Imaging the Spatio-Temporal Dynamics of Supragranular Activity in the Rat Somatosensory Cortex in Response to Stimulation of the Paws

Abstract

We employed voltage-sensitive dye (VSD) imaging to investigate the spatio-temporal dynamics of the responses of the supragranular somatosensory cortex to stimulation of the four paws in urethane-anesthetized rats. We obtained the following main results. (1) Stimulation of the contralateral forepaw evoked VSD responses with greater amplitude and smaller latency than stimulation of the contralateral hindpaw, and ipsilateral VSD responses had a lower amplitude and greater latency than contralateral responses. (2) While the contralateral stimulation initially activated only one focus, the ipsilateral stimulation initially activated two foci: one focus was typically medial to the focus activated by contralateral stimulation and was stereotaxically localized in the motor cortex; the other focus was typically posterior to the focus activated by contralateral stimulation and was stereotaxically localized in the somatosensory cortex. (3) Forepaw and hindpaw somatosensory stimuli activated large areas of the sensorimotor cortex, well beyond the forepaw and hindpaw somatosensory areas of classical somatotopic maps, and forepaw stimuli activated larger cortical areas with greater activation velocity than hindpaw stimuli. (4) Stimulation of the forepaw and hindpaw evoked different cortical activation dynamics: forepaw responses displayed a clear medial directionality, whereas hindpaw responses were much more uniform in all directions. In conclusion, this work offers a complete spatio-temporal map of the supragranular VSD cortical activation in response to stimulation of the paws, showing important somatotopic differences between contralateral and ipsilateral maps as well as differences in the spatio-temporal activation dynamics in response to forepaw and hindpaw stimuli.

Optical imaging of the propagation patterns of neural responses in the rat sensory cortex: Comparison under two different anesthetic conditions

“…The longer latency of our optical signal is thus attributed to this experimental situation. Morales-Botello et al (2012) obtained hind-limbevoked cortical optical responses with a latency of 23 ms, much shorter than ours and in good agreement with that of the evoked potentials. While their recording procedures were similar to ours, one of the main differences was the choice of VSD; they used RH1691.…”

“…It differs among sensory cortices; 220 mm/s in the visual cortex, 380 mm/s in the auditory cortex (Song et al, 2006;Wanger et al, 2013) and much smaller values were reported in the somatosensory cortex. Morales-Botello et al (2012) reported spreading velocities of 80 mm/s and 120 mm/s for the hind-limb and fore-limb responses, respectively. Petersen et al (2003) reported spreading velocities of 60 mm/s and 33 mm/s in the barrel cortex (in the barrel cortex the velocity differs according to the direction of propagation; the former along the row and the latter along the arc).…”

Serotonergic pharmacotherapy promotes cortical reorganization after spinal cord injury

“…This expansion of the FLS cortex into the FLM cortex is not completely unexpected, because in physiological conditions cortical responses to forepaw stimuli propagate both toward the HLSM cortex and toward the FLM cortex, as recently measured with voltage-sensitive dye imaging in rats (Morales-Botello et al, PLoS ONE 2012). The reorganization was not limited to regions of FLM cortex adjacent to forelimb sensory cortex.…”

Increased cortical responses to forepaw stimuli immediately after peripheral deafferentation of hindpaw inputs

“…Similarly, hindpaw stimuli also evoked small responses in the forepaw cortex (Table 1). These observations are consistent with our previous results6710.…”

“…Interestingly, these increased responses to forepaw stimuli are not observed only in the hindpaw cortex, which is deafferented by the thoracic spinal cord injury, but also in the forepaw cortex, which would be classically considered as non-deafferented. However, in control animals the hindpaw cortex does display small responses to forepaw stimuli6910, suggesting that the loss of input from the hindpaw produces a partial deafferentation also of the forepaw cortex. If this interpretation is correct, an increment in the responses evoked in the forepaw cortex by forepaw stimuli should be observed not only after central hindpaw deafferentation due to spinal cord injury, as in our previous studies, but also immediately after peripheral hindpaw deafferentation due to nerve injury.…”