| Hhippocampus is a key structure involved in learning and memory processes, particularly in spatial learning and memory. The mammalian hippocampus is mainly divided into the CA1, CA3, and dentate gyrus (DG), and each subregion has different function in spatial learning and memory. As the entry point of spatial information into the hippocampus, the DG encodes and processes spatial information and plays an essential role in spatial learning and memory. However, the chemical mediators in the DG responsible for mediating spatial learning and memory are incompletely understood. Dopamine (DA) is an important neurotransmitter in central nervous system, and hippocampus receives dopaminergic projections from the ventral tegmental area and substantia nigra, and high expression of Dl and D2 receptors has been detected in the hippocampus. There are many lines of evidence showing that D1 receptor in the hippocampal CA1 play an important modulatory role in LTP formation and maintenance as well as several learning and memory processes, but the roles of the D1 receptors in the hippocampal DG in modulation of spatial learning and memory have not been reported. Therefore, in part I of present study, we used an in vivo brain microdialysis technique to determine DA release in the DG region during MWM test, and then the antagonist or agonist of the Dl receptors was microinjected into the DG region, and the spatial learning and memory was measured.Vascular dementia (VD) is a kind of acquired intellectual impairment syndrome caused by brain tissue damage related to cerebral vascular factors, and it mainly manifests dysfunction of memory and cognitive. Learning and memory is a foundation for cognitive function, and it is necessary index in the study of cognitive function. Although recent studies suggest that the hippocampal DG has important roles in spatial learning and memory deficits of VD, the chemical mediators in the DG responsible for the spatial learning and memory impairment of VD are incompletely understood. Therefore, in part â…¡ of present study, we established a rat model of VD by permanent bilateral carotid occlusion (2-VO), and have used microdialysis, high performance liquid chromatography (HPLC), immunohistochemical staining, microinjection, Morris water maze (MWM) and transmission electron microscope methods to study the effects of DA and D1 receptor in the hippocampal DG on spatial learning and memory impairment of VD.Five parts of experiment were carried out:1. The extracellular concentrations of DA in the DG were examined by in vivo microdialysis and HPLC methods during MWM test.2. The effects of microinjection of SCH23390 (an antagonist of D1 receptors) and SFK38393 (an agonist of D1 receptors) into the DG in the spatial learning and memory were demonstrated.3. The SCH23390 was microinjected into the hippocampal DG, and the extracellular concentrations of amino acids in the DG, including aspirate (Asp), glutamate (Glu), and y-aminobutyric acid (GABA), were measured during MWM test.4. VD model rats prepared with 2-VO, and then 1) spatial learning and memory abilities of rats were assessed by MWM; 2) the extracellular concentrations of DA in the DG were examined by in vivo microdialysis and HPLC methods; 3) the expressions of D1 receptors in the DG were measured by immunohistochemistry; 4) the ultrastructure of DG was detected by transmission electron microscope.5. The effects of microinjection of SFK38393 into the DG in the spatial learning and memory impairment of VD rats were demonstrated.The experimental results are as follows:1. In the place navigation trail of MWM, the escape latency was decreased with the increase in training days; and the extracellular concentration of DA in the DG was significantly increased (P<0.05).2. The microinjection of SCH23390 into the DG elicited a significant increase in escape latency and a significant decrease in number of platform crossings in MWM test (P<0.05, respectively).3. The microinjection of SFK38393 into the DG did not affect the escape latency, but increases the number of platform crossings in MWM test.4. In control group, the extracellular concentrations of Asp and Glu in the DG were significantly increased during MWM test, and the increases were attenuated in SCH23390 group (P<0.05, respectively).5. The extracellular concentrations of GABA in the DG were decreased with the increase in training days in both groups, but compared with control group, the GABA in the DG was significantly increased in SCH23390 group (P<0.05, respectively).6. In the place navigation trail, the mean escape latency was significantly longer in VD group than sham-operated group; in the spatial probe trail, the number of platform crossings was markedly lessened in VD group compared with the sham-operated group.7. The changes in ultrastructure of hippocampal DG in VD rats were as follows:1) the number of synaptic vesicles was reduced; 2) the rough endoplasmic reticulum was reduced and arranged in disorder, and free ribosome was increased; 3) the membrane and cristae of mitochondria in the synaptosoma were blurred, and the cristae was fractured; 4) the synaptic cleft was blurred.8. Compared with the sham-operated group, the extracellular concentration of DA in hippocampal DG was significantly decreased in the VD model group.9. The number of D1 receptor-positive cells in the DG hilus in VD model group was increased compared to the sham-operated group, whereas the expression of D1 receptor in DG granule cell layer did not changed.10. The microinjection of SFK38393 into the DG induced a significant decrease in mean escape latency and a significant increase in number of platform crossings in VD model rats during MWM test.Conclusions1. DA in the DG region facilitates the spatial learning and memory via D1 receptor.2. Activation of D1 receptors in DG enhances the transmission of excitatory amino acids and attenuates the inhibition of GABA during spatial learning and memory.3. Spatial learning and memory impairment of VD model rats is related to decrease in DA concentrations and disorder of ultrastructure in the hippocampal DG.4. Activation of D1 receptors in DG partially reverses spatial learning and memory impairment in VD model rats.5. The expression of Dl receptors in the DG hilus was increased in VD model rats. |
