Superficial Gray Layer Research Articles

Neural Mechanisms of Stimulus Velocity Tuning in the Superior Colliculus

Superior colliculus (SC)-mediated control of visuomotor behavior depends on neuronal selectivity for stimulus velocity. However, the mechanism responsible for velocity tuning in SC neurons is unclear. It was shown in a previous study of anesthetized, decorticate hamsters that the number and distribution of feed-forward retinal inputs are not critical for velocity tuning. Here the alternate hypothesis that inhibition from the surround determines velocity tuning of SC neurons was tested. Surround inhibition was present in 65% (43/66) of SC neurons recorded in the superficial gray layer. Neurons within this group that were selective for slowly moving stimuli exhibited spatially asymmetric surround inhibition, and their velocity tuning arose by preferential suppression of responses to rapidly moving stimuli. In the other 35% (23/66) of SC neurons recorded, surround inhibition was weak or absent and did not play a role in velocity tuning. Most neurons with surround inhibition were nonselective for the duration of stationary flashed stimuli, whereas neurons without surround inhibition were selective for stimulus duration. The majority of neurons that preferred intermediate or rapidly moving stimuli exhibited spatially symmetric surround inhibition. In these neurons, occluding the surround reduced velocity selectivity by enhancing responses to slowly moving stimuli. Based on these data, a model is proposed suggesting spatiotemporal interactions between inhibition and excitation that could underlie velocity tuning.

Electrophysiological and morphological properties of identified crossed tecto-reticular neurons in the rat superior colliculus

Previously we classified randomly sampled neurons in the intermediate layer (SI) of the rat superior colliculus (SC) into six subclasses according to their firing responses to depolarizing current pulses and five subclasses based on their morphological properties in slice preparations. In the present study, we investigated properties of a major output cell group of the rat SC (PND 17–24), crossed tecto-reticular neurons (cTRNs), which project to the contralateral medial pontine reticular formation. The cTRNs were identified by retrograde labeling with a fluorescent tracer ( n = 112). We compared their properties with those of presumed interneurons ( n = 127). We found that a majority of cTRNs were regular spiking neurons with moderate firing frequency (73%) and were multipolar-shaped (66%). The cTRNs had larger membrane capacitance, larger soma size and lower input impedance than presumed interneurons. Electrical stimulation of the superficial gray layer induced oligosynaptic EPSPs in the cTRNs. When bicuculline was added to the extracellular solution, the EPSPs were markedly enhanced and bursting spike responses were induced. The bursting responses were suppressed by applying d-2-amino-5-phosphonovalerate. These results suggest that the cTRNs exhibit NMDA receptor-dependent bursting responses to visual inputs. These observations give insights into the neuronal mechanism of generating burst activity in cTRNs, which triggers orienting behaviors.

Organization of Interlaminar Interactions in the Rat Superior Colliculus

Our previous studies have shown that when slices of the rat superior colliculus (SC) are exposed to a solution containing 10 microM bicuculline and a low concentration of Mg2+ (0.1 mM), most neurons in the intermediate gray layer (stratum griseum intermediale; SGI), wide-field vertical (WFV) cells in the optic layer (stratum opticum; SO), and a minor population of neurons in the superficial gray layer (stratum griseum superficiale; SGS) exhibit spontaneous depolarization and burst firing, which are synchronous among adjacent neurons. These spontaneous and synchronous depolarizations were thought to share common mechanisms with presaccadic burst activity in SGI neurons. In the present study, we explored the site responsible for generation of synchronous depolarization of SGI neurons by performing dual whole cell recordings under different slice conditions. A pair of SGI neurons recorded in a small rectangular piece of the SGI punched out from the SC slice showed synchronous depolarization but far less frequently than those recorded in a small rectangular piece including SGS and SO. This suggests that the superficial layers are needed for triggering synchronous depolarization in the SGI. Furthermore, we recorded spontaneous depolarizations in pairs of neurons belonging to the different layers. Analysis of their synchronicity revealed that WFV cells in the SO exhibit synchronous depolarizations with both SGS and SGI neurons, and the onset of spontaneous depolarization in WFV cells precedes those of neurons in other layers. Further, when SGS and SGI neurons exhibit synchronous depolarizations, SGI neurons usually precede the SGS neurons. These observations give further evidence to the existence of interlaminar interaction between superficial and deeper layers of the SC. In addition, it is suggested that WFV cells can trigger burst activity in other layers of the SC and that there is an excitatory signal transmission from the deeper layers to the superficial layers.

