Vasoactive intestinal peptide alters membrane potential and cyclic nucleotide levels in retinal horizontal cells

Retinal VIP-amacrine cells: their development, structure, and function

Amacrine cells (ACs) are the most structurally and functionally diverse neuron type in the retina. Different ACs have distinct functions, such as neuropeptide secretion and inhibitory connection. Vasoactive intestinal peptide (VIP) -ergic -ACs are retina gamma-aminobutyric acid (GABA) -ergic -ACs that were discovered long ago. They secrete VIP and form connections with bipolar cells (BCs), other ACs, and retinal ganglion cells (RGCs). They have a specific structure, density, distribution, and function. They play an important role in myopia, light stimulated responses, retinal vascular disease and other ocular diseases. Their significance in the study of refractive development and disease is increasing daily. However, a systematic review of the structure and function of retinal VIP-ACs is lacking. We discussed the detailed characteristics of VIP-ACs from every aspect across species and providing systematic knowledge base for future studies. Our review led to the main conclusion that retinal VIP-ACs develop early, and although their morphology and distribution across species are not the same, they have similar functions in a wide range of ocular diseases based on their function of secreting neuropeptides and forming inhibitory connections with other cells.

Neuromodulation: Actions of Dopamine, Retinoic Acid, Nitric Oxide, and Other Substances on Retinal Horizontal Cells

Whereas excitation and inhibition of neurons are well understood, it is clear that neuromodulatory influences on neurons and their synapses play a major role in shaping neural activity in the brain. Memory and learning, emotional and other complex behaviors, as well as cognitive disorders have all been related to neuromodulatory mechanisms. A number of neuroactive substances including monoamines such as dopamine and neuropeptides have been shown to act as neuromodulators, but other substances thought to play very different roles in the body and brain act as neuromodulators, such as retinoic acid. We still understand little about how neuromodulatory substances exert their effects, and the present review focuses on how two such substances, dopamine and retinoic acid, exert their effects. The emphasis is on the underlying neuromodulatory mechanisms down to the molecular level that allow the second order bipolar cells and the output neurons of the retina, the ganglion cells, to respond to different environmental (ie lighting) conditions. The modulation described affects a simple circuit in the outer retina, involves several neuroactive substances and is surprisingly complex and not fully understood.

A Life in Vision

I was drawn into research in George Wald's laboratory at Harvard, where as an undergraduate and graduate student, I studied vitamin A deficiency and dark adaptation. A chance observation while an assistant professor at Harvard led to the major research of my career-to understand the functional organization of vertebrate retinas. I started with a retinal circuit analysis of the primate retina with Brian Boycott and intracellular retinal cell recordings in mudpuppies with Frank Werblin. Subsequent pharmacology studies with Berndt Ehinger primarily with fish focused on dopamine and neuromodulation. Using zebrafish, we studied retinal development, neuronal connectivity, and the effects of genetic mutations on retinal structure and function. Now semi-retired, I have returned to primate retinal circuitry, undertaking a connectomic analysis of the human fovea in Jeffrey Lichtman's laboratory.

Cellular localization and function of DARPP-32 in the rodent retina

The goal of the present study was to elucidate the role of DARPP-32 (dopamine- and cyclic adenosine 3'-5'-monophosphate-regulated phosphoprotein, 32 kDa) in retinal function. We examined mouse and rat retinas for the presence of DARPP-32 by immunocytochemistry. In both rodent retinas DARPP-32 immunoreactivity was localized to horizontal and AII amacrine neurons and to the Mueller glial cells, using immuno-double labelling. Additional unidentified neurons in the amacrine cell layer also showed DARPP-32 immunoreactivity. Using mice entrained to a 12-12 h light-dark cycle, we found that exposure to light presented during the dark phase significantly enhanced phosphorylation of DARPP-32 at threonine (Thr) 34 and phosphorylation of the ionotropic glutamate receptor subunit GluR1 at serine (Ser) 845, as measured by immunoblots. However, light also increased Ser 845-GluR1 phosphorylation in DARPP-32-knockout mice. When a dopamine D1 receptor antagonist was injected into the eye prior to light exposure, phosphorylation of both Thr 34-DARPP-32 and Ser 845-GluR1 was significantly reduced. These data indicate that DARPP-32 participates in dopamine-mediated modifications of retinal function. We also tested for a possible circadian rhythm of Thr 34- and Thr 75-DARPP-32 and Ser 845-GluR1 expression. No significant circadian rhythm of either DARPP-32 or GluR1 phosphorylation was found.

