration of anticholinergic drug SC was used. The main aim was to see whether adopting ADT procedure could withstand the deterioration in spatial memory caused by anticholinergic drugs. SC was administered separately during acquisition and during retention phases in different groups. The lack of a post-acquisition impairment after hippocampectomy30 or scopolamine injection30,31 might reflect the use by the animal of non-spatial strategies in some types of maze. A large number of studies support the role for acetylcholine in short-term memory and the acquisition of new information.32 It was also shown that radial arm maze performance might be more susceptible to impairment than performance on the Morris water maze.33 In present study the scopolamine was used to see the participation of cholinergic systems in performance of a novel ‘alternated dual task’.
Rivastigmine (RVM) is a carbamate inhibitor of acetylcholinesterase (AChE) with a relatively selective action on the enzyme in brain compared with that in the heart and skeletal muscle.34, 35 RVM causes long inhibition on AChE of up to 10 hours.36 Bejar, Wang, & Weinstock, in 1999 37 showed that RVM (1.5 mg/Kg) antagonized the memory deficits induced by scopolamine (1 mg/Kg). They also showed that RVM at a dose of 1.5 mg/Kg have least adverse effects like diarrhea and tremor. In this study similar to SC, RVM was also used to see the participation of cholinergic systems in performance of a novel ‘alternated dual task’.
The present study also compared the effects of a cholinesterase inhibitor, RVM, with that of ‘alternated dual task’, against SC induced learning and memory impairments in RAM and T maze tasks.
Materials and methods
Subjects: A total of 168 male Wistar albino rats were used for this study. They were housed in groups, in propylene cages in an acclimatized (25 - 27C) room and were maintained on a 12 hr light / dark cycle. Food and water was given ad libitum until they aged 60 days at the beginning of the experiment. Average weight of the rats was 180 ± 30 g.
Rats were grouped into four based on the behavioural task, with 42 rats in each group. First group performed only T maze task and designated as TM alone group, and second group had only radial arm maze task and designated as RAM alone group. Third and fourth groups performed both the behavioural tasks. Third group did both the tasks alternatively, hence designated as alternated dual task (ADT) group, but the fourth group did one task first and after completing one phase of it, the second task was done, and designated as non alternated dual task (NADT) group.
Each of the above mentioned four groups were further subdivided into seven groups based on drug administration, with 6 rats in each group. The seven groups were; control group with only saline injection, scopolamine (SC) administered during acquisition group, SC administered during retention group, rivastigmine (RVM) administered during acquisition group, RVM administered during retention group, RVM & SC administered during acquisition group and RVM & SC administered during retention group.
T-maze: The T-maze used for the study was made of wood with smooth polished surface, consisted of a stem (3512 cm), a choice area (1212 cm) and two arms (3512 cm). The end of each arm contained a food well. The sidewalls are 40 cm high. The choice area is separated from the arms by a sliding door. 2