Inhibition of superior colliculus neurons by a GABAergic input from the pretectal nuclear complex in the rat

The mammalian pretectal nuclear complex (PNC) is a visual and visuomotor control structure which is strongly connected to other subcortical visual structures. This indicates that the PNC also controls subcortical visual information flow during the execution of various oculomotor programs. A prominent, presumably GABAergic, projection from the PNC targets the superficial grey layer of the superior colliculus (SC), which itself is a central structure for visual information processing necessary for the generation of saccadic eye movements. In order to characterize the pretectotectal projection in vitro, we performed whole-cell patch-clamp recordings from SC and PNC neurons in slices obtained from 3-6-week-old pigmented rats. Focal glutamate injections into the PNC and electrical PNC stimulation were used to induce postsynaptic responses in SC neurons. Electrical stimulation of the SC allowed electrophysiological identification of PNC neurons that provide the inhibitory pretectotectal input. Only inhibitory postsynaptic currents could be elicited in SC neurons both by pharmacological and by electrical activation of the ipsilateral PNC. Concomitantly, a small number of PNC neurons could be antidromically activated from the ipsilateral SC. Most SC cells postsynaptic to the prectectal input showed the dendritic morphology of wide-field and narrow-field cells and are therefore regarded as projection neurons. All inhibitory currents evoked by PNC activation could be completely blocked by bath application of the selective GABA(A) receptor antagonist bicuculline. Together these results indicate that SC projection neurons receive a direct inhibitory input from the ipsilateral PNC and that this input is mediated by GABA(A) receptors.

GABA receptor ρ subunit expression in the developing rat brain

Ionotropic GABA C receptors are composed of ρ 1, ρ 2 and ρ 3 subunits. Although the distribution of ρ subunit mRNAs in the adult brain has been studied, information on the developmental regulation of different ρ subunits in the brain is scattered and incomplete. Here, GABA C receptor ρ subunit expression was studied in the developing rat brain. In situ hybridization on postnatal brain slices showed ρ 2 mRNA expression from newborn in superficial gray layer (SGL) of superior colliculus (SuC), and from the first postnatal week in the hippocampal CA1 region and pretectal nucleus of the optic tract. ρ 2 mRNA was also expressed in the adult dorsal lateral geniculate nucleus. Quantitative RT-PCR revealed expression of all three ρ subunits in the hippocampus and superior colliculus from the first postnatal day. In the hippocampus, ρ 2 mRNA expression clearly dominated over ρ 1 and ρ 3, whereas in the superior colliculus, ρ 1 mRNA expression levels were similar to ρ 2. In both areas, a clear up-modulation of ρ 2 and ρ 3 mRNA during the first postnatal week was detected. GABA C receptor protein expression was confirmed in adult hippocampus, superior colliculus and dorsal lateral geniculate nucleus by immunohistochemistry. Our results demonstrate for the first time the expression of all three ρ subunit mRNAs in several regions of the developing and adult rat brain. Our quantitative data allows assessment of putative subunit combinations in the superior colliculus and hippocampus. From the selective distribution of ρ subunits, it may be hypothesized that GABA C receptors are specifically involved in aspects of visual image motion processing in the rat brain.

Systemic administration of the potent mGlu8 receptor agonist (S)-3,4-DCPG induces c-Fos in stress-related brain regions in wild-type, but not mGlu8 receptor knockout mice

The effect of a novel and potent metabotropic glutamate 8 (mGlu8) receptor agonist, (S)-3,4-dicarboxyphenylglycine (DCPG), was studied in vivo in mouse brain. c-Fos expression was used as a marker of neuronal activity in specific brain regions 2 h after systemic (S)-3,4-DCPG (3–100 mg/kg, i.p.). The selectivity of (S)-3,4-DCPG on mGlu8 receptors was determined in mGlu8 receptor knockout mice. In wild-type mice, (S)-3,4-DCPG (100 mg/kg) significantly increased c-Fos expression in several stress-related brain regions: paraventricular nucleus of the hypothalamus, central nucleus of the amygdala, lateral parabrachial nucleus and locus coeruleus. In the central nucleus of the amgydala, more than 92% of c-Fos positive neurons were identified as GABAergic inhibitory neurons after (S)-3,4-DCPG. Moreover, (S)-3,4-DCPG significantly induced c-Fos in the superficial gray layer of the superior colliculus, a central visual region. c-Fos expression was unchanged by (S)-3,4-DCPG in mGlu8 receptor knockout mice. Our results indicate that systemic (S)-3,4-DCPG alters neuronal excitability in specific brain regions via mGlu8 receptor. The prominent activation of stress areas suggests a role for mGlu8 receptors in the central integration of stress responses. Furthermore, our results indicate that systemic (S)-3,4-DCPG can be used as a tool to explore behavioral and cellular consequences of mGlu8 receptor activation.

