Orientation Tuning—A Crooked Path to the Straight and Narrow

AbstractNeurons in visual cortex are selective for the orientation of a visual stimulus, while the receptive fields of their thalamic input are circular. Cortical orientation selectivity arises from the organization of both thalamic input and local cortical circuits. In this issue of Neuron, Schummers and colleagues provide evidence that the local circuit mechanisms contributing to orientation selectivity differ depending on the local organization of the orientation map

Francis Crick's Legacy for Neuroscience: Between the α and the Ω

The legacy of Francis Crick is explored by two scientists who were influenced by his wor

Retrograde Tracing with Recombinant Rabies Virus Reveals Correlations Between Projection Targets and Dendritic Architecture in Layer 5 of Mouse Barrel Cortex

A recombinant rabies virus was used as a retrograde tracer to allow complete filling of the axonal and dendritic arbors of identified projection neurons in layer 5 of mouse primary somatosensory cortex (S1) in vivo. Previous studies have distinguished three types of layer 5 pyramids in S1: tall-tufted, tall-simple, and short. Layer 5 pyramidal neurons were retrogradely labeled from several known targets: contralateral S1, superior colliculus, and thalamus. The complete dendritic arbors of labeled cells were reconstructed to allow for unambiguous classification of cell type. We confirmed that the tall-tufted pyramids project to the superior colliculus and thalamus and that short layer 5 pyramidal neurons project to contralateral cortex, as previously described. We found that tall-simple pyramidal neurons contribute to corticocortical connections. Axonal reconstructions show that corticocortical projection neurons have a large superficial axonal arborization locally, while the subcortically projecting neurons limit axonal arbors to the deep layers. Furthermore, reconstructions of local axons suggest that tall-simple cell axons have extensive lateral spread while those of the short pyramids are more columnar. These differences were revealed by the ability to completely label dendritic and axonal arbors in vivo and have not been apparent in previous studies using labeling in brain slices

Regulation of Competitive Interactions During Neuromuscular Synapse Elimination

The subsequent chapters of this thesis address a number of issues related to the phenomena that occur during neuromuscular synapse elimination and to the rules and mechanisms that govern them. The results they describe are therefore based on observations of developmental processes in both normal animals and in those whose normal developmental interactions have been perturbed by alteration of activity for some of the elements involved. Chapter 2 addresses the question of whether the rate of neuromuscular synapse elimination might normally depend on the level of postsynaptic activity. Previous studies had strongly implicated activity in regulation of synapse elimination rate; e.g., increased activity evoked by chronic stimulation increased the elimination rate; but these studies did not differentiate between pre- and postsynaptic activity. Duxson (1982) treated the rat soleus muscle with α-bungarotoxin (α-BGT), completely blocking postsynaptic activity during the normal period of synapse elimination and reported that the number of terminal profiles per endplate observed in electron micrographs did not decrease as it does normally. I did not consider this study to be conclusive because complete activity blockade might invoke influences that are not normally present in active muscles during synapse elimination, and because the assay was indirect - an increase in the number of terminal profiles per endplate might reflect an increase in terminal complexity rather than maintenance of more terminals. For the experiments described in Chapter 2, postsynaptic activity was partially blocked by α-BGT superfusion of the neonatal rabbit soleus muscle. The toxin treatment resulted in slower synapse elimination, as assessed both physiologically and anatomically, even for muscle fibers whose activity was not completely blocked. While the interpretation of this result is dependent on the possibility that α-BGT has influences other than decreased activity, it appears quite likely that a partial block of postsynaptic activity can slow the rate of neuromuscular synapse elimination. Chapter 3 describes a separate series of experiments in which motor unit twitch tensions were assayed for the soleus muscles of neonatal rabbits. Synapse loss could be assayed separately for fast and slow populations by separating the motor units, based on their twitch rise times. Estimates of the rate of synapse elimination for the two populations suggested that slow muscle fibers were initially more heavily polyinnervated than fast fibers and that they lost synapses at a faster rate, so that both populations of fibers became predominantly singly innervated at about the same time. The remainder of the issues in Chapter 3 are related to the question of whether there are particular attributes of motor neurons that might place the inputs from some neurons at a competitive advantage over others. The first such issue was whether motor neurons with relatively large axonal arbors are at an advantage or disadvantage in the competition for synaptic sites. If this were the case, it would be expected that the diversity in motor unit sizes would decrease during synapse elimination if a large arbor were a disadvantage, and thediversity would increase if it were an advantage. Contrary to both hypotheses, no significant change in the diversity of motor unit sizes was observed. The next issue was whether motor neurons from particular positions in the spinal cord were at an advantage or disadvantage compared to the others. To test this issue, mean sizes of motor units from both rostral and caudal extremes of the soleus motor pool were compared to the mean sizes for those from the middle of the pool. At the earliest age tested, when the soleus is still heavily polyinnervated, the motor units from the extremes were no smaller than those from the middle. However, just four days later, the units from each extreme were significantly smaller than those from the middle; this difference persisted in older, singly innervated muscles. There was no significant difference between the rostral and caudal motor units at any age tested. It is concluded that motor neurons from rostral and caudal extremes are at a disadvantage when in competition with those from the middle of the motor pool during synapse elimination in the rabbit soleus. Finally, a small portion of the rabbit soleus motor neurons was inactivated by implantation of a tetrodotoxin-laden Silastic plug during synapse elimination. Since there was nearly complete overlap between the inactivated and active motor units at the time of the implant, this allowed a test of whether the level of activity of a motor neuron can influence the ability of its terminals to compete for sole occupancy of endplates. It was found that the inactive motor units ended up significantly larger than their active counterparts in normal and control implanted animals, and remained larger even after the endplates were virtually all singly innervated. It is concluded that inactivity can result in a significant competitive advantage during synapse elimination. The generality of these conclusions and their implications in terms of the ways in which neuromuscular synapse elimination might be regulated are discussed in detail.</p

