Search

It is now well established that nitric oxide is involved in a variety of physiopathological processes in the central nervous system, which mainly result from the interaction of this gaseous molecule with the heme group of soluble guanylyl cyclase and the elevation of intracellular cGMP in target neurons. During the last decade, several studies have monitored extracellular cGMP, by means of intracerebral microdialysis, to investigate in vivo the functioning and modulation of this neurochemical pathway under different experimental conditions and in various brain regions. In this review, we summarise some of the most relevant results obtained in this research field.

To understand the events underlying the clinical efficacy of deep brain stimulation (DBS) of the subthalamic nucleus (STN), electrophysiological recordings and microdialysis evaluations were carried out in the substantia nigra pars reticulata (SNr), one of the two basal ganglia (BG) nuclei targeted by STN output, in patients with Parkinson's disease (PD). Clinically effective STN-DBS caused a significant increase of the SNr firing rate. The poststimulus histogram (PSTH) showed an excitation peak at 1.92–3.85 ms after the STN stimulus. The spontaneous discharge of SNr neurons was driven at the frequency of the stimulation (130 Hz), as shown in the autocorrelograms (AutoCrl). The fast Fourier transform (FFT) analysis showed a peak at 130 Hz, and a less pronounced second one at 260 Hz. Accordingly, in the distribution of the interspike intervals (ISIs), the mode was earlier, and skewness more asymmetric. Biochemically, the increased excitatory driving from the STN was reflected by a clear-cut increase in cyclic guanosine 3',5'-monophosphate (cGMP) levels in the SNr. These results indicate that the beneficial effect of DBS in PD patients is paralleled with a stimulus-synchronized activation of the STN target, SNr. Our findings suggest that, during STN-DBS, a critical change towards a high-frequency oscillatory discharge occurs. [ABSTRACT FROM AUTHOR]

Using intracerebral microdialysis, we reported previously that acutein vivoactivation of NMDA glutamate receptors triggers rapid and transient releases of prostaglandin E2 (PGE2) andF2-isoprostane 15-F2t-IsoP in the hippocampus of freely moving rats. The formation of the two metabolites– produced through cyclo-oxygenase (COX) enzymatic activity and free radical-mediated peroxidation of arachidonic acid (AA), respectively,– was prevented by the specific NMDA antagonist MK-801, and was largely dependent on COX-2 activity. Here, we demonstrate that besides COX-2, which is the prominent COX isoform in the brain and particularly in the hippocampus, the constitutive isoform, COX-1 also contributes to prostaglandin (PG) synthesis and oxidative damage followingin vivoacute activation of hippocampal NMDA glutamate receptors. The relative contribution of the two isoforms is dynamically regulated, as the COX-2 selective inhibitor NS398 immediately prevented PGE2 and 15-F2t-IsoP formation during the application of NMDA, whereas the COX-1 selective inhibitor SC560 was effective only 1 h after agonist infusion. Our data suggest that, although COX-2 is the prominent isoform, COX-1 activity may significantly contribute to excitotoxicity, particularly when considering the amount of lipid peroxidation associated with its catalytic cycle. We suggest that both isoforms should be considered as possible therapeutic targets to prevent brain damage caused by excitotoxicity. [ABSTRACT FROM AUTHOR]

In the cerebellum, infusion of NMDA (200 μM) for 20 min evoked a marked (200%) increase of extracellular cyclic GMP (cGMP) levels. The selective GABAA receptor agonist muscimol (0.01-100 μM) was able to counteract the NMDA effect with an EC50 of 0.65 μM ; the inhibitory effect of muscimol (10 μM) was prevented by bicuculline (50 μM). Diazepam (10 μM) significantly potentiated the muscimol (1 μM) inhibition ; furthermore, when coinfused with 0.1 μM muscimol (a concentration not affecting, on its own, the cGMP response to NMDA), diazepam (10 μM) reduced the NMDA effect. Similar results were obtained with zolpidem (0.1-1 μM). Finally, local infusion of the benzodiazepine site antagonist flumazenil (10 μM), together with muscimol and diazepam, almost completely restored the effect of NMDA on extracellular cGMP levels. It is concluded that GABAA receptors potently control the NMDA/nitric oxide/cGMP pathway in the cerebellum in vivo. In terms of the α subunit composition, we can deduce that the cerebellar GABAA receptor does not contain α6 or β4 subunits because it is diazepam-sensitive. Moreover, the observation that zolpidem is active at a rather low concentration, in combination with localization studies present in the literature, tend to exclude the presence of α5 subunits in the receptor composition and suggest the involvement of an α1 subunit. [ABSTRACT FROM AUTHOR]

