Masaaki Tokuda

Calcineurin inhibitors, FK506 and cyclosporin A, suppress the NMDA receptor-mediated potentials and LTP, but not depotentiation in the rat hippocampus

The effects of FK506, a Ca2+/calmodulin-dependent phosphatase 2B (calcineurin) inhibitor, on the NMDA receptor-mediated potentials and synaptic plasticity were investigated in the CA1 region of the rat hippocampus. Bath application of FK506 (50 microM) produced a 45% inhibition on the NMDA receptor-mediated potentials. FK506 also inhibited the induction of long-term potentiation (LTP), but had no effect on the depotentiation in the CA1 hippocampus. Cyclosporin A (100 microM), another calcineurin inhibitor, mimicked the effects of FK506 on the NMDA responses and synaptic plasticity. These results suggest that FK506 inhibits the activity of NMDA receptors via the involvement of calcineurin. The differential effects of FK506 on LTP and depotentiation may attribute to the partial inhibition on the activity of NMDA receptors and the subsequent attenuation of intracellular Ca2+ increase.

Localization and developmental changes in the neuron-specific cyclin-dependent kinase 5 activator (p35nck5a) in the rat brain

Mammalian brains contain a cde2-like protein kinase which is a heterodimer of cyclin-dependent kinase 5 (Cdk5) and a brain-specific regulatory subunit with a molecular weight of 35,000. In this study, we examined the temporal and spatial expression patterns of p35nck5a in the developing rat brain. Northern blot analysis showed that p35nck5a messenger RNA expression was low in the brain of 12-day postcoitum rats, and increased to a much higher level from 18 days postcoitum to two weeks after birth, and then declined at three weeks after birth. These developmental changes in p35nck5a expression correlated with the changes in Cdk5-associated kinase activity during brain development. These data suggest that p35nck5a is the specific activator for Cdk5 in the brain. Immunohistochemical and in situ hybridization studies demonstrated the presence of p35nck5a protein in postmitotic neurons but not in glial cells at all stages of brain development, indicating that p35nck5a is a neuron-specific protein. In the adult brain, the protein was rich in cell bodies and dendrites, and only very low amounts were detected in axons. In fetal and neonatal brains, however, axonal pathways such as the corpus callosum and external capsule were also stained with anti-p35nck5a antibody. Our findings suggest that p35nck5a is neuron specific, and a specific activator for Cdk5, and the subcellular localization of the two is strictly regulated depending on brain development. Neuronal Cdc2-like kinase may play key roles in neuronal maturation, synaptic formation, and neuronal plasticity.

Analysis of Dopamine and Neuropeptides in 6-OHDA-Lesioned Rats

Cholecystokinin (CCK) and dopamine (DA) coexist in the ventral tegmental area and medial substantia nigra neurons.1 In the brain of Parkinson’s disease patients, a prominent decrease in CCK content of the substantia nigra has been reported,2 and it is thought that DA and CCK might be related to the etiology of Parkinson’s disease. The precise functional interaction between DA and CCK, however, is not clarified. Based on the hypothesis that CCK has a functional role as one of the modulators of DA metabolism, ceruletide (CLT), which is a CCK-related decapeptide, is currently under trial for the treatment of diseases with involuntary movements including Parkinson’s disease.3 In the present study we attempted to clarify the mechanism of action of CLT and CCK with respect to their receptors as well as their effect on behavior in a Parkinson’s disease model, namely, the 6-OHDA unilaterally lesioned rat.