Maturation of NADPH-d activity in the rat's barrel-field cortex and its relationship to cytochrome oxidase activity

Nitric oxide controls excitatory/inhibitory balance in the hypoglossal nucleus during early postnatal development

Synaptic remodeling during early postnatal development lies behind neuronal networks refinement and nervous system maturation. In particular, the respiratory system is immature at birth and is subjected to significant postnatal development. In this context, the excitatory/inhibitory balance dramatically changes in the respiratory-related hypoglossal nucleus (HN) during the 3 perinatal weeks. Since, development abnormalities of hypoglossal motor neurons (HMNs) are associated with sudden infant death syndrome and obstructive sleep apnea, deciphering molecular partners behind synaptic remodeling in the HN is of basic and clinical relevance. Interestingly, a transient expression of the neuronal isoform of nitric oxide (NO) synthase (NOS) occurs in HMNs at neonatal stage that disappears before postnatal day 21 (P21). NO, in turn, is a determining factor for synaptic refinement in several physiopathological conditions. Here, intracerebroventricular chronic administration (P7–P21) of the broad spectrum NOS inhibitor l-NAME (N(ω)-nitro-l-arginine methyl ester) differentially affected excitatory and inhibitory rearrangement during this neonatal interval in the rat. Whilst l-NAME led to a reduction in the number of excitatory structures, inhibitory synaptic puncta were increased at P21 in comparison to administration of the inactive stereoisomer d-NAME. Finally, l-NAME decreased levels of the phosphorylated form of myosin light chain in the nucleus, which is known to regulate the actomyosin contraction apparatus. These outcomes indicate that physiologically synthesized NO modulates excitatory/inhibitory balance during early postnatal development by acting as an anti-synaptotrophic and/or synaptotoxic factor for inhibitory synapses, and as a synaptotrophin for excitatory ones. The mechanism of action could rely on the modulation of the actomyosin contraction apparatus.

Neuronal nitric oxide synthase expressing neurons: a journey from birth to neuronal circuits

Nitric oxide (NO) is an important signaling molecule crucial for many physiological processes such as synaptic plasticity, vasomotricity, and inflammation. Neuronal nitric oxide synthase (nNOS) is the enzyme responsible for the synthesis of NO by neurons. In the juvenile and mature hippocampus and neocortex nNOS is primarily expressed by subpopulations of GABAergic interneurons. Over the past two decades, many advances have been achieved in the characterization of neocortical and hippocampal nNOS expressing neurons. In this review, we summarize past and present studies that have characterized the electrophysiological, morphological, molecular, and synaptic properties of these neurons. We also discuss recent studies that have shed light on the developmental origins and specification of GABAergic neurons with specific attention to neocortical and hippocampal nNOS expressing GABAergic neurons. Finally, we summarize the roles of NO and nNOS-expressing inhibitory neurons.

The emergence of somatotopic maps of the body in S1 in rats: the correspondence between functional and anatomical organization

Most of what we know about cortical map development and plasticity comes from studies in mice and rats, and for the somatosensory cortex, almost exclusively from the whisker-dominated posteromedial barrel fields. Whiskers are the main effector organs of mice and rats, and their representation in cortex and subcortical pathways is a highly derived feature of murine rodents. This specialized anatomical organization may therefore not be representative of somatosensory cortex in general, especially for species that utilize other body parts as their main effector organs, like the hands of primates. For these reasons, we examined the emergence of whole body maps in developing rats using electrophysiological recording techniques. In P5, P10, P15, P20 and adult rats, multiple recordings were made in the medial portion of S1 in each animal. Subsequently, these functional maps were related to anatomical parcellations of S1 based on a variety of histological stains. We found that at early postnatal ages (P5) medial S1 was composed almost exclusively of the representation of the vibrissae. At P10, other body part representations including the hindlimb and forelimb were present, although these were not topographically organized. By P15, a clear topographic organization began to emerge coincident with a reduction in receptive field size. By P20, body maps were adult-like. This study is the first to describe how topography of the body develops in S1 in any mammal. It indicates that anatomical parcellations and functional maps are initially incongruent but become tightly coupled by P15. Finally, because anatomical and functional specificity of developing barrel cortex appears much earlier in postnatal life than the rest of the body, the entire primary somatosensory cortex should be considered when studying general topographic map formation in development.

