J.D. van Willigen

Ascending projections from the solitary tract nucleus to the hypothalamus. A Phaseolus vulgaris lectin tracing study in the rat

The course of the ascending pathways originating from the anterior gustatory and posterior visceral sensory part of the solitary tract nucleus and the topographic organization of the projections to the hypothalamus in the rat were studied with anterogradely transported Phaseolus vulgaris lectin. In general, the posterior visceral sensory part of the solitary tract nucleus has ascending projections as far as the septum-diagonal band complex and gives rise to heavy input to the bed nucleus of the stria terminals, and to the dorsomedial and paraventricular hypothalamic nuclei. A more moderate projection is aimed at a variety of other hypothalamic nuclei, to the medial and central amygdaloid nuclei and to the paraventricular nucleus of the thalamus. The ventromedial hypothalamic nucleus is strikingly missing an afferent input from the nucleus of the solitary tract. Furthermore, it was shown that whereas the caudal solitary tract nucleus has predominant long ascending connections, the projections from the anterior taste related region of the nucleus of the solitary tract have only limited forebrain projections which do not reach beyond the level of the anterior dorsal hypothalamic nucleus.

Projections from the rostral parvocellular reticular formation to pontine and medullary nuclei in the rat: Involvement in autonomic regulation and orofacial motor control

The efferent connections of the rostral parvocellular reticular formation to pontine and medullary nuclei in the rat were studied with anterogradely transported Phaseolus vulgaris leucoagglutinin. Dense innervations from the rostral parvocellular reticular formation were found in the mesencephalic trigeminal nucleus, the supratrigeminal area, the motor trigeminal nucleus, the motor trigeminal nucleus, the facial, hypoglossal and parabrachial nuclei and specific parts of the caudal parvocellular reticular formation, including nucleus linearis and the dorsal reticular nucleus of the medulla. The raphe nuclei, nucleus of the solitary tract, inferior olive, dorsal principal sensory, spinal trigeminal nuclei and gigantocellular reticular nucleus and the ventral reticular nucleus of the medulla received moderate projections. In general, the projections from the rostral parvocellular reticular formation were bilateral with an ipsilateral dominance. The dorsal motor vagus and the ambiguus nuclei were not labeled. It is concluded that the rostral parvocellular reticular formation participates in regulation of orofacial motor control and in neural networks for limbic control of metabolic homeostasis.

Neurotransmitters and neuropeptides within the mesencephalic trigeminal nucleus of the rat: An immunohistochemical analysis

In order to determine which neurotransmitters and neuropeptides are utilized by the neurons of the mesencephalic trigeminal nucleus and by the fibres making synaptic contact with these primary sensory cells, we have set up an immunohistochemical study using antibodies against 17 major neurotransmitters and neuropeptides in the rat. Apart from some intracellular immunostaining for glutamate, no immunoreactivity to any of the tested neurotransmitters and neuropeptides could be detected inside mesencephalic nucleus of the trigeminal nerve neurons. Our immunohistochemical observations indicate that mesencephalic nucleus of the trigeminal nerve neurons receive input from various nerve fibres that appear to utilize serotonin, GABA, dopamine, noradrenaline (and likely glutamate) as transmitters. The innervation appeared randomly distributed over all mesencephalic nucleus of the trigeminal nerve neurons. The presence of substance P, cholecystokinin, vasoactive intestinal polypeptide, bombesin/gastrin releasing peptide, [Leu]enkephalin and neuropeptide Y observed in some fibres that contact with mesencephalic nucleus of the trigeminal nerve neurons, presumably reflect the co-existence of these peptides with one of the neurotransmitters.

Ultrastructure of the Rat Mesencephalic Trigeminal Nucleus

The subcellular morphology of the mesencephalic trigeminal (Me5) nucleus in the rat was studied by transmission electron microscopy. Most neurons in the thin rostral as well as in the major caudal part of Me5 appeared as large (40-50 microns), round- to ovoid-shaped unipolar cells. A few neurons (estimated 5%) appeared to be multipolar, usually bipolar. The Me5 neurons had a large, round, centrally located nucleus, and their cytoplasm was characterized by a dense network of lamellar granular endoplasmic reticulum, an abundant Golgi apparatus, many mitochondria and neurofilaments suggesting very active cells with a high rate of synthesis and axoplasmatic transport. Numerous small spinous processes covered the surface of the Me5 neurons. Clustering of 2 or 3 cells was accomplished by maculae, i.e. zones of gap junctions and close cell appositions. Boutons contacting the soma of Me5 neurons and boutons contacting large and small dendrites were defined as axosomatic and axodendritic synapses, respectively. Four types of synaptic boutons were distinguished: (1) S boutons, with round vesicles and asymmetrical as well as symmetrical synapses, (2) F boutons, with pleomorphic admixture of flattened and spherical vesicles and asymmetrical synapses, (3) P boutons, which resembled the F-type boutons but contained predominantly spherical vesicles and symmetrical synapses, and (4) G boutons, characterized by a heterogeneous population of vesicles. This description of the Me5 nucleus is particularly useful for future studies that attempt to correlate the structure of a particular synapse with its function.

Arrangement and Connections of Mesencephalic Trigeminal Neurons in the Rat

The morphology of the mesencephalic trigeminal nucleus was examined microscopically in serial frozen sections. The nucleus extends over a length of about 4.5 mm, and its cell number was calculated to range from 1,000 to 1,600. 60% of the cells were located in the caudal third of the nucleus. Clustering of large unipolar cells was seen throughout the nucleus. Small spindle-shaped multipolar cells were found in the pontine part of the nucleus. The efferent connections of the mesencephalic trigeminal neurons were investigated by means of iontophoretically delivered Phaseolus vulgaris leuco-agglutinin or horseradish peroxidase after electrophysiological identification of mesencephalic trigeminal neurons. All projections were found ipsilateral to the injection site; they were confined to the trigeminal motor nucleus, especially to its lateral part, and to the dorsolateral reticular formation. The latter projection area included the supratrigeminal nucleus, the nucleus of Probst, and the parvocellular reticular zone. There were no direct projections to the facial or hypoglossal motor nuclei. It is concluded that proprioceptive input from one side is mediated polysynaptically to the bilateral oral final common-path neurons, with the exception of the ipsilateral trigeminal motoneurons.

