Biomedical subjects

David González-Forero

Publications and source records attributed to David González-Forero.

8 recordsLinked to original sources

PubMed2006

Nitric oxide and synaptic dynamics in the adult brain: physiopathological aspects.

The adult brain retains the capacity to rewire mature neural circuits in response to environmental changes, brain damage or sensory and motor experiences. Two plastic processes, synaptic remodeling and neurogenesis, have been the subject of numerous studies due to their involvement in the maturation of the nervous system, their prevalence and re-activation in adulthood, and therapeutic relevance. However, most of the research looking for the mechanistic and molecular events underlying synaptogenic phenomena has been focused on the extensive synaptic reorganization occurring in the developing brain. In this stage, a vast number of synapses are initially established, which subsequently undergo a process of activity-dependent refinement guided by target-derived signals that act as synaptotoxins or synaptotrophins, promoting either loss or consolidation of pre-existing synaptic contacts, respectively. Nitric oxide (NO), an autocrine and/or paracrine-acting gaseous molecule synthesized in an activity-dependent manner, has ambivalent actions. It can act by mediating synapse formation, segregation of afferent inputs, or growth cone collapse and retraction in immature neural systems. Nevertheless, little information exists about the role of this ambiguous molecule in synaptic plasticity processes occurring in the adult brain. Suitable conditions for elucidating the role of NO in adult synaptic rearrangement include physiopathological conditions, such as peripheral nerve injury. We have recently developed a crush lesion model of the XIIth nerve that induces a pronounced stripping of excitatory synaptic boutons from the cell bodies of hypoglossal motoneurons. The decline in synaptic coverage was concomitant with de novo expression of the neuronal isoform of NO synthase in motoneurons. We have demonstrated a synaptotoxic action of NO mediating synaptic withdrawal and preventing synapse formation by cyclic GMP (cGMP)-dependent and, probably, S-nitrosylation-mediated mechanisms, respectively. This action possibly involves the participation of other signaling molecules working together with NO. Brain-derived neurotrophic factor (BDNF), a target-derived synaptotrophin synthesized and released postsynaptically in an activity-dependent form, is a potential candidate for effecting such a concerted action. Several items of evidence support an interrelationship between NO and BDNF in the regulation of synaptic remodeling processes in adulthood: i) BDNF and its receptor TrkB are expressed by motoneurons and upregulated by axonal injury; ii) they promote axon arborization and synaptic formation, and modulate the structural dynamics of excitatory synapses; iii) NO and BDNF each control the production and activity of the other at the level of individual synapses; iv) the NO/cGMP pathway inhibits BDNF secretion; and finally, v) BDNF protects F-actin from depolymerization by NO, thus preventing the collapsing and retracting effects of NO on growth cones. Therefore, we propose a mechanism of action in which the NO/BDNF ratio regulates synapse dynamics after peripheral nerve lesion. This hypothesis also raises the possibility that variations in this NO/BDNF balance constitute a common hallmark leading to synapse loss in the progression of diverse neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's and Parkinson's diseases.

Animals↗

PubMed2005

Transynaptic effects of tetanus neurotoxin in the oculomotor system.

The question whether general tetanus arises from the independent sum of multiple local tetani or results from the actions of the transynaptic tetanus neurotoxin (TeNT) in higher brain centres remains unresolved. Despite the blood-borne dissemination of TeNT from an infected wound, the access to the central nervous system is probably prevented by the blood-brain barrier. However, several long-term sequelae (e.g. autonomic dysfunction, seizures, myoclonus, and sleep disturbances) present after the subsidence of muscle spasms might be indicative of central actions that occur farther away from lower motoneurons. Subsequently, the obvious entry route is the peripheral neurons followed by the transynaptic passage to the brain. We aimed at describing the pathophysiological correlates of TeNT translocation using the oculomotor system as a comprehensive model of cell connectivity and neuronal firing properties. In this study, we report that injection of TeNT into the medial rectus muscle of one eye resulted in bilateral gaze palsy attributed to firing alterations found in the contralaterally projecting abducens internuclear neurons. Functional alterations in the abducens-to-oculomotor internuclear pathway resembled in part the classically described TeNT disinhibition. We confirmed the transynaptic targeted action of TeNT by analysing vesicle-associated membrane protein2 (VAMP2) immunoreactivity (the SNARE protein cleaved by TeNT). VAMP2 immunoreactivity decreased by 94.4% in the oculomotor nucleus (the first synaptic relay) and by 62.1% presynaptic to abducens neurons (the second synaptic relay). These results are the first demonstration of physiological changes in chains of connected neurons that are best explained by the transynaptic action of TeNT on premotor neurons as shown with VAMP2 immunoreactivity which serves as an indicator of TeNT activity.

