Human retinas synthesize and release acetylcholine

Protein kinase C regulates the expression of M1 receptors and BDNF in rat retinal cells

Protein kinase C (PKC) plays a key role in cellular events including proliferation, survival and differentiation. Our previous study showed the effect of phorbol 12-myristate 13-acetate (PMA), a PKC activator, inducing a decrease in retinal cells proliferation. This effect was mediated by muscarinic type 1 receptors (M1) activation and brain derived neurotrophic factor (BDNF) treatment also induced a decrease in cell proliferation. Based on these results we analyzed the expression of either M1 receptors or BDNF following PMA treatment of retinal cell cultures. Our data demonstrated that PMA induced a decrease in both protein expressions after 48 h in culture. However, after 45 min, PMA induced a transient increase in BDNF expression and a decrease in M1 receptors expression. Analyzing the expression of M1 receptors and BDNF during the postnatal development in vivo, we observed a decrease in both proteins. Taken together our results suggest the involvement of PKC in the control of M1 expression in retinal cells.

Rabbit retinal ganglion cell responses to nicotine can be mediated by beta2-containing nicotinic acetylcholine receptors

Acetylcholine (ACh) affects the response properties of many retinal ganglion cells (GCs) through the activation of nicotinic acetylcholine receptors (nAChRs). To date there have been few studies directly correlating the expression of specific nAChR subtypes with the physiological and morphological characteristics of specific retinal GCs. This study was designed to correlate responses to nicotine application with immunohistochemical evidence of nAChR expression in physiologically and morphologically identified ganglion cells. Extracellular recordings were used to physiologically identify rabbit retinal GCs, based on responses to light stimulation. Cells were then tested for responses to nicotine application and/or for expression of nAChRs, as judged by immunoreactivity to mAb210, an nAChR antibody. The morphologies of many physiologically identified cells were also determined by dye injection. More than three-fourths of ganglion cells tested responded to nicotine application under cobalt-induced synaptic blockade. The nicotine sensitivity was consistent with nAChR immunoreactivity and was also correlated with specific morphological subgroups of GCs. Overall, approximately two-thirds of all physiologically identified GCs that were processed for immunohistochemistry displayed immunoreactivity. In total, 18 of 22 physiologically identified cells demonstrated both sensitivity to nicotine application under synaptic blockade and mAb210 immunoreactivity (mAb210-IR). Thus, mAb210-IR is likely to represent functional nAChRs that can modulate retinal information processing and visual functioning via direct excitation of a number of GC classes.

Human keratinocytes synthesize, secrete, and degrade acetylcholine

We previously reported that normal human keratinocytes express muscarinic receptors, and that acetylcholine induces attachment of these cells to each other. We have now studied the ability of human keratinocytes to synthesize, secrete, and degrade acetylcholine. To detect and localize the synthesizing enzyme choline acetyltransferase and degrading enzyme acetylcholinesterase, cultured cells and cryostat sections of normal human skin were pre-incubated with specific monoclonal antibodies and stained with an avidin-biotin complex/alkaline phosphatase. The choline acetyltransferase activity was assessed by the conversion of [3H]acetyl CoA to [3H]acetylcholine, and newly synthesized [3H]acetylcholine was detected using thin-layer chromatography. The acetylcholinesterase activity was measured spectrophotometrically. Both cholinergic enzymes were present in cultured keratinocytes, and in basal, spinous and granular epidermal cell layers. Choline acetyltransferase was visualized in the vicinity of cell nuclei, and acetylcholinesterase was observed in or near cell membranes. Newly synthesized acetylcholine was detected in both cell homogenates and culture supernatants. The estimated Vmax of the synthesis of labeled acetylcholine by homogenized keratinocytes was about 20 pmoles acetylcholine produced/mg protein/min at 37 degrees C. A single keratinocyte synthesized a mean of 2 x 10(-17) moles, and released 7 x 10(-19) moles acetylcholine per minute. Both cell homogenates and culture supernatants exhibited similar acetylcholinesterase activities indicating that human keratinocytes secrete acetylcholinesterase, too. Thus, we have demonstrated that normal human keratinocytes possess choline acetyltransferase and acetylcholinesterase, and synthesize, store, release, and degrade acetylcholine. Because human keratinocytes can also respond to acetylcholine, we believe that keratinocyte acetylcholine works in the epidermis as a local hormone.

