The cholinergic system in retinal transplants in rats

Acute and chronic effects of intravitreally injected beta-amyloid on the neurotransmitter system in the retina

The potential cytotoxic effect of aggregated Abeta(1-42) to neurons that express classical neurotransmitters, including acetylcholine, gamma-amino butyric acid, catecholamines and serotonin was assessed. The cholinergic system has been the central focus of the therapeutic drug strategies in amyloid-depositing pathologies such as Alzheimer's disease. Aggregated Abeta(1-42) has a multisystem cytotoxic effect causing non-specific reduction in immunoreactivity, dysfunction, or loss of retinal nerve cells. The extent of this was investigated using immunocytochemistry, TUNEL staining for apoptosis, and measurement of cell density as well as retinal surface area. There was a differential acute and/or chronic effect of Abeta on choline acetyltransferase, gamma-aminobutyric acid and 5-tryptamine hydroxylase systems, observed with the increasing time course of 6h to 5 months, and a bilateral/systemic effect. In contrast, the overall pattern of catecholaminergic system, as revealed by tyrosine hydroxylase immunoreactivity of the retina, appears to have remained relatively unaffected by Abeta (however this may reflect neuronal loss due to reduction in the retinal surface). This is the first in vivo evidence in a CNS model to show that not only all major neurotransmitter systems are differentially affected by Abeta aggregates but the effect may vary from one transmitter system to another under the same experimental conditions in situ and in a dose- and time-dependent manner.

Dual substrate model for novel approach towards a kinetic study of acetylcholinesterase inhibition by diazinon

Limited reports as compared to other insecticides appear in the literature for acetylcholinesterase (AChE) inhibition by diazinon. In the current study, new kinetic parameters of AChE inhibition by diazinon have been investigated. The assay was done with bovine retinal AChE using two different substrate (ASCh) concentrations in the absence and presence of diazinon (0.08-1.28 mM). The optical density was monitored up to 25 min (reaction time) for the assay. New kinetic parameters k'(oms), K'(sms), k(oms), K(sms), K'(asms) and K(asms) ) were calculated from these experimental data.

NADPH-diaphorase activity in normally developing and intracranially transplanted retinas

The activity and distribution of nicotinamide dinucleotide phosphate diaphorase (NADPH-d), an enzyme that is widely distributed in the central nervous system and involved in the production of the free radical nitric oxide, were investigated histochemically in the normal developing and intracranially transplanted retinas. In the normal rat retina, NADPH-d activity was first detected in cells in the ganglion cells layer (GCL) and blood vessels on the first postnatal day (P0). A small but distinct population of NADPH-d positive cells were observed along the inner border of the inner nuclear layer at P7. NADPH-d positive sublaminae began to appear in the inner plexiform layer during the second postnatal week, and several strongly reactive sublaminae resembling those observed in the adult were observed by the fourth postnatal week. The overall spatio- temporal sequence of development of NADPH-d positive cells in the transplanted retina was similar to that of the normal retina, except a lack of reactive in the inner plexiform layer in more mature transplants as compared with normal retinas of corresponding ages. These results indicate that the time course of development and distribution of NADPH-d cells in early postnatal retina requires signals mainly of intraretinal origin and is independent of influence from the surroundings. While this finding is supportive to the notion that neurons that are rich in NADPH-d are resistant to injury or perturbation, the observation of a lack of well organized NADPH-d reactive sublaminae in the inner plexiform layer in older transplants suggests a possible alteration in the synaptic circuitry in the inner retina with increasing postgrafting survival time.

Evaluation of the nature of camel retinal acetylcholinesterase: inhibition by hexamethonium

Acetylcholinesterase (AChE, EC 3.1.1.7) has been demonstrated in retinas of several species, however, the nature of the interaction of AChE with specific inhibitors are very limited in the literature and the mode of inhibition of camel retinal AChE by hexamethonium has been studied. Hexamethonium reversibly inhibited AChE in a concentration dependent manner, the IC50 value being c. 2.52 mM. The Km for the hydrolysis of acetylthiocholine iodide was found to be 0.087 mM and the Vmax was 0.63 mumol/min/mg protein. Dixon, as well as Lineweaver-Burk, plots and their secondary replots indicated that the nature of the inhibition is of the hyperbolic (partial) mixed type, which is considered to be a partial competitive and non-competitive mixture. The values of Ki(slope) and KI(intercept) from a Lineweaver-Burk plot were estimated as 0.30 mM and 0.17 mM, respectively, while Ki from a Dixon plot was estimated as 0.725 mM. The Ki was greater than KI indicating that hexamethonium has a greater affinity of binding for the active site than the peripheral site of the camel retina AChE.

