Denervation-induced formation of adrenergic synapses in the superior cervical sympathetic ganglion of the rat and the enhancement of this effect by postganglionic axotomy

The nicotinic activation of the denervated sympathetic neuron of the rat

Nicotinic responses to endogenous acetylcholine and to exogenously applied agonists have been studied in the intact or denervated rat sympathetic neuron in vitro, by using the two-microelectrode voltage-clamp technique. Preganglionic denervation resulted in progressive decrease of the synaptic current (excitatory postsynaptic current, EPSC) amplitude, which disappeared within 24 h. These effects were accompanied by changes in ion selectivity of the nicotinic channel (nAChR). The extrapolated EPSC null potential (equilibrium potential for acetylcholine action, E(Syn)) shifted from a mean value of -15.9+/-0.7 mV, in control, to -7.4+/-1.6 mV, in denervated neurons, indicating a decrease of the permeability ratio for the main components of the synaptic current (P(K)/P(Na)) from 1.56 to 1.07. The overall properties of AChRs were investigated by applying dimethylphenylpiperazinium or cytisine and by examining the effects of endogenous ACh, diffusing within the ganglion after preganglionic tetanization in the presence of neostigmine. The null potentials of these macrocurrents (equilibrium potential for dimethylphenylpiperazinium action, E(DMPP); and equilibrium potential for diffusing acetylcholine, E(ACh), respectively) were evaluated by applying voltage ramps and from current-voltage plots. In normal neurons, E(Syn) (-15.9+/-0.7 mV) was significantly different from E(DMPP) (-26.1+/-1.0) and E(ACh) (-31.1+/-3.3); following denervation, nerve-evoked currents displayed marked shifts in their null potentials (E(Syn)=-7.4+/-1.6 mV), whereas the amplitude and null potential of the agonist-evoked macrocurrents were unaffected by denervation and its duration (E(DMPP)=-26.6+/-1.2 mV). It is suggested that two populations of nicotinic receptors, synaptic and extrasynaptic, are present on the neuron surface, and that only the synaptic type displays sensitivity to denervation.

Remodelling of connections in pelvic ganglia after hypogastric nerve crush

Pelvic ganglia innervate the urogenital organs and contain both sympathetic and parasympathetic neurons. Previous studies have shown that within days of cutting either the lumbar or sacral preganglionic axons that innervate pelvic ganglia, many axon collaterals grow and appear to form specific connections with denervated pelvic neurons. Here we have examined the longer term consequences of partial deafferentation by studying pelvic ganglia up to 7 weeks after hypogastric nerve (HGN) crush, a lesion which also allows faster regeneration of spinal axons. Noradrenergic neurons were denervated by HGN crush, as demonstrated by loss of varicosities immunostained for the synaptic proteins, synaptophysin and synapsin. A week after HGN crush, axon collaterals grew from parasympathetic pelvic ganglion neurons, shown by the presence of numerous varicose fibers immunostained for vasoactive intestinal peptide (VIP). These VIP fibers were poorly stained or unstained for synaptophysin, even after 7 weeks. At early post-operative times the VIP fibers grew irregularly; however, with longer post-operative times they appeared to target particular VIP-negative, noradrenergic neurons. Our results also indicate that some lumbar preganglionic axons regenerated during the post-operative period, although this only affected a minority of sympathetic neurons. These reinnervated sympathetic neurons were not associated with VIP fibers, suggesting that the new intrinsic connections may have precluded regeneration or targeting of preganglionic axons. Together these results demonstrate that there is considerable remodelling within pelvic ganglia after partial deafferentation. This occurs under conditions where spinal preganglionic axons can regenerate. New intra-ganglionic connectivity may be permanent and may impact on this regeneration.

Postdecentralization plasticity of voltage-gated K+ currents in glandular sympathetic neurons in rats

This paper presents the kinetic and pharmacological properties of voltage-gated K(+) currents in anatomically identified glandular postganglionic sympathetic neurons isolated from the superior cervical ganglia in rats. The neurons were labelled by injecting the fluorescent tracer Fast Blue into the submandibular gland. The first group of neurons remained intact, i.e. innervated by the preganglionic axons until the day of current recordings (control neurons). The second group of neurons was denervated by severing the superior cervical trunk 4-6 weeks prior to current recordings (decentralized neurons). In every control and decentralized neuron three categories of voltage-dependent K(+) currents were found. (i) The I(Af) K(+) current, steady state, inactivated at hyperpolarized membrane potentials. This current was fast activated and fast time-dependently inactivated, insensitive to TEA and partially depressed by 4-AP. (ii) The I(As) K(+) current, which was steady-state inactivated at less hyperpolarized membrane potentials than I(Af). The current activation and time-dependent inactivation kinetics were slower than those of I(Af). I(As) was blocked by TEA and partially inhibited by 4-AP. (iii) The IK K(+) current did not undergo steady-state inactivation. In decentralized compared to control neurons the maximum I(Af) K(+) current density (at +50 mV) increased from 116.9 +/- 8.2 to 189.0 +/- 11.5 pA/pF, the 10-90% current rise time decreased from 2.3 to 0.7 ms and the recovery from inactivation was faster. Similarly, in decentralized compared to control neurons the maximum I(As) K(+) current density (at +50 mV) increased from 49.9 +/- 3.5 to 74.3 +/- 5.0 pA/pF, the 10-90% current rise time shortened from 29 to 16 ms and the recovery from inactivation of the current was also faster. The maximum density (at +50 mV) of I(K) in decentralized compared to control neurons decreased from 76.6 +/- 3.9 to 60.7 +/- 6.3 pA/pF. We suggest that the upregulation of voltage-gated time-dependently-inactivated K(+) currents and their faster recovery from inactivation serve to restrain the activity of glandular sympathetic neurons after decentralization.