Deletion of the citron kinase gene selectively affects the number and distribution of interneurons in barrelfield cortex

Citron kinase (CIT-K), a ser/thr kinase, is required during neurogenesis for cytokinesis of neuronal precursors. Deletion of the cit-k gene in mice (cit-k(-/-) mice) leads to a severe malformative central nervous system syndrome characterized by microencephaly, ataxia, and epileptic seizures; affected mice die by the third week of postnatal life. We have used NADPH-diaphorase histochemistry, immunostaining for calbindin, calretinin, parvalbumin, and glutamic acid decarboxylase 67 (GAD67), and histological staining to undertake qualitative and quantitative analyses of the morphology and distribution of interneurons in the barrelfield cortex of cit-k(-/-) mice. By postnatal day 13, lack of CIT-K results in profoundly altered cortical cell morphology: the infragranular layers are populated by large, binucleate interneurons bearing many more dendrites than in control mice, an anatomical profile that has also been reported for the cortex of humans with cortical dysplasias and epilepsy. Tessellation analyses reveal that a clustered distribution of interneurons is maintained in cit-k(-/-) mice, but that their nearest neighbor distance is significantly increased, and thus their density is reduced; the overall number of interneurons is more dramatically decreased in the absence of CIT-K than would be predicted on the basis of the reduced brain size of affected mice. This reduction of inhibitory gamma-aminobutyric acid (GABA)ergic neurons likely underlies the occurrence of epileptic seizures in the cit-k(-/-) mice. Furthermore, the altered distribution of NADPH-diaphorase-positive interneurons could be responsible for an impaired coupling of cortical activity to blood flow, also affecting cortical growth and functioning.

Subbarrel patterns of thalamocortical innervation in rat somatosensory cortical barrels: Organization and postnatal development

Barrel hollows in the posteromedial barrel subfield of adult rat somatosensory cortex typically encompass two or three metabolically and structurally distinct regions, termed subbarrels. We used immunohistochemical staining for vesicular glutamate transporter 2 and the neuronal serotonin transporter, in conjunction with cytochrome oxidase (CO) histochemistry, to investigate the distribution of thalamocortical (TC) axon terminals in relation to subbarrel domains. We found, first, that CO-dark subbarrels are more intensely immunoreactive for thalamocortical terminals than the CO-light clefts that separate them. Second, during the first postnatal week, immunoreactivity for markers of TC terminals is relatively homogeneous throughout the barrel hollow; subbarrel patterns of distribution only become recognizable between P-8 and P-10. These observations extend previous findings that subbarrels denote barrel regions enriched in synaptic contacts. The data also indicate that allocation of TC terminals into subbarrel domains does not occur immediately upon thalamic axon ingrowth. Instead, refinement of TC arbors into subbarrels is a gradual process, the outcome of which is not manifest until the second week of postnatal life.

Subbarrel domains in rat somatosensory (S1) cortex

We used cytochrome oxidase (CO) histochemistry in conjunction with other histological methods to investigate the histochemoarchitecture of barrel hollows in rat somatosensory cortex. We found that individual large barrels in the posteromedial barrel subfield encompass two or three discrete subbarrel domains. Detailed analysis revealed, further, that subbarrel domains are relatively consistent in size, each having average dimensions that approximate those of large barrels in mouse S1. Unexpectedly, subbarrel domains are organized into a few distinct, repeated patterns. The small barrels in rat anterolateral barrel subfield and all barrel hollows in mouse S1 appear to consist of single CO domains. Subbarrel domains revealed here by CO are columnar entities that correspond with cyto- and myeloarchitectonic inhomogeneities within the barrels and are enriched in thalamocortical axon terminals. The present findings together with existing data indicate that barrels in rat posteromedial barrel subfield are structurally and functionally heterogeneous.

