SAPAPs. A family of PSD-95/SAP90-associated proteins localized at postsynaptic density

PSD-95/SAP90 is a member of membrane-associated guanylate kinases localized at postsynaptic density (PSD) in neuronal cells. Membrane-associated guanylate kinases are a family of signaling molecules expressed at various submembrane domains which have the PDZ (DHR) domains, the SH3 domain, and the guanylate kinase domain. PSD-95/SAP90 interacts with N-methyl-D-aspartate receptors 2A/B, Shaker-type potassium channels, and brain nitric oxide synthase through the PDZ (DHR) domains and clusters these molecules at synaptic junctions. However, neither the function of the SH3 domain or the guanylate kinase domain of PSD-95/SAP90, nor the protein interacting with these domains has been identified. We have isolated here a novel protein family consisting of at least four members which specifically interact with PSD-95/SAP90 and its related proteins through the guanylate kinase domain, and named these proteins SAPAPs (SAP90/PSD-95-Associated Proteins). SAPAPs are specifically expressed in neuronal cells and enriched in the PSD fraction. SAPAPs induce the enrichment of PSD-95/SAP90 to the plasma membrane in transfected cells. Thus, SAPAPs may have a potential activity to maintain the structure of PSD by concentrating its components to the membrane area.     

Immunocytochemical localization of polyamines in the tiger salamander retina

The polyamines spermine and spermidine are present in neural tissue, but their functions there are not well understood. Recent work suggests that the NMDA subtype of glutamate receptors, other glutamate receptor subtypes, and certain K(+)-channels, are neural targets for polyamines. To better understand the neuron-specific roles of polyamines, we have developed antibodies that interact with spermine and spermidine in aldehyde-fixed tissue and used these antibodies in immunocytochemical studies to determine the cellular localization of these polyamines in the tiger salamander retina. The affinity-purified, polyclonal antibodies were highly specific for spermine and spermidine, exhibiting < 1% cross reactivity with putrescine, and virtually no cross-reactivity with GABA, arginine, lysine, or glutaraldehyde. Polyamine labeling was most abundant in cells in the inner half of the inner nuclear layer and in the ganglion cell layer. Some cells in the outer half of the inner nuclear layer are labeled, and there was some labeling in both synaptic layers. Double-labeling experiments indicated (1) all GABAergic amacrine cells were polyamine-positive; and (2) all ganglion cells (identified by back-filling after microinjections of rhodamine in the optic nerve) were polyamine-positive. These results are consistent with a role for polyamines as modulators of NMDA receptor function and channel function in the inner retina.     

Immunocytochemical and histochemical localization of nitric oxide synthase in the turtle retina

Recent interest in nitric oxide and its relationship to cGMP has produced many attempts to anatomically localize the enzyme synthesizing nitric oxide, nitric oxide synthase. In the retina, numerous previous studies have used the NADPH-diaphorase enzyme activity of nitric oxide synthase as a histochemical method to localize nitric oxide synthase. However, all NADPH-diaphorase activity is not necessarily nitric oxide synthase, because several enzymes have similar biochemical activity. Additionally, various histochemical methods have been used to demonstrate NADPH-diaphorase activity, which makes comparisons between studies difficult. The purpose of this study was twofold. First, we wanted to examine the histochemical labeling of NADPH-diaphorase in the turtle retina to allow comparisons to previous studies. Second, we wanted to compare the histochemical localization of NADPH-diaphorase activity to the immunocytochemical localization of nitric oxide synthase in the turtle retina. Our histochemical localization of NADPH-diaphorase activity and our localization of nitric oxide synthase-like immunoreactivity in the turtle retina both produced similar results. Both the histochemistry and immunocytochemistry consistently labeled photoreceptor inner segments, at least three amacrine cell types, and processes in the inner plexiform layer. In optimized double-labeled preparations, all cells with NADPH-diaphorase activity were also positive for nitric oxide synthase-like immunoreactivity, although some somata in the ganglion cell layer only had nitric oxide synthase-like immunoreactivity. The immunocytochemical localization of nitric oxide synthase in photoreceptors, amacrine cells, and putative ganglion cells indicates that nitric oxide may function at several levels of visual processing in the turtle retina.     

