The function of the cholinergic system in the developing mammalian retina

On the Role of LGN/V1 Spontaneous Activity as an Innate Learning Pattern for Visual Development

Correlated, spontaneous neural activity is known to play a necessary role in visual development, but the higher-order statistical structure of these coherent, amorphous patterns has only begun to emerge in the past decade. Several computational studies have demonstrated how this endogenous activity can be used to train a developing visual system. Models that generate spontaneous activity analogous to retinal waves have shown that these waves can serve as stimuli for efficient coding models of V1. This general strategy in development has one clear advantage: The same learning algorithm can be used both before and after eye-opening. This same insight can be applied to understanding LGN/V1 spontaneous activity. Although lateral geniculate nucleus (LGN) activity has been less discussed in the literature than retinal waves, here we argue that the waves found in the LGN have a number of properties that fill the role of a training pattern. We make the case that the role of “innate learning” with spontaneous activity is not only possible, but likely in later stages of visual development, and worth pursuing further using an efficient coding paradigm.

Spontaneous Afferent Activity Carves Olfactory Circuits

Electrical activity has a key role in shaping neuronal circuits during development. In most sensory modalities, early in development, internally generated spontaneous activity sculpts the initial layout of neuronal wiring. With the maturation of the sense organs, the system relies more on sensory-evoked electrical activity. Stimuli-driven neuronal discharge is required for the transformation of immature circuits in the specific patterns of neuronal connectivity that subserve normal brain function. The olfactory system (OS) differs from this organizational plan. Despite the important role of odorant receptors (ORs) in shaping olfactory topography, odor-evoked activity does not have a prominent role in refining neuronal wiring. On the contrary, afferent spontaneous discharge is required to achieve and maintain the specific diagram of connectivity that defines the topography of the olfactory bulb (OB). Here, we provide an overview of the development of olfactory topography, with a focus on the role of afferent spontaneous discharge in the formation and maintenance of the specific synaptic contacts that result in the topographic organization of the OB.

Inhibition of acetylcholinesterase attenuated retinal inflammation via suppressing NF-κB activation

Elevated inflammatory cytokines contribute to the pathogenesis of various retinal diseases such as diabetic retinopathy, retinal vasculitis and retinitis. However, the underlying mechanism of retinal inflammation remains largely unknown. Recent studies demonstrated that acetylcholinesterase (ACHE) is an inflammatory indicator in central neural system. This study was aimed to dissect the role of ACHE in retinal inflammation, and its mechanism of action. Retinal inflammation was induced by intravitreal injection of tumor necrosis factor-α (TNF-α) in heterozygous ACHE knockdown mice (ACHE^+/-) and wild type mice (ACHE^+/+). Donepezil, a well-known ACHE inhibitor, was administrated by daily gavage. Expression of ACHE and intercellular adherent molecule-1 (ICAM-1), infiltration of CD11b⁺ inflammatory cells, retinal leukostasis and vascular leakage was determined in both ACHE^+/- and ACHE^+/+ mice. ARPE-19 cells, a human retinal pigment epithelial cell line, were cultured for in vitro assay. Knockdown of ACHE was achieved by lipofectamine-mediated siRNA transfection and pharmaceutical suppression of ACHE was manipulated by donepezil. Cellular expression and distribution of ACHE, ICAM-1, and phosphorylation of NF-κB, IκB and IKKα/β were detected by western-blot analysis or immunocytochemistry. Retinal expression of ACHE was dramatically upregulated, in parallel with increased ICAM-1 expression, enhanced leukostasis and augmented CD11b⁺ inflammatory cell infiltration as well as vascular hyperpermeability in ACHE^+/+ mice injected with TNF-α. However, TNF-α-injected ACHE^+/- mice showed lower level of ICAM-1, less leukostasis and fewer infiltrated CD11b⁺ cells. Moreover, TNF-α-induced retinal vascular leakage was significantly reduced in ACHE^+/- mice. Similarly, TNF-α-induced retinal inflammatory response were also attenuated by donepezil intervention. In addition, TNF-α treatment resulted in significant induction of ACHE, upregulation of ICAM-1 and nuclear translocation of NF-κB, phosphorylation of IκB and IKKα/β in ARPE-19 cells. However, inhibition of ACHE reduced TNF-α-induced phosphorylation of NF-κB, IκB and IKKα/β in ARPE-19 cells. The present study reveals a pivotal role of ACHE in retinal inflammation. Inhibition of ACHE attenuates retinal inflammation and retinal leakage likely through suppressing NF-κB signaling activation.

Glutamate Activity Regulates and Dendritic Development of J-RGCs

Retinal ganglion cells (RGCs) have a wide variety of dendritic architectures, which are critical for the formation of their function-specific synaptic circuitry. The developmental regulation of the dendrites of RGCs is thought to be subtype dependent. The purpose of this study is to characterize the dendritic development of a genetically identified RGC subtype, JamB RGCs (J-RGCs), and the roles of glutamate receptor activity on the dendritic development of these cells. We show that the dendrites of J-RGCs are strictly ramified in the outer portion of the inner plexiform layer (IPL) of the retina at the age of postnatal day 8 (P8), mimicking the ramification pattern of adults. However, several other important features of dendrites undergo substantial developmental refinement after P8. From P8 to P13, the dendritic development of J-RGCs is characterized by a dramatic increase of dendritic length and the size of the dendritic field. After eye opening, the dendritic development of J-RGCs is characterized by a tremendous decrease of the number of dendritic protrusions (spine-like structures) and a consolidation of the size of the dendritic field. To determine whether the dendritic development of J-RGCs is regulated by glutamatergic activity, we conditionally knocked out the expression of an obligatory subunit of N-methyl-D-aspartate receptors (NMDARs), NR1 (Grin1), in J-RGCs. We found that J-RGCs with the NMDAR mutation have decreased dendrite outgrowth and dendritic field expansion but increased number of dendritic protrusions before eye opening. To determine if visual experience regulates the development of J-RGC dendrites, we raised the mice in complete darkness after birth. Light deprivation prevented the decrease in the number of dendritic protrusions and the consolidation of the dendritic field of wild type (WT) mice after eye opening. However, light deprivation has no additional effect on the number of dendritic protrusions or the size of the dendritic field of J-RGCs with NMDAR mutation. Together, these results revealed the roles of light stimulation and NMDAR activity on the dendritic development of J-RGCs.

