Catecholaminergic axo-axonic synapses in the nucleus of the tractus solitarius (pars commissuralis) of the cat: possible relation to presynaptic regulation of baroreceptor reflexes

Studies on the excitability of sinus nerve afferent terminals

1. The excitability of sinus nerve afferent terminals within the nucleus of the tractus solitarius has been studied in cats and rabbits using the technique of antidromic activation. 2. Conditioning stimuli to the hypothalamic defence area increased the excitability of some glossopharyngeal nerve afferents, though no such effects were observed on sinus nerve terminals. 3. Although the excitability of superior laryngeal nerve afferent terminals was observed to fluctuate in phase with the central respiratory cycle, no equivalent variation in sinus nerve terminal excitability were observed. 4. It is concluded that sinus nerve afferent terminals are not influenced by presynaptic mechanisms. Possible sites for the observed modulations of baroreceptor and chemoreceptor reflexes are discussed in the light of these results.

Catecholaminergic depressant effects on bulbar respiratory mechanisms

On the basis of histochemical and pharmacological studies, catecholamines have been implicated in central mechanisms controlling respiration. This hypothesis was tested in iontophoretic studies on neurones located in bulbar respiratory centres. Adrenaline and noradrenaline had a predominantly depressant effect on respiratory as well as on closely situated non-respiratory units. These depressions were mimicked by the application of isoproterenol and clonidine; acetylcholine and serotonin had inconsistent effects on these neurones. In control experiments, microinjections, using a Hamilton syringe, were made in the area of bulbar respiratory centres: noradrenaline, but not serotonin, depressed the central respiratory activity reflected in the phrenic nerve discharge. These results suggest that specific adrenergic and noradrenergic depressant mechanisms could affect both respiratory and other physiological centres at the bulbar level.

Aqueous aldehyde (Faglu) methods for the fluorescence histochemical localization of catecholamines and for ultrastructural studies of central nervous tissue

Aqueous solutions combining a high concentration of formaldehyde (4%) with low concentrations of glutaraldehyde (0.5--01%) have been used to simultaneously localize amines by the formation of fluorescent products and to fix central nervous tissue for electron microscopy. The fluorescence reaction is produced by the aldehyde mixture at room temperature and the fluorescence is stable when the tissue is maintained in aqueous solution. This means that nerve cell bodies and terminal fields which contain catecholamines can be located accurately in vibratome sections at the light microscope level and, after further processing, can be examined under the electron microscope. With 1% glutaraldehyde in the aldehyde mixture, ultrastructural details are well preserved; there is no significant distortion of any component of the tissue. If vibratome or cryostat sections are dried against glass slides, the intensity of the fluorescence reaction is enhanced and the sections can be permanently mounted.

Nuclei of the solitary tract: efferent projections to the lower brain stem and spinal cord of the cat

The efferent projections from the solitary complex to the lower brain stem and spinal cord were studied in the cat with the autoradiographic anterograde axonal transport and retrograde horseradish peroxidase (HRP) techniques. A revised cytoarchitectonic description of the caudal two-thirds of the complex is presented in which the complex was subdivided into six nuclei: lateral, ventrolateral, intermediate, medial, parvocellular, and commissural solitary tract nuclei. Following injections of 3H amino acids into electrophysiologically defined regions of the complex in which cardiac or respiratory units were recorded, labeled fibers could be traced to a number of sites in the caudal brain stem including the medial and lateral parabrachial nuclei, Kölliker-Fuse nucleus and the area ventral to this nucleus, lateral periaqueductal gray matter, ambiguus complex, which consists of the retrofacial, ambiguus and retroambiguus nuclei, ventrolateral reticular nucleus (in an area equivalent to the A1 cell group of Dahlström and Fuxe, '64), medial accessory olive, paramedian reticular formation, and lateral cuneate nucleus. Descending solitario-spinal projections have been traced bilaterally, but predominantly to the contralateral side, to the region of the phrenic motor neurons in the C4-C6 ventral horn, to the thoracic ventral horn, and intermediolateral cell column. Confirmatory evidence of some of these projections was obtained from a series of HRP experiments. Mainly small neurons of the parvocellular, medial and commissural solitary tract nuclei project to the region of the parabrachial and Kölliker-Fuse nuclei. The lateral solitary nucleus projects almost exclusively to the ipsilateral medial accessory olive. It was not possible to interpret conclusively the labeling seen in the medium and large neurons of the ventrolateral solitary nucleus after HRP injections made in the region of the ambiguus-retroambiguus complex due to the problem of fibers of passage. Following injections of HRP into the cervical, thoracic, lumbar, or sacral spinal cord, retrograde cell labeling was seen in the solitary complex. Cells in the intermediate and commissural nuclei were labeled after all four types of experiments. In the ventrolateral nucleus, medium sized neurons were predominantly labeled after the cervical spinal cord experiments, while large sized neurons were labeled mainly after the thoracic spinal cord injections. The potential physiological significance of these connections is discussed in terms of central control of cardiovascular and respiratory functions.

