Expression of the neurokinin 1 receptor in the rabbit retina

Network Architecture of Gap Junctional Coupling among Parallel Processing Channels in the Mammalian Retina

Gap junctions are ubiquitous throughout the nervous system, mediating critical signal transmission and integration, as well as emergent network properties. In mammalian retina, gap junctions within the Aii amacrine cell-ON cone bipolar cell (CBC) network are essential for night vision, modulation of day vision, and contribute to visual impairment in retinal degenerations, yet neither the extended network topology nor its conservation is well established. Here, we map the network contribution of gap junctions using a high-resolution connectomics dataset of an adult female rabbit retina. Gap junctions are prominent synaptic components of ON CBC classes, constituting 5%-25% of all axonal synaptic contacts. Many of these mediate canonical transfer of rod signals from Aii cells to ON CBCs for night vision, and we find that the uneven distribution of Aii signals to ON CBCs is conserved in rabbit, including one class entirely lacking direct Aii coupling. However, the majority of gap junctions formed by ON CBCs unexpectedly occur between ON CBCs, rather than with Aii cells. Such coupling is extensive, creating an interconnected network with numerous lateral paths both within, and particularly across, these parallel processing streams. Coupling patterns are precise with ON CBCs accepting and rejecting unique combinations of partnerships according to robust rulesets. Coupling specificity extends to both size and spatial topologies, thereby rivaling the synaptic specificity of chemical synapses. These ON CBC coupling motifs dramatically extend the coupled Aii-ON CBC network, with implications for signal flow in both scotopic and photopic retinal networks during visual processing and disease.SIGNIFICANCE STATEMENT Electrical synapses mediated by gap junctions are fundamental components of neural networks. In retina, coupling within the Aii-ON CBC network shapes visual processing in both the scotopic and photopic networks. In retinal degenerations, these same gap junctions mediate oscillatory activity that contributes to visual impairment. Here, we use high-resolution connectomics strategies to identify gap junctions and cellular partnerships. We describe novel, pervasive motifs both within and across classes of ON CBCs that dramatically extend the Aii-ON CBC network. These motifs are highly specific with implications for both signal processing within the retina and therapeutic interventions for blinding conditions. These findings highlight the underappreciated contribution of coupling motifs in retinal circuitry and the necessity of their detection in connectomics studies.

The role of histamine in the retina: studies on the Hdc knockout mouse

The role of histamine in the retina is not well understood, despite it regulating a number of functions within the brain, including sleep, feeding, energy balance, and anxiety. In this study we characterized the structure and function of the retina in mice that lacked expression of the rate limiting enzyme in the formation of histamine, histidine decarboxylase (Hdc^−/− mouse). Using laser capture microdissection, Hdc mRNA expression was assessed in the inner and outer nuclear layers of adult C57Bl6J wildtype (WT) and Hdc^−/−-retinae. In adult WT and Hdc^−/−-mice, retinal fundi were imaged, retinal structure was assessed using immunocytochemistry and function was probed by electroretinography. Blood flow velocity was assessed by quantifying temporal changes in the dynamic fluorescein angiography in arterioles and venules. In WT retinae, Hdc gene expression was detected in the outer nuclear layer, but not the inner nuclear layer, while the lack of Hdc expression was confirmed in the Hdc^−/− retina. Preliminary examination of the fundus and retinal structure of the widely used Hdc^−/−mouse strain revealed discrete lesions across the retina that corresponded to areas of photoreceptor abnormality reminiscent of the rd8 (Crb1) mutation. This was confirmed after genotyping and the strain designated Hdc^rd8/rd8. In order to determine the effect of the lack of Hdc-alone on the retina, Hdc^−/− mice free of the Crb1 mutation were bred. Retinal fundi appeared normal in these animals and there was no difference in retinal structure, macrogliosis, nor any change in microglial characteristics in Hdc^−/− compared to wildtype retinae. In addition, retinal function and retinal blood flow dynamics showed no alterations in the Hdc^−/− retina. Overall, these results suggest that histamine plays little role in modulating retinal structure and function.

