Neuronal responses to amino acid iontophoresis in the deafferented spinal trigeminal nucleus.

Facial paresthesias or anesthesia dolorosa occurring after retrogasserian trigeminal rhizotomy for trigeminal neuralgia have been attributed to neuronal hyperactivity in the deafferented spinal trigeminal complex. To gain insight into the mechanism of the deafferentation hyperactivity, the responses of spinal trigeminal neurons to amino acid iontophoresis via multibarrel micropipets were studied in 20 cats 2 to 4 weeks after unilateral retrogasserian trigeminal rhizotomy. A population of spontaneously hyperactive neurons (type 2) had markedly decreased sensitivity to the exogenous amino acid transmitters. Amino acid hyposensitivity was nonspecific for both excitatory and inhibitory agents, except in a small subgroup (type 2B) which was selectively hyposensitive to γ-aminobutyric acid. Some type 2 neurons were identified as trigeminothalamic projection neurons. Silent (type 1) neurons had normal responses to amino acids compared to those of controls. Endogenous corticofugal inhibition remained intact for all neuronal groups in the deafferented nucleus. The data do not support Cannon's law of denervation hypersensitivity as the mechanism of postrhizotomy hyperactivity, but seem more consistent with transneuronal atrophy in the deafferented spinal trigeminal nucleus.     

Ultrastructural changes in acetylcholinesterase activity in the deafferented spinal trigeminal nucleus

Acetylcholinesterase (AChE) activity was investigated in synaptic areas of the cat spinal trigeminal nucleus (pars interpolaris and pars caudalis) ipsilateral and contralateral to complete retrogasserian rhizotomy. Vibratome sections of tissue taken from animals of 1, 3, 6, 14, and 21 days survival were examined by electron microscopy following a histochemical reaction for AChE activity employing a method based on the Karnovsky-Roots technique for demonstrating reaction product. As degeneration progressed with survival time, enzymatic activity was initially reduced in synaptic clefts of injured afferent terminals and subsequently was enhanced throughout the extracellular space, including within synaptic clefts of possibly reinnervated sites. These changes in enzymatic activity with primary deafferentation are discussed in relation to the process of reinnervation, the development of neuronal hyperactivity, and possible noncholinergic functions of AChE.     

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.     

Synapse development within the spinal trigeminal nucleus

Pars interpolaris of the spinal trigeminal nucleus of kittens has been studied with the electron microscope at birth and at several subsequent ages during the first month of life. Attention has been given to ultrastructural maturational changes that occur in this neuropil, especially events in synaptogenesis. The results of this investigation include the following observations: (1) the neuropil, even at the earliest ages studied (three-hour-old kittens), is strikingly mature, necessitating a quantitative assessment in order to determine subtle developmental changes in synaptic patterns; (2) the number of axoaxonic contacts at birth are few, and their emergence is essentially a postnatal phenomenon; (3) it appears that the immature Gray type II or symmetrical synapse possesses distinct cleft material and dense, parallel membrane specializations. Synaptic vesicle accumulation at this contact appears to occur after the membrane specializations have formed. A previous study by Kerr26 has shown a reduced potential for primary afferent reorganization with the spinal trigeminal nucleus when kittens are subjected to trigeminal rhizotomy after three days of age. Our observations on the development of axoaxonic synaptic arrangements in the neonatal period may provide an explanation for these earlier results.     

Edinger-Westphal nucleus: cholecystokinin immunocytochemistry and projections to spinal cord and trigeminal nucleus in the cat

Immunocytochemical methods were used to determine the distribution of cells with cholecystokinin-like immunoreactivity (CCK-LI) in the cat Edinger-Westphal complex (EW). Numerous cells with CCK-LI are found throughout the length of EW. The distribution and frequency of such cells are similar to the pattern of EW neurons that show substance P-like immunoreactivity (SP-LI). Companion retrograde transport experiments reveal that EW neurons which project to spinal cord or the region of the caudal trigeminal nucleus are found throughout the length of EW, and that some EW neurons which project to spinal cord also show CCK-LI.     

Projections from the spinal trigeminal nucleus to the entire length of the spinal cord in the rat

In order to confirm the multiple neurotransmitter biosynthetic ability, the possibility to separation of the activities of tyrosine hydroxylase (TH), choline acetyltransferase and glutamic acid decarboxylase was tested by subcloning of a clonal rat pheochromocytoma PC12 cell line. All of 9 subclones obtained showed significant activities of above 3 enzymes, indicating that the PC12 cell has multi-functional properties of neurotransmitter syntheses. One of the subclones, designated PC12h, was demonstrated to have nerve growth factor- (NGF) responsive TH activity. The ED50 value of NGF to increase the TH activity was 1.7 ng/ml (6.5 X 10-11 M). A simultaneous addition of saturating amounts of NGF (50 ng/ml) and dexamethasone (10-6 M) resulted in the increase of TH activity that is equal to the sum of those achieved when either effector was added separately, indicating that the NGF- mediated increase of TH activity in PC12h cells was independent upon the effect of dexamethasone. And also, the TH activity increased by NGF was somewhat potentiated in PC12h cells cultured in a hormone- supplemented serum-free medium.     

