Axonal bias at a representational border in adult rat somatosensory cortex (S1)

Abstract

The cortex is a highly organized structure and this organization is integral to cortical function. However, the circuitry underlying cortical organization is only partially understood, thus limiting our understanding of cortical function. Within the somatosensory cortex, organization is manifest as a map of the body surface. At the level of the cortical circuitry the horizontal connections of Layer 2/3 express a physiological bias that reflects discontinuities within the somatosensory map. Both excitation and inhibition are smaller when evoked from across a representational border, as compared to when they are evoked from within the representation. This physiological bias may be due to a bias in either the strength or number of synapses and/or the number of axons that cross this border and the extent of their arborization. In this study we used both an anterograde (Phaseolus vulgaris leucoagglutinin) and a retrograde (cholera toxin B) tracer to examine Layer 2/3 horizontal projections in rat S1. We determined that there is a bias in the amount of horizontal axonal projections that cross the forepaw/lower jaw border as compared to projections remaining within an individual representation. This bias in axonal projection and the correlated bias in excitation and inhibition may underlie the expression of the representational border.

Presynaptic GABAB receptors decrease neurotransmitter release in vestibular nuclei neurons during vestibular compensation

“…Independent thresholds for GABA B R2 immunolabeling (red channel) and MAP2 immunolabeling (green channel) were set for each image analyzed based on the background intensity of the cell nucleus (Popratiloff et al, 2003; Shao et al, 2009) and were equal to the mean plus 3× the standard deviation of the background intensity (Suzuki and Bekkers, 2010; Steen et al, 2007). The thresholds were determined from the nucleus of a single principal cell body present in the section.…”

“…Independent thresholds for GABA B R2 immunolabeling (red channel) and Syt1 or Syt2 immunolabeling (green channel) were established based on background intensities of the nucleus and cytoplasm of the principal cells, respectively. Thresholds were equal to the mean plus 3x the standard deviation of the background level (Suzuki and Bekkers, 2010; Steen et al, 2007). The cell cytoplasm rather than the cell nucleus was used to determine the background level for the Syt1 and Syt2 channels because the principal cell body cytoplasm contained weak, uniform, but non-punctate labeling for Syt1 and Syt2, which was more intense than that in the nucleus but dramatically less intense than that found in the terminals (Popratiloff and Peusner, 2011).…”

Plasticity of Horizontal Connections at a Functional Border in Adult Rat Somatosensory Cortex

“…Thus, one electrode stimulated fibers that crossed the border (discontinuous representation pathway) while the other stimulated fibers that were within the representation of the neuron that was being recorded from (continuous representation pathway). These pathways have also been referred to as cross border (CB) and noncross border (NCB) [9–12, 15, 16]. All cells were recorded in current clamp mode and current was injected to keep the resting membrane potential at −70 mV, except during the pairing or depolarization protocol.…”

“…Both the morphological and functional aspects of the circuitry surrounding a border are altered by the border's presence [9–12]. Neurons located in close proximity of a border have both dendritic [11, 13] and axonal [9, 14] biases towards the center of their home column. Additionally, the presence of a border limits the spread of excitation and inhibition to adjacent representations [10, 12, 14].…”

“…Additionally, the presence of a border limits the spread of excitation and inhibition to adjacent representations [10, 12, 14]. This phenomenon may be partially explained by the fewer axons projecting into discontinuous representations [9]. Interestingly, the morphological and functional biases of the original border relocate with the reorganized border following sensory loss [13, 15–17].…”