Callosal axon arbors in the limb representations of the somatosensory cortex (SI) in the agouti (Dasyprocta primnolopha)

Abstract

The present report compares the morphology of callosal axon arbors projecting from and to the hind- or forelimb representations in the primary somatosensory cortex (SI) of the agouti (Dasyprocta primnolopha), a large, lisencephlic Brazilian rodent that uses forelimb coordination for feeding. Callosal axons were labeled after single pressure (n = 6) or iontophoretic injections (n = 2) of the neuronal tracer biotinylated dextran amine (BDA, 10 kD), either into the hind- (n = 4) or forelimb (n = 4) representations of SI, as identified by electrophysiological recording. Sixty-nine labeled axon fragments located across all layers of contralateral SI representations of the hindlimb (n = 35) and forelimb (n = 34) were analyzed. Quantitative morphometric features such as densities of branching points and boutons, segments length, branching angles, and terminal field areas were measured. Cluster analysis of these values revealed the existence of two types of axon terminals: Type I (46.4%), less branched and more widespread, and Type II (53.6%), more branched and compact. Both axon types were asymmetrically distributed; Type I axonal fragments being more frequent in hindlimb (71.9%) vs. forelimb (28.13%) representation, while most of Type II axonal arbors were found in the forelimb representation (67.56%). We concluded that the sets of callosal axon connecting fore- and hindlimb regions in SI are morphometrically distinct from each other. As callosal projections in somatosensory and motor cortices seem to be essential for bimanual interaction, we suggest that the morphological specialization of callosal axons in SI of the agouti may be correlated with this particular function.

S1 to S2 hind- and forelimb projections in the agouti somatosensory cortex: Axon fragments morphological analysis

“…This implies in incomplete reconstructions from which only metric features that do not depend on the order of the segments are suitable for analysis. As previously recommended elsewhere (Amorim and Picanco-Diniz, 1997;Gomes-Leal et al, 2002;Rocha et al, 2007) we have done similar choice in the present morphometric analysis, providing consistent quantitative data through density values in [ ( ) T D $ F I G ] , and field area (E) of axon trees of the S1 to S2 somatosensory ipsilateral projections in the agouti. Significant distinctions between axon fragments Types I and II (left column) and between hind and forelimb (right column) are indicated by asterisk (two-tailed Student's t-test, p < 0.05).…”

“…Except by area measurements, all morphometric features were expressed in density values (number of occurrences of each feature per fragment, divided by total length of the fragment in millimeters). Thus, the result in each case was not affected by terminal size, incompleteness of labeling or visualization (Gomes-Leal et al, 2002;Rocha et al, 2007).…”

“…To investigate this hypothesis, computer-assisted reconstructions of axon fragments from different regions in S2 were assembled following a previously established protocol (Amorim and PicancoDiniz, 1997;Gomes-Leal et al, 2002;Rocha et al, 2007). More specifically, we sampled BDA-labeled axon arbors in S2 after injection of the tracer in S1, either in the representation of the forepaw or the hindpaw wich were localized by multiunit electrophysiological recording.…”

Morphometric analysis of feedforward pathways from the primary somatosensory area (S1) of rats

“…To assess the homogeneity of the axonal population in each area in relation to defined morphometric variables, we first performed a Multivariate Analysis of Variance (MANOVA). Next, an exploratory cluster analysis of morphological terminal types was performed using hierarchical clustering analysis (HCA) to explore whether specific groups of terminals existed in our sample based on the morphometric variables mentioned above (Steele and Weller, 1995;Schweitzer and Renehan, 1997;Gomes-Leal et al, 2002;Rocha et al, 2007). The significance of the classification performed by the HCA was tested with a MANOVA.…”

Characterization of NADPH Diaphorase- and Doublecortin-Positive Neurons in the Lizard Hippocampal Formation

“…Briefly, 3-dimensional reconstructions were performed in a Nikon AFX-DX Optiphot microscope (Tokyo, Japan) (×60 oil Plan-Apochromat objective -Nikon NA 1.4), equipped with a motorized stage (MAC200; Ludl Electronic Products, Haw- Color version available online thorne, NY, USA) and coupled to a computer running the Neurolucida software (MBF Bioscience Inc., Williston, VT, USA) [Rocha et al, 2007]. Eighty NADPHd+ neurons were reconstructed: 40 each from the medial cortex (MC) and dorsomedial cortex (DMC).…”