Retinal dopamine depletion in young quail mimics some of the effects of ageing on visual function

The hypothesis that retinal dopaminergic (DA) neurones are involved in the visual functions of interest was tested. The retinal DA in young quail was partially depleted by intravitreal injection of 6-hydroxydopamine (6-OHDA). It was found that the refractive state of 6-OHDA-treated birds became more myopic than normal (untreated) young, whereas the pupil diameter was not affected. The contrast sensitivity of 6-OHDA treated quail was significantly lowered (two to three times) at all spatial frequencies studied (0.25-5 c/d), and the peak latency of pattern electro-retinogram (PERG) response was prolonged by 3-4 msec (9%). Furthermore, the visual acuity and maximal amplitude of PERG response of the 6-OHDA-treated young quail were lower than those of normals. From histochemical studies, it was revealed that the morphology of the DA cells of 6-OHDA-treated young appeared similar to those of the old quail; the DA cells of 6-OHDA-treated retinae were less fluorescent and 2.5-5 times less numerous than respective controls. Combining the PERG and the morphological results, it would seem that the retinal DA plays an important role in the visual functions studied, and that loss of retinal DA could underlie some of the visual changes which occur during ageing.

Quantitative analysis of the expression of a VIP transgene

We have analyzed the expression of a transgene bearing 2 kilobases of the 5' flanking region of the human vasoactive intestinal polypeptide (VIP) gene coupled to beta-galactosidase. Expression was assayed by beta-galactosidase histochemistry and by mRNA quantitation using polymerase chain reaction (PCR)-mediated amplification; we compared beta-galactosidase activity against both transgene and endogenous VIP mRNA levels. We found that the human 5' flanking sequence in this construct is able to direct tissue-specific expression of beta-galactosidase similar to the pattern for endogenous VIP. However, the transgene is also expressed in smooth muscle and Schwann cells, where VIP mRNA is rare. In various tissues where the transgene and endogenous gene are both active, the ratio between their message levels differs dramatically--transgene mRNA is more abundant where VIP is relatively scarce, but is much less abundant than the endogenous message at sites where VIP mRNA is most concentrated. These results suggest that sequence elements that may restrict VIP transcription or cause tissue-specific VIP mRNA accumulation are missing from the transgene. In the testis there is a high level of transgene message but no significant beta-galactosidase activity; this discrepancy is caused by transcription from a cryptic promoter within the beta-galactosidase sequence.

VIP as a cell-growth and differentiation neuromodulator role in neurodevelopment

In addition to its commonly recognized status as a neuromodulator of virtually all vital functions, including neurobiological, the neuropeptide VIP plays a role in the control of cell growth and differentiation and of neuronal survival. Through these actions, VIP, whose impact appears early in ontogeny, may possess developmental functions. VIP can be stimulatory or inhibitory on cell growth in function of the model considered. The growth regulatory actions of VIP, which are often independent of cAMP, are most likely significant when mitogenic or trophic factors, eventually released by nontarget cells, are simultaneously present in the extracellular medium. The intracellular mechanisms that mediate these actions of VIP may involve different transduction cascades triggered by subsets of VIP binding sites that may coexist in the same tissue.

Vasoactive intestinal polypeptide modulates GABAA receptor function through activation of cyclic AMP

Vasoactive intestinal polypeptide (VIP) has been shown to potentiate current responses elicited by activation of the GABAA receptor (IGABA) in freshly dissociated ganglion cells of the rat retina. Here we tested the hypothesis that this heteroreceptor cross talk is mediated by an intracellular cascade of events that includes the sequential activation of a stimulatory guanine nucleotide binding (Gs) protein and adenylate cyclase, the subsequent increase in levels of cyclic AMP and, finally, the action of the cyclic AMP-dependent protein kinase (PKA). Intracellular dialysis of freshly dissociated ganglion cells with GTP gamma s irreversibly potentiated IGABA, while GDP beta s either decreased or had no effect on IGABA. Additionally, GDP beta s blocked the potentiation of IGABA by VIP. Cholera toxin rendered VIP ineffective in potentiating IGABA, while pertussis toxin had no effect on the VIP-induced potentiation of IGABA. Extracellular application of either forskolin or 8-bromo-cyclic AMP potentiated IGABA, as did the introduction of cyclic AMP directly into the intracellular compartment through the recording pipet. Intracellular application of cyclic AMP-dependent protein kinase (PKA) potentiated IGABA, while a PKA inhibitor blocked the potentiating effect of VIP. These results lead us to conclude that activation of a cyclic AMP-dependent second-messenger system mediates the modulation of GABAA receptor function by VIP in retinal ganglion cells.