Lower urinary cotinine level is associated with a trend toward more myopic refractive errors in Korean adolescents

PurposeTo investigate the association between urinary cotinine levels as an objective biological marker for exposure to nicotine and refractive status.Patients and methodsThis cross-sectional study analyzed data from the Korea National Health and Nutrition Examination Survey between 2008 and 2011. A total of 1139 Korean adolescents aged 12-18 years were enrolled. Urinary cotinine concentrations and other potential risk factors for myopia were examined. Correlation analyses and multivariate regression analysis were performed to investigate the association between urinary cotinine level and refractive error.ResultsSpherical equivalent correlated significantly with urinary cotinine concentration (r=0.104, P=0.011). Lower urinary cotinine level was associated with a trend toward more myopic refractive errors (P for trend=0.003). After adjusting for age, sex, area of residence, physical activity, serum 25-hydroxyvitamin D level, parental income level, and receipt of basic livelihood security, subjects with a low urinary cotinine level had a significantly increased risk of myopia <-0.5 D (odds ratio (OR) 1.95, 95% confidence interval (CI) 1.18-3.21), <-3.0 D (OR 2.03, 95% CI 1.29-3.2), and <-6.0 D (OR 2.2, 95% CI, 1.15-4.23) when compared with subjects with a high urinary cotinine level. As urinary cotinine level decreased, the risks of myopia <-0.5 D, <-3.0 D, and <-6.0 D increased significantly (P for trend <0.05).ConclusionA trend toward less myopic refractive error was observed among Korean adolescents with higher urinary cotinine levels. This result provides the epidemiologic evidence implying nicotine as a potential modulator related with refractive development. Further studies with full consideration for myopia-associated risk factors are required to yield clear answers on the direct effect of smoking to the refractive status.

Extrasynaptic glutamate and inhibitory neurotransmission modulate ganglion cell participation during glutamatergic retinal waves

During the first 2 wk of mouse postnatal development, transient retinal circuits give rise to the spontaneous initiation and lateral propagation of depolarizations across the ganglion cell layer (GCL). Glutamatergic retinal waves occur during the second postnatal week, when GCL depolarizations are mediated by ionotropic glutamate receptors. Bipolar cells are the primary source of glutamate in the inner retina, indicating that the propagation of waves depends on their activation. Using the fluorescence resonance energy transfer-based optical sensor of glutamate FLII81E-1μ, we found that retinal waves are accompanied by a large transient increase in extrasynaptic glutamate throughout the inner plexiform layer. Using two-photon Ca(2+) imaging to record spontaneous Ca(2+) transients in large populations of cells, we found that despite this spatially diffuse source of depolarization, only a subset of neurons in the GCL and inner nuclear layer (INL) are robustly depolarized during retinal waves. Application of the glutamate transporter blocker dl-threo-β-benzyloxyaspartate (25 μM) led to a significant increase in cell participation in both layers, indicating that the concentration of extrasynaptic glutamate affects cell participation in both the INL and GCL. In contrast, blocking inhibitory transmission with the GABAA receptor antagonist gabazine and the glycine receptor antagonist strychnine increased cell participation in the GCL without significantly affecting the INL. These data indicate that during development, glutamate spillover provides a spatially diffuse source of depolarization, but that inhibitory circuits dictate which neurons within the GCL participate in retinal waves.

Role of adenylate cyclase 1 in retinofugal map development

The development of topographic maps of the sensory periphery is sensitive to the disruption of adenylate cyclase 1 (AC1) signaling. AC1 catalyzes the production of cAMP in a Ca2+/calmodulin-dependent manner, and AC1 mutant mice (AC1−/−) have disordered visual and somatotopic maps. However, the broad expression of AC1 in the brain and the promiscuous nature of cAMP signaling have frustrated attempts to determine the underlying mechanism of AC1-dependent map development. In the mammalian visual system, the initial coarse targeting of retinal ganglion cell (RGC) projections to the superior colliculus (SC) and lateral geniculate nucleus (LGN) is guided by molecular cues, and the subsequent refinement of these crude projections occurs via an activity-dependent process that depends on spontaneous retinal waves. Here, we show that AC1−/− mice have normal retinal waves but disrupted map refinement. We demonstrate that AC1 is required for the emergence of dense and focused termination zones and elimination of inaccurately targeted collaterals at the level of individual retinofugal arbors. Conditional deletion of AC1 in the retina recapitulates map defects, indicating that the locus of map disruptions in the SC and dorsal LGN of AC1−/− mice is presynaptic. Finally, map defects in mice without AC1 and disrupted retinal waves (AC1−/−;β2−/− double KO mice) are no worse than those in mice lacking only β2−/−, but loss of AC1 occludes map recovery in β2−/− mice during the second postnatal week. These results suggest that AC1 in RGC axons mediates the development of retinotopy and eye-specific segregation in the SC and dorsal LGN.

