Projection neurons in the superficial layers of the superior colliculus in the rat: a topographic and quantitative morphometric analysis

Neuronal correlates of attention and its disengagement in the superior colliculus of rat

Orienting attention to a new target requires prior disengagement of attention from the current focus. Previous studies indicate that the superior colliculus (SC) plays an important role in attention. However, recordings of responses of SC neurons during attentional disengagement have not yet been reported. Here, we analyzed rat SC neuronal activity during performance of an attention-shift task with and without disengagement. In this task, conditioned stimuli (CSs; right and/or left light-flash or sound) were sequentially presented. To obtain an intracranial self-stimulation reward, rats were required to lick a spout when an infrequent conditioned stimulus appeared (reward trials). In the disengagement reward trials, configural stimuli consisting of an infrequent stimulus and frequent stimulus in the former trials were presented; in the non-disengagement reward trials, only an infrequent stimulus was presented. Of the 186 SC neurons responding to the CSs, 41 showed stronger responses to the CSs in the disengagement reward trials than in the non-disengagement reward trials (disengagement-related neurons). Furthermore, lick latencies in the disengagement reward trials were negatively correlated with response magnitudes to the CSs in half of the disengagement-related neurons. These disengagement-related neurons were located mainly in the deep layers of the SC. Another 70 SC neurons responded to the CSs in both disengagement and non-disengagement reward trials, suggesting that these neurons were involved in attention engagement. Our results suggest complementary mechanisms of attentional shift based on two subpopulations of neurons in the SC.

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.

Frequency-dependent release of substance P mediates heterosynaptic potentiation of glutamatergic synaptic responses in the rat visual thalamus

To investigate the interaction between peptides and glutamatergic synapses in the dorsal thalamus, we compared the frequency-dependent plasticity of excitatory postsynaptic potentials (EPSPs) in the tectorecipient zone of rodent lateral posterior nucleus (LPN), which is densely innervated by axons that contain the neuromodulator substance P (SP). Immunocytochemistry and confocal and electron microscopy revealed that neurokinin 1 (NK1) receptors are distributed on the dendrites of LPN cells, whereas SP is contained in axons originating from the superior colliculus (SC) and is reduced following SC lesions. In vitro whole cell recordings in parasagittal slices revealed that stimulation of the SC or optic radiations (corticothalamic axons [CTXs]) evoked LPN EPSPs that increased in amplitude with increasing stimulation intensity, suggesting convergence. With 0.5- to 10-Hz stimulus trains, CTX EPSP amplitudes displayed frequency-dependent facilitation, whereas SC EPSP amplitudes were unchanged. High-frequency SC stimulation (100 Hz for 0.5 s), or bath application of SP, resulted in gradual increases in both SC and CTX EPSP amplitudes to twofold or greater above baseline within 15-20 min poststimulation/application. This enhancement correlated with increases in input resistance and both the potentiation and resistance change were abolished in the presence of the NK1 antagonist L-703,606. These results indicate that SP is released when SC-LPN neurons fire at high frequency and SP acts postsynaptically via NK1 receptors to potentiate subsequent LPN responses to both cortical and tectal inputs. We suggest that the SP-mediated potentiation of synaptic responses may serve to amplify responses to threatening objects that move across large regions of the visual field.

Effects of superior colliculus ablation on the air-righting reflex in the rat

To examine how the superior colliculus, the motor center of orientation and avoidance, could interact with postural reflexes, we investigated effects of unilateral and bilateral ablations on air-righting reflex movements in otherwise intact rats. Superior colliculus ablations variously modified righting movements: After falling from the supine position, the rats sometimes showed dorsiflexion instead of normal ventriflexion; the motor sequence of rotation from the fore- to the hindquarter was often modified to simultaneous rotation; lateral turn from supine to prone position was occasionally insufficient; body direction that was normally kept constant during falling was often changed; final posture sometimes deviated from the horizontal position. The first three abnormalities occurred almost twice in frequency as lesions increased from unilateral to bilateral ablation, and in unilaterally ablated rats, did so in righting contraversive to the lesions. Multiple influences of tectoreticular input to the air-righting reflex center are discussed.

Synaptic organization of the tectorecipient zone of the rat lateral posterior nucleus

Dorsal thalamic nuclei have been categorized as either "first-order" nuclei that gate the transfer of relatively unaltered signals from the periphery to the cortex or "higher order" nuclei that transfer signals from one cortical area to another. To classify the tectorecipient lateral posterior (LPN), we examined the synaptic organization of tracer-labeled cortical and tectal terminals and terminals labeled with antibodies against the type 1 and type 2 vesicular glutamate transporters (vGLUT1 and vGLUT2) within the caudal/lateral LPN of the rat. For this zone, we found that all tracer-labeled cortical terminals, as well as vGLUT1 antibody-labeled terminals, are small profiles with round vesicles (RS profiles) that innervate small-caliber dendrites. Tracer-labeled tecto-LPN terminals, as well as vGLUT2 antibody-labeled terminals, were medium-sized profiles with round vesicles (RM profiles). Tecto-LPN terminals were significantly larger than cortico-LPN terminals and contacted significantly larger dendrites. These results indicate that, within the tectorecipient zone of the rat LPN, cortical terminals are located distal to tectal terminals and that vGLUT1 and vGLUT2 antibodies may be used as markers for cortical and tectal terminals, respectively. Finally, comparisons of the synaptic patterns formed by tracer-labeled terminals with those of vGLUT antibody-labeled terminals suggest that individual LPN neurons receive input from multiple cortical and tectal axons. We suggest that the tectorecipient LPN constitutes a third category of thalamic nucleus ("second-order") that integrates convergent tectal and cortical inputs. This organization may function to signal the movement of novel or threatening objects moving across the visual field.

