Heterogeneity of muscarinic cholinergic receptors in the developing chick embryo retina

Muscarinic calcium mobilization in the regenerating retina of adult newt

We used optical recording with a Ca(2+)-sensitive dye, fura2, in living slice preparations from the newt retina at different stages of regeneration. ACh produced the most pronounced [Ca2+]i rise in progenitor cells and premature ganglion cells of the earlier stage of retinal regeneration, but less pronounced Ca2+ response in ganglion cells just before, or at the beginning of, synaptogenesis. The [Ca2+]i rise to ACh was mediated by mAChRs. This was shown by inhibition of the ACh-induced Ca2+ response with a preincubation of the mAChR antagonist atropine as well as with direct stimulation of the [Ca2+]i rise by the mAChR agonist muscarine. This muscarine-induced [Ca2+]i rise was more greatly suppressed by the M1 and/or M3 preferring mAChR antagonists than by the M2 preferring mAChR antagonist. The [Ca2+]i rise due to muscarine was not suppressed in the absence of extracellular Ca2+, but suppressed in part in the presence of the L-type voltage-gated Ca2+ channel blockers, verapamil or nicardipine. Furthermore, thapsigargin (TG), a Ca-ATPase inhibitor, abolished the muscarine-induced [Ca2+]i rise in the absence of extracellular Ca2+. These results suggest that the mAChR-mediated [Ca2+]i rise is mainly a result of a release of Ca2+ from intracellular stores. TG produced a slow rise in the resting level of [Ca2+]i. This [Ca2+]i raise was suppressed as extracellular Ca2+ was omitted, whereas a rapid rise in [Ca2+]i occurred when extracellular Ca2+ was reintroduced, suggesting the occurrence of the capacitative Ca2+ influx in the progenitor cells and premature ganglion cells of the regenerating newt retina.

Muscarinic receptor protein expression in the ocular tissues of the chick during normal and myopic eye development

Muscarinic receptor signalling has been implicated in both the embryonic and postnatal development of ocular structures as well as in myopic eye growth. A radioligand binding assay was used to determine whether changes in muscarinic receptor density and/or affinity occurred in the chick retina, choroid and sclera during early post-hatching development or with the induction of myopia. Specific receptor binding sites were saturable with increasing concentrations of the muscarinic receptor ligand [3H]N-methyl-scopolamine in the retina and choroid but not in the sclera. In normal eyes, binding density in the retina was not altered from age P5 to P10 (447+/-14 vs. 411+/-13 fmol/mg of protein, P=0.07). However, in the choroid, the number of receptor binding sites significantly increased between P5 and P10 (637+/-39 vs. 1125+/-121 fmol/mg of protein, P<0.01). Binding affinity (K(D)) was not altered with age in either the retina or choroid. Myopia was induced in chicks by deprivation of form vision, using translucent diffusers, from age P3. Despite the induction of significant degrees of ocular elongation and myopia at P5 (-8.7+/-0.3 D, P<0.01) and P10 (-22.5+/-1.3 D, P<0.01), neither muscarinic receptor density nor affinity were altered in the retina or choroid of myopic eyes. These findings indicate that regulation of muscarinic receptor numbers in the chick choroid is occurring in normal post-hatching development of this tissue. However, myopic eye enlargement was not associated with changes in muscarinic receptor protein expression in the chick retina and choroid.

Muscarinic acetylcholine receptors in the normal, developing and regenerating newt retinas

Immunoreactivity for m2 and m4 muscarinic acetylcholine receptors (mAChRs) was demonstrated in the adult newt retina. The m2 mAChR was localized to somata on either side of the inner plexiform layer (IPL), especially ganglion cells, and also distributed into two bands within the IPL. The distal band at a depth of 0-15% IPL co-localized with one of two choline acetyltransferase (ChAT) immunoreactive bands, while the proximal band at 85-100% depth did not overlap with either of the ChAT-ir bands. The m4 mAChR was localized to somata closely apposed to either side of the IPL, probably amacrine cell somata, and no immunoreactivity was detectable throughout the IPL. The time course of appearance of the m2 and m4 mAChRs was examined in both developing and regenerating retinas. Like acetylcholinesterase (AChE), the m2 was first detected in somata located at the most proximal level of the retina well before ChAT-ir cholinergic neurons appeared, while the m4 was detected at the time of appearance of ChAT, in both developing and regenerating retinas. When the outer plexiform layer (OPL) began to form, somata in the horizontal cell layer became transiently immunoreactive to the m2. The discrepancy in distribution of the m2 and ChAT in the IPL suggests that mAChR may play a role other than cholinergic neurotransmission. Furthermore, the similarity in time course of appearance of the m2 and m4, as well as other cholinergic system components [4], in both developing and regenerating retinas would suggest that the mechanisms that control neuronal differentiation during retinal development and regeneration are similar.

