A quantitative comparison of stimulus-response relationships of vibrissa-activated neurons in subnuclei oralis and interpolaris of the rat's trigeminal sensory complex: receptive field properties and threshold distributions

Interactions of Whisking and Touch Signals in the Rat Brainstem

Perception is an active process, requiring the integration of both proprioceptive and exteroceptive information. In the rat's vibrissal system, a classical model for active sensing, the relative contribution of the two information streams was previously studied at the peripheral, thalamic, and cortical levels. Contributions of brainstem neurons were only indirectly inferred for some trigeminal nuclei according to their thalamic projections. The current work addressed this knowledge gap by performing the first comparative study of the encoding of proprioceptive whisking and exteroceptive touch signals in the oralis (SpVo), interpolaris (SpVi), and paratrigeminal (Pa5) brainstem nuclei. We used artificial whisking in anesthetized male rats, which allows a systematic analysis of the relative contribution of the proprioceptive and exteroceptive information streams along the ascending pathways in the absence of motor or cognitive top-down modulations. We found that (1) neurons in the rostral and caudal parts of the SpVi convey whisking and touch information, respectively, as predicted by their thalamic projections; (2) neurons in the SpVo encode both whisking and (primarily) touch information; and (3) neurons of the Pa5 encode a complex combination of whisking and touch information. In particular, the Pa5 contains a relatively large fraction of neurons that are inhibited by active touch, a response observed so far only in the thalamus. Overall, our systematic characterization of afferent responses to active touch in the trigeminal brainstem approves the hypothesized functions of SpVi neurons and presents evidence that SpVo and Pa5 neurons are involved in the processing of active vibrissal touch.SIGNIFICANCE STATEMENT The present work constitutes the first comparative study of the encoding of proprioceptive (whisking) and exteroceptive (touch) information in the rat's brainstem trigeminal nuclei, the first stage of vibrissal processing in the CNS. It shows that (1) as expected, the rostral and caudal interpolaris neurons convey primarily whisking and touch information, respectively; (2) the oralis nucleus, whose function was previously unknown, encodes both whisking and (primarily) touch touch information; (3) a subtractive computation, reported at the thalamic level, already occurs at the brainstem level; and (4) a novel afferent pathway probably ascends via the paratrigeminal nucleus, encoding both proprioceptive and exteroceptive information.

Comparative study of extracellular recording methods for analysis of afferent sensory information: Empirical modeling, data analysis and interpretation

BACKGROUND

Physiological studies of sensorial systems often require the acquisition and processing of data extracted from their multiple components to evaluate how the neural information changes in relation to the environment changes. In this work, a comparative study about methodological aspects of two electrophysiological approaches is described.

NEW METHOD

Extracellular recordings from deep vibrissal nerves were obtained by using a customized microelectrode Utah array during passive mechanical stimulation of rat´s whiskers. These recordings were compared with those obtained with bipolar electrodes. We also propose here a simplified empirical model of the electrophysiological activity obtained from a bundle of myelinated nerve fibers.

RESULTS

The peripheral activity of the vibrissal system was characterized through the temporal and spectral features obtained with both recording methods. The empirical model not only allows the correlation between anatomical structures and functional features, but also allows to predict changes in the CAPs morphology when the arrangement and the geometry of the electrodes changes.

COMPARISON WITH EXISTING METHOD(S)

This study compares two extracellular recording methods based on analysis techniques, empirical modeling and data processing of vibrissal sensory information.

CONCLUSIONS

This comparative study reveals a close relationship between the electrophysiological techniques and the processing methods necessary to extract sensory information. This relationship is the result of maximizing the extraction of information from recordings of sensory activity.

Principalis, oralis and interpolaris responses to whisker movements provoked by air jets in rats

The present study was designed to compare the electrophysiological characteristics of the principalis, oralis and interpolaris nuclei of the trigeminal sensory complex under stimulation of the vibrissae by air puffs. This stimulus generates deflection profiles resembling those induced by contact with real objects in natural conditions. Three populations of neurons were identified in each nucleus according to their mean spiking frequency at rest. The three nuclei differed in terms of their mean spiking frequencies, the response latencies of their neurons and the proportions of each neuron population observed in single and multi-unit recordings. Findings suggest different information processing tasks for each nucleus.

