Specificity of innervation of iris musculature by sympathetic nerve fibres in tissue culture

Current approaches to development of the autonomic nervous system: clues to clinical problems

A number of different approaches to autonomic development utilizing a variety of experimental models and analytical techniques have been outlined. A scheme, which attempts to delineate a series of events involving separate but sometimes overlapping mechanisms, is proposed for the complex process of formation and maintenance of functional autonomic neuroeffector junctions. The relevance of these basic mechanisms of a variety of clinical abnormalities of autonomic function is discussed.

Sympathetic postganglionic reinnervation of mesenteric arteries and enteric neurones of the ileum of the rat

The reinnervation by sympathetic postganglionic noradrenergic nerve fibres of a mesenteric artery and the ganglionic plexuses of the mid ileum of young rats has been examined using both physiological and anatomical techniques at various times, up to 6 months, after the interruption of their nerve supply by a freezing technique. Reinnervation of the artery was extremely slow and even 6 months after the operation there were fewer fibres over the surface of the artery than before denervation. The responses to nerve stimulation were smaller than those recorded from control vessels. Two distinct post-junctional membrane potential responses to nerve stimulation were recorded from the arterial smooth muscle cells during the process of reinnervation. These responses were similar to those previously recorded during the development of the sympathetic innervation to mesenteric arteries. The responses within the gut wall to sympathetic postganglionic nerve stimulation reappeared by 3 weeks after the operation. At this time, however, a minority of ganglia had been reinnervated as judged by the histochemical localization of catecholamines. The sympathetic fibres reinnervating both the arteries and the intramural plexuses regenerated from the interrupted axons of the paravascular nerve trunks. There was little or no contribution to the reinnervation pattern as a result of sprouting of existing noradrenergic nerve fibres either on the artery proximal to the point of denervations or within adjacent segments of the gut wall.

Fine structure of the synaptic endings between sympathetic axons and skeletal muscle cells and of the varicosities in the bundles of neurites in tissue culture

In cocultures of skeletal muscle and sympathetic ganglia from chick embryos, synaptic boutons on skeletal muscle cells and varicosities in the neuritic bundles were observed electron microscopically. Synaptic endings on skeletal muscles were bulbous. Most synaptic boutons simply made contact with muscle fibers, but some boutons appeared as concave invaginations into the sarcolemma. In the neuritic bundles, numerous varicosities were observed. Close approximations were found between the synaptic boutons and skeletal muscles and between the varicosities and neurites (dendrites or axons), but no membrane thickening nor subsynaptic infolding was observed in either synapse; in our cultures the contacts were characteristic of the autonomic nervous system. In both synapses, three variations were demonstrated by KMnO4 fixation after 5-hydroxydopamine incubation: (i) containing predominantly small dense-core vesicles (noradrenergic type), (ii) predominantly small clear vesicles (cholinergic types), and (iii) a mixture of both small dense-core and small clear vesicles (mixed type). Cytochemically, the varicosities in the neuritic bundles were predominantly noradrenergic at 1 week in culture, and both the synaptic boutons on skeletal muscles and the varicosities in the neuritic bundles contained a mixed population of small dense-core and small clear vesicles at intermediate times, with a gradual shift to cholinergic characteristics. These findings strongly suggest that in this culture system some sympathetic neurons or fibers become cholinergic (neuronal plasticity).

Selective innervation of target regions within fetal mouse spinal cord and medulla explants by isolated dorsal root ganglia in organotypic co-cultures

Correlative electrophysiologic and cytologic analyses demonstrate that a significant group of neurons in isolated (NGF-enhanced) fetal mouse dorsal root ganglia (DRGs) can grow across a collagen substrate and selectively innervate specific dorsal horn and dorsal column nuclei regions in co-cultured explants of deafferented spinal cord and medulla. Neurites from this group of DRG cells from connections in CNS target zones that generate characteristic primary afferent network responses to sensory stimuli, as observed in cultures of spinal cord with attached DRGs. Systematic microelectrode stimulus-mapping tests revealed that many DRG neurites were preferentially distributed in sensory target zones of co-cultured cord and medulla explants and that few collaterals of these DRG neurons were present in neighboring inappropriate regions, especially in the ventral cord. Another group of DRG neurons appears to be responsible for the less prominent, but clear-cut, innervation that developed in some of the co-cultured ventral cord explants. Fetal DRGs were also able to establish characteristic primary afferent dorsal horn or dorsal column nuclei networks when introduced into cultures of deafferented spinal cord and medulla that had been explanted alone for 1-3 weeks prior to introduction of the DRGs. These experiments demonstrate that CNS target neurons remain receptive to DRG innervation even after 1-3 weeks of maturation in vitro. Our electrophysiologic and cytologic analyses of DRG and CNS explants in organotypic co-cultures provide the first systematic attempt to establish conditions under which preferential neuritic growth to and functional innervation of specific CNS target tissues can occur in vitro. This model system should facilitate analyses of mechanisms underlying development, as well as regeneration, of specific synaptic connections in the CNS.

Review lecture. Neurotransmitters and trophic factors in the autonomic nervous system

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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.

