Effect of destruction of the postganglionic sympathetic neurons in neonatal rats on development of choline acetyltransferase and survival of preganglionic cholinergic neurons

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.

Acetylcholinesterase immunolesioning: regional vulnerability of preganglionic sympathetic neurons in rat spinal cord

Rats given antibodies against acetylcholinesterase (AChE) develop sympathetic dysfunction stemming from losses of preganglionic neurons in spinal cord. Central effects of AChE antibodies are surprising since IgG does not readily cross the blood-brain barrier, and lesions of peripheral terminals should not cause cell death. This study was designed to explore the distribution of central neural damage and to investigate features that might account for vulnerability. Rat spinal cord and brainstem were stained for choline acetyltransferase (ChAT) and nitric oxide synthase (NOS) immunoreactivity. Four months after administration of AChE antibodies, ChAT-positive neurons in the intermediolateral nucleus (IML) were 61-66% fewer throughout the thoracolumbar cord (T1, T2, T8, T12, L1). NOS-positive neurons in these loci were affected to the same extent by antibody-treatment, although they were only two-thirds as numerous. By contrast, neurons in the central autonomic nucleus of the thoracolumbar cord were scarcely affected. These results point to immunochemical differences in the central autonomic outflow, which may partially explain the puzzling selectivity of neural damage in AChE immunolesioning. Different results were obtained after guanethidine sympathectomy, which ablated nearly all neurons in the superior cervical ganglion without any effect on preganglionic neurons in the IML. Therefore, if the central effects of antibodies are indirectly mediated by loss of trophic support from the periphery, this support cannot arise from adrenergic neurons but must come from other ganglionic cells.

Differences in developmental cell death between somatic and autonomic motor neurons of rat spinal cord

Considerable knowledge concerning developmental cell death has come from the study of somatic motor neurons (SMNs), but a related set of spinal neurons, the autonomic motor neurons (AMNs), have been studied less extensively in this respect. In the present study, we used three different approaches to determine the amount of AMN cell death during normal development in the rat. First, target dependency was studied in organotypic slice cultures, and it was found that AMNs survived for at least 12 days after removal of their postsynaptic targets. No factors were added to the serum-free medium to substitute for the ablated targets, indicating that AMNs were able to survive without target-derived trophic factors. Such target-independent survival is not characteristic of neurons that undergo typical developmental cell death. Second, AMNs were counted in double-stained choline acetyltransferase immunocytochemical and NADPH diaphorase histochemical preparations at ages (postnatal days 4-22) encompassing the period when AMN postsynaptic target cells undergo developmental death. Neuron numbers were essentially identical at all ages examined, indicating that no AMN cell death occurred postnatally. Finally, from embryonic day 13 to postnatal day 22, animals were analyzed by using terminal transferase-mediated nick-end labeling to identify dying cells. Many fewer labeled cells were observed among AMNs than among SMNs. Thus, all three approaches indicated that there is a significant SMN/AMN difference in developmental cell death. The phenotypic trait(s) that underlies this difference may also be important in the relative resistance of AMNs to pathological conditions that induce death of SMNs, e.g., those involved in amyotrophic lateral sclerosis and excitotoxicity.

Effects of neonatal sympathectomy with 6-hydroxydopamine or guanethidine on survival of neurons in the intermediolateral cell column of rat spinal cord

The effects of removing target cells on survival of, and inputs to, sympathetic preganglionic neurons were studied in rats that were sympathectomized with 6-hydroxydopamine (6-OHDA) or guanethidine sulfate. Separate groups of neonatal and 1-week male rats were given injections of 6-OHDA for 10 days and of guanethidine for 3 weeks (5 days/week), respectively. Histofluorescence results suggest that catecholaminergic neurons in most ganglia are destroyed with treatment except for adrenal medulla, which is unaffected [14], and the pelvic ganglion where only partial destruction occurs. Cells in the intermediolateral cell column from representative spinal cord segments of treated and control adult rats were counted. In 6-OHDA-treated rats, cells decreased in number in all segments compared to controls. In guanethidine-treated rats, cells were also decreased in number; in some segments the decrease was significantly greater than with 6-OHDA. Sympathectomy had no effect on neurons in the intermediate gray of L5 or in the ventral horn of T3. The results of this study demonstrate that peripheral sympathectomy causes loss of sympathetic preganglionic neurons and that guanethidine is slightly more effective than 6-OHDA.