Developmental mechanisms that regulate retinal ganglion cell dendritic morphology

One of the fundamental features of retinal ganglion cells (RGCs) is that dendrites of individual RGCs are confined to one or a few narrow strata within the inner plexiform layer (IPL), and each RGC synapses only with a small group of presynaptic bipolar and amacrine cells with axons/dendrites ramified in the same strata to process distinct visual features. The underlying mechanisms which control the development of this laminar-restricted distribution pattern of RGC dendrites have been extensively studied, and it is still an open question whether the dendritic pattern of RGCs is determined by molecular cues or by activity-dependent refinement. Accumulating evidence suggests that both molecular cues and activity-dependent refinement might regulate RGC dendrites in a cell subtype-specific manner. However, identification of morphological subtypes of RGCs before they have achieved their mature dendritic pattern is a major challenge in the study of RGC dendritic development. This problem is now being circumvented through the use of molecular markers in genetically engineered mouse lines to identify RGC subsets early during development. Another unanswered fundamental question in the study of activity-dependent refinement of RGC dendrites is how changes in synaptic activity lead to the changes in dendritic morphology. Recent studies have started to shed light on the molecular basis of activity-dependent dendritic refinement of RGCs by showing that some molecular cascades control the cytoskeleton reorganization of RGCs.

From retinal waves to activity-dependent retinogeniculate map development

A neural model is described of how spontaneous retinal waves are formed in infant mammals, and how these waves organize activity-dependent development of a topographic map in the lateral geniculate nucleus, with connections from each eye segregated into separate anatomical layers. The model simulates the spontaneous behavior of starburst amacrine cells and retinal ganglion cells during the production of retinal waves during the first few weeks of mammalian postnatal development. It proposes how excitatory and inhibitory mechanisms within individual cells, such as Ca(2+)-activated K(+) channels, and cAMP currents and signaling cascades, can modulate the spatiotemporal dynamics of waves, notably by controlling the after-hyperpolarization currents of starburst amacrine cells. Given the critical role of the geniculate map in the development of visual cortex, these results provide a foundation for analyzing the temporal dynamics whereby the visual cortex itself develops.

Development of excitatory and inhibitory neurotransmitters in transitory cholinergic neurons, starburst amacrine cells, and GABAergic amacrine cells of rabbit retina, with implications for previsual and visual development of retinal ganglion cells

Starburst amacrine cells (SACs), the only acetylcholine (ACh)-releasing amacrine cells (ACs) in adult rabbit retina, contain GABA and are key elements in the retina's directionally selective (DS) mechanism. Unlike many other GABAergic ACs, they use glutamic acid decarboxlyase (GAD)(67), not GAD(65), to synthesize GABA. Using immunocytochemistry, we demonstrate the apoptosis at birth (P0) of transitory putative ACs that exhibit immunoreactivity (IR) for the ACh-synthetic enzyme choline acetyltransferase (ChAT), GAD(67), and the GABA transporter, GAT1. Only a few intact, displaced ChAT-immunoreactive SAC bodies are detected at P0. At P2, ChAT-IR is detected in the two narrowly stratified substrata of starburst dendrites in the inner plexiform layer (IPL). Quantitative analysis reveals that in the first postnatal week, only a small fraction of SACs cells express ChAT- and GABA-IR. Not until the end of the second week are they expressed in all SACs. At P0, a three-tiered stratification of GABA-IR is present in the IPL, entirely different from the adult pattern of seven substrata, emerging at P3-P4, and optimally visualized at P13. At P0, GAD(65) is detectable in normally placed AC bodies. At P1, GAD(65)-IR appears in dendrites of nonstarburst GABAergic ACs, and by P5 is robust in the adult pattern of four substrata in the IPL. GAD(65)-IR never co-localizes with ChAT-IR. In a temporal comparison of our data with physiological, pharmacological, and ultrastructural studies, we suggest that transitory ChAT-immunoreactive cells share with SACs production of stage II (nicotinic) waves of previsual synchronous activity in ganglion cells (GCs). Further, we conclude that (1) GAD(65)-immunoreactive, non-SAC GABAergic ACs are the most likely candidates responsible for the suppression of stage III (muscarinic/AMPA-kainate) waves and (2) DS responses first appear in DS GCs, when about 50% of SACs express ChAT- and GABA-IR, and in 100% of DS GCs, when expression occurs in all SACs.

The immune protein CD3zeta is required for normal development of neural circuits in the retina

Emerging evidence suggests that immune proteins regulate activity-dependent synapse formation in the central nervous system (CNS). Mice with mutations in class I major histocompatibility complex (MHCI) genes have incomplete eye-specific segregation of retinal ganglion cell (RGC) axon projections to the CNS. This effect has been attributed to causes that are nonretinal in origin. We show that a key component of MHCI receptor, CD3zeta, is expressed in RGCs. CD3zeta-deficient mice have reduced RGC dendritic motility, an increase in RGC dendritic density, and a selective defect of glutamate-receptor-mediated synaptic activity in the retina. Disrupted RGC synaptic activity and dendritic motility is associated with a failure of eye-specific segregation of RGC axon projections to the CNS. These results provide direct evidence of an unrecognized requirement for immune proteins in the developmental regulation of RGC synaptic wiring and indicate a possible retinal origin for the disruption of eye-specific segregation found in immune-deficient mice.