ORIGIN, DISTRIBUTION AND MORPHOLOGY OF SEROTONERGIC AFFERENTS TO THE MESENCEPHALIC TRIGEMINAL NUCLEUS OF THE RAT

We have studied the localization, the morphology and sources of serotonergic input on the primary afferent neurons in the mesencephalic trigeminal nucleus (Me5) of the rat with light and electronmicroscopy immunocytochemistry and with anterograde and retrograde neuroanatomical tract tracing methods. Me5 neurons were found to receive a serotonergic input that is part of a serotonergic fibre plexus extending over the neighbouring parabrachial nucleus and locus coeruleus. These serotonergic afferents originate predominantly from serotonergic cells in the dorsal raphe nucleus.

DOPAMINERGIC AFFERENTS TO THE MESENCEPHALIC TRIGEMINAL NUCLEUS OF THE RAT - A LIGHT AND ELECTRON-MICROSCOPE IMMUNOCYTOCHEMISTRY STUDY

The localization and sources of dopaminergic projections on the primary afferent neurons in the mesencephalic trigeminal nucleus (Me5) of the rat were studied using light and electron microscopic immunocytochemical staining techniques combined with anterograde and retrograde neuroanatomical tract tracing methods. Me5 neurons were found to receive a dopaminergic input that is part of a dopaminergic fibre plexus extending over the neighbouring nucleus parabrachialis and locus coeruleus. These dopaminergic afferents originate from the substantia nigra, the ventral tegmental area and the medial hypothalamus.

FACTORS INFLUENCING JAW POSITION SENSE IN MAN

The relative contribution of muscle sensors, joint sensors and trigeminal exterosensors to the position sense of the mandible was investigated. Subjects compared the sizes of gauges placed between the teeth with the size of a previously explored standard gauge of 4.75 mm. The experimental conditions were either anaesthesia of both mandibular joints and the oral exterosensors, or load on the jaw-closing muscles, or adaptation to an extreme jaw position. The results were compared with control experiments under normal conditions. (1) All subjects matched to standard fairly well in control conditions, the imprecision of the match being approximately 0.5 mm. Under anaesthesia, the matches were 1-1.5 mm greater than the controls, the imprecision being almost unaffected. Anaesthetizing the joints and oral exterosensors before or after exploring the standard gauge did not affect the match. Imposing various loads (less than or equal to 3.4 kg) did not change the match to the standard. Adaptation to a wide open position introduced a considerable mismatch of the standard, leaving the imprecision of the match unaffected. (2) The experiments provide evidence that muscle afferents are important sources in sensing jaw position and fulfil the criteria for an independent position servo in the trigeminal system. It is suggested that the engram of the standard is built up mainly from muscle sensor information.

Distribution of Synaptic Boutons in the Mesencephalic Trigeminal Nucleus of the Rat – A Quantitative Electron-Microscopical Study

The distribution of synapses and synaptic bouton types in the mesencephalic trigeminal (Me5) nucleus was examined in a quantitative electron-microscopical study. Of 588 terminal boutons that were counted in the compact caudal part of the Me5 nucleus, less than 8% formed synapses on the somata of the predominantly unipolar Me5 neurons. About 79% formed synapses on fibres located between the Me5 somata, while about 13% of the vesicle-containing terminals had no clear synaptic specialization. All of these non-synaptic terminals were G type boutons, with pleomorphic and large characteristic dense-core vesicles. Approximately 60% of the axosomatic synapses were of the S type, containing spherical vesicles and an asymmetrical or symmetrical synaptic specialization. About 20, respectively 15% of the axosomatic synapses, were of the F, respectively P type; both are symmetrical synapse types containing either a majority of flat or pleomorphic vesicles. Less than 10% of the axosomatic synapses were of the G type. Although some proportional differences were noted, an almost similar bouton type distribution pattern was found for the axodendritic synapses suggesting that the axosomatic and axodendritic synapses in the Me5 nucleus are part of the same afferent fibre plexus covering the Me5 nucleus.

Locus coeruleus projections to the dorsal motor vagus nucleus in the rat

The origin of the noradrenergic innervation of the preganglionic autonomic nuclei in the medulla oblongata and spinal cord is still controversial. In this investigation descending connections of the locus coeruleus to the dorsal motor vagus nucleus in the rat are studied with Phaseolus vulgaris leucoagglutinin and horseradish peroxidase as neuroanatomical tracers. Locus coeruleus projections in the motor vagus nucleus are found in the medial part at rostral levels and in the lateral part at intermediate levels of this nucleus. The terminal labeling in the lateral intermediate part of the vagus nucleus appears in an area where possibly preganglionic parasympathetic cardiac neurons are located, suggesting that the locus coeruleus might be involved in regulation of cardiovascular functions. After small iontophoretic injections of horseradish peroxidase in the motor vagus nucleus, retrogradely labeled cells are found in the ventral part of the locus coeruleus and occasionally in the dorsal part of the nucleus. The results show that the locus coeruleus-dorsal motor vagus nucleus pathway may participate in the inhibition of the cardiac preganglionic neurons in the dorsal motor vagus nucleus by the hypothalamic paraventricular nucleus.