Specificity of nerve fibre 'attraction' to autonomic effector organs in tissue culture

Formation of contacts between mast cells and sympathetic neurons in vitro

Functional interactions between mast cells and peripheral nerves may occur at sites of association seen in vivo. To study the interactions, we developed a tissue culture model of murine sympathetic neurons co-cultured with rat basophilic leukaemia (RBL-2H3) cells (homologues of mucosal mast cells) or rat peritoneal mast cells. In co-cultures of up to 3 days, light microscopy identified neurite contacts with peritoneal mast cells or RBL-2H3 cells, but not with glial cells or fibroblasts. Electron microscopy confirmed membrane-membrane contact between neurites and RBL-2H3 cells. Time-lapse analysis of interactions between neurons and RBL-2H3 cells showed that 60-100% of the cells in a given field acquired neurite contact within 17 h. In matching control studies, there was no increase in the frequency of neurite contact with cells of the rat plasmacytoma line (YB2/0): these were not selected as targets, and contacts were broken if formed. Time-lapse records of the derivation of neurites from their path suggested a neurotropic effect of mast cells, with neurite contact ensuing when the intervening distance was less than 36 +/- 4 microns. Once formed, contacts were invariably maintained throughout the period of examination (up to 72 h), in contrast to YB2/O or fibroblast contacts. We conclude that neurons selectively form and maintain connections with cells representative of rat connective tissue-type and mucosal mast cells in vitro. Similar interactions in vivo could promote nerve/mast cell contacts, which may allow bidirectional communication between the nervous and immune systems.

Extension of sympathetic neurites in vitro towards explants of embryonic and neonatal mouse heart and stomach: ontogeny of neuronotrophic factors

In order to establish when target organs first produce neuronotrophic factors, extension of neurites in vitro from sympathetic ganglia (superior cervical and coeliac) of 1-day neonatal mice towards explants of 10-, 11-, 14- and 17-day embryonic and 1-day neonatal atrium and stomach was examined in co-cultures. Longer neurites extended from ganglia towards, than away from, atrial targets at all stages examined, and was most marked towards 17-day embryonic and neonatal explants. Treatment of atrial co-cultures with antiserum to nerve growth factor (NGF) almost totally blocked preferential neurite outgrowth. Directional growth of neurites towards stomach explants in co-cultures was not as pronounced as that towards atrium; extension of neurites was most marked when stomach was provided by 11-, 14- and 17-day embryos. Such outgrowth was only partially blocked by antiserum to NGF, significant preferential extension of neurites towards stomach persisting in the presence of the antiserum. These results indicate that atrium and stomach produce neuronotrophic factors from the earliest ages studied; the evidence indicates that in the case of atrium, NGF predominates but that stomach produces NGF as well as another factor immunologically distinct from NGF. It is of interest that both types of target explanted before they receive sympathetic innervation show evidence of producing NGF in culture.

Relationship between the sympathetic nervous system and vascular smooth muscle: a morphometric study of adult and juvenile spontaneously hypertensive rat/Wistar-Kyoto rat caudal artery

The relationship between the sympathetic nervous system and vascular smooth muscle has been assessed in adult and juvenile spontaneously hypertensive rats (SHR) and compared with age-matched Wistar-Kyoto rats (WKY) using ultrastructural and light microscopic morphometric analysis of the caudal artery. The absolute volume of smooth muscle in the caudal artery of adult SHR (14-19) months was 169% greater than that in WKY vessels. As well, the axonal volume was 89% greater than that in the WKY. There was also a 51% increase in the number of vesicles per volume of varicosity in SHR compared to WKY. At 3 weeks of age the volume of both smooth muscle and axons within the caudal artery of SHR and WKY was not significantly different. However, there was a significantly greater number of vesicles (25%) per unit volume of varicosity in the SHR compared to the WKY. Thus, in the caudal artery there appears to be a relationship between smooth muscle cell volume and axonal volume. An increase in arterial smooth muscle volume (whether it be due to growth or hypertrophy) is accompanied by an increase in axonal volume, or vice versa. The significant increase in the number of vesicles per unit volume of varicosity in the SHR, compared to the WKY reported here, is consistent with other published data indicating an increased availability or turnover of transmitter in these animals. Since the blood pressures of the SHR and WKY are similar at 3 weeks, the apparent increase in sympathetic nerve activity observed suggests that this may be an initiating factor in the development of high blood pressure in SHR.

