Immunohistochemical localization of nitric oxide synthase and soluble guanylyl cyclase in the ventral cochlear nucleus of the rat

Induction of Activity-Dependent Plasticity at Auditory Nerve Synapses

Exposure to nontraumatic noise in vivo drives long-lasting changes in auditory nerve synapses, which may influence hearing, but the induction mechanisms are not known. We mimicked activity in acute slices of the cochlear nucleus from mice of both sexes by treating them with high potassium, after which voltage-clamp recordings from bushy cells indicated that auditory nerve synapses had reduced EPSC amplitude, quantal size, and vesicle release probability (P r). The effects of high potassium were prevented by blockers of nitric oxide (NO) synthase and protein kinase A. Treatment with the NO donor, PAPA-NONOate, also decreased P r, suggesting NO plays a central role in inducing synaptic changes. To identify the source of NO, we activated auditory nerve fibers specifically using optogenetics. Strobing for 2 h led to decreased EPSC amplitude and P r, which was prevented by antagonists against ionotropic glutamate receptors and NO synthase. This suggests that the activation of AMPA and NMDA receptors in postsynaptic targets of auditory nerve fibers drives release of NO, which acts retrogradely to cause long-term changes in synaptic function in auditory nerve synapses. This may provide insight into preventing or treating disorders caused by noise exposure.SIGNIFICANCE STATEMENT Auditory nerve fibers undergo long-lasting changes in synaptic properties in response to noise exposure in vivo, which may contribute to changes in hearing. Here, we investigated the cellular mechanisms underlying induction of synaptic changes using high potassium and optogenetic stimulation in vitro and identified important signaling pathways using pharmacology. Our results suggest that auditory nerve activity drives postsynaptic depolarization through AMPA and NMDA receptors, leading to the release of nitric oxide, which acts retrogradely to regulate presynaptic neurotransmitter release. These experiments revealed that auditory nerve synapses are unexpectedly sensitive to activity and can show dramatic, long-lasting changes in a few hours that could affect hearing.

Wnt Signaling Through Nitric Oxide Synthase Promotes the Formation of Multi-Innervated Spines

Structural plasticity of synapses correlates with changes in synaptic strength. Dynamic modifications in dendritic spine number and size are crucial for long-term potentiation (LTP), the cellular correlate of learning and memory. Recent studies have suggested the generation of multi-innervated spines (MIS), in the form of several excitatory presynaptic inputs onto one spine, are crucial for hippocampal memory storage. However, little is known about the molecular mechanisms underlying MIS formation and their contribution to LTP. Using 3D enhanced resolution confocal images, we examined the contribution of Wnt synaptic modulators in MIS formation in the context of LTP. We show that blockage of endogenous Wnts with specific Wnt antagonists supresses the formation of MIS upon chemical LTP induction in cultured hippocampal neurons. Gain- and loss-of-function studies demonstrate that Wnt7a signaling promotes MIS formation through the postsynaptic Wnt scaffold protein Disheveled 1 (Dvl1) by stimulating neuronal nitric oxide (NO) synthase (nNOS). Subsequently, NO activates soluble guanylyl cyclase (sGC) to increase MIS formation. Consistently, we observed an enhanced frequency and amplitude of excitatory postsynaptic currents. Collectively, our findings identify a unique role for Wnt secreted proteins through nNOS/NO/sGC signaling to modulate MIS formation during LTP.

Nitric oxide increases gain in the ventral cochlear nucleus of guinea pigs with tinnitus

Previous work has led to the hypothesis that, during the production of noise-induced tinnitus, higher levels of nitric oxide (NO), in the ventral cochlear nucleus (VCN), increase the gain applied to a reduced input from the cochlea. To test this hypothesis, we noise-exposed 26 guinea pigs, identified evidence of tinnitus in 12 of them and then compared the effects of an iontophoretically applied NO donor or production inhibitor on VCN single unit activity. We confirmed that the mean driven firing rate for the tinnitus and control groups was the same while it had fallen in the non-tinnitus group. By contrast, the mean spontaneous rate had increased for the tinnitus group relative to the control group, while it remained the same for the non-tinnitus group. A greater proportion of units responded to exogenously applied NO in the tinnitus (56%) and non-tinnitus groups (71%) than a control population (24%). In the tinnitus group, endogenous NO facilitated the driven firing rate in 37% (7/19) of neurons and appeared to bring the mean driven rate back up to control levels by a mechanism involving N-methyl-D-aspartic acid (NMDA) receptors. By contrast, in the non-tinnitus group, endogenous NO only facilitated the driven firing rate in 5% (1/22) of neurons and there was no facilitation of driven rate in the control group. The effects of endogenous NO on spontaneous activity were unclear. These results suggest that NO is involved in increasing the gain applied to driven activity, but other factors are also involved in the increase in spontaneous activity.

