Jan G. Wolters

Distribution of catecholamines in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study based on the use of antibodies to tyrosine hydroxylase

Antibodies to tyrosine hydroxylase were used to study the distribution of nerve cells, fibers and terminals, containing catecholamines, in the lizard Varanus exanthematicus, by means of the indirect immunofluorescence technique. Tyrosine hydroxylase-containing cell bodies occurred in the hypothalamus, the ventral and dorsal tegmentum mesencephali, the substantia nigra, the isthmic reticular formation, in and ventrolaterally to the locus coeruleus, in the nucleus tractus solitarii and in a lateral part of the nucleus reticularis inferior. In addition tyrosine hydroxylase-containing cell bodies were found throughout the spinal cord, ventral to the central canal. Tyrosine hydroxylase-immunoreactive terminal areas in the brain stem were seen in the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus raphes superior, the locus coeruleus, several parts of the reticular formation and the nucleus descendens nervi trigemini. Ascending catecholaminergic pathways could be traced from the ventral mesencephalic tegmentum as well as from the dorsal isthmic tegmentum rostralwards, through the lateral hypothalamus. These pathways correspond to the mesostriatal and isthmocortical projections respectively, as described in mammals. Furthermore, ascending catecholaminergic fibers could be traced from the catecholaminergic cell groups in the medulla oblongata to the isthmus, where they intermingle with the locus coeruleus neurons. These pathways correspond to the medullohypothalamic projection and to the dorsal periventricular system in mammals. Descending catecholaminergic fibers to the spinal cord pass via the dorsomedial part of the lateral funiculus, and mainly terminate in the dorsal horn. The results obtained in the present study have been placed in a comparative perspective, which illustrates the constancy of catecholaminergic innervation throughout phylogeny.

Distribution of serotonin in the brain stem and spinal cord of the lizard Varanus exanthematicus: An immunohistochemical study

The distribution of serotonin-containing nerve cell bodies, fibers and terminals in the lizard Varanus exanthematicus was studied with the indirect immunofluorescence technique, using antibodies to serotonin. Most of the serotonin-containing cell bodies were found in the midline, in both of the raphe nuclei, i.e. the nuclei raphes superior and inferior. A considerable number of more laterally shifted serotonergic neurons was found particularly at three levels of the brain stem, viz. in the caudal mesencephalic tegmentum, at the isthmic level, and over a long distance in the medulla oblongata. These laterally situated serotonin-positive neurons were partly found within the confines of the substantia nigra, the nucleus reticularis superior and the lateral part of the nucleus reticularis medius and ventrolateral part of the nucleus reticularis inferior, respectively. No serotonergic cell bodies were found in the spinal cord. In the brain stem a dense serotonergic innervation was observed in all of the motor nuclei of the cranial nerves, in two layers of the tectum mesencephali, in the nucleus interpeduncularis pars ventralis, the nucleus profundus mesencephali pars rostralis, the periventricular grey, the nucleus parabrachialis, the vestibular nuclear complex, the nucleus descendens nervi trigemini, the nucleus raphes inferior, and parts of the nucleus tractus solitarii. Descending serotonergic pathways could be traced into the spinal cord via the dorsolateral, ventral and ventromedial funiculi, and were found to innervate mainly three parts of the spinal grey throughout the spinal cord, i.e. the dorsal part of the dorsal horn, the motoneuron area in the ventral horn, and the intermediate zone just lateral to the central canal. The results obtained in the present study suggest a close resemblance of the organization of the serotonergic system in reptiles and mammals, especially as to the serotonergic innervation of the spinal cord.

Distribution of some peptides (substance P, [Leu]enkephalin, [Met]enkephalin) in the brain stem and spinal cord of a lizard, Varanus exanthematicus

The distribution of substance P-like and [Leu]- and [Met]enkephalin-immunoreactive cell bodies, fibers and terminal structures in the brain stem and spinal cord of a lizard, Varanus exanthematicus, was studied with the indirect immunofluorescence technique, using antibodies to these peptides. Substance P-like immunoreactive cell bodies were found in the hypothalamus, in a periventricular cell group in the rostral mesencephalon, in the interpeduncular nucleus, in and ventral to the descending nucleus of the trigeminal nerve, in and directly ventral to the nucleus of the solitary tract, scattered in the brain stem reticular formation and in the trigeminal and spinal ganglia. A rather widespread distribution of substance P-like immunoreactivity was found in the brain stem and spinal cord, mainly concentrated in striatotegmental projections related to visceral and/or taste information (nucleus of the solitary tract, parabrachial region), in the descending nucleus of the trigeminal nerve and in the dorsal horn of the spinal cord (areas I and II). In the spinal cord also around the central canal (area X and adjacent parts of area V-VI) a distinct substance P innervation was found. The ventral horn receives only a very sparse substance P innervation. The distribution of [Leu]- and [Met]enkephalin in the brain stem and spinal cord of Varanus exanthematicus is less impressive than that of substance P. Enkephalinergic cell bodies were found particularly in the caudal hypothalamus. Small populations of enkephalinergic cell bodies were found in the vestibular nuclear complex, in the nucleus of the solitary tract, in and around the descending nucleus of the trigeminal nerve and throughout the rhombencephalic reticular formation. Enkephalins are likely to be present in efferent projections of the striatum, in projections related to taste and/or visceral information (nucleus of the solitary tract, parabrachial region) and in descending pathways to the spinal cord. Enkephalinergic fibers are present in the lateral funiculus and enkephalin-immunoreactive cell bodies are found in the reticular formation, particularly the inferior reticular nucleus which is known to project to the spinal cord. In the spinal cord enkephalinergic terminal structures were found especially in the superficial layer of the dorsal horn (areas I and II) and around the central canal. The ventral horn including the motoneuron area receives only a relatively sparse enkephalinergic innervation.