Synaptic vesicle proteins and neuronal plasticity in adrenergic neurons

The neurons in the superior cervical ganglion are active in plasticity and re-modelling in order to adapt to requirements. However, so far, only a few studies dealing with synaptic vesicle related proteins during adaptive processes have been published. In the present paper, changes in content and expression of the synaptic vesicle related proteins in the neurons after decentralization (cutting the cervical sympathetic trunk) or axotomy (cutting the internal and external carotid nerves) were studied. Immunofluorescence studies were carried out using antibodies and antisera against integral membrane proteins, vesicle associated proteins, NPY, and the enzymes TH and PNMT. For colocalization studies, the sections were simultaneously double labelled. Confocal laser scanning microscopy was used for colocalization studies as well as for semi-quantification analysis, using the computer software. Westen blot analysis, in situ 3'-end DNA labelling, and in situ hybridization were also employed. After decentralization of the ganglia several of the synaptic vesicle proteins (synaptotagmin I, synaptophysin, SNAP-25, CLC and GAP-43) were increased in the iris nerve terminal network, but with different time patterns, while TH-immunoreactivity had clearly decreased. In the ganglia, these proteins had decreased at 1 day after decentralization, probably due to degeneration of the pre-ganglionic nerve fibres and terminals. At later intervals, these proteins, except SNAP-25, had increased in the nerve fibre bundles and re-appeared in nerve fibres outlining the principal neurons.

Spontaneous synaptogenesis in ex vivo sympathetic ganglion and the blockade by serum treatment

Central denervation for more than 1 month has been shown to cause an increase in the number of adrenergic synapses in sympathetic ganglia in vivo. Here, we report several lines of evidence that adrenergic synapses may be generated de novo in ex vivo superior cervical ganglion (SCG) of adult rats only several hours after the isolation. Structures immunoreactive for synaptophysin, a marker of presynaptic elements, were drastically decreased 6 days after the preganglionic denervation. A significant increase in number of synaptophysin positive boutons was observed over 3-8 hours in the denervated SCGs maintained ex vivo at 36 degrees C in oxygenated physiologic saline, and this increase was blocked by adding normal serum in the saline. Electron microscopic analysis confirmed that the number of adrenergic synapses specifically labeled with 5-hydroxydopamine was increased by several-fold under the same condition. Intracellular labeling of SCG neurons revealed an increase in the incidence (from 8 to 50%) of neurons having dendritic plexus after the in vitro incubation. No evidence of axonal sprouting within the ganglion was observed. Intracellular recordings from single neurons of denervated SCGs revealed that maximum amplitudes of inhibitory postsynaptic potentials, which were completely blocked by yohimbine, an alpha2-adrenoceptor antagonist, in response to focal stimulation were increased over the several hours. These results suggest that dendrites of SCG neurons rapidly develop and exhibit local efferent characteristics that underlie the inhibitory synaptic transmission once they are subjected to serum deprivation.

Stimulant-induced exocytosis from neuronal somata, dendrites, and newly formed synaptic nerve terminals in chronically decentralized sympathetic ganglia of the rat

Loss of preganglionic neurones underlies the autonomic failure of human multiple system atrophy. In rat sympathetic ganglia decentralization leads to new synapse formation. We explored whether these synapses are functional, and whether chronically decentralized neurones respond normally to activation, in terms of exocytosis. Potassium depolarization and cholinergic agonists were applied to freshly excised rat superior cervical sympathetic ganglia, preganglionically denervated with prevented reinnervation 5 months earlier. Ganglia were incubated and stimulated in the presence of tannic acid, which stabilizes released vesicle cores for subsequent electron microscopy. In denervated ganglia exocytosis was observed from newly formed synaptic nerve terminals, and from nonsynaptic surfaces of neurone somata and dendrites. The results demonstrated that the new intraganglionic synapses, which are mostly catecholaminergic, can function and that chronically decentralized sympathetic neurones remain capable of stimulant-induced exocytosis from somata and dendrites. The maximal release upon potassium depolarization did not differ significantly between denervated and contralateral ganglia. Relative to this, the exocytotic responses of decentralized somata and dendrites to nicotine resembled those of contralateral ganglia. Responses to muscarine were significantly less in denervated than in contralateral ganglia, indicating inhibition in dendrites. Responses to carbachol suggested interactions between nicotinic and excitatory muscarinic effects. Nerve terminals in denervated ganglia showed high basal release. Their responses to muscarine and carbachol resembled those of the decentralized neurones, from which most may originate. Their response to nicotine evidenced inhibition. Their actions, coupled with nonsynaptic effects of soma-dendritic exocytosis, might modulate responses of the decentralized neurone population to other surviving inputs. This modulation could be influential in disease-induced decentralization in man.