Projections from the spinal trigeminal nucleus to the cochlear nucleus in the rat

The integration of information across sensory modalities enables sound to be processed in the context of position, movement, and object identity. Inputs to the granule cell domain (GCD) of the cochlear nucleus have been shown to arise from somatosensory brain stem structures, but the nature of the projection from the spinal trigeminal nucleus is unknown. In the present study, we labeled spinal trigeminal neurons projecting to the cochlear nucleus using the retrograde tracer, Fast Blue, and mapped their distribution. In a second set of experiments, we injected the anterograde tracer biotinylated dextran amine into the spinal trigeminal nucleus and studied the resulting anterograde projections with light and electron microscopy. Spinal trigeminal neurons were distributed primarily in pars caudalis and interpolaris and provided inputs to the cochlear nucleus. Their axons gave rise to small (1-3 microm in diameter) en passant swellings and terminal boutons in the GCD and deep layers of the dorsal cochlear nucleus. Less frequently, larger (3-15 microm in diameter) lobulated endings known as mossy fibers were distributed within the GCD. Ventrally placed injections had an additional projection into the anteroventral cochlear nucleus, whereas dorsally placed injections had an additional projection into the posteroventral cochlear nucleus. All endings were filled with round synaptic vesicles and formed asymmetric specializations with postsynaptic targets, implying that they are excitatory in nature. The postsynaptic targets of these terminals included dendrites of granule cells. These projections provide a structural substrate for somatosensory information to influence auditory processing at the earliest level of the central auditory pathways.

2-DG uptake patterns related to single vibrissae during exploratory behaviors in the hamster trigeminal system

Stimulation of one or several whiskers activates discrete foci throughout the trigeminal (V) neuraxis. These foci contribute to patterns, corresponding to the patterns of vibrissae, that have been directly related to aggregates of cells and axon terminals in the "barrel" cortex. Here, we combine high-resolution, 2-deoxyglucose (2DG) mapping and cytochrome oxidase (CO) staining to determine whether the known pattern of V primary afferent projections is sufficient to deduce the functional activation of their targets during exploratory behavior. Four adult hamsters had all of their large mystacial vibrissae trimmed acutely, except for C3 on the left, and B2 and D4 on the right; in two others, the left C3 and right A1 and E4 whiskers were spared. After fasting overnight, 2DG was injected and the animals behaved freely in the dark for 45 minutes. The brainstem, thalamus, and cortices were sectioned, then processed for both CO staining and 2DG autoradiography. Image-processing microscopy was used to separate the autoradiographic silver grains from the histochemical staining. CO patches were patterned in a whisker-like fashion in the full rostrocaudal extent of V nucleus principalis and in caudal portions of spinal V subnuclei interpolaris and caudalis, but absent in subnucleus oralis. 2DG silver grains were densest above those CO patches in the pattern corresponding to the active whiskers. There were no consistent 2DG foci in subnuclei oralis or rostral caudalis. In these same cases, prominent 2DG labeling was restricted to the appropriate barrels in the contralateral cortex. Only one case, however, displayed a clear and appropriate region of heightened 2DG uptake in contralateral ventroposteromedial thalamus (VPM) and the adjacent part of the reticular thalamic nucleus. Patterns of increased glucose utilization with single whisker stimulation are well matched to the CO patterns that mirror distributions of neurons associated with a vibrissa in the V brainstem complex, thalamus, and cortex. Single whiskers are represented by relatively homogeneous longitudinal columns of 2DG labeling in the V brainstem nuclei. The columns are not continuous through the axial extent of the V brainstem complex; rather, they occur separately within principalis, interpolaris, and caudalis. While whisker columns were consistently labeled in interpolaris and caudalis in all animals, the labeling was increasingly variable in principalis, barrel cortex, and VPM, respectively. This suggests that the behaving animal can and does significantly modulate activity in this major, synaptically secure pathway.

Barrelettes--architectonic vibrissal representations in the brainstem trigeminal complex of the mouse. II. Normal post-natal development

Vibrissal representations in the brainstem trigeminal complex (BTC) of rodents are manifested as architectural sub-units called barrelettes. The development of barrelettes was studied by using Nissl staining, cytochrome oxidase histochemistry, and Golgi-impregnation methods. On the day of birth (PND-1), barrelettes are manifested as longitudinal, histochemical cylinders in sub-nuclei principalis, interpolaris and caudalis of the BTC. One day later (PND-2), fully formed histochemical barrelette formations are seen in the three sub-nuclei. The development of cytoarchitectural barrelettes lags behind histochemical barrelettes by about two days. Between PND-2 and PND-3, longitudinal cytoarchitectonic cylinders begin to appear. By PND-3, BTC neurons segregate into five rows of barrelettes in the coronal plane. Segmentation of rows into individual barrelettes begins on PND-4, and complete cytoarchitectonic barrelette formations are seen by PND-5. Golgi-impregnation shows that on the day of birth, primary afferent terminals and dendritic arbors of second-order trigeminal neurons within the BTC are short and poorly ramified. Over the next five post-natal days, lengthening of these processes as well as elaboration into secondary and tertiary branches take place. Growth of these processes continues for two additional weeks, contributing to the increase in barrelette neuropils (hollows). As the neuropils expand, neuronal somata are pushed toward barrelette sides. Morphometric measurements show that there is a relatively constant rate of growth of barrelettes over the first three post-natal weeks. The growth rate of the barrelette formations is identical to that of BTC as a whole. Thus, at the time of birth, the volume of neural tissue in the brainstem allotted to vibrissae is fixed relative to that allotted to other sensory receptors. Several features of the early development of barrelettes are identified: (1) Chemoarchitectural barrelettes appear before cytoarchitectural barrelettes, suggesting that terminal arbors of primary trigeminal afferents are organized before their target neurons form barrelettes. (2) Early cytoarchitecture is manifested in the form of unsegmented rows, suggesting that rough, row-based topological maps are first formed, which are then fine-tuned into individual sub-units. Recent evidence shows that other vibrissal representations--thalamic barreloids and cortical barrels--also follow these "afferent-before-target" and "row-before-individual units" sequences of development. This gradual, afferent-dependent fine-tuning of topological organization is analogous to similar events during the early development of the visual system, and may be a general feature of developing sensory systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Parvalbumin and calbindin immunocytochemistry reveal functionally distinct cell groups and vibrissa-related patterns in the trigeminal brainstem complex of the adult rat