Selective activation of noradrenergic neurons in the brainstem and spinal cord of young spontaneously hypertensive rats

In young spontaneously hypertensive rats (SHR), dopamine beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT) activities were examined in the brainstem nuclei. Activation of noradrenergic neurons in the locus coeruleus, A2 and spinal intermediolateral cell areas, resulting in enhanced sympathetic nervous activity in the periphery, initiates hypertension. Adrenergic neurons, unchanged in these and A1 cell areas of young SHR, are not involved in the development of hypertension in SHR.

Synaptic structures and quantification of catecholaminergic axons in the nucleus tractus solitarius of the rat: possible modulatory roles of catecholamines in baroreceptor reflexes

The synaptic organization in the nucleus tractus solitarius (NTS) of the rat at the level of the obex was examined by fluorescence and electron microscopy in 5 groups of animals: (1) normal control, (2) intraventricular injection of 5-OHDA, (3) intraventricular injection of 6-OHDA, (4) intracranial denervation of the IXth and Xth cranial nerves, and (5) intraventricular injection of 5-OHDA 48 h after intracranial denervation of the IXth and Xth cranial nerves. A dense network of catecholaminergic nerves was observed in the NTS and several catecholaminergic neurons were seen to be scattered in the lateral portion of the NTS. Nerve cells in the NTS were small in size (15-20 micrometer in diameter). In addition to ordinary axodendritic and axo-somatic synapses, serial synapses were occasionally encountered. The first presynaptic site in some of the serial synapses was identified as catecholaminergic by 6-OHDA treatment. After treatment with 5-OHDA, 2.4% of axon varicosities were identified as catecholaminergic by small dense-cored vesicles. After administration of 6-OHDA, 5.19% of dense degenerated axon varicosities were counted. After intracranial deafferentation of the IXth and Xth cranial nerves, 5.3% of dense degenerated axon varicosities were found. The total number of axon varicosities in 6800 sq.mum area was decreased by 9% after the injection of 6-OHDA and 11% after deafferentation of the IXth and Xth cranial nerves. At least 3 types of axons could be identified: (1) catecholaminergic axons with small dense-cored vesicles after 5-OHDA administration, (2) afferent axons from the IXth and Xth cranial nerves with small clear vesicles, and (3) axons with small clear vesicles probably originated from the supramedullary nucleus. The results of the present study suggest that catecholamines modulate reflex blood pressure regulation within the NTS of the rat.

3H estradiol in catecholamine neurons of rat brain stem: combined localization by autoradiography and formaldehyde-induced fluorescence

3H setradion-17beta was localized in nuclei of catecholamine (CA) neurons of the rat lower brain stem, i.e., midbrain, pons and medulla, by employing a combined formaldehyde-induced fluorescence (FIF)-autoradiography approach which permits the direct visualization of 3H estradiol uptake in relation to CA-neuron perikarya and terminals. In the medulla, noradrenalin (NA)-containing neurons in and adjacent to the nucleus (n.) reticularis lateralis (group A1) and in the vicinity of the n. tractus solitarii (group A2) concentrated 3H estradiol. In the pons, NA neurons dorsal to the n. olivaris superior (group A5), in the locus ceruleus (group A6) and ventral to the pedunculus cerebellaris superior (group A7) displayed 3H estradiol uptake. In the midbrain, dopamine (DA)-containing neurons in the n. cuneiformis (group A8), in the zona compacta of the substantia nigra (group A9) and adjacent to the n. interpeduncularis (group A10), were not seen to concentrate 3H estradiol. This is in contrast to DA neurons in the n. arcuatus of the hypothalamus (group A12), previously reported to concentrate 3H estradiol. In addition, certain non-CA-containing neurons with nuclear uptake of 3H estradiol were observed to be surrounded by CA terminals in such areas as substantia grisea centralis, n. tractus solitarii and n. tractus spinalis nervi trigemini. This suggests a second mode of interaction between CA neurons and estrogen target neurons. The estradiol-CA neuron relationships described may provide morphological substrates underlying functional interactions between estradiol and CA neurons in the rat brain.

Simultaneous fixation and production of catecholamine fluorescence in central nervous tissue by perfusion with aldehydes

Perfusion with a mixture of formaldehyde (4%) and glutaraldehyde (0.5%) is shown both to fix central nervous tissue and to produce, with no further treatment, a fluorescence histochemical localization of catecholamines. After perfusion, serial sections can be readily taken through the whole brain with a Vibratome. Landmarks which are apparent at low power with white-light illumination can be seen when the sections are viewed in the fluorescence microscope. Catecholamine-containing nerve cell bodies, varicose axon terminals and non-varicose nerve fibres appear brightly fluorescent and well localized. The method has been applied to rats, guinea-pigs and rabbits and is ideally suited to the accurate mapping of central catecholamine neurons and their processes.