Specific targeting of ganglion cell sprouts provides an additional mechanism for restoring peripheral motor circuits in pelvic ganglia after spinal nerve damage

The pelvic ganglia contain both sympathetic and parasympathetic neurons and provide an interesting model in which to study the effects of a distributed spinal nerve lesion. Previous animal studies have suggested that after either lumbar or sacral nerve injury, some functional connections are restored between preganglionic and postganglionic neurons. It has been proposed that this is because of intact preganglionic axons sprouting collaterals to supply denervated ganglion cells. However, this has never been demonstrated, and our study has investigated whether the ganglion cells themselves contribute to axogenesis and restoration of peripheral circuitry. We have monitored the growth of axons from pelvic ganglion cells after lumbar or sacral nerve injury (partial decentralization), or a combination of the two (total decentralization). These new processes were distinguished from intact preganglionic terminals by their immunoreactivity for substances present only in pelvic ganglion neurons (vasoactive intestinal peptide, neuropeptide Y, and tyrosine hydroxylase). The proportion of pelvic neurons surrounded by these immunostained fibers was then assessed. Complete removal of preganglionic terminals provides the biggest stimulus for growth of new axon processes (sprouts), which grow profusely within just a few days. These arise from each of the main chemical classes of pelvic neurons but grow at different rates and have different distributions. Importantly, some chemical classes of sprouts preferentially supply neurons of dissimilar histochemistry, suggesting the presence of very specific targeting mechanisms rather than random growth. These sprouts are transient, however, those formed after partial decentralization appear to be maintained. Moreover, after lesion of either lumbar or sacral spinal nerves, many sprouts arise from neurons with intact spinal connections and innervate neurons that have lost their preganglionic inputs. This provides a very different alternative mechanism to reestablish communication between preganglionic and postganglionic neurons. In conclusion, we have demonstrated a rapid and selective axogenesis within the pelvic ganglion after spinal nerve injury. This may allow the development of novel strategies by which autonomic nerve pathways can be experimentally manipulated, to facilitate more rapid return of appropriate peripheral reflex control.

Exocytotic release from neuronal cell bodies, dendrites and nerve terminals in sympathetic ganglia of the rat, and its differential regulation

Stimulant-induced exocytosis has been demonstrated in sympathetic ganglia of the rat by in vitro incubation of excised ganglia in the presence of tannic acid, which stabilizes vesicle cores after their exocytotic release. Sites of exocytosis were observed along non-synaptic regions of the surfaces of neuron somata and dendrites, including regions of dendrosomatic and dendrodendritic apposition, as well as along the surfaces of nerve terminals About half the exocytoses associated with nerve terminals were parasynaptic or synaptic, and these appeared mostly to arise from the presynaptic terminal, but occasionally from the postsynaptic element. The results demonstrated that the neurons of sympathetic ganglia release materials intraganglionically in response to stimulation, that release from different parts of the neuron is subject to independent regulation, at least via cholinergic receptors, and that release is partly diffuse, potentially mediating autocrine or paracrine effects, and partly targeted toward other neurons, but that the latter mode is not necessarily, and not evidently, synaptic. Specifically, exocytosis from all locations increased significantly during incubation in modified Krebs' solution containing 56 nm potassium. Observation of the effects of cholinergic agonists (nicotine, carbachol, oxotremorine) and antagonists (atropine, AF-DX 116) showed that nicotinic and muscarinic excitation each, independently, increased the incidence of exocytosis from somata and dendrites. Exocytosis from nerve endings was not altered by nicotine, but was enhanced or, at high initial rates of exocytosis, decreased, by muscarinic stimulation. Evidence was obtained for muscarinic auto-inhibition of exocytosis from nerve terminals, occurring under basal incubation conditions, and for a muscarinic excitatory component of somatic exocytosis, elicitable by endogenous acetylcholine. The M2-selective muscarinic antagonist AF-DX 116 was found to modify the exocytotic response of the dendrites to oxotremorine, widening the range of its variation; this effect is consistent with recent evidence for the presence of M2-like muscarinic binding sites, in addition to M1-like binding, upon these dendrites [Ramcharan E. J. and Matthews M. R. (1996) Neuroscience 71, 797-832]. Over all conditions, disproportionately more sites of somatic and dendritic exocytosis were found to be located in regions of dendrosomatic and dendrodendritic apposition than would be expected from the relative extent of the neuronal surface occupied by these relationships. Such mechanisms of intraganglionic release may be expected to contribute to the regulation and integration of the behaviour of the various functionally distinctive populations of neurons in these ganglia, by autocrine, paracrine, and focal, neuroneuronal, routes of action. Similar phenomena of exocytotic soma-dendritic release might prove to subserve integrative neuroneuronal interactions more widely throughout the nervous system.