NADPH-diaphorase activity in the superficial layers of the superior colliculus of rats during aging

Neurons in the superficial layers of the superior colliculus are key elements in the visual system of rodents since they receive extensive afferent projections from retinal ganglion cells. The NADPH-diaphorase histochemical technique was used to detect differences in neuronal nitric oxide synthase (nNOS) in the superficial layers of the superior colliculus (sSC) of young adult (3 months) and aged (24 and 26 months) rats. The orientation of the dendritic processes of NADPH-diaphorase-positive neurons, cross-sectional area, and number of neurons per mm2 were analyzed. NADPH-d histochemistry revealed a high number of NADPH-d-positive cells in the stratum zonale and stratum griseum superficiale in adult and aged animals. NADPH-d-positive neurons were classified into the following morphological types: marginal, horizontal, pyriform, narrow-field vertical, wide-field vertical, and stellate. During aging, narrow field vertical and wide field vertical neurons present somatic atrophy and an increase in dendritic processes with dorsoventral orientation, whereas wide field vertical neurons show a decrease in those with lateromedial orientation. Marginal neurons undergo somatic hypertrophy at 26 months when compared with those at 3 months. The remaining types of neurons do not undergo size changes. Finally, the number of NADPH-d-positive neurons per mm2 in the various types of morphology does not significantly change with age. It is suggested to be likely that the aging process in the nitrergic neurons of the sSC does not lead to significant changes in the synthesis of NO from the constitutive NOS isoforms.

Intrinsic connectivity of human superior colliculus

The superior colliculus (SC) is believed to play an important role in sensorimotor integration and orienting behavior. It is classically divided into superficial layers predominantly containing visual neurons and deep layers containing multisensory and premotor neurons. Investigations of intrinsic connectivity within the SC in non-human species initially led to controversy regarding the existence of interlaminar connections between superficial and deep layers. It now seems more likely that such connections exist in a number of species, including non-human primates. In the latter, anatomical data concerning intrinsic SC connectivity are restricted to a limited number of intracellularly labeled neurons. No studies have been conducted to investigate the existence of intrinsic connections of human SC. In the present study, DiI (1,1'-dioctadecyl-3,3,3',3'- tetramethylindocarbocyanine perchlorate) and BDA (biotinylated dextran amine) were two tracers used in post-mortem human brains to examine intrinsic SC connections. Injections into the superficial layers revealed tangential connections within superficial layers and radial superficial-layer to deep-layer connections. Within superficial layers, horizontal connections were found over the entire rostro-caudal axis and were mostly directed laterally, i.e. toward the brachium of the inferior colliculus. Superficial-layer to deep-layer connections were more prominent in sections containing the injection site or located close to it. In these sections, an axon bundle having roughly the same diameter as the injection site crossed all deep layers, and individual axons displayed en passant or terminal boutons. The present results suggest that intrinsic connections within superficial layers and radial superficial-layers to deep-layers exist in human SC. The putative roles of these connections are discussed with regard to visual receptive field organization, as well as visuomotor and multisensory integration.

Visually-induced NGFI-A protein expression in the calbindin-, parvalbumin- and nitric oxide synthase-neuronal populations of the rat superior colliculus

The expression of the immediate early gene NGFI-A in the nervous system is induced by sensory stimulation and seems to be related to long-term synaptic plasticity. We have used double-labeling immunohistochemistry to identify calbindin (CB)(+), parvalbumin (PV)(+) and neuronal nitric oxide synthase (nNOS)(+) neurons that also expressed the protein encoded by this immediate early gene after light-exposure on in the superficial layers of the rat superior colliculus (sSC). The majority of the NGFI-A(+) cells were not double-labeled for the tested markers. In the stratum zonale+stratum griseum superficiale (SZ/SGS), only 17.8%, 8.0% and 12.1% of NGFI-A(+) cells were also labeled for CB, PV or nNOS, respectively. In the stratum opticum (SO), only 10.5% of the NGFI-A(+) cells were also CB(+). Furthermore, only a small subset of each population expressed the NGFI-A protein after light-exposure. In the SZ/SGS, 35.7% of the CB(+), 32.1% of the PV(+) and 26.6% of the nNOS(+) neurons also expressed the NGFI-A. In the SO, 31.7% of the CB(+) neurons also expressed the NGFI-A. The proportional distribution of the nNOS(+)/NGFI-A(+) neurons throughout the SZ/SGS layers showed a slight but significant rostro-caudal gradient. No significant difference was observed for the other markers, indicating homogeneous activation of these populations throughout the retinotopic map. Our results suggest that the visually-driven NGFI-A expression is not restricted to a specific population of the sSC and that visual processing in this structure, as assessed by the expression of this candidate-plasticity protein, involves the activation of subsets of ascending and non-ascending projection neurons.

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.

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.