Evidence for Excitatory Amino Acid Neurotransmitters in Forward and Feedback Corticocortical Pathways within Rat Visual Cortex

It is a commonly accepted notion that cells which make projections between the multiple cortical areas found in the mammalian visual system are excitatory, but there is little direct evidence that this is the case. Here we demonstrate using retrograde tracing with D-[³H]aspartate that connections in the rat which project from lower to higher visual areas (i.e. forward) and those which project from higher to lower areas (i.e. feedback) may use excitatory amino acid neurotransmitters. Following injection into the primary visual cortex, clusters of retrogradely labelled cells were found in several extrastriate areas within the cytoarchitectonic subdivisions 18a (‘areas’ LM, AL, PX, FLX, RL, AX) and 18b (‘area’ MX), and in the retrosplenial cortex. In all of these areas D-[³H]aspartate-labelled cells were surrounded by diffuse label which may represent anterograde labelling of axon terminals. This suggests that both legs of reciprocal intracortical circuits have similar chemospecificity. To directly demonstrate excitatory amino acid localization in forward projections, D-[³H]aspartate was injected into extrastriate area LM. As expected, the results revealed retrogradely labelled neurons within area 17. Outside area 17, LM injections labelled neurons in AL, PX, FLX, RL, AX and MX. Taken in the context of the hierarchy of areas in rat cerebral cortex (Coogan and Burkhalter, J. Neurosci., 13, 3749–3772, 1993), these results show that D-[³H]aspartate labels: (1) forward connections from area 17 to LM, AL, PX, RL, AX and MX, (2) feedback connections from LM, AL, FLX, PX, RL, AX and MX to area 17, (3) feedback connections from AL, PX, RL, AX and MX to LM, and (4) lateral connections between FLX and LM. These findings strongly indicate that both forward and feedback connections as well as lateral connections at several different levels of the cortical hierarchy use excitatory amino acid neurotransmitters.     

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.     

Neurotransmitter-specific uptake and retrograde transport of [H]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 injections 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 include 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.     

Time course of embryonic midbrain and thalamic auditory connection development in mice as revealed by carbocyanine dye tracing

Central auditory connections develop in mice before the onset of hearing, around postnatal day 7. Two previous studies have investigated the development of auditory nuclei projections and lateral lemniscal nuclear projections in embryonic rats, respectively. Here, we provide detail for the first time of the initiation and progression of projections from the inferior colliculus (IC) to the medial geniculate body (MGB) and from the MGB to the auditory cortex (AC). Overall, the developmental progression of projections follows that of terminal mitoses in various nuclei, suggesting the consistent use of a developmental timetable at a given nucleus, independent of that of other nuclei. Our data further suggest that neurons project specifically and reciprocally from the MGB to the AC as early as embryonic day 14.5. These projections develop approximately a day before the reciprocal connections between the MGB and IC and before development of projections from the auditory nuclei to the IC. The development of IC projections is prolonged and progresses from rostral to caudal areas. Brainstem nuclear projections to the IC arrive first from the lateral lemniscus nuclei then the superior olive and finally the cochlear nuclei. Overall, the auditory connection development strongly suggests that most of the overall specificity of nuclear connections is set up at least 2 weeks before the onset of sound-mediated cochlea responses in mice and, thus, is likely governed predominantly by molecular genetic clues.     

Direct synaptic connections of axons from superior colliculus with identified thalamo-amygdaloid projection neurons in the rat: Possible substrates of a subcortical visual pathway to the amygdala

