It is well established that inflammatory changes contribute to brain ageing, and an increased concentration of proinflammatory cytokine, interleukin-1b (IL-1b), has been reported in the aged brain associated with a deficit in long-term potentiation (LTP) in rat hippocampus. The precise age at which changes are initiated is unclear. In this study, we investigate parallel changes in markers of inflammation and LTP in 3-, 9-and 15-month-old rats. We report evidence of increased hippocampal concentrations of the proinflammatory cytokines IL-1a, IL-18 and interferon-c (IFNc), which are accompanied by deficits in LTP in the older rats. We also show an increase in expression of markers of microglial activation, CD86, CD40 and intercellular adhesion molecules (ICAM). Associated with these changes, we observed a significant impairment of hippocampal LTP in the same rats. The importance of microglial activation in the attenuation of long-term potentiation (LTP) was demonstrated using an inhibitor of microglial activation, minocycline; partial restoration of LTP in 15-month-old rats was observed following administration of minocycline. We propose that signs of neuroinflammation are observed in middle age and that these changes, which are characterized by microglial activation, may be triggered by IL-18.
There is compelling evidence to suggest that inflammation significantly contributes to neurodegenerative changes. Consistent with this is the observation that several neurodegenerative disorders are accompanied by an increase in the concentration of interleukin (IL)-1b. IL-1b has a negative impact on synaptic plasticity and therefore an increased concentration of IL-1b, such as that in the hippocampus of the aged rat, is associated with a deficit in long-term potentiation (LTP). IL-1b is derived mainly from activated microglia but the trigger leading to this activation, specifically in the aged brain, remains to be identified. Here we examined the possiblity that interferon (IFN)c may stimulate microglial activation and increase IL-1b concentration, thereby inhibiting LTP. The IFNc concentration was increased in hippocampus prepared from aged, compared with young, rats and inversely correlated with the ability of rats to sustain LTP. Intracerebroventricular injection of IFNc inhibited LTP, and increased microglial activation was observed in both IFNc-injected and aged rats. The age-related increase in IFNc was accompanied by a decrease in the hippocampal concentration of insulin-like growth factor (IGF)-1. The evidence presented suggests that IGF-1 acts to antagonize the IFNc-induced microglial activation, the accompanying increase in IL-1b concentration and the consequent deficit in LTP.
Huntington's disease is characterized by death of striatal projection neurons. We used a Cre/Lox transgenic approach to generate an animal model in which D1 dopamine receptor (Drd1a)؉ cells are progressively ablated in the postnatal brain. Striatal Drd1a, substance P, and dynorphin expression is progressively lost, whereas D2 dopamine receptor (Drd2) and enkephalin expression is up-regulated. Magnetic resonance spectroscopic analysis demonstrated early elevation of the striatal choline/creatine ratio, a finding associated with extensive reactive striatal astrogliosis. Sequential MRI demonstrated a progressive reduction in striatal volume and secondary ventricular enlargement confirmed to be due to loss of striatal cells. Mutant mice had normal gait and rotarod performance but displayed hindlimb dystonia, locomotor hyperactivity, and handling-induced electrographically verified spontaneous seizures. Ethological assessment identified an increase in rearing and impairments in the oral behaviors of sifting and chewing. In line with the limbic seizure profile, cell loss, astrogliosis, microgliosis, and down-regulated dynorphin expression were seen in the hippocampal dentate gyrus. This study specifically implicates Drd1a؉ cell loss with tail suspension hindlimb dystonia, hyperactivity, and abnormal oral function. The latter may relate to the speech and swallowing disturbances and the classic sign of tongueprotrusion motor impersistence observed in Huntington's disease. In addition, the findings of this study support the notion that Drd1a and Drd2 are segregated on striatal projection neurons. striatum ͉ Cre ͉ Huntington's disease T he nigrostriatal pathway projects from the midbrain substantia nigra pars compacta to innervate the dorsal striatum. Dopamine is released from dopaminergic terminals in the striatum to regulate motor activity and eating behavior (1). Early studies suggested that dopamine D1 (Drd1a) and D2 receptors (Drd2) are segregated on striatal projection neurons; the Drd1a is expressed on substance P and dynorphin-positive striatal neurons, which project directly to the substantia nigra pars reticulata/entopeduncular complex (known as the direct pathway), whereas Drd2 is preferentially expressed on enkephalin-positive striatopallidal projecting neurons (2). Neurons within the globus pallidus then project to the subthalamic nucleus, which in turn relays to the substantia nigra pars reticulata/entopeduncular complex (known as the indirect pathway). Dopamine also modulates the activity of glutamatergic corticostriatal input on striatal projection neurons.Idiopathic Parkinson's disease is characterized by the death of dopaminergic neurons; however, rare Parkinsonian syndromes have been identified in which the defect is associated with cell death in the dopamine-responsive neurons in the striatum (3-6). Huntington's disease (HD) (7-10), a neurodegenerative condition with motor, cognitive, and psychiatric disturbances, also involves death of dopamine receptor-expressing striatal projection neurons. Several ...
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