Corticotropin-releasing hormone stimulates adenylyl cyclase activity in the retinas of different animal species

In the present study we investigated the presence of corticotropin-releasing hormone (CRH)-stimulated adenylyl cyclase activity in the retinas of different animal species. CRH significantly stimulated adenylyl cyclase activity in homogenates of calf, pig, rabbit and guinea pig retinas. The stimulatory effects were concentration-dependent with half-maximal responses occurring at 20-30 nM CRH. The enzyme activities increased by 37-80% at the maximal concentration of CRH (1 microM). On the other hand, adenylyl cyclase activities of chicken and pigeon retinas were poorly stimulated by CRH. In calf, pig and rabbit retinas, the CRH effect was completely antagonized by the CRH receptor antagonist alpha-helical CRH 9-41 and required the presence of GTP. The stimulatory response elicited by CRH was also found to be not additive with that produced by either vasoactive intestinal peptide or dopamine. These results provide evidence for the presence in retinas of different animal species of functional CRH receptors, an important criterion for the classification of CRH as a retinal neurotransmitter.

Substance P modulates calcium current in retinal bipolar neurons

Retinal bipolar cells are non-spiking interneurons that relay information from photoreceptors to amacrine and ganglion cells. In turn, bipolar cells receive extensive synaptic feedback from amacrine cells, some of which contain neuropeptides, including substance P. We have examined the effect of substance P on single bipolar neurons isolated from goldfish retina and find that substance P (0.1-1 nM) produced a voltage-dependent inhibition of calcium current in these cells. The inhibition was strongest at negative potentials, with the peak suppression occurring at -20 to -30 mV; at potentials positive to 0 mV, there was little effect on calcium current. Thus, the net effect was to shift the voltage range of activation of calcium current toward more positive potentials. The inhibition of calcium current by substance P required GTP in the patch pipette and was blocked by internal GDP-beta-S. Similar effects on calcium current were observed with somatostatin and metenkephalin, which are also found in amacrine cells.

Peptidergic neurons of teleost retinas

In retinas of teleost fish, neuropeptides typically have subtle, modulatory actions. The peptide effects typically have long latencies and durations, and, in some instances, they are known to be mediated by second messengers. Peptidergic neurons in teleost retinas have certain morphological features in common that are consistent with their function. Most peptidergic neurons are stratified amacrine cells with long, varicose processes; the processes of peptidergic centrifugal axons are also narrowly stratified and ramify extensively in the retina. The peptidergic amacrine cells are relatively infrequent, and, likewise, the centrifugal axons originate from a small number of perikarya in the brain. Cells that are so sparsely distributed and whose processes overlap so extensively are better-suited for modulation than for conveying detailed representations of visual space.

Morphology and mosaics of VIP-like immunoreactive neurons in the retina of the rhesus monkey

Vasoactive intestinal peptide (VIP) is a 28-amino acid peptide that has been demonstrated to reside in cells ( = VIP+ cells) of the retinae of various vertebrate species. In an attempt to study the morphology and distribution of VIP+ cells in the retina of the rhesus monkey in more detail, we subjected VIP+ cells observed in cryostat sections or wholemounts rhesus monkey retinae to a quantitative analysis. VIP+ cells were found to reside in the innermost row of the inner nuclear layer (INL) and in the ganglion cell layer (GCL) in similar numbers (estimate: 50 cells/mm2 at 6-10 mm eccentricity each) and only on rare occasions (12% of all VIP+ cells) in varying positions within the inner plexiform layer (IPL). Somata of VIP+ cells were circular and had a mean diameter of 9.1 microns. They gave rise to 1-3 main dendrites, which were usually oriented toward the IPL. Main dendrites ramified widely into thin fibers (dendritic field diameter less than = 1 mm), carrying varicose swellings. The fibers that contributed to one and the same plexus of VIP+ fibers preferred the middle third of the IPL, independent of the positions of the parent somata. A quantitative analysis of nearest-neighbour distances in the retinal wholemount preparation suggested that VIP+ cells in the GCL and in the INL might be distributed according to 2 independent mosaics. A comparison with Golgi-stained material leads to the tentative equation of VIP+ cells with the "spiny" A12 amacrine cell of Mariani ('90). Whereas the low density and large dendritic field size of VIP+ cells might suggest a more widespread function, the varicose dendritic morphology seems to be more compatible with functionally independent dendritic subunits mediating localized effects.

VIP-mediated increase in cAMP prevents tetrodotoxin-induced retinal ganglion cell death in vitro

Afferent influences on natural cell death were modeled in retinal cultures derived from neonatal rats. Tetrodotoxin (TTX) blockade of electrical activity produced a significant reduction in surviving retinal ganglion cell (RGC) neurons during a critical period of development, similar in magnitude to the reduction observed during natural cell death in the intact retina at a similar developmental stage. The addition of vasoactive intestinal peptide (VIP) protected the RGCs from the lethal action of TTX. This effect was specific, since the related peptides PHI-27 and secretin produced no significant increase in RGC survival. Radioimmunoassay of cyclic nucleotides showed that TTX decreased culture levels of cAMP and that this trend was reversed by VIP. Decreases in RGC survival associated with TTX electrical blockade were prevented by 8-bromo:cAMP or forskolin. Furthermore, VIP10-28, the C-terminal fragment that inhibits VIP stimulation of adenylate cyclase, reduced the number of surviving RGCs. Thus, our results suggest that VIP, acting by increasing cAMP, has a neurotrophic effect on electrically blocked RGCs and may be an endogenous factor modulating normal cell death in the retina.