The aim of the present study was to identify synaptic contacts from axons originating in the superior colliculus with thalamic neurons projecting to the lateral nucleus of the amygdala. Axons from the superior colliculus were traced with the anterograde tracers Phaseolus vulgaris leucoagglutinin or the biotinylated and fluorescent dextran amine “Miniruby.” Thalamo-amygdaloid projection neurons were identified with the retrograde tracer Fluoro-Gold. Injections of Fluoro-Gold into the lateral nucleus of the amygdala labeled neurons in nuclei of the posterior thalamus which surround the medial geniculate body, viz. the suprageniculate nucleus, the medial division of the medial geniculate body, the posterior intralaminar nucleus, and the peripeduncular nucleus. Anterogradely labeled axons from the superior colliculus terminated in the same regions of the thalamus. Tecto-thalamic axons originating from superficial collicular layers were found predominantly in the suprageniculate nucleus, whereas axons from deep collicular layers were detected in equal density in all thalamic nuclei surrounding the medial geniculate body. Double-labeling experiments revealed an overlap of projection areas in the above-mentioned thalamic nuclei. Electron microscopy of areas of overlap confirmed synaptic contacts of anterogradely labeled presynaptic profiles originating in the superficial layers of the superior colliculus with retrogradely labeled postsynaptic profiles of thalamo-amygdaloid projection neurons. These connections may represent a subcortical pathway for visual information transfer to the amygdala. J. Comp. Neurol. 403:158–170, 1999. © 1999 Wiley-Liss, Inc.     

Sources of presumptive glutamatergic/aspartatergic afferents to the magnocellular basal forebrain in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of [3H]-D-aspartate into the magnocellular basal forebrain of the rat was compared with the distribution of neurons labeled by comparable injections of the nonspecific retrograde axonal tracer wheat germ agglutinin conjugated to horseradish peroxidase. Cells retrogradely labeled by wheat germ agglutinin-horseradish peroxidase were found in a wide range of limbic and limbic-related structures in the forebrain and brainstem. In the telencephalon, labeled neurons were seen in the orbital, medial prefrontal, and agranular insular cortical areas, the amygdaloid complex, and the hippocampal formation. Labeled cells were also seen in the olfactory cortex, the lateral septum, the ventral striatopallidal region, and the magnocellular basal forebrain itself. In the diencephalon, neurons were labeled in the midline nuclear complex of the thalamus, the lateral habenular nucleus, and the hypothalamus. In the brainstem, labeled cells were found bilaterally in the ventral midbrain, the central gray, the reticular formation, the parabrachial nuclei, the raphe nuclei, the laterodorsal tegmental nucleus, and the locus coeruleus. A significant fraction of the afferents to the magnocellular basal forebrain appear to be glutamatergic and/or aspartatergic. Only a few of the regions labeled with wheat germ agglutinin-horseradish peroxidase were not also labeled with [3H]-D-aspartate in the comparable experiments. Most prominent among the non-glutamatergic/aspartatergic projections were those from fields CA1 and CA3 of the hippocampus, the hilus of the dentate gyrus, the dorsal subiculum, the tuberomammillary nucleus, and the ventral pallidum. In addition, most of the lateral hypothalamic and brainstem projections to the magnocellular basal forebrain were not significantly labeled with [3H]-D-aspartate. In addition to these inputs, a commissural projection from the region of the contralateral nucleus of the horizontal limb of the diagonal band was confirmed with both wheat germ agglutinin-horseradish peroxidase and the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin. This projection did not label with [3H]-D-aspartate or [3H]-GABA, suggesting that it is not glutamatergic/aspartatergic or GABAergic. Furthermore, double labeling experiments with the fluorescent retrograde tracer True Blue and antibodies against choline acetyltransferase indicate that the projection is not cholinergic.     

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat.

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.     

Colocalization of excitatory and inhibitory neurotransmitter markers in striatal projection neurons in the rat