Immunocytochemical localization of the NMDA-R2A receptor subunit in the cat retina

Immunocytochemical studies were performed to determine the distribution and cellular localization of the NMDA-R2A receptor subunit (R2A) in the cat retina. R2A-immunoreactivity (R2A-IR) was noted in all layers of the retina, with specific localizations in the outer segments of red/green and blue cone photoreceptors, B-type horizontal cells, several types of amacrine cells, Müller cells and the majority of cells in the ganglion cell layer. In the inner nuclear layer, 48% of all cells residing in the amacrine cell layer were R2A-IR including a cell resembling the GABAergic A17 amacrine cell. Interestingly, the AII rod amacrine cell was devoid of R2A-IR. Although the localization of the R2A subunit was anticipated in ganglion cells, amacrines and Müller cells, the presence of this receptor subunit to the cells in the outer retina was not expected. Here, both the R2A and the R2B subunits were found to be present in the outer segments of cone photoreceptors and to the tips of rod outer segments. Although the function of these receptor subunits in rod and cone photoreceptors remains to be determined, the fact that both R2A and R2B receptor subunits are localized to cone outer segments suggests a possible alternative pathway for calcium entry into a region where this cation plays such a crucial role in the process of phototransduction. To further classify the cells that display NR2A-IR, we performed dual labeling experiments showing the relationship between R2A-labeled cells with GABA. Results showed that all GABAergic-amacrines and displaced amacrines express the R2A-subunit protein. In addition, approximately 11% of the NR2A-labeled amacrines, did not stain for GABA. These findings support pharmacological data showing that NMDA directly facilitates GABA release in retina and retinal cultures [I.L. Ferreira, C.B. Duarte, P.F. Santos, C.M. Carvalho, A.P. Carvalho, Release of [3H]GABA evoked by glutamate receptor agonist in cultured chick retinal cells: effect of Ca2+, Brain Res. 664 (1994) 252-256; G.D. Zeevalk, W.J. Nicklas, Action of the anti-ischemic agent ifenprodil on N-methyl-d-aspartate and kainate-mediated excitotoxicity, Brain Res. 522 (1990) 135-139; R. Huba, H.D. Hofmann, Transmitter-gated currents of GABAergic amacrine-like cells in chick retinal cultures, Vis. Neurosci. 6 (1991) 303-314; M. Yamashita, R. Huba, H.D. Hofmann, Early in vitro development of voltage- and transmitter-gated currents in GABAergic amacrine cells, Dev. Brain Res. 82 (1994) 95-102; R. Ientile, S. Pedale, V. Picciurro, V. Macaione, C. Fabiano, S. Macaione, Nitric oxide mediates NMDA-evoked [3H]GABA release from chick retina cells, FEBS Lett. 417 (1997) 345-348; R.C. Kubrusly, M.C. deMello, F.G. deMello, Aspartate as a selective NMDA agonist in cultured cells from the avian retina, Neurochem. Intl. 32 (1998) 47-52] or reduction of GABA in vivo [N.N. Osborn, A.J. Herrera, The effect of experimental ischaemia and excitatory amino acid agonist on the GABA and serotonin immunoreactivities in the rabbit retina, Neurosci. 59 (1994) 1071-1081]. Since the majority of GABAergic synapses in the inner retina are onto both rod and cone bipolar axon terminals [R.G. Pourcho, M.T. Owzcarzak, Distribution of GABA immunoreactivity in the cat retina: A light and electron-microscopic study, Vis. Neurosci. 2 (1989) 425-435], we hypothesize that the NMDA-receptor plays a crucial role in providing feedback inhibition onto rod and cone bipolar cells.     