Characterization of vasoactive intestinal peptide receptors in retina

Vasoactive intestinal peptide (VIP) is a neuropeptide having a wide range of effects on a large number of tissues. To gain insight into the role VIP plays in retinal function, VIP receptors in bovine retinal membranes were analyzed in competition binding assays and by affinity labeling studies and compared to VIP receptors in rat liver membranes. In both membrane preparations, high affinity VIP binding sites (KD approximately 1 nM) were detected. Secretin and glucagon, each having close structural homology to VIP, were found to have negligible effects on [125I]VIP binding in retina. In contrast, secretin (KD = 70 nM) was modestly effective in inhibiting [125I]VIP binding to rat liver membranes. Affinity labeling analysis revealed a VIP binding site of 59 kDa in both bovine retinal and rat liver membranes. Digestion of affinity-labeled receptor proteins with endoglycosidase F generated final cleavage products of approx. 45 kDa for both receptors. These results indicate that the retina expresses a high affinity, highly selective VIP receptor thereby supporting a specific function for VIP in this tissue.

VIP: molecular biology and neurobiological function

In the mammalian brain, a major regulatory peptide is vasoactive intestinal peptide (VIP). This 28 amino acid peptide, originally isolated from the porcine duodenum, was later found in the central and peripheral nervous systems and in endocrine cells, where it exhibits neurotransmitter and hormonal roles. Increasing evidence points to VIP's importance as a mediator or a modulator of several basic functions. Thus, VIP is a major factor in brain activity, neuroendocrine functions, cardiac activity, respiration, digestion, and sexual potency. In view of this peptide's importance, the mechanisms controlling its production and the pathways regulating its functions have been reviewed. VIP is a member of a peptide family, including peptides such as glucagon, secretin, and growth hormone releasing hormone. These peptides may have evolved by exon duplication coupled with gene duplication. The human VIP gene contains seven exons, each encoding a distinct functional domain on the protein precursor or the mRNA. VIP gene transcripts are mainly found in neurons or neuron-related cells. VIP gene expression is regulated by neuronal and endocrine signals that contribute to its developmental control. VIP exerts its function via receptor-mediated systems, activating signal transduction pathways, including cAMP. It can act as a neurotransmitter, neuromodulator, and a secretagog. As a growth and developmental regulator, VIP may have a crucial effect as a neuronal survival factor. We shall proceed from the gene to its multiple functions.

Peptide histidine isoleucine (PHI) immunoreactivity in the rat retina: identification and characterisation by radioimmunoassay, immunohistochemistry and high-performance liquid chromatography

The presence of peptide histidine isoleucine immunoreactivity (PHI-IR) in the retinae of male Wistar rats has been demonstrated using a specific radioimmunoassay (RIA), immunohistochemistry (IHC) and high performance liquid chromatography (HPLC). PHI levels (mean +/- S.E.M.) of 16.68 +/- 1.61 ng/g wet weight of normal retinal tissue were measured by RIA. IHC showed PHI-IR to be localized to a population of amacrine cells distributed along the proximal aspect of the inner nuclear layer and to processes in laminae I and IV of the inner plexiform layer. PHI-IR in retinal extracts was shown to coelute with a synthetic porcine PHI standard on a reverse phase HPLC system. Additionally levels of PHI-IR were significantly increased (P less than 0.001) with light adaptation.

Vasoactive intestinal peptide-like immunoreactivity in rodent taste cells

The neuropeptide vasoactive intestinal peptide was localized to taste buds of the posterior tongue regions of hamsters and rats by immunocytochemical techniques. Tissue sections, taken from foliate and circumvallate papillae, generally revealed taste buds in which all cells were immunoreactive; however, occasionally some taste buds were found to contain highly reactive individual cells adjacent to non-reactive cells. Additionally, some non-reactive taste buds were observed. Taste buds that displayed vasoactive intestinal peptide-like immunoreactivity usually had a tendency for much darker staining at the apical ends of the cells than the basal ends, suggesting a polar cytoplasmic distribution of the peptide. The multi-functional roles of vasoactive intestinal peptide in other physiological systems combined with both its cytoplasmic localization in taste cells and the known histochemistry/ultrastructure of taste cells raises interesting speculations of this peptide's function in gustation that include secretion, stimulation of a second messenger system, and neuromodulation.

