Cortical reintegration after facial allotransplantation.

Neural plasticity of the brain or its ability to reorganize following injury has likely coincided with the successful clinical correction of severe deformity by facial transplantation since 2005. In this study, we present the cortical reintegration outcomes following syngeneic hemifacial vascularized composite allograft (VCA) in a small animal model. Specifically, changes in the topographic organization and unit response properties of the rodent whisker-barrel somatosensory system were assessed following hemifacial VCA. Clear differences emerged in the barrel-cortex system when comparing naïve and hemiface transplanted animals. Neurons in the somatosensory cortex of transplanted rats had decreased sensitivity albeit increased directional sensitivity compared with naïve rats and evoked responses in transplanted animals were more temporally dispersed. In addition, receptive fields were often topographically mismatched with the indication that the mismatched topography reorganized within adjacent barrel (same row-arc bias following hemifacial transplant). These results suggest subcortical changes in the thalamus and/or brainstem play a role in hemifacial transplantation cortical plasticity and demonstrate the discrete and robust data that can be derived from this clinically relevant small animal VCA model for use in optimizing postsurgical outcomes.NEW & NOTEWORTHY Robust rodent hemifacial transplant model was used to record functional changes in somatosensory cortex after transplantation. Neurons in the somatosensory cortex of face transplant recipients had decreased sensitivity to stimulation of whiskers with increased directional sensitivity vs. naive rats. Transplant recipient cortical unit response was more dispersed in temporary vs. naive rats. Despite histological similarities to naive cortices, transplant recipient cortices had a mix of topographically appropriate and inappropriate whiskered at barrel cortex relationships.

An investigational report into the causes of pine mouth events in US consumers.

Between July 2008 and June 2012, the US Food and Drug Administration received 501 consumer reports of prolonged taste disturbances consistent with pine mouth syndrome. Consumers consistently reported a delayed bitter or metallic taste beginning hours to days following consumption of pine nuts that recurred with intake of any food or meal. This dysgeusia lasted in some cases up to a few weeks, but would eventually resolve without serious health consequences. To evaluate these reports, a questionnaire was developed to address various characteristics of the pine nuts consumed, pertinent medical history of complainants and other dysgeusia-related factors. Pine nut samples associated with 15 complaints were collected for analysis. The investigation of reports found no clear evidence of an underlying medical cause or common trigger that could adequately explain the occurrence of dysgeusia in complainants. Rather, the results of our investigation suggest that the occurrence of "pine mouth syndrome" in US consumers is correlated with the consumption of the pine nut species Pinus armandii.

Consistency of angular tuning in the rat vibrissa system.

Each region along the rat mystacial vibrissa pathway contains neurons that respond preferentially to vibrissa deflections in a particular direction, a property called angular tuning. Angular tuning is normally defined using responses to deflections of the principal vibrissa, which evokes the largest response magnitude. However, neurons in most brain regions respond to multiple vibrissae and do not necessarily respond to different vibrissae with the same angular tuning. We tested the consistency of angular tuning across the receptive field in several stations along the vibrissa-to-cortex pathway: primary somatosensory (barrel) cortex, ventroposterior medial nucleus of the thalamus (VPM), second somatosensory cortex, and superior colliculus. We found that when averaged across the population, neurons in all of these regions have low (superior colliculus and second somatosensory cortex) or statistically insignificant (barrel cortex and VPM) angular tuning consistencies across vibrissae. Nevertheless, in each region there are a small number of neurons that display consistent angular tuning for at least some vibrissae. We discuss the relevance of these findings for the transformation of inputs along the vibrissa trigeminal pathway and for the detection of sensory cues by whisking animals.

Subthreshold receptive field properties distinguish different classes of corticothalamic neurons in the somatosensory system.

Most corticothalamic (CT) neurons in somatosensory cortex are silent in lightly anesthetized and even awake animals, making it difficult to investigate CT function and the underlying circuitry. Here we use juxtasomal recording and stimulation techniques to probe subthreshold response properties of antidromically identified CT neurons in the rat whisker/barrel system. When neuronal firing is facilitated by depolarizing juxtasomal currents, silent neurons become responsive to whisker stimuli, permitting identification of three functional classes of CT cells: those having a short-latency excitatory response to whisker deflection, those having a long-latency response, and neurons whose firing is suppressed by whisker deflection. During sensorimotor behaviors when the CT system may be active, cells having excitatory vs inhibitory receptive fields may participate in push-pull corticothalamic circuits that, acting together, selectively enhance sensory signaling in the thalamocortical system.

Response properties of mouse trigeminal ganglion neurons.

We used controlled whisker deflections to examine the response properties of 208 primary afferent neurons in the trigeminal ganglion of adult mice. Proportions of rapidly adapting (RA, 47%) and slowly adapting (SA, 53%) neurons were equivalent, and most cells had low or no spontaneous activity. We quantified angular tuning and sensitivity to deflection amplitude and velocity. Both RA and SA units fired more frequently to larger deflections and faster deflections, but RA units were more sensitive to differences in velocity whereas SA units were more sensitive to deflection amplitudes. Almost all neurons were tuned for deflection angle, and the average response to the maximally effective direction was more than fourfold greater than the average response in the opposite direction; SA units were more tuned than RA units. Responses of primary afferent whisker-responsive neurons are qualitatively similar to those of the rat. However, average firing rates of both RA and SA neurons in the mouse are less sensitive to differences in deflection velocity, and RA units, unlike those in the rat, display amplitude sensitivity. Subtle observed differences between mice and rats may reflect greater mechanical compliance in mice of the whisker hairs and of the tissue in which they are embedded.

