Transmitter-related retrograde labeling in the pigeon optic lobe: a high resolution autoradiographic study

Mouse Fgf8-Cre-LacZ lineage analysis defines the territory of the postnatal mammalian isthmus

The isthmus is recognized as the most rostral segment of the hindbrain in non-mammalian vertebrates. In mammalian embryos, transient Fgf8 expression defines the developing isthmic region, lying between the midbrain and the first rhombomere, but there has been uncertainty about the existence of a distinct isthmic segment in postnatal mammals. We attempted to find if the region of early embryonic Fgf8 expression (which is considered to involve the entire extent of the prospective isthmus initially) might help to identify the boundaries of the isthmus in postnatal animals. By creating an Fgf8-Cre-LacZ lineage in mice, we were able to show that Fgf8-Cre reporter expression in postnatal mice is present in the same nuclei that characterize the isthmic region in birds. The 'signature' isthmic structures in birds include the trochlear nucleus, the dorsal raphe nucleus, the microcellular tegmental nuclei, the pedunculotegmental nucleus, the vermis of the cerebellum, rostral parts of the parabrachial complex and locus coeruleus, and the caudal parts of the substantia nigra and VTA. We found that all of these structures were labeled with the Fgf8-Cre reporter in the mouse brain, and we conclude that the isthmus is a distinct segment of the mammalian brain lying caudal to the midbrain and rostral to rhombomere 1 of the hindbrain.

Columnar projections from the cholinergic nucleus isthmi to the optic tectum in chicks (Gallus gallus): a possible substrate for synchronizing tectal channels

The cholinergic division of the avian nucleus isthmi, the homolog of the mammalian nucleus parabigeminalis, is composed of the pars parvocellularis (Ipc) and pars semilunaris (SLu). Ipc and SLu were studied with in vivo and in vitro tracing and intracellular filling methods. 1) Both nuclei have reciprocal homotopic connections with the ipsilateral optic tectum. The SLu connection is more diffuse than that of Ipc. 2) Tectal inputs to Ipc and SLu are Brn3a-immunoreactive neurons in the inner sublayer of layer 10. Tectal neurons projecting on Ipc possess "shepherd's crook" axons and radial dendritic fields in layers 2-13. 3) Neurons in the mid-portion of Ipc possess a columnar spiny dendritic field. SLu neurons have a large, nonoriented spiny dendritic field. 4) Ipc terminals form a cylindrical brush-like arborization (35-50 microm wide) in layers 2-10, with extremely dense boutons in layers 3-6, and a diffuse arborization in layers 11-13. SLu neurons terminate in a wider column (120-180 microm wide) lacking the dust-like boutonal features of Ipc and extend in layers 4c-13 with dense arborizations in layers 4c, 6, and 9-13. 5) Ipc and SLu contain specialized fast potassium ion channels. We propose that dense arborizations of Ipc axons may be directed to the distal dendritic bottlebrushes of motion detecting tectal ganglion cells (TGCs). They may provide synchronous activation of a group of adjacent bottlebrushes of different TGCs of the same type via their intralaminar processes, and cross channel activation of different types of TGCs within the same column of visual space.

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.

Neurotransmitter-specific uptake and retrograde transport of [3H]glycine from the inferior colliculus by ipsilateral projections of the superior olivary complex and nuclei of the lateral lemniscus

Neurotransmitter-specific uptake and retrograde axonal transport of [3H]glycine were used to identify glycinergic projections to the inferior colliculus in chinchillas and guinea pigs. Six h after injection of [3H]glycine in the inferior colliculus, autoradiographically labeled cells were found ipsilaterally in the ventral nucleus of the lateral lemniscus, the lateral superior olive and the dorsomedial periolivary nucleus. These 3 regions accounted for 95% of the labeled projection neurons, with the remainder scattered elsewhere in the ipsilateral superior olivary complex. No labeled cells were found contralaterally even after survival times as long as 24 h. Retrograde transport of HRP from the inferior colliculus in these same cases confirmed the presence of additional projections that did not accumulate [3H]glycine. These included ipsilateral projections from the medial superior olive and cochlear nucleus and contralateral projections from the inferior colliculus, dorsal nucleus of the lateral lemniscus, lateral superior olive, periolivary nuclei and cochlear nucleus. The results implicate uncrossed projections from the ventral nucleus of the lateral lemniscus, lateral superior olive, and dorsomedial periolivary nucleus as the principal sources of inhibitory glycinergic inputs to the inferior colliculus.

