Electron microscopic serial analysis of GABA presynaptic terminals on the axon hillock and initial segment of labeled abducens motoneurons in the rat

Morphological Characterization of the Action Potential Initiation Segment in GnRH Neuron Dendrites and Axons of Male Mice

GnRH neurons are the final output neurons of the hypothalamic network controlling fertility in mammals. In the present study, we used ankyrin G immunohistochemistry and neurobiotin filling of live GnRH neurons in brain slices from GnRH-green fluorescent protein transgenic male mice to examine in detail the location of action potential initiation in GnRH neurons with somata residing at different locations in the basal forebrain. We found that the vast majority of GnRH neurons are bipolar in morphology, elaborating a thick (primary) and thinner (secondary) dendrite from opposite poles of the soma. In addition, an axon-like process arising predominantly from a proximal dendrite was observed in a subpopulation of GnRH neurons. Ankyrin G immunohistochemistry revealed the presence of a single action potential initiation zone ∼27 μm in length primarily in the secondary dendrite of GnRH neurons and located 30 to 140 μm distant from the cell soma, depending on the type of process and location of the cell body. In addition to dendrites, the GnRH neurons with cell bodies located close to hypothalamic circumventricular organs often elaborated ankyrin G-positive axon-like structures. Almost all GnRH neurons (>90%) had their action potential initiation site in a process that initially, or ultimately after a hairpin loop, was coursing in the direction of the median eminence. These studies indicate that action potentials are initiated in different dendritic and axonal compartments of the GnRH neuron in a manner that is dependent partly on the neuroanatomical location of the cell body.

Differential organization of gamma-aminobutyric acid type A and glycine receptors in the somatic and dendritic compartments of rat abducens motoneurons

Premotor inhibitory neurons responsible for the decrease in the firing discharge during fast or slow eye movements selectively target the cell bodies and the dendrites of abducens motoneurons. Gamma-aminobutyric acid (GABA) and glycine, the main inhibitory synaptic neurotransmitters in the central nervous system, act via glycine and GABAA receptors, assembled from various types of subunits, which determine the kinetics of the currents mediated. Therefore, our hypothesis was that the expression of the inhibitory receptors on the somatic and the dendritic compartments, involved in different functions, may differ. In this study, we compared the subcellular patterns of expression of the main GABAA receptor subunits (GABAARalpha1, alpha2, alpha3, alpha5), glycine receptors (GlyRalpha1), and gephyrin in the somatic and dendritic compartments of rat abducens motoneurons, using double or triple immunocytochemical experiments with confocal microscopy. Significant differences exist in the patterns of organization and the synaptic expression of the GlyR and GABAAR subunits in the cell bodies and dendrites of abducens motoneurons. In the somata, only the GABAARalpha1 subunit was expressed, whereas both GABAARalpha1 and GABAARalpha3 were present in the dendrites. The GlyRalpha1 to GABAARalpha1 density ratio was reversed in the somatic and dendritic compartments (0.9 vs. 2.3). A quantitative electron microscopy study showed that the modes whereby gephyrin reaches its postsynaptic inhibitory synaptic target differ between the somata and the dendrites. Therefore, our results support the idea that a structure-function adaptation occurs at the single-neuron level.

Axon initial segment ensheathed by extracellular matrix in perineuronal nets

Perineuronal nets of extracellular matrix are associated with distinct types of neurons in the cerebral cortex and many subcortical regions. Large complexes of aggregating proteoglycans form a chemically specified microenvironment around the somata, proximal dendrites and the axon initial segment, including the presynaptic boutons attached to these domains. The subcellular distribution and the temporal course of postnatal formation suggest that perineuronal nets may be involved in the regulation of synaptic plasticity. Here we investigate structural and cytochemical characteristics of the extracellular matrix around axon initial segments virtually devoid of synaptic contacts. Wisteria floribunda agglutinin staining, the immunocytochemical detection of aggrecan and tenascin-R, as well as affinity-labeling of hyaluronan were used to analyze perineuronal nets associated with large motoneurons in the mouse superior colliculus. The molecular composition of perineuronal nets was divergent between neurons but was identical around the different cellular domains of the individual neurons. The axon initial segments largely devoid of synapses were covered by a continuous matrix sheath infiltrating the adjacent neuropil. The periaxonal zone penetrated by matrix components often increased in diameter along the initial segment from the axon hillock toward the myelinated part of the axon. The axonal and somatodendritic domains of perineuronal nets were concomitantly formed during the first three weeks of postnatal development. The common molecular properties and major structural features of subcellular perineuronal net domains were retained in organotypic midbrain slice cultures. The results support the hypothesis that the aggrecan-related extracellular matrix of perineuronal nets provides a continuous micromilieu for different subcellular domains performing integration and generation of the electrical activity of neurons.