Development of muscarinic acetylcholine receptors in the ferret retina

The development of muscarinic acetylcholine receptor protein in the ferret retina was studied using biochemical, autoradiographic, and light and electron microscopic immunohistochemical techniques. The development of retinal muscarinic cholinergic receptor proteins involves transient shifts in their number and distribution, as well as changes in the relative abundance of two molecular weight variants. Receptor binding assays demonstrate changes in the number and affinity of retinal binding sites for the muscarinic cholinergic ligand [3H]quinuclidinylbenzilate ([3H]QNB). Light microscopic immunohistochemical studies reveal the presence of muscarinic acetylcholine receptor-like (mAChR-like) immunoreactivity in the adult inner plexiform layer. During development, the mAChR-like immunoreactivity appears in a number of other retinal layers. Electron microscopic immunohistochemical studies indicate that muscarinic acetylcholine receptor-like immunoreactivity is found at amacrine-amacrine cell contacts. Both autoradiographic and gel slice electrophoretic studies were carried out after labeling of developing and adult retinal muscarinic receptors with [3H]propylbenzilylcholine mustard ([3H]propylbenzilylcholine mustard ([3H]PrBCM), which irreversibly labels the muscarinic acetylcholine receptor. Polyacrylamide gel electrophoresis under reducing, denaturing conditions resolved two peaks of radioactivity corresponding to [3H]PrBCM-labeled protein; both were eliminated by pre- and co-incubation of labeled adult retinas with excess atropine. Combined with the results of earlier studies, these observations suggest that the subtypes, number and distribution of muscarinic receptor proteins changes during retinal synaptogenesis.

Cholinergic inositol phosphate formation in striatal neurons is mediated by distinct mechanisms

In murine striatal neurons devoid of functional synapses (6 days in vitro) the cholinergic agonists carbachol and arecoline evoked dose-dependent inositol phosphate (InsP) responses with mean log EC50s of -4.1 +/- 0.5 and -4.48 +/- 0.1, respectively. Carbachol (1 mM) and arecoline (1 mM) responses were insensitive to tetrodotoxin, a voltage-sensitive Na+ channel blocker, and were blocked by pirenzepine with relatively low affinity (logIC50 = -5.9 +/- 0.3 for the carbachol response and logIC50 = -5.8 +/- 0.3 for the arecoline response). After synaptogenesis (13 days in vitro) the maximal carbachol effect doubled whereas the arecoline response remained unchanged. This additional effect was sensitive to tetrodotoxin and the voltage-dependent Ca2+ channel blocker, omega-conotoxin. The tetrodotoxin-sensitive carbachol response was blocked by lower concentrations of pirenzepine than the tetrodotoxin-insensitive carbachol response. More than 75% of the InsP response evoked by low concentrations of muscarine (1 and 10 microM) was sensitive to tetrodotoxin whereas only 38% of the InsP response stimulated by 1 mM of muscarine could be blocked by tetrodotoxin. These results suggest that there are at least two different mechanisms (depending on the stage of development), activated most probably by two different muscarinic receptors responsible for the carbachol-induced InsP formation in striatal neurons.

Muscarinic receptor subclasses in retinal cultures: effect of corticosterone

We have previously shown that exogenously administered corticosterone affects muscarinic receptor binding in the chick embryo retina. Analysis with the selective antagonist pirenzepine has shown that both muscarinic receptor subclasses M1 and M2 are present in treated retinas. On the contrary, only M2 is detectable in controls. Moreover, receptor affinity for agonists is modified by hormone treatment. Since these studies did not show whether or not the hormone directly influences retinal cells, a similar study was performed on retinal tissue cultures. Cells were treated at day 5 in vitro for 24 hr with 1.10(-8) M corticosterone. Scatchard analysis of results obtained with 3H-quinuclidinyl benzilate (3H-QNB) binding showed no difference between treated and control cultures. However, displacement experiments demonstrated that both M1 and M2 were present in treated cultures, whereas controls had only M2. Also, receptor affinity for the agonist carbachol was modified, as already observed with in vivo studies. In addition, a new phenomenon was observed: treated cultures had a significantly higher number of cells. The possibility that the hormone can prevent cell death or stimulate cell mitosis is considered.

