Laminar and segregated distribution of immunoreactivities for some neuropeptides and adenosine deaminase in the superior colliculus of the rat

Differential expression of vesicular glutamate transporters 1 and 2 may identify distinct modes of glutamatergic transmission in the macaque visual system

Glutamate is the primary neurotransmitter utilized by the mammalian visual system for excitatory neurotransmission. The sequestration of glutamate into synaptic vesicles, and the subsequent transport of filled vesicles to the presynaptic terminal membrane, is regulated by a family of proteins known as vesicular glutamate transporters (VGLUTs). Two VGLUT proteins, VGLUT1 and VGLUT2, characterize distinct sets of glutamatergic projections between visual structures in rodents and prosimian primates, yet little is known about their distributions in the visual system of anthropoid primates. We have examined the mRNA and protein expression patterns of VGLUT1 and VGLUT2 in the visual system of macaque monkeys, an Old World anthropoid primate, in order to determine their relative distributions in the superior colliculus, lateral geniculate nucleus, pulvinar complex, V1 and V2. Distinct expression patterns for both VGLUT1 and VGLUT2 identified architectonic boundaries in all structures, as well as anatomical subdivisions of the superior colliculus, pulvinar complex, and V1. These results suggest that VGLUT1 and VGLUT2 clearly identify regions of glutamatergic input in visual structures, and may identify common architectonic features of visual areas and nuclei across the primate radiation. Additionally, we find that VGLUT1 and VGLUT2 characterize distinct subsets of glutamatergic projections in the macaque visual system; VGLUT2 predominates in driving or feedforward projections from lower order to higher order visual structures while VGLUT1 predominates in modulatory or feedback projections from higher order to lower order visual structures. The distribution of these two proteins suggests that VGLUT1 and VGLUT2 may identify class 1 and class 2 type glutamatergic projections within the primate visual system (Sherman and Guillery, 2006).

The dorsal tectal longitudinal column (TLCd): a second longitudinal column in the paramedian region of the midbrain tectum

The tectal longitudinal column (TLC) is a longitudinally oriented, long and narrow nucleus that spans the paramedian region of the midbrain tectum of a large variety of mammals (Saldaña et al. in J Neurosci 27:13108–13116, 2007). Recent analysis of the organization of this region revealed another novel nucleus located immediately dorsal, and parallel, to the TLC. Because the name “tectal longitudinal column” also seems appropriate for this novel nucleus, we suggest the TLC described in 2007 be renamed the “ventral tectal longitudinal column (TLCv)”, and the newly discovered nucleus termed the “dorsal tectal longitudinal column (TLCd)”. This work represents the first characterization of the rat TLCd. A constellation of anatomical techniques was used to demonstrate that the TLCd differs from its surrounding structures (TLCv and superior colliculus) cytoarchitecturally, myeloarchitecturally, neurochemically and hodologically. The distinct expression of vesicular amino acid transporters suggests that TLCd neurons are GABAergic. The TLCd receives major projections from various areas of the cerebral cortex (secondary visual mediomedial area, and granular and dysgranular retrosplenial cortices) and from the medial pretectal nucleus. It densely innervates the ipsilateral lateral posterior and laterodorsal nuclei of the thalamus. Thus, the TLCd is connected with vision-related neural centers. The TLCd may be unique as it constitutes the only known nucleus made of GABAergic neurons dedicated to providing massive inhibition to higher order thalamic nuclei of a specific sensory modality.

The TLC: a novel auditory nucleus of the mammalian brain

We have identified a novel nucleus of the mammalian brain and termed it the tectal longitudinal column (TLC). Basic histologic stains, tract-tracing techniques and three-dimensional reconstructions reveal that the rat TLC is a narrow, elongated structure spanning the midbrain tectum longitudinally. This paired nucleus is located close to the midline, immediately dorsal to the periaqueductal gray matter. It occupies what has traditionally been considered the most medial region of the deep superior colliculus and the most medial region of the inferior colliculus. The TLC differs from the neighboring nuclei of the superior and inferior colliculi and the periaqueductal gray by its distinct connections and cytoarchitecture. Extracellular electrophysiological recordings show that TLC neurons respond to auditory stimuli with physiologic properties that differ from those of neurons in the inferior or superior colliculi. We have identified the TLC in rodents, lagomorphs, carnivores, nonhuman primates, and humans, which indicates that the nucleus is conserved across mammals. The discovery of the TLC reveals an unexpected level of longitudinal organization in the mammalian tectum and raises questions as to the participation of this mesencephalic region in essential, yet completely unexplored, aspects of multisensory and/or sensorimotor integration.

Honeycomb-like structure of the intermediate layers of the rat superior colliculus: afferent and efferent connections

There is increasing evidence that acetylcholinesterase is organised in a lattice-like fashion in the intermediate layers of the mammalian superior colliculus. In a recent study, we described this organisation in rat by showing that it comprises a well formed honeycomb-like lattice with about 100 cylindrical compartments or modules occupying both the intermediate collicular layers. Considering this enzyme domain as a reference marker for comparing the organisation of collicular input-output systems, the present study investigates whether the principal sensori-motor systems in intermediate layers also have honeycomb-like arrangements. In 33 animals, the distributions of afferents (visual from extrastriate cortex; somatic from the primary somatosensory cortex, the trigeminal nucleus and the cervical spinal cord) and efferents (cells of origin of the crossed descending bulbospinal tract and uncrossed pathway to the pontine gray, the ascending system to the medial dorsal thalamus) were examined in a tangential plane following applications of horseradish peroxidase-wheatgerm agglutinin conjugate (used as an anterograde and retrograde tracer). In 22 of the 33 rats, axonal tracing was made within single tangential sections also stained for cholinesterasic activity in order to compare the neuron profiles with the cholinesterasic lattice.The results show that these afferent and efferent systems are also organised in honeycomb-like networks. Moreover, those related to the cortical, trigeminal and some of the spinal afferents are aligned with the cholinesterasic lattice. Likewise most of colliculo-pontine, colliculo-bulbospinal and half of colliculo-diencephalic projecting cells also tend to be in spatial register with the enzyme lattice. This indicates that the honeycomb-like arrangement is a basic architectural plan in the superior colliculus for the organisation of both acetylcholinesterase and major sensori-motor systems for orientation.