Differential distribution of NADPH-diaphorase histochemistry in human cerebral cortex

Beta-nicotinamidedinucleotide phosphate diaphorase (NADPH-d) colocalizes with NOS in the central nervous system. Two types of NADPH-d-positive neurons are present in the primate cerebral cortex: type 1, intensely and Golgi-like labeled neurons, a subset of GABAergic interneurons; type 2, lightly labeled neurons (divided into two subclasses, a first one having a lightly stained cell body bearing only one short process, and a second one showing intense NADPH-d staining with short processes extending radially). We have analyzed the distribution of NADPH-d activity in human frontal, temporal, and occipital cortical areas, finding remarkable laminar and interareal differences in cell size and distribution of the different cell types. There was a clear bias for type 1 neurons in infragranular layers in all areas considered; both in supra- and infragranular layers, their density was highest in frontal, and lowest in temporal cortex. The density of type 2 neurons was lower supragranularly in temporal cortex and infragranularly in occipital cortex. The overall density of type 2 cells was remarkably higher in occipital cortex than in the temporal and frontal ones. Type 1 neurons were significantly larger than type 2, and were smaller in the supragranular than in the infragranular subzone in occipital and temporal cortex. Type 1 cells were significantly larger in frontal cortex than in occipital and temporal cortex, and type 2 cells were significantly smaller in occipital than in temporal and frontal cortex. These area-related differences might reflect differences between heterotypic and homotypic cortex in the regulation of cortical blood flow.

Quantitative spatial analysis of the distribution of NADPH-diaphorase-positive neurons in the developing and mature rat retina

NADPH-diaphorase (NADPH-d) histochemistry labels a subpopulation of nitric oxide-synthesizing amacrine cells in the inner nuclear layer of the rat retina. We have studied their morphology and distribution in postnatal and adult rats in whole-mounted retinae. NAPDH-d-positive neurons are detected as early as postnatal day (P)5, especially in the peripheral retina; intense labeling of somata and long lengths of dendrites is obtained between P10 and P18, after which only the somata exhibit NADPH-d activity. The density and number of these cells increase progressively from P7 to P14, with a significantly higher density in the central retina as compared to the periphery. The sociology of these cells was analyzed quantitatively studying the Voronoi domains: a polygon area can be drawn that delineates the territory of the map that is closer to the cell than to any other cell of the map. In addition, we calculated the conformity ratio of Cook, i.e., the mean nearest neighbor distance/standard deviation of all the nearest neighbor distances, in order to reveal whether or not these cells are regularly distributed through the retina. We find that the distribution of the NADPH-d-positive cells tends to be regular throughout the retina: the local coefficient of variation (obtained by comparing the size of each Voronoi polygon area to those of its neighbors) tends to regularity at P14 and remains unaltered through maturity. Therefore, as other cell types, NADPH-d-positive amacrine cells are almost regularly distributed from the time of eye opening and nitric oxide may play a role in the development of retinal circuitry and in the regulation of retinal blood flow.

Distribution of neuronal nitric oxide synthase-immunoreactive neurons in the cerebral cortex and hippocampus during postnatal development

Although many reports have argued a role for nitric oxide (NO) during postnatal development, there has been no combined demonstration in the cerebral cortex and hippocampus. We have investigated the distribution and morphology of neurons and fibers expressing neuronal NO synthase (nNOS) in the cerebral cortex and hippocampal formation of rats during the postnatal development, and correlated these findings with developmental events taking place in these regions. In the cerebral cortex, the nNOS-immunoreactive cells could be divided into two classes : heavily stained neurons and lightly stained neurons. For the lightly stained nNOS-positive neurons, only the cell bodies were observed, whereas for the heavily stained neurons, the cell bodies and their dendrites were visible. During the postnatal days, heavily stained neurons reached their typical morphology in the second week and appeared in all layers except for layer I. In the hippocampus, there was a transient expression of nNOS in the pyramidal cell layer at P3-P7, and this expression disappeared during following days. The adult pattern of staining developed gradually during the postnatal period. This study suggested that these alterations might reflect a region-specific role of NO and a potential developmental role in the postnatal cerebral cortex and hippocampus.