The localization of guanylyl cyclase-activating proteins in the mammalian retina

PURPOSE: To explore the distribution of guanylyl cylase-activating proteins 1 and 2 (GCAP1 and GCAP2) in the mammalian retina. METHODS: Cryostat and vibratome vertical sections and wholemount retinas from mouse, rat, cat, bovine, monkey, and human eyes were prepared for immunocytochemistry and viewing by light and confocal microscopy. RESULTS: In all mammalian retinas investigated, intense GCAP1 immunoreactivity (GCAP1-IR) was seen in cone photoreceptor inner and outer segments, cell bodies, and synaptic regions. Intensity of the GCAP1-IR was strong in inner segments of rods in all species but weaker in outer segments-particularly so in primates and cats. GCAP2 immunoreactivity (GCAP2-IR) was weak in bovine, mouse, and rat cones but was intense in human and monkey cones. In all species except primates, GCAP2 staining was intense in rod inner and outer segments. In primates GCAP2-IR was intense in the rod inner segment but faint in the rod outer segment. A striking difference from the GCAP1 pattern of immunoreactivity was seen with GCAP2 antibodies as far as the inner retina was concerned. GCAP2-IR was evident in certain populations of bipolar, amacrine, and ganglion cells in all species. CONCLUSIONS: GCAP1 and GCAP2, which are involved in Ca2+-dependent stimulation and inhibition of photoreceptor guanylyl cyclase, can be detected in mammalian photoreceptor inner and outer segments, consistent with their physiological function. The occurrence of both GCAPs in the synaptic region of the photoreceptors indicates participation of these proteins in pathways other than regulation of phototransduction. The occurrence of GCAP2 in inner retinal neurons is indicative of second-messenger chemical transduction, possibly in metabotropic glutamate, gamma-aminobutyric acid (GABA) receptor, and nitric oxide-activated neural circuits.     

Presynaptic and postsynaptic localization of GABA(B) receptors in neurons of the rat retina

The recently cloned GABA(B) receptors were localized in rat retina using specific antisera. Immunolabelling was detected in the inner and outer plexiform layers (IPL, OPL), and in a number of cells in the inner nuclear layer and the ganglion cell layer. Double-labelling experiments for GABA (gamma-aminobutyric acid) and GABA(B) receptors, respectively, demonstrated a co-localization in horizontal cells and amacrine cells. Electron microscopy showed that GABA(B) receptors of the OPL were localized presynaptically in horizontal cell processes invaginating into photoreceptor terminals. In the IPL, GABA(B) receptors were present presynaptically in amacrine cells, as well as postsynaptically in amacrine and ganglion cells. The postnatal development of GABA(B) receptors was also studied, and immunoreactivity was observed well before morphological and synaptic differentiation of retinal neurons. The present results suggest a presynaptic (autoreceptor) as well as postsynaptic role for GABA(B) receptors. In addition, the extrasynaptic localization of GABA(B) receptors could indicate a paracrine function of GABA in the retina.     

Immunocytochemical localization of GABA and glycine in amacrine and displaced amacrine cells of macaque monkey retina

Recent studies have varied widely in the percentages of GABA- and glycine-immunoreactive (GABA+, GLY+) amacrines reported for primate retina. We compared the distributions of GABA+ and GLY+ amacrines and displaced amacrines at seven locations along the horizontal meridian of macaque retina using postembedding immunogold labeling with silver intensification. The percentage of GABA+ amacrine profiles was higher in central retina (50-55%) than peripheral retina (30-40%), whereas the percentage of GLY+ amacrine profiles did not vary much with eccentricity (52-57%). GABA and glycine were colocalized in 5-20% of amacrines, depending on the eccentricity, whereas 5-30% of amacrines were not immunoreactive for either neurotransmitter. GABA+ amacrines were slightly larger than GLY+ amacrines or Müller cells. In the ganglion cell layer, 5-20% of neurons were labeled for either GABA or glycine and were identified as displaced amacrines. Of these, 53% were GABA+ only, 11% were GLY+ only, and 37% were double-labeled. A few large, very lightly labeled GABA+ cells were identified as ganglion cells. Other features that varied with eccentricity included the linear density of GABA+ and GLY+ amacrines, and the ratio of amacrines to Müller cells.     