Plasticity of neuropeptide Y in the rat superior cervical ganglion in response to nerve lesion

Axotomy of the rat superior cervical ganglion results in a two-fold increase of neuropeptide tyrosine as determined by radioimmunoassay. On the other hand, treatment of sympathetic neuron cultures with leukemia inhibitory factor, a cytokine that is known to be involved in the up-regulation of galanin after axotomy in vivo, decreases neuropeptide tyrosine messenger RNA. These, apparently contradictory findings, prompted us to investigate the regulation of neuropeptide tyrosine in the axotomized superior cervical ganglion in vivo. For comparison, the regulation of galanin was examined under the same conditions. Compared to control ganglia, the number of neuropeptide tyrosine-positive cell bodies decreased while the density of immunoreactive neuronal processes increased one week after transection of the major postganglionic nerves. The nerve fibres were identified as axons by the absence of MAP2, a somatodendritic marker protein. They extended into both carotid nerves and ramified at the lesion site. In situ hybridization revealed that, although the number of neuropeptide tyrosine messenger RNA-positive neurons was not different from controls, the average grain density/neuron decreased by 40%. When axotomized ganglia were decentralized simultaneously, a three-fold elevation of neuropeptide tyrosine immunoreactivity was detectable by radioimmunoassay and an additional increase in numerical density of neuropeptide tyrosine-immunoreactive nerve fibres was observed. Levels of neuropeptide tyrosine messenger RNA were significantly reduced within postganglionic neurons. This synergistic effect of combined axotomy and decentralization on peptide content was also detected for the neuropeptide galanin that, in contrast to neuropeptide tyrosine, is induced by axotomy or decentralization on protein and messenger RNA level. Therefore, while neuropeptide tyrosine messenger RNA is reduced in axotomized ganglia (most likely in response to leukemia inhibitory factor), the peptide accumulates in axonal processes resulting in increased peptide levels as determined by radioimmunoassay.

SNAP25 and GAP-43 behave differently in decentralized rat superior cervical ganglia

The behaviour of synaptosome associated protein of mol. wt 25 kDa (SNAP25) in decentralized rat superior cervical ganglia (SCG) was investigated in order to observe its possible involvement in adrenergic postganglionic neuronal plasticity. Immunofluorescence and immunoblot results showed that the protein was increased in the nerve terminals in irides 8 days after operation. The intra-ganglionic nerve terminals and nerve fibres differed in their content of GAP-43 and SNAP25: GAP-43, which could not be observed at 1 day, appeared at 3 days after cutting the cervical sympathetic trunk, whilst SNAP25-immunoreactive material was still undetectable at this time. Immunoblot data also showed that SNAP25 did not reach control levels at 3 and 8 days after decentralization, in contrast to GAP-43. This observation may imply that SNAP25 is rapidly transported to its functional destinations immediately after synthesis and is possibly mainly involved in the remodelling of long projection pathways.

Clathrin light chain and synaptotagmin I in rat sympathetic neurons

Clathrin light chain (clathrin LC) and synaptotagmin I in sympathetic neurons in rat superior cervical ganglia (SCG) were studied using immunofluorescence and confocal microscopy. The distributions of clathrin LC and synaptotagmin I were compared with that of tyrosine hydroxylase (TH) and neuropeptide Y (NPY) in double label experiments. The influence of preganglionic regulation on the expression of clathrin LC and synaptotagmin I in post-ganglionic adrenergic neurons was investigated after cutting the cervical sympathetic trunk. In SCGs and irides of control animals, the calthrin LC- and synaptotagmin-I-positive structures were present in a granular pattern in nerve fibers and varicose terminals. In principal neurons, the two proteins were present in a perinuclear network (the Golgi complex). After decentralization, the synaptotagmin-I- and clathrin LC-positive granules normally present in preganglionic nerve terminals outlining the neuronal somata were no longer observed on day 1, but reappeared, and were increased above control in number and intensity, in axon bundles in the ganglia, on day 3 and up to day 28 post-decentralization. In irides, the fluorescence intensity and density of clathrin LC- and synaptotagmin-I-positive nerve terminals in the dilator plate, were semi-quantified using the confocal microscopy software. It was found that both proteins increased shortly after decentralization. Immunoblot data confirmed the immunohistochemical/confocal microscopy observations. Fast axonal transport of clathrin LC- and synaptotagmin I in preganglionic sympathetic neurons was demonstrated in crush-operated cervical sympathetic trunk. Both proteins rapidly accumulated proximally as well as distally to the crush, demonstrating fast anterograde and retrograde axonal transport (recycling). Thus, clathrin LC and synaptotagmin I are normally present in pre- as well as post-ganglionic sympathetic neurons. The colocalization of clathrin LC with synaptotagmin I in the Golgi complex of the adrenergic neurons may imply that clathrin participates in the synthesis/sorting of the fast transported materials in these neurons. Possible explanations for the increase of the two proteins after decentralization are discussed.

Reinnervation of frog sympathetic ganglia after selective denervation of B or C neurons

