Tumor necrosis factor-␣ (TNF-␣

Brain-derived neurotrophic factor (BDNF) plays a critical role in synaptic plasticity such as long-term potentiation (LTP), a form of synaptic correlate of learning and memory. BDNF is also implicated in learning and memory. We have demonstrated that radial arm maze training in rats for spatial learning and memory results in a significant increase in the BDNF mRNA expression in the hippocampus. Moreover, antisense BDNF oligonucleotide treatment impaired not only acquisition, but also maintenance and/or recall of spatial memory in the maze. Although these results suggest a role of BDNF for spatial memory processes, the signal transduction mechanisms that mediate the actions of BDNF remain unknown. Here we show that phosphorylation of BDNF receptor tyrosine kinase B (TrkB), phosphatidylinositol 3-kinase (PI3-K) and Akt, a target of PI3-K, in the hippocampus increased in parallel with spatial memory formation. Moreover, an activation of translational processes was suggested in the hippocampus after the maze training. When spatial learning was inhibited by antisense BDNF oligodeoxynucleotide, the activation was diminished. Chronic treatment with PI3-K inhibitor wortmannin impaired spatial learning. Our findings suggested that activation of TrkB/PI3-K and protein synthesis signaling pathway by BDNF in the hippocampus is important for spatial memory.

No abstract

Amyloid β‐peptide (Aβ) plays a critical role in the development of Alzheimer's disease. However, the molecular mechanisms of Aβ‐induced brain damage in vivo remain to be elucidated. Here, we investigated whether overproduction of nitric oxide (NO) catalyzed by inducible NO synthase (iNOS) is involved in Aβ‐induced brain dysfunction. Chronic intracerebroventricular infusion of Aβ1‐40 induced iNOS mRNA expression in the hippocampus on days 3 and 5 after the infusion. An accumulation of NO metabolites was observed, and the peak correlated with expression of iNOS mRNA. Measurement of NOS activities revealed an increase in Ca2+‐independent, but not Ca2+‐dependent, activity. Immunohistochemistry identified numerous iNOS‐immunoreactive microglia and astrocytes in the dentate gyrus and to a lesser extent in the CA1 subfield of the hippocampus. Daily treatment with the iNOS inhibitor aminoguanidine (AG, 100 mg/kg/day, i.p.) or S‐methylisothiourea (10 mg/kg/day, i.p.) during Aβ infusion prevented an impairment of nicotine‐evoked acetylcholine release induced by Aβ, whereas the neuronal NOS inhibitor 7‐nitroindazole (30 mg/kg/day, i.p.) had no effect. Daily treatment with AG also ameliorated the impairment of spatial learning of Aβ‐infused rats in a radial arm maze. Our findings suggest that overproduction of NO catalyzed by iNOS is responsible for Aβ‐induced brain dysfunction.

To investigate whether histaminergic neurons influence the activity of cholinergic neurons, the ventral striatum was superfused through a push-pull cannula and the release of endogenous acetylcholine was determined in the superfusate. Local inhibition of histamine synthesis by superfusion with alpha-fluoromethylhistidine (FMH) gradually decreased the release rate of acetylcholine. Superfusion with histamine increased the release of acetylcholine. The releasing effect of histamine was greatly inhibited when the striatum was simultaneously superfused with the D2/D3 agonist quinpirole and the D1 antagonist (+/-)-7-bromo-1-(fluoresceinylthioureido)phenyl-8-hydroxy-3-methyl -2,3,4,5-tetrahydro-1H-3-benzapine (SKF 83566). The effect of histamine on acetylcholine release was abolished by the GABA(A) receptor antagonist bicuculline. Superfusion with the H3 receptor agonists imetit or immepip increased acetylcholine release rate in the striatum. The releasing effects of the two H3 agonists were FMH resistant, while superfusion with quinpirole and SKF 83566 abolished the H3 receptor agonist-induced acetylcholine release. Superfusion with the H3 receptor antagonist thioperamide enhanced acetylcholine release rate. The releasing effect of thioperamide was abolished after inhibition of histamine synthesis by FMH. The release of acetylcholine by thioperamide was also abolished on simultaneous superfusion with quinpirole and SKF 83566. The findings show that, in the striatum, the activity of cholinergic neurons is permanently modulated by neighbouring histaminergic nerve terminals and axons. The release of acetylcholine is also permanently inhibited by neighbouring GABAergic neurons. The enhanced release of acetylcholine by the H3 receptor agonists imetit and immepip is due to stimulation of H3 heteroreceptors, while the increase of acetylcholine release by the H3 receptor antagonist thioperamide is elicited via blockade of H3 autoreceptors. Histamine released from histaminergic nerve terminals increases the release of acetylcholine in part by inhibition of dopamine release which, in turn, decreases GABAergic transmission. A dopamine-independent way seems also to be involved in the histamine-evoked acetylcholine release.