A morphometric study of the effects of maturation and aging on synaptic patterns in the spinal trigeminal nucleus of the cat

Timecourse of development of the wallaby trigeminal pathway. I. Periphery to brainstem

The development of the vibrissae and their innervation and the maturation of the brainstem trigeminal sensory nuclei have been studied in the wallaby, Macropus eugenii, from birth to adulthood. At birth, developing vibrissal follicles consist of solid epidermal pegs surrounded by dermal condensations. The developing follicles and adjacent skin are innervated by trigeminal afferents. Ten days after birth the follicle contains a dermal papilla and the deep vibrissal nerve can be recognised. A hair cone is present at postnatal day (P) 30 and hairs are apparent on the skin surface by P35. By P63 the deep vibrissal nerve can be seen innervating Merkel cells in the outer root sheath; in addition, the first signs of the blood sinus can be recognised. Innervation of the inner conical body and lanceolate and lamellated receptors supplying the mesenchymal sheath and waist region are not seen until P119, when the follicle resembles that seen in the adult. At birth, central processes of the trigeminal ganglion cells have entered the trigeminal tract and extend from the rostral pons to the upper cervical cord. Labelling with a carbocyanine dye at P0 shows afferents extending medially from the tract into the trigeminal subnuclei at all levels. At this stage the trigeminal nuclei appear as areas of increased cell density in the lateral brainstem. By P30-40 the four subnuclei can be distinguished on the basis of shape, cytoarchitecture, and succinic dehydrogenase reactivity. Adult morphology is not fully established until P210. In mature animals, nucleus principalis contains closely packed, polymorphic cells, frequently aligned parallel to thick fibre bundles that traverse the nucleus obliquely. Subnuclei oralis and interpolaris contain sparsely distributed, medium to large cells, randomly oriented, as well as prominent rostrocaudally directed fibre bundles. Subnucleus caudalis consists of the marginal layer, substantia gelatinosa, and magnocellular layers as described in other species. Patches of increased succinic dehydrogenase or cytochrome oxidase reactivity, presumably corresponding to the vibrissae, are present in subnuclei principalis, interpolaris, and caudalis in developing and adult animals, although the pattern is less clear than in rats. The brainstem patches are first seen at P40, approximately 6 weeks before the corresponding vibrissal-related pattern develops in the cortex. This suggests that the onset of patch formation may be regulated independently at different levels of the pathway.

Electron microscopic localization of nerve growth factor receptor (p75)-immunoreactivity in pars caudalis/medullary dorsal horn of the cat

Previous studies have demonstrated the presence of nerve growth factor receptor [NGFr(p75)]-immunoreactivity (IR) in the spinal trigeminal nucleus of both 8-10 week-old kittens and mature cats. Most of the NGFr(p75)-IR is lost following retrogasserian rhizotomy, indicating that the majority of the NGFr(p75)-IR within the spinal trigeminal nucleus is of trigeminal primary afferent origin. Here, we examined the ultrastructural localization of NGFr(p75)-IR within lamina II outer of pars caudalis/medullary dorsal horn in the mature cat. Lamina II outer represents a location where dense NGFr(p75)-IR is seen with the light microscope. The NGFr(p75)-IR identified with the electron microscope was located within small thinly myelinated and unmyelinated axons and within axon terminals. The terminals with NGFr(p75)-IR typically formed asymmetric synaptic specializations onto dendritic profiles and at times were postsynaptic to other axon terminals at symmetric synaptic specializations. The terminals with NGFr(p75)-IR were either simple (associated with a single profile) or more complex, such as those that typically formed the central element in synaptic glomeruli. The NGFr(p75)-IR in terminals was especially prominent on microtubules and the plasmalemma and these findings are consistent with proposed roles for NGFr(p75) in axoplasmic/neuronal transport and as a membrane protein, respectively. The profiles with NGFr(p75)-IR seen with the electron microscope indicate a primary afferent origin and show some similarities when compared to other markers of primary afferent fibers such as calcitonin gene-related peptide. In addition, a possible role for NGFr(p75) in the transmission of nociceptive stimuli is also discussed.