Disulfide-linked head-to-head multimerization in the mechanism of ion channel clustering by PSD-95

The PSD-95/SAP90 family of PDZ-containing proteins is directly involved in the clustering of specific ion channels at synapses. We report that channel clustering depends on a conserved N-terminal domain of PSD-95 that mediates multimerization and disulfide linkage of PSD-95 protomers. This N-terminal multimerization domain confers channel clustering activity on a single PDZ domain. Thus, channel clustering depends on aggregation of PDZ domains achieved by head-to-head multimerization of PSD-95, rather than by concatenation of PDZ domains in PSD-95 monomers. This mechanism predicts that PSD-95 can organize heterogeneous membrane protein clusters via differential binding specificities of its three PDZ domains. PSD-95 and its relative chapsyn-110 exist as disulfide-linked complexes in rat brain, consistent with head-to-head multimerization of these proteins in vivo.     

Requirement of N-terminal cysteines of PSD-95 for PSD-95 multimerization and ternary complex formation, but not for binding to potassium channel Kv1.4

The PSD-95 family of PSD-95/Discs large/ZO-1 (PDZ) domain-containing proteins plays a role in the clustering and localization of specific ion channels and receptors at synapses. Previous studies have shown that PSD-95 forms multimers through an N-terminal region (termed the N-segment) and that the multimerization of PSD-95 is critical for its ability to cluster Shaker-type potassium channel Kv1.4 in heterologous cells. We show here that the PSD-95 N-segment functions as a multimerization domain only when located at the N-terminal end of a heterologous protein. A pair of N-terminal cysteines, Cys3 and Cys5, is essential for the ability of PSD-95 to self-associate and to form cell surface clusters with Kv1.4. However, PSD-95 mutants lacking these cysteine residues retain their ability to associate with membranes and to bind to Kv1.4. Unlike wild type PSD-95, the cysteine mutant of PSD-95 cannot form a ternary complex with Kv1.4 and the cell adhesion molecule Fasciclin II. These results suggest that the N-terminal cysteines are essential for PSD-95 multimerization and that multimerization is required for simultaneous binding of multiple membrane protein ligands by PSD-95.     

Immunocytochemical localization of a growth-associated protein (GAP-43) in rat adrenal gland

We have localized at light and electron-microscopic level the growth-associated protein GAP-43 in adrenal gland using single and double labelling immunocytochemistry. Clusters of GAP-43-immunofluorescent chromaffin cells and many immunofluorescent fibres were observed in the medulla. GAP-43-immunoreactive fibres also formed a plexus under the capsule, crossed the cortex and ramified in the zona reticulata. Double labelled sections showed the coexpression of GAP-43 with a subpopulation of tyrosine hydroxylase- and of dopamine-beta-hydroxylase-immunoreactive chromaffin cells. Dual colour immunofluorescence for GAP-43 and calcitonin gene-related peptide (CGRP) revealed that some of the GAP-43-immunoreactive fibres also express CGRP. Pre-embedding electron microscopy showed GAP-43 immunoreactivity associated with the plasma membranes and cytoplasm of noradrenaline-producing chromaffin cells, and with processes of nonmyelin-forming Schwann cells. Immunoreactive unmyelinated axons and terminals were also observed. The immunostained terminals made symmetrical synaptic contacts with chromaffin cells. Immunoreactive unmyelinated fibres and small terminals were present in the cortex. Our results show that GAP-43 is expressed in noradrenergic chromaffin cells and in various types of nerve fibres that innervate the adrenal. Likely origins for these fibres include preganglionic sympathetic fibres which innervate chromaffin cells, postganglionic sympathetic fibres in the cortex, and CGRP containing sensory fibres.     