Immunocytochemistry for calbindin (CA) and parvalbumin (PA) was combined with retrograde tracing from the thalamus, superior colliculus (SC), and cerebellum to define the ascending projections of neurons in the rat's trigeminal (V) brainstem complex that express immunoreactivity for these calcium binding proteins. Many PA-immunoreactive neurons were observed in trigeminal nucleus principalis (PrV). Many of these cells projected to thalamus and a few sent axons to SC. In ventral PrV, PA-immunoreactive neurons were arranged in a vibrissa-related pattern. A very small number of large CA-immunoreactive neurons were observed in dorsomedial PrV. None of these cells were labeled by our tracer deposits. Small neurons in V subnucleus oralis (SpO) were also immunoreactive for PA, but none were retrogradely labeled. A small percentage of the large neurons in SpO were CA-immunoreactive; many of these were retrogradely labeled by tracer injections in the thalamus and/or SC. In V subnucleus interpolaris (SpI), many small to medium sized cells were PA-positive and they were arrayed in a vibrissae-like pattern. None of these neurons were retrogradely labeled from any of the above-listed targets, but many were retrogradely labeled by tracer injections into ipsilateral PrV. SpI also contained many large CA-immunoreactive cells. Many of these projected to the thalamus and/or SC and some were also retrogradely labeled by tracer injections into ipsilateral PrV. In V subnucleus caudalis (SpC), very dark PA-immunoreactive neurons were located in the inner part of lamina II and less often in laminae I. Lightly labeled cells were located in the magnocellular laminae and formed vibrissa-related aggregates. None of these neurons were retrogradely labeled by our tracer injections. CA-immunoreactive cells were located throughout the depth of lamina II in SpC and smaller numbers were also visible in lamina I and layers III-V. A small percentage of the CA-positive cells in lamina I and in the magnocellular layers were retrogradely labeled from the thalamus. These data indicate that PA and CA antisera identify two cell populations in whisker-related regions of the V brainstem complex and that PA cells are somatotopically patterned in PrV, SpI, and SpC. These markers also distinguish two cell groups in superficial laminae of the medullary dorsal horn.

The barrelettes--architectonic vibrissal representations in the brainstem trigeminal complex of the mouse. I. Normal structural organization

The organization of the brainstem trigeminal complex (BTC) of the mouse is described, with emphasis on the normal organization of the vibrissal representations. Thionin staining for Nissal substance was employed to reveal the cytoarchitecture. Cytochrome oxidase histochemistry was used to reveal the chemoarchitecture. Golgi impregnation methods, in combination with thionin staining, were used to examine the neuronal dendritic morphology within a defined cytoarchitectonic context. An in vitro horseradish peroxidase labelling method was used to study the distribution and morphology of primary trigeminal afferent terminals within the BTC. The BTC consists of four distinct subnuclei: principalis (nVp), oralis (nVo), interpolaris (nVi), and caudalis (nVc). The present study shows that these sub-nuclei can be distinguished from each other on the basis of several anatomical criteria, including the distribution and density of neuronal size classes, histochemical staining intensity, morphology and orientation of neuronal dendrites, and size and texture of primary afferent terminal arbors. Anatomical manifestation of vibrissal representations within the BTC can be described in nVp, nVi, and nVc, but not in nVo. Within the three subnuclei where they are found, anatomical vibrissal representations are composed to architectural subunits that form an overall pattern homeomorphic to the pattern of vibrissae on the face of the animal. Each sub-unit forms a cylindrical tube running in a rostrocaudal orientation within the BTC. These sub-units will be called barrelettes. Cytologically, each barrelette consists of cell-dense "sides," surrounding a practically cell-free "hollow." Individual sub-units are separated by narrow, cell-free "septa." Histochemically, each subunit is manifested as a discrete patch of positive-staining reaction products. Differential interference contrast optics shows that these patches correspond precisely to the barrelette hollows. Evidence is presented to show that the barrelettes are the functional units for the processing of vibrissal sensory information. Terminal arborizations of individual primary afferents seem to be confined to the hollow of single barrelettes. The majority of neurons that form the sides of a barrelette have bitufted dendritic arbors, which project predominantly into the barrelette hollow, although a minority of neurons, particularly in nVi and nVc, also extend part of their dendritic arbors into adjacent barrelette hollows. The barrelette hollows are thus the principal neuropil region in which primary afferents and their target neurons interact. Contacts are made mainly between en passant varicosities and terminal boutons on primary afferent collaterals and dendritic spines and shafts of second order neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Chronic functional consequences of adult infraorbital nerve transection for rat trigeminal subnucleus interpolaris

