Prandial increase of leptin in the brain activates spatial learning and memory.

Leptin is well known to be involved in the control of feeding, thermogenesis, reproduction and neuroendocrine functions through its actions on the rodents hypothalamic receptors. Leptin facilitated the presynaptic transmitter release and postsynaptic sensitivity to the transmitters in the hippocampal CA1 neurons. Thus long-term potentiation (LTP) and the phosphorylation of Ca(2+)/calmodulin protein kinase II (CaMKII) were facilitated in the CA1 neurons. Therefore behavioral performance related to spatial learning and memory was improved by leptin in vivo applications. We also investigated LTP and spatial learning and memory function in two leptin receptor-deficient rodents, Zucker fatty rats and db/db mice. The CA1 region of both strains showed impairments of LTP and leptin application did not improve these impairments. These strains showed lower basal levels of CaMKII activity in the CA1 region than the respective controls, and the levels did not respond to a brief tetanic stimulation. These strains also showed impaired spatial learning and memory. The present studies suggest that leptin signaling in the brain may have important implications for cognitive function.

Prandial increases of leptin and orexin in the brain modulate spatial learning and memory.

Leptin is well known to be involved in the inhibition of feeding, hermogenesis, reproduction and neuroendocrine functions through its actions on the rodents hypothalamic receptors. Leptin facilitated the presynaptic transmitter release and postsynaptic sensitivity to the transmitters in the hippocampal CA1 neurons. Thus long-term potentiation (LTP) and the phosphorylation of Ca2+/calmodulin protein kinase II (CaMK II) were facilitated in the CA1 neurons. Therefore behavioral performance related to spatial learning and memory was improved by leptin in vivo applications. Orexin-A produced by glucose-sensitive neurons in the lateral hypothalamic area (LHA) and released during food intake facilitates feeding. Orexin-A suppressed LTP and CaMK II phosphorylation without affecting the presynaptic transmitter release. Therefore behavioral performance on learning and memory was impaired. The present studies suggest that leptin and orexin signalings in the brain may have important implications for cognitive function.

Synapsin I is phosphorylated at Ser603 by p21-activated kinases (PAKs) in vitro and in PC12 cells stimulated with bradykinin.

The function of synapsin I is regulated by phosphorylation of the molecule at multiple sites; among them, the Ser(603) residue (site 3) is considered to be a pivotal site targeted by Ca(2+)/calmodulin-dependent kinase II (CaMKII). Although phosphorylation of the Ser(603) residue responds to several kinds of stimuli, it is unlikely that many or all of the stimuli activate the CaMKII-involved pathway. Among the several stimulants tested in PC12 cells, bradykinin evoked the phosphorylation of Ser(603) without inducing the autophosphorylation of CaMKII, which was determined using phosphorylation site-specific antibodies against phospho-Ser(603)-synapsin I (pS603-Syn I-Ab) and phospho-Thr(286/287)-CaMKII. The bradykinin-evoked phosphorylation of Ser(603) was not suppressed by the CaMKII inhibitor KN62, whereas high KCl-evoked phosphorylation was accompanied by CaMKII autophosphorylation and inhibited by KN62. Thus, we attempted to identify Ser(603) kinase(s) besides CaMKII. We consequently detected four and three fractions with Ca(2+)/calmodulin-independent Ser(603) kinase activity on the DEAE column chromatography of bovine brain homogenate and PC12 cell lysate, respectively, two of which were purified and identified by amino acid sequence of proteolytic fragments as p21-activated kinase (PAK) 1 and PAK3. The immunoprecipitants from bovine brain homogenate with anti-PAK1 and PAK3 antibodies incorporated (32)P into synapsin I in a Cdc42/GTPgammaS-dependent manner, and its phosphorylation site was confirmed as Ser(603) using pS603-Syn I-Ab. Additionally, recombinant GST-PAK2 could phosphorylate the Ser(603) residue in the presence of Cdc42/GTPgammaS. Finally, we confirmed by immunocytochemical analysis that the transfection of constitutively active rat alphaPAK (PAK1) in PC12 cells evokes the phosphorylation of Ser(603) even in the resting mutant cells and enhances it in the bradykinin-stimulated cells, whereas that of dominant-negative alphaPAK quenches the phosphorylation. These results raise the possibility that Ser(603) on synapsin I is alternatively phosphorylated by PAKs, not only by CaMKII, in neuronal cells in response to some stimulants.