Direct projections from the caudal spinal trigeminal nucleus to the striatum in the cat

The results of a WGA-HRP and HRP study in the cat indicated that some neurons in the marginal zone (lamina I) of the caudal spinal trigeminal nucleus sent their axons contralaterally to the striatum; mainly to the dorsal part of the putamen, and additionally to the ventrolateral part of the caudate nucleus, at the stereotaxic rostrocaudal levels of A 13.0-A 15.5.     

Mesencephalic trigeminal neuron responses to γ-aminobutyric acid

Mesencephalic trigeminal neurons are primary sensory neurons which have cell somata located within the brain stem. In spite of the presence of synaptic terminals on and around the cell somata, applications of a variety of neurotransmitter substances in earlier studies have failed to demonstrate responses. Using intracellular recording in a brain slice preparation, we have observed prominent depolarizations and decreases in input resistance in response to applications of γ-aminobutyric acid (GABA) in most recorded mesencephalic trigeminal neurons. Those cells failing to respond were located deeply within the slice, and the low responsiveness was shown to be related to uptake of GABA in the slice. The responses were direct, since they remained during perfusion with a low calcium, high magnesium solution that blocks synaptic transmission. The responses were mimicked by the GABA_A receptor agonist isoguvacine, and blocked by GABA_A receptor antagonists. The GABA_B receptor agonist baclofen evoked no changes in membrane potential or input resistance in neurons exhibiting depolarizations with GABA application. Tests of neuronal excitability during GABA applications indicated that the excitatory effects of the depolarization prevail over the depressant effects of the increase in membrane conductance. In situ hybridization histochemistry indicated that the GABA_A receptors in Me5 cells are comprised of α₂, β₂ and γ₂ subunits.© 1997 Elsevier Science B.V. All rights reserved.     

Facial thermal input to the trigeminal spinal nucleus of rabbits and rats.

In rabbits and rats under urethane anesthesia, a systematic survey was made of the caudal trigeminal nucleus, using glass coated tungsten microelectrodes. This revealed many neurons in the marginal layer sensitive either to warming or cooling the facial skin. The majority of these neurons were specifically temperature sensitive. In rats, a somatotopic arrangement of cold receptive fields was evident within the marginal layer of the trigeminal nucleus, with the ophthalmic division of the trigeminal nerve represented most laterally, the mandibular most medially and the maxillary division represented over most of the recording region. This arrangement is similar to the mediolateral distribution seen in cats. In rabbits, the distribution was less rigorous but the ophthalmic division tended to be represented caudally and the mandibular division rostrally. The maxillary division was represented over most of the recording region. All receptive fields were ipsilateral and showed spatial convergence of input. Both rabbits and rats possessed a concentration of thermal receptive-fields around the nose, whisker pad and mouth. At steady skin temperatures, marginal units gave bell-shaped intensity functions which were very similar to those reported for the cold and warm receptors. With rapid changes of temperature, neurons responded with a dynamic outburst which corresponded to the equivalent receptor response.     

Baclofen in trigeminal neuralgia: its effect on the spinal trigeminal nucleus: a pilot study

Experiments with cats showed that baclofen resembles carbamazepine and phenytoin sodium in its ability to depress excitatory synaptic transmission in the spinal trigeminal nucleus. Baclofen was, therefore, given to 14 patients with refractory trigeminal neuralgia. Ten patients were relieved of the paroxysms of tic douloureux while taking 60 to 80 mg/day of baclofen. A reduction in the dosage of baclofen in six of these patients resulted in a recurrence of painful paroxysms in five patients. Seven patients have been pain-free or almost pain-free on a regimen of baclofen for four to 12 months. Our results suggest that baclofen may be a useful drug in the treatment of trigeminal neuralgia and that our experimental model may successfully predict the efficacy of a drug in the treatment of this condition.     

Golgi studies in the substantia gelatinosa neurons in the spinal trigeminal nucleus.

