dc.description.abstract | Alzheimer’s disease (AD) is present in 46.8 million people in the world, and is
characterized by the progressive decline of cognition, mainly episodic memory. AD is
multifactorial, and the accumulation of amyloid peptides (Aβ) is the main proposed
mechanism underlying neurodegeneration. Aβ are the main components of the amyloid
plaques in the brain, that are physiopathological hallmarks of the disease. Intracerebral
infusion of Aβ in rats is usually used as an animal model of DA and generates the
accumulation of these peptides in the brain together with spatial memory impairment.
However, most studies show moderate to severe deficits after chronic Aβ infusion,
without evaluation of possible subtle initial deficits. The study of the initial stages of AD
is relevant for mechanicistic and therapeutic investigations. The Barnes maze has been
used for investigating deficits in the hippocampal function in rats. The animals are
exposed to a circular apparatus with holes in its periphery. One of the holes is connected
to a safe place. In this task, rats navigate guided by distal cues to find this safe
compartment. The evaluation is conduct by parameters of general performance (latency
and distance to reach target, number of errors, among others) and by a specific analysis
of search strategies. In this way, the animal use spatial information and move directly
towards the target (direct strategy), visit sequential holes until reach the target (serial
strategy) or visit holes in a non-systematic fashion until reach the target (aleatory search).
The analysis of strategies allows the detection of alterations in the mode of solution of
the task. In the present study, we aimed to investigate initial cognitive signs of AD in
Wistar rats submitted to intracerebral infusions of Aβ, as well as the effects of a
potentially neuroprotective treatment, by the evaluation of spatial memory in the Barnes
maze. We used the alkaloid extract of Erythrina velutina (“mulungu”), which was
previously studied for anxiolytic, anti-inflammatory, antioxidant and cholinergic actions.
First, we standardized the task to our experimental conditions, and verified a possible
influence of repeated expositions to the apparatus (for future long-term protocols) or of
the implantation of the cannulas in the brain. Rats went through (or not) to implantation
of cannulas (bilaterally in the hippocampal CA1 and in the lateral ventricle) and were
exposed to 5 sequences of exposition to the Barnes maze (4 trainning sessions with 4
trials each, a 24h test and a 10-day retest sessions). Both groups showed task retrieval,
and there was a slightly improved performance in the implanted group. We concluded
that the task can be held repeatedly, and in implanted animals, without altering the
performance. In the second phase, we investigated the use of different strategies in the
Barnes maze by rats submitted to the presence of absence of distal cues. Rats were
exposed to the training phase, and in the probe session (24 h later) half the animals were
exposed to the maze in the presence of the same distal cues used in training (spatial
group), while the other rats went through the probe test without those cues (a black curtain
was placed around the maze – non-spatial group). Both groups learned the task, but the
spatial group preferred the used of direct strategies, while the non-spatial group preferred
other strategies. We concluded that the removal of distal cues does not hinder the
execution of the task, and the animals use alternative search modes under this condition.
In the third phase (two experiments) we verified the effects of the intracerebral infusion
of Aβ on the acquisition and retrieval of Barnes task. In experiment 1, rats received 15 daily i.c.v. infusions of saline or Aβ (30, 100 or 300 pmol) plus bilateral CA1 infusions
in the first day, and were exposed to 3 sequences of Barnes task (training, 24h test and
10-day retest in each sequence). Sequence I was held before surgery (all the animals
learned the task), II started at 11th infusion and III started 10 days after II. The behavior
of the Aβ-treated animals varied greatly at sequences II and III, and hence no differences
were observed. There was high mortality due to treatment. At the end of the behavioral
sessions, saline and Aβ 30pmol groups were euthanized for Aβ immunohistochemistry.
The analysis by relative optical density showed increased Aβ staining in the hippocampus
and neocortex. In experiment 2 we investigated the effects of Aβ (30 pmol) infusion on
the search strategies in the Barnes maze. Animals went through one sequence of Barnes
task (4 trainings with 2 trials each and a 3-day test, in order to increase difficulty). In the
probe test, although Aβ animals showed some retrieval, they showed preference for nonspatial
strategies, opposed to saline-treated rats. We concluded that Aβ infusion induced
subtle alterations in spatial memory, compatible with the initial stages of AD, which is
relevant for investigations of potential neuroprotective approaches. In the last stage,
animals submitted to the same infusion protocol described above were concomitantly
treated orally with 200 mg/kg of E. velutina (mulungu) extract and went through 2 Barnes
task sequences. In general, no differences were observed among the groups in acquisition
or retrieval. Thus, we concluded that the protocol used here was not able to detect a
beneficial effect of mulungu extract in the Aβ infusion AD model. | |