GABA and glycine co-release optimizes functional inhibition in rat brainstem motoneurons in vitro

Whole-cell patch clamp recordings of miniature inhibitory postsynaptic currents (mIPSCs) were obtained in identified abducens motoneurons (aMns) from young rats (P5-P13). Three types of mIPSC were distinguished according to their kinetics and their sensitivity to receptor antagonists: faster decaying events mediated by glycine receptors (glyRs), slower decaying events mediated by GABA(A) receptors (GABA(A)Rs), and mIPSCs displaying two components corresponding to GABA and glycine co-release. Dual component events accounted for approximately 30 % of mIPSCs, independently of the rat's age and were also identified during evoked transmitter release. In contrast, the kinetics of glyR- and GABA(A)R-mediated mIPSCs became faster during development. Monosynaptic inhibitory postsynaptic potentials (IPSPs) were able to fully inhibit motoneuron discharge elicited by current pulses. When the GABA(A)R-mediated component or the glyR-mediated component of the IPSP was blocked, the inhibition of motoneuron firing was reduced. The 20-80 % rise time and duration of GABA(A)R-mediated IPSPs were significantly longer than those mediated by glyRs. The time window of inhibition for each component was determined using single postsynaptic action potentials elicited with various delays from the onset of the IPSP. GlyR-mediated IPSPs induced fast transient inhibition whereas GABA(A)R-mediated IPSPs induced slow sustained suppression of firing. Using a modelling approach, we found that the two components summated non-linearly. We conclude that in developing aMns, co-release of GABA and glycine determines the strength and timing of inhibition through non-linear interactions between the two components, thus optimizing inhibition of motoneuron function.

Increased tyrosine nitration of the brain in chronic renal insufficiency: reversal by antioxidant therapy and angiotensin-converting enzyme inhibition

Interaction of reactive oxygen species with nitric oxide promotes nitric oxide inactivation and generation of cytotoxic reactive nitrogen species that attack DNA, lipids, and proteins. Nitration of free tyrosine and tyrosine residues of proteins results in production of nitrotyrosine, which can lead to excitotoxicity and frequently is found in the brain of patients and animals with various degenerative, ischemic, toxic, and other neurologic disorders. According to earlier studies, reactive oxygen species activity is increased and neuronal NO synthase expression in the brain is elevated in animals with chronic renal failure (CRF). It was hypothesized, therefore, that tyrosine nitration must be increased in the uremic brain. This hypothesis was tested, through determination of nitrotyrosine abundance (by Western blot analysis), as well as distribution (by immunohistology), in the cerebrum of rats with CRF 6 wk after 5/6 nephrectomy. The results were compared with those of sham-operated controls and antioxidant (lazaroid)-treated and captopril-treated rats with CRF. Western blot analysis revealed a significant increase in nitrotyrosine abundance in the cerebral cortex of rats with CRF. This was accompanied by an intense nitrotyrosine staining of the neuronal processes, including proximal segments of dendrites, axons, and axon terminals of the cortical neurons. Both antioxidant therapy and captopril administration alleviated oxidative stress (as evidenced by normalization of plasma lipid peroxidation product malondialdehyde) and significantly reduced nitrotyrosine abundance in the cerebral cortex of the treated CRF group. In conclusion, CRF resulted in oxidative stress and increased tyrosine nitration in the cerebral cortex. Antioxidant therapy and angiotensin-converting enzyme inhibition alleviated the CRF-induced oxidative stress and mitigated tyrosine nitration in the rats with CRF.

Convergent properties of vestibular-related brain stem neurons in the gerbil

Three classes of vestibular-related neurons were found in and near the prepositus and medial vestibular nuclei of alert or decerebrate gerbils, those responding to: horizontal translational motion, horizontal head rotation, or both. Their distribution ratios were 1:2:2, respectively. Many cells responsive to translational motion exhibited spatiotemporal characteristics with both response gain and phase varying as a function of the stimulus vector angle. Rotationally sensitive neurons were distributed as Type I, II, or III responses (sensitive to ipsilateral, contralateral, or both directions, respectively) in the ratios of 4:6:1. Four tested factors shaped the response dynamics of the sampled neurons: canal-otolith convergence, oculomotor-related activity, rotational Type (I or II), and the phase of the maximum response. Type I nonconvergent cells displayed increasing gains with increasing rotational stimulus frequency (0.1-2.0 Hz, 60 degrees /s), whereas Type II neurons with convergent inputs had response gains that markedly decreased with increasing translational stimulus frequency (0.25-2.0 Hz, +/-0.1 g). Type I convergent and Type II nonconvergent neurons exhibited essentially flat gains across the stimulus frequency range. Oculomotor-related activity was noted in 30% of the cells across all functional types, appearing as burst/pause discharge patterns related to the fast phase of nystagmus during head rotation. Oculomotor-related activity was correlated with enhanced dynamic range compared with the same category that had no oculomotor-related response. Finally, responses that were in-phase with head velocity during rotation exhibited greater gains with stimulus frequency increments than neurons with out-of-phase responses. In contrast, for translational motion, neurons out of phase with head acceleration exhibited low-pass characteristics, whereas in-phase neurons did not. Data from decerebrate preparations revealed that although similar response types could be detected, the sampled cells generally had lower background discharge rates, on average one-third lower response gains, and convergent properties that differed from those found in the alert animals. On the basis of the dynamic response of identified cell types, we propose a pair of models in which inhibitory input from vestibular-related neurons converges on oculomotor neurons with excitatory inputs from the vestibular nuclei. Simple signal convergence and combinations of different types of vestibular labyrinth information can enrich the dynamic characteristics of the rotational and translational vestibuloocular responses.