Mechanisms underlying spontaneous patterned activity in developing neural circuits

Patterned, spontaneous activity occurs in many developing neural circuits, including the retina, the cochlea, the spinal cord, the cerebellum and the hippocampus, where it provides signals that are important for the development of neurons and their connections. Despite there being differences in adult architecture and output across these various circuits, the patterns of spontaneous network activity and the mechanisms that generate it are remarkably similar. The mechanisms can include a depolarizing action of GABA (gamma-aminobutyric acid), transient synaptic connections, extrasynaptic transmission, gap junction coupling and the presence of pacemaker-like neurons. Interestingly, spontaneous activity is robust; if one element of a circuit is disrupted another will generate similar activity. This research suggests that developing neural circuits exhibit transient and tunable features that maintain a source of correlated activity during crucial stages of development.

Synaptic activity, visual experience and the maturation of retinal synaptic circuitry

A fundamental feature of the synaptic organization of retina is the laminar-specific structure, in which specific types of retinal neurons form highly selective synapses to transfer distinct synaptic signals. In mature vertebrate retina, the dendrites of most retinal ganglion cells (RGCs) are narrowly stratified and ramified in specific strata of the inner plexiform layer (IPL) of retina to synapse with distinct subtypes of bipolar cells (BCs). However, little is known of how retinal neurons form this laminar-specific synaptic structure during development. Recent studies showed that the formation of retinal synaptic circuitry is regulated by both gene expression and neuronal activity. Here I will briefly discuss the recent advances in our understanding of how synaptic activity modulates the maturation of RGC synaptic connections.

GABA(A) receptor-mediated signaling alters the structure of spontaneous activity in the developing retina

Ambient GABA modulates firing patterns in adult neural circuits by tonically activating extrasynaptic GABA(A) receptors. Here, we demonstrate that during a developmental period when activation of GABA(A) receptors causes membrane depolarization, tonic activation of GABA(A) receptors blocks all spontaneous activity recorded in retinal ganglion cells (RGCs) and starburst amacrine cells (SACs). Bath application of the GABA(A) receptor agonist muscimol blocked spontaneous correlated increases in intracellular calcium concentration and compound postsynaptic currents in RGCs associated with retinal waves. In addition, GABA(A) receptor agonists activated a tonic current in RGCs that significantly reduced their excitability. Using a transgenic mouse in which green fluorescent protein is expressed under the metabotropic glutamate receptor subtype 2 promoter to target recordings from SACs, we found that GABA(A) receptor agonists blocked compound postsynaptic currents and also activated a tonic current. GABA(A) receptor antagonists reduced the holding current in SACs but not RGCs, indicating that ambient levels of GABA tonically activate GABA(A) receptors in SACs. GABA(A) receptor antagonists did not block retinal waves but did alter the frequency and correlation structure of spontaneous RGC firing. Interestingly, the drug aminophylline, a general adenosine receptor antagonist used to block retinal waves, induced a tonic GABA(A) receptor antagonist-sensitive current in outside-out patches excised from RGCs, indicating that aminophylline exerts its action on retinal waves by direct activation of GABA(A) receptors. These findings have implications for how various neuroactive drugs and neurohormones known to modulate extrasynaptic GABA(A) receptors may influence spontaneous firing patterns that are critical for the establishment of adult neural circuits.

Choline acetyltransferase-immunoreactive neurons in the retina of normal and dark-reared turtle

Visual deprivation alters retinal-ganglion-cell response properties through changes in spontaneous wave-like activity (Sernagor and Grzywacz [1996] Curr Biol 6:1503-1508). This activity depends on cholinergic synaptic transmission in the turtle retina (ibid; Sernagor and Mehta [ 2001] J Anat 199:375-383). We studied the expression of choline acetyltransferase (ChAT) by immunocytochemistry and Western blot in developing retinas of control and dark-reared turtles. At postnatal day 0 (P0), right after hatching, ChAT-immunoreactivity was present in the ganglion cell layer (GCL), in the inner nuclear layer (INL), and in two distinct bands of the inner plexiform layer (IPL). In P14- and P28-control, and P14- and P28-dark-reared retinas, ChAT-immunoreactivity showed similar patterns to those in P0. However, in P14- and P28-dark-reared retinas the density of ChAT-immunoreactive cells was higher in both the INL and GCL than in P14- and P28-control retinas, respectively. Moreover, Western blotting showed that ChAT protein levels were significantly increased in the dark-reared retina compared to those of the control. TUNEL studies indicated that the difference between normal and dark-reared conditions was not due to extra apoptosis in the former. In turn, proliferating-cell nuclear antigen immunocytochemistry showed no extra proliferating cells in the latter. Finally, nearest-neighbor analysis revealed that the denser population of cholinergic cells in dark-reared turtles formed a mosaic as regular as the normal ones in the GCL. Thus, light deprivation increases the expression of ChAT, increasing the apparent density of cholinergic neurons in the developing turtle retina.

Gene expression and protein localization of calmodulin-dependent phosphodiesterase in adult rat retina

Calcium calmodulin-dependent cyclic nucleotide phosphodiesterase (PDE1) was identified in crude extract and immunolabeled sections of rat retina. Both cAMP and cGMP PDE activities were stimulated by calcium-calmodulin (4.7-fold and 2.3-fold, respectively). To characterize PDE1 isoforms in retinal cells further, we used antibodies that specifically recognize PDE1 gene products. PDE1B antibody stained a band at molecular mass of 63 kDa whereas PDE1C antibody recognized two bands at 74- and 70-kDa molecular masses. Two PDE1A antibodies (against N-terminal and C-terminal peptides) detected a band at 79 kDa never described before. Immunohistochemical analysis showed a distribution of PDE1A in the outer retina with a bright fluorescence in the outer segments of photoreceptors. PDE1B is uniformly distributed across the retina. PDE1C is confined mainly to the inner retina, with a precise localization in the inner nuclear layer. Immunostaining with choline acetyltransferase antibody indicates localization in cholinergic amacrine cell. The present data provide evidence of expression of PDE1 isoforms in mammalian retina with a complementary distribution of PDE1A and PDE1C, suggesting different roles in retinal function.