Binding pattern of alpha-bungarotoxin on horizontal cells of a marine teleost retina

A conjugate of alpha-bungarotoxin and a fluorescent marker (fluorescein isothiocyanate) has been used to localize "nicotinic" acetylcholine receptors on neurons in the outer plexiform layer of marine teleost retina. Toxin binding was confined to bipolar cell dendrites and to intermediate horizontal cells. The arrangement of labeled horizontal cells appears irregular in the whole retina, with a peak density in the ventral and dorsal quandrants. Alpha-bungarotoxin receptors on horizontal cells differ from those on bipolar cells and from those on dendrites in the inner plexiform layer in their sensitivity to agonists and antagonists such as d-tubocurarine and nicotine. They constitute a different type of "nicotinic" receptor that probably has a different function.

The electrophysiological effects of vasoactive intestinal polypeptide in the guinea-pig inferior mesenteric ganglion

1. The effects of vasoactive intestinal polypeptide (VIP) on the inferior mesenteric ganglion of the guinea-pig were studied in vitro. 2. In 67% of the neurones tested, application of VIP (1-7.5 X 10(-5) M) by pressure ejection caused a depolarization of the membrane potential which averaged 8.6 +/- 0.4 mV. 3. In 52% of the cells that were responsive to VIP, the membrane depolarization was accompanied by a decrease in membrane input resistance. In another 48% of the cells tested, there was an increase in membrane input resistance. 4. Membrane depolarization caused by VIP enhanced the excitability of post-ganglionic neurones and converted subthreshold electrotonic and subthreshold synaptic potentials to action potentials. 5. The effects of VIP persisted during nicotinic and muscarinic synaptic blockade. The effects of VIP also persisted in a low-Ca2+, high-Mg2+ solution. Thus, the site of action of VIP was on the postsynaptic membrane. 6. Electrical stimulation of the lumbar colonic nerves evoked a slow noncholinergic depolarization of the membrane potential. 7. VIP appeared to be one of the transmitters involved in the electrically evoked e.p.s.p. because both prior desensitization with exogenous VIP and VIP antiserum reduced the amplitude of the slow, non-cholinergic e.p.s.p. 8. Radial distension of a segment of colon attached to the inferior mesenteric ganglion (i.m.g.) evoked a non-cholinergic depolarization of the membrane potential in neurones in the i.m.g. 9. The distension-induced non-cholinergic depolarization was reduced by VIP antiserum. 10. The data support the hypothesis that a population of the mechanosensory afferent nerves running between the colon and the i.m.g. utilize VIP or a VIP-like peptide as a transmitter to modulate reflex activity between the colon and the i.m.g.

Epinephrine regulates cholinergic transmission mediated by rat retinal neurons in culture

The purpose of this study was to investigate the effects of epinephrine on neurotransmission mediated by cholinergic neurons derived from the rat retina. We used a culture system in which striated muscle cells served as postsynaptic targets for cholinergic neurons of the embryonic retina. This culture system permitted the physiological monitoring of acetylcholine released by retinal neurons. Here, we report that epinephrine facilitates evoked transmission across retina-muscle synapses. This facilitation of cholinergic transmission by epinephrine is reversible, can be mimicked by isoproterenol (a beta adrenoceptor agonist) and blocked by propranolol (a beta adrenoceptor antagonist). Neither the alpha-2 adrenoceptor blocker, yohimbine, nor the dopamine receptor antagonist, haloperidol, blocked this effect of epinephrine. Since epinephrine was found not to influence the membrane potential of muscle cells nor the responses of myotubes to acetylcholine, epinephrine appeared to have mediated its facilitatory effect on cholinergic transmission by affecting retinal cells. Because previous findings indicated that adenosine 3',5'-cyclic monophosphate may be involved in the modulation of transmission at retina-muscle synapses, the effect of epinephrine on adenosine 3',5'-cyclic monophosphate levels was investigated. Our biochemical studies demonstrated that epinephrine could increase adenosine 3',5'-cyclic monophosphate levels markedly in cultured retinal cells. The accumulation of adenosine 3',5'-cyclic monophosphate induced by epinephrine could be blocked by propranolol, but not by yohimbine nor haloperidol. Taken together, the results indicate that the facilitatory effect of epinephrine is mediated via a beta adrenoceptor and may involve an increase in adenosine 3',5'-cyclic monophosphate levels. Our findings are in agreement with the hypothesis that epinephrine may be a modulatory neurotransmitter in the rat retina.

