Neuroendocrine plasticity in GnRH release during rat estrous cycle: correlation with molecular markers of synaptic remodeling

Agonists of serotonin (5-HT)-1 receptors modulate the synaptic strength of the connection between retinal ganglion cells and neurons of the frog optic tectum in brain slices (Brain Res. 1998;781:167-181). We have now used autoradiographic receptor binding techniques to determine the location of 5-HT1A and 5-HT1B binding sites in the laminated optic tectum. 5-HT1A binding sites, as labeled with [3H]8-hydroxy-dipropylaminotetralin (8-OH-DPAT), were highest in the superficial, retinorecipient layers of the tectum, intermediate in layers 6 and 7 and low in the remaining layers. Binding densities in all of these layers were unaffected by optic nerve lesion. 5-HT1B binding sites were visualized using [125I]iodocyanopindolol (ICYP). Binding densities were highest in the plexiform layers 5 and 7 and intermediate in layers 6 and 8. Binding sites were present at low levels in layer 9; however, optic nerve lesion resulted in a strong upregulation of these sites in this layer. Pharmacological manipulation of receptor activation resulted in changes in the activity-dependent visual map that is created at the tectum by retinal ganglion cell terminals. Chronic treatment of the tectum with SB-224289, a selective antagonist of 5-HT1B receptors, disrupted the topographic map. In contrast, exposure to WAY-100635, a selective antagonist of 5-HT1A receptors, refined it. We conclude that both 5-HT1A and 5-HT1B receptors are present in the adult frog tectum and that changes in their activation levels can produce changes in retinotectal transmission levels that drive visual plasticity in opposite directions.     

Permanent disorganization of the regenerating optic tract in the frog

The organization of the optic tract as it regenerates following optic nerve transection in adult frogs was studied using microelectrode recordings from optic nerve arborizations in the tectum. In normal frogs, a cut extending from the midline partway across the rostromedial margin of the tectum severs optic axons with receptive fields in the temporosuperior quadrant of the visual field. During regeneration, however, a similar cut spares many axons with temporosuperior fields. This result implies that some fibers which normally enter the tectum via the most medial parts of the optic tract regenerate through other parts of the tract. Despite their anomalous routes, many of these fibers eventually terminate at the appropriate locations in the tectum.     

Distribution and development of nicotinic acetylcholine receptor subtypes in the optic tectum of Rana pipiens

Acetylcholine allows the elicitation of visually evoked behaviors mediated by the frog optic tectum, but the mechanisms behind its effects are unknown. Although nicotinic acetylcholine receptors (nAChRs) exist in the tectum, their subtype has not been assessed. By using quantitative autoradiography, we examined the binding of [(3)H]cytisine and [(125)I]alpha-bungarotoxin in the laminated tectum. In mammalian systems, these radioligands bind with high affinity to alpha4 nAChR subunits and alpha7 nAChR subunits, respectively. [(3)H]Cytisine demonstrated high specific binding in adult frogs in retinorecipient layer 9, intermediate densities in layer 8, and low binding in layers 1-7 of the tectum. [(3)H]Cytisine binding was significantly higher in the tecta of adults than in those of tadpoles. Lesioning the optic nerve for 6 weeks decreased [(3)H]cytisine binding in layers 8/9 by 70+/-1%, whereas 6-month lesions decreased binding by 76+/-3%. Specific binding of [(125)I]alpha-bungarotoxin in adults was present only at intermediate levels in tectal layers 8 and 9, and undetectable in the deeper tectal layers. However, the nucleus isthmi, a midbrain structure reciprocally connected to the tectum, exhibited high levels of binding. There were no significant differences in tectal [(125)I]alpha-bungarotoxin binding between tadpoles and adults. Six-week lesions of the optic nerve decreased tectal [(125)I]alpha-bungarotoxin binding by 33+/-10%, but 6-month lesions had no effect. The pharmacokinetic characteristics of [(3)H]cytisine and [(125)I]alpha-bungarotoxin binding in the frog brain were similar to those demonstrated in several mammalian species. These results indicate that [(3)H]cytisine and [(125)I]alpha-bungarotoxin identify distinct nAChR subtypes in the tectum that likely contain non-alpha7 and alpha7 subunits, respectively. The majority of non-alpha7 receptors are likely associated with retinal ganglion cell terminals, whereas alpha7-containing receptors appear to have a different localization.     