Synaptic connections of calbindin-immunoreactive cone bipolar cells in the inner plexiform layer of rabbit retina

In the mammalian retina, information concerning various aspects of an image is transferred in parallel, and cone bipolar cells are thought to play a major role in this parallel processing. We have examined the synaptic connections of calbindin-immunoreactive (IR) ON cone bipolar cells in the inner plexiform layer (IPL) of rabbit retina and have compared these synaptic connections with those that we have previously described for neurokinin 1 (NK1) receptor-IR cone bipolar cells. A total of 325 synapses made by calbindin-IR bipolar axon terminals have been identified in sublamina b of the IPL. The axons of calbindin-IR bipolar cells receive synaptic inputs from amacrine cells through conventional synapses and are coupled to putative AII amacrine cells via gap junctions. The major output from calbindin-IR bipolar cells is to amacrine cell processes. These data resemble our findings for NK1 receptor-IR bipolar cells. However, the incidences of output synapses to ganglion cell dendrites of calbindin-IR bipolar cells are higher compared with the NK1-receptor-IR bipolar cells. On the basis of stratification level and synaptic connections, calbindin-IR ON cone bipolar cells might thus play an important role in the processing of various visual aspects, such as contrast, orientation, and approach sensing, and in transferring rod signals to the ON cone pathway.

Neurokinin A and neurokinin B in the human retina

Very recently, the authors found levels of neurokinin (NK) A-like immunoreactivities in the human retina which were more than five times higher than those of substance P (SP). The present study aimed to find out how many of these immunoreactivities can be attributed to NKA and NKB and then the exact distribution pattern of both NKA and NKB was evaluated in the human retina and compared with that of SP. For this purpose, NKA-like immunoreactivities were characterized in the human retina by reversed phase HPLC followed by radioimmunoassay using the K12 antibody which recognizes both NKA and NKB. Furthermore, the retinae from both a 22- and 70-year-old donor were processed for double-immunofluorescence NKA/SP and NKB/SP. The results showed that NKA contributes to approximately two thirds and NKB to approximately one third of the immunoreactivities measured with the K12 antibody. NKA was found to be localized in sparse amacrine cells in the proximal inner nuclear layer, in displaced amacrine cells in the ganglion cell layer with processes ramifying in stratum 3 of the inner plexiform layer and also in sparse ganglion cells. By contrast, staining for NKB was only observed in ganglion cells and in the nerve fiber layer. Double-immunofluorescence revealed cellular colocalization of NKA with SP and also of NKB with SP. Thus, the levels of NKA and NKB are more than three and two times higher than those of SP, respectively. Whereas the distribution pattern of NKA is typical for neuropeptides, the localization of NKB exclusively in ganglion cells is atypical and unique.

Histamine receptors in mammalian retinas

Mammalian retinas are innervated by histaminergic axons that originate from perikarya in the posterior hypothalamus. To identify the targets of these retinopetal axons, we localized histamine receptors (HR) in monkey and rat retinas by light and electron microscopy. In monkeys, puncta containing HR3 were found at the tips of ON-bipolar cell dendrites in cone pedicles and rod spherules, closer to the photoreceptors than the other neurotransmitter receptors. This is the first ultrastructural localization of any histamine receptor and the first direct evidence that HR3 is present on postsynaptic membranes in the central nervous system. In rat retinas, most HR1 were localized to dopaminergic amacrine cells. The differences in histamine receptor localization may reflect the differences in the activity patterns of the two species.

