Convergence of forepaw somatosensory and motor cortical projections in the striatum, claustrum, thalamus, and pontine nuclei of cats

The basal ganglia and pontocerebellar systems regulate somesthetic-guided motor behaviors, and receive prominent inputs from sensorimotor cortex. Additionally, the claustrum and thalamus are forebrain subcortical structures that have connections with somatosensory and motor cortices. Our previous studies in rats have shown that primary and secondary somatosensory cortex (S1 and S2) send overlapping projections to the neostriatum and pontine nuclei, whereas overlap of primary motor cortex (M1) and S1 was much weaker. Additionally, we have shown that M1, but not S1, projects to the claustrum in rats. The goal of the current study was to compare these rodent projection patterns with connections in cats, a mammalian species that evolved in a separate phylogenetic superorder. Three different anterograde tracers were injected into the physiologically identified forepaw representations of M1, S1, and S2 in cats. Labeled fibers terminated throughout the ipsilateral striatum (caudate and putamen), claustrum, thalamus, and pontine nuclei. Digital reconstructions of tracer labeling allowed us to quantify both the normalized distribution of labeling in each subcortical area from each tracer injection, as well as the amount of tracer overlap. Surprisingly, in contrast to our previous findings in rodents, we observed M1 and S1 projections converging prominently in striatum and pons, whereas S1 and S2 overlap was much weaker. Furthermore, whereas rat S1 does not project to claustrum, we confirmed dense claustral inputs from S1 in cats. These findings suggest that the basal ganglia, claustrum, and pontocerebellar systems in rat and cat have evolved distinct patterns of sensorimotor cortical convergence.

Distinctive Spatial and Laminar Organization of Single Axons from Lateral Pulvinar in the Macaque

Pulvino-cortical (PC) projections are a major source of extrinsic input to early visual areas in the macaque. From bulk injections of anterograde tracers, these are known to terminate in layer 1 of V1 and densely in the middle cortical layers of extrastriate areas. Finer, single axon analysis, as reviewed here for projections from the lateral pulvinar (PL) in two macaque monkeys (n = 25 axons), demonstrates that PL axons have multiple arbors in V2 and V4, and that these are spatially separate and offset in different layers. In contrast, feedforward cortical axons, another major source of extrinsic input to extrastriate areas, are less spatially divergent and more typically terminate in layer 4. Functional implications are briefly discussed, including comparisons with the better investigated rodent brain.

Postnatal development of retrosplenial projections to the parahippocampal region of the rat

The rat parahippocampal region (PHR) and retrosplenial cortex (RSC) are cortical areas important for spatial cognition. In PHR, head-direction cells are present before eye-opening, earliest detected in postnatal day (P)11 animals. Border cells have been recorded around eye-opening (P16), while grid cells do not obtain adult-like features until the fourth postnatal week. In view of these developmental time-lines, we aimed to explore when afferents originating in RSC arrive in PHR. To this end, we injected rats aged P0-P28 with anterograde tracers into RSC. First, we characterized the organization of RSC-PHR projections in postnatal rats and compared these results with data obtained in the adult. Second, we described the morphological development of axonal plexus in PHR. We conclude that the first arriving RSC-axons in PHR, present from P1 onwards, already show a topographical organization similar to that seen in adults, although the labeled plexus does not obtain adult-like densities until P12.

Telencephalic projections to the nucleus of the basal optic root and pretectal nucleus lentiformis mesencephali in pigeons

In birds, the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) and the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow and the generation of the optokinetic response. In several species, it has been shown that the AOS and pretectum receive input from visual areas of the telencephalon. Previous studies in pigeons using anterograde tracers have shown that both nBOR and LM receive input from the visual Wulst, the putative homolog of mammalian primary visual cortex. In the present study, we used retrograde and anterograde tracing techniques to further characterize these projections in pigeons. After injections of the retrograde tracer cholera toxin subunit B (CTB) into either LM or nBOR, retrograde labeling in the telencephalon was restricted to the hyperpallium apicale (HA) of the Wulst. From the LM injections, retrograde labeling appeared as a discrete band of cells restricted to the lateral edge of HA. From the nBOR injections, the retrograde labeling was more distributed in HA, generally dorsal and dorso-medial to the LM-projecting neurons. In the anterograde experiments, biotinylated dextran amine (BDA) was injected into HA and individual axons were reconstructed to terminal fields in the LM and nBOR. Those fibers projecting to the nBOR also innervated the adjacent ventral tegmental area. However, tracing of BDA-labeled axons revealed no evidence that individual neurons project to both LM and nBOR. In summary, our results suggest that the nBOR and LM receive input from different areas of the Wulst. We discuss how these projections may transmit visual and/or somatosensory information to the nBOR and LM.

Projections From Primary Somatosensory Cortex to the Neostriatum: The Role of Somatotopic Continuity in Corticostriatal Convergence

We characterized the organization of corticostriatal projections from rodent primary somatosensory cortex (SI), testing the hypothesis that projections from SI areas representing subcomponents of the forelimb exhibit greater neostriatal overlap than projections from areas representing separate body parts. The anterograde tracers Fluoro-Ruby (FR), Alexa Fluor (AF), and biotinylated dextran amine (BDA) were injected into physiologically identified regions of rat SI. Injection locations were confirmed by examining the SI barrel fields and limb representations in tangential sections processed for cytochrome oxidase (CO). Experimental animals were divided into two groups: one group received multiple tracer injections in neighboring SI regions that represent separate body parts (whiskers, forepaw, and hindpaw); the other group received injections in SI areas that represent different components of the forelimb (forepaw, antebrachium, and brachium). The distribution of labeled terminals and their varicosities in the neostriatum and in the thalamus were plotted and quantitatively analyzed. For most animals, tracer overlap in the thalamus was either minimal or completely absent. In the neostriatum, projections from the whisker, forelimb, and hindlimb representations terminated in regions that rarely overlap with each other, while those originating from different parts of the forelimb representation were more likely to terminate in overlapping parts of the neostriatum. To the extent that neostriatal activation depends on corticostriatal convergence, the corticostriatal projections in the sensorimotor channel appeared to be organized so that neostriatal neurons may signal when multiple components of the same body part are activated simultaneously.

Spatial reciprocity of connections between areas 17 and 18 in the cat

We examined whether the interconnections between areas 17 and 18 are spatially reciprocal, i.e., whether a column of cells in area 17 receives from the same region of area 18 as it sends projections to, and vice versa. We addressed this question by making side by side injections of retrograde fluorescent tracers in area 18, calculating the convergence and divergence of the connections from area 17 to 18. We compared these values with previously reported values of divergence and convergence of the projections from area 18 to area 17. The results demonstrate that there is a good match between the convergence and divergence of the area 17 to area 18 connection and, respectively, the divergence and convergence of the reverse connection. We confirmed directly the spatial reciprocity by injecting simultaneously in area 17 a retrograde and an anterograde tracer and by analyzing quantitatively the density of anterograde and retrograde labeling across the surface of area 18. There was an excellent match between the density maps of retrogradely labeled cells and anterogradely labeled axon terminals in area 18. Connections between areas 17 and 18 therefore exhibit large degrees of convergence and divergence and are spatially reciprocal. Thus, a given column of cells within one of these two areas is reciprocally interconnected with a large region of the opposite area. Such an organization may provide the basis for synchronization of firing of neurons across these two areas, as revealed by cross-correlation studies.Key words: double labeling, fluorescent tracers, retrograde and anterograde tracers, convergence and divergence.