Nitric oxide regulates the firing rate of neuronal subtypes in the guinea pig ventral cochlear nucleus

The gaseous free radical, nitric oxide (NO) acts as a ubiquitous neuromodulator, contributing to synaptic plasticity in a complex way that can involve either long term potentiation or depression. It is produced by neuronal nitric oxide synthase (nNOS) which is presynaptically expressed and also located postsynaptically in the membrane and cytoplasm of a subpopulation of each major neuronal type in the ventral cochlear nucleus (VCN). We have used iontophoresis in vivo to study the effect of the NOS inhibitor L-NAME (L-NG-Nitroarginine methyl ester) and the NO donors SIN-1 (3-Morpholinosydnonimine hydrochloride) and SNOG (S-Nitrosoglutathione) on VCN units under urethane anaesthesia. Collectively, both donors produced increases and decreases in driven and spontaneous firing rates of some neurones. Inhibition of endogenous NO production with L-NAME evoked a consistent increase in driven firing rates in 18% of units without much effect on spontaneous rate. This reduction of gain produced by endogenous NO was mirrored when studying the effect of L-NAME on NMDA(N-Methyl-D-aspartic acid)-evoked excitation, with 30% of units showing enhanced NMDA-evoked excitation during L-NAME application (reduced NO levels). Approximately 25% of neurones contain nNOS and the NO produced can modulate the firing rate of the main principal cells: medium stellates (choppers), large stellates (onset responses) and bushy cells (primary-like responses). The main endogenous role of NO seems to be to partly suppress driven firing rates associated with NMDA channel activity but there is scope for it to increase neural gain if there were a pathological increase in its production following hearing loss.

Nitric Oxide-Mediated Plasticity of Interconnections Between T-Stellate cells of the Ventral Cochlear Nucleus Generate Positive Feedback and Constitute a Central Gain Control in the Auditory System

T-stellate cells in the ventral cochlear nucleus (VCN) form an ascending pathway that conveys spectral information from the cochlea to brainstem nuclei, the inferior colliculi, and the thalamus. The tonotopic array of T-stellate cells enhances the encoding of spectral peaks relative to their auditory nerve fiber inputs. The alignment of local collaterals and T-stellate cell dendrites within the isofrequency lamina suggests that the cells make connections within the isofrequency lamina in which they reside. Recordings from pairs of T-stellate cells in mice of both sexes revealed that firing in the presynaptic cell evoked responses in the postsynaptic cell when presynaptic firing was paired with depolarization of the postsynaptic cell. After such experimental coactivation, presynaptic firing evoked EPSCs of uniform amplitude whose frequency depended on the duration of depolarization and diminished over minutes. Nitric oxide (NO) donors evoked EPSCs in T-stellate cells but not in the other types of principal cells. Blockers of neuronal nitric oxide synthase (nNOS) and of NMDA receptors blocked potentiation, indicating that NO mediates potentiation. nNOS and its receptor, guanylate cyclase (NO-GC), are expressed in somata of T-stellate cells. Excitatory interconnections were bidirectional and polysynaptic, indicating that T-stellate cells connect in networks. Positive feedback provided by temporarily potentiated interconnections between T-stellate cells could enhance the gain of auditory nerve excitation in proportion to the excitation, generating a form of short-term central gain control that could account for the ability of T-stellate cells to enhance the encoding of spectral peaks.SIGNIFICANCE STATEMENT T-stellate cells are interconnected through synapses that have a previously undescribed form of temporary, nitric oxide-mediated plasticity. Coactivation of neighboring cells enhances the activation of an excitatory network that feeds back on itself by enhancing the probability of EPSCs. Although there remain gaps in our understanding of how the interconnections revealed in slices contribute to hearing, our findings have interesting implications. Positive feedback through a network of interconnections could account for how T-stellate cells are able to encode spectral peaks over a wider range of intensities than many of their auditory nerve inputs (Blackburn and Sachs, 1990; May et al., 1998). The magnitude of the gain may itself be plastic because neuronal nitric oxide synthase increases when animals have tinnitus (Coomber et al., 2015).