Selective transection of the B or C preganglionic nerve fibres respectively innervating the B and C sympathetic neurons was carried out on the last two ganglia of the sympathetic chain of the frog Rana esculenta. At different times thereafter, the cross-reinnervation of one type of denervated neuron by nerve endings sprouting within the ganglia from intact fibres innervating the other type was investigated by both the quantitative morphology of the synaptic contacts and related structures and electrophysiological recordings of ganglionic transmission. As there are no fine ultrastructural criteria for distinguishing B from C neurons, the overall density of synapse, simple contact, and 'vacated' postsynaptic differentiation profiles was measured in the two cases of selective section and compared with the values for normal ganglia, therefore permitting the progress of cross-reinnervation with time for each type of neuron to be followed. At ten days after section of the C preganglionic fibres, immunocytochemistry showed that there were no anti-LH-RH-like peptide containing fibres within the ganglia. The B myelinated preganglionic fibres were able to reinnervate the denervated C neurons, with return to normal values of synaptic density and fully efficient transmission at two months in all tested C neurons. However, the latency of orthodromic action potentials was close to that of normally innervated B neurons. In contrast, the C non-myelinated preganglionic fibres reinnervated the denervated B neurons with limited efficiency, the synaptic density being two-thirds the normal value after five months, while subthreshold excitatory postsynaptic potentials or action potentials were only recorded in 44% of the tested B neurons. The latency of these orthodromic responses was close to that of normally innervated C neurons. It is postulated that the poor cross-reinnervation of B neurons could be due to insufficient sprouting of C fibres and/or lack of 'affinity' between C fibres and B neurons. In addition, these experiments demonstrated that the subsynaptic apparatus, fairly characteristic of frog ganglionic synapses, is present in both types of sympathetic neurons, although predominantly in B neurons.

Dual character, asynaptic and synaptic, of the dopamine innervation in adult rat neostriatum: a quantitative autoradiographic and immunocytochemical analysis

Dopamine (DA) axon terminals (varicosities) in the neostriatum of adult rats were examined for shape, size, content, synaptic incidence, type of junction, synaptic targets, and microenvironment after electron microscopic identification either by [3H]DA uptake autoradiography or by immunocytochemistry with monoclonal antibodies against DA-glutaraldehyde-protein conjugate. Both approaches yielded comparable results. Whether they were from the paraventricular or the mediodorsal neostriatum, respectively, the [3H]DA-labeled and DA-immunostained varicosities were generally oblong and relatively small; more than 60% contained one or more mitochondria. Sixty to seventy percent were asynaptic, and 30-40% were endowed with a synaptic membrane differentiation (junctional complex), as inferred by stereological extrapolation from single thin sections (both approaches) or observed directly in long, uninterrupted series of thin sections (immunocytochemistry). The synaptic DA varicosities always displayed symmetrical junctions: 67% with dendritic branches, 30% with dendritic spines, and 2-3% with neuronal cell bodies. DA varicosities juxtaposed to one another were frequent. Other axonal varicosities were more numerous in the immediate vicinity of DA varicosities than around randomly selected, unlabeled terminals. The respective microenvironments of DA and unlabeled varicosities also showed enrichment in the preferred synaptic targets of both groups of varicosities, with dendritic branches for DA and dendritic spines for the unlabeled ones. These data suggest a dual mode of operation that is diffuse as well as synaptic for the nigrostriatal DA system. In such a densely DA-innervated brain region, they also lead to the hypothesis that a basal level of extracellular DA might be maintained permanently around every tissue constituent and, thus, contribute to the mechanisms of action, properties, and functions (or dysfunctions) of DA within the neostriatum itself and as part of the basal ganglia circuitry.

Ultrastructure of neuronal circuitry in sympathetic ganglia

To elucidate the intraganglionic circuitry in sympathetic ganglia, attempts have been made to define the nature and source of those neuronal elements that establish synaptic connections there. Intracellular labeling of sympathetic cells is of particular value for this purpose. Dendrites of principal neurons in the rat superior cervical ganglion exhibit a varying complexity in their morphology and arborization. Some dendrites show specializations such as a glomerular plexus, where extensively-branched dendritic collaterals form synaptic connections comprising not only axodendritic synapses between preganglionic axons and principal cell dendrites, but also dendrodendritic synapses between principal cell dendrites. A few of these may represent reciprocal synapses. Most presynaptic elements of adrenergic synapses observed by conventional methods appear to represent these specialized dendritic collaterals of principal neurons. These presynaptic dendrites may be an important addition to the conventional scheme of intraganglionic synaptic organization. However, there seem to be extreme species and even strain differences in the number of these adrenergic synapses, and in the sophistication of the specialized local circuits within sympathetic ganglia. Sympathetic ganglia may thus function as more than a simple relay station, with specialized neuronal circuitry that may be involved in the modulation of cholinergic transmission.

Localization and axotomy-induced regulation of the peptide secretoneurin in the rat superior cervical ganglion