Neurotrophic molecules

Neurotrophic molecules have a profound influence on developmental events such as naturally occurring cell death, differentiation, and process outgrowth. Despite their striking effects on developing neurons, a role for these molecules in the pathogenesis or therapy of neurological disease has not yet been defined. However, a variety of recent advances promise to provide the techniques necessary to assess the potential relevance of neurotrophic molecules to clinical neurology. In this article we review recent investigations into the biological effects, regulation of production, and mechanisms of action of the best characterized trophic molecule, nerve growth factor. In addition we review studies characterizing brain-derived neurotrophic factor and other putative neurotrophic molecules. Finally, we discuss how pharmacological effects of these molecules may be relevant to the therapy of disease states as well as neural regeneration.

The development of cholinergic neurons

Motoneuron precursors acquire some principles of their spatial organization early in their cell lineage, probably at the blastula stage. A predisposition to the cholinergic phenotype in motoneurons and some neural crest cells is detectable at the gastrula to neurula stages. Cholinergic expression is evident upon cessation of cell division. Cholinergic neurons can synthesize ACh during their migration and release ACh from their growth cones prior to target contact or synapse formation. Neurons of different cell lineages can express the cholinergic phenotype, suggesting the importance of secondary induction. Early cholinergic commitment can be modified or reversed until later in development when it is amplified during interaction with target. Motoneurons extend their axons and actively sort out in response to local environmental cues to make highly specific connections with appropriate muscles. The essential elements of the matching mechanism are not species-specific. A certain degree of topographic matching is present throughout the nervous system. In dissociated cell culture, most topographic specificity is lost due to disruption of local environmental cues. Functional cholinergic transmission occurs within minutes of contact between the growth cone and a receptive target. These early contacts contain a few clear vesicles but lack typical ultrastructural specializations and are physiologically immature. An initial stabilization of the nerve terminal with a postsynaptic AChR cluster is not prevented by blocking ACh synthesis, electrical activity, or ACh receptors, but AChR clusters are not induced by non-cholinergic neurons. After initial synaptic contact, there is increasing deposition of presynaptic active zones and synaptic vesicles, extracellular basal lamina and AChE, and postjunctional ridges over a period of days to weeks. There is a concomitant increase in m.e.p.p. frequency, mean quantal content, metabolic stabilization of AChRs, and maturation of single channel properties. At the onset of synaptic transmission, cell death begins to reduce the innervating population of neurons by about half over a period of several days. If target tissue is removed, almost all neurons die. If competing neurons are removed or additional target is provided, cell death is reduced in the remaining population. Pre- or postsynaptic blockade of neuromuscular transmission postpones cell death until function returns.(ABSTRACT TRUNCATED AT 400 WORDS)

Central neurotoxic effects of guanethidine: altered serotonin and enkephalin neurons within the area postrema

This study reveals that the 5-HT and enkephalin cell body population of the area postrema (AP) is dramatically reduced via exposure to the peripherally circulating neurotoxin guanethidine. Efforts to restore cell body numbers to control levels, with colchicine and monoamine oxidase inhibitors, subsequent to guanethidine treatment are ineffective. However, 5-HT and enkephalin immunostaining of surrounding bulbar nuclei appears normal, implying that the neurotoxic effect of guanethidine is restricted to the AP. In addition, gamma-aminobutyric acid and neurotensin immunostaining within the AP appears normal, subsequent to guanethidine treatment, suggesting that the neurotoxic effect is restricted to specific AP cell populations.

Target organ destruction enhances recovery of choline acetyltransferase activity in adult rat sympathetic ganglia after denervation

We studied the effect of destruction of the adrenergic neuronal population on the recovery of preganglionic choline acetyltransferase activity in adult rat sympathetic ganglia. To produce a partial destruction of the adrenergic system, rats were injected with guanethidine for 4 weeks; the preganglionic nerve to the superior cervical ganglion was then crushed and the guanethidine injections were continued for an additional 3 days to 6 weeks. To determine that the drug was effective, tyrosine hydroxylase activity was assessed; enzymic activity was reduced by 76% or more after guanethidine administration. In addition, electron microscopy studies showed that the number of principal cell-synaptic contacts and vesicle-containing varicosities were decreased by 90% after guanethidine administration. Those measures indicated the drug effectively destroyed the postsynaptic adrenergic neurons. In contrast, crushing the preganglionic nerve in animals not treated with guanethidine did not change tyrosine hydroxylase activity, suggesting minimal nonspecific damage to the ganglion as a result of the lesion. Choline acetyltransferase activity was measured as an index of presynaptic cholinergic integrity. After crush of the preganglionic nerve, there was a gradual recovery of ganglionic choline acetyltransferase activity in the saline-injected rats from 5% of control 3 days after the crush to 49% of control after 6 weeks. On the other hand, in the ganglia of rats administered guanethidine, there was a much enhanced recovery of choline acetyltransferase activity after the nerve crush compared with saline-injected animals; in the guanethidine-injected rats, the ganglionic choline acetyltransferase activity 3 days and 6 weeks after the nerve crush was 15 and 96%, respectively, compared with the uncrushed side. These results demonstrate after destruction of the adrenergic target tissue, recovery of presynaptic choline acetyltransferase activity in the adult rat sympathetic ganglion can still occur after denervation; however, the mechanism(s) that controls the regeneration is altered, so that enzymic activity is enhanced.

