Descending phenylethanolamine-N-methyltransferase projections to the monkey spinal cord: an immunohistochemical double labeling study

Insulin-responsive autonomic neurons in rat medulla oblongata

Low blood glucose activates brainstem adrenergic and cholinergic neurons, driving adrenaline secretion from the adrenal medulla and glucagon release from the pancreas. Despite their roles in maintaining glucose homeostasis, the distributions of insulin-responsive adrenergic and cholinergic neurons in the medulla are unknown. We fasted rats overnight and gave them insulin (10 U/kg i.p.) or saline after 2 weeks of handling. Blood samples were collected before injection and before perfusion at 90 min. We immunoperoxidase-stained transverse sections of perfused medulla to show Fos plus either phenylethanolamine N-methyltransferase (PNMT) or choline acetyltransferase (ChAT). Insulin injection lowered blood glucose from 4.9 ± 0.3 mmol/L to 1.7 ± 0.2 mmol/L (mean ± SEM; n = 6); saline injection had no effect. In insulin-treated rats, many PNMT-immunoreactive C1 neurons had Fos-immunoreactive nuclei, with the proportion of activated neurons being highest in the caudal part of the C1 column. In the rostral ventrolateral medulla, 33.3% ± 1.4% (n = 8) of C1 neurons were Fos-positive. Insulin also induced Fos in 47.2% ± 2.0% (n = 5) of dorsal medullary C3 neurons and in some C2 neurons. In the dorsal motor nucleus of the vagus (DMV), insulin evoked Fos in many ChAT-positive neurons. Activated neurons were concentrated in the medial and middle regions of the DMV beneath and just rostral to the area postrema. In control rats, very few C1, C2, or C3 neurons and no DMV neurons were Fos-positive. The high numbers of PNMT-immunoreactive and ChAT-immunoreactive neurons that express Fos after insulin treatment reinforce the importance of these neurons in the central response to a decrease in glucose bioavailability.

Molecular cloning and characterization of an L-epinephrine transporter from sympathetic ganglia of the bullfrog, Rana catesbiana

Chemical signaling by dopamine (DA) and L-norepinephrine (L-NE) at synapses is terminated by uptake via specialized presynaptic transport proteins encoded by the DA transporter (DAT) and L-NE transporter (NET) genes, respectively. In some vertebrate neurons, particularly the sympathetic neurons of amphibians, L-NE is converted to L-epinephrine (L-Epi, adrenaline) and released as the primary neurotransmitter. Although evidence exists for a molecularly distinct L-Epi transporter (ET) in the vertebrate brain and peripheral nervous system, a transporter specialized for extracellular L-Epi clearance has yet to be identified. To pursue this issue, we cloned transporter cDNAs from bullfrog (Rana catesbiana) paravertebral sympathetic ganglia and characterized functional properties via heterologous expression in non-neuronal cells. A cDNA of 2514 bp (fET) was identified for which the cognate 3.1 kb mRNA is highly enriched in frog sympathetic ganglia. Sequence analysis of the fET cDNA reveals an open reading frame coding for a protein of 630 amino acids. Inferred fET protein sequence bears 75, 66, and 48% amino acid identity with human NET, DAT, and the 5-hydroxytryptamine transporter (SERT), respectively. Transfection of fET confers Na+- and Cl--dependent catecholamine uptake in HeLa cells. Uptake of [3H]-L-NE by fET is inhibited by catecholamines in a stereospecific manner. L-Epi and DA inhibit fET-mediated [3H]-L-NE uptake more potently than they inhibit [3H]-L-NE uptake by human NET (hNET), whereas L-NE exhibits equivalent potency between the two carriers. Moreover, fET exhibits a greater maximal velocity (Vmax) for the terminal products of catecholamine biosynthesis (L-Epi > L-NE >> DA), unlike hNET, in which a Vmax rank order of L-NE > DA > L-Epi is observed. fET-mediated transport of catecholamines is sensitive to cocaine and tricyclic antidepressants, with antagonist potencies significantly correlated with hNET inhibitor sensitivity. Amino acid conservation and divergence of fET with mammalian catecholamine transporters help define residues likely to be involved in catecholamine recognition and translocation as well as blockade by selective reuptake inhibitors.