The College News, 1966-09-30, Vol. 53, No. 03

Bryn Mawr College student newspaper. Merged with The Haverford News in 1968 to form the Bi-college News (with various titles from 1968 on). Published weekly (except holidays) during the academic year

Functional Specialization of Seven Mouse Visual Cortical Areas

SummaryTo establish the mouse as a genetically tractable model for high-order visual processing, we characterized fine-scale retinotopic organization of visual cortex and determined functional specialization of layer 2/3 neuronal populations in seven retinotopically identified areas. Each area contains a distinct visuotopic representation and encodes a unique combination of spatiotemporal features. Areas LM, AL, RL, and AM prefer up to three times faster temporal frequencies and significantly lower spatial frequencies than V1, while V1 and PM prefer high spatial and low temporal frequencies. LI prefers both high spatial and temporal frequencies. All extrastriate areas except LI increase orientation selectivity compared to V1, and three areas are significantly more direction selective (AL, RL, and AM). Specific combinations of spatiotemporal representations further distinguish areas. These results reveal that mouse higher visual areas are functionally distinct, and separate groups of areas may be specialized for motion-related versus pattern-related computations, perhaps forming pathways analogous to dorsal and ventral streams in other species

A systematic topographical relationship between mouse lateral posterior thalamic neurons and their visual cortical projection targets.

Higher-order visual thalamus communicates broadly and bi-directionally with primary and extrastriate cortical areas in various mammals. In primates, the pulvinar is a topographically and functionally organized thalamic nucleus that is largely dedicated to visual processing. Still, a more granular connectivity map is needed to understand the role of thalamocortical loops in visually guided behavior. Similarly, the secondary visual thalamic nucleus in mice (the lateral posterior nucleus, LP) has extensive connections with cortex. To resolve the precise connectivity of these circuits, we first mapped mouse visual cortical areas using intrinsic signal optical imaging and then injected fluorescently tagged retrograde tracers (cholera toxin subunit B) into retinotopically-matched locations in various combinations of seven different visual areas. We find that LP neurons representing matched regions in visual space but projecting to different extrastriate areas are found in different topographically organized zones, with few double-labeled cells (~4-6%). In addition, V1 and extrastriate visual areas received input from the ventrolateral part of the laterodorsal nucleus of the thalamus (LDVL). These observations indicate that the thalamus provides topographically organized circuits to each mouse visual area and raise new questions about the contributions from LP and LDVL to cortical activity

