A substance P projection from the superior colliculus to the parabigeminal nucleus in the rat and hamster

Dissecting the Tectal Output Channels for Orienting and Defense Responses

Electrical stimulation and lesion experiments in 1980’s suggested that the crossed descending pathway from the deeper layers of superior colliculus (SCd) controls orienting responses, while the uncrossed pathway mediates defense-like behavior. To overcome the limitation of these classical studies and explicitly dissect the structure and function of these two pathways, we performed selective optogenetic activation of each pathway in male mice with channelrhodopsin 2 (ChR2) expression by Cre driver using double viral vector techniques. Brief photostimulation of the crossed pathway evoked short latency contraversive orienting-like head turns, while extended stimulation induced body turn responses. In contrast, stimulation of the uncrossed pathway induced short-latency upward head movements followed by longer-latency defense-like behaviors including retreat and flight. The novel discovery was that while the evoked orienting responses were stereotyped, the defense-like responses varied considerably depending on the environment, suggesting that uncrossed output can be influenced by top-down modification of the SC or its target areas. This further suggests that the connection of the SCd-defense system with non-motor, affective and cognitive structures. Tracing the whole axonal trajectories of these two pathways revealed existence of both ascending and descending branches targeting different areas in the thalamus, midbrain, pons, medulla, and/or spinal cord, including projections which could not be detected in the classical studies; the crossed pathway has some ipsilaterally descending collaterals and the uncrossed pathway has some contralaterally descending collaterals. Some of the connections might explain the context-dependent modulation of the defense-like responses. Thus, the classical views on the tectal output systems are updated.

GABAergic cell types in the superficial layers of the mouse superior colliculus

To begin to unravel the complexities of GABAergic circuits in the superior colliculus (SC), we utilized mouse lines that express green fluorescent protein (GFP) in cells that contain the 67 kDa isoform of glutamic acid decarboxylase (GAD67-GFP), or Cre-recombinase in cells that contain glutamic acid decarboxylase (GAD; GAD2-cre). We used Cre-dependent virus injections in GAD2-Cre mice and tracer injections in GAD67-GFP mice, as well as immunocytochemical staining for gamma amino butyric acid (GABA) and parvalbumin (PV) to characterize GABAergic cells that project to the pretectum (PT), ventral lateral geniculate nucleus (vLGN) or parabigeminal nucleus (PBG), and interneurons in the stratum griseum superficiale (SGS) that do not project outside the SC. We found that approximately 30% of SGS neurons in the mouse are GABAergic. Of these GABAergic neurons, we identified three categories of potential interneurons in the GAD67-GFP line (GABA+GFP ~45%, GABA+GFP + PV ~15%, and GABA+PV ~10%). GABAergic cells that did not contain GFP or PV were identified as potential projection neurons (GABA only ~30%). We found that GABAergic neurons that project to the PBG are primarily located in the SGS and exhibit narrow field vertical, stellate, and horizontal dendritic morphologies, while GABAergic neurons that project to the PT and vLGN are primarily located in layers ventral to the SGS. In addition, we examined GABA and GAD67-containing elements of the mouse SGS using electron microscopy to further delineate the relationship between GABAergic circuits and retinotectal input. Approximately 30% of retinotectal synaptic targets are the presynaptic dendrites of GABAergic interneurons, and GAD67-GFP interneurons are a source of these presynaptic dendrites.

Intergeniculate leaflet and suprachiasmatic nucleus organization and connections in the golden hamster

The intergeniculate leaflet (IGL) is a distinct subdivision of the lateral geniculate complex which receives retinal input and projects upon a circadian pacemaker, the suprachiasmatic nucleus (SCN). In the present study, we have analyzed the organization of the IGL and its connections in the hamster, a species commonly used in circadian rhythm studies. The location of the IGL is defined by the presence of retinal afferents demonstrated by anterograde transport of cholera toxin-HRP, neuropeptide Y-containing neurons and axons, cells retrogradely labeled from the regions of the SCN and contralateral IGL, and substance P-containing axons. It is a long nucleus extending the entire rostrocaudal axis of the geniculate. The most rostral IGL lies between the lateral dorsal thalamus, ventrolateral part, and the horizontal cerebral fissure. It then enlarges ventral to the rostral dorsal lateral geniculate, medial to the optic tract. The mid-portion of the leaflet is a thin lamina intercalated between the dorsal and ventral geniculate nuclei. The extended caudal portion of the nucleus lies lateral and ventral to the medial geniculate and is contiguous with the zona incerta and the lateral terminal nucleus. The IGL contains populations of neuropeptide Y (NPY+) and enkephalin (ENK+) neurons which project to the retinorecipient portion of the SCN. In addition to the immunoreactive perikarya, the IGL contains plexuses of NPY+, ENK +, substance P-, serotonin-, and glutamic acid decarboxylase-immunoreactive axons.