Modulating central gain in tinnitus: changes in nitric oxide synthase in the ventral cochlear nucleus

A significant challenge in tinnitus research lies in explaining how acoustic insult leads to tinnitus in some individuals, but not others. One possibility is genetic variability in the expression and function of neuromodulators – components of neural signaling that alter the balance of excitation and inhibition in neural circuits. An example is nitric oxide (NO) – a free radical and potent neuromodulator in the mammalian brain – that regulates plasticity via both pre-synaptic and postsynaptic mechanisms. Changes in NO have previously been implicated in tinnitus generation, specifically in the ventral cochlear nucleus (VCN). Here, we examined nitric oxide synthase (NOS) – the enzyme responsible for NO production – in the guinea pig VCN following acoustic trauma. NOS was present in most cell types – including spherical and globular bushy cells, small, medium, and large multipolar cells, and octopus cells – spanning the entire extent of the VCN. The staining pattern was symmetrical in control animals. Unilateral acoustic over-exposure (AOE) resulted in marked asymmetries between ipsilateral and contralateral sides of the VCN in terms of the distribution of NOS across the cochlear nuclei in animals with behavioral evidence of tinnitus: fewer NOS-positive cells and a reduced level of NOS staining was present across the whole extent of the contralateral VCN, relative to the ipsilateral VCN. The asymmetric pattern of NOS-containing cells was observed as early as 1 day after AOE and was also present in some animals at 3, 7, and 21 days after AOE. However, it was not until 8 weeks after AOE, when tinnitus had developed, that asymmetries were significant overall, compared with control animals. Asymmetrical NOS expression was not correlated with shifts in the threshold hearing levels. Variability in NOS expression between animals may represent one underlying difference that can be linked to whether or not tinnitus develops after noise exposure.

Neural changes accompanying tinnitus following unilateral acoustic trauma in the guinea pig

Animal models of tinnitus allow us to study the relationship between changes in neural activity and the tinnitus percept. Here, guinea pigs were subjected to unilateral noise trauma and tested behaviourally for tinnitus 8 weeks later. By comparing animals with tinnitus with those without, all of which were noise-exposed, we were able to identify changes unique to the tinnitus group. Three physiological markers known to change following noise exposure were examined: spontaneous firing rates (SFRs) and burst firing in the inferior colliculus (IC), evoked auditory brainstem responses (ABRs), and the number of neurons in the cochlear nucleus containing nitric oxide synthase (NOS). We obtained behavioural evidence of tinnitus in 12 of 16 (75%) animals. Both SFRs and incidences of burst firing were elevated in the IC of all noise-exposed animals, but there were no differences between tinnitus and no-tinnitus animals. There were significant decreases in ipsilateral ABR latencies in tinnitus animals, contrary to what might be expected with a small hearing loss. Furthermore, there was an ipsilateral-contralateral asymmetry in NOS staining in the ventral cochlear nucleus (VCN) that was only apparent in tinnitus animals. Tinnitus animals had a significantly greater number of NOS-containing neurons on the noise-exposed side, whereas no-tinnitus animals did not. These data suggest that measuring NOS in the VCN and recording ABRs supplement behavioural methods for confirming tinnitus in animals, and that nitric oxide is involved in plastic neural changes associated with tinnitus.