Cerebral expression of the α2-subunit of soluble guanylyl cyclase is linked to cerebral maturation and sensory pathway refinement during postnatal development

Soluble guanylyl cylase (sGC) has been identified for being a receptor for the gaseous transmitters nitric oxide and carbon monoxide. Currently four subunits alpha1, alpha2, beta1, and beta2 have been characterized. Heterodimers of alpha and beta-subunits as well as homodimers of the beta2-subunit are known to constitute functional sGC which use GTP to form cGMP a potent signal molecule in a multitude of second messenger cascades. Since NO-cGMP signaling plays a pivotal role in neuronal development we analyzed the maturational expression pattern of the newly characterized alpha2-subunit of sGC within the brain of Wistar rats by means of RNase protection assay and immunohistochemistry. alpha2-subunit mRNA as well as immunoreactive alpha2-protein increased during postnatal cerebral development. Topographical analysis revealed a selective high expression of the alpha2-subunit in the choroid plexus and within developing sensory systems involving the olfactory and somatosensory system of the forebrain as well as parts of the auditory and visual system within the hindbrain. In cultured cortical neurons the alpha2-subunit was localized to the cell membrane, especially along neuronal processes. During the first 11 days of postnatal development several cerebral regions showed a distinct expression of the alpha2-subunit which was not paralleled by the alpha1/beta1-subunits especially within the developing thalamo-cortical circuitries of the somatosensory system. However, at later developmental stages all three subunits became more homogenously distributed among most cerebral regions, indicating that functional alpha1/beta1 and alpha2/beta1 heterodimers of sGC could be formed. Our findings indicate that the alpha2-subunit is an essential developmentally regulated constituent of cerebral sensory systems during maturation. In addition the alpha2-subunit may serve other functions than forming a functional heterodimer of sGC during the early phases of sensory pathway refinement.

A morphometric study of the progressive changes on NADPH diaphorase activity in the developing rat's barrel field

The distribution of NADPH diaphorase (NADPH-d)/nitric oxide synthase (NOS) neurons was evaluated during the postnatal development of the primary somatosensory cortex (SI) of the rat. Both cell counts and area measurements of barrel fields were carried out throughout cortical maturation. In addition, NADPH-d and cytochrome oxidase (CO) activities were also compared in both coronal and tangential sections of rat SI between postnatal days (P) 10 and 90. Throughout this period, the neuropil distributions of both enzymes presented a remarkable similarity and have not changed noticeably. Their distribution pattern show the PMBSF as a two-compartmented structure, displaying a highly reactive region (barrel hollows) flanked by less reactive regions (barrel septa). The number of NADPH-d neurons increased significantly in the barrel fields between P10 and P23, with peak at P23. The dendritic arborization of NADPH-d neurons became more elaborated during barrel development. In all ages evaluated, the number of NADPH-d cells was always higher in septa than in the barrel hollows. Both high neuropil reactivity and differential distribution of NADPH-d neurons during SI development suggest a role for nitric oxide throughout barrel field maturation.

Differential expression of nitric oxide in serotonergic projection neurons: neurochemical identification of dorsal raphe inputs to rodent trigeminal somatosensory targets

The dorsal raphe (DR) is invested with nitric oxide synthase (NOS)-expressing profiles. To characterize the connections of NO-containing cells and further assess neurochemical relationships maintained by DR, the transmitter identity of the raphe projection to the trigeminal somatosensory system was examined. Rats were injected with retrograde tracer into vibrissae-related target areas or with anterograde tracer into DR. NADPH-diaphorase (NADPHd) histochemistry or NOS-immunostaining was combined with serotonin (5HT)- or serotonin transporter (SERT)-immunolabeling to examine: 1) the presence of NO in 5HT-containing axons from DR; 2) the distribution of NO-containing fibers with respect to other nitrergic profiles in the somatosensory system; and 3) the propensity for individual projection neurons in specific subdivisions of DR to colocalize 5HT and NO. Results confirm that "barrel-like" patches can be identified in several adult trigeminal relay nuclei by NADPHd histochemistry and demonstrate that fibers from DR contain 5HT and NO. Observations include a high percentage of cortical midline projection neurons which contained NADPHd (70-80%) and coexpressed 5HT. In contrast, approximately 40% of retrogradely labeled DR-thalamus cells in the lateral wing demonstrated NADPHd or 5HT expression, but not both in the same neuron. Colocalization of NADPHd and 5HT within individual DR projection neurons indicates that: i) DR is a source of nitrergic input to trigeminal structures, and ii) NO and 5HT may be simultaneously released to influence information-processing within somatosensory targets. Disparities in NADPHd expression between retrogradely labeled DR neuronal subpopulations further suggest functional differences in the impact of NO on cortical and subcortical targets.

