The role of excitatory amino acid evoked second messengers -inositol phosphates and nitric oxide in cerebellar plasticity
Authors:   , Li Jun , Smith Sheryl S. 
Published: July 2024

In this dissertation, the roles of inositol phosphates (IPs) and nitric oxide (NO) evoked by excitatory amino acid (EAA) have been investigated in two types of neuronal plasticity: (1) development and (2) learning. First, phosphoinositide (PI) turnover stimulated by several agonists of the glutamate (Glu) receptor (GluR) was assessed using (3H) -inositol in 160 [mu]m cross chopped slices of cerebellar tissue from rats of different ages (postnatal day (PND) 7-10; PND30; adults 200-220 grams). The stimulation of PI hydrolysis by quisqualate (Quis) and Glu was concentration dependent. Quis was more potent than, but of the same efficacy as Glu. A peak in PI turnover evoked by Quis and Glu was seen at PND 7-10. PI turnover stimulated by Glu or Quis were not different from that at PND30, and in adults. NMDA, alone, did not alter the level of PI turnover. However, the GABAB receptor agonist baclofen (BAC) at a low concentration (1 [mu]M) potentiated NMDA (100 [mu]M)-stimulation of PI hydrolysis only in neonatal cerebellum (PND 7-10), which implies that stimulation of PI turnover by NMDA is condition-dependent. This NMDA stimulation can be inhibited by nitric oxide synthase (NOS) inhibitors, suggesting that this effect of NMDA on PI turnover may be mediated by NO. Ca²⁺ influx occurs upon the activation of the GluR. This Ca²⁺ influx may also stimulate PI hydrolysis by directly activating phospholipase C (PLC). Calcimycin (CM), a calcium ionophore, was utilized to address this issue. CM-stimulated PI hydrolysis also reached maximal levels at PND 7-10, but was absent at PND30 and in adulthood, suggesting that robust stimulation of PI hydrolysis by Glu is also produced by Ca²⁺ influx occurring during the activation of GluRs. These data portray an interesting profile in that GluR stimulation has a robust stimulatory effect on PI turnover around PND 7-10. During this period, maximal levels of synaptogenesis, dendritic growth and cell migration are also taking place. This temporal coincidence suggests that the PI hydrolysis stimulated by EAA may play a role in cerebellar development. NO also serves as a second messenger for GluR. To examine the role of NO during cerebellar development, changes in NOS were studied as NO is a gaseous molecule and very difficult to quantify. NOS was localized in rat cerebellum from PND1 to adult by using the nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) staining technique. NADPH-d stained neurons were first visualized at PND5 and gradually increased throughout the cerebellar cortex during development. At later ages, this increase followed the course of granule cell (GC) migration, suggesting that NO may be involved in the processing of GC migration/termination, In addition, NADPH-d stained GC were uniformly distributed initially in all GC layers. But, these stained GCs were re-organized as 'patches' upon PND19, remaining so until adulthood. In the adult, these NADPH-d stained patches were found to be overlapped with the mossy fiber projections (traced by cholera toxin B) from specific nuclei of the brain stem. (Abstract shortened by UMI.).