Gonadotropin-releasing hormone (GnRF), molluscan cardioexcitatory peptide (FMRFamide), enkephalin and related neuropeptides affect goldfish retinal ganglion cell activity

Characterization of neuropeptide Y-like immunoreactivity in vertebrate retina

We have determined the endogenous levels of neuropeptide Y-like immunoreactivity (NPY-LI) in retinal extracts from pigs, cats, rabbits, chickens, and frogs by radioimmunoassay (RIA). The NPY-LI levels varied among the species. The highest concentration was found in frog retina, where seasonal variations were seen, 861 +/- 31 pmol g(-1) wet weight in the autumn and 334 +/- 26 pmol g(-1) wet weight in the spring. Lower levels were demonstrated in chicken and pig retina, 4.1 +/- 0.4 and 3.6 +/- 0.3 pmol g(-1) wet weight, respectively. The lowest concentration was demonstrated in rabbit retina, 2.0 +/- 0.3 pmol g(-1) wet weight. (All values are expressed as mean +/- S.E.M.). The NPY-LI in pig, rabbit, chicken and frog retina was characterized by high-performance liquid chromatography (HPLC). The main part of the extracted NPY-LI had an elution volume close to that to the porcine NPY. We have also analysed the evoked release of endogenous NPY from frog retina, induced either with light flashes (3 Hz, 300 lx), or with potassium depolarization of the neurons (40 mM). Light flashes and potassium induced an increased release of NPY-LI of 61 and 77%, respectively. NPY-LI in the efflux had the same HPLC retention time as that extracted directly from the retina.

Distribution of Met-enkephalin, Leu-enkephalin, substance P, neuropeptide Y, FMRFamide, and serotonin immunoreactivities in the optic tectum of the Atlantic salmon (Salmo salar L.)

The distribution of the neuropeptides methionine- and leucine-enkephalins, substance P, FMRFamide, neuropeptide Y, and vasoactive intestinal peptide, as well as the biogenic amine serotonin was studied in the optic tectum of the Atlantic salmon by means of immunocytochemistry. Peroxidase-antiperoxidase and indirect immunofluorescence methods were used to compare the differential laminar distribution of each of these substances. Nine parts of the optic tectum were selected for analysis on frontal sections: median, dorsolateral, and ventrolateral areas at rostral, medial, and caudal levels. Methionine- and leucine-enkephalin immunoreactive fibers were found in discrete sublayers in the following strata: stratum opticum, stratum fibrosum et griseum superficiale, stratum griseum centrale, stratum, and album centrale. Most of the substance P-, serotonin-, and vasoactive intestinal peptide-immunoreactive fibers were found in the stratum album centrale, whereas the FMRFamide- and neuropeptide Y-immunoreactive fibers were more or less randomly distributed within most of the strata of the optic tectum. Neuropeptide Y-immunoreactive cell bodies were located in the stratum periventriculare. We suggest an extrinsic origin for most of the immunoreactive fibers observed in the optic tectum, except for the neuropeptide Y-immunoreactive fibers that probably originate in the periventricular neurons. Although retinal peptidergic input to the optic tectum has been proposed in other vertebrates, there is no evidence that any of the neuropeptidelike or serotonin immunoreactive fibers in the optic tectum of the salmon should be of retinal origin. Differences and similarities with the distribution of neuropeptides in the optic tectum in representatives of other vertebrate classes are discussed.

Distribution of FMRFamide-like immunoreactivity in the brain, retina and nervus terminalis of the sockeye salmon parr, Oncorhynchus nerka