Adrenergic innervation of aortic patch-grafts in rats

The regeneration of vascular adrenergic nerves was studied using the glyoxylic acid-induced fluorescence method for the specific demonstration of adrenergic nerves in syngeneic patch-grafts of the right atrium of the heart, vena cava and glutaraldehyde-treated vena cava transplanted into the abdominal aorta of the rat. Glutaraldehyde-treated segments of the supradiaphragmatic inferior vena cava were transplanted into the abdominal aorta of rats as well. At the end of the observation period of 24 weeks limited, patchy and defective innervation was observed in the syngeneic vena cava and atrial patches. No adrenergic nerves were found in the glutaraldehyde-treated vein patch-grafts or vein grafts. Owing to the very poor innervation of atrial and venous patch-grafts the results are not entirely in agreement with the target organ concept of adrenergic nerve regeneration. In this study the suture line around the patch graft probably hampers regeneration of vascular adrenergic nerves in the patches.

Tropic interactions between sympathetic nerves and vascular smooth muscle

Explants of sympathetic ganglia from 3-9 day old rats were grown on collagen-coated coverslips in modified Rose chambers for 5 days, either alone or 2 mm away from explants of 6 week old rat caudal artery and aorta. Nerve fibre growth was stimulated on the side of the ganglion explant near the caudal artery explants but was not stimulated near the aorta. To determine the source of the nerve growth stimulation, explants of whole intact wall of the caudal artery, separated adventitia and medial layers, enzyme-dispersed cells, homogenized cells and medium pre-conditioned by caudal artery explants, were combined with ganglia. Explants of whole caudal artery and dispersed cells were also precultured prior to combination with ganglia. These combinations allowed analysis of the role of smooth muscle cells, existing nerve fibres, necrotic cells and connective tissue. Results suggested that degenerating nerve fibres within the blood vessels caused the increase in the number and the 'attraction' of the nerve fibres growing from the sympathetic ganglia. In contrast, both caudal artery and aorta from 3-9 day old rats caused stimulation of nerve fibre growth. Since these vessels were not yet innervated, the effect cannot be due to degenerating nerve terminals and a different mechanism must be involved.

The effects of conditioned media on spinal neurites: substrate-associated changes in neurite direction and adherence

The effects of conditioned media and other growth factors on spinal cord neurite adherence, growth rate, and direction were studied. Skeletal muscle-conditioned medium (SMCM) significantly increased neurite adherence and altered the direction of elongation. Lung- and skin-conditioned media, nerve growth factor, and polyornithine binding neurite promoting factor did not affect neurite adherence or direction. The positive effects of SMCM on neurite adherence and direction may be due to substrate-bound substances, since SMCM depleted of poly-L-lysine-adsorbable materials decreased neurite adherence and direction, and the directional response was greatly diminished on SMCM-conditioned substrate. Haptotaxis may be the basis of the turning response, since it appears to involve substrate-bound molecules which slightly increase adherence. It was also observed that the measured length of neurite involved in a turn toward SMCM was much greater than the calculated length of neurite (based on neurite elongation rates). Therefore, it appears as if the growth cone has been displaced, dragging the attached neurite with it. These observations suggest that the growth cone may have extensive locomotive capabilities which affect growth cone migration.

Cell death of asynaptic neurons in regenerating spinal cord

The weakly electric fish Sternarchus albifrons possesses a unique class of asynaptic neurons, the electromotor neurons, whose axons constitute the electric organ. The cell bodies of origin of the electrocyte axons are located in the spinal cord. Both spinal cord and electromotor neurons ( electrocytes ) regenerate after amputation of the tail. Sternarchus spinal cords which have regenerated for 1 or more years show a progression in number of perikarya of electromotor neurons along the rostro-caudal axis. The most recently regenerated region of the cord is at the caudal end, which consists of a tube of ependyma. Progressing rostrally along regenerated spinal cord from the caudal end, numerous cells are generated and large numbers of electromotor neurons differentiate. The maximum number of electromotor neurons per transverse section of regenerated cord is five times higher than in normal mature cord. Rostral to this, the number of electromotor neurons decreases gradually to the normal number near the transition zone (the border with unregenerated cord). As the more rostral regenerated cord has presumably had a longer period of regeneration, we conclude that excess numbers of electromotor neurons are generated initially, and that subsequently the number of these neurons is decreased by cell death. This conclusion is supported by the fact that younger regenerates (2-4 months) have larger-than-normal numbers of perikarya of electromotor neurons extending up to the transition zone (Anderson and Waxman , 1981). No evidence of migration or depletion of electromotor neurons from unregenerated cord rostral to the amputation has been observed. Since the axons of the electromotor neurons in Sternarchus do not normally form any synapses, this study provides evidence that factors other than synaptic competition must be responsible for determining cell death during regeneration of these spinal neurons.