In adult rats, transection of the infraorbital nerve and subsequent regeneration have been shown to result in altered somatotopic organization and changes in response properties of primary afferents within the trigeminal ganglion. The present study examined how these changes affect the postsynaptic targets of these neurons within subnucleus interpolaris of the trigeminal brainstem. Extracellular recordings were made from 330 cells in normal rats and 424 cells in rats surviving 57-290 days after transection of the infraorbital nerve in adulthood. Adult infraorbital nerve transection resulted in significant functional reorganization within subnucleus interpolaris. Relative to normal rats, the major changes can be summarized as follows: (1) a decrease in the dorsoventral extent of infraorbital representation; (2) a disruption of inter- and intradivisional somatotopic organization; (3) an increase in the proportion of cells with no discernible receptive field; (4) an increase in receptive field size for cells with infraorbital receptive field components; (5) the appearance of a significant proportion of cells with discontinuous receptive fields; (6) an increase in the proportion of cells exhibiting interdivisional convergence; (7) significant changes in the types of receptor surfaces activating local-circuit neurons with infraorbital receptive field components; (8) the appearance of a significant proportion of cells exhibiting convergence of different receptor surfaces; (9) significant changes in the dynamic response characteristics of cells with infraorbital receptive field components; and (10) an increase in the proportion of spontaneously active infraorbital-responsive cells. The changes observed were quite similar to those reported in adult subnucleus interpolaris following neonatal infraorbital nerve transection. The majority of changes observed in both studies can be most parsimoniously explained by alterations of primary afferents. However, central mechanisms may be more likely substrates for others. Regardless of the mechanism, the mature rodent trigeminal system appears capable of considerable functional reorganization following peripheral nerve damage.

Cell structure and response properties in the trigeminal subnucleus oralis

Extra- and intracellular recording, electrical stimulation, receptive field mapping, and horseradish peroxidase injection techniques were used to study the structure of functionally identified neurons in trigeminal (V) brainstem subnucleus oralis of the rat. Of 15 heavily labeled cells located within oralis, 4 were local-circuit neurons with receptive fields restricted to either an incisor, guard hairs, one vibrissa, or deep facial tissue (nociceptors). Their morphologies were highly varied, with expansive and spiny dendritic trees and recurrent and intersubnuclear axon collaterals. Oralis local-circuit neurons therefore most closely resembled non-vibrissa-sensitive local-circuit cells in adjacent subnucleus interpolaris. Six other stained cells projected to contralateral thalamus, and two others projected to ipsilateral cerebellum. They typically had intramodality convergent receptive fields (i.e., spanning more than one receptor organ, such as multiple vibrissae or teeth) with widespread dendritic trees, and were therefore indistinguishable from similarly projecting cells in interpolaris. Two other cells projected to the ipsilateral spinal cord, as well as other V brainstem subnuclei. One of these responded to high-threshold mechanical stimulation of teeth; the other was discharged by deflection of one mystacial vibrissa. Their dendrites were very widespread and ended in spiny and bulbous appendages. Local axon collaterals were also extensive. The remaining oralis cell had two axons, one projecting to the thalamus, the other to the spinal cord. Its receptive field expressed convergence from multiple receptor organs, including vibrissae, guard hairs, and skin. Its somadendritic morphology was similar to that of oralis cells projecting only to thalamus. We conclude that, with some exceptions, the extensive dendritic trees, axon branching, convergence, and functional diversity of oralis cells approximate those described previously for functionally equivalent neurons in interpolaris (Jacquin et al., 1989a,b). Such anatomical and physiological properties are rarely seen, however, in nucleus principalis (Jacquin et al., 1988a). The structure and function of three atypical principalis cells with structural and functional characteristics typical of oralis cells are also described. It is argued that such cells are rostrally displaced oralis cells.