Organization,development, and effects of infraorbital nerve transection on galanin binding sites in the trigeminal brainstem complex

Previous experiments from this laboratory have indicated that transection of the infraorbital nerve (ION, the trigeminal \[V] branch that supplies the mystacial vibrissae follicles) at birth and in adulthood has markedly different effects on galanin immunoreactivity in the V brainstem complex. Adult nerve transection increases galanin immunoreactivity in the superficial layers of V subnucleus caudalis (SpC) only, while neonatal nerve transection results in increased galanin expression in vibrissae-related primary afferents throughout the V brainstem complex. The present study describes the distribution of binding sites for this peptide in the mature and developing V ganglion and brainstem complex and determines the effects of neonatal and adult ION damage and the associated changes in galanin levels upon their distribution and density. Galanin binding sites are densely distributed in all V brainstem subnuclei and are particularly dense in V subnucleus interpolaris and the superficial layers of SpC. They are present at birth (P-0) and their distribution is similar to that in adult animals. Transection of the ION in adulthood and examination of brainstem 7 days later indicated marked reductions in the density of galanin binding sites in the V brainstem complex. With the exception of the superficial laminae of SpC, the same reduction in density remained apparent in rats that survived 45 days after nerve cuts. Transection of the ION on P-0 resulted in no change in the density of galanin binding sites in the brainstem after either 7 or 60 days survival. These results indicate that densely distributed galanin binding sites are present in the V brainstem complex of both neonatal and adult rats, that they are located in regions not innervated by galanin-positive axons, and that their density is not significantly influenced by large lesion-induced changes in the primary afferent content of their natural ligand.     

A comparative analysis of vibrissa count and infraorbital foramen area in primates and other mammals

Vibrissae are specialized sensory “hairs” that respond to mechanical stimuli. Sensory information from vibrissae is transmitted to the brain via the infraorbital nerve, which passes through the infraorbital foramen (IOF). Several analyses have documented that primates have smaller IOFs than non-primate mammals, and that haplorhines have smaller IOFs than strepsirrhines. These grade shifts in IOF area were attributed to differences in “vibrissa development.” Following earlier analyses, IOF area has been used to derive a general estimate of “whiskeredness” in extinct primates, and consequently, IOF area has been used in phylogenetic and paleoecological interpretations. Yet, the relationship between IOF area and vibrissa count has not been tested, and little is known about how IOF area and vibrissa counts vary among mammals. This study explores how relative IOF area and vibrissa count differ among 25 mammalian orders, and tests for a correlation between IOF area and vibrissa count. Results indicate that primates and dermopterans (Primatomorpha) have smaller IOFs than most non-primate mammals, but they do not have fewer vibrissae. In addition, strepsirrhines and haplorhines do not differ from one another in relative IOF area or vibrissa counts. Despite different patterns documented for IOF area and vibrissa count variation across mammals, results from this study do confirm that vibrissa count and IOF area are significantly and positively correlated (p < 0.0001). However, there is considerable scatter in the data, suggesting that vibrissa counts cannot be predicted from IOF area. There are three implications of these finding. First, IOF area reflects all mechanoreceptors in the maxillary region, not just vibrissae. Second, IOF area may be an informative feature in interpretations of the fossil record. Third, paleoecological interpretations based on vibrissae are not recommended.     

Characterizing the functional significance of the neonatal rat vibrissae prior to the onset of whisking

The present series of experiments assessed how information from the whiskers controls and modulates infant rat behavior during early learning and attachment. Passive vibrissal stimulation can elicit behavioral activity in pups throughout the first two postnatal weeks, although orienting to the source of stimulation is evident only after ontogenetic emergence of whisking. In addition, while pups were capable of demonstrating learning in a classical conditioning paradigm pairing vibrissa stimulation with electric shock, no corresponding changes were detected in the anatomy of the barrel cortex as determined by cytochrome oxidase (CO) staining. Finally, the role of whiskers in a more naturalistic setting was determined in postnatal day (PN)3-5 and PN11-12 pups. Our results showed that both nipple attachment and huddling were disrupted in whisker-clipped PN3-5 pups but only marginally altered in PN11-12 pups. Together, these results suggest that the neonatal whisker system is behaviorally functional and relevant for normal mother-infant interactions, though it lacks the sophistication of a mature whisker system that evokes very specific and directed responses.     