Autoradiographic imaging of the serotonin transporter in the brain of rats and pigs using S-([ 18F]fluoromethyl)-(+)-McN5652

The [ 18F]fluoromethyl analogue of (+)-McN5652 ([ 18F]FMe-McN) has recently been developed as a radioligand for imaging the neuronal serotonin transporter (SERT) with positron emission tomography (PET). We describe here the autoradiographic evaluation of [ 18F]FMe-McN in the brain of rats and pigs. Autoradiographic studies of [ 18F]FMe-McN performed on rat and pig brain in vitro showed a high accumulation of radioactivity in the regions rich in SERT, such as amygdala, hypothalamus, superficial gray layer of the superior colliculus, various nuclei of thalamus and substantia nigra. The binding of [ 18F]FMe-McN was reduced by citalopram, a highly selective inhibitor for SERT. Similar regional specific binding densities of [ 18F]FMe-McN were observed in both species. The regional distribution and specific binding of this radiotracer correlates well with the distribution and regional brain binding of [ 3H]citalopram. Region-to-cerebellum ratios of [ 18F]FMe-McN in vitro reached a maximum value of 20.6 in the rat and 14.5 in the pig. In addition, ex vivo autoradiography of the rat brain was performed 90 min after i.v. administration of [ 18F]FMe-McN. The highest regional uptake of [ 18F]FMe-McN was observed in the hypothalamic area, substantia nigra and amygdaloid area. There is a high correlation between the in vitro and in vivo binding. The region-to-cerebellum ratio in vivo reached a maximum value of 5.1 in the substantia nigra, the highest yet reported for an 18F-labelled SERT tracer in vivo in this region. Furthermore, the distribution volume of [ 18F]FMe-McN calculated from the PET data in various regions of the porcine brain is highly correlated with the SERT density as determined by in vitro autoradiography with [ 3H]citalopram. Thus, [ 18F]FMe-McN has a clear potential as a radiotracer for studies of the SERT distribution in man with PET.

Cytoarchitecture and fiber pattern of the superior colliculus are disrupted in the Shaking Rat Kawasaki

Shaking Rat Kawasaki ( SRK) is a Reelin-deficient rat, that shows significant cytoarchitectural abnormalities in the cerebral and cerebellar cortices in a similar manner to the reeler malformation. In the present study, we investigated the cytoarchitecture and myeloarchitecture of the superior colliculus (SC) of this mutant rat. The Nissl staining clearly showed that neuronal components in the superficial layers of the SC in SRK rat were intermingled with each other and that the boundaries between these superficial layers were blurred. The MBP immunohistochemistry showed an abnormal fiber pattern in the superficial layers of the SC of this mutant rat. In the normal rat, myelinated fibers passed rostrocaudally through the optic layer, and only a few myelinated fibers were recognized in the uppermost two layers, i.e., the zonal and superficial gray layers. By contrast, in SRK rat, the myelinated fibers were distributed throughout the entire thickness of the superficial layers of the SC. Anterograde labeling of retinotectal fibers with an injection of Cholera Toxin subunit B into the retina revealed that this abnormal fiber pattern was associated with the anomalous course of the retinotectal fibers. No distinct differences in the cytoarchitecture and fiber pattern in the deep layers of the SC were seen. In conclusion, the present study demonstrated that the cytoarchitecture and fiber patterning in the superficial layers of the SC were disrupted in SRK rat, suggesting that Reelin protein regulates the formation of the superficial layers of the SC.

Immunocytochemical localization of calretinin in the superficial layers of the cat superior colliculus.