Viral vector-based reversible neuronal inactivation and behavioral manipulation in the macaque monkey

Viral vectors are promising tools for the dissection of neural circuits. In principle, they can manipulate neurons at a level of specificity not otherwise achievable. While many studies have used viral vector-based approaches in the rodent brain, only a few have employed this technique in the non-human primate, despite the importance of this animal model for neuroscience research. Here, we report evidence that a viral vector-based approach can be used to manipulate a monkey's behavior in a task. For this purpose, we used the allatostatin receptor/allatostatin (AlstR/AL) system, which has previously been shown to allow inactivation of neurons in vivo. The AlstR was expressed in neurons in monkey V1 by injection of an adeno-associated virus 1 (AAV1) vector. Two monkeys were trained in a detection task, in which they had to make a saccade to a faint peripheral target. Injection of AL caused a retinotopic deficit in the detection task in one monkey. Specifically, the monkey showed marked impairment for detection targets placed at the visual field location represented at the virus injection site, but not for targets shown elsewhere. We confirmed that these deficits indeed were due to the interaction of AlstR and AL by injecting saline, or AL at a V1 location without AlstR expression. Post-mortem histology confirmed AlstR expression in this monkey. We failed to replicate the behavioral results in a second monkey, as AL injection did not impair the second monkey's performance in the detection task. However, post-mortem histology revealed a very low level of AlstR expression in this monkey. Our results demonstrate that viral vector-based approaches can produce effects strong enough to influence a monkey's performance in a behavioral task, supporting the further development of this approach for studying how neuronal circuits control complex behaviors in non-human primates

Short Promoters in Viral Vectors Drive Selective Expression in Mammalian Inhibitory Neurons, but do not Restrict Activity to Specific Inhibitory Cell-Types

Short cell-type specific promoter sequences are important for targeted gene therapy and studies of brain circuitry. We report on the ability of short promoter sequences to drive fluorescent protein expression in specific types of mammalian cortical inhibitory neurons using adeno-associated virus (AAV) and lentivirus (LV) vectors. We tested many gene regulatory sequences derived from fugu (Takifugu rubripes), mouse, human, and synthetic composite regulatory elements. All fugu compact promoters expressed in mouse cortex, with only the somatostatin (SST) and the neuropeptide Y (NPY) promoters largely restricting expression to GABAergic neurons. However these promoters did not control expression in inhibitory cells in a subtype specific manner. We also tested mammalian promoter sequences derived from genes putatively coexpressed or coregulated within three major inhibitory interneuron classes (PV, SST, VIP). In contrast to the fugu promoters, many of the mammalian sequences failed to express, and only the promoter from gene A930038C07Rik conferred restricted expression, although as in the case of the fugu sequences, this too was not inhibitory neuron subtype specific. Lastly and more promisingly, a synthetic sequence consisting of a composite regulatory element assembled with PAX6 E1.1 binding sites, NRSE and a minimal CMV promoter showed markedly restricted expression to a small subset of mostly inhibitory neurons, but whose commonalities are unknown

Centrifugal Inputs to the Main Olfactory Bulb Revealed Through Whole Brain Circuit-Mapping

Neuronal activity in sensory regions can be modulated by attention, behavioral state, motor output, learning, and memory. This is often done through direct feedback or centrifugal projections originating from higher processing areas. Though, functionally important, the identity and organization of these feedback connections remain poorly characterized. Using a retrograde monosynaptic g-deleted rabies virus and whole-brain reconstructions, we identified the organization of feedback projecting neurons to the main olfactory bulb of the mouse. In addition to previously described projections from regions such as the Anterior Olfactory Nucleus (AON) and the piriform cortex, we characterized direct projections from pyramidal cells in the ventral CA1 region of hippocampus and the entorhinal cortex to the granule cell layer (GCL) of the main olfactory bulb (MOB). These data suggest that areas involved in stress, anxiety, learning and memory are all tethered to olfactory coding, two synapses away from where chemical compounds are first detected. Consequently, we hypothesize that understanding olfactory perception, even at the earliest stages, may require studying memory and behavior in addition to studying the physiochemical features of odors