Distribution of immunoreactivity for enkephalin, substance P and vasoactive intestinal peptide in fibres surrounding splanchnic sympathetic preganglionic neurons in rats

Neuropeptide coding of sympathetic preganglionic neurons; focus on adrenally projecting populations

Chemical coding of sympathetic preganglionic neurons (SPN) suggests that the chemical content of subpopulations of SPN can define their function. Since neuropeptides, once synthesized are transported to the axon terminal, most demonstrated chemical coding has been identified using immunoreactive terminals at the target organ. Here, we use a different approach to identify and quantify the subpopulations of SPN that contain the mRNA for pituitary adenylate cyclase activating polypeptide (PACAP) or enkephalin. Using double-labeled immunohistochemistry combined with in situ hybridization (ISH) we firstly identified the distribution of these mRNAs in the spinal cord and determined quantitatively, in Sprague-Dawley rats, that many SPN at the T4-T10 spinal level contain preproPACAP (PPP+, 80 ± 3%, n=3), whereas a very small percentage contain preproenkephalin (PPE+, 4 ± 2%, n=4). A similar neurochemical distribution was found at C8-T3 spinal level. These data suggest that PACAP potentially regulates a large number of functions dictated by SPN whereas enkephalins are involved in few functions. We extended the study to explore those SPN that control adrenal chromaffin cells. We found 97 ± 5% of adrenally projecting SPN (AP-SPN) to be PPP+ (n=4) with only 47 ± 3% that were PPE+ (n=5). These data indicate that adrenally projecting PACAPergic SPN regulate both adrenal adrenaline (Ad) and noradrenaline (NAd) release whereas the enkephalinergic SPN subpopulation must control a (sub) population of chromaffin cells - most likely those that release Ad. The sensory innervation of the adrenal gland was also determined. Of the few adrenally projecting dorsal root ganglia (AP-DRG) observed, 74 ± 12% were PPP+ (n=3), whereas 1 ± 1% were PPE+ (n=3). Therefore, if sensory neurons release peptides to the adrenal medulla, PACAP is most likely involved. Together, these data provide a neurochemical basis for differential control of sympathetic outflow particularly that to the adrenal medulla.

Enkephalin-immunoreactive interneurons extensively innervate sympathetic preganglionic neurons regulating the pelvic viscera

Enkephalin (ENK)-immunoreactive (IR) axons occur in regions containing spinal autonomic neurons and endogenous opiates contribute to spinal regulation of bladder function. To identify possible spinal sites of opiate action, we used immunocytochemistry for ENK with retrograde tracing from the major pelvic ganglion (MPG), a key location for postganglionic neurons controlling pelvic viscera, with cholera toxin B subunit (CTB) or CTB-horseradish peroxidase (CTB-HRP). We compared the relationship of ENK-IR axons with sympathetic preganglionic neurons (SPNs) projecting to the MPG between intact spinal cords and cords with 2- or 11-week complete transections between thoracic segments 4 and 5. By light microscopy, sections of intact cord showed dense networks of ENK-IR axons surrounding CTB-IR SPNs in the intermediolateral cell column (IML), intercalated nucleus, and central autonomic area of lower thoracic and upper lumbar cord. This staining pattern was similar in rats with 2- or 11-week transections. Ultrastructurally, ENK-IR axons formed synapses on SPNs in all three autonomic subnuclei of intact cord. In the IML, ENK-IR varicosities contributed 52% of the synapses on the somata of MPG-projecting SPNs. In 2-week transected cord, synapses from ENK-IR axons persisted on SPNs and the proportion of input to IML SPNs had increased to 67%, probably reflecting loss of supraspinal input. These results suggest that endogenous opioids could play a major role in controlling sympathetic outflow to the bladder through a direct action on SPNs. The persistence of the dense ENK innervation after complete cord transection indicates that the ENK-IR input to SPNs arises predominantly from intraspinal sources.