This study demonstrates the localization and regulation of a novel neuropeptide of 33 amino acids, secretoneurin (SN), in the rat superior cervical ganglion. Gel filtration chromatography of ganglion proteins followed by a specific radioimmunoassay revealed that SN is the predominant cleavage product of secretogranin II, a member of the chromogranin/secretogranin protein family, in adult ganglia. SN was detected within the majority of nerve endings surrounding postganglionic neurons that were identified by the presence of synaptophysin and, in part, colocalized leu-encephalin. Applying immuno-electronmicroscopy, SN was localized to large dense core vesicles of neuronal and small intensely fluorescent (SIF) cells. In situ hybridization revealed the presence of secretogranin II mRNA in postganglionic neurons and, to a lesser extent, in SIF cells. One week after transection of the postganglionic branches SN levels were not significantly altered; however, a decrease of secretogranin II mRNA was observed in postganglionic neurons but not in SIF cells. After decentralization of the ganglion, SN-immunoreactive nerve terminals disappeared and intraganglionic SN levels were reduced by 70%, indicating the preganglionic origin of SN-positive nerve fibres and varicosities. Secretogranin II mRNA was slightly reduced under this condition. Combined axotomy and decentralization further diminished intraganglionic secretogranin II mRNA, although peptide levels increased significantly above control values under these conditions. Double-labelling immunofluorescence with antibodies against the somatodendritic marker microtubule-associated protein 2 (MAP2) revealed that the increase in SN immunoreactivity was due to an accumulation of SN in axonal processes of postganglionic neurons. SN immunoreactivity was also detected in dissociated neonatal superior cervical ganglion cultures and increased significantly upon treatment with nerve growth factor, the survival and differentiation factor of sympathetic neurons during perinatal development. Co-culture with non-neuronal cells or addition of leukaemia inhibitory factor, a cytokine known to stimulate synthesis of various peptides after nerve transection, did not influence SN immunoreactivity. Therefore, since no fixed relationship between SN and any of the known neuropeptides or neurotransmitters expressed in sympathetic neurons was observed, the expression of this novel peptide appears to be independently regulated.

Autoradiographic localization of functional muscarinic receptors in the rat superior cervical sympathetic ganglion reveals an extensive distribution over non-synaptic surfaces of neuronal somata, dendrites and nerve endings

Fast synaptic transmission in sympathetic ganglia is mediated by acetylcholine, acting on nicotinic receptors, yet muscarinic receptors are also present and are involved in the production of slow postsynaptic potentials. In order further to elucidate the role of muscarinic receptors in ganglionic transmission their distribution in the rat superior cervical sympathetic ganglion was investigated autoradiographically by use of the tritiated irreversible muscarinic ligand propylbenzilylcholine mustard. It was observed that this agent blocked the carbachol-evoked hydrolysis of inositol phospholipids in the ganglion and that this response to carbachol is itself inhibitable by selective muscarinic antagonists with a potency sequence which indicates involvement primarily of M1 receptors. Light microscope autoradiography showed that labelling inhibitable by atropine and by the M1-selective muscarinic antagonist pirenzepine was essentially confined to the margins of neuronal somata and regions of dendritic arborization, which include synaptic contacts. Quantitative electron microscope autoradiography showed that binding of the radioligand, of which approximately 70% was inhibitable by atropine and 68% by pirenzepine, was associated predominantly with surface membranes of neuronal somata, dendrites, other neurites (including axons and uncharacterized dendrites) and nerve terminal profiles, in the approximate ratios 95:85:52:45. Of the inhibitable binding over neuronal membranes in the ganglion little more than 3% was found to be synaptically located, and this involved para- or peri-synaptic regions of nerve terminal contacts rather than the specialized synaptic zone. About 5% of the inhibitable binding over neuronal membranes involved non-synaptic surfaces of nerve terminals and preterminal axon segments; almost 70% was distributed over non-synaptic surfaces of neuronal somata and dendrites, and about 21% upon other neurites. Binding sites were found not to be more highly concentrated at or adjacent to synapses than over other regions of neuronal surface membranes. About 50%, possibly more, of the binding on non-synaptic surfaces of nerve endings, and about 7% of binding upon dendritic membranes, was of non-M1, possibly M2 type, inhibitable by atropine but not by pirenzepine. Non-synaptic neuro-neuronal appositions, which involve dendrites and somata and often lie adjacent to synapses, showed rather more than twice the binding expected for each membrane individually; and neuronal membrane exposed to basal lamina lining ganglionic tissue spaces showed high levels of binding. Little inhibitable binding was seen over membranes of satellite and Schwann cells, or over cytoplasmic territories or ganglionic interstitial tissue. A model was constructed of the distribution of label, which showed that the observed results for total binding could be approximately matched by assuming the following relative densities of ligand binding sites: interstitial tissue space and supporting cells 1, soma cytoplasm 3, cytoplasm of dendrites, neurites and nerve terminals 4.5, surfaces of mesodermal elements 15, surfaces of neurites and nerve endings including sites of synapse 45, surfaces of dendrites 90, surfaces of neuronal somata 120, non-synaptic neuro-neuronal appositions 180. It is concluded that functional muscarinic receptors in this sympathetic ganglion, predominantly of the M1 type linked with slow depolarizations, but including some non-M1 receptors, are widely distributed over non-synaptic surfaces of the neuronal somata and dendrites and are not concentrated at synapses. Presynaptic autoreceptors are also present, of which half or more are of non-M1, possibly M2, type which might be inhibitory. The presence of M4 receptors is not excluded...

Correlation between dendrodendritic synapses of adrenergic type and synaptically evoked hyperpolarization in the sympathetic ganglion of adult rats

Intracellular recording and labeling with biocytin followed by electron microscopic observation were used to examine the nature and the morphological basis of a synaptically evoked hyperpolarization following spikes in the rat superior cervical ganglion neurons. A large hyperpolarization (the amplitude > 8 mV; the duration > 1 s following spikes) was elicited by repetitive stimulation of the preganglionic nerves in 8% of cells examined (n = 50). The alpha 2-adrenoceptor antagonist, yohimbine, reversibly attenuated the hyperpolarization, without affecting spikes. A nicotinic antagonist, hexamethonium, blocked both the hyperpolarization and spikes. Atropine had no effect of these responses. Electron microscopic observation of dendrites of these cells revealed that they received synaptic inputs of adrenergic type besides a cholinergic one from the preganglionic axons. Some dendrites served as presynaptic elements. These results strongly suggest that the hyperpolarization is an inhibitory postsynaptic potential and that this disynaptic response to the preganglionic stimulation is mediated mainly by two transmitters, acetylcholine and noradrenaline that are released from axodendritic and dendrodendritic synapses, respectively. We conclude that there appears to be an adrenergic inhibitory local circuit that modulates cholinergic transmission in the sympathetic ganglia.