The principle neuronal output of the neostriatum comes from medium spiny neurons that project from the caudate/putamen to the globus pallidus and substantia nigra. Although current evidence generally indicates that γ-aminobutyric acid (GABA) is the principal neurotransmitter in this pathway, this cannot account for the excitatory synaptic activity present among cultures of striatal neurons or the short latency excitatory postsynaptic potentials which often proceed or obscure inhibitory activity evoked by striatal stimulation. In this study, retrograde transport of [³H]D -aspartate has been used to demonstrate striatopallidal and striato-nigral neurons that possess a high-affinity uptake system for glutamate and aspartate and are therefore putatively glutamatergic. Injections of [³H]D -aspartate into the globus pallidus or substantia nigra, pars reticularis of the rat retrogradely labeled mediumsized neurons throughout the rostral-caudal extent of the neostriatum. To characterize this population further, adjacent sections were immunoreacted with antibodies to either GABA, glutamic acid decarboxylase (GAD), calbindin, or parvalbumin prior to autoradiographic processing. Under these conditions, autoradiographically labeled neurons displayed positive immunoreactivity for GABA, GAD, or calbindin. Autoradiographic label did not colocalize with parvalbumin immunoreactivity. The colocalization of anatomical markers of GABAergic and glutamatergic neurotransmission raises the possibility that both neurotransmitters are functionally expressed within single striatal projection neurons. © 1994 Wiley-Liss, Inc.     

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.     

Cysteine sulfinic acid-induced release ofd-[H]aspartate and [C]GABA in hippocampus slices: the role of sodium channels and cAMP

Cysteine sulfinic acid, a putative transmitter in the brain induces release ofd-[³H]aspartate and [¹⁴C]GABA without the help of any general depolarizing agent. Tetrodotoxin partially blocks the release ofd-[³H]aspartate and completely blocks the induced release of [¹⁴C]GABA. Withdrawal of Ca²⁺ from the medium does not affect thed-[³H]aspartate release, but increases the extent of inhibition by tetrodotoxin. In contrast, removal of Ca²⁺ increases the cysteine sulfinic acid-induced [¹⁴C]GABA release, which remains totally blocked by the toxin.Anemonia sulcata toxin type II, which slows down Na⁺ channel inactivation, acts in synergism with cysteine sulfinic acid to increase the rate of release of both of the labeled amino acids. Comparison of glutamate with cysteine sulfinic acid in the same experiments indicates a different action pattern of the two acidic amino acids. Forskolin plus isobutyl methyl xanthine, which are known to raise intracellular cyclic adenosine monophosphate (cyclic AMP) levels, caused little release of the labeled amino acids on their own, but strongly enhanced the cysteine sulfinic acid-induced release. The experiments conducted by double labeling withd-[³H]aspartate and [¹⁴C]GABA, revealed several characteristic differences between the glutamatergic and the GABAergic neurons. It is tentatively concluded that cysteine sulfinic acid brings about excitation of the glutamatergic as well as the GABAergic neurons, leading to opening of Na⁺ channels which play a role in the release in both systems. Cyclic AMP, presumably by initiating phosphorylation of a specific component, has a remarkable potentiating effect on the release.     

Projections to the subcortical forebrain from anatomically defined regions of the medial geniculate body in the rat

Although the auditory cortex is believed to be the principal efferent target of the medial geniculate body (MG), our recent behavioral studies indicate that in rats the conditioned coupling of emotional responses to an acoustic stimulus is mediated by subcortical projections of the MG. In the present study we have therefore used WGA-HRP as an anterograde and retrograde axonal marker to (1) define the full range of subcortical efferent projections of the MG; (2) identify the cells of origin within the MG of each projection; and (3) determine whether the subregions of the MG that project to subcortical areas receive inputs from acoustic relay nuclei of the mid-brain, particularly the inferior colliculus. The rat MG was first parcelled into three major cytoarchitectural areas: the ventral, medial, and dorsal divisions. The suprageniculate nucleus, located within the body of the MG just dorsal to the medial division, was also identified. Efferent projections of the MG were determined by combined anterograde and retrograde tracing methods. Injections of WGA-HRP in the MG produced anterograde transport to cortex and several subcortical areas, including the posterior caudate-putamen and amygdala, the ventromedial nucleus of the hypothalamus, and the subparafascicular thalamic nucleus. The cells of origin of the subcortical projections were then mapped retrogradely after injections in the anterogradely labeled areas. Injections in the caudate-putamen or amygdala retrogradely labeled the medial division of the MG and the suprageniculate nucleus, as well as several adjacent areas of the posterior thalamus surrounding the MG. In contrast, injections in the ventromedial nucleus of the hypothalamus or the subparafascicular thalamic nucleus only produced labeling in the areas surrounding MG. Afferents to MG from the inferior colliculus were then identified. The central nucleus of the inferior colliculus, the main lemniscal acoustic relay nucleus in the midbrain, was found to project to the ventral and medial divisions of the MG. In contrast, the dorsal cortex and external nucleus of the inferior colliculus project to each division of the MG and to several additional nuclei in adjacent areas of the posterior thalamus. These data demonstrate that the medial division of MG, the suprageniculate nucleus, and immediately adjacent areas of the posterior thalamus provide a direct linkage between auditory neurons in the inferior colliculus and subcortical areas of the forebrain and thereby support the view that thalamic sensory nuclei relay afferent signals to subcortical as well as cortical areas.     