Expression of messenger RNAs for glutamic acid decarboxylase, preprotachykinin, cholecystokinin, somatostatin, proenkephalin and neuropeptide Y in the adult rat superior colliculus

The mammalian superior colliculus is an important subcortical integrator of sensorimotor behaviours. It is multi-layered, each layer containing specific neuronal types and possessing distinct input/output relationships. Here we use in situ hybridisation methods to map the distribution of seven neurotransmitters/neuromodulator systems in adult rat superior colliculus. Coronal sections were probed for preprotachykinin, cholecystokinin, somatostatin, proenkephalin, neuropeptide Y and the enzymes glutamic acid decarboxylase and choline acetyltransferase, markers for GABA and acetylcholine respectively. Cells expressing glutamic acid decarboxylase messenger RNA were the most abundant, the highest density being found in the superficial layers. Many cells containing proprotachykinin messenger RNA were found in stratum zonale and the upper two-thirds of stratum griseum superficiale; cells were also located in deeper tectal laminae, particularly caudomedially. Most cholecystokinin messenger RNA expressing cells were located in the superficial layers with a prominent band in the middle third of stratum griseum superficiale. Cells expressing moderate to high levels of somatostatin messenger RNA formed a dense band in the lower third of stratum griseum superficiale/upper stratum opticum; two less distinct tiers of labelling were seen in deeper layers. These in situ hybridisation data reveal three distinct sub-laminae in rat stratum griseum superficiale. Cells expressing moderate to low levels of proenkephalin messenger RNA were located in lower stratum griseum superficiale/upper stratum opticum and intermediate laminae. A cluster of enkephalinergic cells was located medially in the deep tectal laminae. Expression of neuropeptide Y messenger RNA was relatively low and mostly confined to cells in stratum griseum superficiale and stratum opticum. No choline acetyltransferase messenger RNA was detected. This in situ analysis of seven different neurotransmitters/neuromodulator systems sheds new light on the neurochemical organisation of the rat superior colliculus. The data are related to what is known anatomically and physiologically about intrinsic and extrinsic tectal circuitry, and the potential involvement of different neuropeptides in these circuits is discussed. The work forms the basis for future developmental studies examining the effects of transplantation and visual deprivation/deafferentation on tectal neurochemistry and function.

Honeycomb-like structure of the intermediate layers of the rat superior colliculus, with additional observations in several other mammals: AChE patterning

The aim of the present study was to reinvestigate the stereometric pattern of acetylcholinesterase (AChE) activity staining in the intermediate layers of the superior colliculus in several mammalian species. A pioneering study in the cat and the monkey by Graybiel (1978) stressed the regular arrangement of AChE staining in the deep collicular layers. According to her description, made in the frontal plane, the enzyme was arranged in a mediolateral series of patches, the cores of which tended to line up in the longitudinal axis of the structure, so they formed roughly parallel bands. As exhaustive a description as possible of the AChE distribution was undertaken in the rat by compiling observations in the frontal, sagittal, and tangential planes. It emerged that AChE-positive elements are organized in the form of a conspicuous honeycomb-like network that is divided into about 100 rounded compartments, over virtually the full extent of the intermediate layers. The generality of the rat model was then tested in other rodents such as mouse and hamster and also in cat and monkey. For these species we resorted to a single tangential cutting plane, which proved to be more appropriate for disclosing such a modular arrangement. The data revealed that in all species AChE staining followed the same architectural plan and identified the striking similarity in the number of compartments that compose the various honeycomb-like lattices. In conclusion, the present findings support a unified model of the AChE arrangement within the intermediate layers of the mammalian colliculus; the model comprehensively incorporates the classical description of the patchy and stripy features of the enzyme distribution. We hypothesize here that the modular AChE arrangement might be the anatomical basis for collicular vectorial encoding of orienting movements.

Physiological properties of neurons in the optic layer of the rat's superior colliculus

We made intracellular recordings from 74 neurons in the optic layer of the rat superior colliculus (SC). Resting membrane potentials were -62.3 +/- 6.2 (SD) mV, and input resistances were 37.9 +/- 10.1 MOmega. Optic layer neurons had large sodium spikes (74.2 +/- 12.3 mV) with an overshoot of 12 mV and a half-amplitude duration of 0.75 +/- 0.2 ms. Each sodium spike was followed by two afterhyperpolarizations (AHPs), one of short duration and one of longer duration, which were mediated by tetraethylammonium (TEA)-sensitive (IC) or apamin-sensitive (IAHP) calcium-activated potassium currents, respectively. Sodium spikes were also followed by an afterdepolarization (ADP), which was only revealed when the AHPs were blocked by TEA or apamin. In response to hyperpolarizing current pulses, optic layer neurons showed an inward rectification mediated by H channels. At the break of the current pulse, there was a rebound low-threshold spike (LTS) with a short duration of <25 ms. The LTS usually induced two sodium spikes (doublet). Most optic layer neurons (84%) behaved as intrinsically bursting cells. They responded to suprathreshold depolarization with an initial burst (or doublet) followed by a train of regular single spikes. The remaining 16% of cells acted as chattering cells with high-frequency gamma (20-80 Hz) rhythmic burst firing within a narrow range of depolarized potentials. The interburst frequency was voltage dependent and also time dependent, i.e., showed frequency adaptation. Unmasking the ADP with either TEA or apamin converted all of the tested intrinsically bursting cells into chattering cells, indicating that the ADP played a crucial role in the generation of rhythmic burst firing. Optic layer neurons receive direct retinal excitation mediated by both N-methyl--aspartate (NMDA) and non-NMDA receptors. Optic tract (OT) stimulation also led to gamma-aminobutyric acid-A (GABAA) receptor-mediated inhibition, the main effect of which was to curtail the excitatory response to retinal inputs by shunting the excitatory postsynaptic current. Intracellular staining with biocytin showed that the optic layer neurons that we recorded from were mostly either wide-field vertical neurons or other cells with predominately superficially projecting dendrites. These cells were similar to calbindin immunoreactive cells seen in the optic layer. The characteristics of these optic layer neurons, such as prominent AHPs, strong shunting effect of inhibition, and short-lasting LTS, suggest that they respond transiently to retinal inputs. This is consistent with a function for these cells as the first relay station in the extrageniculate visual pathway.

Neuropeptide Y and enkephalin immunoreactivity in retinorecipient nuclei of the hamster pretectum and thalamus

This investigation was stimulated by the historical confusion concerning the identity of certain pretectal nuclei and by large differences reported between species with respect to which nuclei receive retinal innervation. Subcortical visual nuclei were studied using immunohistochemistry to identify retinal projections labeled following intraocular injection of cholera toxin, b fragment. In addition, neuropeptide Y (NPY) or enkephalin (ENK) immunoreactive cells and fibers were also evaluated in the retinorecipient pretectal and thalamic areas. The results confirm the established view that the retina directly innervates the nucleus of the optic tract (NOT), posterior (PPT), and olivary pretectal (OPT) nuclei. However, the retina also innervates the hamster medial (MPT) and anterior (APT; dorsal division) pretectal nuclei, results not previously reported in rodents. A commissural pretectal area (CPT) sparsely innervated by retina is also described. The data show for the first time that the posterior limitans nucleus (PLi) receives a moderately dense, direct retinal input. The PLi does not project to the cortex and appears to be a pretectal, rather than thalamic, nucleus. All retinal projections are bilateral, although predominantly contralateral. The PLi contains a moderately dense plexus of NPY- and ENK-IR fibers and terminals. However, peptidergic fibers also traverse the ATP and connect with the dorsomedial pretectium. The OPT contains ENK- and NPY-IR neurons and fibers, but is specifically identifiable by a moderately dense plexus of ENK-IR terminals. Numerous ENK-IR neurons are found in the NOT and PPT. The latter also has moderate numbers of ENK-IR fibers and terminals, but few NPY-IR neurons or fibers. The MPT contains modest numbers of ENK-IR fibers. The APT has no NPY-IR neurons or terminals, but an occasional ENK-IR neuron is seen and there is sparse ENK-IR innervation. Peptidergic innervation of the visual nuclei does not appear to be derived from the retina. The results show a set of retinally innervated, contiguous nuclei extending from the thalamic ventrolateral geniculate nucleus dorsomedially to the midbrain CPT. These nuclei plus the superior colliculus comprise a dorsal "visual shell" embracing a central core of caudal thalamus and rostral midbrain.

