Serotonin and whisking

Rhythmic whisker movements, called "whisking," are produced by a brainstem central pattern generator (CPG) that uses serotonin to induce periodic firing in facial motorneurons. During active touch, motor cortex could regulate whisking frequency by controlling the rate of firing of the serotonergic neurons.     

One whisker whisking: unit recording during conditioned whisking in rats

Understanding of the functional neurobiology of the rodent whisker system would be advanced by neurobehavioral studies in awake, behaving animals that combine unit recording from structures at various levels of the system with quantitative characterization of the kinematics and temporal organization of whisking. Such studies require the solution of a number of methodological problems. These include: chronic recording procedures ensuring unit isolation, stability and maximum yield, monitoring and display of unit activity and whisker movements within the same (ms) timeframe and behavioral paradigms which bring whisking movement parameters under the control of the experimenter rather than the rat. Here we describe a head-fixed rodent preparation which makes possible chronic recording of unit activity in the awake, whisking rat, combined with real-time, high resolution monitoring of whisker and pad movements in two dimensions and under behavioral control. While the head-fixed "whisking" preparation has some inherent limitations, it may be used to address a number of important neurobehavioral problems. We suggest that it should contribute significantly to understanding the functional neurobiology of the whisker system.     

Serotonin regulates rhythmic whisking

Many rodents explore their environment by rhythmically palpating objects with their mystacial whiskers. These rhythmic whisker movements ("whisking"; 5-9 Hz) are thought to be regulated by an unknown brainstem central pattern generator (CPG). We tested the hypothesis that serotonin (5-HT) inputs to whisking facial motoneurons (wFMNs) are part of this CPG. In response to exogenous serotonin, wFMNs recorded in vitro fire rhythmically at whisking frequencies, and selective 5-HT2 or 5-HT3 receptor antagonists suppress this rhythmic firing. In vivo, stimulation of brainstem serotonergic raphe nuclei evokes whisker movements. Unilateral infusion of selective 5-HT2 or 5-HT3 receptor antagonists suppresses ipsilateral whisking and substantially alters the frequencies and symmetry of whisker movements. These findings suggest that serotonin is both necessary and sufficient to generate rhythmic whisker movements and that serotonergic premotoneurons are part of a whisking CPG.     

Sniffing behaviors in Mahale chimpanzees

Although it is difficult for observers to determine how non-human primates use olfaction in a natural environment, sniffing is one clue. In this study, the sniffing behaviors of wild chimpanzees were divided into six categories, and sex differences were found in most categories. Males sniffed more frequently than females in sexual and social situations, while females sniffed more often during feeding and self-checking. Chimpanzees sniffed more frequently during the dry season than during the wet season, presumably due to the low humidity. This suggests that the environment affects olfactory use by chimpanzees and that chimpanzees easily gather new information from the ground via sniffing.     

Sniffing and spatiotemporal coding in olfaction

The act of sniffing increases the air velocity and changes the duration of airflow in the nose. It is not yet clear how these changes interact with the intrinsic timing within the olfactory bulb, but this is a matter of current research activity. An action of sniffing in generating a high velocity that alters the sorption of odorants onto the lining of the nasal cavity is expected from the established work on odorant properties and sorption in the frog nose. Recent work indicates that the receptor properties in the olfactory epithelium and olfactory bulb are correlated with the receptor gene expression zones. The responses in both the epithelium and the olfactory bulb are predictable to a considerable extent by the hydrophobicity of odorants. Furthermore, receptor expression in both rodent and salamander nose interacts with the shapes of the nasal cavity to place the receptor sensitivity to odorants in optimal places according to the aerodynamic properties of the nose.     

Cortical barrel field ablation and unconditioned whisking kinematics

The effects of "barrel cortex" ablation upon the biometrics of "exploratory" whisking were examined in three head-fixed rats which had previously sustained unilateral ablation of the left cortical "barrel field" under electrophysiological control. Unconditioned movements of a pair of bilaterally homologous whiskers (C-1, Right, Left) were monitored, optoelectronically, with other whiskers present. Whisking movements on the intact and ablated side were analyzed with respect to kinematics (protraction amplitude and velocity) whisking frequency and phase relationships between whisking movement on the two sides of the face. Histological analysis confirmed complete removal of S-1 "barrel cortex". In normal animals whisking movements have a characteristic rhythm (6-9 Hz), and protractions on the two sides of the face tend to be both synchronous and of very similar amplitudes. In the lesioned animals, whisking frequency was unchanged and whisking movements remained bilaterally synchronous. However, there was a significant difference between the amplitude of Right and Left whisker movements which was evident many months postoperatively. Our results suggest that the deficits in vibrissa-mediated tactile discrimination reported after "barrel" field ablation may reflect an impairment in the animal's ability to modulate whisking parameters on the two sides of the face to meet the functional requirements of a discriminative whisking task. The effects upon whisking amplitude seen after unilateral barrel field ablation are consistent with a model in which the activity of a whisking Central Pattern Generator is modulated by descending inputs to achieve sensorimotor control of whisking movement parameters.     

