Search

The receptive fields (RFs) of polymodal nociceptor units of canine testis consist of small fascicles of branching axons ending as clusters within the thin vascular layer overlying the seminiferous tubules. This propitious arrangement enabled serial reconstruction of electron micrographs of a flat, punctate zone, identified during recording of impulse discharge of a single physiologically characterized nociceptor "unit." The RFs showed multiple axons, with some branching and sequential vesicle-containing swellings, similar to what have been called terminals and assumed to constitute impulse generating sites in other tissues. Consistent with this interpretation is the local presence of: (1) clusters of small mitochondria, (2) glycogen granules, and (3) sectors of axolemma denuded of Schwann cell processes and thus "free" endings directly contacting the epithelial basal lamina. The most distal sectors showed additional morphologic properties: (1) zones of vesicles embedded in an extensive granular matrix, (2) preterminal arrays of "axonal reticulum" derived from the smooth endoplasmic reticulum displacing axonal cytoskeletal elements, (3) a variety of sizes and electron lucency in clear, spherical vesicles and a few granular or dense-core vesicles, and (4) specialization in the last Schwann cell of the fascicle. These latter features may reflect differences in the specialized receptor mechanisms of nociceptors that are difficult to detect without extensive serial electron microscopic analysis. Alternatively, these features may constitute a regional specialization of the testis. The term free nerve ending is perhaps an insufficient and inaccurate descriptor of the morphology of nociceptors. These findings are considered in the context of their possible relation to the sensitized vanilloid receptor mechanism unique to nociceptors., (Copyright 2003 Wiley-Liss, Inc.)

Regional distribution of gene expression of the axonal growth-associated protein, GAP-43, was studied in adult rat brains by in situ hybridization autoradiography to determine the features of mature neuronal populations that synthesize GAP-43 protein. Such synthesis appears to correlate with axonal growth during maturation and regrowth after axotomy. In most adult neurons, the sharp decline in GAP-43 gene expression implies a reduced capacity for axonal growth. Neurons capable of extending axonal knobs in the absence of injury may indicate a "plasticity" underlying dynamic processes of interaction between neurons and their synaptic targets. Antisense and sense (control) riboprobes were used on serial sections in the three principal axes, and the magnitude of hybridization signal was examined to determine regional patterns. GAP-43 mRNA levels are pronounced in diverse neuronal groups including the locus coeruleus, raphé nn., dopaminergic nigral and ventral tegmental nn., mitral cells, hippocampal CA3, inferior olivary n., vagal motor n. and other parasympathetic preganglionic neurons, select thalamic midline and intralaminar nn., several specific nn. of the hypothalamus and basal forebrain, the granular layer of cerebellar cortex, the infragranular neocortex, and the granular olfactory paleocortex; there is a substantial range in the magnitude of expression. Regions revealing minimal signal include most thalamic sensory relay nuclei, the granule neurons of the olfactory bulb and dentate gyrus, and the caudate and putamen. Possible concomitants of GAP-43 expression include regulation of ion flux and neurotransmitter release. Those neurons with long, extensively dispersed and numerous synaptic connections display the strongest signals and may possess the greatest propensity for continuous growth and turnover of their axon terminals, in contrast to short-axon and specific projection neurons exhibiting minimal levels. These data may enable inferring which populations display normal or experimentally induced axonal growth.