Neurons displaying FMRFamide(Phe - Met - Arg - Phe - NH2)-like immunoreactivity have recently been implicated in neural plasticity in salmon. We now extend these findings by describing the extent of the FMRF-like immunoreactive (FMRF-IR) system in the brain, retina and olfactory system of sockeye salmon parr using the indirect peroxidase anti-peroxidase technique. FMRF-IR perikarya were found in the periventricular hypothalamus, mesencephalic laminar nucleus, nucleus nervi terminalis and retina (presumed amacrine cells), and along the olfactory nerves. FMRF-IR fibers were distributed throughout the brain with highest densities in the ventral area of the telencephalon, in the medial forebrain bundle, and at the borders between layers III/IV and IV/V in the optic tectum. High densities of immunoreactive fibers were also observed in the area around the torus semicircularis, in the medial hypothalamus, median raphe, ventromedial tegmentum, and central gray. In the retina, immunopositive fibers were localized to the inner plexiform layer, but several fiber elements were also found in the outer plexiform layer. The olfactory system displayed FMRF-IR fibers in the epithelium and along the olfactory nerves. These findings differ from those reported in other species as follows: (i) FMRF-IR cells in the retina have not previously been reported in teleosts; (ii) the presence of FMRF-IR fibers in the outer plexiform layer of the retina is a new finding for any species; (iii) the occurrence of immunopositive cells in the mesencephalic laminar nucleus has to our knowledge not been demonstrated previously.

Electron microscopic study of immunocytochemically labeled centrifugal fibers in the goldfish retina

The centrifugal fibers innervating the goldfish retina were studied quantitatively by light and electron microscopy. These fibers originating from cell bodies in the olfactory bulb were labeled by antiserum to the tetrapeptide Phe-Met-Arg-Phe-NH2 (FMRFamide). The number of FMRFamide-immunoreactive (ir) centrifugal fibers in each eye of the adult goldfish (body length: 12-15 cm) was 65 +/- 14 (mean +/- S.D., n = 7). All of these fibers in the optic nerve and the retina were unmyelinated. Each FMRFamide-ir centrifugal fiber runs along the optic fiber layer and gives several terminal arborizations in the outermost layer (layer 1) of the inner plexiform layer. Layer 1 is, therefore, densely covered by a plexus of terminal arborizations. Along these terminal arborizations, we found output synapses characterized by a cluster of small clear vesicles (40 nm in diameter) at the presynaptic site and a thickened membrane in the apposed retinal cell processes. In a sample area of 2,000 microns 2, such synapses occurred at a density of one per 105 microns 2, or about 13,000 per centrifugal fiber. Thus, the FMRFamide-ir centrifugal fibers are likely to modulate retinal cell activity through an estimated total of 840,000 output synapses per retina.

Substance P-like immunoreactivity in the ganglion cells of the tench terminal nerve

The distribution of substance P (SP) in the olfactory bulb of the tench Tinca tinca was studied by using an indirect immunoperoxidase technique. Many perikarya and processes of the ganglion cells of the nervus terminalis (NT) were strongly labeled. In addition, SP-like immunopositive fibers were observed in the proximity of these neurons and extending along the olfactory nerves and the olfactory tracts. The ganglion cells of the NT were not immunoreactive for methionine- and leucine-enkephalin, motilin, vasoactive intestinal polypeptide, neuropeptide Y, cholecystokinin-8, and tyrosine hydroxylase.

Synaptic organization of enkephalinlike-immunoreactive amacrine cells in the goldfish retina

Immunoelectron microscopy was used to examine the synaptic organization of enkephalinlike-immunoreactive amacrine cells in the goldfish retina. Enkephalin-immunostained processes sometimes contained dense-cored vesicles (115-145 nm) in addition to a generally homogeneous population of small, round, clear synaptic vesicles. A total of 194 synaptic relationships were observed that involved the immunostained processes of enkephalin-amacrine cells. The large majority of these were observed in sublayer 5 of the inner plexiform layer. In greater than 95% of the synaptic relationships, the enkephalin-immunostained profile served as the presynaptic element. In 58.8% of these relationships, enkephalin processes synapsed onto amacrine cell processes, while 30.4% of their synapses were onto processes that lacked synaptic vesicles. They also occasionally formed synaptic contacts (6.7%) onto the somas of cells located either in the inner nuclear or in the ganglion cell layers. Enkephalin profiles received synaptic input only from amacrine cells (4.1%), while no direct synaptic interaction was observed between enkephalin processes and bipolar cells. However, in sublayer 1, enkephalin profiles were found to synapse onto amacrine cell processes that were presynaptic to bipolar cell terminals. In the proximal inner plexiform layer, enkephalin processes were presynaptic to amacrine cell processes that as a group surrounded and sometimes provided synaptic input to extremely large and round bipolar cell endings.