Intrinsic circuitry in the deep layers of the cat superior colliculus

The mammalian superior colliculus is involved in the transformation of sensory signals into orienting behaviors. Sensory and motor signals are integrated in the colliculus to produce movements of the eyes, head, and neck. While there is a considerable amount of information available on the afferent and efferent connections of the colliculus, almost nothing is known about its intrinsic circuitry, particularly that of its deepest layers. It is likely that intrinsic connections in these deeper layers of the colliculus participate in the sensory-motor transformations leading to orienting movements. In this study, we used the neuroanatomical tracer biocytin to label small groups of neurons in the deeper layers of the cat superior colliculus and examine the distribution of their axons and terminals. We found a broadly distributed network of intrinsic projections throughout the deep layers of the superior colliculus. While the majority of terminals were found in a 1-2 mm radius around the injection site, labeled terminals were found throughout the deep layers of the colliculus up to 5 mm from the injection site. In addition, these injections sometimes labeled terminals in the superficial tectum. Extensive projections were demonstrated by the more superficial injections, but few terminals were found when injections were confined to the deepest layers of the colliculus. There was no evidence of anisotropy in the distribution of terminals from injections made at different rostrocaudal or mediolateral locations; neurons located in any one region in the colliculus could potentially influence any other region. This network of intrinsic connections in the cat superior colliculus could provide a means for deeper-layer efferent neurons to associate, and to modulate or coordinate their output. Interneurons could also provide a substrate for mutual inhibition between neurons at the rostral pole of the colliculus that are active during fixation, and more caudally located neurons whose activity is associated with saccadic eye movements.

Adenosine deaminase in rodent median eminence: detection by antibody to the mouse enzyme and co-localization with adenosine deaminase-complexing protein (CD26)

Adenosine deaminase in the hypothalamic tuberomammillary nucleus and median eminence of rat and mouse brains was investigated with two different antibodies generated against the enzyme derived from either calf or mouse. Both antibodies labelled neurons in the tuberomammillary nucleus and, as determined in rat, they immunolabelled the same neurons. In the median eminence, immunopositive fibres and terminals were detected with anti-mouse adenosine deaminase in both rat and mouse, while no such staining was seen in either species with antibody against the calf enzyme. These fibres were most concentrated in the external median eminence, had a more restricted distribution than those containing either galanin or tyrosine hydroxylase and only partially overlapped with oxytocin-positive fibres. By electron microscopy, adenosine deaminase was found in terminals containing both small, clear vesicles with diameters of 35 to 45 nm and large dense-core vesicles with diameters of 100 to 140 nm. Preadsorption of antibodies with purified enzyme derived from the species against which they were directed eliminated all staining in rat, while antibody adsorptions across species were less effective. Preadsorption of anti-mouse adenosine deaminase antibody with the mouse deaminase led to increased labelling in mouse median eminence, suggesting an interaction between tissue components and antibody-linked enzyme. Tests for the presence of adenosine deaminase-complexing protein (CD26) with an antibody against this protein gave positive labelling in the median eminence of both species and this labelling was co-distributed with that seen for adenosine deaminase. These results confirm the expression of adenosine deaminase in restricted populations of neurons in rodent brain as revealed with a novel antibody, suggest the presence of a distinct form or localization of the enzyme in the median eminence, and raise the possibility that it contributes, perhaps along with CD26, to purinergic regulation of hormone secretion in this structure.

The mosaic architecture of the superior colliculus

The superior colliculus is a midbrain structure serving visual, multisensory and sensorimotor processing. Throughout various collicular layers, visual afferents are linked together with afferents related to other sensory modalities as well as with afferents from sources not easily subsumed under the term 'sensory'. These inputs are orchestrated in a topographic fashion and led to premotor neurons that are important elements in generating saccadic eye movements and orientation movements of other kinds. Using immunocytochemical techniques to chart the distribution of various substances serving neurotransmission and neuromodulation, it was found that many of them, e.g. acetylcholinesterase (AChE), choline acetyltransferase, the enkephalins, substance P, and parvalbumin, relate to repetitive structural islands, or modules, in the superior colliculus. From studies on the distribution of three further neuroactive substances in rat superior collicular tissue: the calcium binding protein calretinin, the growth and plasticity related protein neuromodulin (GAP-43), and a glutamate receptor of the NMDA-type, we were led to conclude (1) that the intermediate layers of the superior colliculus are composed not of two, but of at least three disjunct types of modules, (2) that not just the intermediate layers but more or less the whole superior colliculus is an assemblage of modules, and (3) that, besides topographic connectivity and laminar structuring, the modules constituting an iterative partitioning represent a third major feature of superior collicular architecture. The origin of the collicular mosaic is considered under an evolutionary perspective, and a hypothesis is presented stating that the pattern of AChE-rich modules on the level of the multimodal collicular layers can be predicted from retinal ganglion cell topography.

Non-uniform distribution of cellular phenotypes in the optic tectum of the leopard frog

Regional specialization in the retina have been described in a number of species. We have investigated whether such specializations can be found in the optic tectum, an area of the brain responsible for the processing of visual information. Using the tectum of Rana pipiens, we have examined the distribution of three different cell types defined on the basis of their immunoreactivity to somatostatin, substance P, and serotonin antibodies. These three immunoreactive cell populations had differing, nonuniform distributions in the optic tectum. Somatostatin-line immunoreactive cells were largely restricted to the caudal one-third of the tectum, whereas both substance P-like immunoreactive (SP-ir) and serotonin-like immunoreactive (5-HT-ir) cells were found unequally represented throughout the tectum. The percentage of SP-ir cells decreased significantly in both the posterior and medial directions from its high in the anterior lateral tectum. Although serotonin-like immunoreactivity was also greatest in the lateral tectum and decreased significantly medially, it was largely constant in the anterior-to-posterior dimension. The populations of SP-ir and 5-HT-ir cells were nonoverlapping. Our results suggest that information may be processed differently in different regions of the optic tectum.