Primary gustatory neurons and their peripheral and central processes were evaluated histochemically in the geniculate and petrosal cranial nerve ganglia, lingual fungiform taste buds, and the nucleus of the solitary tract (NST) using 1) the plant lectin Griffonia simplicifolia I-B4, which binds specifically to D-galactose residues and selectively labels primarily nonpeptide-containing peripheral somatosensory neurons, and 2) calcitonin gene-related peptide immunoreactivity (CGRP-IR), which labels most peptidergic somatosensory neurons. Lectin reactivity was expressed by the vast majority of geniculate and petrosal ganglion cells, while CGRP-IR labeled very few cells. Peripherally, gustatory intragemmal axons penetrating fungiform taste buds were labeled only by the lectin and were depleted following chorda tympani transection. However, both lectin-labeled and CGRP-IR subpopulations of somatosensory perigemmal axons surrounding the taste buds were observed and were eliminated by section of the lingual nerve. The differing brainstem projection patterns of lectin-reactive vs. CGRP-IR central axons reflected their distinct ganglionic origins and the differential distributions of lectin reactivity and CGRP-IR among taste buds. Central lectin-reactive terminals were found throughout the entire rostrocaudal extent of the NST, including its rostral lateral "gustatory" zone; the extensive lectin-reactive visceral afferent projection can be presumed to have originated mainly from the large proportion of lectin-labeled neurons in the nodose ganglion. The lectin also prominently and selectively labeled the area postrema. CGRP-IR central terminals, however, was relatively sparse and restricted primarily to the caudal and medial "visceral" divisions of the NST. The results are discussed with respect to the possible functional implications of cell surface glycoconjugate expression by gustatory axons innervating taste bud receptor cells of the tongue.

The distribution of calcitonin-gene-related peptide-like immunoreactivity (CGRP-IR) was studied in sections of decalcified rat head and selected whole-mount preparations in order to address the complex peptidergic innervation patterns in peripheral cephalic specialized zones and to examine neuronal ganglia in situ. Labeled neuron somata in trigeminal, glossopharyngeal, and vagal ganglia comprised a large proportion of small to medium size type B ganglion cells. Parasympathetic ganglia (ciliary, otic, sphenopalatine, submandibular) revealed a small population of labeled somata and numerous perisomatic IR axons, whereas sympathetic ganglion cells (superior cervical) were devoid of label though richly innervated by perisomatic IR axons. The gustatory geniculate ganglion contained only a few labeled neurons and axons. Coarse peripheral CGRP-IR axons were traced to skeletal muscle motor end plates (e.g., lingual, tensor tympani, etc.), and thin sensory axons most densely innervated the cornea, iris, general integument, all mucosal epithelia lining the tympanic, nasal, sinus and oropharyngeal cavities, and the cerebral meninges. Blood vessels, glands, ducts, and their orifices were often heavily innervated, and specific specializations and exceptions are discussed. Distinctive patterns of IR innervation characterized the various specialized sensory systems, including 1) cochlear and vestibular hair cells; 2) lingual, palatal, oropharyngeal, and laryngoepiglottal taste buds; 3) main olfactory epithelium and axons projecting to glomeruli in specific sectors of main olfactory bulb; 4) septal-olfactory organ; 5) vomeronasal organ; and 6) the nervus terminalis system. Secretory epithelia (ciliary body, choroid plexus, and stria vascularis) were notably lacking in CGRP-IR. Despite the multiplicity of functionally distinct CGRP neuronal and axonal populations, certain generalizations merit consideration. The extensive innervation of chemosensory nasal and oral epithelia may contribute to specific chemical sensitivities (e.g., relating to olfactory and gustatory senses) as well as evoking "nociceptive" responses to chemical irritants as part of a "common chemical sense." An efferent role for some of these peptidergic afferent axons may also be inferred from their specific distributions. Sites involved in regulating access to and sensitivity of sense organs to external stimuli (e.g., cochlear and vestibular hair cells, taste bud orifices, and main olfactory epithelium) are heavily innervated. Other IR axons are in position to exert control over airflow through nasal turbinates, glandular secretion, blood circulation, and duct transport systems.(ABSTRACT TRUNCATED AT 400 WORDS)