Acetylcholine and dopamine promote the production of platelet activating factor in immature cells of chick embryonic retina

We have previously shown that platelet-activating factor (PAF), a naturally occurring lipid mediator of cell-to-cell communication, was produced by 3-day-old chick retina stimulated with acetylcholine (ACh) and dopamine (DA), but not with other neurotransmitters. ACh and DA stimulated PAF synthesis via a dithiothreitol (DTT)-insensitive cholinephosphotransferase, without affecting the acetyltransferase pathway, which was stimulated only by the calcium ionophore A23187. Therefore, we attempted to study the effects of neurotransmitters on PAF production and on the activities of the DTT-insensitive cholinephosphotransferase and acetyltransferase in the developing chick embryo retina up to hatching. Our results show that PAF was produced already at 8 days of development, when retinal cells are still rather immature and ganglion and Mueller cells are the only differentiated cells. The stimulation of PAF production occurred with ACh and not with other neurotransmitters. In older stages, DA also stimulated PAF production, as already described in the chick after hatching. DTT-insensitive cholinephosphotransferase and acetyltransferase activities were present in 8-day-old embryos, the earliest stage analyzed. Both enzymatic activities increased with age; DTT-insensitive cholinephosphotransferase increased rapidly from day 12 up to day 18, whereas acetyltransferase activity increased linearly up to the time of hatching. To promote PAF production, ACh and DA activate DTT-insensitive cholinephosphotransferase, but not acetyltransferase.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinoceptive neurons in the retina of the chick: an immunohistochemical study of the nicotinic acetylcholine receptors

Monoclonal antibodies directed against nicotinic acetylcholine receptors (nAChRs) were used to identify and characterize cholinoceptive neurons in the chick retina. Two monoclonal antibodies (mAbs), mAb 210 and mAb 270, stained many neurons in both the inner nuclear layer (INL) and ganglion cell layer (GCL). A class of large labeled cells in the inner INL were positioned at the INL/IPL (inner plexiform layer) border and resembled displaced ganglion cells (DGCs). Their identity was confirmed with injections of rhodamine-labeled microspheres into the ventral tectum and nucleus of the basal optic root (nBOR). Four days after the injection, large nAChR-positive neurons in the inner INL were labeled with beads. The distribution of these cells matched that reported for DGCs in the chicken and pigeon (Reiner et al., 1979; Fite et al., 1981). Many smaller cells in the INL also exhibited nAChR immunoreactivity. These cells were not retrogradely labeled after bead injections into retinal recipient areas. Their processes entered IPL where they arborized in a band comprised of the inner leaflet of lamina 1 and all of lamina 2. In some instances, a process continued inward to lamina 4. These neurons were tentatively identified as amacrine cells because of their position and branching pattern. Approximately 12-18% of the cells in the GCL exhibited nAChR immunoreactivity. Many of these cells could be classified as ganglion cells as their axons were also labeled following exposure to nAChR antibodies. Their distribution mirrored that of all ganglion cells with a higher density of cells in the central retina than in the periphery (Ehrlich, 1981). A "double label" technique was used to compare the distribution of nAChR-positive neurons with that of the choline acetyltransferase-positive (ChAT), cholinergic neurons in the chick retina. The two antigens were visualized with two different fluorophores: FITC and RITC. We were unable to find any cells in either the INL or GCL that exhibited both ChAT- and nAChR-like immunoreactivity. The nAChR-positive cells and the ChAT-positive cells both arborized in two bands within the IPL. The patterns were in perfect register in the inner IPL (lamina 4). But, in the outer IPL, the nAChR-positive dendrites were observed in the inner leaflet of lamina 1 and in all of lamina 2 while the ChAT-positive dendrites did not extend into the innermost portion of lamina 2.

Acetylcholine as a neurotransmitter in the vertebrate retina

The evidence for the existence of acetylcholine as a neurotransmitter in the vertebrate retina is reviewed. There is evidence for the existence of a cholinergic system in every retina studied to date; therefore, it appears that acetylcholine is both essential and ubiquitous at this level of the visual system. Particular attention is directed to descriptions of the possible functions of acetylcholine in the retina, and formation of testable models which will serve to elucidate some of the details of cholinergic neurotransmission in the retina.

Localization of choline acetyltransferase-like immunoreactivity in the embryonic chick retina

Putative sites of acetylcholine synthesis in the retina of the embryonic and posthatched chick were localized immunohistochemically with antisera to choline acetyltransferase; the resultant choline acetyltransferase-like immunoreactivity (ChAT-IR) was compared to demonstrated sites of acetyltransferase (AChE) activity, and changes were followed in localization during development. The results confirmed the early and rapid course of development of the chick's retinal cholinergic system described in previous biochemical and morphological studies. Immunoreactivity was first detected at embryonic day 6.5 in cells close to the retina's vitreal surface. By 8 days it was present in cells in two juxtaposed rows; by the ninth day the two rows were separated and immunoreactivity was evident in two subliminae of the inner plexiform layer. On the tenth day distribution was like that in the posthatched chicken, in type I cholinergic cells in the inner nuclear layer and in type II cells in the ganglion cell layer (Millar et al.: Neurosci. Lett. 61:311-316, '85), and similar to that of most vertebrates. Three days before hatching, a third population of weakly immunoreactive cells (type III cells) appeared within the inner nuclear layer. The onset of localizable ChAT-IR occurred in amacrine cells and in their processes, before the period of synaptogenesis. Acetylcholinesterase activity was localized at an earlier age than ChAT-IR, and at all ages was present in more cells. The results obtained support the view that "displaced" cholinergic amacrine cells begin to differentiate at the same time and in the same retinal region as type I cholinergic cells. Separation of the two groups is a consequence of the ramification of processes of amacrine and ganglion cells rather than a result of the secondary migration of cells between layers.

