Synaptic organization of neurotensin immunoreactive amacrine cells in the chicken retina

The anti-apoptotic role of neurotensin

The neuropeptide, neurotensin, exerts numerous biological functions, including an efficient anti-apoptotic role, both in the central nervous system and in the periphery. This review summarizes studies that clearly evidenced the protective effect of neurotensin through its three known receptors. The pivotal involvement of the neurotensin receptor-3, also called sortilin, in the molecular mechanisms of the anti-apoptotic action of neurotensin has been analyzed in neuronal cell death, in cancer cell growth and in pancreatic beta cell protection. The relationships between the anti-apoptotic role of neurotensin and important physiological and pathological contexts are discussed in this review.

Expression of axonin-1 in developing amacrine cells in the chick retina

This study focused on the temporal and spatial pattern of expression of the cell adhesion molecule axonin-1 in amacrine cells and the identification of these cells in the developing chick retina. We analyzed 5-20-day-old chick embryos. The antigen was localized and visualized by the indirect immunogold and the immunofluorescence technique. Colocalization studies with antibodies against tyrosine hydroxylase, acetylcholinesterase, choline acetyltransferase, parvalbumin, calbindin, and calretinin served to characterize these cells further and to explore whether they have other properties in common. Axonin-1 was expressed in amacrine cells from E8 onward in the inner nuclear, in the inner plexiform, and in the ganglion cell layer. Their maturation showed a gradient similar to that found for amacrinogenesis. Expression was closely correlated with the period when the cells develop and shape their processes. The interneurons were classified with reference to Cajal, and most of the morphological types described by him were found. In addition, some cells were considered as axon-bearing amacrine cells. However, the total number of labeled cells was rather small. At least two morphologically different types terminated in each of the inner plexiform sublayers. Narrow- and wide-field arbors indicated the existence of a diversified network. The colocalization studies revealed that the neurotransmitters and neuropeptides overlapped partially with axonin-1 expression. This indicated that axonin-1-immunoreactive amacrine cells were also functionally diverse.

Development of the enkephalin-, neurotensin- and somatostatin-like (ENSLI) amacrine cells in the chicken retina

The development of the enkephalin-, neurotensin- and somatostatin-like immunoreactive (ENSLI) amacrine cells in the chicken retina has been investigated by radioimmunoassay (RIA) and immunocytochemistry (ICC). By RIA, enkephalin-like immunoreactivity (ENK-LI) was detected at embryonic day (E) 5 at only very low levels, which gradually increased until E17. From E18 to E21, there was a relatively rapid increase in ENK-LI levels, and just after hatching, there was a very steep rise. By ICC, the cell bodies of the ENSLI amacrine cells were first detected in the inner nuclear layer on E18, with no immunostaining in the inner plexiform layer (IPL). On E21, more cells were detected and processes in the IPL were visible, but detailed arborisations were not clear. On postnatal day (P) 1, the ENSLI amacrine cells showed a morphology similar to that in mature retina in both the density of cell bodies and the ramification pattern of processes. Antibodies to neurotensin and somatostatin revealed a similar developmental pattern. Thus, the three peptides appear to follow a similar developmental pattern in the ENSLI amacrine cells, suggesting that the three peptides respond similarly to developmental stimuli, just as they are released in parallel in response to physiological stimulation from mature ENSLI amacrine cells. After hatching, higher levels of ENK-LI were detected by RIA and more ENSLI amacrine cell bodies and processes were detected by ICC in animals kept in the light than in those kept in the dark. In retinas kept in the light for 12 h, it was found that immunoreactive processes in the IPL formed strongly stained patches, but this was not observed in retinas kept in the dark for 12 h.

Neurotensin induces calcium oscillations in cultured amacrine cells

The peptide, neurotensin, is found in a class of amacrine cells synapsing chiefly with other amacrine cells in the chicken retina (Li & Lam, 1990; Watt et al., 1991). To investigate the possible effects of neurotensin, we have used Ca2+ imaging to measure cytosolic Ca2+ concentrations in cultured chick amacrine cells. Following a delay of about 2 min, neurotensin (300 nM) induced oscillations in Ca2+ concentration that typically had a period of 2 min and peak values of about 300 nM when averaged over the cell body. The phospholipase C inhibitors U-73, 112 and 4'-bromophenacyl bromide terminated oscillations induced by neurotensin but the protein kinase inhibitors H7 and staurosporine did not inhibit oscillations, increasing their frequency instead. In the absence of external Ca2+, neurotensin induced only a single Ca2+ transient, much briefer than when external Ca2+ was present. Together these results suggest that neurotensin activates phospholipase C, thereby producing IP3 that triggers Ca2+ release from an internal store. Although this released Ca2+ contributes to periodic Ca2+ peaks, the majority of cytosolic Ca2+, even in the first peak, comes from Ca2+ influx across the plasmalemma.