Axon-target interactions maintain synaptic gene expression in retinae transplanted to intracranial regions of the rat

In the present study we examined the effects of optic axon-CNS target interactions on gene expression in the rat retina. These studies took advantage of a transplantation paradigm that allowed us to assay gene expression in retinae transplanted to different intracranial locations in the neonatal rat that either promoted (dorsal midbrain) or precluded (cerebral cortex) the formation of retino-collicular connections. Using in situ hybridization experiments, we observed that transplantation to the dorsal midbrain resulted in a relatively normal pattern of nicotinic acetylcholine receptor (nAChR) beta-3 subunit and glutamate receptor 3 (GluR3) gene expression. In contrast, retinae transplanted to the cerebral cortex (which did not result in normal retino-collicular interactions) showed a dramatic reduction in nAChR beta-3 subunit and GluR3 gene expression. These results agree with those obtained in the adult goldfish retina, where it has been demonstrated that an optic nerve-optic tectum interaction is responsible for the re-induction nAChR and NMDA receptor gene expression during optic nerve regeneration. Taken together, these results support the hypothesis that proper axon-target interactions are required for maintenance of nAChR and glutamate receptor gene expression in the mature vertebrate retina.

Thermodynamic investigation of camel retina acetylcholinesterase inhibition by cyclophosphamide

The present work addresses the estimation of energy parameters such as Gibb's free energy change (delta G), enthalpy change (delta H); entropy change (delta S) and activation energy (E a) of acetylcholinesterase (AChE, EC 3.1.1.7) from camel retina in the absence and presence of the antineoplastic drug cyclophosphamide (CP). A spectrophotometric method was used for the determination of AChE activity, which was the basis for determination of these parameters. The PZ factor (number of sterically and energetically favorable collisions occurring between CP and AChE) have also been studied in this investigation. The energy parameters obtained in the present investigation were compared with the values reported elsewhere.

The inhibitory effect of cyclophosphamide on camel retina acetylcholinesterase activity

Kinetic parameters for the effect of cyclophosphamide (CP) on the camel retina acetylcholinesterase (AChE) activity were investigated for the first time in the present study. It was found that 18 micrograms of retina protein and an incubation time of 4.0 min were suitable conditions for linear of AChE activity. The CP effect was independent of time of incubation with AChE before the addition of substrate, which shows it reversible action. Moreover, dilution data prove that CP is a reversible inhibitor of camel retina AChE. Cyclophosphamide (0.2-2.4 mM) inhibited activity of camel retina in a concentration-dependent fashion, the IC50 being about 1.17 mM. The Michaelis constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.106 mM and the Vmax was 0.765 mumol/min/mg protein. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the pure noncompetitive type. The value of Ki was estimated as 0.763 mM by the primary Dixon and secondary replots of the Lineweaver-Burk plot.

Kinetics for camel (Camelus dromedarius) retina acetylcholinesterase inhibition by methotrexate in vitro

This work addresses the kinetic analysis of the interaction of methotrexate (MTX) with camel retina acetylcholinesterase (A ChE, EC 3.1.1.7). It was found that the MTX effect was reversible in nature. The IC50 was determined, by two methods, to be 1.362 mM. The Michaelis-Menten constant (Ks) for the hydrolysis of acetylthiocholine iodide (ASCh) by AChE was 0.123 mM in the control system, and the MTX-treated systems showed a 10-35% decrease in this value. The Vmax was 0.789 mumol/min/mg protein for the control system, while it was decreased by 23-76% in the MTX-treated systems. The Lineweaver-Burk plot, Dixon plot and their secondary replots indicated that the inhibition was a linear mixed type; i.e., uncompetitive and noncompetitive. The values of Ki and KI were estimated as 0.782 and 0.404 mM, respectively. The use of camel retina as a model for the study of human retina may open new avenues for studying various aspects of AChE.