Spatial density and distribution of choline acetyltransferase immunoreactive cells in human, macaque, and baboon retinas

Whole-mounted human, macaque, and baboon retinas were labelled with an antiserum to human choline acetyltransferase (ChAT), by the immunoperoxidase technique. Previous work in nonprimate species has shown that these cells correspond to the starburst amacrine cells. Labelled somata were disposed on either side of the inner plexiform layer, and their processes formed two narrow zones within it. In human retinas, the ratio of labelled somata in the ganglion cell layer (GCL) to those in the inner nuclear layer (nominal Sb/Sa ratio) was about 60/40 at all locations, similar to that found in nonprimate mammalian species. The density of labelled cells in the human GCL ranged from 1,000 to 1,150 mm-2 near the fovea to 300 to 400 mm-2 in the periphery. Labelling tended to be more erratic in macaque retinas. Nevertheless the Sb/Sa ratio was as high as 70/30 and spatial densities were similar to those of humans. The overlap factor in macaque retinas outside the nasal quadrant was about 10 at all retinal eccentricities, based upon dendritic-field sizes from a Golgi study. About each labelled soma there was a region 20 to 120 microns in diameter in which the probability of the occurrence of other labelled somata was lower than elsewhere. No such nonrandomness was found between labeled cells in the GCL and those in the amacrine cell layer. The packing factor was about 0.3 in well-labelled regions, independent of retinal position or spatial density. Published data on ChAT-labelled cells in rabbit and rat show a similar value. This invariance is consistent with the hypothesis that this nonrandomness is a residual consequence of somal contiguity at an early developmental stage.

A comparison of methodologies for the study of functional transmitter neurochemistry in human brain

A number of different approaches to the study of functional neurochemistry in human brain are discussed. The advantages and disadvantages of three main techniques are contrasted: (i) using animal tissue preparations as models of the human brain; (ii) using human peripheral tissue preparations as models of dynamic CNS processes; and (iii) studying human tissue, obtained postmortem, directly. Animal models are often readily obtained and reliable, and the high degree of inbreeding of common laboratory animals ensures that they usually yield consistent results. However, there are a number of human disorders for which animal models are either poor or unavailable, and species differences make extrapolation from the animal to the human case difficult. Human peripheral tissue models rely on a degree of homology between peripheral and CNS processes; in most cases, the evidence for such homologies derives from animal, rather than human, studies. Moreover, several examples are known where a peripheral process mimics the equivalent glial cell activity more closely than the neuronal, which can be a serious drawback for studies of neurotransmission. The use of postmortem human brain tissue presents a number of obvious difficulties, resulting from variations in the patient's age, agonal state, sex, preterminal medication, postmortem delay, etc. Human beings are genetically and nutritionally heterogeneous, so that data variability is usually greater here than when using tissue from laboratory animals. However, it is possible to control for a number of these factors, for example, by matching samples for basal metabolic rate and tissue integrity, and recently developed tissue freezing and storage techniques permit the use of within-subject experimental designs to help reduce experimental variation. A range of neurotransmitter functions are well retained in such tissue samples, so that regional variations, differential transmitter activities, drug effects, etc., can be studied in normal tissue samples, as well as in samples taken from cases of neurological and psychiatric disease. This allows, for example, changes in neuroanatomical indices to be correlated with localised alterations in a specific neurotransmitter function. A systematic approach to the analysis and matching of tissue samples is advocated. The three approaches should be considered to be complementary, especially for the study of human brain diseases.

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.

Cholinergic neurons in the human retina

Choline acetyltransferase (ChAT)-like immunoreactivity in the human retina can be demonstrated using a polyclonal antiserum to ChAT isolated from chick brain. There is a population of ChAT-like immunoreactive cells along both margins of the inner plexiform layer (IPL). The labeled cells have a morphology and position characteristic of the cholinergic amacrine- and displaced amacrine cells demonstrated by other workers in the mammalian retina. Non-immune rabbit serum or pre-absorbed antiserum, used in place of the primary antiserum, verified the specificity of the method. Human retinas can also be labeled with the fluorescent dye 4',6-diamidino-2-phenylindole (DAPI), which has been reported to bind selectively to DNA in the nuclei of cholinergic cells. The fluorescent cells are similar in morphology, position, and distribution to the cells which show ChAT-like immunoreactivity. In addition, we have localized the presence of [3H]choline and [3H]choline metabolites in freeze-dried, vapor-fixed tissue using 'dry' autoradiographic techniques. Incubation in [3H]choline was followed by either stimulation or inhibition of calcium-dependent transmitter release during a 1-hr 'chase' period. Using tissue incubated in a chase designed to retain labeled neurotransmitters, silver grains were concentrated over a population of cell bodies at either margin of the IPL (i.e. in the same position as putative ChAT-immunoreactive cells and DAPI-labeled cells). In contrast, tissue incubated in a chase designed to release labeled acetylcholine was labeled uniformly throughout the neural retina, with a heavy band of label over the pigment epithelium. Taken together, the results presented here indicate that three independent markers for cholinergic cells (i.e. ChAT immunoreactivity, DAPI binding, and choline uptake) are present in a population of cells in the human retina. This suggests that acetylcholine may be a neurotransmitter synthesized by amacrine and displaced amacrine cells in the retina.