The role of the periaqueductal grey in vocal behaviour

This is a review of our current knowledge about the role of the periaqueductal grey (PAG) in vocal control. It shows that electrical stimulation of the PAG can evoke species-specific calls with short latency and low habituation in many mammals. The vocalization-eliciting region contains neurones the activity of which is correlated with the activity of specific laryngeal muscles. Lesioning studies show that destruction of the PAG and laterally bordering tegmentum can cause mutism without akinesia. Neuroanatomical studies reveal that the PAG lacks direct connections with the majority of phonatory motoneurone pools but is connected with the periambigual reticular formation, an area which does have direct connections with all phonatory motor nuclei. The PAG receives a glutamatergic input from several sensory areas, such as the superior and inferior colliculi, solitary tract nucleus and spinal trigeminal nucleus. Glutamatergic input, in addition, reaches it from numerous limbic structures the stimulation of which also produces vocalization, such as the anterior cingulate cortex, septum, amygdala, hypothalamus and midline thalamus. Pharmacological blocking of this glutamatergic input causes mutism. The glutamatceptive vocalization-controlling neurones are under a tonic inhibitory control from GABAergic neurones. Removal of this inhibitory input lowers the threshold for the elicitation of vocalization by external stimuli. A modulatory control on vocalization threshold is also exerted by glycinergic, opioidergic, cholinergic, histaminergic and, possibly, noradrenergic and dopaminergic afferents. It is proposed that the PAG serves as a link between sensory and motivation-controlling structures on the one hand and the periambigual reticular formation coordinating the activity of the different phonatory muscles on the other.

Pattern formation in the developing superior colliculus: ontogeny of the periodic architecture in the intermediate layers

The superior colliculus of mammals contains a striking neurochemical architecture in which histochemically identifiable compartments are distributed in an iterative arrangement in the intermediate layers. We used stains for acetylcholinesterase activity as a compartment marker to trace ontogenesis of this architecture during pre- and postnatal development in the domestic cat. We found that compartmentation in the intermediate collicular layers is virtually absent at birth, and only gradually emerges during the first weeks of postnatal life. Over the same postnatal period, acetylcholinesterase activity shifts from a predominantly perikaryal expression pattern immediately postnatally to a nearly exclusive localization in the neuropil at maturity. Remarkably, a striking compartmentation of the superior colliculus was readily apparent with acetylcholinesterase histochemistry prenatally. The first appearance of a periodic architecture in the superior colliculus was observed at embryonic day 34, a time at which the collicular plate had not yet become laminated. The compartments characterized by high levels of acetylcholinesterase activity then gained in prominence until late in the prenatal period, when they receded and disappeared. The loss of the acetylcholinesterase-positive compartments in the perinatal period did not reflect a loss of compartmentation altogether. Neonatally, there was a distinct compartmental architecture visible with enkephalin immunohistochemistry. The virtual absence of acetylcholinesterase-positive compartments in the superior colliculus at birth therefore reflects developmental regulation of enzyme expression in the compartments, not regulation of the compartments as structural entities. We conclude that the periodic architecture, which characterizes the intermediate collicular layers in the adult cat, arises early in ontogenesis. These observations raise the possibility that the histochemical compartments are ontogenetic units that undergo remodeling as the superior colliculus matures.

Enkephalin-positive and acetylcholinesterase-positive patch systems in the superior colliculus have matching distributions but distinct developmental histories

Histochemical stains for acetylcholinesterase activity and enkephalin-like immunoreactivity both demonstrate a high degree of patterning in the superior colliculus, particularly in the intermediate and deep layers. Both markers occur predominantly in the neuropil of these layers, and both are principally distributed in distinct macroscopic compartments. We report here that patches of heightened acetylcholinesterase activity correspond to patches of high enkephalin-like immunoreactivity. The two markers thus delineate largely the same domain in the intermediate and deep layers. The most prominent zones of staining for enkephalin-like peptide and for acetylcholinesterase also coincided in the dorsolateral periaqueductal gray matter. These findings suggest a close interlocking of one or more acetylcholinesterase-containing systems with one or more pathways related to endogenous opioids in the superior colliculus. As the acetylcholinesterase expression in the patches is known to match in detail choline acetyltransferase expression, our results also suggest the possibility of local cholinergic-opiatergic interactions. In some sections, blood vessels associated with enkephalin-rich and acetylcholinesterase-rich patches extended beyond the colliculus into the periaqueductal gray matter, where they again became surrounded by dense fibrous labeling. This pattern suggests that neurohumoral signal exchange might occur through blood vessels even in a sensory-motor structure such as the colliculus. In a postnatal developmental series of kitten brains we found that enkephalin-like immunoreactivity was already distinctly compartmental in the intermediate layers at birth and continued to show this distribution throughout postnatal development. By contrast, acetylcholinesterase staining was nearly homogeneous at birth and became compartmental gradually during the first postnatal weeks. Thus, despite the eventual near coincidence of the enkephalin-rich and acetylcholinesterase-rich compartments of the superior colliculus, they mark systems that follow distinct programs of neurochemical development.

Immunohistochemical localization of substance P receptor in the superior colliculus. A light and electron microscope study in the rat

The superficial layers of the superior colliculus (SC) have been known to contain many axons showing substance P-like immunoreactivity (SP-LI). We, therefore, immunohistochemically examined the distribution of SP receptor (SPR) in the superficial layers of the SC in the rat by using a specific antibody against SPR. The majority of SC neurons with SPR-LI were distributed in the zonal and the superficial gray layers, the rest of them were in the optic layer. Electron microscopy revealed that SPR-immunoreaction products in SC neurons were distributed not only in postsynaptic sites, but also in non-synaptic regions of perikaryal and dendritic profiles.

Projections of tachykinin- and glutaminase-containing rat retinal ganglion cells

Glutamate (Glu) and the tachykinin substance P (SP) have been proposed as neurotransmitters or neuromodulators of the retinal projection to the brain. In the present study, we demonstrate that tachykinin-like (TK) immunoreactivity (IR) accumulates in rat retinal axons following electrical lesions to the optic tract, indicating that SP is conveyed in the optic nerve to its central targets. In addition, we show that eye enucleation causes a dramatic decrease in TK-IR fibers in the pretectal olivary nucleus (PON), but not in other retinorecipient nuclei of the thalamus and the midbrain, and that Fluorogold injected into the pretectum is retrogradely transported to the somata of TK-IR retinal ganglion cells (RGCs), indicating an important projection of TK-IR RGCs to the PON. We also show that most rat RGCs are labeled with antibodies against phosphate-activated glutaminase, an enzyme considered to generate the transmitter pool of glutamate. Unlike TK-IR fibers, phosphate-activated glutaminase-IR structures disappear in most retinorecipient nuclei following eye enucleation. The present results give neuroanatomical support to the idea that glutamate is a neurotransmitter in the retinal projection and suggest an important role for TK-IR RGCs in the relay of visual information to the PON.