Identification of active thoracic spinal segments responsible for tonic and bursting sympathetic discharge in neonatal rats

The isolated thoracic cord of a neonatal rat in vitro generates tonic sympathetic activities in the splanchnic nerves. This tonic sympathetic nerve discharge (SND) has a prominent quasi-periodic oscillation at approximately 1-2 Hz. Bath application of bicuculline and strychnine, which removes endogenous GABA(A) and glycine receptor activities, transforms the quasi-periodic tonic SND into synchronized bursts (bSND). Picrotoxin, another GABA(A) receptor antagonist, also induces bSND. Serial transections of the thoracic cord (T1-12) were performed to identify the cord segments responsible for these tonic and bursting SNDs. Removal of T1-5 did not affect tonic SND. Nerve-cord preparation with either T6-8 or T10-12 segments could generate a substantial amount of tonic SND that retained comparable oscillating patterns. On the other hand, removal of T1-5 significantly reduced bSND amplitude without affecting its rhythmicity. Either T6-8 or T10-12 segments alone could generate bSND. Mid-point transection of T6-12 at T9 might split bSND rhythmogenesis, leading to the occurrence of bSND that could be attributed to two independent oscillators. Our results demonstrated that three segments within the T6-12 cord were sufficient to generate a rudimentary tonic and bursting SNDs. The thoracic cord segments, however, are dynamically interacting so that a full size bSND could only be produced with the intact thoracic cord.

Preproenkephalin mRNA is expressed by C1 and non-C1 barosensitive bulbospinal neurons in the rostral ventrolateral medulla of the rat

The autonomic regions of the thoracolumbar spinal cord receive a dense enkephalinergic (ENK) innervation from supraspinal sources, including the rostral ventrolateral medulla (RVLM). In the present study, we sought to determine whether the barosensitive bulbospinal (BSBS) neurons of the RVLM express preproenkephalin (PPE) mRNA. After injection of Fluoro-Gold (FG) into the upper thoracic spinal cord, neurons with PPE mRNA (PPE(+) neurons) were retrogradely labeled throughout the ventrolateral medulla. At the most rostral RVLM level, 29% of bulbospinal PPE+ cells were tyrosine hydroxylase-immunoreactive (TH-ir) and the latter constituted 19.4% of the bulbospinal TH-ir cells. We determined whether the bulbospinal PPE(+) RVLM neurons are barosensitive in two ways. First, we examined Fos production by FG-labeled RVLM neurons after 2 hours of hydralazine-induced hypotension (to 73 +/- 2 mm Hg) in conscious rats. Hydralazine (10 mg/kg i.v.) increased the number of Fos-ir neurons by two- to eightfold at all levels of the ventrolateral medulla examined. In the RVLM, 54% of bulbospinal PPE(+) neurons were Fos-ir, whereas such cells were more rarely found at caudal ventrolateral medullary levels. Second, we recorded individual BSBS RVLM units extracellularly in anesthetized rats and filled them juxtacellularly with biotinamide. Most biotinamide-filled neurons were PPE(+) (10 of 17), and the PPE(+) BSBS cells had a faster axonal conduction velocity than those without PPE mRNA (4.2 vs. 0.67 m/sec). Four of the 10 PPE(+) BSBS RVLM neurons were TH-ir. In summary, PPE mRNA is predominantly expressed by RVLM BSBS neurons with lightly myelinated spinal axons. PPE mRNA is present in most noncatecholaminergic BSBS neurons and also in approximately 20% of the bulbospinal C1 neurons. BSBS RVLM neurons most likely provide a major ENK input to sympathetic preganglionic neurons and PPE mRNA is the first identified positive phenotype of the non-C1 BSBS RVLM neurons.

Differential distribution of nerve terminals immunoreactive for substance P and cholecystokinin in the sympathetic preganglionic cell column of the filefish Stephanolepis cirrhifer