An excitatory amino acid projection from rat prefrontal cortex to periaqueductal gray

High affinity D-[3H]aspartate and [3H]GABA uptake, and amino acid concentrations were examined in synaptosome-enriched preparations of rat periaqueductal gray matter 6-7 days following N-methyl-D-aspartate lesions confined to medial prefrontal cortex. Specific reductions were observed in the high affinity uptake of D-[3H]aspartate (78% of control, p less than 0.025), but not of [3H]GABA. Concentrations of glutamate, aspartate, GABA, glycine and alanine were not significantly reduced in lesioned animals. These results suggest the presence of glutamatergic and/or aspartatergic projections from medial prefrontal cortex to periaqueductal grey matter.     

Retrograde transport of [3H]glycine from the cochlear nucleus to the superior olive in the guinea pig

The origins of descending glycinergic projections to the guinea pig cochlear nucleus were investigated using retrograde labelling techniques. To identify the cell groups that provide descending projections to the cochlear nucleus, horseradish peroxidase, a nonspecific retrograde neuronal marker, was injected into the cochlear nucleus. After 24 or 48 hours, labelled cell bodies were evident bilaterally in all of the periolivary nuclei that surround the lateral and medial superior olive. The largest numbers of labelled neurons were located in the ventral nucleus of the trapezoid body bilaterally and in the lateral nucleus of the trapezoid body and dorsal periolivary nucleus ipsilaterally. Labelled cells were also present in the inferior colliculus bilaterally and in the contralateral cochlear nucleus. [3H]Glycine was employed as a retrograde tracer to identify the cell groups providing descending glycinergic projections to the cochlear nucleus. Three to 48 hours after injection of 19, 190, or 380 microM [3H]glycine into the cochlear nucleus, retrogradely labelled cell bodies were observed ipsilaterally in all of the periolivary nuclei. No labelled neurons were found in the inferior colliculus. After injections of the highest concentration of [3H]glycine, labelled cells were also found contralaterally in the ventral and lateral nuclei of the trapezoid body and also in the contralateral cochlear nucleus. We conclude that descending glycinergic projections to the cochlear nucleus originate mostly in ipsilateral periolivary cell groups. Minor glycinergic projections originate from the contralateral cochlear nucleus and also from the contralateral ventral and lateral nuclei of the trapezoid body.     

Aspartate and glutamate as possible neurotransmitters in the visual cortex

To identify possible neurotransmitters in the visual cortex, high pressure liquid chromatography was used to measure the release of endogenous compounds from a tissue slice preparation of the visual cortex of the rat. When synaptic release was induced, either by raising the K+ concentration in the medium or by adding veratridine, of the compounds measured, marked increases (6- to 35- fold) in release rate were observed for aspartate, glutamate, and gamma-aminobutyric acid (GABA). This increased release was blocked either with a low Ca2+/high Mg2+ or a tetrodotoxin-containing medium. To label possible aspartate or glutamate pathways, D-[3H]aspartate and D-[3H]glutamate were injected in the lateral geniculate nucleus (LGN), superior colliculus, and visual cortex. Following injections in the LGN, labeling was observed in the pyramidal cells in cortical layer 6 and in a diffuse band in layer 4, whereas no cortical cells were labeled after injections in the superior colliculus. When D-[3H]aspartate was injected in the cortex, the uptake again was concentrated in the layer 6 cells, but not labeled cell bodies were seen in the LGN, confirming the specificity of the uptake and retrograde filling process. Diffuse labeling was present in the LGN, however, presumably produced by anterograde filling process. Diffuse labeling was present in the LGN, however, presumably produced by anterograde transport from the layer 6 cells. These results suggest that layer 6 cells in the cortex, which are the source of the recurrent projection to the thalamus, may use aspartate or glutamate as their transmitter. Analysis of the function of the corticothalamic pathway may be facilitated by these findings.     

