Interaction between enkephalin and gamma-aminobutyric acid in the chicken retina: a double-label immunoelectron microscopic analysis

Mammal retinal distribution of ENKergic amacrine cells and their neurochemical features: evidence from the PPE-GFP transgenic mice

The neuroactive peptide enkephalin (ENK) has been postulated to play important roles in modulating visual information. The retinal presence of ENKergic cells has been revealed with conventional morphological protocols targeting ENK molecule especially in avian, however, the detailed distribution of ENKergic cells and their specific neurochemical features in the mammal retina remain unclear because of the difficulties in visualizing ENKergic cells efficiently and reliably. To address this question, we took advantage of the preproenkephalin-green fluorescent protein (PPE-GFP) transgenic mice previously generated and identified in our group, and identified the neurochemical characteristics of retinal ENKergic cells. The majority of ENKergic cells occupied the proximal inner nuclear layer with a few displaced in the ganglion cell layer. Further double labeling revealed that most of these ENKergic amacrine cells used inhibitory glycine or gamma-aminobutyric acid as the primary neurotransmitter. However, some of them also utilized excitatory glutamate as the primary neurotransmitter. The present findings suggest that the retinal ENKergic cells fall into a subpopulation of amacrine cells and show predominantly inhibitory as well as less dominantly excitatory neurochemical features. Our findings offered comprehensive morphological evidence for the function of ENKergic amacrine cells of mammal species.

Colocalization of GABA, enkephalin and neuropeptide Y in the tectum of the green frog Rana esculenta

The colocalization of GABA, enkephalin and neuropeptide Y immunoreactivity in neurons in the pretectal area and in the mesencephalic tectum of the green frog (Rana esculenta) was studied. Several Met-enkephalin immunoreactive perikarya were found in layer 6 of the tectum and every third of these neurons showed GABA-ir as well. Colocalization of GABA and NPY could also be shown in half of the neuropeptide Y immunopositive cells in the 6th layer of the tectum, but only a few cells were double stained in layers 9 and 4. In the pretectal area no colocalization of the investigated peptides and GABA was found.

Morphological substrates underlying opioid, epinephrine and gamma-aminobutyric acid inhibitory actions in the rat locus coeruleus

The locus coeruleus (LC) has been implicated in attentional processes related to orienting behaviors, learning and memory, anxiety, stress, the sleep-wake cycle, and autonomic control, as well as to contributing to the affective state. Direct activation of LC neurons causes desynchronization of the electroencephalogram, suggesting that the LC is an important modulator of the behavioral state. The LC has been an intensely studied neuronal system, as the physiology and pharmacology of this nucleus is well understood. This is mainly because of the similarity in neurochemical composition of LC cells which all contain norepinephrine in the rat. However, the homogeneity in neurotransmitter content in LC neurons is sharply contrasted by the heterogeneity of neurochemicals found in its afferent processes. Among these are axon terminals that contain inhibitory and excitatory amino acids, monoamines, and neuropeptides, many of which have been shown to exert differential physiological effects on LC discharge activity. Although much attention has focused on physiological activation of LC neurons, substantial evidence indicates that diverse afferents prominently inhibit noradrenergic cellular activity. Such inhibitory neurochemicals, which arise from local and extrinsic sources, include gamma-aminobutyric acid (GABA) and epinephrine as well as the neuropeptides methionine5-enkephalin and leucine5-enkephalin. Inhibitory transmission in the LC has widespread implications for norepinephrine release at diverse postsynaptic targets, and clinically useful pharmacological agents such as clonidine, an alpha2 adrenergic receptor agonist that potently inhibits the firing of LC neurons, alleviate some negative physical symptoms observed following withdrawal from opiates. In the present review, the synaptic and functional organization of selected inhibitory-type neurotransmitters in the LC obtained from immunoelectron microscopic data will be discussed.