Immunoreactivity for substance P and cholecystokinin-8 was examined in the nerve fibers in the central autonomic nucleus, a cell column for sympathetic preganglionic neurons, in the filefish Stephanolepis cirrhifer. Substance P-immunoreactive fibers were distributed throughout the entire rostrocaudal extent, but were more abundant in the caudal part of the column, where substance P-immunoreactive varicosities sometimes made contacts with the sympathetic preganglionic neurons. Cholecystokinin-8-immunoreactive fibers were found almost entirely in the rostral part of the column, where a dense network of varicosities was in close apposition to a considerable number of the sympathetic preganglionic neurons. Double labeling immunohistochemistry showed that substance P fibers and cholecystokin-8 fibers were entirely different, and distinct from serotonin-immunoreactive fibers. By using immunoelectron microscopy, synaptic specialization was sometimes observed between the dendrites of preganglionic neurons and varicosities immunoreactive for substance P and cholecystokinin-8. Substance P- and cholecystokinin-8 fibers were seen from the descending trigeminal tract, through the dorsolateral funiculus and the ventral portion of the dorsal horn, to the central autonomic nucleus. After colchicine treatment, substance P-immunoreactive perikarya were found in the cranial and spinal sensory ganglia. These results suggest that the sympathetic preganglionic neurons of the filefish receive innervation by substance P fibers and cholecystokinin fibers, and that the former might be of primary sensory origin. Topographical distribution of cholecystokinin-8-immunoreactive terminals in the central autonomic nucleus along the rostrocaudal extent might underlie the differential regulation of sympathetic activity via a distinct population of sympathetic preganglionic neurons.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

Co-localization of substance P and dopamine beta-hydroxylase with growth-associated protein-43 is lost caudal to a spinal cord transection

After spinal cord injury, abnormal responses of spinal cord neurons to sensory input lead to conditions such as autonomic dysreflexia, urinary bladder dyssynergia, muscle spasticity and chronic pain syndromes. These responses suggest that the spinal cord undergoes marked reorganization after an injury. In previous studies, we demonstrated changes in individual patterns of immunoreactivity for growth-associated protein-43, dopamine beta-hydroxylase and substance P that suggest growth and/or changes in expression of neurotransmitter enzymes and peptides in the cord caudal to a transection injury. In the present study we determined whether (i) growth-associated protein-43 and dopamine beta-hydroxylase or substance P were co-expressed in the same neurons prior to cord injury, and (ii) these patterns of expression changed after injury. A change in co-localization patterns caudal to an injury would suggest diversity in responses of different populations of spinal neurons. We used double-labelling immunocytochemistry to determine whether either dopamine beta-hydroxylase or substance P was co-localized with growth-associated protein-43 in control rats and in rats one, two or six weeks after spinal cord transection. We focused on the intermediate gray matter, especially the sympathetic intermediolateral cell column. In control rats, fibres travelling in a stereotyped ladder-like pattern in the thoracic gray matter contained growth-associated protein-43 co-localized with dopamine beta-hydroxylase or substance P. In spinal rats, such co-localization was also observed in spinal cord segments rostral to the cord transection. In contrast, caudal to the transection, substance P and growth-associated protein-43 were found in separate reticular networks. Immunoreactivity for dopamine beta-hydroxylase disappeared in fibres during this time, but was clearly present in somata. Immunoreactivity for growth-associated protein-43 was also found in somata, but never co-localized with that for dopamine beta-hydroxylase. These observations demonstrated co-localization of growth-associated protein-43 with dopamine beta-hydroxylase and substance P in descending spinal cord pathways. Caudal to a cord transection, this co-localization was no longer found, although each substance was present either in an abundant neural network or in somata. One population of spinal neurons responded to cord injury by expressing the growth-associated protein, whereas two others changed in the intensity of their expression of neurotransmitter peptides or enzymes or in the abundance of fibres expressing them. Thus, three populations of spinal neurons had distinct responses to cord injury, two of them increasing their potential input to spinal sensory, sympathetic or motor neurons. Such responses would enhance transmission through spinal pathways after cord injury.

Identification of lamina V and VII interneurons presynaptic to adrenal sympathetic preganglionic neurons in rats using a recombinant herpes simplex virus type 1