Changing distribution of GABA-like immunoreactivity in pigeon visual areas during the early posthatching period and effects of retinal removal on tectal GABAergic systems

The distribution of GABA-like immunoreactivity in the pigeon visual system was studied during the first 9 days after hatching using a mouse monoclonal antibody, mAb 3A12, to glutaraldehyde linked GABA (Matute & Streit, 1986). GABA-like immunoreactivity was seen in cell bodies as well as in neuropil at the level of both the retina and central visual regions at any posthatching age. However, the distribution of putative GABAergic cells and processes varied with age reaching the adult pattern at around 9 days. As a general observation, almost no cell bodies in the retina (except for some perikarya in the ganglion cell layer) were labeled at hatching but densely packed immunostained processes were present in the inner plexiform layer. During the next few days, GABA-immunoreactive amacrine and horizontal cells appeared and the adult distribution of GABA-like immunoreactivity was reached at around 9 days. In the other visual regions examined, the general trend in the variation of GABA-like immunoreactivity included: (1) a progressive decrease in the density of immunostained cell bodies and (2) an almost parallel increase in the concentration of stained neuropil. Since in pigeons the adult organization of visual pathways and the final distribution of putative GABAergic systems are reached at around the same age, we suggest the possibility that incoming ganglion cell axons play a role in regulating the distribution of GABA-like immunoreactivity in visual areas. This hypothesis is supported by the fact that the distribution of GABA-like immunoreactivity in the superficial layers of the optic tectum was altered following ablation of the contralateral retina immediately after hatching.

Avian nucleus isthmi ventralis projects to the contralateral optic tectum

Injections of HRP throughout the upper tectal strata led in 4 cases to the appearance of retrogradely labeled neurons within n.isthmi ventralis, contralateral to the experimental side. An additional case proved that this projection courses through the ventral supraoptic commissure. This is the first description of a crossed isthmo-tectal projection in birds.

Muscarinic action of acetylcholine in the pigeon optic tectum

Eighty visual units were extracellularly recorded from the pigeon optic tectum, and the effects of iontophoretically applied acetylcholine and its antagonists, atropine and tubocurarine, on these units were examined. The results showed that acetylcholine in the avian tectum functions as an excitatory transmitter or modulator and acts predominantly through a muscarinic mode of action.

Selective retrograde labeling with D-[3H]-aspartate in afferents to the mammalian superior colliculus

The selectivity previously reported for retrograde labeling patterns obtained following D-[3H]-aspartate injections and proposed to be related to the transmitter specificity of the labeled pathways was tested in afferents to the superior colliculus (SC) of rats and rabbits. In rats selectivity was assessed by comparing retrograde perikaryal labeling patterns observed in D-[3H]-aspartate experiments with those found after administration of a nonselective tracer, horseradish-peroxidase-labeled wheat germ agglutinin (HRP-WGA), and of the tritiated neurotrasmitter gamma-aminobutyric acid (GABA). Following D-[3H]-aspartate injections into the SC labeling was intense in a large number of cortical and hypothalamic neurons. Other afferents to the SC, however, such as those originating from the ventrolateral geniculate nucleus, the pars reticulata of the substantia nigra, the locus coeruleus, the pontine nuclei, or the retinal ganglion cells, were not labeled. Similar results were obtained in rabbits. In cats, the analysis was focused on the cerebral cortex, since in an earlier investigation no retrograde labeling had been detected in the visual cortex following D-[3H]-aspartate injections in the SC. In the present work, however, retrogradely labeled neurons were observed in various cortical areas including a few in visual cortex. This report shows selective retrograde labeling for a part of the afferents to the SC. This selectivity does not display major differences among the mammalian species studied. Moreover, according to the information available about the distribution of neurotransmitters in the brain, the findings reported here favour the idea that D-[3H]-aspartate is a retrograde tracer selective for glutamatergic and/or aspartatergic pathways.