Expression of substance P, neurokinin 1 receptors (NK1) and neurokinin 3 receptors in the developing mouse retina and in the retina of NK1 knockout mice

To complete a series of studies on the expression of substance P and neurokinin receptors in mammalian retinas, we investigated the occurrence of these molecules in developing mouse retinas and in retinas of mice with genetic deletion of the neurokinin 1 receptor, the preferred substance P receptor. Using semi-quantitative reverse transcription-polymerase chain reaction, we measured detectable levels of the gamma isoform of preprotachykinin A (a substance P precursor) mRNA at postnatal day 4. Neurokinin 1 receptor and neurokinin 3 receptor mRNAs were also detected at postnatal day 4. While gamma preprotachykinin A and neurokinin 1 receptor mRNA levels significantly increased up to eye opening (postnatal day 11), neurokinin 3 receptor mRNA levels remained constant throughout development. Substance P, neurokinin 1 receptor and neurokinin 3 receptor immunoreactivities were present at postnatal day 5. Substance P was in amacrine cells, neurokinin 1 receptor in developing amacrine and bipolar cells and neurokinin 3 receptor in OFF-type cone bipolar cells. Interestingly, a transient increase in the density of neurokinin 1 receptor immunoreactive processes was observed at eye opening in lamina 3 of the inner plexiform layer, suggesting a role of substance P and neurokinin 1 receptor in this developmental phase. However, in neurokinin 1 receptor knockout retinas, besides a significant increase of the gamma preprotachykinin A mRNA levels, no major changes were detected: neurokinin 3 receptor mRNA levels as well as substance P and neurokinin 3 receptor immunostainings were similar to wild types. Together with previous studies, these observations indicate that there are major differences in neurokinin 1 receptor expression patterns among developing mammalian retinas. The observations in neurokinin 1 receptor knockout mice may not be applicable to rats or rabbits, and substance P and neurokinin 1 receptor may play different developmental roles in different species.

Vascular NK-1 receptor occurrence in normal and chronic painful Achilles and patellar tendons: studies on chemically unfixed as well as fixed specimens

It is not known as to whether the Achilles and patellar tendons contain neurokinin-1 (NK-1) receptors. This is a drawback when considering the fact that pain symptoms are frequent in these and as recent studies show that the pain symptoms might be cured via interference with blood vessel function. In the present study, the human Achilles and patellar tendons were examined concerning immunohistochemical expression of the NK-1 receptor. Chemically unfixed and fixed specimens, TRITC and PAP stainings and a battery of NK-1 receptor antibodies, including antibodies against the C-terminus and the N-terminal region, were utilized. NK-1 receptor immunoreaction could be detected in inner parts of the walls of large blood vessels and in the walls of small blood vessels. To some extent, NK-1 immunoreaction was also detectable in small nerve fascicles and in tenocytes. It was found to be of utmost importance to apply both chemically unfixed and fixed specimens. The use of chemically unfixed tissue was found advantageous in order to depict the immunoreactions in the blood vessel walls. The observations represent new findings and are of relevance as substance P (SP) is known to be of importance where neurogenic angiogenesis contributes to diseases and as SP on the whole has profound effects concerning blood vessel regulation.

Synaptic connections of cone bipolar cells that express the neurokinin 1 receptor in the rabbit retina

We have investigated and further characterized, in the rabbit retina, the synaptic connectivity of the ON-type cone bipolar cells that are immunoreactive for an antibody against the neurokinin-1 receptor (NK1R). NK1R-immunoreactive bipolar cell axons terminate in stratum 4 of the inner plexiform layer. The axons of NK1R-positive bipolar cells receive synaptic inputs from amacrine cells through conventional synapses and from putative AII amacrine cells via gap junctions. The major outputs from NK1R-positive bipolar cells make contacts with amacrine cell processes. The most frequent postsynaptic dyads comprise two amacrine cell processes. Double-labeling experiments with antibodies against NK1R and either calretinin or glycine have demonstrated that NK1R-immunoreactive bipolar cells form gap junctions with AII amacrine cells. Thus, NK1R-positive cone bipolar cells, together with calbindin-positive cone bipolar cells, may play an important role in transferring rod signals to the ON-type ganglion cells of the cone pathway in the rabbit retina.