Target selection by surgically misdirected optic fibers in the tectum of goldfish

This study tested the capacity of regenerating optic fibers to read tectal markers and thereby grow to their appropriate tectal loci when initial position, optic pathway, and interfiber interactions are eliminated as useful cues. The stability of these markers with long-term optic denervation of the tectum was also examined. In adult goldfish optic fibers innervating lateroposterior optic tectum were dissected free of tectum and inserted into the medial anterior region of the opposite "host" tectum. Normally, fibers at this position either innervate medial anterior tectum or follow the medial division of the optic pathway into medioposterior tectum. Host tectum was denervated of all other optic fibers by enucleating its contralateral eye either at the time of the deflection or at various times up to 18 months prior to deflection. The regeneration of these deflected fibers into host tectum was examined by autoradiography and electrophysiology at 1 to 11 months later. At the insertion site deflected fibers split into two groups of roughly equal size. One group directly entered the optic layers of medial tectum and grew posterolaterally across the medial half of tectum into the lateral half. The second group followed an almost direct path to the lateral tectum, sometimes traversing through the deep cell layers of tectum in which optic fibers are not usually found. These fibers subsequently entered the optic layers at the lateral edge of tectum and grew posteriorly. This second path was not seen in controls in which optic fibers from medioposterior tectum were similarly deflected. Instead growth was almost entirely posteriorly directed. On the average by 1.5 months deflected lateroposterior fibers were preferentially distributed in the lateral half of the tectum. Densitometric measurements indicated nearly a 4-fold difference in lateroposterior compared with medial posterior labeling. By contrast, controls in which medial posterior fibers were deflected had 4 times more grains medially than laterally. There was also a posterior over anterior preference, but this was weak. There was no suggestion that long periods of optic denervation prior to deflection or long postoperative periods after deflection of lateroposterior fibers diminished the lateral over medial preference. These findings support the idea that stable tectal markers exist which are differentially read by medial and lateral optic fibers. However, in no case was the innervation by deflected fibers as selective as in the normal projection.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of axotomy on the expression of NADPH-diaphorase in the visual pathway of the tench

The distribution of NADPH-diaphorase (ND) positive elements was analyzed throughout the visual pathway of the tench in normal conditions and after optic nerve transection. In the control retina, ND-labeled elements were observed in the photoreceptor, inner nuclear, outer nuclear and ganglion cell layers. In the optic nerve of control animals, small and numerous ND-positive glial cells that were identified as presumably astrocyte-like cells were observed. In the optic tracts and optic tectum, a different type of ND-positive glial cell was detected. Axotomy induced severe changes in the ND staining pattern in the visual pathway. A decrease in the number of ND-stained cells was detected in the retina. In the optic nerve of lesioned animals, the number of small cells gradually decreased, whereas the number of large cells did not change. Two new ND-positive cell populations were observed after the lesion: microglial-like cells appeared close to the lesioned area from 24 h to 7 days after transection, and astrocyte-like cells were found throughout the optic nerve from 14 days up to at least 120 days. The total number of ND-stained glial cells increased at 30 and 60 days and returned to control parameters at 120 days. In addition, the number of ND-positive cells increased at the same survival times in the optic tracts and in the retinorecipient strata of the optic tectum with respect to control animals. Thus, degenerative/regenerative processes in the fish visual pathway are accompanied by an increase in the number of ND-positive cells. Synthesis of nitric oxide is elicited in microglial-like cells as a response to axon injury, whereas the expression in astrocyte-like cells seems to be associated with both normal processes under physiological conditions and with the regenerative phase after the lesion.     