Retrograde transport studies demonstrate that, in addition to the NPY+ neurons, there is a population of non-NPY+ neurons projecting upon the SCN. There is also a reciprocal projection upon the IGL from neurons in the SCN region, particularly the retrochiasmatic area. The hamster SCN differs from the rat in containing a distinct subdivision of substance P-immunoreactive neurons.

The mammalian superior colliculus: laminar structure and connections

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

Cross-modal neuroplasticity in neonatally enucleated hamsters: structure, electrophysiology and behaviour

Potential auditory compensation in neonatally bilaterally enucleated Syrian hamsters was explored anatomically, electrophysiologically and behaviourally. Gross morphology of the visual cortex appeared normal and no obvious cytoarchitectural malformation was discerned. However, enucleation induced a significant increase in the spontaneous firing rate of visual cortex cells. Further, auditory stimuli elicited field potentials and single unit responses in the visual cortex of enucleated, but not normal, animals. About 63% of the cells isolated in the visual cortex of 16 enucleated hamsters responded to at least one type of auditory stimulus. Most of the responses were less vigorous and less time-locked than those of auditory cortex cells, and thresholds were typically higher. Projection tracing with WGA-HRP disclosed reciprocal connections between the visual cortex and the dorsal lateral geniculate nucleus in both intact and enucleated animals. However, in the enucleated animals retrogradely labelled cells were also found in the inferior colliculus, the major midbrain auditory nucleus. Behaviourally determined auditory sensitivity across the hearing range did not differ between enucleated and intact hamsters. Minimum audible angle, as determined by a conditioned suppression task, ranged from around 17 to 22 degrees, with no significant difference between normal and enucleated animals. The two groups also did not differ with regard to the direction of their unconditioned head orientating response to intermittent noise. However, the enucleated animals showed a more vigorous response and were slower to habituate to the noise. These results show that bilateral enucleation of newborn hamsters results in auditory activation of visual targets, in addition to the typical activation of the intact auditory pathway. Behaviourally it appears that enucleated hamsters, compared with their normal littermates, are slower to habituate in their response to an unexpected source of sound.

Distribution and regulation of substance P-related peptide in the frog visual system

Modulation of visual signal activity has consequences for both signal processing and for activity-dependent structuring mechanisms. Among the neuromodulatory agents found in visual areas are substance P (SP)-related peptides. This article reviews what is known about these substances in the amphibian retina and optic tectum with special emphasis on the leopard frog, Rana pipiens. It is found that the distribution of these SP-related peptides is remarkably similar to that seen in mammals. This suggests that study of model amphibian systems may significantly enhance our understanding of how neuropeptides contribute to visual system function and organization.

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.

Connections of the superior colliculus with the tegmentum and the cerebellum in the hedgehog tenrec

Different tracer substances were injected into the superior colliculus (CoS) in order to study its afferents and efferents with the meso-rhombencephalic tegmentum, the precerebellar nuclei and the cerebellum in the Madagascan hedgehog tenrec. The overall pattern of tectal connectivity in tenrec was similar to that in other mammals, as, e.g. the efferents to the contralateral paramedian reticular formation. Similarly the origin of the cerebello-tectal projection in mainly the lateral portions of the tenrec's cerebellar nuclear complex corresponded to the findings in species with little binocular overlap. In comparison to other mammals, however, the tenrec showed a consistent projection to the ipsilateral inferior olivary nucleus, in addition to the classical contralateral tecto-olivary projection. The tenrec's CoS also appeared to receive an unusually prominent monoaminergic input particularly from the substantia nigra, pars compacta. There was a reciprocal tecto-parabigeminal projection, a distinct nuclear aggregation of parabigeminal neurons, however, was difficult to identify. The dorsal lemniscal nucleus did not show perikaryal labeling in contrast to the paralemniscal region. Similar to the cat but unlike the rat there were a few neurons in the nucleus of the central acoustic tract. Unlike the cat, but similar to the rat there was a distinct, predominantly ipsilateral projection to the magnocellular reticular field known to project spinalward.