Dissociated GABAergic retinal interneurons exhibit spontaneous increases in intracellular calcium

Early in development, before the retina is responsive to light, neurons exhibit spontaneous activity. Recently it was demonstrated that starburst amacrine cells, a unique class of neurons that secretes both GABA and acetylcholine, spontaneously depolarize. Networks comprised of spontaneously active starburst cells initiate correlated bursts of action potentials that propagate across the developing retina with a periodicity on the order minutes. To determine whether other retinal interneurons have similar “pacemaking” properties, we have utilized cultures of dissociated neurons from the rat retina. In the presence of antagonists for fast neurotransmitter receptors, distinct populations of neurons exhibited spontaneous, uncorrelated increases in intracellular calcium concentration. These increases in intracellular calcium concentration were sensitive to tetrodotoxin, indicating they are mediated by spontaneous membrane depolarizations. By combining immunofluorescence and calcium imaging, we found that 44% of spontaneously active neurons were GABAergic and included starburst amacrine cells. Whole cell voltage clamp recordings in the absence of antagonists for fast neurotransmitters revealed that after 7 days in culture, individual retinal neurons receive bursts of GABA-A receptor mediated synaptic input with a periodicity similar to that measured in spontaneously active GABAergic neurons. Low concentrations of GABA-A receptor antagonists did not alter the inter-burst interval despite significant reduction of post-synaptic current amplitude, indicating that pacemaker activity of GABAergic neurons was not influenced by network interactions. Together, these findings indicate that spiking GABAergic interneurons can function as pacemakers in the developing retina.

A transient network of intrinsically bursting starburst cells underlies the generation of retinal waves

Pharmacologically isolated starburst amacrine cells (SACs) in perinatal rabbit retinas spontaneously generated semiperiodic calcium spikes and long-lasting after-hyperpolarizations (AHPs), mediated by calcium-activated, cyclic AMP-sensitive potassium currents. These AHPs, rather than a depletion of neurotransmitters (as was previously believed), produced the refractory period of spontaneous retinal waves and set the upper limit of the wave frequency. Each SAC received inputs from roughly 10-30 neighboring SACs during a wave. These inputs synchronized and reshaped the intrinsic bursts to produce network oscillations at a rhythm different from that of individual SACs. With maturation, the semiperiodic bursts in SACs disappeared, owing to reduced intrinsic excitability and increased network inhibition. Thus, retinal waves are generated by a transient and specific network of cell-autonomous oscillators synchronized by reciprocally excitatory connections.

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.

Expression and function of the neuronal gap junction protein connexin 36 in developing mammalian retina

With the advent of transgenic mice, much has been learned about the expression and function of gap junctions. Previously, we reported that retinal ganglion cells in mice lacking the neuronal gap junction protein connexin 36 (Cx36) have nearly normal firing patterns at postnatal day 4 (P4) but many more asynchronous action potentials than wild-type mice at P10 (Torborg et al. [2005] Nat. Neurosci. 8:72-78). With the goal of understanding the origin of this increased activity in Cx36-/- mice, we used a transgenic mouse (Deans et al. [2001] Neuron 31:477-485) to characterize the developmental expression of a Cx36 reporter in the retina. We found that Cx36 was first detected weakly at P2 and gradually increased in expression until it reached an adult pattern at P14. Although the onset of expression varied by cell type, we identified Cx36 in the glycinergic AII amacrine cell, glutamatergic cone bipolar cell, and retinal ganglion cells (RGCs). In addition, we used calcium imaging and multielectrode array recording to characterize further the firing patterns in Cx36-/- mice. Both correlated and asynchronous action potentials in P10 Cx36-/- RGCs were significantly inhibited by bath application of an ionotropic glutamate receptor antagonist, indicating that the increase in activity was synaptically mediated. Hence, both the expression patterns and the physiology suggest an increasing role for Cx36-containing gap junctions in suppressing RGC firing between waves during postnatal retinal development.

Spontaneous patterned retinal activity and the refinement of retinal projections

A characteristic feature of sensory circuits is the existence of orderly connections that represent maps of sensory space. A major research focus in developmental neurobiology is to elucidate the relative contributions of neural activity and guidance molecules in sensory map formation. Two model systems for addressing map formation are the retinotopic map formed by retinal projections to the superior colliculus (SC) (or its non-mammalian homolog, the optic tectum (OT)), and the eye-specific map formed by retinal projections to the lateral geniculate nucleus of the thalamus. In mammals, a substantial portion of retinotopic and eye-specific refinement of retinal axons occurs before vision is possible, but at a time when there is a robust, patterned spontaneous retinal activity called retinal waves. Though complete blockade of retinal activity disrupts normal map refinement, attempts at more refined perturbations, such as pharmacological and genetic manipulations that alter features of retinal waves critical for map refinement, remain controversial. Here we review: (1) the mechanisms that underlie the generation of retinal waves; (2) recent experiments that have investigated a role for guidance molecules and retinal activity in map refinement; and (3) experiments that have implicated various signaling cascades, both in retinal ganglion cells (RGCs) and their post-synaptic targets, in map refinement. It is likely that an understanding of retinal activity, guidance molecules, downstream signaling cascades, and the interactions between these biological systems will be critical to elucidating the mechanisms of sensory map formation.