Effect of chemical destruction of adrenergic neurones on some cholinergic mechanisms in adult rat sympathetic ganglia

Rats were treated for 2-6 weeks with guanethidine after which their superior cervical ganglia were removed. Ganglionic tyrosine hydroxylase and alpha-bungarotoxin binding sites were reduced by the guanethidine treatment indicating adrenergic cell body destruction. Choline acetyltransferase activity and acetylcholine content of ganglia were not clearly changed by the guanethidine treatment, indicating that the drug does not destroy presynaptic terminals and that these presynaptic indicators do not adapt markedly to postsynaptic loss. The cholinesterase in the ganglia was reduced by guanethidine treatment, but such ganglia retained their ability to accumulate surplus acetylcholine when they were incubated with physostigmine. This is interpreted as indicating surplus acetylcholine accumulation is a presynaptic phenomenon. Choline uptake by resting ganglia was not reduced as a result of guanethidine treatment nor was it affected by preganglionic denervation. This is interpreted as indicating that during rest, choline uptake is into supporting cells or intraganglionic cells rather than cholinergic nerve terminals or adrenergic cell bodies.

Neural control of the heart-rate-orienting response in preweanling rats

The neural origins of heart-rate responses to auditory and painful stimuli were studied in preweanling and neonatal rats. Bradycardia and tachycardia in response to auditory and painful stimuli were found to be mediated by the 10th cranial nerve. Bilateral vagotomy abolished both responses, as did the peripherally acting cholinergic antagonist methyl scopolamine. Neither peripheral sympathectomy with guanethidine nor beta adrenergic receptor blockade with propranolol significantly altered the cardiac responses. Thus, both the tachycardia and bradycardia evoked by psychologically relevant stimuli appear to be under direct nervous system control exerted through cholinergic vagal efferents to the heart.

Time of appearance during development of differences in nigrostriatal tyrosine hydroxylase activity in two inbred mouse strains

The greater activity of tyrosine hydroxylase (TH) in substantia nigra and corpus striata of adult BALB/cJ than CBA/J mice, is attributable to differences in the number of dopamine neurons in the ventral midbrain tegmentum. To determine if strain differences in TH activity develop postnatally we have measured the development of TH in the midbrain (SN) and in the corpus striatum (CS). In the midbrain neonatal TH activity was 20% of adult levels. Thereafter, TH activity increased rapidly achieving adult levels by 11 days. A 25% "overshoot' above adult values at 15 days was followed by a gradual decrease to adult activity at 4 weeks. In the CS neonatal activity was about 10% of adult levels and increased slowly to reach adult values at 4 weeks. Striatal choline acetyltransferase (CAT) activity in the neonate was only 3.7% of adult values and at 21 days had only reached 70% of adult activity. Neonatal glutamic acid decarboxylase (GAD) activity was relatively high in both brain regions and increased gradually to adult activity by 4 weeks. Strain differences in TH activity were not present at birth but first appeared at 9 days in SN and 11 days in CS. Once established, the differences were maintained. These results suggest that strain differences in TH are most probably a consequence of differences in postnatal neuron survival, although the possibility that some neurons lose their phenotypic expression of TH cannot be excluded.

Quantitative studies of sympathetic ganglia and spinal cord intermedio-lateral gray columns in familial dysautonomia

In adult patients with familial dysautonomia the mean volume of superior cervical sympathetic ganglia is reduced to 34% of the normal of 222 mm3. Packing density of neurons is reduced to 37% of normal. The mean total number of ganglionic neurons is 120,000 as compared to 1,060,000 in controls. The mean totals of preganglionic neurons in the first three thoracic cord segments are 13,600 in patients and 25,150 in controls. Deficits in sympathetic neurons account for many of the clinical, pharmacological and biochemical manifestations of familial dysautonomia.