PSD-95 promotes Fyn-mediated tyrosine phosphorylation of the N-methyl-D-aspartate receptor subunit NR2A

Our previous study has shown that the phases of circadian rhythms of ocular melatonin and dopamine are always opposite and intraocular melatonin injection suppresses dopamine release. Therefore, it is possible that dopamine rhythms result from inhibitory action of melatonin. We have examined this possibility in the following experiments. In the first experiment effects of continuous light on melatonin and dopamine release were examined. The data indicated that continuous light exposure resulted in loss of circadian rhythmicity of melatonin and dopamine by suppressing melatonin and enhancing dopamine levels throughout the day. To further examine the effects of light in the second experiment, 2 h light pulse was applied during the night, then temporal changes of melatonin and dopamine release were studied. The light pulse rapidly suppressed melatonin release, whereas it rapidly increased dopamine release. These changes occurred within 30 min in both melatonin and dopamine. However, the recovery after the cessation of the light stimulus was slower in melatonin than dopamine. In the third experiment it was tested if dopamine release was increased by lowering melatonin release with an intraocular injection of the D2 agonist, quinpirol. Although quinpirol strongly inhibited melatonin release independently of the time of injection, dopamine did not always increase by the inhibition of melatonin. These results indicate that ocular dopamine rhythms are not simply produced by melatonin inhibitory action.     

Circadian rhythms in cultured mammalian retina

Many retinal functions are circadian, but in most instances the location of the clock that drives the rhythm is not known. Cultured neural retinas of the golden hamster (Mesocricetus auratus) exhibited circadian rhythms of melatonin synthesis for at least 5 days at 27 degrees celsius. The rhythms were entrained by light cycles applied in vitro and were free-running in constant darkness. Retinas from hamsters homozygous for the circadian mutation tau, which shortens the free-running period of the circadian activity rhythm by 4 hours, showed a shortened free-running period of melatonin synthesis. The mammalian retina contains a genetically programmed circadian oscillator that regulates its synthesis of melatonin.     

Beta B2-crystallin in the mammalian retina

beta-crystallins are abundant lens proteins in most, if not all vertebrate species. We have previously reported the presence of low levels of beta-crystallins in chick non-lens tissues, both ocular and extra-ocular, including the expression of beta B2-crystallin in the retina. Here we report that extralenticular beta-crystallin expression is also found in mammals. beta B2-crystallin is expressed in mouse and cat neural and pigmented retinas and in cat iris. Although present at levels lower than those found in the lens, the appearance and accumulation of beta B2-crystallin in the neural retina coincides with the functional maturation of this tissue.     

Immunohistochemical localization of ganciclovir in the human retina

Facial asymmetry (facedness) of selected academic faculty members was studied in relation to brain asymmetry and cognitive specialization. Comparisons of facedness were made among humanities faculty (H), faculty members of mathematics and physics (M-P), psychologists (P), and a group of randomly selected individuals (R). Facedness was defined in terms of the relative sizes (in square centimeters) of the two hemifaces. It was predicted that the four groups would show differences in facedness, namely, H, right face bias; M-P, left face bias; P, no bias; and R, no bias. The predictions were confirmed, and the results interpreted in terms of known differences in hemispheric specialization of cognitive functions as they relate to the dominant cognitive activity of each of the different groups. In view of the contralateral control of the two hemifaces (below the eyes) by the two hemispheres of the brain, the two sides of the face undergo differential muscular development, thus creating facial asymmetry. Other factors, such as gender, also may affect facial asymmetry. Suggestions for further research on facedness are discussed.     