The chemo- and somatotopic architecture of the Galago cuneate and gracile nuclei

The pattern of peripheral nerve inputs into the dorsal column nuclei, cuneate and gracile, was investigated in the prosimian Galago garnetti. The major findings were, that there is a greater segregation of the inputs from the fingers/hand within the cuneate compared with input form the toes/foot within the gracile. In both nuclei, cell clusters can be identified as cytochrome oxidase dense blotches, reactive also for the activity-dependent enzyme nitric oxide synthase. In the cuneate, cell clusters were apparent as six main cytochrome oxidase/nitric oxide synthase-reactive ovals arranged in a medial to lateral sequence. In contrast in the gracile, a higher degree of parcellation was noted and several cytochrome oxidase/nitric oxide synthase blotches were distributed along the rostrocaudal axis of the nucleus. This different architecture parallels differences in the organization of the inputs from the hand and from the foot. In the cuneate, cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals from the glabrous and hairy skin of digits d1 to d5 segregated in each of the five most lateral cytochrome oxidase/nitric oxide synthase blotches. Afferents from the thenar, palmar pads and hypothenar overlapped with those from digit 1, digit 2 to digit 4 and digit 5, respectively. Inputs from wrist arm and shoulder were segregated in the most medial blotch. In the gracile, multiple foci of cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals were observed upon injections of single sites in the toes or plantar pads. Although in multiple foci, inputs from different toes segregated from one another as well. Terminals from the plantar pads appeared to converge on the same cytochrome oxidase/nitric oxide synthase blotches targeted by inputs from the toes. In both the cuneate and the gracile, cytochrome oxidase/nitric oxide synthase blotches also presented intense immunoreactivity for GABA, calbindin, parvalbumin, and brain derived neurotrophic factor. Finally, in the cuneate the cell cluster region presented similarities in prosimian galagos and four species of New World monkeys, whereas it appeared more differentiated and complex in the Old Word macaque monkeys. In conclusion, the different pattern of segregation of the inputs from the hand and from the foot can be related to the different metabolic organization of the cuneate and of the gracile, respectively.

NOS inhibition during postnatal development leads to increased ipsilateral retinocollicular and retinogeniculate projections in rats

Synthesis of nitric oxide (NO) occurs downstream from activation of N-methyl-D-aspartate (NMDA) receptors; NO reportedly acts as a retrograde messenger, influencing the refinement and stabilization of coactive afferent terminals. Cells and neuropil in the rat superior colliculus (SC) and lateral geniculate body (LGB) show intense, developmentally regulated activity for NO synthase (NOS). To study the role of NO in the development of retinogeniculate and retinotectal axon arbors, we examined primary visual projections of rats that had received intraperitoneal injections of Nomega-nitro-L-arginine (L-NoArg, an NOS inhibitor) on postnatal day 0, and daily thereafter for 4-6 weeks. Treated rats showed significant alterations in ipsilateral retinotectal projections, in the mediolateral and anteroposterior axes; there was an increase in the density of fibres entering the SC, in branch length, and in the numbers of boutons on retinotectal arbors in the treated group. Ipsilaterally projecting retinal axons also showed an increase in density and distribution in the dorsal nucleus of the LGB. If animals were allowed to survive for several months after stopping treatment, similar changes were also noted, but these were much less striking. Our results support the hypothesis that, in the mammalian visual system, NO released from target neurons in the SC and LGB serves as a retrograde signal which feeds back on retinal afferents, influencing their growth. The effects of NOS inhibition are partially reversed after treatment is stopped, indicating that lack of NO synthesis delays the maturation of retinofugal connections, and also that NO plays a constitutive role in their development.