The effect of chronic stimulation of serotonin receptor type 7 on recognition, passive avoidance memory, hippocampal long-term potentiation, and neuronal apoptosis in the amyloid β protein treated rat

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by memory impairment, neuronal death, and synaptic loss in the hippocampus. Long-term potentiation (LTP), a type of synaptic plasticity, occurs during learning and memory. Serotonin receptor type 7 (5-HTR7) activation is suggested as a possible therapeutic target for AD. The aim of the present study was to examine the effects of chronic treatment with the 5-HTR7 agonist, AS19, on cognitive function, memory, hippocampal plasticity, amyloid beta (A beta) plaque accumulation, and apoptosis in an adult rat model of AD. AD was induced in rats using A beta (single 1 mu g/mu L intracerebroventricular (icv) injection during surgery). The following experimental groups were included: control, sham-operated, A beta + saline (1 mu L icv for 30 days), and A beta + AS19 (1 mu g/mu L icv for 30 days) groups. The animals were tested for cognition and memory performance using the novel object recognition and passive avoidance tests, respectively. Next, anesthetized rats were placed in a stereotaxic apparatus for electrode implantation, and field potentials were recorded in the hippocampal dentate gyrus. Lastly, brains were removed and A beta plaques and neuronal apoptosis were evaluated using Congo red staining and TUNEL assay, respectively. Administration of AS19 in the A beta rats increased the discrimination index of the novel object recognition test. Furthermore, AS19 treatment decreased time spent in the dark compartment during the passive avoidance test. AS19 also enhanced both the population spike (PS) amplitude and the field excitatory postsynaptic potential (fEPSP) slope evoked potentials of the LTP components. A beta plaques and neuronal apoptosis were decreased in the AS19-treated A beta rats. These results indicate that chronic treatment with a 5-HTR7 agonist can prevent A beta-related impairments in cognition and memory performance by alleviating A beta plaque accumulation and neuronal apoptosis, hence improving neuronal plasticity. AS19 may be useful as a therapeutic agent for AD.