Glutamate containing neurons in the cat superior colliculus revealed by immunocytochemistry

Glutamate is the probable neurotransmitter of both retinal and cortical afferents to the cat superior colliculus (SC). The present study shows that glutamate is also contained in many postsynaptic neurons in SC. The distribution, morphology, and ultrastructure of neurons in SC were examined using glutamate antibody immunocytochemistry. Labeled cells were widely distributed throughout, but a specific laminar pattern was evident. Relatively few cells were found in the zonal and upper superficial gray layers (SGL). A dense band of intensely labeled neurons was found within the deep superficial gray and upper optic layers. Many cells were also labeled in the deeper layers. Labeled cells had varied sizes and morphologies. Soma diameters ranged from 9-67 microns, with a mean of 22 microns. Cells with stellate, vertical fusiform, and multipolar morphologies were labeled. Cells in the deep subdivision all had morphologies and sizes typical of projection neurons. To determine if labeled cells in the dense band were also projection neurons, WGA-HRP was injected into the lateral posterior nucleus and these sections were double-labeled with the glutamate antibody. Over one-half of cells in the dense band that were labeled by HRP were also obviously labeled by antibody. At the electron-microscope level, both medium- and large-sized neurons were also labeled by glutamate antibodies. These cells had different but characteristic morphologies.

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.

Birth dates and survival after axotomy of neurochemically defined subsets of trigeminal ganglion cells

Trigeminal (V) ganglion cells with different neurochemical phenotypes or different birth dates are affected differently by neonatal axonal transection. The aim of the present study was to determine if V ganglion cell birth date and neurochemical phenotype were correlated and if these two variables could be related to responses to neonatal axonal transection. Immunocytochemistry, histochemistry, and [3H]thymidine labelling were used to determine the birth dates of V ganglion cells recognized by antibodies directed against neurofilament protein (NF), calcitonin gene-related peptide (CGRP), and substance P (SP) and those that bound the lectin Bandierea simplicifolia-I (BS-I). All V ganglion cells were born between embryonic days (E-) 9.5 and 14.5. All ganglion cells were born between E-9.5 and E-14.5. In a normalized population (percentages normalized to equal 100%), over 90% of NF-positive V ganglion cells were born between E-10.5 and E-12.5. The majority of CGRP-positive and SP-positive ganglion cells (> 90%) were generated from E-13.5 to E-14.5 and E-12.5 through E-14.5, respectively. Almost 85% of BS-I-positive ganglion cells were generated on E-12.5 through E-14.5. Previous results and additional data from this study indicated that NF- and BS-I-positive ganglion cells are proportionally more likely to be lost after neonatal axotomy and that SP-positive cells are more likely to remain. The percentage of CGRP-positive cells in the V ganglion was not significantly altered by neonatal infraorbital nerve transection. Overall, these findings do not indicate a strong relationship between cell birth date and the probability of survival after neonatal axonal damage for all V ganglion cell phenotypes.

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.

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.

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.

Ultrastructural studies of the primate parabigeminal nucleus: electron microscopic autoradiographic analysis of the tectoparabigeminal projection in Galago crassicaudatus

The normal ultrastructure of the parabigeminal nucleus and the morphology and synaptic relationships of tectoparabigeminal terminals have been examined. Five different morphological types of terminals have been observed within the parabigeminal nucleus. Three of these profiles contain round vesicles and make asymmetrical synapses, while two contain pleomorphic vesicles and make symmetrical synapses. Electron microscopic autoradiographic data indicate that labeled tectoparabigeminal terminals represent only one of the three profiles containing round vesicles. Such terminals are primarily presynaptic to dendritic shafts, and several labeled profiles have been observed presynaptic to the same dendrite.

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.

Neuronal activity in narcolepsy: identification of cataplexy-related cells in the medial medulla

Narcolepsy is a neurological disorder characterized by sleepiness and episodes of cataplexy. Cataplexy is an abrupt loss of muscle tone, most often triggered by sudden, strong emotions. A subset of cells in the medial medulla of the narcoleptic dog discharged at high rates only in cataplexy and rapid eye movement (REM) sleep. These cells were noncholinergic and were localized to ventromedial and caudal portions of the nucleus magnocellularis. The localization and discharge pattern of these cells indicate that cataplexy results from a triggering in waking of the neurons responsible for the suppression of muscle tone in REM sleep. However, most medullary cells were inactive during cataplexy but were active during REM sleep. These data demonstrate that cataplexy is a distinct behavioral state, differing from other sleep and waking states in its pattern of brainstem neuronal activity.

Tectospinal neurons in hamster contain glutamate-like immunoreactivity

Immunocytochemistry with a monoclonal antibody directed against glutamate revealed numerous immunoreactive cells in the hamster's superior colliculus (SC). A large number of these neurons were located in the deep layers and many were in the stratum album intermedium (SAI). These neurons appeared similar to the large multipolar cells that have been shown to project to the cervical spinal cord in this species. The combination of retrograde tracing using either Fluoro-Gold- or FITC-labelled latex beads with immunocytochemistry for glutamate revealed that many of the immunoreactive cells did, in fact, project into the predorsal bundle and extend axons as far as the cervical spinal cord.