The nature of the inhibition of camel retina acetylcholinesterase (EC 3.1.1.7) activity by tetrahydroaminoacridine

The nature of the inhibition of camel retina acetylcholinesterase (AChE) activity by tetrahydro-aminoacridine (THA, tacrine) has been investigated in the present study. The non-significant change of the percent inhibition of AChE by THA with respect to various lengths of the preincubation period showed the type of the reversible inhibition. THA reversibly inhibited AChE activity in a concentration dependent manner; IC50 was 0.23 microM while the IC100 was 14.22 microM. The K(m) for the hydrolysis of acetylthiocholine iodide was found to be 62.6 microM in the control system; a value increased in the THA treated systems. The Vmax was 0.472 mumole/min/mg protein for the control system, while it decreased in the THA treated systems. Dixon, as well as Lineweaver-Burk, plots and their secondary replots indicated that the nature of the inhibition is of the linear mixed type, which is considered to be a partial competitive and pure non-competitive mixture. The values of Ki(slope) and K'i(intercept) were estimated as 0.068 microM and 0.181 microM, respectively. The K'i was greater than Ki indicating that THA has a greater affinity of binding for the peripheral site than the active site of the camel retina AChE. The use of camel retina as a good experimental animal model may open new avenues for studying acetylcholine and AChE metabolism.

Development of gamma-aminobutyric acid-immunoreactive neurons in normal and intracranially transplanted retinas in rats

Retinas from embryonic day 14 Sprague-Dawley rats were transplanted intracranially to the midbrain or cortex of newborn (P0) rats with right eyes enucleated at the time of transplantation and the gamma-aminobutyric acid (GABA) immunoreactivity in developing retinal transplants, host as well as normal retinas, was studied. The results showed that GABA-immunoreactive neurons were identified in retinas of normal and host rats from the day of birth (P0) onward and that their somata were distributed primarily in the inner half of the internal nuclear layer and in the ganglion cell layer. The adult pattern of GABA immunoreactivity was first observed at P16 when several immunoreactive sublaminae were clearly identifiable in the inner plexiform layer. In contrast, gamma-aminobutyric acid-immunoreactive somata could not be identified in retinal transplants until P4, with a significant reduction in the density and number of GABAergic neurons detected by P12. Moreover, only two immunoreactive sublaminae were observed in the inner plexiform layer in all transplants at P12, as well as in more mature stages. These results suggest that significant changes occurred in the GABA system of the transplanted retina, despite the fact that the overall pattern of organization of the GABAergic neurons and their processes in the retinal transplants was comparable to that of the normal retina.

Development of choline acetyltransferase-immunoreactive neurons in normal and intracranially transplanted retinas in rats

Retinas from embryonic day 14 (E14) Sprague-Dawley rats were transplanted to the tectum of newborn (P0) recipient rats, and the distribution pattern of choline acetyltransferase immunoreactivity (ChAT-I) in developing transplants was studied and compared with those observed in the retinas of normal developing rats. In normal retinas, ChAT-I cells were first identified in restricted regions in the ganglion cell layer (GCL) at P4, but were found to cover the entire GCL by P6. A second population of ChAT-I cells was detected in the inner nuclear layer (INL) at P8, and they were observed in most parts of the INL on P10 when two immunoreactive sublaminae began to appear in the inner plexiform layer (IPL). The adult pattern of having two distinct populations of ChAT-I cells, organized in mirror symmetrical fashion in the inner retinal layers was basically established by P12. The time course of development and overall distribution pattern of ChAT-I cells in developing retinal transplants on the whole were very similar to those observed in normal retinas. The first identification of these cells and the establishment of their final distribution pattern were made at stages corresponding to P4 and P12 of normal developing retinas respectively. However, ChAT-I somata were located in the INL at a much earlier stage compared with their counterparts in the normal retina, and a transient population of immunoreactive cells with their processes extending to retinal layers other than the IPL was observed in some transplants from P6 to P10. These features were not observed in normal developing retinas. These results suggest that the development of cholinergic neurons, especially the expression of their characteristic antigen and their final distribution pattern is largely determined by programmes which are intrinsic to the original retinal tissue, despite some minor deviation or variation in the developmental process which may occur under certain abnormal conditions.