Immunocytochemical localization of the GABA(C) receptor rho subunits in the cat, goldfish, and chicken retina

Polyclonal antibodies against the N-terminus of the rat rho1 subunit were used to study the distribution of gamma-aminobutyric acid C (GABA(C)) receptors in the cat, goldfish, and chicken retina. Strong punctate immunoreactivity was present in the inner plexiform layer (IPL) of all three species. The punctate labelling suggests a clustering of the GABA(C) receptors at synaptic sites. Weak label was also found in the outer plexiform layer (OPL) and over the cell bodies of bipolar cells. Double immunostaining of vertical sections with an antibody against protein kinase C (PKC) showed the punctate immunofluorescence to colocalize with bipolar cell axon terminals. In the goldfish retina, the axon terminals of Mb1 bipolar cells were enclosed by rho-immunoreactive puncta. In the chicken retina, several distinct strata within the IPL showed a high density of rho-immunoreactive puncta. The results suggest a high degree of sequence homology between the rho subunits of different vertebrate species, and they show that the retinal localization of GABA(C) receptors is similar across different species.

Electron microscopic evidence for coexistence of leucine5-enkephalin and gamma-aminobutyric acid in a subpopulation of axon terminals in the rat locus coeruleus region

We recently described ultrastructural evidence for morphologically heterogeneous axon terminals containing the endogenous opioid peptide, methionine5-enkephalin (ENK), that formed synapses with neurons containing the catecholamine synthesizing enzyme, tyrosine hydroxylase, in the locus coeruleus (LC) of the rat brain. The morphological characteristics of these terminals suggested that ENK may be co-localized with either an excitatory or inhibitory amino acid. To further test this hypothesis, we combined immunogold-silver localization of gamma-aminobutyric acid (GABA) and immunoperoxidase labeling for ENK in single sections through the LC, in the present study, to determine whether ENK and GABA were contained within single axon terminals. Light microscopic analysis of ENK and GABA immunoreactivities in the LC indicated that both transmitters were enriched in the dorsal pons. Although electron microscopy revealed that ENK and GABA were located primarily in axon terminals, some dendrites also contained immunolabeling for GABA. The dense core vesicles were consistently the most immunoreactive in ENK containing axon terminals and were identified toward the periphery of the axon terminal distal to the synaptic specialization. Axon terminals containing either ENK or GABA immunoreactivities contained pleomorphic vesicles as well as large dense core vesicles, varied in size and formed heterogeneous types of synaptic specializations (i.e. asymmetric vs. symmetric). Approximately 38% (n = 76) of the axon terminals containing ENK immunoreactivity (n = 200) also contained GABA. Some axon terminals containing peroxidase labeling for ENK (22%; n = 44) converged on common targets with GABA-labeled axon terminals. Finally, a few ENK-labeled axon terminals (14%; n = 28) formed asymmetric (excitatory-type) synapses with dendrites containing gold-silver labeling for GABA. The results, therefore, indicate that the opioid peptide, ENK, and the inhibitory amino acid, GABA, may influence LC neurons by concerted actions via (1) release from a common axon terminal, and (2) via separate sets of afferents converging on similar portions of the plasmalemma of target neurons. Furthermore, these studies also suggest a cellular substrate for opioid inhibition of LC neurons via activation (i.e. asymmetric synapses) of inhibitory GABAergic neurons. Future studies are required to determine whether the receptive sites for ENK and GABA are located at similar sites on the plasma membranes of LC neurons pre- or postsynaptically and whether there is differential release of either transmitter from single terminals in the LC.

Preproenkephalin messenger RNA-containing amacrine cells in the chicken retina identified with in situ hybridization histochemistry

Enkephalin peptides are present in the retina of several vertebrate species. In the avian retina, enkephalin immunoreactivity is primarily localized to a population of amacrine cells. In the present study, we determined the localization of cells expressing preproenkephalin (PPE) mRNA, which encodes the precursor of enkephalin peptides, in adult as well as in embryonic chicken retinas. The localization of PPE mRNA-expressing cells to the proximal inner nuclear layer (INL) in the adult chicken retina is similar to that of enkephalin-immunoreactive cells observed in previous studies, indicating that amacrine cells expressing PPE mRNA synthesize Met5- and Leu5-enkephalin peptides and related extended forms. Specific hybridization signal is absent in retinas at embryonic day (E) 11, but it is detected in retinas at E 15 and at hatching. PPE mRNA-expressing cells at these ages are located in the proximal INL, and they can be classified as amacrine cells on the basis of their soma size and laminar position. These findings confirm and extend previous observations on the presence of opioid peptides in amacrine cells of the chicken retina. The presence of PPE mRNA at embryonic ages, together with the evidence that enkephalins influence developmental processes, suggests that these peptides modulate retinal maturation in birds.