Heterogeneity of muscarinic cholinergic receptors in the developing human fetal brain: regional distribution and characterization

The ontogeny of muscarinic cholinergic receptors has been studied in different regions of the human fetal brain. For a comparison, the same study has been carried out on newborn and premature brain. Regarding on the areas examined (frontal cortex, cerebellum, hippocampus, thalamus and basal ganglia) either an increase or a decrease of receptor density during gestation was observed. Thus, the ontogeny of the receptors follows a different pattern in areas which differ in function, cholinergic innervation and embryological origin. However, in all the regions the affinity of the binding site for the ligand [3H]quinuclidinyl benzilate [3H]QNB was very similar to that reported for muscarinic receptors from adult mammalian brain. Data obtained from agonist binding (acetylcholine and carbachol) revealed the presence of a high (H)- and a low-affinity binding site (L) from 10 weeks of gestation. The selective antagonist pirenzepine (PZ) also distinguished two different muscarinic receptor subtypes, which however had higher affinity than that seen in adult brain. In conclusion, during ontogeny, the muscarinic acetylcholine receptor shares some but not all of the pharmacological properties shown in the adult brain.

Pharmacological characterization of cholinergic receptors in a human neuroblastoma cell line

A human neuroblastoma cell line, IMR32, has been characterized as far as morphology, membrane receptors for neurotransmitters, and uptake and release of [3H]3,4-dihydroxyphenylethylamine ([3H]dopamine). These cells expressed at their surface both nicotinic and muscarinic cholinergic receptors, revealed by [125I]alpha-bungarotoxin and [3H]quinuclidinylbenzilate ([3H]QNB) binding, respectively. [125I]alpha-Bungarotoxin binding was efficiently inhibited by alpha-bungarotoxin, nicotine, carbachol, and d-tubocurarine. [3H]QNB binding was competitively inhibited by atropine, pirenzepine, and carbachol. Hexamethonium did not affect the binding of either ligand. In competition experiments with [3H]QNB, pirenzepine recognized only one binding site with "low affinity," and carbachol recognized two sites with different affinities. beta-adrenergic receptors were present in a very low amount, whereas alpha-adrenergic and dopaminergic receptors were not detectable. IMR32 cells had an imipramine-sensitive [3H]dopamine uptake, but carbachol, high levels of K+, the calcium ionophore A23187, and alpha-latrotoxin were not able to induce release of [3H]dopamine that had been taken up. The ultrastructural analysis showed that IMR32 cells contained very few dense-core vesicles, suggesting a low storage capacity for neurotransmitter. These cells could be an useful in vitro model for studying neurotransmitter receptors of the human CNS.

Muscarinic receptor subclasses in the chick embryo retina: influence of corticosterone treatment

The present study was performed on retinas of chick embryos receiving at day 8 of incubation an intracerebral injection of 0.02 microgram of corticosterone. We had previously shown with the use of [3H]quinuclidinylbenzilate [( 3H]QNB) that such treatment induced the appearance of two muscarinic binding sites in the treated retinas, whereas only one was detectable in the controls. In the present study we investigated muscarinic cholinergic receptor subclasses with agonist and antagonist binding. Agonist binding was studied by varying the concentrations of carbachol and acetylcholine (10(-9) M-10(-5) M) in the presence of a constant concentration (0.2 nM) of [3H]QNB. Two subpopulations of receptors were revealed, a high- and a low-affinity receptor, in both treated and control retinas. However, in the hormone-treated retinas, the two subpopulations significantly differed from the controls in their affinity and in their relative percentage among the total receptor population. Moreover, using pirenzepine, an antagonist known to have the capacity to distinguish between muscarinic cholinergic subclasses, two receptor subpopulations were found to be present in the hormone-treated retinas but a single one in the controls. It is suggested that hormone treatment can either induce the appearance of a new subclass of muscarinic cholinergic receptors or favor the maturation of a population of retinal cells having these receptors. Pirenzepine binding in retinas from intact embryos of 7, 9, and 11 days of incubation revealed one receptor subpopulation. Thus, these findings are more consistent with the hypothesis that corticosterone effects the target cells, either inducing changes in muscarinic receptor and/or modifying the receptor environment.