Synaptic receptors and intracellular signal transduction in the cerebellum

Ethanol selectively interferes with the trophic action of NMDA and carbachol on cultured cerebellar granule neurons undergoing apoptosis

Exposure of mature rat cerebellar granule neurons to non-depolarizing conditions (5 mM K+) for 24 h resulted in the onset of apoptosis. NMDA, forskolin, carbachol and GABA attenuated low K+-induced toxicity, although to a different extent, with NMDA and GABA being the most effective agents. When cells were co-exposed for 24 h to ethanol, the survival promoting action of NMDA and carbachol, but not that of forskolin and GABA, was attenuated. By contrast, a 24 h cell pre-treatment with ethanol, followed by its removal prior to K+ deprivation, was ineffective towards the neurotrophic action of NMDA and carbachol. The concomitant presence of alcohol and neurotrophic factors was not required for the pro-apoptotic effect of ethanol to be manifest after a long-term alcohol exposure: inhibition of NMDA- and carbachol-mediated neurotrophism was still observed when cells were pre-exposed for 72 h to alcohol in depolarizing conditions, prior to the challenge with 5 mM K+-containing medium and the test compounds in the absence of ethanol. The present study shows that ethanol promotes apoptotic cell death of cultured cerebellar neurons by selectively inhibiting the neurotrophic effect of NMDA and carbachol, and suggests that alcohol may cause permanent changes in the control mechanisms of apoptosis: this finding may have significant implications for the in vivo toxicity of prenatal ethanol exposure on the developing cerebellum.

Cerebral blood flow increases evoked by electrical stimulation of rat cerebellar cortex: relation to excitatory synaptic activity and nitric oxide synthesis

The purpose of this study was to examine mechanisms involved in the coupling of neuronal activity to cerebral blood flow (CBF). CBF was measured in rat cerebellum using laser-Doppler flowmetry during stimulus-evoked neuronal activity and related to the distribution of the extracellular field potential. Local electrical stimulation of the cerebellar cortex activated a narrow beam of parallel fibers (PFs) 100 microns across and evoked increases of CBF along (On-B) and perpendicular (Off-B) to the beam. Increases of CBF and field potentials were recorded for a distance of up to 1500 microns along the activated beam, and perpendicular to the beam, in a zone approximately 1000 microns wide, i.e. about 10 times wider than the zone in which synaptic excitation took place. CBF increased as a function of stimulus frequency up to 75 Hz, the response being larger On-B than Off-B. TTX abolished both the field potentials and the CBF responses at all frequencies, suggesting that action potentials were mechanistically related to the evoked CBF increases. CBF changes were unchanged by picrotoxin, a blocker of GABA(A) receptors, consistent with the idea that inhibitory synaptic activity does not contribute to CBF increases. The latency to the CBF rise was much shorter On-B than Off-B for the same distance from the stimulating electrode. This may suggest that the CBF response Off-B is dependent on diffusion of vasoactive substances from neuronal structures activated by the parallel fibers On-B. Nitric oxide (NO) synthase inhibition with NG-nitro-L-Arginine increased the time latency to onset of CBF rise by 2-4 times and attenuated the evoked CBF increase by approximately 50%. Sodium nitroprusside, a NO donor, increased baseline CBF, but did not reverse the effects of L-NNA. Thus the initial part of the evoked CBF rise is probably mediated by NO, which also contributes to the later part of the response. This study provides insight into the distribution and mechanism of neurally evoked increases of CBF, of putative importance for the interpretation of activation studies in animals and humans.

Importance of nitric oxide for local increases of blood flow in rat cerebellar cortex during electrical stimulation

The endothelium-derived relaxing factor, probably nitric oxide (NO), is a potent vasodilator that regulates the vascular tone in several vascular beds, including the brain. We explored the possibility that NO might be of importance for the increase of cerebral blood flow (CBF) associated with activity of the well-defined neuronal circuits of the rat cerebellar cortex. Laser-Doppler flowmetry was used to measure increases of cerebellar blood flow evoked by trains of electrical stimulations of the dorsal surface. The evoked increases of CBF were frequency-dependent, being larger on than off the parallel fiber tracts, suggesting that conduction along parallel fibers and synaptic activation of target cells were important for the increase of CBF. This was verified experimentally since the evoked CBF increases were abolished by tetrodotoxin and reduced by 10 mM Mg2+ and selective antagonists for non-N-methyl-D-aspartate receptors. The cerebellar cortex contains high levels of NO synthase. This raised the possibility that NO was involved in the increase of CBF associated with neuronal activation. NO synthase inhibition by topical application of NG-nitro-L-arginine attenuated the evoked CBF increase by about 50%. This effect was partially reversed by pretreatment with L-arginine, the natural substrate for the enzyme, while NG-nitro-D-arginine, the inactive enantiomer, had no effect on the evoked CBF increases. Simultaneous blockade of non-N-methyl-D-aspartate receptors and NO synthase had no further suppressing effect on the blood flow increase than either substance alone, suggesting that the NO-dependent flow rise was dependent on postsynaptic mechanisms. These findings are consistent with the idea that local synthesis of NO is involved in the transduction mechanism between neuronal activity and increased CBF.

Structure, expression, and functional analysis of a Na(+)-dependent glutamate/aspartate transporter from rat brain

Transport systems specific for L-glutamate and L-aspartate play an important role in the termination of neurotransmitter signals at excitatory synapses. We describe here the structure and function of a 66-kDa glycoprotein that was purified from rat brain and identified as an L-glutamate/L-aspartate transporter (GLAST). A GLAST-specific cDNA clone was isolated from a rat brain cDNA library. The cDNA insert encodes a polypeptide with 543 amino acid residues (59,697 Da). The amino acid sequence of GLAST suggests a distinctive structure and membrane topology, with some conserved motifs also present in prokaryotic glutamate transporters. The transporter function has been verified by amino acid uptake studies in the Xenopus laevis oocyte system. GLAST is specific for L-glutamate and L-aspartate, shows strict dependence on Na+ ions, and is inhibited by DL-threo-3-hydroxy-aspartate. In situ hybridization reveals a strikingly high density of GLAST mRNA in the Purkinje cell layer of cerebellum, presumably in the Bergmann glia cells, and a less dense distribution throughout the cerebrum. These data suggest that GLAST may be involved in the regulation of neurotransmitter concentration in central nervous system.

Enhancement of optokinetic and vestibuloocular responses in the rabbit by cholinergic stimulation of the flocculus

Bilateral microinjections into the cerebellar flocculus of the rabbit of carbachol, a general cholinergic agonist, profoundly affect vestibuloocular (VOR) and optokinetic (OKR) reflexes. For sinusoidal stimuli (0.15 Hz, 5 deg peak to peak), the gain of the OKR was strongly increased, while the gain of the VOR was moderately increased. These effects were partially mimicked by floccular injection of the acetylcholinesterase inhibitor eserine. Floccular injection of the muscarinic blocker atropine significantly lowered the gain of the OKR. The effects of the nicotinic blocker mecamylamine were not significant. Optokinetic nystagmus (OKN) in response to constant stimulus velocities (1-30 deg/second) showed a markedly accelerated buildup and a shortened optokinetic after-nystagmus (OKAN) after floccular injections of carbachol. The steady-state gain of OKN remained unaffected. None of the described effects occurred after floccular injection of the solvent, saline. It is postulated that cholinergic cerebellar afferents, one probable source of which are the vestibular nuclei, enhance the optokinetic and vestibular modulation of floccular Purkinje cells.