The aim of this work was to assess whether ammonia concentrations similar to the increase found in the brain of hyperammonemic rats (100 microM), impair N-methyl-D-aspartate (NMDA) receptor-mediated signal transduction. We first measured glutamate neurotoxicity, which in these neurons is mediated by activation of NMDA receptors, as an initial parameter reflecting activation of NMDA receptor-mediated pathways. Long-term treatment of cultured neurons with ammonia prevents glutamate-induced neuronal death. The EC50 was 20 microM, and at 100 microM the protection was complete. The induction of the protective effect was not immediate, but took several hours. Treatment with 100 microM ammonia did not prevent a glutamate- or NMDA-induced rise of intracellular calcium. Ammonia impaired the glutamate-nitric oxide-cGMP (3',5'-cyclic guanosine monophosphate) pathway in a dose- and time-dependent manner. Glutamate-induced formation of cGMP was reduced by 42%, while activation of nitric oxide synthase was not affected. Ammonia reduced by 31% cGMP formation induced by S-nitroso-N-acetyl-penicillamine (SNAP), a NO-generating agent, confirming that the interference occurs at the level of guanylate cyclase activation by nitric oxide. To assess whether chronic moderate hyperammonemia in vivo also impairs the glutamate-nitric oxide-cGMP pathway, we determined by in vivo brain microdialysis in freely moving rats the formation of cGMP induced by NMDA. In hyperammonemic rats, the formation of cGMP induced by NMDA and SNAP was reduced by ca. 60 and 41%, respectively, indicating that chronic hyperammonemia in the animal in vivo also impairs the glutamate-nitric oxide-cGMP pathway. Impairment of this pathway can contribute to the neurological alterations found in hyperammonemia and hepatic encephalopathy.

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Dorsal hippocampal theta rhythm (theta) and extracellular unit activity from CA1 pyramidal layer were recorded in awake guinea pigs, both during standing and during walking on a conveyor belt at increasing speeds. Amplitude, frequency and rhythmicity of theta increased linearly with the movement speed. In this preparation we found the same three types of unit discharge patterns that have been described in anesthetized rats in the presence of spontaneous or induced hippocampal theta: type 1, rhythmic at theta frequency and phase-locked with theta; type 2, discharging non-rhythmically but phase-locked with theta; and type 3, discharging at random. Furthermore, all units modified their firing pattern when the animals walked, either by increasing their rhythmicity and/or phase-locking with theta or by increasing their firing frequency. During walking, some type 3 units changed into type 2 or type 1, type 2 units changed to type 1, and type I increased their rhythmicity. Consequently, the unit discharge rhythmicity and phase-locking with theta increased with the speed of movement. The mean rate of neuronal discharges increased linearly as a function of walking speed. In this paper we show that the progressive spectral theta changes determined by the intensity of movement are concomitant with the increase in rhythmicity of hippocampal cells. Moreover, the firing rate of these cells, and the amplitude, frequency and rhythmicity of theta, increased linearly as a function of walking speed, suggesting that neuronal excitation may be basically responsible for these changes in theta properties.

Synaptic dysfunction is a core deficit in Alzheimer's disease, preceding hallmark pathological abnormalities. Resting-state magnetoencephalography (MEG) was used to assess whether functional connectivity patterns, as an index of synaptic dysfunction, are associated with CSF biomarkers [i.e., phospho-tau (p-tau) and amyloid beta (A␤42) levels]. We studied 12 human subjects diagnosed with mild cognitive impairment due to Alzheimer's disease, comparing those with normal and abnormal CSF levels of the biomarkers. We also evaluated the association between aberrant functional connections and structural connectivity abnormalities, measured with diffusion tensor imaging, as well as the convergent impact of cognitive deficits and CSF variables on network disorganization. One-third of the patients converted to Alzheimer's disease during a follow-up period of 2.5 years. Patients with abnomal CSF p-tau and A␤42 levels exhibited both reduced and increased functional connectivity affecting limbic structures such as the anterior/posterior cingulate cortex, orbitofrontal cortex, and medial temporal areas in different frequency bands. A reduction in posterior cingulate functional connectivity mediated by p-tau was associated with impaired axonal integrity of the hippocampal cingulum. We noted that several connectivity abnormalities were predicted by CSF biomarkers and cognitive scores. These preliminary results indicate that CSF markers of amyloid deposition and neuronal injury in early Alzheimer's disease associate with a dual pattern of cortical network disruption, affecting key regions of the default mode network and the temporal cortex. MEG is useful to detect early synaptic dysfunction associated with Alzheimer's disease brain pathology in terms of functional network organization.

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