Although indirect evidence suggests that the control of sympathetic preganglionic neurons is mediated to a great extent through interneurons, little is known about the location, morphology or neurotransmitter phenotype of such interneurons. This limitation seriously impedes our understanding of spinal synaptic circuits crucial to control of arterial pressure and other visceral functions. We used a highly neurotropic, minimally cytopathic recombinant herpes simplex virus type-1 to study spinal "sympathetic" interneurons labelled by trans-synaptic transport of the virus from the adrenal gland in rats. Approximately 120-320 infected neurons/rat were identified by immunocytochemical detection of the viral antigen. We distinguished between virus-infected preganglionic neurons and infected interneurons by (i) their location within the spinal laminae, (ii) their size and shape and (iii) the presence or absence of immunoreactivity for the acetylcholine-synthesizing enzyme, choline acetyltransferase, a marker of sympathetic preganglionic neurons. Virus-labelled sympathetic preganglionic neurons were found within the known spinal preganglionic nuclei. Non-cholinergic, virus-labelled neurons were located throughout lamina VII and in the ventral portion of lamina V. These putative interneurons were found in the major spinal preganglionic nuclei, usually intermingled with the preganglionic neurons. Sometimes, they were located in clusters separate from the preganglionic neurons. The interneurons were approximately 15 microm in diameter, smaller than the average preganglionic neuron (diameter=25 microm), and had a few fine processes emanating from them. These non-cholinergic interneurons constituted approximately one-half of the population of virus-infected neurons. In summary, with the use of a recombinant herpes simplex virus, we identified a large number of non-cholinergic interneurons close to, or intermingled with, adrenal sympathetic preganglionic neurons. The neurotransmitter phenotype of these neurons remains to be determined but they likely integrate much of the supraspinal and primary afferent inputs to spinal preganglionic neurons that control arterial pressure and other visceral functions.

Changes in immunoreactivity for growth associated protein-43 suggest reorganization of synapses on spinal sympathetic neurons after cord transection

Cervical or high thoracic spinal cord injury often results in autonomic dysreflexia, a condition characterized by exaggerated spinal reflexes and episodic hypertension, that may be caused by reorganization of synapses on sympathetic preganglionic neurons after loss of supraspinal input. To assess remodelling of synaptic input to identified preganglionic neurons, immunoreactivity for growth associated protein-43 was examined by fluorescent and electron microscopy in control rats with intact spinal cords and in rats seven to 30 days after midthoracic cord transection. This protein is found in mature bulbospinal axons that supply spinal sympathetic nuclei and it is also known to be up-regulated in growing or sprouting axons. In the thoracic cord of control rats, fibres containing growth associated protein-43 surrounded histochemically- or retrogradely-labelled preganglionic neurons and formed a ladder-like pattern in the gray matter. Fibres travelled rostrocaudally along the lateral horn and, at approximately regular intervals, they coursed mediolaterally to form "rungs" of a ladder. Electron microscopy revealed concentrated growth associated protein-43 in many intervaricose axon segments in the intermediolateral cell column. Less frequently, faint immunoreactivity for this protein was found in varicosities, some of which synapsed on retrogradely-labelled sympathoadrenal preganglionic neurons. Electron microscopy of conventionally processed tissue was used to determine the time-course of degeneration of severed axon terminals in the intermediolateral cell column. In spinal rats, terminals with ultrastructural signs of degeneration were numerous in the intermediolateral cell column three days after transection, but were rare at seven days and absent at 14 days. Degenerating terminals were never found in this region in control rats. Thus virtually all supraspinal inputs to preganglionic neurons had been eliminated by seven days after transection. At longer times after injury, terminals containing immunoreactivity for growth associated protein-43 must therefore arise from intraspinal neurons. The distribution of fibres immunoreactive for growth associated protein-43 changed markedly in the first 30 days after cord transection. By 14 days, the ladder-like pattern was distorted rostral to the transection by enlarged masses of immunoreactive fibres surrounding preganglionic neurons, suggesting sprouting of bulbospinal or intraspinal axons or accumulation of this protein in their terminals after the parent axon had been severed. Caudal to the transection, the ladder-like arrangement of fibres was completely replaced by a reticular network of immunoreactive fibres that extended throughout the intermediate gray matter and increased in density between 14 and 30 days. In the intermediolateral cell column, at fourteen days after transection, axons with the ultrastructural features of growth cones contained intense growth associated protein-43 immunoreactivity. Although varicosities of bulbospinal axons containing this protein had degenerated by 14 days, weak immunoreactivity was still found in varicosities that synapsed on labelled sympathoadrenal neurons. Furthermore, immunoreactivity appeared in numerous somata of presumed interneurons throughout the intermediate gray matter by 14 days and the number of somata increased by 30 days. These interneurons may be the source of this protein in the reticular network, and in growth cones and synapses. The loss of supraspinal inputs by seven days after cord transection, and the new intraspinal network of immunoreactive fibres, synapses and cells are consistent with new synapse formation on preganglionic neurons. New synpases on preganglionic neurons may be crucial for the development of autonomic dysreflexia.