Choline acetyltransferase-immunoreactive patches overlap specific efferent cell groups in the cat superior colliculus

Fibers containing acetylcholine (ACh) form distinct patches in the dorsal intermediate gray layer (IGL) of the cat superior colliculus (SC). Although these patches are known to overlap several afferent projections to SC, it is not known whether they are associated with specific postsynaptic cell groups. We have examined the relationship of these ACh fiber patches to specific efferent cell groups by combining retrograde transport of horseradish peroxidase (HRP) with choline acetyltransferase (ChAT) immunocytochemistry. Successful HRP injections were made into the predorsal bundle (PB), the tecto-pontine-bulbar pathway (TPB) and the cuneiform region (CFR), the inferior olive (IO), the dorsolateral pontine gray nucleus (PGD), and the pedunculopontine tegmental nucleus (PPTN). The distribution of HRP-labeled neurons which project to these targets was mapped by a computer-based microscope plotter. Distinct clusters of HRP-labeled neurons in the IGL were seen after three injections into the mesencephalic reticular formation that involved the caudal TPB and cuneiform region (CFR), and after one injection into the medial accessory nucleus of IO. As many as seven clusters of labeled neurons were found in some sections through the caudal one-half of SC after the TPB/CFR injections. Each cluster consisted of 3-20 cells, all of which were small to medium in size. In sections also tested for ChAT, the cell clusters in the TPB/CFR cases were found to overlap precisely the ACh patches in the IGL. In addition, SC neurons projecting to the IO formed clusters above the ChAT patches and in the intermediate white layer (IWL) of SC. None of the other HRP injections produced any obvious cell clusters in the deep layers of SC. These results are the first to show that specific cell groups, distinguished by size and projection site, form clusters that match the patch-like innervation of cholinergic afferents to SC. This modular organization may correspond to saccade-related cells that have also been reported to be organized into clusters in the cat SC.

Postnatal development of substance-P immunoreactivity in the rat superior colliculus

Immunocytochemical techniques have been used to examine the distribution of substance-P (SP)-labeled neurons in the superior colliculus of rats from birth to adulthood. At birth, there are almost no SP-immunopositive neurons in the tectum. A small number of SP neurons appear over the next several days. However, the vast majority of SP neurons appear between P9 and P10, and by P12 have attained adult-like numbers and distribution. Neurons are confined to the superficial layers of the colliculus, specifically the upper two-thirds of the stratum griseum superficiale (SGS). There is no indication of a differential developmental sequence along rostrocaudal or mediolateral axes. Neuronal types can be distinguished as early as P6 and include horizontal, vertical, and multipolar cells. Substance-P-immunoreactive axons and boutons are also present in the superior colliculus at birth, and are for the most part confined to the deep layers. Boutons are generally of the en-passant type. The density of labeled axons and boutons increases progressively, and by P10 there is an almost adult-like lamination and patchiness. In the adult, labeled axons and boutons are most dense in the stratum opticum and stratum griseum intermedium. Bridges of dorsoventrally oriented labeled axons span the relatively label-poor stratum album intermedium. SP label in the stratum griseum profundum is dense and patchy, and there is also dense label in the stratum album profundum bordering the periaqueductal grey. The role of substance-P-labeled neurons in the superior colliculus is still a matter of speculation. The findings of this study indicate that SP neurons may play a role in intrinsic collicular circuitry.

Organization and synaptic connections of cholinergic fibers in the cat superior colliculus

The cat superior colliculus (SC) receives a dense cholinergic input from three brainstem nuclei, the pedunculopontine tegmental nucleus, the lateral dorsal tegmental nucleus, and the parabigeminal nucleus (PBG). The tegmental inputs project densely to the intermediate gray layer (IGL) and sparsely to the superficial layers. The PBG input probably projects only to the superficial layers. In the present study, the morphology of choline acetyltransferase (ChAT)-immunoreactive axons and synaptic endings in the superficial and deep layers of the SC was examined by light and electron microscopy to determine whether these cholinergic afferents form different types of synapses in the superficial and deep layers. Two types of fibers were found within the zonal (ZL) and upper superficial gray layers (SGL): small diameter fibers with few varicosities and larger diameter fibers with numerous varicosities. Quantitative analysis demonstrated a bimodal distribution of axon diameters, with one peak at approximately 0.3-0.5 micron and the other at 0.9-1.0 micron. On the other hand, ChAT-immunoreactive fibers in the IGL were almost all small and formed discrete patches within the IGL. Two types of ChAT-immunoreactive synaptic profiles were observed within the ZL and upper SGL using the electron microscope. The first type consisted of small terminals containing predominantly round synaptic vesicles and forming asymmetric synaptic contacts, mostly on dendrites. The second type was comprised of varicose profiles that also contained round synaptic vesicles. Their synaptic contacts were always symmetric in profile. ChAT-immunoreactive terminals in the IGL patches contained round or pleomorphic synaptic vesicles, and the postsynaptic densities varied from symmetric to asymmetric, including intermediate forms. However, no large varicose profiles were observed. This study suggests that cholinergic fibers include at least two different synaptic morphologies: small terminals with asymmetric thickenings and large varicose profiles with symmetric terminals. The large varicose profile in the superficial layers is absent in the IGL. This result suggests that the cholinergic inputs that innervate the superficial layers and the patches in the IGL of the cat SC differ in their synaptic organization and possibly also in their physiological actions.

Immunochemical heterogeneity in the tecto-LP pathway of the rat

The projection from the rat's superior colliculus (SC) to the lateral posterior nucleus of the thalamus (LP) has previously been described as arising from a morphologically homogeneous population of neurons in the stratum opticum (SO). The present study combined immunocytochemistry with retrograde tracing and lesion techniques to determine whether or not the SC-->LP projection arose from neurons that were also neurochemically homogeneous. The combination of retrograde tracing and immunocytochemistry with an antibody directed against calbindin-D 28K (CBD) showed that 64.4% of the neurons that project from SC to LP contain this calcium-binding protein. Retrograde tracing and immunocytochemistry for adenosine deaminase (ADA) showed that a smaller number of tecto-LP cells (15.7%) were immunoreactive (IR) for this enzyme. Moreover, nearly all (93.0%) of the ADA-IR tecto-LP cells also contained CBD-IR. Adenosine deaminase-IR axons in LP were restricted to the dorsomedial portion of the nucleus and their density was substantially reduced after ablation of the ipsilateral superficial SC laminae. The lateral posterior nucleus contained numerous CBD-IR cells and fibers throughout its extent and it was thus difficult to determine the extent to which the extra-perikaryal CBD-IR in this nucleus was dependent upon the tecto-LP pathway. Nevertheless, destruction of the ipsilateral SC did reduce the density of CBD-IR in LP. These results suggest that the SC-->LP projection in rat arises from at least four neurochemically distinct cell groups: 1) those that contain CBD, 2) those that contain both CBD and ADA, 3) a very small population that contains only ADA, and 4) a group that is not recognized by either of these markers. Our results further suggest that ADA containing fibers may have a more restricted terminal distribution in LP than axons that contain only CBD.