Reinnervation of frog sympathetic ganglia with somatic nerve fibres

The formation of synapses in the last two ganglia of the sympathetic chain of the frog Rana esculenta was investigated after anastomosing the 6th spinal nerve to the denervated ganglia in order to evaluate the reinnervation of deafferented sympathetic neurons with somatic cholinergic axons. The same ganglia were examined both electrophysiologically and morphologically from 25 to 280 days after the operation. In response to electrical stimulations of the anastomosed spinal nerve, synaptic transmission was analysed with intracellular microelectrodes placed into B or C sympathetic neurons. Synaptic density was quantified using electron microscopy by a synaptic index defined as the ratio of the number of synapses encountered to the number of perykarya examined. After ganglionic deafferentation, post-synaptic membrane differentiations persisted without any pre-synaptic element and an index of the 'vacated' post-synaptic differentiations was calculated. Although somatic axons were growing into all ganglia studied, no sign of neuronal reinnervation was detected in ganglia of 8 of the 31 frogs (26%) taken from 29 to 210 days after the anastomosis. Moreover, in 18 out of 31 frogs (58%) analysed at different times after the operation, the ganglia were reinnervated with regenerating preganglionic axons in spite of care taken to avoid it. However, even after 3 months, certain neurons of these ganglia were not reinnervated and the synaptic index approximated the value of normal ganglia only in the 8th ganglion. In addition, post-synaptic membrane differentiations could still persist and coexist with normal synapses. It was only beyond three months after the anastomosis that the ganglia of 5 of the 31 frogs (16%) were reinnervated with regenerating somatic axons. Reinnervated B and C neurons were polyinnervated. But in 3 out of these 5 frogs the ganglia were also reinnervated with preganglionic axons and several B and C neurons received a double reinnervation. The synaptic indices were far from the value of normal ganglia except for the 8th ganglion of one frog reinnervated by both types of axons and the indices of vacated differentiations were close to that of ganglia with no reinnervated neurons. Contrary to mammals, frog somatic axons are, therefore, relatively ineffective at reinnervating sympathetic neurons, probably because in amphibian ganglia, synapses between the pre- and post-synaptic elements require higher specificity.

Effects of axotomy, deafferentation, and reinnervation on sympathetic ganglionic synapses: a comparative study

The main physiological and morphological features of the synapses in the superior cervical ganglia of mammals and the last two abdominal ganglia of the frog sympathetic chain are summarized. The effects of axotomy on structure and function of ganglionic synapses are then reviewed, as well as various changes in neuronal metabolism in mammals and in the frog, in which the parallel between electrophysiological and morphological data leads to the conclusion that a certain amount of synaptic transmission occurs at "simple contacts." The effects of deafferentation on synaptic transmission and ultrastructure in the mammalian ganglia are reviewed: most synapses disappear, but a number of postsynaptic thickenings remain unchanged. Moreover, intrinsic synapses persist after total deafferentation and their number is strongly increased if axotomy is added to deafferentation. In the frog ganglia, the physiological and morphological evolution of synaptic areas is comparable to that of mammals, but no intrinsic synapses are observed. The reinnervation of deafferented sympathetic ganglia by foreign nerves, motor or sensory, is reported in mammals, with different degrees of efficiency. In the frog, the reinnervation of sympathetic ganglia with somatic motor nerve fibers is obtained in only 20% of the operated animals. The possible reasons for the high specificity of ganglionic connections in the frog are discussed.

The effect of chronic decentralization on the enkephalin immunoreactive plexus around penile ganglionic neurons

Target organ responses to sympathetic nerve stimulation are altered following partial decentralization of the pelvic plexus in the rat. One possible explanation for the new responses is that nerve injury has led to a reorganization of synaptic connections within pelvic ganglia. Since one measure of synaptic influence is the occurrence of a pericellular plexus of varicose fibers around autonomic ganglion cells, the present study has used immunocytochemistry for enkephalin (ENK), a peptide present in nerve fibers in the pelvic plexus, to follow changes in the innervation of penile ganglionic neurons after interruption of preganglionic pathways. Penile ganglion cells were located by the injection of the tracer Fluorogold into the penile crura. Four days after lesion of the pelvic nerve, innervation of penile neurons falls from 76% to 20%. This number increases however, to 31% in chronically (6 weeks) lesioned animals. In the totally decentralized ganglia, ENK immunoreactive (IR) fibers enclose fewer than 12% of the penile neurons 4 days after nerve lesion. However, this value rises to 35% in the chronically decentralized pelvic ganglion. Therefore, recovery of an enkephalin plexus occurs irrespective of whether the pelvic nerve, or both the hypogastric and pelvic nerve have been cut. Although these findings suggest sprouting within partially decentralized ganglia, the similar incidence of an ENK plexus in ganglia subjected to chronic partial or total decentralization indicates that preganglionic fibers are not responsible for the emergent fibers.