Catecholamine enzymes and neuropeptides are expressed in fibres and somata in the intermediate gray matter in chronic spinal rats

Spinal cord injury disrupts control of sympathetic preganglionic neurons because bulbospinal input has been lost and the remaining regulation is accomplished by spinal circuits consisting of dorsal root afferent and spinal neurons. Moreover, an initial retraction and regrowth of dendrites of preganglionic neurons in response to deafferentation creates the potential for remodelling of spinal circuits that control them. Although catecholamines and neuropeptide Y are found in descending inputs to the preganglionic neurons, their presence in spinal circuits has not been established. Spinal circuits controlling preganglionic neurons contain substance P but participation of these peptidergic neurons in remodelling responses has not been examined. Therefore, we compared immunoreactivity for the catecholamine-synthesizing enzyme dopamine beta-hydroxylase, for neuropeptide Y and for substance P in the intermediate gray matter of the spinal cord in control rats and in rats seven or fourteen days after transection at the fourth thoracic cord segment. Sympathetic preganglionic neurons were retrogradely labelled by intraperitoneal injection of the tracer FluoroGold. These experiments yielded three original findings. 1) At one and two weeks after cord transection, fibres and terminals immunoreactive for dopamine beta-hydroxylase and neuropeptide Y were consistently found in the intermediolateral cell column in segments caudal to the transection. The area of fibres and terminals containing these immunoreactivities was markedly reduced compared to control rats or to segments rostral to the transection in the spinal rats. 2) Immunoreactivity for substance P was increased after cord transection and the distribution of fibres immunoreactive for this peptide in segments caudal to the transection extended more widely through the intermediate gray matter. These reactions demonstrated a plastic reaction to cord transection by spinal neurons expressing substance P. 3) Dopamine beta-hydroxylase expression was up-regulated in somata within the intermediate gray matter of spinal segments caudal to the transection. The numbers of somata immunoreactive for this enzyme increased six-fold by 14 days after cord transection, compared to the few somata counted in control rats. In conclusion, the presence of a catecholamine synthesizing enzyme and neuropeptides in fibres surrounding sympathetic preganglionic neurons caudal to a cord transection suggests a source of catecholamines and these peptides within spinal circuits in the chronic spinal rat. The presence of dopamine beta-hydroxylase in a markedly greater number of neuronal somata after cord transection reflects significant up-regulation of gene expression and may indicate a switch by these neurons to an adrenergic phenotype, revealing a plastic response to injury within the spinal cord.

Spinal control of penile erection

Smooth muscle relaxation of penile arteries, the corpus cavernosum, and the corpus spongiosum, leading to penile erection, results from parasympathetic neural pathway activation and, likely, simultaneous inhibition of sympathetic outflow. Proerectile parasympathetic outflow is reflexively activated by sensory information of peripheral origin, conveyed by the dorsal penile nerve, and reflexive erections are supported by an intraspinal circuitry. Supraspinal influences modulate the reflex. Information integrated at or originating from supraspinal structures may also elicit penile erection. Several neurotransmitters are involved in either the modulation of the spinal reflex or the mediation of supraspinal influences. Spinal cord injury differently alters reflexive penile erection or erection from a central origin, depending on the neurologic level of injury.

Distribution of immunoreactivity for the NK1 receptor on different subpopulations of sympathetic preganglionic neurons in the rat

The distribution of immunoreactivity to the receptor for substance P, the neurokinin 1 (NK1) receptor, was examined in preganglionic sympathetic neurons of the rat by using immunohistochemistry and retrograde neuronal tracing. About one-third of all sympathetic preganglionic neurons were NK1 receptor immunoreactive, and most of the NK1 receptor-immunoreactive neurons were also nitric oxide synthase immunoreactive. The proportions of sympathetic preganglionic neurons projecting to the superior and inferior mesenteric ganglia, adrenal gland, and lumbar sympathetic chain which were NK1 receptor-immunoreactive were determined. Most (89%) of the preganglionic neurons projecting to the adrenal glands were NK1 receptor immunoreactive. Few (17%) of the preganglionic neurons projecting to the L5 sympathetic chain ganglion were immunoreactive for the receptor, while preganglionic neurons projecting to the prevertebral ganglia were NK1 receptor immunoreactive at intermediate frequencies (61-64%). Thus, substance P acting on NK1 receptors is likely to be important in the preganglionic pathways to the adrenal medulla and viscera via the prevertebral ganglia, but is unlikely to be important in pathways to the lumbar sympathetic chain. The co-localisation of the NK1 receptor with the enzyme nitric oxide synthase was also examined. The majority of NK1 receptor-immunoreactive neurons were also nitric oxide synthase immunoreactive. Thus NK1 receptors occur on preganglionic neurons over many spinal segments and in a range of preganglionic pathways, as well as in a range of combinations with nitric oxide synthase. The heterogeneity of preganglionic neurons showing NK1 receptor immunoreactivity may reflect the involvement of NK1-mediated transmission in a variety of functional pathways, most notably the preganglionic projections to the adrenal medulla and to the viscera.