Development of cholinergic amacrine cells is visual activity-dependent in the postnatal mouse retina

In the present study, we used immunocytochemistry to study the temporal and spatial arrangement of mouse cholinergic amacrine cells during postnatal retinal development under normal light/dark cycles and during visual deprivation. Choline acetyltransferase (ChAT)-immunolabeled cells were detected in the neuroblastic layer (NBL) and in the ganglion cell layer (GCL) at postnatal day 0 (P0). Between P3-5, two characteristic cholinergic bands were clearly identified in the inner plexiform layer (IPL). The signal intensity of somas and processes progressively increased over the first 2 postnatal weeks. Around eye opening at P12, cholinergic neurons were mature-like. This early developmental process was not altered by visual deprivation. After eye opening, the space between the two cholinergic bands increased continuously and the spatial regularity index changed constantly, indicating that the cholinergic neurons possibly underwent refinement during later postnatal development. The changes occurring following eye opening were retarded by visual deprivation. The morphologies of photoreceptors, horizontal cells, recoverin-positive OFF-cone bipolar cells, rod bipolar cells, dopaminergic amacrine cells, and Müller cells appeared normal. Their stratification in the outer plexiform layer (OPL) and the IPL was not affected by visual deprivation. However, glial cells grew vertically across the entire thickness of dark-reared retinas. Our results suggest that the development of cholinergic neurons before eye opening is independent of the lighting conditions. Their development after eye opening is greatly impeded by visual deprivation. This visual activity-dependent phase of development may be a critical period for the maturation and synaptic wiring of cholinergic amacrine cells in the mammalian retina.

Retinal waves: mechanisms and function in visual system development

A characteristic feature of developing neural networks is spontaneous periodic activity. In the developing retina, retinal ganglion cells fire bursts of action potentials that drive large increases in intracellular calcium concentration with a periodicity of minutes. These periodic bursts of action potentials propagate across the developing inner retina as waves, driving neighboring retinal ganglion cells to fire in a correlated fashion. Here we will review recent progress in elucidating the mechanisms in mammals underlying retinal wave propagation and those regulating the periodicity with which these retinal waves occur. In addition, we will review recent experiments indicating that retinal waves are critical for refining retinal projections to their primary targets in the central visual system and may be involved in driving developmental processes within the retina itself.

Impaired formation of the inner retina in an AChE knockout mouse results in degeneration of all photoreceptors

Blinding diseases can be assigned predominantly to genetic defects of the photoreceptor/pigmented epithelium complex. As an alternative, we show here for an acetylcholinesterase (AChE) knockout mouse that photoreceptor degeneration follows an impaired development of the inner retina. During the first 15 postnatal days of the AChE-/- retina, three major calretinin sublaminae of the inner plexiform layer (IPL) are disturbed. Thereby, processes of amacrine and ganglion cells diffusely criss-cross throughout the IPL. In contrast, parvalbumin cells present a nonlaminar IPL pattern in the wild-type, but in the AChE-/- mouse their processes become structured within two 'novel' sublaminae. During this early period, photoreceptors become arranged regularly and at a normal rate in the AChE-/- retina. However, during the following 75 days, first their outer segments, and then the entire photoreceptor layer completely degenerate by apoptosis. Eventually, cells of the inner retina also undergo apoptosis. As butyrylcholinesterase (BChE) is present at a normal level in the AChE-/- mouse, the observed effects must be solely due to the missing AChE. These are the first in vivo findings to show a decisive role for AChE in the formation of the inner retinal network, which, when absent, ultimately results in photoreceptor degeneration.

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.

Identification of ON-OFF direction-selective ganglion cells in the mouse retina

We identified the ON-OFF direction-selective ganglion cells (DSGCs) in the mouse retina and characterized their physiological, morphological and pharmacological properties. These cells showed transient responses to the onset and termination of a stationary flashing spot, and strong directional selectivity to a moving rectangle. Application of various pharmacological reagents demonstrated that the ON-OFF DSGCs in the mouse retina utilize a similar array of transmitters and receptors to compute motion direction to their counterparts in the rabbit retina. Voltage clamp recording showed that ON-OFF DSGCs in the mouse retina receive a larger inhibitory input when the stimulus is moving in the null direction and a larger excitatory input when the stimulus is moving in the preferred direction. Finally, intracellular infusion of neurobiotin revealed a bistratified dendritic field with recursive dendrites forming loop-like structures, previously classified as RG(D2) by morphology. Overall, the ON-OFF DSGCs in the mouse retina exhibit almost identical properties to their counterparts in the rabbit retina, indicating that the mechanisms for computing motion direction are conserved from mouse to rabbit, and probably also to higher mammals. This first detailed characterization of ON-OFF DSGCs in the mouse retina provides fundamental information for further study of maturation and regulation of the neuronal circuitry underlying computation of direction.