Selective retrograde labeling in some afferents to the rabbit lateral geniculate nucleus following injections of tritiated neurotransmitter-related compounds

Retrogradely labeled neurons were found in the visual cortex and superior colliculus following D-[3H]aspartate injections in the lateral geniculate nucleus (LGN). In these experiments labeling was also observed over the optic tract. [3H]dopamine and [3H]serotonin injections in the LGN caused weak labeling in a small number of superior colliculus neurons. Furthermore, in [3H]serotonin cases, labeled neurons were also found in the dorsal raphé nucleus. In contrast, when other amino acids or monoamines were injected, no retrograde labeling occurred in any of the afferents to the LGN. These results are largely consistent with the idea of D-[3H]aspartate being a useful marker for pathways using excitatory amino acids as neurotransmitters. The findings in [3H]dopamine and [3H]serotonin experiments indicate that these substances may induce retrograde labeling patterns, which are not related to the transmitter specificity of the pathways concerned.

Selective retrograde labelling of the rat olivocerebellar climbing fiber system with D-[3H]aspartate

Selective retrograde labelling of the olivocerebellar climbing fiber system with D-[3H]aspartate has been observed in the rat, and the results have implications for the identification of a transmitter candidate as well as the neuroanatomical understanding of these cerebellar afferents. Microinjections of D-[3H]aspartate (50 nl, ca 10-2 M) were made into various parts of cerebellar cortex. Survival times were 6, 12 or 24 h. Pronounced diffusion of the tracer resulted in large injection sites. Within the zone of injection, glial elements were labelled over background. Most granule cells exposed to the tracer were unlabelled; the small numbers demonstrating labelling were believed to have been injured by the micropipette penetration. Beneath injection sites, large numbers of well-labelled nerve fibers appeared in the white matter and could be followed through the brainstem to the contralateral inferior olive, where labelled perikarya were found. After the inferior olivary neurons had been effectively destroyed with 3-acetylpyridine, evidence of cerebellar afferent labelling with D-[3H]aspartate was missing. Retrograde labelling of the olivocerebellar system was also observed after superfusion of the vermis with D-[3H]aspartate at concentrations in the range of Km for high affinity uptake (10(-5) or 10(-4) M, for 2 h). Mossy fiber or monoaminergic afferents to the cerebellum were never labelled with D-[3H]aspartate. The distribution of labelled cells in the olivary subnuclei after injections in different cerebellar areas was in line with the olivocerebellar organization previously described in the cat. Moreover, it was demonstrated that fibers from the different subnuclei follow different routes through the brainstem towards the cerebellum. Labelling of climbing fiber collaterals in uninjected parts of cerebellum indicated that some of the retrogradely migrating D-[3H]aspartate was directed in anterograde direction at axonal branching points. Collaterals were demonstrated in all deep cerebellar and Deiters' nuclei, and the results of intranuclear injections suggested that virtually every olivary neuron sends collaterals to these nuclei. Intracortical collaterals were organized in sagittal zones. Midline injections into the anterior lobe and VI lobule labelled collaterals in several zones of the posterior lobe spinal area and uninjected parts of the anterior lobe vermis. Hemispheral injection into copula pyramidis labelled collaterals in two prominent bundles in the anterior lobe.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective retrograde axonal transport of free glycine in identified neurons of Aplysia

The specific retrograde axonal transport of free glycine within the identified neurons R3-14 of Aplysia californica was studied. The soma of the R3-14 neurons are located in the parietovisceral ganglion and their axons project down the branchial nerve to end in a large peripheral field. Using a double-chambered apparatus, the peripheral tissue was incubated in medium containing a 3H-amino acid for 4-48 hr, while the nerve and ganglion were isolated and perfused with plain or chemically altered medium. The nerve and ganglion were then either rapidly frozen for scintillation counting or fixed for autoradiography. When 3H-glycine was used, radioactivity entered the nerve rapidly, reached the ganglion in 3 hr, and was transported largely (greater than 80%) in the free amino acid form [trichloroacetic acid (TCA) soluble]. The right parietovisceral hemiganglion accumulated up to nine times more radioactivity than the left hemiganglion, reflecting the presence of the R3-14 axons and soma. Two phases of radioactivity were observed, a fast component moving at about 3 mm/hr and a slower (but larger) component moving at about 0.4 mm/hr. Light microscope autoradiography on nerves containing 3H-glycine revealed that the R3-14 axons accounted for more than 30% of the total label in the nerve but occupied less than 7% of the total cross-sectional area of the axonal core. Electron microscope autoradiography showed a close association of silver grains and dense core vesicles in the R3-14 axons. Retrograde axonal transport of free glycine was inhibited by (in decreasing order of effectiveness) mercuric chloride, vinblastine, colchicine, Nocodazole, and 2,4-dinitrophenol (2,4-DNP). Comparative studies of other amino acids [3H-leucine, 3H-serine, 3H-glutamic acid, 3H-gamma-aminobutyric acid (3H-GABA), and 3H-alanine] showed that 3H-glycine is the only amino acid that is rapidly axonally transported in large quantities within the R3-14 axons. This work demonstrates, for the first time, that a free amino acid, glycine, is transported in the retrograde direction within a select group of axons. The significance of this transport of glycine is discussed in relation to its use as a neural messenger by neurons R3-14.