A non-cholinergic transmitter, pituitary adenylate cyclase-activating polypeptide, utilizes a novel mechanism to evoke catecholamine secretion in rat adrenal chromaffin cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) is the most potent non-cholinergic neurotransmitter to stimulate catecholamine secretion from rat chromaffin cells; however, the mechanism of action is not clear. We used amperometric detection of exocytosis and indo-1 monitoring of [Ca2+]i to identify PACAP actions in cultured chromaffin cells. PACAP (100 nM) required external Ca2+ to evoke secretion. However, unlike nicotine and KCl which caused immediate and relatively brief secretion, PACAP has a latency of 6.8 +/- 0.96 s to the first secretory response and secretion continued for up to 2 min. PACAP elevation of [Ca2+]i showed similar latency and often remained above base line for several minutes following a brief exposure. ZnCl2 (100 microM) selectively inhibited PACAP-stimulated secretion and [Ca2+]i with little effect on nicotine-evoked responses. Nifedipine (10 microM) had little effect on PACAP-evoked secretion but inhibited nicotine-evoked secretion by more than 80%, while omega-conotoxin (100 nM) failed to affect either agonist. PACAP-stimulated cAMP levels required 5 s to significantly increase, consistent with the latency of exocytotic and Ca2+ responses. Forskolin (10 microM) caused responses similar to PACAP. PACAP-evoked exocytosis was blocked by the protein kinase A inhibitor adenosine 3'5'-cyclic monophosphorothioate Rp-diastereomer (Rp-cAMPS). These data showed that PACAP stimulates exocytosis by a mechanism distinctly different from cholinergic transmitters that appears to involve cAMP-mediated Ca2+ influx. Differences in receptor coupling mechanisms and pharmacology of Ca2+ entry stimulated by cholinergic and peptidergic agonists support the idea that the peptidergic system maintains catecholamine secretion under conditions where the cholinergic system desensitizes or otherwise fails.

Secretoneurin-immunoreactivity in nerve terminals apposing identified preganglionic sympathetic neurons in the rat: colocalization with substance P and enkephalin

Preganglionic sympathetic neurons projecting to the superior cervical ganglion are innervated by nerve fibers containing classical neurotransmitters as well as neuropeptides. In this study we examined the possible participation of a novel peptide, secretoneurin (a cleavage product of secretogranin II), in regulation of sympathetic outflow to head and neck by using a retrograde labelling-technique combined with immunohistochemistry. In addition, the coexistence of secretoneurin with substance P and leu-enkephalin, peptides known to innervate preganglionic neurons, was investigated. The majority of retrogradely labeled neurons were localized in the nucleus intermediolateralis of spinal cord segments T1-T3 (maximum at T2). Nearly all of Fast Blue positive neuronal perikarya were apposed by nerve fibers and terminals exhibiting immunoreactivity to secretoneurin. The main secretoneurin-immunoreactive form found in the upper thoracic segments corresponded to the free peptide secretoneurin as revealed by chromatography and radioimmunoassay. More than half of labeled neurons were surrounded by nerve endings containing in addition substance P or leu-enkephalin which were also, however, less frequently colocalized. Our results suggest that secretoneurin influences the activity of preganglionic sympathetic neurons projecting to the superior cervical ganglion. Regarding their frequent colocalization with substance P and leu-enkephalin, functional interactions of these peptides on preganglionic sympathetic nerve activity have to be considered.