Substance P and the neurokinin-1 receptor in relation to eosinophilia in ulcerative colitis

Substance P (SP) has been implicated in the pathophysiology of ulcerative colitis (UC) and it has been suggested that blocking of its effect would be advantageous in this disease. Eosinophils have also been implicated in the pathophysiology of UC. In the present study, specimens from the sigmoid colon of UC patients were investigated by the use of antisera against SP and the neurokinin-1 receptor (NK-1R) and staining for demonstration of eosinophils. The degrees of SP innervation and NK-1R immunoreaction, as well as the levels of eosinophil infiltration, varied between different patients. Interestingly, NK-1R immunoreaction in the epithelium was often seen to be the most marked where there were numerous eosinophils in the underlying mucosa and where the mucosa showed a marked morphologic derangement. The observations suggest that there are marked fluctuations in effects of SP and eosinophils during the disease. The infiltrating eosinophils may be involved in the destruction of the mucosal tissue. Furthermore, for the majority of cases where there is marked derangement of the mucosa, it is apparent that there is an upregulation of the NK-1 receptor in the epithelium in parallel with the infiltration of the eosinophils.

Expression of the neurokinin 1 receptor in the mouse retina

Previous studies have revealed that the expression pattern of the neurokinin 1 receptor (the preferred receptor for substance P, SP) varies in different mammalian retinas. We investigated NK1 receptor expression in the mouse retina to provide background information for future studies in transgenic mice on SP functional roles in the retina. Mouse retinal sections were treated for single and double-label immunofluorescence. NK1 receptor immunoreactivity was in bipolar cells and in numerous amacrine cells. Double-label studies showed that NK1 receptor-expressing bipolar cells constituted a population of ON-type cone bipolar cells, since they were distinct from rod bipolar cells and contained glycine. They were nonrandomly distributed with highest density in central retina. These cells were similar and may correspond to the population of NK1 receptor-expressing bipolar cells of the rabbit retina. Different subsets of NK1 receptor-expressing amacrine cells were identified on the basis of the expression of selected neurotransmitter substances: i) about 23% of NK1 receptor-expressing amacrine cells also contained glycine; ii) the remaining 77% were likely to be GABAergic, although some inconsistency was observed in the GABA immunostaining obtained with two different GABA antibodies; iii) all dopaminergic amacrine cells also expressed NK1 receptors; iv) about one third of SP-containing amacrine cells also expressed NK1 receptors. These findings confirm and expand previous observations in rat and rabbit retinas. In particular, common to all three species is the expression of NK1 receptors in dopaminergic amacrine cells, indicating that SP neurotransmission may be a universal feature of the circuitry of the dopaminergic amacrine cell. Peculiar to the mouse retina is the presence of putative NK1 autoreceptors expressed by SP-containing amacrine cells.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Evidence that certain retinal bipolar cells use both glutamate and GABA

Retinal bipolar neurons release the excitatory transmitter, glutamate. However, certain bipolar cells contain GABA, raising the question whether a neuron might release both transmitters and, if so, what function might the inhibitory transmitter play in a particular circuit? Here we identify a subset of cone bipolar cells in cat retina that contain glutamate, plus its vesicular transporter (VGLUT1), and GABA, plus its synthetic enzyme (GAD(65)) and its vesicular transporter (VGAT). These cells are negative for a marker of ON bipolar cells and restrict their axons to the OFF strata of the inner synaptic layer. They do not colocalize with the neurokinin 3 receptor that stains a type (or two) of OFF bipolar cells. By "targeted injection," we identified two types of OFF bipolar cell with the machinery to make and package both transmitters. One of these types costratifies with a dopamine plexus.