Immunocytochemical localization of desmin in human fetal skeletal muscle

INTRODUCTION: A comparative evaluation of four air samplers was performed using bioaerosol collection in the outdoor environment. METHODS: Test samplers used included a Rotorod, a Kramer-Collins suction trap, an all-glass impinger (AGI-30), and a high-volume cyclonic liquid impinger (SpinCon). All samples were analyzed microscopically for spores and pollen. The two collectors providing a liquid sample (AGI-30 and SpinCon) also were analyzed for specific allergen content by enzyme-linked immunoassay. RESULTS: The SpinCon collected a larger number of spores than the other devices. The number of spores collected by this unit per volume of air sampled was comparable to the AGI-30. The Rotorod and Kramer-Collins collected a lower number of spores per unit of air but collected a larger number of pollen grains per volume sampled. Alternaria allergens Alt a I and GP70 were collected by both liquid impingers; however, the SpinCon collected more Alt a I and the AGI-30 collected more GP70. CONCLUSIONS: The SpinCon is a device that is capable of efficiently sampling a high volume of air and concentrating it in a form that can be analyzed for the presence of spores and fungal allergens. It is less useful for collecting intact pollen grains. Pollen allergen quantitation has not yet been performed on the SpinCon effluent.     

The number of unidentified amacrine cells in the mammalian retina

The three largest known populations of amacrine cells in the rabbit retina were stained with fluorescent probes in whole mounts and counted at a series of retinal eccentricities. The retinas were counterstained using a fluorescent DNA-binding molecule and the total number of nuclei in the inner nuclear layer were counted in confocal sections. From the total number of inner nuclear layer cells and the known fraction of them occupied by amacrine cells, the fraction of amacrine cells made up by the stained populations could be calculated. Starburst cells made up 3%, indoleamine-accumulating cells made up 4%, and AII cells made up 11% of all amacrine cells. By referring four smaller populations of amacrine cells to the number of indoleamine-accumulating cells, they were estimated to make up 4% of all amacrine cells. Thus, 78% of all amacrine cells in the rabbit's retina are known only from isolated examples, if at all. This proportion is similar in the retinas of the mouse, cat, and monkey. It is likely that a substantial fraction of the local circuit neurons present in other regions of the central nervous system are also invisible as populations to current techniques.     

Differential localization and expression of alpha and beta isoenzymes of protein kinase C in the rat retina

Expression and cellular localization of three isoenzymes of Ca2+-dependent protein kinase C (PKCalpha, PKCbeta, and PKCgamma) in the adult rat retina were revealed by immunohistochemistry and in situ hybridization histochemistry with isoenzyme-specific antibodies and cRNA probes. Immunoreactivities and mRNA signals for PKCalpha were conspicuous in rod bipolar cells. A subgroup of amacrine cells expressed PKCalpha. The cells in the ganglion cell layer also displayed PKCalpha gene products. Positive immunoreactivities for PKCbeta were localized as stripe patterns in the inner plexiform layer, corresponding to the stratification levels of axon terminals of cone bipolar cells. The somata of cone bipolar cells expressed PKCbeta. Amacrine cells and retinal ganglion cells also displayed PKCbeta gene products. The results obtained by immunohistochemistry were confirmed with colocalization of mRNA signals for PKCalpha and PKCbeta on the somata. The cell membranes showed stronger immunoreactivities than did the cytoplasms for both PKCalpha and PKCbeta. Neither immunoreactivities nor mRNA signals for PKCgamma were detected in all retinal regions. The differential roles of Ca2+-dependent PKC isoenzymes could be revealed in physiological defined retinal neurons.     