Nicotinic Acetylcholine Receptor Subtypes Expression during Rat Retina Development and Their Regulation by Visual Experience

By acting through retinal nicotinic acetylcholine receptors (nAChRs), acetylcholine plays an important role in the development of both the retina and central visual pathways. Ligand binding and immunoprecipitation studies with subunit-specific antibodies showed that the expression of alphaBungarotoxin (alphaBgtx) and high-affinity epibatidine (Epi) receptors is regulated developmentally and increases until postnatal day 21 (P21). The increase in Epi receptors is caused by a selective increase in the subtypes containing the alpha2, alpha4, alpha6, beta2, and beta3 subunits. Immunopurification studies revealed three major populations of Epi receptors on P21: alpha6(*) receptors (26%), which contain the alpha6beta3beta2, alpha6alpha4beta3beta2, and alpha6alpha3/alpha2beta3beta2 subtypes; alpha4(non-alpha6)(*) receptors (60%), which contain the alpha2alpha4beta2 and alpha4beta2 subtypes; and (non-alpha4/non-alpha6)(*) receptors (14%), which contain the alpha2beta2/beta4 and alpha3beta2/beta4 subtypes. These three populations can be pharmacologically discriminated using alphaconotoxin MII, which binds the alpha6(*) population with high affinity. In situ hybridization showed that the transcripts for all of the subunits are heterogeneously distributed throughout retinal neurons at P21, with alpha3, alpha6, and beta3 transcripts preferentially concentrated in the ganglion cell layer, alpha5 in the inner nuclear layer, and alpha4 and beta2 distributed rather homogeneously. To investigate whether nAChR expression is affected by visual experience, we also studied dark-reared P21 rats. Visual deprivation had no effect on the expression of alphaBgtx receptors or the developmentally regulated Epi receptors containing the alpha2, alpha6, and/or beta3 subunits but significantly increased the expression of the Epi receptors containing the alpha4 and beta2 subunits. Overall, this study demonstrates that the retina is the rat neural region that expresses the widest array of nAChR subtypes. These receptors have a specific distribution, and their expression is finely regulated during development and by visual experience.

Spontaneous Waves in the Ventricular Zone of Developing Mammalian Retina

Spontaneous rhythmic waves in the developing mammalian retina are thought to propagate among differentiated neurons in the inner retina (IR) and play an important role in activity-dependent visual development. Here we report a new form of rhythmic Ca2+wave in the ventricular zone (VZ) of the developing rabbit retina. Ca2+imaging from two-photon optical sections near the ventricular surface of the whole-mount retina showed rhythmic Ca2+transients propagating laterally as waves. The VZ waves had a distinctively slow Ca2+dynamics (lasting ∼20 s) but shared a similar frequency and propagation speed with the IR waves. Simultaneous Ca2+imaging in VZ and multi-electrode array recording in the ganglion cell layer (GCL) revealed close spatiotemporal correlation between spontaneous VZ and IR waves, suggesting a common source of initiation and/or regulation of the two waves. Pharmacological studies further showed that all drugs that blocked IR waves also blocked VZ waves. However, the muscarinic antagonist atropine selectively blocked VZ but not IR waves at this developmental stage, indicating that IR waves were not dependent on VZ waves, but VZ waves likely relied on the initiation of IR waves. Eliciting IR waves with puffs of nicotinic or non- N-methyl-d-aspartate agonists in GCL produced atropine-sensitive waves in the VZ, demonstrating a unique, retrograde signaling pathway from IR to VZ. Thus differentiated neurons in the IR use spontaneous, rhythmic waves to send both forward signals to the central visual targets and retrograde messages to the developing cells in the VZ.

Development of On and Off retinal pathways and retinogeniculate projections

A fundamental functional feature of the visual system, one recognized in the very first electrophysiological retinal recordings ever made, is that some cells respond to light increments (On cells) while others are activated by light decrements (Off cells). The circuitry underlying On and Off responses in the mature retina have been well-established. In particular, it is known that the dendrites of On- and Off-center retinal ganglion cells (RGCs) stratify in different sublamina of the inner plexiform layer (IPL), where they are innervated by spatially segregated On- and Off-cone bipolar cell inputs. Also, segregated into On and Off sublaminae of the IPL are the processes of starburst amacrine cells. In some species (notably ferret and mink) the retinogeniculate projections are also segregated into sublayers of the dorsal lateral geniculate nucleus (dlgn). The mature organizational features summarized above arise gradually during the course of normal development. Thus, the dendrites of immature RGCs initially ramify throughout the IPL before becoming stratified into On or Off sublamina. This developmental event is regulated by the release of glutamate by developing bipolar cells. Treating the developing retina with the glutamate analog 2-amino-4-phosphonobutyric acid (APB) has been found to prevent the stratification of RGC dendrites. In the mature retina APB binds with mGluR6 receptors expressed by On cone and rod bipolar cells which hyperpolarizes these retinal interneurons and blocks their release of glutamate. The effects of short-term APB treatment are reversible by subsequent normal visual experience, while those of long-term treatment appear to be permanent. At the time that developing RGCs are multistratified they respond to both light onset as well as light offset, suggesting that these neurons are initially functionally innervated by On as well as Off-cone bipolar cells. In the dark-adapted state, On-Off responses of immature multistratified RGCs are completely blocked by APB, while at maturity only On responses are APB-sensitive. This suggests that an APB-resistant Off pathway (possibly from rods to Off-cone bipolar cells) is formed relatively late in development, after RGCs attain their stratified state. In contrast to the activity-regulated refinement of stratified On and Off RGCs, the segregated ingrowth of On- and Off-cone bipolar cells occurs in a highly specific manner, and is not dependent on the presence of either RGCs or cholinergic amacrine cells. It is suggested that the directed ingrowth of bipolar cell axons is guided by molecular cues expressed in the extracellular matrix whose identity is yet to be established. There is also evidence that the later formation of segregated On and Off retinogeniculate projections in the ferret is regulated by an activity-dependent Hebbian type mechanism. Blockade of RGC discharges as well as NMDA receptors in the dlgn perturbs the formation of such segregated inputs. Moreover, On and Off RGCs show distinct correlated firing patterns during the developmental period when the intermingled projections of these neurons are being sorted into sign specific sublayers. Collectively, the available evidence indicates that different developmental mechanisms operate on the different components of retinal and retinogeniculate On and Off pathways to attain the segregated state characteristic of the mature visual system.