The motor thalamus (MTh) plays a crucial role in the basal ganglia (BG)-cortical loop in motor information codification. Despite this, there is limited evidence of MTh functionality in normal and Parkinsonian conditions. To shed light on the functional properties of the MTh, we examined the effects of acute and chronic dopamine (DA) depletion on the neuronal firing of MTh neurons, cortical/MTh interplay and MTh extracellular concentrations of glutamate (GLU) and gamma-aminobutyric acid (GABA) in two states of DA depletion: acute depletion induced by the tetrodotoxin (TTX) and chronic denervation obtained by 6-hydroxydopamine (6-OHDA), both infused into the medial forebrain bundle (MFB) in anesthetized rats. The acute TTX DA depletion caused a clear-cut reduction in MTh neuronal activity without changes in burst content, whereas the chronic 6-OHDA depletion did not modify the firing rate but increased the burst firing. The phase correlation analysis underscored that the 6-OHDA chronic DA depletion affected the MTh-cortical activity coupling compared to the acute TTX-induced DA depletion state. The TTX acute DA depletion caused a clear-cut increase of the MTh GABA concentration and no change of GLU levels. On the other hand, the 6-OHDA-induced chronic DA depletion led to a significant reduction of local GABA and an increase of GLU levels in the MTh. These data show that MTh is affected by DA depletion and support the hypothesis that a rebalancing of MTh in the chronic condition counterbalances the profound alteration arising after acute DA depletion state. [ABSTRACT FROM AUTHOR]

Cortical spreading depression (CSD) is a temporary disruption of local ionic homeostasis that propagates slowly across the cerebral cortex, and may contribute to the pathophysiology of stroke and migraine. Previous studies demonstrated that nitric oxide (NO) formation promotes the repolarisation phase of CSD, and this effect may be cyclic GMP (cGMP)-mediated. Here, we have examined how phosphodiesterase (PDE) inhibition, either alone or superimposed on NO synthase (NOS) inhibition, alters CSD and the associated changes in extracellular cGMP. Microdialysis probes incorporating an electrode were implanted into the frontoparietal cortex of anaesthetised rats for quantitative recording of CSD, pharmacological manipulations, and dialysate sampling for cGMP measurements. CSD was induced by cathodal electrical stimulation in the region under study by microdialysis. Extracellular cGMP increased, but only slightly, during CSD. Perfusion of either zaprinast or sildenafil through the microdialysis probe, at concentrations that inhibited both PDE5 and PDE9 (and possibly other PDE), increased significantly extracellular cGMP. Unexpectedly, these levels remained high when NOS was subsequently inhibited with Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME, 1 mM). The most interesting pharmacological effect on CSD was obtained with sildenafil. This drug altered neither CSD nor the subsequent characteristic effect of NOS inhibition, i.e. a marked widening of CSD. The fact that NOS inhibition still widened CSD in the presence of the high extracellular levels of cGMP associated with PDE inhibition, suggests that NO may promote CSD recovery, independently of cGMP formation. [Copyright &y& Elsevier]

We have investigated the functional relationships between NMDA receptors and the NOS/sGC system in the rat pre-frontal cortex in vivo by microdialysis. cGMP basal levels were sensitive to NOS or sGC inhibitors (l-NARG or ODQ) or NO donors (SNAP) when enzymatic breakdown was blocked by the phosphodiesterase inhibitor IBMX, indicating that basal cGMP production derives, at least in part, from the NOS/sGC pathway activity and that the pre-frontal cortex possesses a very efficient degradation system for cGMP. The glutamate receptor agonist NMDA did not alter extracellular cGMP either in absence or presence of IBMX. cGMP was not augmented when NMDA was co-infused with the NOS substrate l-arginine, the glycine site agonist d-serine or the glutamate receptor agonist AMPA. Interestingly, the selective GABAA receptor antagonist bicuculline enhanced cGMP production, revealing that the cortical NOS/sGC system is tonically inhibited by endogenous GABA. However, in the presence of bicuculline, NMDA did not increase extracellular cGMP. In the presence of bicuculline, blockade of 5-HT1/2 receptors, known to inhibit the NMDA/NOS/sGC pathway, with the antagonist methiothepin did not unmask cGMP elevations by NMDA. Thus, it would seem that NMDA receptors do not regulate cortical NOS/sGC activity that, on the other hand, is modulated by endogenous GABA acting at GABAA receptors. [Copyright &y& Elsevier]