Distribution of transforming growth factor-beta isoforms in the mammalian retina

The distribution of transforming growth factor-beta (TGF-beta) was examined in the posterior segment of the monkey, human, and feline eye using antisera to TGF-beta 1, TGF-beta 2, or TGF-beta 3. A number of different antibodies, tissue processing methods, immunolocalization techniques, and microscopic imaging systems were used in an attempt to gain a more comprehensive picture of TGF-beta isoform distribution in the retina and retinal pigmented epithelium (RPE). The results are generally consistent in identifying one or more of the three TGF-beta isoforms in the cytoplasm of a small, overlapping subset of cells. RPE cells, photoreceptors, Mueller cells, ganglion cells, hyalocytes, and cells associated with choroidal and retinal vessels are all represented in this immunoreactive population. No evidence of extracellular labeling was noted. The intracellular distribution of the three isoforms is distinctly different in photoreceptors. Anti-TGF-beta 1 precursor and anti-TGF-beta 2 immunoreactivity is confined primarily to rod outer segments, whereas anti-TGF-beta 3 immunoreactivities are restricted to mitochondria within inner segments. In the RPE, clusters of anti-TGF-beta 2 positive cytoplasmic granules are located near the cells' lateral borders, whereas anti-TGF-beta 3 labeling is concentrated apically. These results provide baseline information from which new hypotheses regarding the function(s) of TGF-beta isoforms in the retina can be formulated.     

Noise removal at the rod synapse of mammalian retina

Mammalian rods respond to single photons with a hyperpolarization of about 1 mV which is accompanied by continuous noise. Since the mammalian rod bipolar cell collects signals from 20-100 rods, the noise from the converging rods would overwhelm the single-photon signal from one rod at scotopic intensities (starlight) if the bipolar cell summed signals linearly (Baylor et al., 1984). However, it is known that at scotopic intensities the retina preserves single-photon responses (Barlow et al., 1971; Mastronarde, 1983). To explore noise summation in the rod bipolar pathway, we simulated an array of rods synaptically connected to a rod bipolar cell using a compartmental model. The performance of the circuit was evaluated with a discriminator measuring errors in photon detection as false positives and false negatives, which were compared to physiologically and psychophysically measured error rates. When only one rod was connected to the rod bipolar, a Poisson rate of 80 vesicles/s was necessary for reliable transmission of the single-photon signal. When 25 rods converged through a linear synapse the noise caused an unacceptably high false positive rate, even when either dark continuous noise or synaptic noise where completely removed. We propose that a threshold nonlinearity is provided by the mGluR6 receptor in the rod bipolar dendrite (Shiells & Falk, 1994) to yield a synapse with a noise removing mechanism. With the threshold nonlinearity the synapse removed most of the noise. These results suggest that a threshold provided by the mGluR6 receptor in the rod bipolar cell is necessary for proper functioning of the retina at scotopic intensities and that the metabotropic domains in the rod bipolar are distinct. Such a nonlinear threshold could also reduce synaptic noise for cortical circuits in which sparse signals converge.     

Immunocytochemical localization of polyamines in the gastrointestinal tracts of rats and mice

An immunocytochemical method using a recently produced monoclonal antibody (ASPM-29) with an antibody specificity to spermine (Spm) and spermidine (Spd) fixed in situ, was used to demonstrate an immunocytochemical localization of polyamine (PA) pools in the gastrointestinal tracts of rats and mice. High PA immunoreactivity was always found in the cytoplasm of cells not only at the cell proliferative zone or the precursor cell zone but also at the neighboring non-proliferative premature cell zone of the epithelium, and a gradient of decreasing PA levels was noticed from these cells to the fully mature differentiated gastric surface mucous cells and absorptive cells of the small and large intestines. Also, strong staining for PAs was seen in the cytoplasm of fully differentiated gastric chief cells and neurons of both the myenteric and submucous plexuses, whereas the nuclei of the cells remained virtually unstained. These results may suggest that PAs are closely associated with the high biosynthetic activity in the cells of the gastrointestinal mucosa of normal rats and mice. This seems to be consistent with the PA immunocytochemical results previously obtained for neoplastic cells and active protein- or peptide-secreting cells, including exocrine or endocrine cell types.