Characterization of NO-sensitive guanylyl cyclase: expression in an identified interneuron involved in NO-cGMP-dependent memory formation

In a number of neuronal models of learning signalling by endogenous nitric oxide (NO), produced by the enzyme NO synthase (NOS), is essential for the formation of long-term memory (LTM). For example, in the molluscan model system Lymnaea, NO is required for LTM formation in the first few hours after one-trial reward conditioning. Furthermore, conditioning leads to transient up-regulation of the NOS gene in identified modulatory neurons, the cerebral giant cells (CGCs), which are known to be involved in LTM formation. In Lymnaea nothing is known however about the structure and localization of the major receptor for NO, the soluble guanylyl cyclase (sGC). Here we report on the cloning and characterization of both alpha and beta subunits of NO-sensitive sGC and show that they are coexpressed in the CGCs. Furthermore, our electrophysiological experiments on isolated CGCs show that these neurons respond to NO by generating a prolonged depolarization of the membrane potential. Moreover, we demonstrate that this depolarization is blocked by ODQ, supporting our hypothesis that it is mediated by sGC.

Distribution of NADPH-diaphorase and nitric oxide synthase reactivity in the central nervous system of the goldfish (Carassius auratus)

The nitrergic system has been inferred from cells positive to nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry and/or to the neuronal isoform of nitric oxide synthase (nNOS) immunohistochemistry in different species of vertebrates. The aim of the present work was to systematically study the distribution of cell producing nitric oxide in the goldfish (Carassius auratus) brain. To reach this goal, we firstly studied co-localization for NADPHd and nNOS techniques and demonstrated an extensive double labeling. Then, we studied the distribution through the brain by the two separate methods and found labeled cells widely distributed in brain and spinal cord. In the telencephalon, such cells were in both dorsal and ventral areas. In the diencephalon, the cells were found in some nuclei of the preoptic area and hypothalamus, habenula, pretectum, and dorsal and ventral thalamic regions. In the midbrain, cells were observed in the optic tectum, torus longitudinalis, and tegmental nuclei. In the rhombencephalon, cells were found in the cerebellum, the reticular formation, the locus coeruleus, the raphe nuclei, and the nuclei of the cranial nerves. Labeled cells were also observed in the gray area of the spinal cord. Cognizing that a direct comparison of the present results with those reported in other vertebrates is not clear-cut because of homologies; we conclude that the nitrergic system is roughly similar from fish to mammals.

Neuronal nitric oxide synthase expression in the cochlear nucleus in a salicylate model of tinnitus

Although a number of studies suggest that the development of tinnitus is associated with hyperactive neuronal discharges in the brainstem cochlear nucleus (CN), there is relatively little evidence to indicate the neurochemical basis of this phenomenon. While some studies suggest that it may be partly due to a decrease in GABAergic inhibition, it is also possible that increased excitability is a contributing factor. In the current study, we investigated whether the salicylate animal model of tinnitus is associated with changes in the number of CN neurons expressing neuronal nitric oxide synthase (nNOS), one of the NOS isoforms that results in the production of the neurotransmitter, nitric oxide. We used a behavioral conditioning paradigm to confirm that animals receiving salicylate injections experienced tinnitus, and used immunohistochemistry with stereology to quantify the number of nNOS-expressing neurons in the dorsal and ventral CN (DCN and VCN, respectively) in salicylate- and vehicle control-treated animals. We also employed Western blotting to quantify the amount of nNOS protein expression in the total CN (i.e., the DCN and VCN together). We found a significant increase (of approximately 70%) in the number of nNOS-expressing principal neurons in the VCN of salicylate-treated animals compared to controls, with no significant differences in the DCN; nor did we find any significant difference in the overall level of nNOS protein in the total CN using Western blotting. These results suggest that changes in the number of neurons in the VCN expressing nNOS may be implicated in the mechanisms of tinnitus.

Wisteria floribunda lectin is associated with specific cell types in the ventral cochlear nucleus of the gerbil, Meriones unguiculatus

The cochlear nucleus is made up of a number of diverse cell types with different anatomical and physiological properties. A plant lectin, Wisteria floribunda agglutinin, that recognizes specific carbohydrate residues in the extracellular matrix binds to some cell types in the ventral cochlear nucleus but not to cells in the dorsal cochlear nucleus. In the ventral cochlear nucleus, the most intensely labeled cells are octopus cells, a subset of multipolar cells and cochlear root neurons. The multipolar cells that are labeled may correspond to the population that projects to the inferior colliculus.