3H]kainate localization in goldfish brain: receptor autoradiography and membrane binding

The anatomical distribution of specific [3H]kainate binding in goldfish brain was investigated by membrane binding and autoradiographical techniques. Saturation binding of the radioligand was determined in 8 anatomically defined regions and demonstrated a single class of high affinity sites with Kd values ranging from 290 to 650 nM. Kainate receptor densities, however, varied significantly. The cerebellum contained the highest concentration of binding sites (964 pmol/mg prot.), while the optic tectum had the lowest (96 pmol/mg prot.). Binding site distributions determined by autoradiographic studies demonstrated the same regional variation and allowed more specific localization of the binding sites. Within the cerebellum, the molecular layers of the corpus, valvula and lobus caudalis displayed a uniform and highly intense image while the granule cell layers (except for the medial granule cell mass of the lobus caudalis) did not. Other areas of intense binding were the posterior tubercle of the diencephalon, inferior lobes of the hypothalamus and layers 1 and 2 of the optic tectum (deep to the periventricular granule cells).     

Distribution, laminar location, and morphology of tectal neurons projecting to the isthmo-optic nucleus and the nucleus isthmi, pars parvocellularis in the pigeon (Columba livia) and chick (Gallus domesticus): a retrograde labelling study.

Retrograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L), fluorogold, fast blue, rhodamine labelled microspheres, and horseradish peroxidase (HRP) was employed to study the distribution, laminar location within the optic tectum, and morphology of tectal cells projecting upon the isthmo-optic nucleus (ION) and the nucleus isthmi, pars parvocellularis (Ipc), in the pigeon and chick. Following injections into the ION, all retrograde markers labelled tecto-ION neurons and their dendrites in the ipsilateral tectum. The cells were located within a relatively narrow band at the border between layers 9 and 10 of the stratum griseum et fibrosum superficiale (SGFS). Retrogradely labelled neuronal somata were different in both dendritic branching and shape; however, tecto-ION neurons generally possessed non-spiny radially oriented and multi-branched dendrites. The apical processes extended into the retino-recipient layers (2-7) of the SGFS and basal dendrites extended into layers 12-14 of the SGFS. Positive neuronal somata were observed throughout the rostro-caudal extent of the optic tectum. The average distance between adjacent tecto-ION neurons varied from one region to another. Specifically, retrogradely labelled cells were more numerous in the caudal, lateral, and ventral tectum, and less numerous at rostro-dorsal levels. Approximately 12,000 tecto-ION neurons were labelled within the ipsilateral optic tectum following either PHA-L or fluorescent dye injections. While the regional distribution of tecto-Ipc neurons was not examined, the morphology indicated that the cells had a single radially oriented dendritic process. Therefore, the apical dendrites are more restricted than those of tecto-ION cells. Moreover, the dendrites were spiny and arborized within layers 3, 5, and 9 of the ipsilateral optic tectum. The axon of tecto-Ipc cells arise from the apical process as a shepherd's crook and descend into the deep layers of the optic tectum. These results indicate that 1) tecto-ION and tecto-Ipc neurons are possibly monosynaptically activated by retinal input, 2) tecto-ION neurons are heterogeneous in morphology, and 3) there is a differential distribution of the tecto-ION neurons throughout the rostro-caudal extent of the optic tectum, suggesting a greater representation of the caudo-ventral portion of the optic tectum within the ION. The discussion primarily concerns the organization of the retino-tecto-ION-retinal circuit in light of the distribution and morphology of tecto-ION neurons within the optic tectum.     