Effects of chemical stimulation in the periaqueductal gray on vocalization in the squirrel monkey

Twenty-nine agonists and 32 antagonists of more than 10 transmitters known to be present in the periaqueductal gray (PAG) have been injected into the squirrel monkey's PAG in order to test their effects on spontaneous vocalization at sites yielding vocalization with electrical stimulation. Vocalization could be elicited with the glutamate agonists sodium-L-glutamate, L-aspartic acid, L-homocysteic acid, N-methyl-D-aspartic acid, quisqualic acid, and kainic acid, the cholinergic agonists acetylcholine, carbachol, and muscarine, the monoaminergic agonist histamine, and the GABA antagonists bicuculline and picrotoxin. No vocalizations could be obtained with agonists of dopamine, noradrenaline, adrenaline, serotonin, GABA, glycine, nicotinic receptors, and endogenous opioids, as well as with antagonists of glutamate, acetylcholine, dopamine, noradrenaline, adrenaline, serotonin, histamine, glycine, GABA-B, delta- and mu-receptors. Blocking of spontaneous vocalization was obtained with the nonspecific glutamate antagonist kynurenic acid and the GABA-A receptor agonist muscimol. The results indicate that the production of vocalization depends upon the activation of glutamatergic synapses in the PAG. GABAergic afferents seem to have a tonic inhibitory control on the periaqueductal vocalization mechanism, while acetylcholine and histamine seem to exert only a transient modulatory control.

Association of efferent neurons to the compartmental architecture of the superior colliculus

The superior colliculus is a layered structure in the mammalian midbrain serving multimodal sensorimotor integration. Its intermediate layers are characterized by a compartmental architecture. These compartments are apparent through the clustering of terminals of major collicular afferents, which in many instances match the heterogeneous distribution of tissue components such as acetylcholinesterase, choline acetyltransferase, substance P, and parvalbumin. The present study was undertaken to determine whether efferent cells observe this compartmental architecture. It was found that subpopulations of both descending and ascending collicular efferents originate from perikarya situated in characteristic positions relative to the collicular compartments defined by elevated acetylcholinesterase activity and that their dendrites appear to be specifically coordinated with the heterogeneous environment. With the specific interlocking of afferent and efferent neurons through spatially distinguished neural networks, the compartmental architecture apparently constitutes an essential element for the determination of information flow in the superior colliculus.

Sensory responses of intralaminar thalamic neurons activated by the superior colliculus

The intralaminar thalamus of anesthetized rats was explored for neurons activated by stimulation of the superior colliculus and responsive to sensory inputs. Neurons activated by stimulation of the intermediate and deep collicular layers were distributed throughout the intralaminar thalamus. Approximately one half of them responded to tectal as well as sensory inputs. The majority were nociceptive or had a more complex response pattern including responses to auditory stimulation. A small population of low threshold units had contralateral orofacial receptive fields and responded to light taps; these units were preferentially localized anteriorly in the central lateral and paracentral nuclei. Neurons responsive to tectal and sensory stimulation were randomly intermingled with other neurons which had no detectable sensory input. The results indicate that ascending projection neurons of the intermediate and deep layers of the superior colliculus provide an input to functionally diverse subpopulations of intralaminar thalamic neurons. In view of its projections to motor cortex and basal ganglia, the intralaminar thalamus appears directly implicated in basal ganglia and superior colliculus related mechanisms of attention, arousal and postural orienting.

Postnatal development of acetylcholinesterase in, and cholinergic projections to, the cat superior colliculus

The postnatal development of cholinergic afferents to the superior colliculus in neonatal cats was studied by using acetylcholinesterase (AChE) histochemistry, choline acetyltransferase (ChAT) immunohistochemistry, and retrograde transport of horseradish peroxidase (HRP). In the adult cat, the pattern of AChE staining was laminar specific. AChE was distributed continuously in the stratum griseum superficiale (SGS) but was organized as patches in the stratum griseum intermediate (SGI). Diffuse AChE staining also was present in the stratum griseum profundum (SGP) and the dorsolateral periaqueductal gray (PAG). At birth, however, AChE staining was barely detectable in the SGS and, aside from a few isolated labeled neurons, was absent from the SGI, SGP, and PAG. By 7 days postnatal (dpn), staining in the SGS was more apparent but did not change appreciably in the deeper laminae. A substantial increase in AChE staining occurred in the SGS at 14 dpn (several days after eye opening), at which time patches in the SGI first became apparent. By 28 dpn, the complete laminar-specific adult AChE staining pattern was present, though the staining intensity did not reach the adult level until 56 dpn. A protracted maturation of both AChE staining and ChAT immunoreactivity also was observed in the sources of cholinergic afferents to the superior colliculus, which include the parabigeminal nucleus, and the pedunculopontine (PPN) and lateral dorsal tegmental (LDTN) nuclei. AChE and ChAT-immunoreactive staining in each nucleus was weak at birth but increased during the ensuing 2 weeks. At 21 dpn, however, ChAT immunoreactivity virtually disappeared in the parabigeminal nucleus and significantly decreased in PPN and LDTN. The ChAT immunoreactivity in these nuclei then gradually increased reaching maximum levels by 28 dpn. At 35 dpn, AChE staining showed a significant, though temporary (4 weeks), decrease in the parabigeminal nucleus, but not in the PPN and LDTN, that subsequently increased to the adult level of staining at 70 dpn. The absence of AChE in the SGI in neonatal animals was correlated, at least in part, with a paucity of neurons in the brainstem cholinergic cell groups labeled by retrograde transport of HRP from the superior colliculus. Injections of HRP into the superior colliculus retrogradely labeled many neurons in the parabigeminal nucleus, but few, if any, neurons in the PPN or LDTN at 1 dpn. Retrogradely labeled neurons also were observed in the substantia nigra pars reticulata, albeit fewer in neonates than in adults.(ABSTRACT TRUNCATED AT 400 WORDS)

Substance P immunoreactivity identifies a projection from the cat's superior colliculus to the principal tectorecipient zone of the lateral posterior nucleus

Cells in the superficial layers of the superior colliculus innervate multiple visual regions within the pulvinar-lateral posterior complex of the cat. To characterize these neurons we have examined their immunocytochemical properties in conjunction with their projection patterns. In the present study, we show that a monoclonal antibody for substance P recognizes a morphologically diverse population of neurons, which can be classified as granular, stellate, angular, and horizontal or nonhorizontal fusiform cell types. These neurons are distributed throughout the superficial layers of the colliculus, with a peak density corresponding to sublayer 2 of the stratum griseum superficiale. Injections of rhodamine latex micropheres into the pulvinar-lateral posterior complex demonstrate that a substantial proportion of these collicular cells (at least 35%) project to this region of the posterior thalamus. The overall population of substance P-containing cells, as well as the immunoreactive projection neurons, is composed of the same proportions of cell classes, with the exception that granular cells were not found to be projection neurons. A distinct wedge of substance P immunoreactivity, consisting of fiber and diffuse extracellular labeling, was discovered in the pulvinar-lateral posterior complex. This staining was demonstrated to be confined entirely within the medial division of the lateral posterior nucleus, which is considered to be the principal tectorecipient zone of the extrageniculate visual thalamus. Lesions of the superior colliculus largely abolished the substance P immunoreactivity in the ipsilateral tectorecipient zone. These results are consistent with the view that substance P plays a role in the functional organization of the principal tectothalamic pathway of the cat's extrageniculate visual system.