Descending adrenergic input to the primate spinal cord and its possible role in modulation of spinothalamic cells

The present study focuses on 3 different aspects of the descending adrenergic system in the primate: (1) the distribution of adrenergic fibers and terminals in the spinal cord, (2) the source of this input and (3) the possible physiological effects of this system on spinal nociceptive processing. Antibodies to the enzyme phenylethanolamine-N-methyltransferase (PNMT) were employed to map the distribution of epinephrine-containing axonal profiles in the primate spinal cord. Smooth longitudinally oriented fibers were localized to the outer edge of the lateral funiculus. PNMT-containing axonal enlargements were distributed to the superficial dorsal horn, intermediate gray matter and the region surrounding the central canal at all spinal cord levels. PNMT-immunostained profiles were also observed in the intermediolateral cell column. A double labeling study employing retrograde transport of HRP from the spinal cord and PNMT immunohistochemistry identified a small population of HRP-PNMT-labeled neurons in the 'C1' region at the levels of the medulla and ponto-medullary junction. Thus, these cells are a probable source of adrenergic input to the spinal cord. Electrophysiological studies demonstrated that iontophoresis of epinephrine onto identified primate spinothalamic tract neurons in the lumbar dorsal horn resulted in inhibition of the glutamate-induced firing of these cells. The data from these studies support the hypothesis that adrenergic (PNMT-containing) cells in the caudal brainstem project to all levels of the cord and may contribute to descending modulation of nociceptive processing at these levels.

Four groups of tyrosine hydroxylase-immunoreactive neurons in the ventrolateral medulla of rats, guinea-pigs and cats identified on the basis of chemistry, topography and morphology

The data in the preceding paper [Halliday G. M. and McLachlan E. M. (1991) Neuroscience 43, 531-550] suggest that some neurons in the rostral ventrolateral medulla contain some catecholamine-synthesizing enzymes but may not produce catecholamines. The present study addresses this question directly by comparing the anatomical location and morphology of these neurons with those revealed by formaldehyde-induced fluorescence. Catecholamine-containing somata of rats and guinea-pigs have been demonstrated following FAGLU-perfusion in normal untreated animals, in animals pretreated with pargyline (a monoamine oxidase inhibitor), and in animals pretreated with colchicine (to block axoplasmic transport). The number and location of fluorescent somata in the ventrolateral medulla have been determined in serial coronal sections of tissue from the cervical spinal cord to the level of the facial nucleus. Catecholamine-fluorescent neurons at different levels of the ventrolateral medulla varied in their topography and sensitivity to pharmacological manipulation. However, the rostrocaudal distributions in rats and guinea-pigs were quantitatively remarkably similar implying that homologous groups of catecholamine-containing neurons exist. Comparison between these distributions and those of somata stained immunohistochemically for catecholamine-synthesizing enzymes and neuropeptide Y [Halliday G. M. and McLachlan E. M. (1991) Neuroscience 43, 531-550] revealed that the majority of fluorescent neurons in both species probably contain dopamine-beta-hydroxylase and neuropeptide Y as well as tyrosine hydroxylase. Those neurons lying just caudal to the facial nucleus immunoreactive for tyrosine hydroxylase and phenylethanolamine-N-methyltransferase but not dopamine-beta-hydroxylase and neuropeptide Y also lack catecholamine fluorescence. This rostral group of somata can be identified immunohistochemically in cats. The size and morphology of catecholamine-fluorescent neurons have been analysed in detail, and compared with the same features of the immunohistochemically stained neurons. Three morphological types of catecholamine-containing neurons could be distinguished in material prepared by both techniques from rats and guinea-pigs, and in immunohistochemical material from cats. Rostral tyrosine hydroxylase-positive neurons, which differed morphologically from these three types, were present in all three species. On the basis of anatomical location, neuronal morphology and chemical characteristics, four groups of tyrosine hydroxylase-immunoreactive neurons have been identified in the ventrolateral medulla of rats, guinea-pigs and cats. Only the caudal three of these four groups appear to synthesize catecholamine, probably noradrenaline. From published data it seems likely that these four groups of tyrosine hydroxylase-positive neurons have distinct projections and functions related to cardiovascular and respiratory control.