Neurogenesis in the optic tectum of larval Rana pipiens following unilateral enucleation

DNA synthesis and interlayer migrations of cells in the optic tectum of larval Rana pipiens were investigated, using several series of larvae which had been subjected to unilateral enucleation at stage 25, the last embryonic stage. It has been found that DNA synthesis occurs in all cellular layers of the tectum with least activity in peripheral layers. The location of the most active DNA synthesis during the larval period is the same as the location of cell division in the larval tectum, namely, in the layers bordering the ventricle of the optic lobe Unilateral enucleation of stage 25 embryos results in a decrease in DNA synthesis in all cellular layers of the optic tectum contralateral to the operation when compared to the corresponding layers in the ipsilateral tectum. The differences in rate of incorporation of ³H-thymidine between the control and affected lobes become greater during development. Fewer cells are found in each layer of the affected larval tectum than in the corresponding layer of the control tectum. The decrease is greatest in more peripheral layers, whose cells are more intimately associated with the visual circuit than are cells of the deeper layers. Peripheral to layers 1 and 2, the percentages of labeled cells found in each layer are very similar on the two sides, suggesting common factors which control migration. Differences in cell number, therefore, reflect differential cell production rather than differential cell migrations. The distribution of label resulting from ³H-thymidine incorporation at stage III indicates that the distribution of mitotic activity is not uniform in the cephalocaudal axis of the tectum. Greater cell proliferation occurs in the posterior portion of the tectum than in the anterior region throughout larval development. The peripheral control of mitotic divisions in the frog optic tectum remains unknown. The data in the present study, however, support the hypothesis that influences from afferent fibers of the optic tract modify the rates or timing of DNA synthesis in the optic tectum. The data support the notion that the deepest tectal cells respond earliest to the stimulus and these may be ependymal cells which have processes extending to the outer surface of the tectum.     

Evidence for shifting connections during development of the chick retinotectal projection

The pattern in which optic axons invade the tectum and begin synaptogenesis was studied in the chick. The anterogradely transported marker, horseradish peroxidase, was injected into one eye of embryos between 5 and 16 days of development (E5 to E16). This labeled the optic axons in the brain. The first retinal axons arrived in the most superficial lamina of the tectum on E6. They entered the tectum at the rostroventral margin. During the next 6 days of development the axons grew over the tectal surface. First they filled the rostral tectum, the oldest portion of the tectum, and then they spread to the caudal pole. Shortly after the first axons entered the tectum on E6, labeled retinal axons were found penetrating from the surface into deeper tectal layers. In any given area of the tectum, optic axons were seen penetrating deeper layers shortly after arriving in that area. Electron microscopic examination showed that at least some of the labeled axons in rostral tectum formed synapses with tectal cells by E7. These results show two things which contrast with results from previous studies. First, there is no delay between the time the retinal axons enter the tectum and the time they penetrate into synaptic layers of the tectum. Second, the first retinotectal connections are formed in rostral tectum and not central tectum. Retrograde tracing showed the first optic axons that arrived in the tectum were from ganglion cells in central retina. Previous studies have shown that the ganglion cells of central retina project to the central tectum in the mature chick. This opens the possibility that the optic axons from central retina, which connect to rostral tectum in the young embryo, shift their connections to central tectum during subsequent development. As they enter the tectum the growth cones of retinal axons appear to be associated with the external limiting membrane. During the time that connections would begin to shift in the tectum a second population of axons appears at the bottom of stratum opticum, some with characteristics of growth cones. This late-appearing population may represent axons shifting their connections. These results have implications for theories on how the retinotopic pattern of retinotectal connections develops.     

Localization of α-bungarotoxin binding sites to the goldfish retinotectal projection

The optic tectum of the goldfishCarassius auratus is a rich source of α-bungarotoxin (α-Btx) binding protein. In order to determine whether some fraction of these receptors is present at retinotectal synapses, we have compared the histological distribution of receptors revealed by the use of [¹²⁵Iα-Btx radioautography to the distribution of optic nerve terminals revealed by the use of cobalt and horseradish peroxidase (HRP) techniques. The majority of α-Btx binding is concentrated in those tectal layers containing primary retinotectal synapses. The same layers contain high concentrations of acetylcholinesterase (AChE), revealed histochemically. Following enucleation of one eye, there is a loss of α-Btx binding in the contralateral tectum, observed both by radioautography and by a quantitative binding assay of α-Btx binding. Approximately 40% of the α-Btx binding sites are lost within two weeks following enucleation. By contrast, no significant change in AChE activity could be demonstrated up to 6 months enucleation. These results are discussed in light of recent studies which show that the α-Btx binding protein and the nicotinic acetylcholine receptor are probably identical in goldfish tectum. We conclude that the 3 main classes of retinal ganglion cells projecting to the goldfish tectum are nicotinic cholinergic and that little or no postdenervation hypersensitivity due to receptor proliferation occurs in tectal neurons following denervation of the retinal input.     