Earliest sensory nerve fibres are guided to peripheral targets by attractants other than nerve growth factor

Recent studies have shown that developing nerve fibres grow directly to their targets and are guided by specific cues, but the nature of these cues and the mechanism of guidance remain unknown. The growth of sympathetic axons towards an artificial source of nerve growth factor (NGF) in vivo and of sensory neurites up a concentration gradient of NGF in vitro has supported the hypothesis that NGF, produced by target tissues, acts as a chemotactic attractant for these nerve fibres during development. Both these studies and those of the influence of NGF or target tissues on neurite growth in vitro were conducted late in development when, following target encounter, the neurones had become dependent on NGF or target for survival. Here we have co-cultured embryonic mouse sensory neurones and their peripheral target tissue at a stage preceding their contact in vivo. Neurites grew directly and exclusively towards their own target but not to regionally inappropriate peripheral tissue. Antiserum to isogeneric NGF did not reduce this outgrowth but did reduce undirected neurite outgrowth which occurred in co-cultures of older neurones with denervated target tissue. These results demonstrate that agents other than NGF guide neurites of NGF-responsive neurones in development.

Directional growth of sympathetic nerve fibres in vitro towards enteric smooth muscle and heart from mice with congenital aganglionic colon and their normal littermates

Neuronotrophic factors in tissues from normal mice and mice with hereditary aganglionic colon (s1/s1) were assayed by examining neurite extension in vitro from sympathetic ganglia (superior cervical and coeliac) towards explants of stomach, atrium and colon in co-culture. Directional growth of neurites from both ganglia towards all 3 target explants was observed. There were no statistically significant differences between normal and s1/s1 ganglia in the capacity to extend neurites, neither were there differences between the target tissues of these mice in ability to promote neurite extension from ganglia. Since it is known that in s1/s1 mice there is impaired migration of neuroblasts along the developing gut, we conclude that the mechanisms determining neurite extension in culture are different to those regulating neuroblast migration and early differentiation. Studies on normal mice revealed that antiserum to nerve growth factor (NGF) blocked the outgrowth of neurites towards atrium, and reduced but did not abolish the outgrowth towards stomach, suggesting that the gut muscle produces a factor trophic for sympathetic neurones which is immunochemically distinguishable from NGF.

In vivo evidence for a hippocampal adrenergic neuronotrophic factor specifically released on septal deafferentation

Denervation of the hippocampal formation in adult rats through lesions of the septohippocampal pathway was found to induce a trophic growth response in intracortical grafts of sympathetic superior cervical ganglia, and to stimulate regeneration of the intrinsic locus coeruleus adrenergic neurons following chemically induced axotomy. The grafted sympathetic adrenergic neurons grew very poorly into the adjacent hippocampus in animals with the septohippocampal pathways intact. A lesion of the ipsilateral fimbria-fornix or of the medial septum-diagonal band area caused a massive stimulation of axonal growth from the transplanted ganglionic neurons into the denervated hippocampus. This increase was more than 100-fold by 1 month after lesion and about 10-fold by 3 months after lesion. Fluorescence histochemistry revealed that the lesion-induced ingrowth occurred primarily into those areas of the dentate gyrus and hippocampus which were denervated of their septal cholinergic afferents. In addition, the septal and fimbria-fornix lesions induced a marked increase in size and noradrenaline fluorescence of the grafted sympathetic neurons, without any clear-cut effects on the numbers of surviving neurons in the graft. This lesion-induced trophic growth response (increases in axonal outgrowth, cell body size and noradrenaline content) was specific for lesions of the septal (probably primarily cholinergic) innervation of the hippocampus. Thus, extensive denervations induced by lesions of the commissural or perforant path afferents, as well as selective lesions of the intrinsic adrenergic afferents from the locus coeruleus, had no clear-cut effects. The intrinsic central adrenergic neurons were also found to be responsive to the lesion-induced growth-stimulating mechanism. Thus, lesions of the fimbria-fornix or the medial septum-diagonal band area had a marked stimulatory effect on the regeneration of the locus coeruleus noradrenergic neurons after selective chemical axotomy (induced by 5,7-dihydroxytryptamine; 5,7-DHT). Thus, the adrenergic reinnervation of the initially denervated hippocampus was significantly accelerated by 3 weeks after the fimbria-fornix or septal lesions, and the increase persisted for at least 8-10 months after transplantation. These results provide evidence for an adrenergic neuronotrophic factor whose production in the hippocampus normally is under the control of non-adrenergic (probably cholinergic) afferents originating in the septal-diagonal band area. The actions of this putative factor on sympathetic adrenergic neurons resemble those induced by nerve growth factor (NGF). Interestingly, however, the results obtained after 5,7-DHT-induced axotomy indicate that central and peripheral adrenergic neurons are equally responsive, and thus that the putative central adrenergic neuronotrophic factor may play a normal physiological role in the regulation of axonal growth and regeneration within the central nervous system.