The distribution of the neuropeptide calcitonin gene-related peptide (CGRP) was studied in relation to the known subcortical somatosensory pathways and contiguous systems in the central nervous system (CNS) of rats by using peroxidase histochemical methods in order to relate zones of immunoreactivity (IR) to cytoarchitecture. CGRP is the most ubiquitous peptide found to date in sensory ganglion cells: principally small and medium-size neurons emitting thin axons inferred to be largely nociceptive in function on the basis of the peripheral distribution of their terminals. Its apparent absence in sympathetic axons provides an especially useful sensory marker. The distribution of CGRP-IR axons displays remarkable selectivity at each level of the CNS. The trigeminal root distributes axons primarily to the pericornual layers (laminae I and II) of spinal V nucleus caudalis and to subnucleus oralis, evading the subnucleus interpolaris and contributing only few axons to principal V. Although there are only a few CGRP-IR somata at each level, heavily labeled axon trajectories can be traced to the nuclei of the solitary tract, the parabrachial nuclei, several sectors of the caudal medial thalamus, and the central nucleus of the amygdala. A sector of labeled neuron somata lies contiguous to each of these axon terminal zones, the largest of which is a thalamic nucleus containing cells of distinctive dendritic architecture extending from the periaqueductal gray across the posterior group nuclei to the peripeduncular nucleus, forming a linear array at the mesodiencephalic junction. The relation of CGRP-IR axonal distribution to spinothalamic, visceral, and gustatory systems is discussed in the context of a specialized "chemosensory" component of the thin-fiber somatosensory system.

The distribution of calcitonin-gene-related peptide (CGRP) immunoreactivity (IR) was studied in peripheral tissues of rats. The ganglionic origin, somatosensory nature, and anatomic relations of this thin-axon population were evaluated with particular emphasis on possible nociceptive roles. In animals untreated with colchicine, CGRP-IR is found in a vast proportion of small- and medium-diameter sensory ganglion cells that give rise to numerous thinly myelinated and unmyelinated axons that display CGRP-IR throughout the body. The integumentary innervation consists, in part, of an extensive subpapillary network largely traced to dermal blood vessels, sweat glands, and "free" nerve endings, some of which are found within regions containing only mast cells, fibroblasts, and collagen. Dermal papillae contain CGRP-IR axons surrounding each vascular loop; other papillary axons end freely or occasionally surround Meissner corpuscles. Intraepithelial axons enter glabrous epidermal pegs, branching and exhibiting terminals throughout the stratum spinosum. A similar pattern is found in hairy skin with additional innervation entering the base and surrounding the lower third of each hair follicle, but apparently not supplying sebaceous glands and arrector pili muscle. Axons innervating nonkeratinized oral epithelium are similar or greater in number and distribution compared to epidermis, often with more extensive branching. The high density of intraepithelial CGRP-IR innervation does not appear to correlate with the sensitive mechanoreceptor-based increase in spatial sensory discriminative capacities in the distal portions of the limb. In deep somatic tissues, CGRP-IR is principally related to vasculature and motor end plates of striated muscle, but there is an extensive network of thin axons within bone, principally in the periosteum, and focally in joint capsules, but not in relation to muscle spindles or tendon organs. These findings, together with the distribution in cranial tissues described in an accompanying paper (Silverman and Kruger: J. Comp. Neurol. 280:303-330, '89), are considered in the context of a "noceffector" concept incorporating the efferent role of these sensory axons in various tissues. It is suggested that involvement in tissue maintenance and renewal during normal function, as well as following injury, may predominate over the relatively infrequent nociceptive role of this peptidergic sensory system.

Postdeafferentation reorganization in the central terminal fields of spared dorsal root axons was evaluated by examining the intraspinal distribution of horseradish peroxidase-labeled sciatic nerve afferent fibers at various intervals following the removal of several lumbar dorsal root ganglia. The sciatic projection to the spinal cord, as determined by the pattern and density of intraspinal reaction product, was remarkably stable following the ganglionectomies. For as long as 3 months later, there was no evidence that sciatic afferent fibers had formed anomalous connections either with new spinal segments or in denervated areas within normal segments of entry. These findings cast doubt upon the existence of anatomic reorganization within the spinal cord following its partial deafferentation and suggest that physiological processes other than new axonal growth underlie observations such as postdenervation alterations in the response properties of dorsal horn neurons and the recovery of behavioral function.