Abducens Nerve↗

PubMed2005

Differential postnatal maturation of GABAA, glycine receptor, and mixed synaptic currents in Renshaw cells and ventral spinal interneurons.

Renshaw cells (RCs) receive excitatory inputs from motoneurons to which then they inhibit. The gain of this spinal recurrent inhibitory circuit is modulated by inhibitory synapses on RCs. Inhibitory synapses on RCs mature postnatally, developing unusually large postsynaptic gephyrin clusters that colocalize glycine and GABA(A) receptors. We hypothesized that these features potentiate inhibitory currents in RCs. Thus, we analyzed glycinergic and GABAergic "inhibitory" miniature postsynaptic currents (mPSCs) in neonatal [postnatal day 1 (P1) to P5] and mature (P9-P15) RCs and compared them to other ventral interneurons (non-RCs). Recorded neurons were Neurobiotin filled and identified as RCs or non-RCs using post hoc immunohistochemical criteria. Glycinergic, GABAergic, and mixed glycine/GABA mPSCs matured differently in RCs and non-RCs. In RCs, glycinergic and GABA(A) mPSC peak amplitudes increased 230 and 45%, respectively, from P1-P5 to P9-P15, whereas in non-RCs, glycinergic peak amplitudes changed little and GABA(A) amplitudes decreased. GABA(A) mPSCs were slower in RCs (P1-P5, tau = 58 ms; P9-P15, tau = 43 ms) compared with non-RCs (P1-P5, tau = 27 ms; P9-P15, tau = 14 ms). Thus, fast glycinergic currents dominated "mixed" mPSC peak amplitudes in mature RCs, and GABA(A) currents dominated their long decays. In non-RCs, GABAergic and mixed events had shorter durations, and their frequencies decreased with development. Functional maturation of inhibitory synapses on RCs correlates well with increased glycine receptor recruitment to large gephyrin patches, colocalization with alpha3/alpha5-containing GABA(A) receptors, and maintenance of GABA/glycine corelease. As a result, charge transfer in GABA, glycine, or mixed mPSCs was larger in mature RCs than in non-RCs, suggesting RCs receive potent inhibitory synapses.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2005

Nitric-oxide-directed synaptic remodeling in the adult mammal CNS.

In adult mammals, learning, memory, and restoration of sensorimotor lost functions imply synaptic reorganization that requires diffusible messengers-mediated communication between presynaptic and postsynaptic structures. A candidate molecule to accomplish this function is the gaseous intercellular messenger nitric oxide (NO), which is involved in synaptogenesis and projection refinement during development; however, the role of NO in synaptic reorganization processes in adulthood remains to be established. In this work, we tested the hypothesis that this free radical is a mediator in the adult mammal CNS synaptic remodeling processes using a model of hypoglossal axonal injury recently developed by us. Axonal injury-induced disconnection of motoneurons from myocytes produces withdrawal of synaptic inputs to motoneurons and concomitant upregulation of the neuronal isoform of NO synthase (NOS-I). After recovery of the neuromuscular function, synaptic coverage is reestablished and NOS-I is downregulated. We also report, by using functional and morphological approaches, that chronic inhibition of the NO/cGMP pathway prevents synaptic withdrawal evoked by axon injury, despite the persistent muscle disconnection. After successful withdrawal of synaptic boutons, inhibition of NO synthesis, but not of cGMP, accelerated the recovery of synaptic coverage, although neuromuscular disconnection was maintained. Furthermore, protein S-nitrosylation was upregulated after nerve injury, and this effect was reversed by NOS-I inhibition. Our results suggest that during synaptic remodeling in the adult CNS, NO acts as a signal for synaptic detachment and inhibits synapse formation by cGMP-dependent and probably S-nitrosylation-mediated mechanisms, respectively. We also suggest a feasible role of NO in neurological disorders coursing with NOS-I upregulation.