Whisker primary afferents encode temporal frequency of moving gratings.

To investigate the encoding of behaviorally relevant stimuli in the rodent whisker-somatosensory system, we recorded responses to moving gratings from trigeminal ganglion neurons. This allowed us to quantify how spike patterns in these neurons encode behaviorally distinguishable tactile stimuli presented with the variability inherent in a freely moving whisker paradigm. Our stimulus set consisted of three grating plates with raised bars of the same thickness (275 microm) having different spatial periods (1.0, 1.1, and 1.5 mm) swept rostro-caudally past the whiskers at velocities ranging from 50 to 330 mm/s. This resulted in 20 presentations each of nine different temporal frequencies (ranging from 50 to 220 Hz) for every grating plate. We found that despite the additional degrees of freedom introduced in this freely moving whisker paradigm, firing patterns from the majority (83%) of trigeminal ganglion neurons were statistically distinguishable, and corresponded to the temporal frequency of stimulation. The range of velocities (100-160 mm/s) that resulted in the most accurate and least variable representation of stimulus temporal frequency by trigeminal firing patterns closely corresponds to the whisking velocities employed by trained rats performing similar discrimination tasks. This suggests that, during naturally occurring whisking, individual primary afferents faithfully encode temporal frequency evoked by whisker contacts.

Anticipatory activity of motor cortex in relation to rhythmic whisking.

Rats characteristically generate stereotyped exploratory (5-12 Hz) whisker movements, which can also be adaptively modulated. Here we tested the hypothesis that the vibrissal representation in motor cortex (vMCx) initiates and modulates whisking by acting on a subcortical whisking central pattern generator (CPG). We recorded local field potentials (LFPs) in vMCx of behaving Sprague-Dawley rats while monitoring whisking behavior through mystacial electromyograms (EMGs). Recordings were made during free exploration, under body restraint, or in a head-fixed animal. LFP activity increased significantly prior to the onset of a whisking epoch and ended prior to the epoch's termination. In addition, shifts in whisking kinematics within a whisk epoch were often reflected in changes in LFP activity. These data support the hypothesis that vMCx may initiate and modulate whisking behavior through its action on a subcortical CPG.

The role of thalamic inputs in surround receptive fields of barrel neurons.

Controversy exists regarding the relative roles of thalamic versus intracortical inputs in shaping the response properties of cortical neurons. In the whisker-barrel system, this controversy centers on the mechanisms determining the receptive fields of layer IV (barrel) neurons. Whereas principal whisker-evoked responses are determined by thalamic inputs, the mechanisms responsible for adjacent whisker (AW) responses are in dispute. Here, we took advantage of the fact that lesions of the spinal trigeminal nucleus interpolaris (SpVi) significantly reduce the receptive field size of neurons in the ventroposterior thalamus. We reasoned that if AW responses are established by these thalamic inputs, brainstem lesions would significantly reduce the receptive field sizes of barrel neurons. We obtained extracellular single unit recordings from barrel neurons in response to whisker deflections from control rats and from rats that sustained SpVi lesions. After SpVi lesions, the receptive field of both excitatory and inhibitory barrel neurons decreased significantly in size, whereas offset/onset response ratios increased. Response magnitude decreased only for inhibitory neurons. All of these findings are consistent with the hypothesis that AW responses are determined primarily by direct thalamic inputs and not by intracortical interactions.

Response properties of whisker-related neurons in rat second somatosensory cortex.

In addition to a primary somatosensory cortex (SI), the cerebral cortex of all mammals contains a second somatosensory area (SII); however, the functions of SII are largely unknown. Our aim was to explore the functions of SII by comparing response properties of whisker-related neurons in this area with their counterparts in the SI. We obtained extracellular unit recordings from narcotized rats, in response to whisker deflections evoked by a piezoelectric device, and compared response properties of SI barrel (layer IV) neurons with those of SII (layers II to VI) neurons. Neurons in both cortical areas have similar response latencies and spontaneous activity levels. However, SI and SII neurons differ in several significant properties. The receptive fields of SII neurons are at least five times as large as those of barrel neurons, and they respond equally strongly to several principal whiskers. The response magnitude of SII neurons is significantly smaller than that of neurons in SI, and SII neurons are more selective for the angle of whisker deflection. Furthermore, whereas in SI fast-spiking (inhibitory) and regular-spiking (excitatory) units have different spontaneous and evoked activity levels and differ in their responses to stimulus onset and offset, SII neurons do not show significant differences in these properties. The response properties of SII neurons suggest that they are driven by thalamic inputs that are part of the paralemniscal system. Thus whisker-related inputs are processed in parallel by a lemniscal system involving SI and a paralemniscal system that processes complimentary aspects of somatosensation.