Corticothalamic and corticotectal projections from the anterior ectosylvian sulcus (AES) in neonatal cats were studied with anterograde and retrograde neuroanatomical techniques. When the injection site was relatively restricted to the sulcal walls and fundus of the rostral AES (i.e., the SIV cortex), heavy ipsilateral thalamic label was observed in the medial subdivision of the posterior group, in the suprageniculate nucleus, and in the external medullary lamina. No terminal label was seen in the contralateral thalamus although the contralateral homotopic cortex was heavily labeled. Within the ventrobasal complex (VB), dense axonal label was observed in fascicles that traversed VB, but only light terminal label was observed within VB itself. However, in cases where the tracer spread into adjacent SII, terminal label in VB was pronounced. Similarly, when the injection site extended into auditory cortex, terminal label was observed in the lateral and intermediate subdivisions of the posterior group. Rostral AES injections produced distinct, predominantly ipsilateral, terminal label in the superior colliculus that was distributed in two tiers: a discontinuous band in the stratum griseum intermedium and a more diffuse band in stratum griseum profundum. Caudally, dense terminal label was seen in the intercollicular zone and dorsolateral periaqueductal gray. When the injection site did not include rostral AES, no label was observed in the superior colliculus. Horseradish peroxidase injections into the superior colliculus of neonates produced retrogradely labeled neurons throughout the AES, but none was found on the crown of the gyrus where SII is located. Thus, the neonatal corticotectal somatosensory projection arises exclusively from AES and parallels that found in adults. These data indicate that the elaboration of a major descending somatosensory pathway from AES to the thalamus and midbrain is largely a prenatal event. The in utero anatomical maturation of the corticofugal projections from SIV cortex to the superior colliculus contrasts with the protracted postnatal development of the corticotrigeminal projections from SI cortex but is consistent with the mature anatomical state of ascending trigeminotectal projections.

Following axotomy, motoneurons of the opossum spinal cord display an early "axon reaction" or "chromatolysis" characterized by a redistribution of ribosomes accounting for a widespread basophilia and an apparent reduction in the size of two distinct varieties of Nissl bodies. This alteration is accompanied by zones of increased extracellular glycocalyx demonstrable in light and electron microscopy. In addition, large intracellular periodic acid-Schiff-positive vacuolated zones in the neuron periphery possess numerous free ribosomes, glycogen, lipids, and huge vacuolated sacs containing a flocculent matrix material similar to that found within the sacs of granular endoplasmic reticulum. "Artifacts" in the neuronal periphery associated with chromatolysis seen in light microscopy are probably related to polysaccharide alterations and redistribution of granular endoplasmic reticulum.

The pattern of projection of the rat medial lemniscus was studied by axonal transport labeling following injections of tritiated leucine, proline and/or adenosine, or of horseradish peroxidase for retrograde identification of the neurons of origin. The vast majority of neurons in the gracile, cuneate, and principal trigeminal nuclei contribute to an almost totally crossed projection primarily to the thalamic ventrobasal complex. Additional thalamic components were traced to specific sites within the "posterior group," including a medial component largely traversed by lemniscal axons and a caudolateral component lying between the principal nucleus of the medial geniculate and ventral nucleus of the lateral geniculate. We have designated this latter zone "intermediate geniculate," distinguishing a somatosensory portion of the geniculate group on the basis of its myelo- and cytoarchitecture, as well as its connections. Other projections replicated in several animals included the zona incerta and nearby sectors of the substantia nigra; three distinct mesencephalic arrangements within the deep layers of the superior colliculus, the external nucleus of the inferior colliculus, and the intercollicular nucleus; the anterior pretectal nucleus; dorsal sectors of the inferior olivary complex and the ipsilateral cerebellar cortex. The results are compared with findings in other species (with emphasis on the caudal thalamic region) in an attempt to resolve some of the apparent inconsistencies in nomenclature.