Morphological properties of projection specific pyramidal neurons of primate anterior cingulate cortex

Abstract

The anterior cingulate cortex is an important interface of cortical, motor, and limbic networks, and thus is a brain area uniquely situated to affect a wide variety of higher order functions. The aim of this study was to characterize the morphology of two distinct populations of anterior cingulate cortex (ACC) pyramidal neurons, a dorsal-caudal population projecting to the premotor cortex (PMC) and a ventral-rostral population projecting to the amygdala. Retrograde tracers injected into area 6DC of the “cognitive” premotor cortex, and into the basolateral nucleus of the “affective” amygdala were used to label distinct projection neurons in the ACC. Whole-cell patch clamp recording and intracellular filling techniques were used to fill the dendritic arbor of these labeled projection neurons. High resolution confocal microscopy and 3D neuronal reconstructions were used to quantify dendritic morphological parameters. Amygdala projecting neurons were more superficial than premotor projecting neurons, with an average soma-to-pia distance of 498 μm compared to 1,012 μm, respectively (amygdala projecting: 498 ± 139 μm vs. PMC projecting: 1012 ± 113 μm, p<.05). Overall, amygdala and PMC projection neurons had very similar average dendritic lengths, branch points, branch densities, and vertical and horizontal extensions in both apical and basal compartments. Amygdala projecting cells had greater apical tuft branch points than deep PMC projecting cells (8.25 vs. 3.3 apical tuft branch points, p<.05). Superficial PMC projecting cells had smaller total vertical and apical vertical extensions than deep PMC projecting cells (Total vertical: 304.98 vs. 750.96 μm, apical vertical: 241.78 vs 601.95 μm, p<.05). Sholl analyses revealed that the distribution of apical dendritic length as a function of distance from the soma of amygdala projections had bimodal peaks, while that of superficial and deep PMC cells had a single peak. Total spine number of amygdala projecting neurons was greater than PMC projecting cells (~17,000 spines vs. ~2,100 spines). Three major classes of morphology were visualized within the ACC neuron reconstructions dataset: regular-tufted, narrow-tufted, and untufted, with the regular-tufted cells containing more branch points than narrow tufted but less basal branch point density. The work in this study assessing cellular morphological properties of specific amygdala and PMC inputs and outputs within the ACC helps to characterize functional dynamics of both emotional and motor planning networks