A vasoactive intestinal polypeptide system in retinal cell cultures: immunocytochemistry and physiology

The purpose of this study was to search for vasoactive intestinal polypeptide (VIP)-like immunoreactivity and VIP-mediated effects in cultures containing cells from the mammalian retina. VIP-like immunoreactivity was detected by indirect immunocytochemistry within 6 days after plating dissociated retinal cells from embryonic day-19 rats. In electrophysiological experiments, VIP was found to facilitate evoked transmission at cholinergic synapses formed by retinal neurons in culture.

The interplexiform-horizontal cell system of the fish retina: effects of dopamine, light stimulation and time in the dark

Interplexiform cells contact cone horizontal cells in the fish retina and probably release dopamine at synaptic sites. The effects of dopamine, certain related compounds, and light and dark régimes were tested on the intracellularly recorded activity of horizontal cells in the superfused carp retina to elucidate the functional role of the interplexiform cell. Dopamine application onto retinae kept in the dark for 30-40 min increased the size of the responses of cone horizontal cells to small-spot stimuli but decreased response size to large- and full-field stimuli. Dopamine also altered the response waveform of these cells; the transient at response onset increased in size and the depolarizing afterpotential decreased in size. Haloperidol, a dopamine antagonist, blocked these effects of dopamine application. Forskolin, an adenylate cyclase activator, increased the size of the responses of the cells to small-spot stimuli. Superfusion of vasoactive intestinal peptide did not produce any effects on horizontal cells. The results indicate that dopamine produces multiple physiological effects on cone horizontal cells by activation of an intracellular enzyme system. We propose that some of these effects are probably related to an uncoupling of the gap junctions between horizontal cells, but that other effects are most likely not explained on this basis and reflect additional changes induced in the cells by dopamine. After prolonged periods of darkness (100-110 min), compared with short periods (30-40 min), L-type cone horizontal cells exhibited responses similar to those obtained during dopamine application. Dim flickering or continuous light backgrounds did not mimic the effects of dopamine. Although dopamine application onto retinae after short-term darkness produced dramatic effects on L-type cone horizontal cells, little or no effect was observed when dopamine was applied while the effects of a previous dopamine application were still present or after prolonged darkness. These results suggest that interplexiform cells may release dopamine after prolonged darkness and that interplexiform cells may regulate lateral inhibitory effects mediated by L-type cone horizontal cells as a function of time in the dark.

Cyclic nucleotide potentiation of muscarinic responses in neuroblastoma cells

The role of cyclic nucleotides in modulating acetylcholine-induced and dopamine-induced responses was examined with cultured neuroblastoma N1E-115 cells by means of intracellular recording techniques. Acetylcholine-induced muscarinic hyperpolarization and muscarinic depolarization were potentiated by bath application of a dibutyryl analog of adenosine 3',5'-phosphate (cyclic AMP) or phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone. Dibutyryl cyclic AMP did not affect the resting membrane potential and membrane resistance. Acetylcholine-induced nicotinic depolarization was unaffected by dibutyryl cyclic AMP or phosphodiesterase inhibitors. Intracellular pressure injection of cyclic AMP caused a potentiation of muscarinic hyperpolarization and muscarinic depolarization without marked change in the resting membrane potential. Nicotinic depolarization and dopamine depolarization were not affected by cyclic AMP injection. Among the possible metabolites of cyclic AMP, injection of adenosine potentiated muscarinic hyperpolarization, but did not change nicotinic depolarization and dopamine depolarization. Injection of guanosine 3',5'-phosphate (cyclic GMP) potentiated muscarinic hyperpolarization and muscarinic depolarization without effect on nicotinic depolarization and dopamine depolarization. We conclude that cyclic AMP and cyclic GMP enhance muscarinic responses in neuroblastoma cells. It is suggested that synaptic transmission in the nervous system may be modulated postsynaptically by changes in intracellular cyclic nucleotide levels.

Septal-preoptic unit responses to microelectrophoresis of cholecystokinin, gastrin, vasoactive intestinal peptide and secretin in the rat

Effects of microelectrophoretic application of cholecystokinin-8, gastrin-17, vasoactive intestinal peptide and secretin on activities of septal-preoptic neurons were examined in ovariectomized rats. All of the 4 peptides produced either excitatory or inhibitory responses in some neurons tested. No consistent relationship was observed between effects of different peptides, even between the peptides of the same family. These results provide electrophysiologic evidence for the action in the septal-preoptic region of these peptides, and suggest that there may be specific interneurons sensitive to a corresponding peptide with some overlapping.