Animals↗

PubMed2004

Nerve injury reduces responses of hypoglossal motoneurones to baseline and chemoreceptor-modulated inspiratory drive in the adult rat.

The effects of peripheral nerve lesions on the membrane and synaptic properties of motoneurones have been extensively studied. However, minimal information exists about how these alterations finally influence discharge activity and motor output under physiological afferent drive. The aim of this work was to evaluate the effect of hypoglossal (XIIth) nerve crushing on hypoglossal motoneurone (HMN) discharge in response to the basal inspiratory afferent drive and its chemosensory modulation by CO(2). The evolution of the lesion was assessed by recording the compound muscle action potential evoked by XIIth nerve stimulation, which was lost on crushing and then recovered gradually to control values from the second to fourth weeks post-lesion. Basal inspiratory activities recorded 7 days post-injury in the nerve proximal to the lesion site, and in the nucleus, were reduced by 51.6% and 35.8%, respectively. Single unit antidromic latencies were lengthened by lesion, and unusually high stimulation intensities were frequently required to elicit antidromic spikes. Likewise, inspiratory modulation of unitary discharge under conditions in which chemoreceptor drive was varied by altering end-tidal CO(2) was reduced by more than 60%. Although the general recruitment scheme was preserved after XIIth nerve lesion, we noticed an increased proportion of low-threshold units and a reduced recruitment gain across the physiological range. Immunohistochemical staining of synaptophysin in the hypoglossal nuclei revealed significant reductions of this synaptic marker after nerve injury. Morphological and functional alterations recovered with muscle re-innervation. Thus, we report here that nerve lesion induced changes in the basal activity and discharge modulation of HMNs, concurrent with the loss of afferent inputs. Nevertheless, we suggest that an increase in membrane excitability, reported by others, and in the proportion of low-threshold units, could serve to preserve minimal electrical activity, prevent degeneration and favour axonal regeneration.

Action Potentials↗

PubMed2004

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons.

A low or a high dose of tetanus neurotoxin (TeNT) injected in the lateral rectus muscle of the cat causes respectively, functional block of inhibitory synapses only or of both inhibitory and excitatory synapses simultaneously in abducens neurons (González-Forero et al. [2003] J. Neurophysiol. 89:1878-1890). As a consequence, neuronal firing activity increases (at low dose) or decreases (at high dose). We investigated possible structural modifications of inhibitory synapses in response to these activity alterations induced by TeNT. We used immunofluorescence against postsynaptic (gephyrin) and presynaptic (vesicular gamma-aminobutyric acid [GABA] transporter [VGAT]) markers of inhibitory synapses in combination with cell type markers for abducens motoneurons (calcitonin gene-related peptide or choline acetyltransferase) or internuclear neurons (calretinin). Seven days after high-dose treatment, the number of gephyrin-immunoreactive (IR) clusters per 100 microm of membrane perimeter was reduced on the soma of abducens motoneurons by 55.3% and by 60.1% on internuclear neurons. Proximal dendritic gephyrin-IR clusters were also significantly altered but to a lesser degree. Partial synaptic re-establishment was observed 15 days post injection, and complete recovery occurred after 42 days. Coverage by VGAT-IR terminals was reduced in parallel. In contrast, a low dose of TeNT caused no structural alterations. With electron microscopy we estimated that overall synaptic coverage was reduced by 40% in both types of neurons after a high dose of TeNT. However, F-type terminals with postsynaptic gephyrin were preferentially lost. Thus, the ratio between F and S terminals diminished from 1.28 to 0.39 on motoneurons and from 1.26 to 0.47 on internuclear neurons. These results suggest that the maintenance of proximal inhibitory synaptic organization on central neurons is best related to neuronal activity and not to the level of inhibitory synaptic function, which was equally blocked by the high or low dose of TeNT.