We localized calretinin-immunoreactive (IR) fibers and cells in the superior colliculus (SC) of the cat and studied the distribution and effect of enucleation on the distribution of this protein. Calretinin was localized with antibody immunocytochemistry. A dense plexus of anti-calretinin-IR fibers was found within the upper part of the superficial gray layer. Almost all of the labeled fibers were small diameter fibers with few varicosities. Monocular enucleation produced an almost complete reduction of calretinin-IR fibers in the SC contralateral to the enucleation. Furthermore, many calretinin-IR cells appeared in the contralateral SC. The newly appeared cells had small- to medium-sized vertical fusiform, oval or round, or stellate cell bodies. Two-color immunofluorescence revealed that no cells in the superficial layers expressed both calretinin and GABA. Many retinal ganglion cells were labeled after injections of retrograde axonal transport horseradish peroxidase (HRP) in the superficial layers. However, no large cells were double-labeled with calretinin and HRP. More than 95% of the double-labeled cells were small cells (<15 microm). Based on the retinal ganglion cell size, we believe that the vast majority of calretinin-IR retinocollicular fibers in cat SC are small gamma type cells that have W type physiologies.

Expression of the L-type calcium channel in the developing mouse visual system by use of immunocytochemistry

Developmental refinement of the retinogeniculate and retinocollicular pathways is partially dependent upon Ca 2+ channel function [J. Comp. Neurol. 440 (2001) 177–191]. We have examined the development of the L-type voltage gated Ca 2+ channel to determine if the onset of expression matches this period of refinement. Labeling by an antibody directed against the α1C subunit of this channel was examined in the superior colliculus (SC), lateral geniculate nucleus (LGN), visual cortex (CTX), hippocampus (HC) and cerebellum (CB) in mice aged P3–4, P8–9, P15, P21, P28, and adults. At P3–4, labeled cells within the SC were concentrated within a dense band in the retinorecipient zone of the superficial gray layer. More lightly labeled neurons were seen in other layers. This dense band was still seen at P15, while more labeled neurons were seen in other layers. By P21–P28, labeled neurons were fairly uniformly distributed throughout all layers of SC. Neuronal cell types appeared to be labeled at all ages examined within the LGN. Within CTX, putative layer V–VI pyramidal neurons were well labeled at P4 and later ages, and labeled layer II–III pyramids could be distinguished by P9 and later ages. The dendrites and cell bodies of pyramidal neurons within CA1–CA3 of HC, granule neurons in the dentate gyrus, and Purkinje neurons in CB were labeled at all ages examined. We conclude that the L-type Ca 2+ channel is expressed in many neurons within retinorecipient targets as well as in other brain regions during the developmental period in which pathway refinement and synaptic plasticity occurs.

Ultrastructure of neuropeptide-Y immunoreactive elements in the superior colliculus of cat.

Whereas the presence of neuropeptide-Y (NPY) in the superior colliculus (SC) has been established, its participation in the ultrastructural organisation of the neuronal networks in the SC has not been studied. Accordingly, in the present paper light and electron microscopic NPY immunohistochemical studies were performed on the SC of cat. NPY fibres were found to be present predominantly in the superficial grey layer (SGL) of the SC, though a few small NPY cells were found in both the deeper and the upper layers. Ultrastructural observations revealed that the NPY nerve endings establish almost exclusively axo-dendritic synaptic contacts in the SGL of the SC. Thus, the presumably inhibitory impact of the NPY terminals is exerted through the dendrites of the SGL neurons, and not directly to the retinal axons, as thought previously.

Ablation of pituitary pro-opiomelanocortin (POMC) cells produces alterations in hypothalamic POMC mRNA levels and midbrain mu opioid receptor binding in a conditional transgenic mouse model.