Preganglionic sympathetic neurones, innervating the guinea pig adrenal medulla, immunohistochemically contain choline acetyltransferase and also leu-enkephalin

Applying retrograde neuronal tracing combined with double labelling immunofluorescence, preganglionic nerve cell bodies in the intermediate grey matter of the guinea pig thoracic spinal cord, projecting to the adrenal gland, co-exhibited immunolabelling for choline-acetyltransferase (ChAT) and sometimes, also for leu-enkephalin. Likewise, ChAT-immunoreactive nerve fibres, forming a dense meshwork in the adrenal medulla, partly contained immunostaining also for leu-enkephalin. Some of the intramedullary nerve cell bodies were ChAT-positive but were non-reactive for leu-enkephalin. The findings provide evidence for an extrinsic (preganglionic) and an intrinsic (postganglionic) cholinergic nerve system in the guinea pig adrenal medulla, the preganglionic system utilising leu-enkephalin as co-mediator.

Identification of spinal interneurons antecedent to adrenal sympathetic preganglionic neurons using trans-synaptic transport of herpes simplex virus type 1

Control of sympathetic preganglionic neurons appears to be mediated, in part, through polysynaptic pathways using spinal interneurons. To identify spinal interneurons antecedent to adrenal sympathetic preganglionic neurons, we injected herpes simplex virus type 1 into the adrenal gland of hamsters as this virus is an effective trans-synaptic tracer of neural pathways. After a three day survival period, immunocytochemistry was used to visualize virus-infected spinal cord cells. Infected sympathetic preganglionic neurons with somata that were either kite-shaped, elliptical or fusiform and that had extensive dendrite arbors were identified as well as a group of smaller round cells with finer processes. For comparison, in additional hamsters, labelling with the retrograde tracer Fluoro-Gold and histochemical reactions for the enzyme nicotinamide adenine dinucleotide phosphate-diaphorase were used to identify sympathetic preganglionic neurons. Sympathetic preganglionic neurons identified with Fluoro-Gold or herpes virus were present mostly in the nucleus intermediolateralis, pars intermediolateralis and nucleus intermediolateralis, pars funicularis of the spinal cord. The smaller herpes virus-infected cells were found mostly medial to the preganglionic neurons in lamina VII and also dorsally in lamina V of the spinal cord. Assessing immunoreactivity for glial fibrillary acidic protein demonstrated that the smaller herpes virus-infected cells were not reactive astrocytes. Furthermore, these cells were immunoreactive for two neuronal markers, neuron-specific enolase and for microtubule-associated protein 2. These findings suggest that these smaller round cells with finer processes are distinct from sympathetic preganglionic neurons and astrocytes and may be interneurons antecedent to the sympathetic preganglionic neurons.

Reflex and morphological changes in spinal preganglionic neurons after cord injury in rats

Autonomic dysreflexia manifested as episodic hypertension after spinal cord injury may occur because of changes in sympathetic preganglionic neurons (SPNs) in response to loss of bulbospinal inputs. We studied dysreflexia in rats one week after midthoracic spinal cord hemisection or complete transection. After cord hemisection at the fifth thoracic segment all rats had hemiparaplegia and after complete transection they were paraplegic and exhibited dysreflexia characterized by pressor responses to distension of the urinary bladder. Changes in morphology of SPNs retrogradely labelled by cholera toxin and Fluoro Gold were examined and changes also were assessed in expression of the synaptic vesicular protein synaptophysin. A comparison of SPNs rostral and caudal to the lesion revealed significant dendritic degeneration and decreased soma size after the loss of supraspinal input. Expression of synaptophysin was normally observed rostral to a cord hemisection but this immunoreactivity was increased caudal to the lesion. In conclusion significant structural changes in SPNs occur within a week after cord injury. The abnormal cardiovascular control and exaggerated reflex reactions may be due to new synapse formation on these SPNs.

Central sympathetic mechanisms of blood pressure control in hamsters

The goal of this study was to investigate central vasomotor control of blood pressure in golden hamsters. Electrophysiological experiments demonstrated that tonic and reflex firing of renal nerves was controlled by brainstem and spinal circuits in manner similar to control of these nerves in rats, rabbits and cats. These findings confirmed that autonomic neural circuits for vasomotor control in hamsters are functionally similar to those of other well-studied species.