Axon hillocks and initial segments in spinal trigeminal nucleus with emphasis on synapses including axo-axo-axonic contacts

As a part of a continuing study of the feline spinal trigeminal nucleus, the fine structure and synaptic arrangements on the axon hillock and axon initial segment of neurons in this region are described here. Transmission electron microscopy has been used to characterize qualitatively the axon hillock and initial segment and associated synapses in pars interpolaris. Axon hillocks and initial segments are easily identified in continuity with somata or as isolated profiles in the neuropil, and they receive synaptic contacts: these we regard as axo-axonic. The presynaptic terminals contain either mainly round or mainly flattened synaptic vesicles and have Type I (asymmetric) or Type II (symmetric) thickenings respectively at their contacts with the axon hillock or initial segment. I report here also the unusual arrangement of three separate axons in a serial synaptic complex. Some of the round vesicle Type I contacts onto the axon hillock-initial segment region also receive Type II contacts from one or more flattened vesicle terminals, thus forming an axo-axo-axonic complex. These flattened vesicle terminals lack the usual features of a presynaptic dendrite. It has been shown that in this nucleus some round vesicle terminals, especially those postsynaptic to flattened vesicle terminals, are primary afferents from the periphery. Therefore the round vesicle terminal presynaptic to the axon hillock-initial segment region, some of which are included in the axo-axo-axonic complex may also be a primary afferent directly contacting the spike generator area of the relay neuron and under presynaptic control of a flattened vesicle synapse. The latter may possibly be an intrinsic contact. This strategic situation of round vesicle terminals and the axo-axo-axonic complex at the axon hillock or initial segment has major implications relevant to the overall output of these neurons.

Synaptic substrates for enkephalinergic and serotoninergic interactions with dental primary afferent terminals in trigeminal subnucleus interpolaris: an immunocytochemical study using peroxidase and colloidal gold

Pain processing in the trigeminal complex has been thought to reside primarily in the spinal subnucleus caudalis (Vc). However, trigeminal tractotomies eliminating primary afferent input to Vc and severance of secondary trigemino-thalamic fibers from Vc do not disturb pain perception from the central face and oral cavity. Furthermore, large numbers of neurons that are highly responsive to noxious stimuli and suppressed by inputs from the periaqueductal gray and raphe complex have been identified in subnuclei interpolaris (Vi) and oralis (Vo). Therefore, the purpose of this study was to assess the distribution and spatial arrangements of nociceptive modulatory transmitters with nociceptive afferents and trigemino-thalamic relay cells in the rostral portion of the spinal trigeminal nuclear complex. The dental pulp contains predominantly nociceptors that project to all three subdivisions of the trigeminal spinal complex. These projections were visualized by anterograde transganglionic transport of horseradish peroxidase or by degeneration following administration of toxic ricin to the pulp chambers. The spatial arrangements of dental primary afferents with enkephalinergic (ENK) and serotoninergic (5HT) inputs was then assessed by employing avidin-biotin peroxidase and protein-A colloidal gold double-labeling immunocytochemistry. Trigemino-thalamic relay cells were also labeled by retrograde transport of HRP after stereotaxic injections into the ventrobasal thalamus. ENK and 5HT immunoreactivity was found in the ventrolateral quadrant and lateral margin of Vi, together with the adjacent interstitial nucleus (IN). This activity extended from the caudal pole of Vi and the periobex region, where it was most dense, rostrally to a position approximately 2.9 mm from the Obex. Neither ENK nor 5HT immunoreactivity was observed in Vo. Primary dental afferents projected into the ventromedial quadrant of rostral Vi and were found in the ventrolateral quadrant and dorsal aspect of the subnucleus farther caudally. They appeared as simple boutons with single contacts or as larger, sometimes scalloped terminals that formed multiple contacts. Postsynaptic elements were usually small dendritic profiles, although relay cell somata rarely received primary afferent inputs. Many primary afferents entered areas of synaptic clustering and contacted enkephalinergic dendrites, some of which were also postsynaptic to serotoninergic synapses. Alternatively, primary afferents contacted unlabeled processes that were also postsynaptic to the enkephalinergic element to form a triad arrangement. The least common occurrence was axo-axonic contacts in which enkephalinergic synapses were presynaptic to primary afferents. Both enkephalinergic and serotoninergic synaptic categories displayed round vesicles and generally formed asymmetric junctions.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-related loss of synaptic terminals in the rat medial nucleus of the trapezoid body