The hypothalamic-pituitary-adrenal (HPA) axis is regulated by stress-related excitatory inputs, and various inhibitory and negative-feedback controls by glucocorticoids and opioids, including pro-opiomelanocortin (POMC)-derived peptides. The role of POMC-derived peptides of pituitary origin in the modulation of brain POMC mRNA expression and opioid receptor binding was investigated using a line of transgenic mice that express a fusion gene composed of the pituitary expression-specific promoter region of the POMC gene driving the herpes simplex viral-1 thymidine kinase (TK). Male adult mice were treated with the antiherpes agent ganciclovir that selectively ablates cells expressing TK. Following treatment, POMC mRNA levels, measured by quantitative solution hybridization/RNase protection assays, were decreased by 48% in the pituitary of the TK+/+ mice, reflecting an expected loss of the pituitary corticotrope POMC cells. This treatment also significantly lowered pituitary beta-endorphin immunoreactivity content and plasma concentrations of corticosterone. In contrast, POMC mRNA levels were increased by 79% in the hypothalamus of the TK+/+ mice with pituitary POMC cell ablation. Binding of [(3)H]DAMGO to mu opioid receptors, as measured by quantitative autoradiography, was significantly reduced in several brain regions including the central grey, median raphe and superficial grey layer of the superior colliculus. These regions are innervated by hypothalamic POMC neurones. No significant differences in binding to either kappa or delta opioid receptors were found in the brain regions studied. These results suggest that POMC-derived peptides of pituitary origin may exert a tonic negative-feedback effect on hypothalamic POMC neurones. In turn, the downregulation of central mu opioid receptors in this model may be mediated through a mechanism related to hypothalamic POMC overexpression.

Subcellular localization of neuronal nitric oxide synthase in the superficial gray layer of the rat superior colliculus.

The superficial layers of the rat superior colliculus (sSC) receive innervation from retina and include nitric oxide synthase (NOS)-immunoreactive neurons. We used electron microscopic immunocytochemistry to assess the subcellular localization of neuronal NOS (nNOS) in the sSC. nNOS immunoreactivity was detected on the external membrane of mitochondria, endoplasmic reticulum, in pre- and postsynaptic profiles and also diffusely distributed in the cytosol. Postsynaptic labeled regions were often associated with presumptive retinal unlabeled terminals. Microtubules also appeared intensely labeled. These results show that NOS immunoreactive neurons may be innervated by retinal terminals and suggest an association of nNOS with cytoskeletal elements.

Slow IPSC kinetics, low levels of alpha1 subunit expression and paired-pulse depression are distinct properties of neonatal inhibitory GABAergic synaptic connections in the mouse superior colliculus.

Remodelling of visual maps in the superior colliculus (SC) depends on neuronal activity. Synaptic inhibition could contribute to this process because spontaneous spike discharge in the SC was modulated by GABA(A) receptor activation at postnatal days (P) 1-3. To investigate the functional capacity of GABAergic synaptic transmission at this early stage of development, whole-cell patch-clamp recordings were made from wide field neurons (WFNs) in horizontal slices comprising the superficial grey layer of the SC. Focal stimulation in the vicinity of WFNs evoked tetrodotoxin-sensitive stimulus-locked inhibitory postsynaptic currents (eIPSCs). The failure rate of eIPSCs was low ( approximately 0.2), and the maximal amplitude of evoked unitary eIPSCs exceeded the amplitude of average miniature IPSCs (mIPSCs) by a factor of 4-5, suggesting that action potential-mediated GABA release was more effective than spontaneous release. Some of the properties of GABAergic synaptic transmission in the neonatal SC were age-specific. In contrast with eIPSCs in the more mature SC at P20-22, neonatal eIPSCs decayed more slowly, preferentially fluctuated in duration, not amplitude, and mostly lacked temporal summation, due to depression at shorter intervals. The paired-pulse ratio (eIPSC2 : eIPSC1) was inversely related to the duration of eIPSCs. PCR analysis showed, in addition, that the ratio of alpha1 : alpha3 subunit expression was lower in the neonatal SC. Together, these results suggest that, at a young age, efficacy of GABAergic synaptic transmission is primarily constrained by the slow kinetics and the saturation of postsynaptic GABA(A) receptors.

Postnatal alterations of GABA receptor profiles in the rat superior colliculus.