Relaxing effects induced by the soluble guanylyl cyclase stimulator BAY 41-2272 in human and rabbit corpus cavernosum

5-Cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-pyrimidin-4-ylamine (BAY 41-2272) is a potent soluble guanylyl cyclase stimulator in a nitric oxide (NO)-independent manner. The relaxant effect of BAY 41-2272 was investigated in rabbit and human corpus cavernosum in vitro. BAY 41-2272 (0.01-10 microM) relaxed both rabbit (pEC(50)=6.82+/-0.06) and human (pEC(50)=6.12+/-0.10) precontracted cavernosal strips. The guanylyl cyclase inhibitor (ODQ, 10 microM) caused significant rightward shifts in the concentration-response curves for BAY 41-2272 in rabbit (4.7-fold) and human (2.3-fold) tissues. The NO synthesis inhibitor (N-nitro-L-arginine methyl ester (L-NAME), 100 microM) also produced similar rightward shifts, revealing that BAY 41-2272 acts synergistically with endogenous NO to elicit its relaxant effect. The results also indicate that ODQ is selective for the NO-stimulated enzyme, since relaxations evoked by BAY 41-2272 were only partly attenuated by ODQ. The present study shows that both BAY 41-2272 and sildenafil evoke relaxations independent of inhibition of haem in soluble guanylate cyclase. Moreover, there is no synergistic effect of the two compounds in corpus cavernosum.

Localization of mRNAs encoding α and β subunits of soluble guanylyl cyclase in the brain of rainbow trout: comparison with the distribution of neuronal nitric oxide synthase

Detailed distribution of mRNAs encoding alpha and beta subunits of soluble guanylyl cyclase (sGC) was examined in the brain of rainbow trout by in situ hybridization. In addition, distribution of nitric oxide synthase (NOS) was mapped in adjacent parallel sections by neuronal NOS (nNOS) immunocytochemistry and NADPH-diaphorase (NADPHd) histochemistry. Following application of digoxigenin-labeled riboprobes for sGC alpha and beta subunit mRNAs, we found comparatively intense hybridization signals in the telencephalon, preoptic area, thalamus, hypothalamus, pretectum and tegmentum. Both nNOS immunocytochemistry and NADPHd histochemistry showed extensive distribution of nitroxergic neurons in various brain areas, although various degrees of dissociation of nNOS immunoreactivity (ir) and NADPHd staining were detected. In comparison with sGC subunit mRNAs, nNOS signals were more widely distributed in many neurons, including parvocellular neurons in the preoptic area, nucleus anterior tuberis in the hypothalamus, periventricular neurons in the optic tectum, most of the rhombencephalic neurons and pituitary cells. However, wide overlaps of sGC mRNA-containing neurons and nNOS-positive neurons were observed in the olfactory bulb, telencephalon, preoptic area, thalamus, hypothalamus, pretectum, optic tectum, tegmentum and cerebellum. The widespread overlapping in sGC subunit mRNAs and nNOS distribution suggests a role for sGC in various neuronal functions, such as processing of olfactory and visual signals and neuroendocrine function, possibly via NO/cGMP signaling in the brain of rainbow trout.

Distribution of soluble guanylyl cyclase in the rat brain

The diffusible messenger nitric oxide (NO) acts in the brain largely through activation of soluble guanylyl cyclase (sGC), a heterodimer comprising alpha and beta subunits. We used immunohistochemistry to study the distribution of both sGC subunits in the brain of adult rats. alpha and beta subunits gave similar widespread staining throughout the CNS, which was strongest in neostriatum, olfactory tubercle, and supraoptic nucleus. Double-labeling experiments showed striking cellular colocalization in most brain regions, suggesting that the two subunits may be organized into enzymatically active alpha/beta heteromers. Mismatches were observed in cerebellar cortex: Purkinje cells and Bergmann glia were positive for both subunits, whereas granule cells and interneurons in the molecular layer were strongly immunopositive for beta but only weakly stained for the alpha subunit. By using multiple labeling, we compared the localization of sGC with neuronal nitric oxide synthase (NOS-I, the NO-producing enzyme in neurons). In forebrain, the distribution of sGC and NOS-I was complementary, with only occasional colocalization. In contrast, cellular colocalization was common in midbrain and cerebellum. These data support a widespread role for the NO/sGC/cGMP pathway in the CNS and suggest that, in addition to its role as paracrine messenger, NO may also be an intracellular autocrine agent.