Localization and characterization of somatostatin binding sites in the mouse retina

We studied the binding of [125I]Tyr11-somatostatin-14 and [125I]Leu8,D-Trp22,Tyr25-somatostatin-28 to frozen, unfixed sections of C57BL/6J mouse eyes with autoradiography. Specific binding of both ligands occurred in 3 maxima, a broad band extending from the retinal ganglion cell to the inner nuclear layers, a narrow and inconstant band over the outer plexiform layer, and a band over the retinal pigment epithelium and choroid. We quantified the label over the inner plexiform layer and found evidence for a single, saturable binding site after Scatchard analysis of saturation binding data. With [125I]Tyr11-somatostatin-14 the dissociation constant (Kd) was 1.48 nM and the total number of binding sites (Bmax) was 68 fmol/mg protein; in competition experiments the inhibitory binding constant (Ki) was 900 pM for somatostatin-14 and 350 pM for somatostatin-28. With [125I]Leu8,D-Trp22,Tyr25-somatostatin-28, Kd was 625 pM and Bmax was 69 fmol/mg protein; in competition experiments Ki was 4.58 nM for somatostatin-14 and 710 pM for somatostatin-28. These results demonstrate the existence of somatostatin receptors in the inner plexiform layer of the retina that appear to have greater specificity for somatostatin-28 than for somatostatin-14.

Light and electron microscopic immunocytochemistry of neurons in the blowfly optic lobe reacting with antisera to RFamide and FMRFamide

Different antisera to the molluscan cardioexcitatory peptide FMRFamide, and its fragment, RFamide (Arg-Phe-NH2), label a distinct population of neurons in the optic lobe of the blowfly, Calliphora erythrocephala. Seven morphological types of RFamide/FMRFamide-like immunoreactive neurons could be distinguished in the optic lobes based on the locations of their cell bodies, their axonal projections and the distribution of their processes. Of these, two types could be resolved in their entire extent, the others were labeled only in their cell bodies and terminal processes or were partly obscured by other immunoreactive processes. The RF-like immunoreactive neurons in the optic lobes are of two main classes: (1) two types of large field projection neurons and (2) five types of local neurons. One type of projection neurons (five in each lobe) connects the entire projected retinal mosaic of the medulla and lobula in the optic lobe with protocerebral centres associated with the mushroom body calyx. The other type (2-3 invading each lobe) has cell bodies in the protocerebrum and contralateral processes invading optic lobes. Of the class of local neurons there are two amacrine RF-like immunoreactive neurons in each medulla. Each of these amacrines supplies the entire mosaic with fine processes. The remaining local RF-like immunoreactive neurons are present in relatively large numbers (one type in more than 2000 copies in each medulla) and-supply the medulla, lobula and lobula plate neuropils with fine varicose processes. In the medulla the RF-like immunoreactive processes are arranged in strict layers whereas in the lobula complex the distribution is diffuse. Electron microscopic immunocytochemistry, using both pre-embedding immuno peroxidase-antiperoxidase and post-embedding protein A-gold labeling, was employed for analysis of cytology and synaptic connections of RF-like immunoreactive neurons in the medulla. The varicosities of the processes of the large field projection neurons were not found to make chemical synapses with other neurons in the medulla. The spines of the RF-like immunoreactive processes of the large medulla amacrines, however, make pre- and postsynaptic contacts with other neural elements. Our findings indicate that an RFamide/FMRFamide-like substance may be used as a neurotransmitter or neuromodulator by optic lobe neurons of different types. The local and projection RF-like immunoreactive pathways probably play different roles in visual processing.