The effect of aging on axosomatic synaptic terminals in the rat medial nucleus of the trapezoid body was studied using quantitative electron microscopy. In young adult rats (3 months of age), the mean percentage of the surface area of principal cells covered by synaptic terminals is 61.7% (S.E.M. = 4.1) while in aged animals (27-33 months of age) the per cent coverage is 43.7% (S.E.M. = 3.3). Likewise, between 3 and 27-33 months of age, the average number of synaptic terminals present along a 100 micron length of principal cell surface decreases significantly (P less than 0.001) from 28.3 (S.E.M. = 1.3) to 18.9 (S.E.M. = 1.3). Only terminals derived from calyces of Held are lost in the aged animals, displaying a 37% reduction between 3 and 27-33 months of age. The length of apposition by synaptic terminals in the medial nucleus of the trapezoid body does not change significantly with aging. We conclude that because of a significant loss of calycine synaptic endings, the structure of calyces of Held becomes less complex with advancing age in rats. This would presumably result in an age-related partial deafferentation of principal cells, causing significant alterations in the processing of auditory information in the medial nucleus of the trapezoid body.

Trigeminotectal and other trigeminofugal projections in neonatal kittens: an anatomical demonstration with horseradish peroxidase and tritiated leucine

The trigeminal projection to the superior colliculus in neonatal kittens was studied by using both anterograde and retrograde neuroanatomical tracing techniques. Trigeminothalamic observations also were made. In the first series of experiments, horseradish peroxidase was injected into the superior colliculus in kittens on the day of parturition and in adult cats. Retrogradely labeled cells were found throughout the contralateral sensory trigeminal complex: the greatest numbers of cells were concentrated in pars oralis, with fewer in the principal nucleus, and fewer still in pars interpolaris and pars caudalis. Thus, the distribution pattern of trigeminotectal cells in neonates is similar to that in adult animals. In the second series of experiments, we injected tritiated leucine into the rostral portion of the spinal trigeminal nucleus in neonatal kittens and adult cats and compared the laminar and spatial distribution of anterogradely transported label in the superior colliculus and thalamus. Terminal label was observed in both structures in animals as young as 1-2 days postpartum. The label in the superior colliculus was overwhelmingly contralateral and formed a tier of discontinuous patches in the stratum griseum intermediale and, in a more diffuse manner, in the stratum griseum profundum. Most of the patches were located in the rostral 80% of the superior colliculus and were 60-280 micron in width. Although the size of the patches was smaller in the neonates, their distribution was similar to that in adult cats. Thus, with the exception of the difference in patch size, the terminal pattern of trigeminotectal projections is essentially adultlike at birth. The dense pattern of contralateral terminal label in the arcuate division of the ventrobasal complex also was similar to that of the adult cat, as was the trigeminal projection to the supraoculomotor gray. These data indicate that the development of the spatial organization of a major ascending somatosensory pathway to the superior colliculus (and to the thalamus) is largely a prenatal event. It is likely that the further maturation of these systems during postnatal life is limited to fine changes in axonal terminals and synaptic formation within prenatally determined terminal territories. The in utero maturation of these trigeminofugal projections is necessary to enable the newborn kitten to utilize the perioral tactile cues necessary for early orientation and suckling behaviors.