Optic axons ignore foreign denervated sites in goldfish tectum even after removal of some of their normal termination sites

Previous experiments have shown that optic axons ignore foreign denervated territory in the goldfish tectum and preferentially reinnervate their normal layers of termination. In the present experiments ultrastructural morphometry was used to test whether this was still the case if optic axons were deprived of some of their normal termination sites. The stratum fibrosum marginale (SFM) of the tectum was partially denervated to provide foreign synaptic sites and the caudal half of the tectum was ablated at the same time. This removed approximately half the normal sites of optic termination. In the majority of cases optic axons grew into the SFM but there was no evidence of any significant formation of synapses by them in the SFM. Instead they synapsed in their normal layers. Myelinated optic axons in the SFM were present in significantly higher numbers in fish operated on in spring than in fish operated on in autumn after similar survival times. Although removal of some of the normal sites of termination promoted growth of optic axons into foreign denervated territory, the axons did not synapse there. Instead they passed through the foreign region and reinnervated their normal layers even though this meant reorganizing the optic projection within the reduced tectal space.     

The response of optic tract glia during regeneration of the goldfish visual system. II. Tectal factors stimulate optic tract glia

After transection, retinal ganglion cell axons of the goldfish will regenerate by growing into a primary target tissue, the optic tectum. To determine what role the target tissue may play in regulating glial cell growth, we measured biosynthetic activity of optic tract glia following excision of the optic tectum and compared it to activity of glia found in the regenerating visual system. Ablation of the tectum reduced glial incorporation of both [³H]thymidine and [³⁵S]methionine. Tectal ablation also led to nearly 80% reduction of amino acids incorporated by oligodendroglia as well as a decrease in the amount of newly synthetized protein found within multipotential glia and within cytoplasmic projections of astroglia. Since the tectal influence upon optic tract glia was detected at a time when tract and tectum are physically separated, we sought to determine if the optic tectum contained soluble glia-promoting factors. A soluble fraction recovered from tecta of the regenerating visual system increased amino acid incorporation within optic tract glia at 2–3-fold above preparations incubated with fractions from control, intact tecta. Comparisons of radiolabeled proteins separated by sodium dodecyl polyacrylamide gel electrophoresis from regenerating and factor-stimulated optic tract were similar and indicated that a soluble tectal fraction promoted biosynthesis of specific glial proteins. Our findings suggest that during regeneration of the goldfish visual system glia are influenced by humoral factor(s) released from the synaptic target site.     

Regional differences in pigeon optic tract,chiasm, and retino-receptive layers of optic tectum

Electron-microscopic examination of the pigeon optic chiasm, tract, stratum opticum, and retino-receptive layers of the optic tectum revealed regional differences at each level. Axonal size in the fiber pathways paralleled that previously reported for pigeon optic nerve, with mean diameter values of 0.96 μm for optic chiasm and 1.06 μm for optic tract. The dorsolateral aspects of these pathways contained a heterogeneous population of fibers (mean diameter ? 1.44 μm) similar to that found in the nasal portion of optic nerve, while the ventromedial regions were occupied by a more homogeneous population of smaller fibers (mean diameter ? 0.82 μm) resembling those observed in the temporal portion of the nerve. The retino-receptive layers of anteroventral optic tectum (avT) differed ultrastructurally from those of posterodorsal tectum (pdT) with respect to the thickness of horizontal dendrites in layers 2–3, the size of optic terminals in layers 2–7, and the number of synaptic contacts per terminal. These findings point towards a regional variation in the processing of visual information throughout the retino-tectal system and suggest that neurons in avT vs. pdT should show differences in the way they modify the neurophysiological characteristics of their respective optic inputs.     