Neuropeptide-like immunoreactive cells in the retina of the larval tiger salamander: attention to the symmetry of dendritic projections

Light microscopic immunocytochemistry was used to study the morphology of cells that showed immunoreactivity (IR) to antisera against substance P (SP), glucagon (GLU), met enkephalin (ENK), and somatostatin (SS) in the retina of the larval tiger salamander. Both vertical sections and retinal whole mounts were studied. All four antisera labeled amacrine cells in the inner nuclear layer and cells in the ganglion cell layer (GCL). GLU-IR cells had processes stratified throughout the inner plexiform layer (IPL), whereas the other three types had bistratified projections in laminae 1 and 5, with lamina 5 being broader and more dense. Two types of SS-IR amacrine cell were observed. As seen in retinal whole mount, most GLU-IR and ENK-IR amacrine cells had processes that were symmetrically distributed about the soma, whereas processes of SS- and SP-IR amacrine cells were markedly asymmetrical. The dendritic fields of SP-IR amacrine cells were selectively oriented toward the periphery of the retina in the nasal, temporal, and dorsal areas. Immunoreactive cells in the GCL had fine projections into the IPL and in addition gave rise to two large oriented processes proximal to the soma that projected in opposite directions for 100-200 microns. The oriented processes often showed further branching; they appeared to be along the radiated lines from the optic disc but did not enter it. The implications of the selectively oriented processes of SP-IR amacrine cells and cells in the GCL are discussed.

Vasoactive intestinal polypeptide depolarizations in cat bladder parasympathetic ganglia

The effect of vasoactive intestinal polypeptide (VIP) on the neuronal membranes of isolated cat vesical pelvic ganglia and its underlying ionic mechanism were examined by means of intracellular recording and voltage-clamp techniques. Application of VIP (0.05-50 microM) to the neurones by pressure 'puff' ejection through a micropipette placed close to the neurones produced a depolarizing response (2-15 mV) in 83% of neurones tested; this effect was concentration dependent. The VIP-induced depolarization frequently evoked spontaneous action potentials in quiescent neurones and increased the frequency of action potentials in spontaneously firing neurones. The VIP depolarization was not blocked in a Ca2+-free, high-Mg2+ solution or in a solution containing hexamethonium (1 mM) and atropine (1 microM). Tetrodotoxin (TTX; 1 microM) also did not affect the VIP depolarization. The VIP depolarization was associated with an increase in membrane resistance and the slope of a current-voltage relation (I-V curve) was increased by VIP. Conditioning hyperpolarization and depolarization of the membrane increased and decreased the amplitude of the VIP depolarization, respectively. The VIP depolarization reversed polarity around--100 mV. The reversal potential shifted about 20 mV to a more positive level in a high-K+ (10 mM) solution in accord with the Nernst equation. Substituting Cl- with isethionate in the superfusate did not affect the reversal potential of the VIP depolarization. Closure of M-channels does not underlie VIP action since the VIP depolarization was enhanced by muscarine (10 microM) and unchanged in the presence of Ba (5 mM), or intracellular or extracellular Cs+, conditions known to block the M-channels (Adams, Brown & Constanti, 1982a, b). Tetraethylammonium (TEA; 20 mM) also did not affect the VIP depolarization. Voltage-clamp analyses showed that VIP applied by pressure ejection produced an inward current of 80-110 pA associated with a decrease in membrane conductance (from 2.8 to 3.5 nS) at a holding potential of--60 mV. VIP inward current was diminished by either repetitive or continuous application of VIP (5 microM) suggesting desensitization of the VIP receptor. It is concluded that VIP produces a depolarization in neurones of bladder parasympathetic ganglia by decreasing a K+ conductance, the pharmacological characteristics of which are unlike previously described K+ conductance mechanisms.

Adenosine-mediated regulation of cyclic AMP levels in isolated incubated retinas

The effect of adenosine on the modulation of retinal cAMP levels was assessed in several mammalian species including mouse, rat, guinea pig and rabbit. Adenosine had no effect when added to incubated rat, mouse and guinea pig retinas. However, levels of cAMP were elevated in dose-dependent manner by adenosine in both light- and dark-adapted incubated rabbit retinas. Isobutylmethylxanthine (IBMX) blocked the elevation elicited by adenosine. Norepinephrine and dopamine also elevated cAMP in incubated rabbit retinas and these effects were not blocked by IBMX. The elevations of cyclic AMP levels produced by adenosine were additive with the effects of dopamine or norepinephrine. These results indicate that an adenosine-sensitive cAMP system exists in rabbit retina, and it is probably distinct from the dopamine and norepinephrine regulated cyclic AMP systems.

White perch horizontal cells in culture: methods, morphology and process growth

Four types of horizontal cells are recognized in primary cultures of the adult white perch retina. Immediately after isolation, all horizontal cell types show fine terminal processes. However, these fine processes disappear over the next 12-24 h and the cells typically round up partially. After 2-4 days in culture, new processes appear on the horizontal cells, and these usually grow substantially with time. By 14-21 days, processes several hundred micrometers in length are observed in the retinal culture dishes. Processes from different cells intertwine and contacts that appear synaptic in nature are observed.