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.

Developmental expression of heteromeric nicotinic receptor subtypes in chick retina

Acting through nicotinic acetylcholine receptors (nAChRs), acetylcholine plays an important role in retinal development and the formation of retinal connections to target tissues, but very little is known about the nAChR subtypes expressed in vertebrate retina during neuronal development. We used immunoprecipitation and [3H]epibatidine binding to study the expression of chick retina alpha-bungarotoxin-insensitive heteromeric nAChRs during development and adulthood, and found that it is strictly developmentally regulated, reaching a peak on postnatal day 1. The increase in [3H]epibatidine receptors is caused mainly by an increase in the receptors containing the alpha2, alpha6, beta3, and beta4 subunits. The contribution of beta subunits to [3H]epibatidine receptors significantly changes during development: the beta2 subunit is contained in the majority (84%) of receptors on embryonic day (E) 7 but in only 32% on postnatal day (P) 1, whereas the beta4-containing receptors increase from 22% to 78% during the same period. Using a sequential immunodepletion procedure, we purified the beta2- and beta4-containing subtypes and found that they coassemble with alpha4 and/or alpha3 on E11, and also with the alpha2, alpha6, and beta3 on P1. After the immunodepletion of alpha6-containing receptors, the beta2- and beta4-containing receptors have a very similar pharmacological profile on P1. Parallel immunoprecipitation experiments in other brain areas showed that the developmentally regulated receptors in optic lobe are those containing the alpha2, alpha5, and beta2 subunits and those containing the alpha4 and beta2 subunits, whereas the receptors in forebrain-cerebellum contain the alpha4 and beta2 subunits with or without the alpha5 subunit. These results indicate that there is an increase in receptor heterogeneity and complexity in chick retina during development that is also maintained in adulthood.

The role of nAChR-mediated spontaneous retinal activity in visual system development

In the developing vertebrate retina, nAChR synapses are among the first to appear. This early cholinergic circuitry plays a key role in generating "retinal waves," spontaneously generated waves of action potentials that sweep across the ganglion cell layer. These retinal waves exist for a short period of time during development when several circuits within the visual system are being established. Here I review the cholinergic circuitry of the developing retina and the role these early circuits play in the development of the retina itself and of retinal projections to the lateral geniculate nucleus of the thalamus.

Depletion of cholinergic amacrine cells by a novel immunotoxin does not perturb the formation of segregated on and off cone bipolar cell projections

Cone bipolar cells are the first retinal neurons that respond in a differential manner to light onset and offset. In the mature retina, the terminal arbors of On and Off cone bipolar cells terminate in different sublaminas of the inner plexiform layer (IPL) where they form synapses with the dendrites of On and Off retinal ganglion cells and with the stratified processes of cholinergic amacrine cells. Here we first show that cholinergic processes within the On and Off sublaminas of the IPL are present early in development, being evident in the rat on the day of birth, approximately 10 d before the formation of segregated cone bipolar cell axons. This temporal sequence, as well as our previous finding that the segregation of On and Off cone bipolar cell inputs occurs in the absence of retinal ganglion cells, suggested that cholinergic amacrine cells could provide a scaffold for the subsequent in-growth of bipolar cell axons. To test this hypothesis directly, a new cholinergic cell immunotoxin was constructed by conjugating saporin, the ribosome-inactivating protein toxin, to an antibody against the vesicular acetylcholine transporter. A single intraocular injection of the immunotoxin caused a rapid, complete, and selective loss of cholinergic amacrine cells from the developing rat retina. On and Off cone bipolar cells were visualized using an antibody against recoverin, the calcium-binding protein that labels the soma and processes of these interneurons. After complete depletion of cholinergic amacrine cells, cone bipolar cell axon terminals still formed their two characteristic strata within the IPL. These findings demonstrate that the presence of cholinergic amacrine cells is not required for the segregation of recoverin-positive On and Off cone bipolar cell projections.

Functional nicotinic acetylcholine receptor expression in stem and progenitor cells of the early embryonic mouse cerebral cortex

The adult cerebral cortex contains nicotinic acetylcholine (ACh) receptors vital to cortical function. However, little is known about the assembly of embryonic nicotinic receptor subunits into functional receptors or whether they play an active role in cortical development. We now report evidence of functional nicotinic acetylcholine receptor channels in fetal mouse cerebral cortex as early as embryonic day 10 (E10), when the cortex consists of dividing stem and progenitor cells. Patch-clamp electrophysiological measurements indicate that nicotine and ACh evoke sizable inward currents characteristic of nicotinic receptors, that are strongly rectifying with a reversal potential near 0 mV. Three different nicotinic agonists, ACh, nicotine, and dimethylphenylpiperazinium, evoked cytosolic Ca(2+) signals. Agonist-evoked Ca(2+) signals and electrophysiological responses were found in greater than 70% of all E10-E11 cells tested and were blocked by nicotinic receptor antagonists. The Ca(2+) response to nicotinic agonists was markedly prolonged in cells from early embryonic stages relative to later stages of development. alpha3, alpha4, and alpha7 receptor subunit proteins were detected immunocytochemically in cortical cells from E10 to birth. The incidence of each subunit declined with embryonic age, suggesting a role in early development. We discuss the possible function of nicotinic receptors in early cortical development and their role as a target for nicotine in the developmental pathologies associated with the fetal tobacco syndrome.