FMRFamide-like immunoreactive neurons of the nervus terminalis of teleosts innervate both retina and pineal organ

The tetrapeptide FMRFamide (Phe-Met-Arg-Phe-NH2) was first isolated from molluscan ganglia. Subsequently, it has become clear that vertebrate brains also contain endogenous FMRFamide-like substances. In teleosts, the neurons of the nervus terminalis contain an FMRFamide-like substance, and provide a direct innervation to the retina (Proc. Natl. Acad. Sci. U.S.A., 81 [1984] 940-944). Here we report the presence of FMRFamide-immunoreactive axonal bundles in the pineal organ of Coho salmon and three-spined sticklebacks. The largest numbers of axons were observed proximal to the brain, in the pineal stalk, while the distal part of the pineal organ contained only few axons. No FMRFamide-like-immunoreactive (IR) cell bodies were observed in the pineal organ. In adult fish it was not possible to determine the origin of these axons, due to the large numbers of FMRFamide-like IR axons in the teleost brain. However, by following the development of FMRFamide-like IR neurons in the embryonic and larval stickleback brain, it was possible to conclude that, at least in newly hatched fish, FMRFamide-like IR axons that originate in the nucleus nervus terminalis reach the pineal organ. Thus, it seems there is a direct connection between a specialized part of the chemosensory system and both the retina and the pineal organ in teleost fish.

Nervus terminalis innervation of the goldfish retina and behavioral visual sensitivity

The possibility that axon terminals of the nervus terminalis in the goldfish retina regulate visual sensitivity was examined psychophysically. Fish were classically conditioned to respond in darkness to a diffuse red light conditioned stimulus. Bilateral ablation of the olfactory bulb and telencephalon had no significant effect on response threshold which was measured by a staircase method. Retinopetal nervus terminalis fibres thus appear to play no role in maintaining scotopic photosensitivity.

Distribution of synaptic inputs onto goldfish retinal ganglion cell dendrites

Retinal ganglion cells in the goldfish were labeled by retrograde transport of horseradish peroxidase, and areas near the optic disk where the dendrites appeared to be completely filled were analyzed by electron microscopy. Only 6% of their inputs were ribbon synapses from bipolar cells; the other 94% of the inputs were conventional synapses mostly or entirely from amacrine cells. There were three strata of the inner plexiform layer with high densities of inputs to ganglion cells, the first centered at approx. 50% and the second at approx. 80% of the inner plexiform layer depth, as measured from the ganglion cell layer to the inner nuclear layer. These two strata comprised 25% of the volume but contained 41% of the inputs to ganglion cells. There were also two strata with very low densities of ganglion cell inputs located near the boundaries of the inner plexiform layer, from 0- to 15% and 90- to 100% of the depth. These strata, which also comprised 25% of the volume, contained only 7% of the inputs to retinal ganglion cells. These strata near the boundaries of the inner plexiform layer also contained 81% of the processes with large, dense-cored vesicles characteristic of peptidergic neurons. We concluded that each of the two sublaminae, a and b, identified previously by physiological criteria, could be further divided into at least two strata, one near the boundary of the inner plexiform layer with abundant peptidergic terminals and very few ganglion cell synapses and another near the center of the inner plexiform layer with numerous ganglion cell synapses. We also propose a hypothesis that could explain the functions of these additional strata.

Functional-anatomical studies on the terminal nerve projection to the retina of bony fishes

We have explored the structure and actions of terminal nerve (TN) fibers in the teleostean retina, the most accessible of TN projections. Using immunocytochemistry we have shown that the goldfish TN contains neuropeptides related to the molluscan cardioexcitatory peptide (FMRFamide) as well as luteinizing hormone-releasing hormone (LHRH). Retinal TN terminals were found upon major dendrites in the distal inner plexiform layer and neuronal cell bodies in the amacrine cell layer. Electron-microscopic double-labeling revealed TN terminals applied to the surface of [3H]-dopamine-, glycine-, and gamma-aminobutyric acid (GABA)-accumulating cells. Synthetic LHRH and FMRFamide at less than 1 microM modified spontaneous and light-evoked activity of ganglion cells in isolated superfused goldfish retina, especially during the active breeding season. Salmon(I)-LHRH was 10-30 times as potent as mammalian LHRH and caused rapid, prolonged desensitization. We conclude that LHRH- and FMRFamide-like peptides may be released by retinal TN endings, probably in concert with reproductive activity, and that they act independently through horizontal and/or amacrine cell pathways to modify visual information processing in the retina.