Morphological analysis of the hyperpolarization-activated cyclic nucleotide-gated cation channel 1 (HCN1) immunoreactive bipolar cells in the rabbit retina

Hyperpolarization-activated cation currents (I(h)) have been identified in neurons in the central nervous system, including the retina. There is growing evidence that these currents, mediated by the hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN), may play important roles in visual processing in the retina. This study was conducted to identify and characterize HCN1-immunoreactive (IR) bipolar cells by immunocytochemistry, quantitative analysis, and electron microscopy. The HCN1-IR bipolar cells were a subtype of OFF-type cone bipolar cells and comprised 10% of the total number of cone bipolar cells. The axons of the HCN1-IR cone bipolar cells ramified narrowly in the border of strata 1 and 2 of the inner plexiform layer (IPL). These cells formed a regular distribution, with a density of 1,825 cells/mm(2) at a position 1 mm ventral to the visual streak, falling to 650 cells/mm(2) in the ventral periphery. Double-labeling experiments demonstrated that their axons stratified narrowly within and slightly proximal to the OFF-starburst amacrine cell processes. In the IPL, they were presynaptic to amacrine cell processes. The most frequent postsynaptic dyads formed of HCN1-IR bipolar cell axon terminals are pairs composed of both amacrine cell processes. These results suggest that these HCN1-IR cone bipolar cells might be the same as the DAPI-Ba1 bipolar population, and might therefore be involved in a direction-selective mechanism, providing inputs to the OFF-starburst amacrine cells and/or the OFF-plexus of the ON-OFF ganglion cells.

Neurokinin 1 receptor expression and substance p physiological actions are developmentally regulated in the rabbit retina

We investigated the expression of the substance P (SP) receptor (the neurokinin 1 receptor, NK1 receptor) and SP functional effects in developing rabbit retinas. NK1 receptors in adult retinas were in a population of cone bipolar cells and in dopaminergic amacrine cells, as previously described. In contrast, at birth and at postnatal day (PND) 6, NK1 receptors were exclusively expressed by cholinergic amacrine and displaced amacrine cells. NK1 receptor expression in cholinergic cells was still observed at PND10 (eye opening), while at PND21 it was confined to cholinergic cells of the inner nuclear layer. Starting at PND10, NK1 receptors were also in bipolar cells and in dopaminergic amacrine cells. A fully mature NK1 receptor expression pattern was observed at PND35. Dopamine release was assessed in isolated retinas in the presence of SP, the NK1 receptor agonist GR73632 or the NK1 receptor antagonist GR82334. At PND35, extracellular dopamine was significantly increased by 10 microM SP or 0.01-100 microM GR73632, and it was decreased by 0.01-10 microM GR82334. No effects were detected in developing retinas up to PND21. Ca2+ imaging experiments were performed in single cholinergic cells identified by their "starburst" morphology in perinatal retinas. Intracellular Ca2+ levels were significantly increased by 1 microM SP or GR73632. This effect was reversibly inhibited by 1 microM GR82334. These data demonstrate that both NK1 receptor expression and SP physiological actions are developmentally regulated in the retina. SP neurotransmission in the immature retina may subserve developmental events, and SP is likely to represent an important developmental factor for the maturation of retinal neurons and circuitries.

The population of bipolar cells in the rabbit retina

The population of bipolar cells in the rabbit retina was studied using Golgi impregnation and photocatalyzed filling of single cells with dihydrorhodamine, a quantitative sampling technique. The Golgi method revealed the morphology and stratification of cells in detail. The photofilling method allowed us to estimate the frequency of the cell types. From a sample of 243 Golgi-impregnated bipolar cells and 107 photofilled cells, we identified 1 type of rod bipolar cell and 12 types of cone bipolar cells. An analysis based on retinal coverage indicates that this number of types could be contained within the number of bipolar cells known to exist. The dendrites of most cone bipolars contacted all the cones within the individual cone bipolar cell's dendritic field. Types of bipolar cell were encountered at roughly similar frequency, without any one type predominating. The rabbit retina thus contains about a dozen parallel and roughly equipotent through-pathways.