A new paradigm for the reversible blockage of whisker sensory transmission

Callosal inputs generate side-invariant receptive fields in the barrel cortex

Barrel cortex integrates contra- and ipsilateral whiskers' inputs. While contralateral inputs depend on the thalamocortical innervation, ipsilateral ones are thought to rely on callosal axons. These are more abundant in the barrel cortex region bordering with S2 and containing the row A-whiskers representation, the row lying nearest to the facial midline. Here, we ask what role this callosal axonal arrangement plays in ipsilateral tactile signaling. We found that novel object exploration with ipsilateral whiskers confines c-Fos expression within the highly callosal subregion. Targeting this area with in vivo patch-clamp recordings revealed neurons with uniquely strong ipsilateral responses dependent on the corpus callosum, as assessed by tetrodotoxin silencing and by optogenetic activation of the contralateral hemisphere. Still, in this area, stimulation of contra- or ipsilateral row A-whiskers evoked an indistinguishable response in some neurons, mostly located in layers 5/6, indicating their involvement in the midline representation of the whiskers' sensory space.

Multimodal learning of pheromone locations

Memorizing pheromonal locations is critical for many mammalian species as it involves finding mates and avoiding competitors. In rodents, pheromonal information is perceived by the main and accessory olfactory systems. However, the role of somatosensation in context-dependent learning and memorizing of pheromone locations remains unexplored. We addressed this problem by training female mice on a multimodal task to locate pheromones by sampling volatiles emanating from male urine through the orifices of varying dimensions or shapes that are sensed by their vibrissae. In this novel pheromone location assay, female mice’ preference toward male urine scent decayed over time when they were permitted to explore pheromones vs neutral stimuli, water. On training them for the associations involving olfactory and whisker systems, it was established that they were able to memorize the location of opposite sex pheromones, when tested 15 days later. This memory was not formed either when the somatosensory inputs through whisker pad were blocked or when the pheromonal cues were replaced with that of same sex. The association between olfactory and somatosensory systems was further confirmed by the enhanced expression of the activity-regulated cytoskeleton protein. Furthermore, the activation of main olfactory bulb circuitry by pheromone volatiles did not cause any modulation in learning and memorizing non-pheromonal volatiles. Our study thus provides the evidence for associations formed between different sensory modalities facilitating the long-term memory formation relevant to social and reproductive behaviors.

Whisker dependent responsiveness of C57BL/6J mice to different behavioral test paradigms

Whisker trimming is very common in C57BL/6J mice. Dewhiskering may lead to an alteration in the thalamocortical connectivity and relevant behavioral functions. Since C57BL/6J is a commonly used strain for neurobehavioral studies, it is important to examine how whisker dependent heterogeneity affects the internal validity of behavioral phenotypes. The present study aimed to investigate the responsiveness of mice to different behavioral test paradigms in the presence or absence of whiskers. We employed two models of whisker deprivation: Acute Whisker Desensitization (AWD) and Chronic Habitual Dewhiskering (CHD). The AWD model blocks whisker sensation by lidocaine application. For CHD model, mice at the age of 12 weeks were carefully scrutinized for presence or absence of whiskers and divided into three groups, the whiskered mice, partially dewhiskered mice and completely dewhiskered mice. The whisker-dependent behavioral functions were assessed using open field test, novel object recognition test, marble burying test and forced swim test. Our results showed that habitual dewhiskering significantly altered the short-term memory and basal anxiety-like functions. Such behavioral alteration due to dewhiskering was significantly different in fully and partially dewhiskered mice, which is indicative of behavioral adaptation to the whisker desensitization. Contrary to CHD, the Acute Whisker Desensitization ameliorated behavioral compulsivity and basal anxiety. Our results suggest that vibrissal desensitization in the mice may lead to changes in their affective and cognitive state. Since, heterogeneity in whisker status may affect behavioral functions, careful inspection of the whisker status of C57BL/6J mice is recommended to increase the reproducibility and reliability of results obtained from behavioral assessments.

Tactile modulation of hippocampal place fields

Neural correlates of spatial representation can be found in the activity of the hippocampal place cells. These neurons are characterized by firing whenever the animal is located in a particular area of the space, the place field. Place fields are modulated by sensory cues, such as visual, auditory, or olfactory cues, being the influence of visual inputs the most thoroughly studied. Tactile information gathered by the whiskers has a prominent representation in the rat cerebral cortex. However, the influence of whisker-detected tactile cues on place fields remains an open question. Here we studied place fields in an enriched tactile environment where the remaining sensory cues were occluded. First, place cells were recorded before and after blockade of tactile transmission by means of lidocaine applied on the whisker pad. Following tactile deprivation, the majority of place cells decreased their firing rate and their place fields expanded. We next rotated the tactile cues and 90% of place fields rotated with them. Our results demonstrate that tactile information is integrated into place cells at least in a tactile-enriched arena and when other sensory cues are not available.

Precision rodent whisker stimulator with integrated servo-locked control and displacement measurement

We developed a high speed voice coil based whisker stimulator that delivers precise deflections of a single whisker or group of whiskers in a repeatable manner. The device is miniature, quiet, and inexpensive to build. Multiple stimulators fit together for independent stimulation of four or more whiskers. The system can be used with animals under anesthesia as well as awake animals with head-restraint, and does not require trimming the whiskers. The system can deliver 1-2 mm deflections in 2 ms resulting in velocities up to 900 mm/s to attain a wide range of evoked responses. Since auditory artifacts can influence behavioral studies using whisker stimulation, we tested potential effects of auditory noise by recording somatosensory evoked potentials (SEP) with varying auditory click levels, and with/without 80 dBa background white noise. We found that auditory clicks as low as 40 dBa significantly influence the SEP. With background white noise, auditory clicks as low as 50 dBa were still detected in components of the SEP. For behavioral studies where animals must learn to respond to whisker stimulation, these sounds must be minimized. Together, the stimulator and data system can be used for psychometric vigilance tasks, mapping of the barrel cortex and other electrophysiological paradigms.