Ultrastructure and retinal innervation of deafferentation-induced enkephalin-immunoreactive elements in the superficial layers of the rat superior colliculus

Leu-enkephalin-like immunoreactive (ENK-I) elements appearing in the superficial layers of the rat superior colliculus (SC) after eye-enucleation were examined by means of immunoelectronmicroscopy. ENK-I somata were of a single type and formed symmetric and asymmetric synapses with non-immunoreactive axon terminals. Some degenerating retinal terminals made synaptic contacts only with small ENK-I dendrites, suggesting that deafferentation-induced ENK-I neurons in the rat SC receive retinal input onto the distal portions of their dendrites.

Fine structure of synapses and retinal innervation of substance P and adenosin deaminase containing neurons in the superior colliculus of the rat

The fine structure of substance P (SP) and adenosine deaminase (ADA) immunoreactive structures in synaptic contacts localized to the superficial layers of the superior colliculus of the rat was investigated by means of immunoelectron microscopy. We also examined the possibility of retinal innervation of SP- and ADA- containing neurons by immunohistochemistry after degeneration of retinal terminals caused by enucleation. SP-like immunoreactive presynaptic terminals of the stratum griseum superficiale (SGS) formed both asymmetric and symmetric synaptic contacts. Presynaptic dendritelike structures were also observed. SP immunoreactive postsynaptic elements made contacts with terminals showing diverse features. ADA-like immunoreactive structures were seen only as postsynaptic elements to different kinds of nonimmunoreactive terminals and were mostly localized in the ventral third of the SGS and the dorsalmost stratum opticum (SO). After enucleation, degenerating retinal terminals were found to form synaptic contacts with SP and ADA immunoreactive structures. The highest number of such degenerating terminals on ADA immunoreactive structures was observed 2 days after retinal denervation, very few being seen after 5 days. These degenerating terminals were restricted to the ventral SGS and dorsal SO. SP immunoreactive structures postsynaptic to degenerating retinal terminals were most numerous 5 days after enucleation and mainly localized in the dorsal SGS. Occasionally, SP immunoreactive dendritelike processes forming synapses with degenerating retinal terminals were simultaneously presynaptic to other nonimmunoreactive profiles, defining, therefore, serial synapses. The present results suggest that SP-I and ADA-I collicular neurons may be part of distinct channels carrying visual information to the lateral posterior and lateral geniculate nuclei of the thalamus, respectively.

Effects of eye-enucleation on substance P-immunoreactive fibers of some retinorecipient nuclei of the rat in relation to their origin from the superior colliculus

We have previously shown that retinal deafferentation causes a decrease in immunoreactive dendrites of substance P-positive neurons of the superficial superior colliculus of the rat. Since some retinorecipient thalamic and pretectal nuclei are putative targets for substance P-containing cells of the superior colliculus, the present study attempted to ascertain whether substance P-immunoreactive fibers in these nuclei are also affected by retinal denervation. We found that unilateral eye removal produced a progressive increase in fibrous substance P immunoreactivity in the nucleus of the optic tract, lateral posterior nucleus, and lateral geniculate nucleus of the side contralateral to the enucleation. On the other hand, unilateral lesions to the superficial layers of the superior colliculus produced a dramatic reduction in substance P immunoreactivity in the ipsilateral nucleus of the optic tract, lateral posterior nucleus, and dorsal and ventral lateral geniculate nuclei. In bilaterally enucleated animals, unilateral lesion to the superior colliculus produced, as expected, loss of immunoreactive fibers only in the lateral posterior nucleus and the retinorecipient nuclei ipsilateral to the lesion. These results suggest that transneuronal changes in the distribution of substance P in collicular neurons observed after enucleation could be reflected in their projections to the other primary visual centers and to the lateral posterior nucleus.

Possible induction of [Met]enkephalin-Arg6-Gly7-Leu8 immunoreactivity in neurons of the rat superior colliculus following eye enucleation

The distribution of [Met]enkephalin-Arg6-Gly7-Leu8 (MEAGL)-immunoreactive (-IR) neurons and its modification after enucleation have been investigated in the rat superior colliculus. In normal rats and on the ipsilateral side of monocular-enucleated rats, small sized vertically elongated fusiform-shaped weakly immunostained neurons were dispersed throughout the sublamina of the stratum griseum superficiale (SGS). In bilaterally enucleated rats and on the contralateral side of monocular-enucleated rats, many small strongly immunoreactive MEAGL-containing neurons, projecting processes horizontally or obliquely toward the surface, appeared in the deepest part of the SGS and the superficial part of the stratum opticum (SO), in contrast to the disappearance of the fusiform-shaped weakly stained neurons in the SGS. MEAGL-IR fibers increased in density throughout the sublamina of the SGS, being densest in the deep SGS, accompanying their increase in the neighboring SO. Sporadically found MEAGL-IR neurons in the deep SO and the stratum griseum intermediale did not show the detectable change of immunoreactivity. These results indicate that enkephalin biosynthesis is undergone by different type of neurons in the normal and the ocular-derived superior colliculus, and suggest that some neurons in the adult superior colliculus have a potentiality to express the peptidergic phenotype.

Substance P and enkephalins in the superficial layers of the rat superior colliculus: differential plastic effects of retinal deafferentation

In this work we studied the effects of unilateral eye enucleation on the contents and distribution of leu-enkephalin-, met-enkephalin-arg6-gly7-leu8-, and substance-P-like immunoreactivities (L-ENK-I, ENK-8-I, and SP-I, respectively) in the superficial layers of the rat superior colliculus (SC) by means of the unlabelled antibody peroxidase-antiperoxidase method. In the normal rat only a few L-ENK-I neurons appear dispersed in the stratum griseum superficiale. No immunostained somata appear in the stratum opticum. The most striking effect of unilateral enucleation was the dramatic appearance of a laminarly distributed population of L-ENK-I and/or ENK-8-I neurons in the dorsal stratum opticum of the SC contralateral to the enucleated side. This population of immunoreactive cells was observed with all the survival times examined in the present study (3, 7, 15, and 30 days) and was always accompanied by an increase in the immunostaining of L-ENK-I and/or ENK-8-I fibers in the contralateral stratum griseum superficiale. Enucleation also produced a decrease in the immunostaining of SP-I dendrites that only became obvious 15 days after enucleation. However, the number of SP-I somata or terminal-like immunoreactive structures showed no detectable changes. These results show that retinal deafferentation of the superficial layers of the rat SC has different effects on some immunohistochemically distinguishable neuronal subgroups in the SC, suggesting different functional or trophic relationships of the retinal input to these groups of neurons.