Endogenous dopamine inhibits the release of enkephalin-like immunoreactivity from amacrine cells of the chicken retina in the light

The activity of the enkephalin-immunoreactive (ENSLI) amacrine cells of the chicken retina is low in the light and high in the dark, resulting in parallel increases and decreases in the levels of the enkephalins. In vivo, the selective dopaminergic D1 antagonist SCH23390 increased the activity of the ENSLI amacrine cells in the light (ED50; 20 pmol), but had a much lesser effect in the dark, whereas the selective dopaminergic D2 antagonist sulpiride had effects only at very high concentrations (ED50; 39 nmol). In contrast, the non-selective dopamine agonist ADTN hardly affected the activity of the ENSLI amacrine cells in the light, but markedly reduced their activity in the dark. This pattern of effects suggests that dopamine actively inhibits the ENSLI amacrine cells in the light, but exerts much less inhibitory activity in the dark, consistent with the idea that dopamine is released during the exposure of the retina to light. Thus dopaminergic controls over the ENSLI amacrine cells appear to contribute to the light:dark differences in activity of the ENSLI amacrine cells. Results obtained on the dopaminergic control of enkephalin release in vitro were generally consistent with this model, except that ADTN appeared to stimulate the ENSLI amacrine cells in the dark.

[Leu5]enkephalin-like immunoreactive amacrine cells are under nicotinic excitatory control during darkness in chicken retina

Based on the principle that retinal levels of [Leu5]enkephalin-like immunoreactivity (LELI) are set by the rate of release and thus reflect neural activity, we partially defined the dark-associated increase in excitatory control of LELI amacrine cells in chicken. Retinal levels of LELI were measured by radioimmunoassay (RIA). Intravitreal injection of cholinergic antagonists decreased the rate of depletion of LELI during the dark phase, suggesting the presence of cholinergic excitatory control of the LELI neurons. This cholinergic control involves nicotinic rather than muscarinic receptors, as tubocurarine appeared over 100 times more effective than atropine in inhibiting the decrease in retinal levels of LELI in the dark. (The ED50s were estimated at 3.2 and 450 nmol, respectively.) The lack of effect of the antagonists when applied during the light phase, suggest that there is little cholinergic input to the LELI amacrine cells in the light. Superfusing isolated retinas with buffer containing tubocurarine (10 microM) decreased the efflux of LELI by 35%, compared to the spontaneous release during the dark. Atropine (10 microM) had no effect on the release of LELI, and pilocarpine (100 microM) increased the release of LELI from retinas superfused in the light by 20%. We conclude that, in addition to previously reported glycinergic and dopaminergic inhibition, the LELI amacrine cells receive cholinergic excitatory input. A shift in balance between glycinergic and dopaminergic inhibitory, and cholinergic excitatory control may underly the light-driven variation in activity of the LELI neurons in chicken retina.

A double-label analysis demonstrating that all enkephalin-immunoreactive amacrine cells in the chicken retina express neurotensin immunoreactivity

A previous study demonstrated less than 50% co-existence between the populations of enkephalin- and neurotensin-like immunoreactive amacrine cells in the chicken retina. The present study was undertaken with the intent of re-examining this relationship using a more sensitive double-label paradigm. An examination of retinal cryosections collected throughout the retina revealed that all labelled cells express both enkephalin and neurotensin-like immunoreactivity. Therefore, these results indicate the presence of a single population of chicken amacrine cells the members of which express both these neuropeptides.

Coexistence of somatostatin and neurotensin in amacrine cells of the chicken retina

A comparison of previous immunocytochemical studies reveals a striking similarity in the morphologies of the populations of somatostatin-like and neurotensin-like immunoreactive amacrine cells in the chicken retina. A double-label analysis was performed to determine if these two neuroactive peptides coexist in chicken amacrine cells. An examination of retinal cryosections collected throughout the retina revealed that all labelled cells express both somatostatin- and neurotensin-like immunoreactivity. Therefore, these results indicate the presence of a single population of chicken amacrine cells whose members contain both of these neuroactive peptides.