Tactile size discrimination by a California sea lion (Zalophus californianus) using its mystacial vibrissae

The capability of a blindfolded California sea lion (Zalophus californianus) to discriminate diameter differences of circular discs by means of active touch with its mystacial vibrissae was studied. Using a forced choice paradigm the sea lion was required to choose the larger of two simultaneously presented perspex discs. Absolute difference thresholds (D) were determined for 3 standard discs (1.12 cm Ø, 2.52 cm Ø, 8.74 cm Ø) by the psychophysical method of constants. Increasing disc size resulted in an increase in the absolute difference threshold from 0.33 cm for the smallest disc size to 1.55 cm for the largest disc size. The relative difference threshold (Weber fraction) remained approximately constant at a mean value of 0.26. According to a video analysis the sea lion did not move its vibrissae when touching the discs. Instead, it performed precisely controlled lateral head movements, with the touched disc located centrally between the vibrissae of both sides of the muzzle. Since the extent of these head movements was identical at discs to be compared, discs of different size must have led to different degrees of deflection of vibrissae involved in the tactile process, resulting in quantitatively different mechanical stimulations of mechanoreceptors in the follicles. This suggests that the accuracy of the sea lion's size discrimination was determined by the efficiency of two sensory systems: the mechanosensitivity of follicle receptors as well as kinaesthesis.Abbreviations D starting stimulus size - D _i size of the interpolated comparison disc at 75% correct choices - D absolute difference threshold     

Basic fibroblast growth factor-like immunoreactivity in the rat trigeminal sensory system and peri-oral skin with vibrissae

We have characterized an antiserum against basic fibroblast growth factor (bFGF) by immunoblot, investigated the location of bFGF-like immunoreactivity (bFGF-IR) in the trigeminal sensory system and perioral skin endowed with vibrissae, and demonstrated the site of bFGF mRNA expression in the vibrissae by in situ hybridization histochemistry. Light-microscopic immunohistochemistry has demonstrated that bFGF-IR is present not only in trigeminal ganglion neurons and their central and peripheral processes, but also in cells of the matrix, external root sheath and papillae of vibrissae and the stratum basale of the stratified squamous epithelium of the skin. Electron microscopy has revealed intense bFGF-IR mainly in cytoplasmic regions, other than the lumen of rough endoplasmic reticulum and the Golgi apparatus, in trigeminal ganglion neurons, in fibroblast-like cells in the papillae, and in capsules of vibrissae. In contrast, actively proliferating and/or differentiating cells in the matrix of vibrissae have intensely stained euchromatin and weakly labeled cytoplasm that, unlike that of the aforementioned cells, contain immunoreaction products in discrete spots less than 100 nm in diameter, implying the generation of different molecular forms of bFGF in cells of the matrix and papillae. Moreover, the accumulation of bFGF in the euchromatin appears to take place in cells at non-mitotic stages (possibly interphases), characterized by a conspicuous nucleolus and well-developed nuclear envelope. A digoxigenin-labeled cRNA probe for the demonstration of bFGF mRNA gives conspicuous hybridization signals mainly in the matrix of vibrissae. These findings suggest that bFGF is involved in the growth and differentiation of matrix cells during certain periods of the cell cycle and that it acts as a non-mitogenic mediator in the adult trigeminal sensory system.     