Co-induction of growth-associated protein GAP-43 and neuronal nitric oxide synthase in the cochlear nucleus following cochleotomy

In adult animals, cochlear lesioning leads to a reactive synaptogenesis with a reemergence of growth-associated protein, GAP-43, in the auditory brainstem nuclei. In addition, nitric oxide (NO) is also implicated in synaptogenesis. Three isoforms of nitric oxide synthase (NOS) responsible for generating NO have been identified and, in neurons, the predominant isoform is neuronal NOS (nNOS). Studies in visual or olfactory systems have found that the NOS expression often correlates with periods of axonal outgrowth and synapse formation; whether NO plays a similar role in the auditory brainstem needs to be examined. In the present study, a unilateral cochleotomy was performed in adult mice to examine the relationship between the reemergence of GAP-43 and the expression pattern of nNOS. Following surgery, GAP-43 re-emerged in the ipsilateral anterior ventral cochlear nucleus (AVCN) and the immunoreactivity reached a climax around postoperative day (POD) 8; the same expression pattern as that reported in the previous literature is the indicator of synaptogenesis. As for the nNOS immunoreactivity, a dramatic redistribution from a mostly cytoplasmal to a predominantly membranous localization in the ipsilateral AVCN was found especially at POD 4. A similar redistribution pattern in the ipsilateral AVCN for the N-methyl-D-aspartate (NMDA) receptor was also observed at POD 4, corresponding to the fact that the activation of nNOS is coupled to calcium influx via the NMDA-receptor. Furthermore, the expression of cyclic guanosine monophosphate (cGMP) is an indicator for activity of soluble guanylyl cyclase (sGC), the substrate of NO, which reveals the target area of NO. Therefore, cGMP immunoreactivity was also examined and an obvious increase of cytoplasmal cGMP expression was observed around POD 4. Accordingly, it is suggested that nNOS activity correlates closely with the reactive synaptogenesis following a cochleotomy. Further evidence is shown by the results of fluorescent double staining; nNOS-positive cells were surrounded by GAP-43 labeled regions that appeared to be presynaptic boutons, and the vast majority of nNOS-positive cells also expressed cGMP. The former result indicates that, after surgery, there should be new terminal endings projecting onto the nNOS-positive cells in the AVCN. Furthermore, the latter result suggests a possible role of an autocrine mediator for nNOS in the AVCN.

Cell-specific expression and regulation of soluble guanylyl cyclase alpha 1 and beta 1 subunits in the rat ovary

Soluble guanylyl cyclase (sGC) is activated by nitric oxide (NO) and carbon monoxide, resulting in cGMP production. Recent studies indicate that NO and cGMP influence ovarian functions. However, little information is available regarding the ovarian expression of sGC. The present study examined sGC alpha(1) and beta(1) subunit protein levels in the ovary during postnatal development, gonadotropin-induced follicle growth, ovulation, and luteinization as well as in cultured rat granulosa cells. In postnatal rats, sGC alpha(1) subunit immunoreactivity was high in granulosa cells of primordial and primary follicles on Day 5 but low in granulosa cells of larger follicles on Days 10 and 19. Theca cells of developing follicles, but not stromal cells, also demonstrated moderate sGC alpha(1) immunoreactivity. In gonadotropin- treated immature rats, intense sGC alpha(1) subunit staining was similarly observed in granulosa cells of primordial and primary follicles, but such staining was low in granulosa cells of small antral follicles and undetectable in granulosa cells of large antral and preovulatory follicles. Following ovulation, corpora lutea expressed moderate sGC alpha(1) immunoreactivity. Similar ovarian localization and expression patterns were seen for sGC beta(1), indicating regulated coexpression of sGC subunits. Immunoblot analysis revealed no change in total ovarian sGC alpha(1) and beta(1) subunit protein levels during gonadotropin treatment. Similarly, no effect of FSH on sGC subunit protein levels was apparent in cultured granulosa cells. These findings indicate regulated, cell- specific patterns of sGC expression in the ovary and are consistent with roles for cGMP in modulating ovarian functions.

Synaptic localization of nitric oxide synthase and soluble guanylyl cyclase in the hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1.