Dopamine regulates synaptic transmission mediated by cholinergic neurons of the rat retina

The purpose of this study was to investigate the effect of dopamine on the function of synapses formed by cholinergic neurons derived from the rat retina. We used an experimental culture system in which rat striated muscle cells served as postsynaptic targets for cholinergic neurons of the retina. This culture system permitted the physiological monitoring of acetylcholine release at synapses formed by retinal neurons. We found that dopamine could facilitate evoked transmission at retina-muscle synapses. This facilitation by dopamine was reversible and could be blocked by haloperidol, a dopamine receptor antagonist. The adenosine 3':5'-phosphate analogue, 8-bromoadenosine 3':5'-phosphate, mimicked the facilitating effect of dopamine. In addition, dopamine elevated markedly the levels of adenosine 3':5'-phosphate in cultures of rat retinal cells. The results suggest that dopamine can regulate transmission through retinal neurons. Our findings support the hypothesis that a dopamine-induced facilitation of stimulus-evoked transmission involves the activation of dopamine receptors and the intracellular accumulation of adenosine 3':5'-phosphate.

The release of gamma-aminobutyric acid from horizontal cells of the goldfish (Carassius auratus) retina

Isolated horizontal cells from goldfish retinas were prepared by enzymatic dissociation using papain and separated from other cells by velocity sedimentation. In the intact retina, H1 horizontal cells possess a high-affinity mechanism for accumulating gamma-aminobutyric acid (GABA). This property is retained in isolated cells, which also release the accumulated GABA in response to depolarization by elevated external K+. L-Glutamic acid and its analogues are highly effective at micromolar concentrations in eliciting the release of preloaded GABA from isolated cells. At saturating concentrations, L-aspartic acid stimulates about one-third as much release as L-glutamic acid. In contrast, the D-isomers of glutamate and aspartate are ineffective. In the intact retina, micromolar concentrations of L-glutamic acid analogues are also capable of eliciting GABA release from H1 horizontal cells. Release of the accumulated GABA from isolated H1 cells is largely independent of external Ca2+ concentrations. In the intact retina, H1 horizontal cells also possess a K+-stimulated GABA release mechanism that is independent of the Ca2+ concentrations in the medium. In addition, there appears to be a small but significant amount of [3H]GABA release that may be Ca2+ dependent. Under our conditions, [3H]GABA release from isolated cells is unaffected by external Na+ concentrations between 20 and 120 mM. However, concentrations of 10 mM or less significantly diminishes this release, with 70% curtailed in Na+-free solutions. Our results, together with morphological observations by a number of other investigators, suggest that there may be two distinct mechanisms for GABA release from goldfish H1 horizontal cells: one being a conventional vesicular mechanism which is Ca2+ dependent, while the other is Na+ driven and Ca2+ independent. H1 horizontal cells in the intact goldfish retina release the accumulated GABA in response to brief incubations in darkness, which is known to be the natural stimulus that depolarizes these neurones.

Inositol incorporation into phosphoinositides in retinal horizontal cells of Xenopus laevis: enhancement by acetylcholine, inhibition by glycine

The absorption of light by photoreceptor cells leads to an increased incorporation of [2-3H]inositol into phosphoinositides of horizontal cells in the retina of Xenopus laevis in vitro. We have identified several retinal neurotransmitters that are involved in regulating this response. Incubation with glycine, the neurotransmitter of an interplexiform cell that has direct synaptic input onto horizontal cells, abolishes the light effect. This inhibition is reversed by preincubation with strychnine. Acetylcholine added to the culture medium enhances the incorporation of [2-3H]inositol into phosphoinositides in horizontal cells when retinas are incubated in the dark. This effect is inhibited by preincubation with atropine. However, atropine alone does not inhibit the light-enhanced incorporation of [2-3H]inositol into phosphoinositides in the retina. gamma-Aminobutyric acid, the neurotransmitter of retinal horizontal cells in X. laevis, as well as dopamine and norepinephrine, have no effect on the incorporation of [2-3H]inositol into phosphoinositides. These studies demonstrate that the light-enhanced incorporation of [2-3H]inositol into phosphoinositides of retinal horizontal cells is regulated by specific neurotransmitters, and that there are probably several synaptic inputs into horizontal cells which control this process.

Vasoactive intestinal polypeptide (VIP): current status

Research on VIP continues at a rapid pace. Recent progress includes: insights into its biosynthesis (and that of a closely related PHI-like peptide) and its neuronal localization, discovery of novel biological actions, new data on its release and binding to specific receptors, and additional evidence for its roles in physiological regulation and in the pathogenesis of disease.