Sources of GABAergic input to the inferior colliculus of the rat

We have studied the GABAergic projections to the inferior colliculus (IC) of the rat by combining the retrograde transport of horseradish peroxidase (HRP) and immunohistochemistry for γ-amino butyric acid (GABA). Medium-sized (0.06–0.14 μl) HRP injections were made in the ventral part of the central nucleus (CNIC), in the dorsal part of the CNIC, in the dorsal cortex (DCIC), and in the external cortex (ECIC) of the IC. Single HRP-labeled and double (HRP-GABA)-labeled neurons were systematically counted in all brainstem auditory nuclei. Our results revealed that the IC receives GABAergic afferent connections from ipsi- and contralateral brainstem auditory nuclei. Most of the contralateral GABAergic input originates in the IC and the dorsal nucleus of the lateral lemniscus (DNLL). The dorsal region of the IC (DCIC and dorsal part of the CNIC) receives connections mostly from its homonimous contralateral region, and the ventral region from the contralateral DNLL. The commissural GABAergic projections originate in a morphologically heterogeneous neuronal population that includes small to medium-sized round and fusiform neurons as well as large and giant neurons. Quantitatively, the ipsilateral ventral nucleus of the lateral lemniscus is the most important source of GABAergic input to the CNIC. In the superior olivary complex, a smaller number of neurons, which lie mainly in the periolivary nuclei, display double labeling. In the contralateral cochlear nuclei, only a few of the retrogradely labeled neurons were GABA immunoreactive. These findings give us more information about the role of GABA in the auditory system, indicating that inhibitory inputs from different ipsi- and contralateral, mono- and binaural auditory brainstem centers converge in the IC. © 1996 Wiley-Liss, Inc.     

Neurogenetic gradients in the hamster visual pathway

The dorsal lateral geniculate nucleus, suprachiasmatic nucleus and superior colliculus of the hamster were examined autoradiographically after administration of [3H]thymidine for the presence of spatiotemporal gradients of neuron production and for the relationship between neuron cell body size and birthdate. The dorsal lateral geniculate nucleus had a dorsolateral-to-ventromedial (superficial-to-deep) gradient of neuron production, the suprachiasmatic nucleus had a caudoventral to rostrodorsal gradient, and the superior colliculus had a complex laminar gradient. In the lateral geniculate nucleus and in the superior colliculus, labeled neurons were typically larger than unlabeled neurons at early stages and unlabeled neurons were typically larger than labeled neurons at late stages; however, variation in neuron size does not account for the neurogenetic gradients in hamsters.     

An axonal transport study of the ascending projection of medial lemniscal neurons in the rat

The pattern of projection of the rat medial lemniscus was studied by axonal transport labeling following injections of tritiated leucine, proline and/or adenosine, or of horseradish peroxidase for retrograde identification of the neurons of origin. The vast majority of neurons in the gracile, cuneate, and principal trigeminal nuclei contribute to an almost totally crossed projection primarily to the thalamic ventrobasal complex. Additional thalamic components were traced to specific sites within the "posterior group," including a medial component largely traversed by lemniscal axons and a caudolateral component lying between the principal nucleus of the medial geniculate and ventral nucleus of the lateral geniculate. We have designated this latter zone "intermediate geniculate," distinguishing a somatosensory portion of the geniculate group on the basis of its myelo- and cytoarchitecture, as well as its connections. Other projections replicated in several animals included the zona incerta and nearby sectors of the substantia nigra; three distinct mesencephalic arrangements within the deep layers of the superior colliculus, the external nucleus of the inferior colliculus, and the intercollicular nucleus; the anterior pretectal nucleus; dorsal sectors of the inferior olivary complex and the ipsilateral cerebellar cortex. The results are compared with findings in other species (with emphasis on the caudal thalamic region) in an attempt to resolve some of the apparent inconsistencies in nomenclature.     