Funicular Trajectories of Descending Brain Stem Pathways in a Lizard(Varanus exanthematicus)

Publisher Summary This chapter discusses funicular trajectories of descending brain stem pathways in a lizard. Horseradish peroxidase (HRP) slow-release gels have been applied to various parts of the lateral and ventral funiculi of the cervical enlargement in the lizard Varunus exanthematicus to obtain more conclusive evidence on funicular trajectories of various descending pathways. In an earlier study, the preparation of the HRP slow-release gel was slightly modified to obtain a slower release of the enzyme and a less fragile dried gel, which would be easier to handle and to apply. Following the implantation of HRP slow-release gel in the dorsal part of the lateral funiculus, many labeled neurons were observed throughout the brain stem. No labeled neurons were found in the hypothalamus. A few labeled cells were present in the ipsilateral interstitial nucleus of the fasciculus longitudinalis medialis (flm). At the isthmic level, labeled cells were observed in the locus coeruleus and in two reticular cell groups.

Materials and Techniques

The present cytoarchitectonic analysis of the brain stem of the savanna monitor lizard, Varanus exanthematicus (Fig. 1), is based on Nissl (cresylecht violet), Kluver-Barrera (1953), and Haggqvist (1936) stained material. Four transversely sectioned (15 µm) Nissl stained, one Kluver-Barrera, and one Haggqvist stained series were available. In addition, a Nissl and a Kluver-Barrera stained sagittal series (15 µm) were used. For comparison, transversely and sagittally sectioned series of the central nervous system of the tegu lizard, Tupinambis nigropunctatus, were available.

Organization and Connections of the Sensory Trigeminal Nuclei

The sensory part of the trigeminal nuclear complex comprises three nuclei: the descending (or spinal) nucleus, the principal nucleus, and the mesencephalic nucleus. The descending and principal nuclei constitute a column of gray which extends from the entrance of the trigeminal nerve into the rostral part of the spinal cord, where it is continuous with the peripheral part of the dorsal horn.

The Reticular Formation

The reticular formation of the reptilian brain stem is composed of various types of cells, ranging from small to very large, reticulospinal neurons (Sect. 13.1). It contains monoaminergic and peptidergic components (Sect. 13.2). The rhombencephalic reticular formation is the main site of termination of afferents from the spinal cord (see Sect. 4.2.1.1 and Fig. 22), is extensively afferented by the tectum mesencephali (see Sect. 10.1.3.2) and also receives a direct projection from the striatum (see Sect. 11.2 and Fig. 45). Efferent fibers pass to the thalamus and even to the telencephalon (Sect. 13.3), but chiefly to the spinal cord (Sect. 13.4). The bulk of descending pathways to the spinal cord is formed by reticulospinal fibers (ten Donkelaar 1982). Prosencephalic structures exert their influence on brain stem and spinal motor mechanisms via the brain stem reticular formation. It seems likely that, in reptiles and in other lower vertebrates (e.g., Nieuwenhuys 1977), the reticular formation represents a “final common supraspinal” pathway by way of which higher nervous centers can influence spinal motor mechanisms (ten Donkelaar 1982).

The Vestibular Nuclear Complex and Related Structures

The reptilian vestibular complex is usually divided into at least five nuclei (Weston 1936), i.e., the nucleus vestibularis dorsolateralis (also described as the superior vestibular nucleus: Beccari 1911 ; Larsell 1926; Stefanelli 1944a), the nucleus vestibularis ventrolateralis, the nucleus vestibularis tangentialis, the nucleus vestibularis ventromedialis, and the nucleus vestibularis descendens (or inferior: Beccari 1911; Stefanelli 1944a). In turtles, in addition a nucleus vestibularis superior can be distinguished as a subdivision of the dorsolateral vestibular nucleus (Weston 1936; Cruce and Nieuwenhuys 1974; Miller and Kasahara 1979; ten Donkelaar and Nieuwenhuys 1979; Bangma et al. 1983). Stefanelli (1944 a, b; see also Schwab 1979; ten Donkelaar and Nieuwenhuys 1979) noted the considerable differences in the development of the vestibular nuclei and their connections. The ventrolateral and dorsolateral vestibular nuclei are strongly developed in those reptiles with wholly or partly quadrupedal locomotion, whereas the tangential nucleus is particularly large in species with serpentine movements.

The Solitary Tract and Related Nuclei

The reptilian solitary tract is a well-developed, thin-fibered bundle that receives fibers from cranial nerves VII, IX, and X (see Sect. 6.2). Its fibers terminate in the nucleus of the solitary tract and in its mediocaudal extension, the nucleus commissurae infimae (Jacobs 1979; Barbas-Henry 1982; Barbas-Henry and Lohman 1984). In the older literature, based on normal material, an ascending pathway to a sensory visceral nucleus located in the rostral part of the rhomben-cephalon just ventromedial to the nucleus isthmi, has been suggested (Shanklin 1930; Barnard 1936; see also ten Donkelaar and Nieuwenhuys 1979). In a more recent study in Varanus exanthematicus, ten Donkelaar and de Boer-van Huizen (1981b) introduced the term “parabrachial region” for this secondary visceral center.

Spinal Projections to the Brain Stem

Spinal projections reach the brain stem via the dorsal and the lateral funiculi. The dorsal funiculus contains ascending collateral branches of spinal primary afferent (dorsal root) fibers, the lateral funiculus the spinal lemniscus and spino-cerebellar projections. The course and site of termination of spinal projections to the brain stem are shown in Fig. 22 in which the distribution of degenerating fibers and preterminal structures following a hemicordotomy at the third spinal segment is presented.