Midbrain sections taken from Sprague-Dawley rats of varying ages within the first four postnatal weeks were used to determine, immunocytochemically, putative changes of GABA(A) receptor beta2/3 subunits, GABA(B) receptor (R1a and R1b splice variants), and GABA(C) receptor rho1 subunit expression and distribution in the superficial, visual layers of the superior colliculus. Immunoreactivity for the GABA(A) receptor beta2/3 subunits was found in the superficial grey layer from birth. The labelling changed with age, with an overall continuous reduction in the number of cells labelled and a significant increase in the labelling intensity distribution (neuropil vs soma). Further analysis revealed an initial increase in the labelling intensity between postnatal days 0 and 7 in parallel with an overall reduction of labelled neurones. This was followed by a significant decrease in labelling intensity distribution between postnatal days 7 and 16, and a subsequent increase in intensity between postnatal days 16 and 28. The labelling profiles for GABA(B) receptors (R1a and R1b splice variants) and GABA(C) receptors (rho1 subunit) showed similar patterns. Both receptors could be found in the superficial layers of the superior colliculus from birth, and the intensity and distribution of labelling remained constant during the first postnatal month. However, the cell body count showed a significant decrease between postnatal days 7 and 16. These changes may be related to the time-point of eye opening, which occurred approximately two weeks after birth. For all three receptor types, the cell body count remained constant after postnatal day 16. By four weeks of age, there was no significant difference between the cell numbers obtained for the different receptors. Both GABA itself and neurofilament labelling were also obtained in the superficial superior colliculus at birth. Neurofilament, although found at birth, showed very little ordered arrangement until 16days after birth. When slices were double labelled for GABA(C) receptors and neurofilament, some overlap was observed. Double labelling for the presynaptic protein synaptophysin and GABA(C) receptors showed proximity in some places, indicative of a partly synaptic location of GABA(C) receptors. When GABA(C) and GABA(A) receptors were labelled simultaneously, some but not all neurones showed immunoreactivity for both receptor types. In conclusion, all three GABA receptor types were found to be present in the superior colliculus from birth, and all show some form of postnatal modification, with GABA(A) receptors demonstrating the most dramatic changes. However, GABA(B) and GABA(C) receptors are modified significantly around the onset of input-specific activity. Together, this points towards a contribution of the GABAergic system to processes of postnatal maturation in the superficial superior colliculus.

Prenatal and postnatal expression of nitric oxide in the developing kitten superior colliculus revealed with NADPH diaphorase histochemistry.

Nitric oxide (NO) is a neuronal messenger molecule that mediates pathway refinement in some brain regions. We used nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry to examine the development of NO expression in the superior colliculus (SC) of kittens aged E28-E58 and P2-P57 and adults in order to determine if NO expression is correlated with pathway refinement. At E28, labeled cells were seen only within the subventricular zone (SVZ). At E36-E41, labeled cells were also found within the deep gray layer (DGL) of SC. At E51 and E58, a few labeled neurons were also present in the intermediate gray layer (IGL). These neurons already had extensive dendritic fields and well-developed morphologies at the time that they first expressed nitric oxide synthase (NOS). The number of neurons labeled in the DGL and IGL increased postnatally, reaching a peak density between P14 and P35. Neurons within the optic (OL) and superficial gray layers (SGL) were first visible at P7 and increased slightly in number until adulthood. However, SGL-labeled neurons were relatively limited in number and lightly labeled at all ages examined. We conclude that (1) NADPHd expression occurs in SC beginning in the second trimester in kittens and progresses in a ventral to dorsal pattern between E36-P35; (2) few neurons in kitten SGL are labeled by NADPHd and these appear relatively late in postnatal development; and (3) there is no correlation between NOS expression and retinocollicular pathway refinement in kittens, a result different from that seen in rodents.

Superior colliculus projections to midline and intralaminar thalamic nuclei of the rat.

The superior colliculus (SC) projections to the midline and intralaminar thalamic nuclei were examined in the rat. The retrograde tracer cholera toxin beta (CTb) was injected into one of the midline thalamic nuclei-paraventricular, intermediodorsal, rhomboid, reuniens, submedius, mediodorsal, paratenial, anteroventral, caudal ventromedial, or parvicellular part of the ventral posteriomedial nucleus-or into one of the intralaminar thalamic nuclei-medial parafascicular, lateral parafascicular, central medial, paracentral, oval paracentral, or central lateral nucleus. After 10-14 days, the brains from these animals were processed histochemically, and the retrogradely labeled neurons in the SC were mapped. The lateral sector of the intermediate gray and white layers of the SC send axonal projections to the medial and lateral parafascicular, central lateral, paracentral, central medial, rhomboid, reuniens, and submedius nuclei. The medial sector of the intermediate and deep SC layers project to the parafascicular and central lateral thalamic nuclei. The paraventricular thalamic nucleus is innervated almost exclusively by the medial sectors of the deep SC layers. The superficial gray and optic layers of the SC do not project to any of these thalamic areas. The discussion focuses on the role these SC-thalamic inputs may have on forebrain circuits controlling orienting and defense (i.e., fight-or-flight) reactions.