Direct catecholaminergic innervation of primate spinothalamic tract neurons

Catecholaminergic axonal varicosities identified by immunocytochemical staining for dopamine-beta-hydroxylase were observed at the light microscopic level apposing the somata of retrogradely labeled spinothalamic tract neurons in the monkey spinal cord. Three retrogradely labeled and two intracellularly labeled spinothalamic neurons were serially sectioned and examined at selected intervals at the electron microscopic level. Electron microscopic study revealed that axonal boutons directly contacted the somata and/or dendrites of lamina I, IV, and V spinothalamic tract neurons. All of the profiles apposing one of the retrogradely labeled lamina I spinothalamic tract neurons were categorized from eight planes of section spaced at 1-micron intervals. Of the 305 profiles counted that were adjacent to this soma, 17 (5.6%) stained positively for dopamine-beta-hydroxylase. Of these 17 appositions, three were followed in serial sections to confirm that they had synaptic thickenings and alignment of vesicles along the membrane contacting the spinothalamic tract soma. Catecholaminergic boutons were observed apposing the somata and dendrites of intracellularly filled STT cells characterized as high threshold and wide dynamic range neurons. These observations clearly indicate a direct innervation of spinothalamic tract neurons by catecholaminergic neurons, providing anatomical data to support previous physiological findings demonstrating that catecholamines modulate nociceptive transmission.

Quantitative analysis of spinally projecting adrenaline-synthesising neurons of C1, C2 and C3 groups in rat medulla oblongata

Spinal projections of the phenylethanolamine N-methyltransferase (PNMT) immunoreactive neurons of the medulla were investigated using a combination of immunohistochemistry and retrograde transport of colloidal gold particles conjugated to cholera toxin B subunit (CTB-gold). The PNMT-containing adrenergic neurons were localised in three groups, the C1 group in the rostral ventrolateral medulla, the C2 group in the nucleus tractus solitarius/dorsal vagal motor complex in the dorsal medulla and the C3 group in the mediodorsal medulla. The C1 group contained 72% of the medullary PNMT-IR neurons, while C2 comprised 13% and C3 15% of the medullary PNMT-IR neuron population. CTB-gold was injected in the area of the intermediolateral cell column in either upper (T2-T4) or lower (T8-T9) thoracic spinal cord and retrogradely labelled cells were found in the areas of the C1, C2 and C3 groups and in other regions of the medulla which did not contain PNMT-IR neurons. After tracer injections bilaterally at levels T2-T4, retrograde labelling suggested that at least 21% of all medullary PNMT-IR neurons projected to these levels. As a proportion of each group, 26% of C1, 9% of C2 and 33% of C3 neurons projected spinally to T2-T4. After tracer injections bilaterally at levels T8-T9, retrograde labelling suggested that at least 17% of all medullary PNMT-IR neurons projected to these levels. As a proportion of each group, 16% of C1, 9% of C2 and 30% of C3 neurons projected spinally to T8-T9. These figures represent minimum numbers since it is impossible to ensure that every neuron has equal access to the tracer. The results demonstrate that contrary to previous belief, the PNMT-IR innervation of the spinal cord derives from PNMT-IR neurons in the dorsal medulla, as well as from the rostral ventrolateral medulla. Indeed 24% of the PNMT-IR neurons terminating at spinal cord levels T2-T4, and 35% of those terminating at levels T8-T9, derive from the dorsal (C2 and C3) medullary cell groups. Since the PNMT-IR projections are directed towards the intermediolateral cell column, it seems likely that all three groups of medullary adrenaline-containing neurons contribute to the regulation of sympathetic outflow.

Distribution of phenylethanolamine N-methyltransferase cell bodies, axons, and terminals in monkey brainstem: an immunohistochemical mapping study