Goldfish retinotectal system: Continuing development and synaptogenesis

The development of the retina and tectum in goldfish was studied using light and electron microscopy. Soon after hatching the retina is well differentiated in that all the layers of the adult retina are present. The tectum at this time lacks the characteristic layered structure of the adult and innervation in the stratum opticum is extremely sparse, being confined mainly to the rostral region. The retina grows rapidly and retinal layers increase in thickness. This continues into adulthood. Optic innervation of the tectum increases and in fish 19 mm in body length the adult pattern of layers seen by silver staining and by electron microscopy is recognizable. At this time the optic nerve contains large number of unmyelinated axons. The thickness of tectal layers continues to increase over the entire size range of fish studied, well into adulthood. Synaptic densities in the layer of optic termination also change. Density falls in the rostral region as the fish increase in size. In the caudal region there is an initial decrease followed by a small increase. Total numbers of synapses in the main layer of optic termination increase both rostrally and caudally over the entire range of fish studied. Optic and nonoptic fibers contribute to this. The optic nerve at this stage is almost completely myelinated. The continuing growth of both the retina and tectum, including synaptogenesis, may provide a basis for the remarkable regeneration and plasticity shown by this system.     

The development of NADPH-diaphorase and nitric oxide synthase in the visual system of the cichlid fish, Tilapia mariae

The pattern of NADPH-diaphorase expression was studied in the retina and optic tectum of the cichlid fish Tilapia mariae during the first developmental stages. NADPH-diaphorase activity was seen early, at hatching. In the retina a few cell bodies of the retinal inner nuclear layer showed a faint labeling. Scattered labeled cells were found in the stratum periventriculare of the optic tectum, while the optic nerve was unlabeled. Two days after hatching, the number of labeled neurons increased in the inner nuclear layer and a few stained cell bodies were also scattered in the ganglion cell layer. Both the inner and outer plexiform layers showed a diffuse staining and the optic nerve was devoid of labeling. In the optic tectum several positive cells in the periventricular layer, with their dendritic trees extending in the superficial fibrous layer, were found. In 1-month-old Tilapia, NADPH-diaphorase staining and nitric oxide synthase immunoreactivity were found to overlap in both the retina and optic tectum. The density of NADPH-diaphorase labeled neurons in the inner nuclear layer of the retina and in the stratum periventriculare of the optic tectum was largely reduced in comparison with 2 days posthatching embryos. These findings indicated an early and transient production of nitric oxide in the retina and optic tectum of Tilapia, suggesting a functional role for nitric oxide in the development of visual structures in aquatic vertebrates.     

Recovery of tectal nicotinic-cholinergic receptor sites during optic nerve degeneration in goldfish.

The concentration of cholinergic nicotinic-like sites as measured by alphabungarotoxin (alphaBuTX) binding, decreased in the goldfish (Carassius auratus) optic tectum after optic nerve disconnection. Initially, the rate of loss of sites is greater than the rate of tissue or protein degradation in experiments where disconnection was achieved either by unilateral optic nerve crush or by enucleation of one eye. When the crushed optic nerve is allowed to regenerate and form behaviorally potent connections, the number and concentration of these sites appears restored. Pharmacological studies indicate that the alphaButTX binding site in the goldfish optic tectum has a drug binding profile similar to that seen at central or peripheral alphaBuTX sites in other species.     

Ultrastructural localization of synaptic input to the optic lobe of carp (Carassius carassius)

The ultrastructure of the optic lobes of carp (Carassius carassius) is outlined, and the terminal degeneration after lesions of the optic nerve, tectal commissure, cerebellum, forebrain, spinal cord, and labyrinth is described in relation to the seven layers that may be distinguished ultrastructurally. Optic fibers enter at the top of the tectum in the optic layer and terminate mainly in the external grey layer. Commissural fibers enter in the deep white zone at the bottom of the tectum and ascend to terminate mainly in the fibrous marginal layer, with smaller numbers throughout the deeper layers. Other afferents enter mainly in the deep white zone with smaller numbers of forebrain, cerebellar, and labyrinthine fibers entering in the optic layer. These have sparse terminations throughout the other tectal layers.     