Rapid axoplasmic transport of free leucine

Axoplasmic transport of free 3H-leucine has been studied in vivo in the pike olfactory nerve following application of labeled leucine to the olfactory mucosa. A considerable amount of free 3H-leucine is transported at constant velocity along the axon in the form of a distinct peak. The maximum transport velocity for free 3H-leucine is the same as for rapidly transported 3H-protein (130 and 135 mm/day, respectively, at 19 degrees C). Microtubule inhibitors block or significantly reduce the amount of free 3H-leucine transported, but do not influence the transport velocity. Disruption of the oxygen supply abolishes free 3H-leucine transport, so that this phenomenon cannot be explained by diffusion. The amount of free leucine in the rapidly moving peak decreases with time and distance along the axon and is not detectable after 5 h or more. The transported 3H-leucine is not derived from the circulation or from proteolysis of rapidly transported proteins. This study may help to resolve the controversy over the axoplasmic transport of free amino acids since it shows that free 3H-leucine is transported rapidly but does not travel by rapid axoplasmic transport to the end of axons longer than about 30 mm.

The topographical distribution of alanine, aspartate, gamma-aminobutyric acid, glutamate, glutamine, and glycine in the pigeon optic tectum and the effect of retinal ablation

The concentrations of alanine, aspartate, gamma-aminobutyric acid, glutamine, glutamate, and glycine were measured in the pigeon optic nerve and in the individual tectal layers. Characteristic topographical distribution patterns were observed for the different amino acids. After unilateral retinal ablation, the concentration of aspartate and glutamate was decreased in the nerve and contralateral tectum. The reduction was restricted to the superficial part of the tectum, which receives a direct retinal input. The maximal loss was measured in the first two layers, where aspartate was reduced by 51% and glutamate by 75% in comparison with the ipsilateral side 4 weeks after ablation. The results favor a special role for aspartate and glutamate in pigeon retino-tectal afferents.

Selective retrograde labeling indicating the transmitter of neuronal pathways

A number of tritiated transmitter related compounds-amino acids and biogenic amines-were injected into the rat caudoputamen or substantia nigra in order to test (1) for the occurence of autoradiographic perikaryal labeling, (2) for a selectivity of perikaryal labeling relating certain compounds to certain pathways, and (3) for the relation of perikaryal labeling to known transmitter specificitites of the systems involved. Perikaryal labeling was observed after injection of some but not all of the substances used and was best explained by retrograde labeling in pathways projecting to the injection sites. Six hours after injection of high concentrations of tritiated transmitter into the terminal area, perikaryal labeling was observed: (A) in substantial nigra compacta (A9), A10 (rostral) and A8 (all heavy), and in n. raphe dorsalis (light) after [3H]-dopamine and [3H]-norepinephrine injection into caudoputamen; (B) same pattern as in A, but heavy in n. raphe dorsalis after [3H]-serotonin injection into caudoputamen; perikaryal labeling absent in cortex and thalamus after injection of substances mentioned in A and B; (C) only in substantia nigra compacta (minimally) after [3H]-GABA injection into caudoputamen; (D) in cerebral cortex and thalamus but not in substantia nigra, A10, A8, nor in n. raphe dorsalis after injection of [3H]-D-aspartate into caudoputamen; (E) in the rat caudoputamen but not in n. raphe dorsalis after [3H]-GABA injection into substantia nigra; (F) in n. raphe dorsalis but not in caudoputamen after [3H]-serotonin into substantia nigra. These results indicated, indeed, a certain selectivity-partly related to transmitter specificity-for perikaryal labeling patterns. As a method, transmitter specific retrograde tracing could be useful in pathways with dopamine-, serotonin-, and GABA-mediated synaptic transmission.