Pre- and postnatal development of catecholamine-containing and cholinesterase-positive nerves of the rat cornea and iris

Glyoxylic acid-induced fluorescence technique and a copper thiocholine method were used to investigate the ontogenesis of the catecholamine-containing and cholinesterase-positive nerves of the rat iris and cornea. First fluorescent nerve fibres appeared in the iris on the 18th gestation day and in the cornea on the 19th day. A rapid increase in the density of the adrenergic nerve fibres of the iris continued to the age of three weeks, while the number of such fibres were small in the cornea. Acetylcholinesterase-positive fibres appeared both in the cornea and in the iris on the 19th gestation day. Their density increased more rapidly in the iris, especially in the sphincter muscle, than in the cornea. Non-specific cholinesterase activity was localized in the Schwann cells and the reaction was more intense during development than in the nerves of the cornea of adult rats.

Fine structure of the rat carotid body transplanted into the anterior chamber of the eye

Rat carotid bodies transplanted into the anterior chamber of the eye were studied by electron microscopy. Chief and sustentacular cells and a few ganglion cells survived for 3 months and maintained cytological characteristics similar to those in the intact carotid body. The transplant contained many fenestrated capillaries. Chief cells at the periphery of the cell cluster had long cytoplasmic processes which projected into the stroma of the iris. The cell processes became incorporated into bundles containing nerve fibres, which were enveloped by a perineurial sheath. Three types of nerve fibres were identified in the explant. Type I and type II nerve fibres (presumptive cholinergic and adrenergic, respectively) were enclosed by sustentacular and satellite cells. Most of the nerve fibres were completely separated from chief cells and ganglion cells by sustentacular and satellite cells. A few nerve fibres made direct apposition to chief cells and ganglion cells, where some nerves were presynaptic to them. Type III nerve fibres derived from myelinated nerve fibres were also enclosed by sustentacular and satellite cells.

Selectivity of neuronal degeneration produced by chronic guanethidine treatment

Chronic guanethidine treatment of rats produced extensive damage to sympathetic neurons of the superior cervical ganglion and pelvic plexus. No ultrastuctural changes were observed in parasympathetic cholinergic neurons in the ciliary ganglion and pelvic plexus, nor in sensory neurons in nodose and dorsal root ganglia. A total of only six nerve cell bodies free of degenerative changes were observed in sections of superior cervical ganglia from 20 rats. This suggests either that the earlier estimates of 5% cholinergic neurons in the superior cervical ganglion based on acetylcholinesterase staining are too high, or implies that sympathetic cholinergic neurons, unlike parasympathetic neurons, are damaged by chronic guanethidine treatment.

Differences in sensitivity to nerve growth factor of axon formation and tyrosine hydroxylase induction in cultured sympathetic neurons

Superior cervical ganglia from 2-day-old and 3-week-old rats were maintained in vitro for up to 2 weeks in the presence of a range of concentrations of nerve growth factor up to 100 micrograms/ml. Nerve fibre length and density were measured and tyrosine hydroxylase activity of these cultures assayed after various times. Ganglia were also examined for catecholamines and neuronal numbers using fluorescence histochemistry and histology respectively. In cultures maintained without nerve growth factor, or in those containing low concentrations of nerve growth factor (3 ng/ml), tyrosine hydroxylase decreased to 5-10% of the initial levels by 14 days in vitro. The presence of the high concentration of 1 microgram/ml nerve growth factor in the culture medium or the addition of such a concentration during the culture period did not prevent an initial decrease in tyrosine hydroxylase but subsequently increased the enzyme activity. The maximal effect of nerve growth factor on nerve fibre density was at low concentrations whereas its maximal effect on neuronal survival, tyrosine hydroxylase activity or nerve fibre elongation was at high concentrations. After 2 days in culture, maximum neurite production occurred in cultures containing 10 ng/ml, while maximum nerve fibre elongation and tyrosine hydroxylase activity occurred in cultures containing 100 micrograms/ml nerve growth factor. We conclude that low concentrations of nerve growth factor, as occur in plasma, cause maximum axon formation while high concentrations of nerve growth factor, as occur in effector organs, induce maximum tyrosine hydroxylase activity and cell survival. The former process may be mediated via cell surface receptors and the latter via retrograde axonal transport of nerve growth factor to the cell body, following uptake by the terminal regions of the axons.

Do some nerve cells release more than one transmitter?

The concept that each nerve cell makes and releases only one nerve transmitter (widely known as Dale's Principle) has been re-examined. Experiments suggesting that some nerve cells store and release more than one transmitter have been reviewed. Developmental and evolutionary factors are considered. Conceptual and experimental difficulties in investigating this problem are discussed. It is suggested that the term 'transmitter' should be applied to any substance that is synthesised and stored in nerve cells, is released during nerve activity and whose interaction with specific receptors on the postsynaptic membrane leads to changes in postsynaptic activity. Expressed in this way, it seems likely that while many nerves do have only one transmitter, others in some species, during development or during hormone-dependent cycles, employ multiple transmitters.