Abducens Nerve↗

PubMed2002

Correlation between CGRP immunoreactivity and firing activity in cat abducens motoneurons.

A relationship between motoneuron activity and calcitonin gene-related peptide (CGRP) expression was previously suggested based on indirect inferences. We show here a positive correlation between CGRP immunoreactivity and firing activity in an experimental model that used tetanus neurotoxin (TeNT) to alter basal firing levels. A low dose (0.5 ng/kg) of TeNT injected in the lateral rectus muscle raised the basal firing rate of ipsilateral abducens motoneurons, estimated as the firing rate at straight-ahead gaze (F(0)); the firing rate returned to control values after 2 weeks. In contrast, a high dose (5 ng/kg) of TeNT decreased basal firing, which recovered slowly over a 7-week period. Expression of CGRP immunoreactivity by abducens motoneurons, preferentially related to betaCGRP gene expression, was analyzed during these periods of altered firing activity. The number of CGRP-immunofluorescent abducens motoneurons increased to approximately 120% by 7 days after low-dose TeNT, to include all available motoneurons in the nucleus. In addition, the average CGRP immunofluorescence optical density inside motoneurons almost doubled after 4 days and returned toward control values in the following 2 weeks. In contrast, a high-dose injection of TeNT reduced the number of CGRP-immunofluorescent motoneurons to 5.4% of control 7 days post injection, and the number returned to 77.6% after 42 days. CGRP immunofluorescence intensity inside motoneurons was also reduced. Regression analysis of F(0) values with either the number of CGRP-immunolabeled motoneurons, their average immunofluorescence intensity, or both factors combined resulted in positive correlations with regression coefficients of 0.87 or higher. Therefore, CGRP expression and firing activity in abducens motoneurons are positively correlated.

Abducens Nerve↗

PubMed2002

Influence of afferent synaptic innervation on the discharge variability of cat abducens motoneurones.

The discharge variability of abducens motoneurones was studied after blocking inhibitory synaptic inputs or both excitatory and inhibitory inputs by means of an intramuscular (lateral rectus) injection of either a low (0.5 ng kg(-1)) or a high dose (5 ng kg(-1)) of tetanus neurotoxin (TeNT), respectively. Motoneuronal firing increased after low-dose TeNT. High-dose treatment, however, produced a firing depression, and in some cells, a total lack of modulation in relation to eye movements. Firing became increasingly more regular with larger TeNT doses as shown by significant reductions in the coefficient of variation after low- and high-dose treatments. Similarly, autocorrelation histograms of interspike intervals increased the number of resolvable peaks twofold in low-dose-treated motoneurones and sevenfold in high-dose-treated motoneurones. The plots of standard deviation versus the mean instantaneous firing frequency showed an upward deflexion with low firing frequencies. The upward deflexion occurred in controls at 39.9 +/- 4.9 ms, an interval similar to the mean afterhyperpolarisation (AHP) duration (48.4 +/- 8.8 ms). Low-dose TeNT treatment shifted the deflexion point to 20.9 +/- 3.9 ms, whereas the high dose increased it to 60.7 +/- 6.1 ms, in spite of the fact that no differences in AHP parameters between groups were found. The density of synaptophysin-immunoreactive boutons decreased by 14 % after the low-dose treatment and 40.5 % after the high-dose treatment, indicating that protracted synaptic blockade produces elimination of synaptic boutons. It is concluded that abducens motoneurone spike variability during spontaneous ocular fixations depends largely on the balance between inhibitory and excitatory synaptic innervation.

Abducens Nerve↗