Developmental onset of functional activity in the wallaby whisker cortex in response to stimulation of the infraorbital nerve

This study used the extrauterine development of a marsupial wallaby to investigate the onset of functional activity in the somatosensory pathway from the whiskers. In vivo recordings were made from the somatosensory cortex from postnatal day (P) 55 to P138, in response to electrical stimulation of the infraorbital nerve supplying the mystacial whiskers. Current source density analysis was used to localize the responses within the cortical depth. This was correlated with development of cortical lamination and the onset of whisker-related patches, as revealed by cytochrome oxidase. The earliest evoked activity occurred at P61, when layers 5 and 6 are present, but layer 4 has not yet developed. This activity showed no polarity reversal with depth, suggesting activity in thalamocortical afferents. By P72 synaptic responses were detected in developing layer 4 and cytochrome oxidase showed the first hint of segregation into whisker-related patches. These patches were clear by P86. The evoked response at this age showed synaptic activity first in layer 4 and then in deep layer 5/upper layer 6. With maturity, responses became longer lasting with a complex sequence of synaptic activity at different cortical depths. The onset of functional activity is coincident with development of layer 4 and the onset of whisker-related pattern formation. A similar coincidence is seen in the rat, despite the markedly different chronological timetable, suggesting similar developmental mechanisms may operate in both species.     

Object localization with whiskers

Rats use their large facial hairs (whiskers) to detect, localize and identify objects in their proximal three-dimensional (3D) space. Here, we focus on recent evidence of how object location is encoded in the neural sensory pathways of the rat whisker system. Behavioral and neuronal observations have recently converged to the point where object location in 3D appears to be encoded by an efficient orthogonal scheme supported by primary sensory-afferents: each primary-afferent can signal object location by a spatial (labeled-line) code for the vertical axis (along whisker arcs), a temporal code for the horizontal axis (along whisker rows), and an intensity code for the radial axis (from the face out). Neuronal evidence shows that (i) the identities of activated sensory neurons convey information about the vertical coordinate of an object, (ii) the timing of their firing, in relation to other reference signals, conveys information about the horizontal object coordinate, and (iii) the intensity of firing conveys information about the radial object coordinate. Such a triple-coding scheme allows for efficient multiplexing of 3D object location information in the activity of single neurons. Also, this scheme provides redundancy since the same information may be represented in the activity of many neurons. These features of orthogonal coding increase accuracy and reliability. We propose that the multiplexed information is conveyed in parallel to different readout circuits, each decoding a specific spatial variable. Such decoding reduces ambiguity, and simplifies the required decoding algorithms, since different readout circuits can be optimized for a particular variable.     

Slitrk6 expression profile in the mouse embryo and its relationship to that of Nlrr3

Slitrk6 is a member of the Slitrk family of proteins, which are integral membrane proteins possessing two leucine-rich repeat (LRR) domains and a carboxy-terminal domain partially similar to that in the trk neurotrophin receptor proteins. Here, I show that Slitrk6 is uniquely expressed in various organs, different from other Slitrk genes which are predominantly expressed in neural tissues. In the developing mouse embryo, Slitrk6 expression was detected in the otic cyst, lateral trunk epidermis and its underlying mesenchymal tissue, limb bud, maxillary process, pharyngeal arches, cochlea, retina, tongue, tooth primordium, central nervous system (CNS), and the visceral organ primordia including of the lung, gastrointestinal tract (particularly in the enteric neurons) and pancreas. The expression in these organs occurred in a spatially restricted manner. In the CNS, the expression was highly compartmentalized in the dorsal thalamus, cerebellum and medulla. The expression compartment in the thalamus in which Slitrk6 was expressed was closely related to the Gbx2-expressing prosomere 2. Interestingly, the Slitrk6 expression in the CNS, cochlea, tongue, tooth primordial, and other organs was partially complementary to the expression of Nlrr3, which belongs to another family of neuronal LRR-containing transmembrane proteins. The complementary expression of the two proteins in the dorsal thalamus persisted from E13.5 to the adult stage.