Postnatal development of nitric oxide synthase expression in the mouse superior colliculus

Since nitric oxide has a role in the refinement of the retinal projection to the superior colliculus (SC), we studied the onset of neuronal nitric oxide synthase (nNOS) expression in the mouse SC in order to compare its development with that of the refinement process. Sections from animals at ages P1, P5, P8, P11, P15, and P21 and adults were examined with nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry or immunocytochemistry using an antibody directed against nNOS. At all ages there was a wedge of labeled neurons in the dorsolateral periaqueductal gray extending into the deep layers of the SC. At P1 there was also a single superficial band of labeled neurons within the region that will become the intermediate gray layer (IGL). By P5, labeled neurons were also seen in what will become the superficial gray layer. There was a ventral to dorsal progression in nNOS expression with substantial changes in the numbers of labeled neurons in the different laminae between P5 and adulthood. The number of labeled neurons in the IGL peaked at P15, whereas in the superficial layers the numbers continued to increase through P21 and then declined in adults. At all ages these neurons represented a variety of morphological cell types. The onset of nNOS expression in the different laminae is earlier than has been reported in studies using NADPHd as a marker for nNOS. The temporal and spatial patterns of nNOS expression reported here match more closely the time course of pathway refinement in the SC, providing additional evidence for the involvement of nitric oxide in this process. J. Comp. Neurol. 427:581–592, 2000. © 2000 Wiley-Liss, Inc.

The T Cell Oncogene Tal2 Is Necessary for Normal Development of the Mouse Brain

Transcription factors are commonly involved in leukemia by activation through chromosomal translocations and normally function in cell type(s) that differ from that of the tumor. TAL2 is a member of a basic helix–loop–helix gene family specifically involved in T cell leukemogenesis. Null mutations of Tal2 have been made in mice to determine its function during development. Tal2 null mutant mice show no obvious defects of hematopoiesis. During embryogenesis, Tal2 expression is restricted to the developing midbrain, dorsal diencephalon, and rostroventral diencephalic/telencephalic boundary, partly along presumptive developing fiber tracts. The null mutant mice are viable at birth but growth become progressively retarded and they do not survive to reproductive age. Tal2-deficient mice show a distinct dysgenesis of the midbrain tectum. Due to loss of superficial gray and optical layers, the superior colliculus is reduced in size and the inferior colliculus is abnormally rounded and protruding. Death is most likely due to progressive hydrocephalus which appears to be caused by obstruction of the foramen of Monro (the connection between the ventricles of the forebrain). Thus, in addition to its oncogenicity when ectopically expressed, Tal2 normally plays a pivotal role in brain development and without this gene, mice cannot survive to maturity.

Autoradiographic localization of [ 3H]nociceptin binding sites in the rat brain

The binding sites of nociceptin (also named orphanin FQ), the endogenous ligand of ORL1 (opiate receptor like 1), were localized in rat brain, using an autoradiographic procedure. High levels of binding were observed in the cingulate, retrosplenial, perirhinal, insular and occipital cortex, anterior and posteromedial cortical amygdaloid nuclei, basolateral amygdaloid nucleus, amygdaloid complex, posterior hippocampus, dorsal endopiriform, central medial thalamic, paraventricular, rhomboid thalamic, suprachiasmatic, ventromedial hypothalamic nuclei, mammillary complex, superficial gray layer of the superior colliculus, locus coeruleus, dorsal raphe nucleus. More moderate labelling was observed in the prefrontal, fronto–parietal, temporal, piriform cortex, dentate gyrus, anterior olfactory nucleus, olfactory tubercle, shell of nucleus accumbens, claustrum, lateral septum, laterodorsal thalamic, medial habenular, subthalamic, reuniens thalamic nuclei, subiculum, periaqueductal grey matter and pons. A lower binding site density was observed in the anterior and medial hippocampus, olfactory bulb, caudate putamen, the core of the nucleus accumbens, medial septum, ventrolateral, ventroposterolateral and mediodorsal thalamic nuclei, lateral and medial geniculate nuclei, hypothalamic area, substantia nigra, ventral tegmentum area and interpedoncular nucleus. A moderate and similar labelling was found in the dorsal and ventral horn of the spinal cord. No labelling was apparent in the corpus callosum. Thus, it appears that the ORL1 receptor is particularly abundant in the cerebral cortex, limbic system of the rat brain and some areas involved in pain perception.