Characterization of the turning response of dorsal root neurites toward nerve growth factor

This study reports that chick dorsal root ganglion neurites undergo a rapid (20 min) reorientation of their direction of growth in response to nerve growth factor (NGF) concentration gradients in vitro. Dorsal root ganglia from chick embryos were explanted onto a collagen-poly-L-lysine substrate. After 24-48 h in culture, NGF gradients were applied to individual growth cones via a micropipette containing 50 biological units NGF/ml. The growth cones turned and grew toward these NGF sources. This turning response was not caused by the trophic effects of NGF on neurite initiation, survival, or growth rate. Dorsal root neurites also grew toward sources of mono- and dibutyryl cyclic adenosine monophosphate (dB cAMP), cyclic guanosine monophosphate (cGMP), and elevated calcium in the presence of the calcium ionophore A23187. These results are consistent with the hypothesis that intracellular levels of cAMP and /or cGMP and calcium may play a role in the turning response of dorsal root neurites toward NGF, but do not establish a causal relationship between the mechanisms of action of NGF, cyclic nucleotides and calcium. Total growth cone adherence to the substrate was measured using a timed microjet of perfusion medium. NGF increased the adherence of growth cones to the substrate, but caffeine and dB cAMP which also elicit the positive turning response, decreased growth cone adherence. Calcium, which did not elicit the positive turning response, produced a greater growth cone adherence to the substrate than that observed with NGF. Although these results do not rule out a role of adhesion changes in axonal turning to NGF, they show that a general increase in adherence does not correlate well with the rapid turning response observed in this study.

Neuronal chemotaxis: chick dorsal-root axons turn toward high concentrations of nerve growth factor

Micropipettes containing 2 to 50 biological units of beta growth factor (NGF) were placed near growing axons of chick dorsal-root ganglion neurons in tissue culture. The axons turned and grew toward the NGF source within 21 minutes. This turning response to elevated concentrations of NGF appears to represent chemotactic guidance rather than a general enhancement of growth rate.

Some bases of differences in vascular response to sympathetic activity

Basic patterns of neuroeffector organization vary widely in the vasculature, in general, with vessel diameter and type, and confer distinctive properties. The smaller the vessel, the more intimate the neuroeffector relationship, the more localized the action of the released transmitter, and the more important myogenic conduction compared to transmitter diffusion for the coordination of vascular effector response. Seemingly superimposed upon these basic general patterns are other variable features, conferring upon vessels of similar size and type diversity of function. These variables include sensitivity and magnitude and possible location of alpha- and beta-receptors and their subtypes, presence and nature of intrinsic vascular tone, and the density and pattern of adrenergic innervation to mention the more important. Functional diversity in neuroeffector characteristics can, to some extent, be understood in relation to embryological development, neurotrophic influences, effector regulation of innervation, and the mural response to an increase in intravascular pressure.

Attraction of nerve fiber outgrowth from sympathetic ganglia to heart auricles in tissue culture

Sympathetic chain ganglia of 3-day-old rats were cultured in Rose chambers for 5 days. Each chamber contained either two ganglia or a ganglion and a piece of heart atrium. Visual examination failed to show an increased density or length of nerve fibers growing towards the other explant. However, actual measurements showed that the length of nerve fiber outgrowth towards the atrium explant was about 10% longer than the mean outgrowth to all directions, the difference being statistically significant (P less than 0.0001). In ganglion-ganglion pair cultures the nerve fiber outgrowth towards the other ganglion tended to be shorter than that to other directions. It is tentatively concluded that atrium explants exert a growth-promoting influence on sympathetic nerve fibers in cultures, possibly due to diffusion of a growth-promoting substance from the atrium explant.