Electrical signs of an oligosynaptic visual projection to the rat hippocampus

In rats with pons transection photic or optic nerve stimulation elicits a response in dorsal hippocampus with approximately with same latency, amplitude and time course as the response in striate cortex. After optic nerve stimulation a positive polyphasic deflection with the last peak at 10 ms is followed by a larger biphasic major deflection with peaks at 15 and 25 ms and later, slower deflections at intervals of 125 ms. The polyphasic deflection is maximal in the inferior part of the hippocampus but does not reverse polarity; the other deflections reverse above and below an isoelectric point in the hilum of the dentate gyrus, a distribution attributable to depolarization in the molecular layers, perhaps also cell bodies, of that structure. The major deflection is more sensitive to changes in optic nerve stimulus strength than the response in striate cortex and is more resistant to reduction in amplitude during repetitive stimulation, following frequencies up to 50/s. The pathway between retina and hippocampus for all parts of the response is interrupted by lesions in the tectum. The major deflection is abolished by lesions in the posterior cingulum. In the posterior cingulum the pathway has fast and slow components in the lower and upper portions, respectively, associated with the first and second parts, respectively, of the major deflection. The pathway is not interrupted by lesions in the fornix, septal nuclei, anterior cingulum, anterior medial thalamus or medial midbrain ventral to the superficial tectum. There are complex interactions, up to several hundred milliseconds in duration, between the response to optic nerve stimulation and those elicited by stimulation in the cingulum, midbrain and thalamus. Tonic influences on the dentate gyrus from cingulum and tectum are described.     

Glutamate-like Immunoreactivity in the Pigeon Optic Tectum and Effects of Retinal Ablation

The pattern of glutamate-like immunoreactivity was investigated in the pigeon optic tectum. The most impressive aspect of the labelling pattern was an accumulation of immunoreactive terminal-like elements restricted to those superficial tectal layers that correspond to the termination zone of the retinal afferents. These immunoreactive puncta occurred frequently in small clusters. At the level of electron microscopy, many of the labelled nerve endings showed the characteristics of retinal terminals. Moreover, following unilateral retinal ablation a drastic loss of immunoreactive terminal-like puncta was observed in the retinorecipient layers of the tectum contralateral to the lesion. The remaining glutamate-immunoreactive terminal-like elements had the light and electron microscopic features typical of the afferents from the nucleus isthmi, pars parvocellularis (lpc). The relation between the latter result and the transmitter specificity of the afferents from this subtectal nucleus is unclear at present. On the other hand, the light and electron microscopic labelling patterns and the effect of retinal ablation suggest that afferents from retina and from lpc are the only major sources for glutamate-immunoreactive terminals in the pigeon optic tectum. Furthermore, the results are well in line with previous data indicating glutamate as neurotransmitter at least in part of the retinal afferents to the pigeon optic tectum.     

Quantitative autoradiographic analysis of the distribution of [3H]muscimol binding to GABA receptors in chick brain

Quantitative receptor autoradiography was used to investigate the distribution of high-affinity GABA receptors (GABAA) in left and right hemispheres of the brains of 3-week-old chicks. The receptors were labelled with the potent GABA agonist [3H]muscimol. High levels of [3H]muscimol labelling were found throughout the fore-, mid-, and hindbrain, though considerable variation was found in different regions. In the telencephalon the highest concentration of specific binding was found in the hyperstriatum ventrale followed by the neostriatum, and then the lobus parolfactorius of the paleostriatal complex, whilst in the diencephalon highest levels of labelling were present in the infundibulum. In the midbrain distinct lamination was observed in the high levels of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding sites is in good agreement with our previous work on the distribution of GABA-immunoreactivity in the chick brain.