Adrenaline (epinephrine) is an important candidate transmitter in descending spinal control systems. To date intrinsic spinal adrenergic neurons have not been reported; thus adrenergic input is presumably derived from brainstem sites. In this regard, the localization of adrenergic neurons in the brainstem is an important consideration. Maps of adrenergic cell bodies and to a lesser extent axons and terminal fields have been made in various species, but not in monkeys. Thus, the present study concerns the organization of adrenergic systems in the brainstem of a monkey (Macaca fascicularis) immunohistochemically mapped by means of an antibody to the enzyme phenylethanolamine N-methyltransferase (PNMT). PNMT-immunostained cell bodies are distributed throughout the medulla in two principal locations. One concentration of labeled cells is in the dorsomedial medulla and includes the nucleus of the solitary tract (NTS), the dorsal motor nucleus of the vagus (X), and an area ventral to X in a region of the reticular formation (RF) known as the central nucleus dorsalis (CnD) of the medulla. A few scattered cells are observed in the periventricular gray just ventral to the IVth ventricle and on midline in the raphe. The second major concentration of PNMT-immunostained cells is located in the ventrolateral RF, lateral and dorsolateral to the inferior olive (IO), including some cells in the rostral part of the lateral reticular nucleus (LRN). Terminal fields are located in the NTS, X, area postrema (AP), and the floor of the IVth ventricle in the medulla and pons. A light terminal field is also observed in the raphe, particularly raphe pallidus (RP). A heavy terminal field is present in locus coeruleus (LC). Fibers labeled for PNMT form two major fiber tracts. One is in the dorsomedial RF extending as a well-organized bundle through the medulla, pons, and midbrain. A second tract is located on the ventrolateral edge of the medulla and caudal pons. Fibers in this tract appear to descend to the spinal cord. A comparison with maps of other catecholamine neurons in primates is discussed, confirming that the distribution of the adrenergic system in monkeys is similar to that described in the human.

Anatomy of the adrenergic system in the medulla oblongata of the tree shrew: PNMT immunoreactive structures within the nucleus tractus solitarii

The adrenergic system in the medulla oblongata of tree shrews was investigated by immunocytochemistry with an antibody against phenylethanolamine-N-methyltransferase. Two groups of adrenergic cells, which are equivalent to those of other species, were detected: Group C1 in the ventrolateral medulla and group C2 in the dorsomedial medulla. Adrenergic cells in C1 are located around the lateral reticular nucleus or between its subdivisions. They are mostly multipolar with branched processes. In group C2, some immunoreactive cell bodies smaller than those in C1 and many nerve terminals are found in the motor nucleus of the vagus, but most of the adrenergic cells and fibers are observed in the nucleus tractus solitarii. The cytoarchitecture of this nucleus resembles that described before for the rhesus monkey. In contrast to the rat, the subnucleus gelatinosus, which according to other authors receives cardiac and gastric afferents, is a prominent structure in immunocytochemically as well as conventionally stained sections. Adrenergic cell bodies and their fibers form a ring around this nucleus, but no immunoreactive structures are found within it. In the dorsomedial part of the nucleus tractus solitarii, adrenergic neurons are accumulated. They are often located in close proximity to blood vessels. Elongated immunoreactive neurons in the medial subdivision of the nucleus also seem to project in the direction of the dorsal area. Our data give new information about the adrenergic system in the medulla oblongata, especially in the nucleus tractus solitarii of the tree shrew, a species that provides a useful model of a small primate brain.

Distribution of monoamine-synthesizing neurons in the human medulla oblongata

We have employed immunohistochemical and morphometric procedures to study the distribution of monoamine-synthesizing neurons in the medulla oblongata of the adult human, utilizing antibodies to tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase (PNMT), and phenylalanine hydroxylase (PH8). In the human brain, the antigen with which PH8 reacts occurs within neurons that presumably synthesize serotonin (Haan et al., '87). Neurons containing these antigens were mapped and counted in successive coronal sections with the aid of a computer-assisted procedure. The results indicate that monoamine-synthesizing neurons are distributed in the human brain in patterns broadly similar to those described for other species. TH-immunoreactive cells extended caudorostrally for approximately 32 mm commencing at the spinomedullary junction and ending 8 mm caudal to the pontomedullary junction. In coronal sections these TH-immunoreactive neurons were seen in the lateral medulla dorsal to the inferior olive extending in a continuous band to the dorsomedial medulla. Above the obex the majority of these cells apparently synthesize adrenaline since many PNMT-immunoreactive cells were also found in this region. There were few or no PNMT-immunoreactive cells caudal to the obex, indicating that the TH-immunoreactive cells in this region synthesize either noradrenaline or dopamine. Approximately 65% of these TH-immunoreactive neurons contained melanin pigment, whereas few or no PNMT-immunoreactive cells contained melanin pigment. PH8-immunoreactive cells extended throughout the rostrocaudal extent of the medulla oblongata (approximately 40 mm). In coronal sections the majority were found in the medullary raphe nuclei. However, many cells throughout the rostrocaudal extent of the medulla were found laterally intermingled with catecholamine-synthesizing neurons. Occasional neurons in the lateral medulla appeared to contain both PH8- and TH-immunoreactivity.