Cortical barrel field ablation and unconditioned whisking kinematics

The effects of “barrel cortex” ablation upon the biometrics of “exploratory” whisking were examined in three head-fixed rats which had previously sustained unilateral ablation of the left cortical “barrel field” under electrophysiological control. Unconditioned movements of a pair of bilaterally homologous whiskers (C-1, Right, Left) were monitored, optoelectronically, with other whiskers present. Whisking movements on the intact and ablated side were analyzed with respect to kinematics (protraction amplitude and velocity) whisking frequency and phase relationships between whisking movement on the two sides of the face. Histological analysis confirmed complete removal of S-1 “barrel cortex”. In normal animals whisking movements have a characteristic rhythm (6-9 Hz), and protractions on the two sides of the face tend to be both synchronous and of very similar amplitudes. In the lesioned animals, whisking frequency was unchanged and whisking movements remained bilaterally synchronous. However, there was a significant difference between the amplitude of Right and Left whisker movements which was evident many months postoperatively. Our results suggest that the deficits in vibrissa-mediated tactile discrimination reported after “barrel” field ablation may reflect an impairment in the animal's ability to modulate whisking parameters on the two sides of the face to meet the functional requirements of a discriminative whisking task. The effects upon whisking amplitude seen after unilateral barrel field ablation are consistent with a model in which the activity of a whisking Central Pattern Generator is modulated by descending inputs to achieve sensorimotor control of whisking movement parameters.     

Parallel thalamic pathways for whisking and touch signals in the rat

In active sensation, sensory information is acquired via movements of sensory organs; rats move their whiskers repetitively to scan the environment, thus detecting, localizing, and identifying objects. Sensory information, in turn, affects future motor movements. How this motor-sensory-motor functional loop is implemented across anatomical loops of the whisker system is not yet known. While inducing artificial whisking in anesthetized rats, we recorded the activity of individual neurons from three thalamic nuclei of the whisker system, each belonging to a different major afferent pathway: paralemniscal, extralemniscal (a recently discovered pathway), or lemniscal. We found that different sensory signals related to active touch are conveyed separately via the thalamus by these three parallel afferent pathways. The paralemniscal pathway conveys sensor motion (whisking) signals, the extralemniscal conveys contact (touch) signals, and the lemniscal pathway conveys combined whisking–touch signals. This functional segregation of anatomical pathways raises the possibility that different sensory-motor processes, such as those related to motion control, object localization, and object identification, are implemented along different motor-sensory-motor loops.     

Sniffing out disease using the artificial nose

Is there information in the odours that we emit and that circulate around us? More importantly, can doctors gain knowledge of disease by smelling their patients? Machines that emulate the mammalian nose have picked up the scent of several diseases and may drastically change diagnostic procedure.     

Sniffing behavior of mice during performance in odor-guided tasks

Sniffing, a rhythmic inhalation and exhalation of air through the nose, is a behavior thought to play a critical role in shaping how odor information is represented and processed by the nervous system. Although the mouse has become a prominent model for studying olfaction, little is known about sniffing behavior in mice. Here, we characterized mouse sniffing behavior by measuring intranasal pressure transients in behaving mice. Sniffing was monitored during unstructured exploratory behavior and during performance of 3 commonly used olfactory paradigms: a habituation/dishabituation task, a sand digging-based discrimination task, and a nose poke-based discrimination task. We found that respiration frequencies in quiescent mice ranged from 3 to 5 Hz--higher than that reported for rats. During exploration, sniff frequency increased up to approximately 12 Hz and was highly dynamic, with rapid changes in frequency, amplitude, and waveform. Sniffing behavior varied strongly between tasks as well as for different behavioral epochs of each task. For example, mice performing the digging-based task showed little increase in sniff frequency prior to digging, whereas mice performing a nose poke-based task showed robust increases. Mice showed large increases in sniff frequency prior to reward delivery in all tasks. Mice also showed increases in sniff frequency when nose poking in a nonodor-guided task. These results show that mouse sniffing behavior is highly dynamic, varies with behavioral context, and is strongly modulated by olfactory as well as nonolfactory events.