Ultrastructural immunolocalization of adenosine deaminase in histaminergic neurons of the tuberomammillary nucleus of rat

Neurons in the tuberomammillary nucleus (TM) of the rat hypothalamus were immunolabelled for the enzyme adenosine deaminase (ADA) and investigated by electron microscopic immunohistochemical techniques. ADA-immunoreactivity was distributed throughout the somal and dendritic cytoplasm of TM neurons and in the karyoplasm of most, but not all of these neurons. Immunoreactive axons were rarely observed within the tightly packed cell clusters of the TM subdivisions examined. Dense deposition of immunoreaction product together with reasonable preservation of morphological detail facilitated identification of immunoreaction product together with reasonable preservation of morphological detail facilitated identification of immunoreactive profiles and allowed characterization of the ultrastructural features of labelled neurons and the relationships of these with each other and with surrounding unlabelled neuronal and glial elements. Immunolocalization of ADA therefore represents a reliable and convenient method for the identification of TM neurons in EM studies of their ultrastructure and synaptic interactions.

Choline acetyltransferase-like immunoreactivity in the superior colliculus of the cat and its relation to the pattern of acetylcholinesterase staining

Choline acetyltransferase, the biosynthetic enzyme for acetylcholine, is thought to be a marker for cholinergic neurons. This report presents an analysis of the pattern of choline acetyltransferase-like immunoreactivity in the superior colliculus of the cat. A dense network of highly varicose immunoreactive fibers pervaded the superficial gray and optical layer. The density of the fiber network in the superficial layers was heterogeneous, forming a mosaic pattern with a period of about 400 microns. The antigen was also located in numerous small perikarya embedded in this network. This neuronal population reached a density of 2,000 cells/mm3 of the superficial gray layer and suggested the presence of a substantial cholinergic system originating in the superior colliculus. A detailed comparison was made between the pattern of choline acetyltransferase-like immunoreactivity and the distribution of acetylcholinesterase activity. By comparisons of adjacent sections, both staining patterns were found to be similar in all collicular layers. In particular, the compartmental distribution of immunoreactivity in the intermediate collicular layers seemed to mimic the pattern of acetylcholinesterase staining. A double-staining technique demonstrated a near-perfect correlation between the two patterns. In conclusion, there was no indication of heightened acetylcholinesterase activity without an associated elevation in choline acetyltransferase-like immunoreactivity throughout the superior colliculus. In this part of the brain, the presence of the putative cholinergic terminals could fully account for the distribution of acetylcholinesterase activity.

Fluorescent histochemical localization of neutral endopeptidase-24.11 (enkephalinase) in the rat brainstem

Characterization of the distribution of the peptide-degrading enzyme neutral endopeptidase-24.11 (E.C. 3.4.24.11; NEP; enkephalinase) in the rat brainstem was examined by means of a unique fluorescent histochemical method. Enzyme staining was completely blocked by three potent NEP inhibitors (thiorphan, phosphoramidon, and JHF-26) at a concentration of 50 nM, supporting the specificity of this method to visualize sites of NEP activity selectively. At all levels of the brainstem, NEP was localized to cell bodies, cell processes or terminal-like fields and was localized to more than 90 distinct nuclei or subnuclei. In the mesencephalon these included the central gray, cuneiform n., dorsal and lateral tegmental n., inferior colliculus, interpeduncular n., lateral and medial geniculate n., central linear raphe n., mesencephalic n. of the trigeminal nerve, mammillary nuclei, occulomotor n., red n., superior colliculus, ventral n. of the lateral lemniscus, substantia nigra-ventral tegmental area, and the zona incerta. In the pons, NEP staining was restricted to fewer regions or nuclei, including the dorsal and ventral cochlear n., facial n., motor trigeminal n., principal sensory trigeminal n., parabrachial nuclei, pontine n., the oral and caudal pontine reticular n., pontine olivary nuclei, several pontine tegmental nuclei, pontine raphe nuclei, and the trapezoid n. In the cerebellum, staining was localized largely to the granule cell layer of the cerebellar cortex. Scattered staining was observed in the molecular cell layer. The medulla contained extensive NEP staining localized to nuclei that included the ambiguous n., dorsal motor n. of the vagus, hypoglossal n., inferior olivary n., prepositus hypoglossus n., solitary tract n., nuclei of the spinal tract of the trigeminal n., and the lateral, medial, and superior vestibular nuclei. Nuclei of the medullary reticular formation that were also richly stained for NEP included the raphe magnus n., raphe obscurus n., raphe pallidus n., dorsal, lateral, and ventral reticular nuclei of the medulla, and the gigantocellular, lateral paragigantocellular, linear, paramedian and parvicellular reticular nuclei. The widespread distribution of NEP in the brainstem suggests the existence of a number of functional systems, including the pathways involved in the mechanisms of pain and analgesia, which are potential targets of NEP inhibitors. In most regions, the distribution of NEP closely overlapped with that reported for the enkephalins, and showed a more restricted overlap with the reported distribution of substance P.

Appearance of substance P-like immunoreactive neurons in the rat superior colliculus after neonatal eye enucleation

The possibility that eye enucleation within one day after birth has an effect on the postnatal development of substance P (SP)-like-immunoreactive (SP-I) structures in the superior colliculus (SC) was investigated in the rat. Results were compared with those in animals enucleated at postnatal day 15. All the animals were allowed to survive until postnatal day 90, after which changes in SP-I neurons and fibers were identified immunohistochemically. In colchicine-treated rats, the most remarkable changes occurred in SP-I neurons following eye enucleation at birth; large numbers of SP-I neurons appeared in the ventral part of the stratum griseum superficiale (SGS), stratum opticum (SO) and stratum griseum intermediale (SGI) of the deafferentated SC. SP-I neurons did not appear in these layers, when deafferentation of the SC was carried out in rats at postnatal day 15. These findings suggest strongly that eye enucleation at birth affects the production of SP of neurons in the ventral part of the SGS, SO and SGI at the deafferentated SC. The appearance of SP-I neurons in the neonatal eye enucleation may be due, at least partially, to reorganization of another neuronal system in the SC.

Adenosine deaminase containing fiber pathway from the superior colliculus to the lateral posterior nucleus of the rat

Presence of a projection containing adenosine deaminase (ADA)-like immunoreactivity from the stratum opticum (SO) to the dorsomedial portion of the lateral posterior nucleus of the thalamus (LPN) of the rat was demonstrated using a method combining retrograde tracing by horseradish peroxidase (HRP) and immunohistochemistry for ADA. In the caudal two thirds of the LPN, a clearly delimited ADA-like immunoreactive fiber plexus was located. Injection of HRP into the lateral posterior nucleus labeled many neurons in the medial portion of the SO where medium-sized neurons with ADA immunoreactivity were concentrated. Simultaneous immunostaining showed that some of the HRP-labeled neurons are ADA-positive.