This Golgi study identifies three neuronal cell types in the substantia gelatinosa (SG) layer of the spinal trigeminal nucleus. The SG neurons are distinguished from each other based on: (1) dendritic branching pattern, (2) denritic spine distribution, (3) geometric shape of the denritic tree, (4) laminar distribution of the dendrites, (5) axonal branching pattern and (6) laminar distribution of the axonal arbor. The islet cell is found in small clusters and its dendrites and axonal arbor are confined within the SG layer. Its dendrites span the full width of the SG layer and extend up to 500 mum in the long axis of the layer. Dendritic spines are generally sparse with small clusters of spines found on the higher order dendritic branches. The islet cell axon extends for at least 1 mm in the long axis of the layer. Each of its collaterals divide every 50-100 mum with one branch doubling back in the direction of the cell body and the other branch continuing on in the direction of its parent. In this manner each islet cell generates a profuse axonal plexus in the SG layer. The stalked cell is found individually within the SG layer. Its cell body is usually found in the inner half of the SG layer and its sinuous dendrites cross the SG layer and enter the marginal layer. The stalked cell dendrites emit numerous fine stalk-like branches and dentritic spines. Its axon emits branches in the SG and marginal layers. The spiny cell is found singly between groups of islet cells. Its extensive dendritic tree spans up to 500 mum rostrocaudally and mediolaterally crossing into both the marginal and magnocellular layers. Spiny cells have evenly distributed dendritic spines along their dendrites in the SG layer. The spiny cell axon sends branches into all three layers of nucleus caudalis. Numerous branches enter the outer 300 mum of the magnocellular layer where they undergo further branching with some branches returning in recurrent fashion toward the SG layer. The three neuronal cell types of the SG layer satisfy all of the morphological criteria for Golgi type II interneurons. Their highly branched axons generate many collaterals within the confines of their dendritic trees and do not project out of nucleus caudalis. The SG neurons are considered to be inhibitory interneurons interposed between V nerve primary afferent axons which arborize in the SG layer and second order neurons of nucleus caudalis.     

Projections from subnucleus oralis of the spinal trigeminal nucleus to contralateral thalamus via the relay of juxtatrigeminal nucleus and dorsomedial part of the principal sensory trigeminal nucleus in the rat.

Following injection of HRP into contralateral thalamus, retrogradely labeled cells were observed in principal sensory trigeminal nucleus (Vp) and an area of juxtatrigeminal nucleus (JX) formerly described by John and Tracey (1987). When PHA-L was delivered to dorsomedial part of the subnucleus oralis (Vodm), PHA-L labeled terminals were seen in dorsomedial part of the Vp (Vpdm) and in the JX region. Comparing the distribution of PHA-L labeled terminal field with that of HRP labeled JX neurons showed that the labeled terminals and neurons were overlapped closely in the JX. The distribution patterns of the labeled terminals and JX neurons were also the same: viewed on the coronal planes caudal-rostrally, both of the labelings began to appear at the levels where the facial nerve root was just broken. Rostrally, at middle levels of the motor trigeminal nucleus (Vmo), the labelings showed their typical view covering dorsal and ventral JX (dJX, vJX). The labelings disappeared at rostral poles of the Vmo and Vp. When injections of PHA-L into the Vodm and HRP into the contralateral thalamus was made in one rat, the contacts between Vodm projecting terminals labeled with PHA-L and HRP labeled trigemino-thalamic neurons were seen in the JX and also in the Vpdm. Then, electron microscopic (EM) study was done, injections of kainic acid into the Vodm and HRP into the contralateral thalamus was performed simultaneously. After EM embedding, the JX and Vpdm regions were selected, ultrathin sections were cut and observed with EM. In both areas, axo-somatic and axo-dendritic synapses were seen between degenerated boutons and HRP labeled somata or dendrites. Namely, the Vodm projecting terminals synapsed on trigemino-thalamic neurons in the JX and Vpdm. Anyway, axo-dendritic synapses was the main type of observed synapses. Thus, the present work demonstrated 1. the JX containing a group of trigemno-thalamic neurons was a target of special projections froin the Vodm; 2. The Vodm neurons projected to the contralateral thalamus through the relay of JX and Vpdm neurons.     

Organization of the spinal trigeminal nucleus in star‐nosed moles

Somatosensory inputs from the face project to multiple regions of the trigeminal nuclear complex in the brainstem. In mice and rats, three subdivisions contain visible representations of the mystacial vibrissae, the principal sensory nucleus, spinal trigeminal subnucleus interpolaris, and subnucleus caudalis. These regions are considered important for touch with high spatial acuity, active touch, and pain and temperature sensation, respectively. Like mice and rats, the star‐nosed mole (Condylura cristata) is a somatosensory specialist. Given the visible star pattern in preparations of the star‐nosed mole cortex and the principal sensory nucleus, we hypothesized there were star patterns in the spinal trigeminal nucleus subnuclei interpolaris and caudalis. In sections processed for cytochrome oxidase, we found star‐like segmentation consisting of lightly stained septa separating darkly stained patches in subnucleus interpolaris (juvenile tissue) and subnucleus caudalis (juvenile and adult tissue). Subnucleus caudalis represented the face in a three‐dimensional map, with the most anterior part of the face represented more rostrally than posterior parts of the face. Multiunit electrophysiological mapping was used to map the ipsilateral face. Ray‐specific receptive fields in adults matched the CO segmentation. The mean areas of multiunit receptive fields in subnucleus interpolaris and caudalis were larger than previously mapped receptive fields in the mole's principal sensory nucleus. The proportion of tissue devoted to each ray's representation differed between the subnucleus interpolaris and the principal sensory nucleus. Our finding that different trigeminal brainstem maps can exaggerate different parts of the face could provide new insights for the roles of these different somatosensory stations. J. Comp. Neurol. 522:3335–3350, 2014. © 2014 Wiley Periodicals, Inc.