Retrograde transport of [3H]proline: a widespread phenomenon in the central nervous system

[3H]Proline was injected into spinal cord, pons, inferior colliculus, superior colliculus, lateral geniculate nucleus, pulvinar-LP complex and visual cortex of cats or rats. After 1-3 days' survival the animals were perfused with formalin or mixed aldehydes. Autoradiography showed labeling of cell bodies in most regions known to project to the injection sites. The ability to take up and retrogradely transport [3H]proline appears to be a common property of central neurons. We infer that the transport of this compound is unrelated to its possible status as a neurotransmitter.     

The intercollicular pathway in the golden hamster: An anatomical study

The intercollicular pathway of the hamster was studied by means of a combination of horseradish peroxidase (HRP), autoradiographic, and double-labeling (nuclear yellow-HRP) techniques. Small deposits of HRP marked significant numbers of cells in the contralateral colliculus only when the injection site included the laminae ventral to the stratum opticum. Anterior deposits labeled many more neurons than injections into the caudal part of the tectum. Of the cells labeled by our injections, 18.2% were located in the superficial collicular laminae (stratum griseum superficiale and stratum opticum), and the remainder (81.8%) were in the deep layers. A wide variety of morphological cell types contributed axons to the intercollicular projection, and in a given animal the loci of the labeled neurons were generally symmetrical with the injection site. Small deposits of [³H]-leucine resulted in contralateral labeling only when the injection included the deep collicular laminae. The transported label was most dense in the stratum griseum intermediale and stratum griseum profundum, and its location was generally homotopic with the injection site. Experiments in which collicular HRP deposits were combined with large cervical spinal or pontine reticular injections of Nuclear Yellow indicated that intertectal neurons did not, in most cases, contribute axon branches to the spinal or pontine reticular projections of the colliculus. Receptive field data obtained at the time of the HRP and/or [³H]-leucine deposits demonstrated that the collicular representations of the ipsilateral and at least 45° of the contralateral hemifields were encompassed by intercollicular connections. This was also true for the somatosensory representation of the entire head and a portion of the neck.     

Transneuronal degeneration of thalamic neurons following deafferentation: quantitative studies using [3H]thymidine autoradiography.

Transneuronal degeneration of thalamic neurons following partial deafferentation was studied using [3H]thymidine autoradiography. Timed-pregnant female Sprague-Dawley rats received systemic injections of [3H]thymidine on embryonic day (E) 13, 14 and/or 15. On the day of birth, pups were anesthetized by hypothermia and subjected to unilateral enucleation, unilateral removal of the inferior colliculus or sham lesion. Animals were sacrificed on postnatal day 10 or 30 and the brains processed for autoradiography. Material from sham-lesioned animals demonstrates that neurons destined for the dorsal lateral geniculate nucleus (LGd) undergo final mitoses on E13, 14 and 15. Neurons in the ventral medial geniculate nucleus (MGv) undergo final mitoses on E13 and 14. Thirty days following neonatal unilateral eye removal, the contralateral LGd displays a loss of approximately 30-35% of [3H]thymidine labeled neurons. Neonatal unilateral removal of the inferior colliculus results in a loss of approximately 30-40% of labeled neurons in MGv. For both LGd and MGv, shorter survival times reveal less severe cell loss. Late generated (E15) LGd neurons show less severe loss following enucleation than do earlier generated neurons. These results document the degree of cell loss in sensory thalamic nuclei following deafferentation and demonstrate that [3H]thymidine autoradiography provides a useful quantitative method for assessing anterograde transneuronal cell loss in targeted populations of neurons in the developing central nervous system.