Principal neurons as local circuit neurons in the rat superior cervical ganglion: the synaptology of the neuronal processes revealed by intracellular injection of biocytin

To analyze the local circuitry of the sympathetic ganglion, the synaptic relations of the neuronal processes of the principal neurons in the rat superior cervical ganglion were investigated by correlated light and electron microscopy combined with intracellular injection of biocytin. Intracellular iontophoresis of biocytin followed by avidin-biotinylated horseradish peroxidase cytochemistry allowed complete visualization of the neuronal processes of the principal neurons. The stained principal neurons have a single process (axon), which leaves the ganglion, and several intraganglionic processes (dendrites), some of which show specific terminal arborizations. Some terminals of the dendritic collaterals formed pericellular plexuses or intercellular glomerular plexuses. Electron microscopically, the dendrites and their collaterals contain numerous small vesicles. Synaptic membrane specializations were observed between the stained dendritic collaterals and unlabeled neurites. These may be both preganglionic axon terminals and processes of principal neurons. The likely direction of neurotransmission often could not be determined because of the bidirectional synaptic structures. Our findings show that the dendritic collaterals of principal neurons appear to make both post- and presynaptic contacts with both the principal neurons and the preganglionic axons. It is suggested that the principal neurons might participate in local circuits involving not only preganglionic axons but also neighboring principal neurons.

Increased expression of T alpha 1 alpha-tubulin mRNA during collateral and NGF-induced sprouting of sympathetic neurons

We have examined expression of T alpha 1 alpha-tubulin mRNA in the rat superior cervical ganglion (SCG) to determine whether changes in gene expression accompany neuronal sprouting and to investigate factors that regulate growth-associated genes in intact neurons. Northern blot analysis demonstrates that levels of T alpha 1 alpha-tubulin mRNA increase in the uninjured SCG following transection of contralateral neurons that project to bilaterally innervated, but not unilaterally innervated target organs. The observed increase in uninjured neurons is associated with collateral sprouting, as measured by increased tyrosine hydroxylase immunoreactivity within the pineal gland. These data suggest that target-derived factors may regulate T alpha 1 mRNA in sprouting neurons. Consistent with this hypothesis, systemic NGF treatment of neonatal animals over a developmental interval when T alpha 1 alpha-tubulin mRNA normally decreases led to a 5- to 10-fold increase in T alpha 1 mRNA levels in developing sympathetic neurons. In addition, deafferentation of the SCG, which promotes neuronal sprouting in the ganglion, increases T alpha 1 mRNA in ganglia on the ipsilateral and contralateral sides. Together, these data demonstrate that T alpha 1 alpha-tubulin mRNA elevates as a function of neuronal sprouting, and that T alpha 1 mRNA expression in intact neurons can be regulated by extrinsic cues, including NGF and changes in connectivity.

Penile erection in the rat: stimulation of the hypogastric nerve elicits increases in penile pressure after chronic interruption of the sacral parasympathetic outflow

Penile erection, a vascular event mediated by the autonomic nervous system, is often adversely affected by injury to the spinal cord. To further characterize the laboratory rat as an animal model of penile erection and to investigate erectile responses following neural injury, the present study has examined pressor penile responses in intact rats and in animals deprived of sacral parasympathetic outflow. Increases in penile pressure result from graded stimulation of postganglionic parasympathetic fibers. The vasodilator response is insensitive to blockade with atropine, a cholinergic antagonist. Penile tumescence also results from stimulation of the pelvic nerve, but not the hypogastric nerve. However, beginning 3 days after unilateral interruption of the pelvic nerve, stimulation of the ipsilateral hypogastric nerve results in an increase in penile pressure. This novel response, which is blocked by a ganglionic antagonist, is maximally developed at 1 week post-lesion, is stable for at least 3 months and remains confined to the side of the lesion. These results suggest that the rat, although relatively small, can be used to obtain quantitative data on penile erection. Moreover, the model may lend itself to an analysis of the mechanisms of altered control of visceral tissues following injury to the nervous system.

Degeneration and regeneration in the superior cervical sympathetic ganglion after Latrodectus venom

The effects of the venom of the spider Latrodectus mactans hasselti on the superior cervical ganglion were studied in the guinea pig. Under anaesthesia the ganglion was bathed in venom solution for 15 min. Shortly afterwards animals salivated profusely and later developed unilateral ptosis and enophthalmos. Postoperative survival times ranged from 15 min to 10 weeks. Electron microscopy showed acute swelling of preganglionic cholinergic nerve terminals, followed by degeneration with separation of synapses. Other ganglionic elements appeared to be undamaged, although after detachment of synapses the dendritic postsynaptic specializations were reduced in number. Recovery was very rapid; axon growth cones were identifiable at 18 h and synapse reformation was well established by 2 weeks. With longer survival times there was progressive restoration of normal morphology such that by 8 weeks regeneration appeared complete. These experiments indicate that the preganglionic cholinergic nerve terminals are selectively affected by Latrodectus venom and have a considerable capacity for appropriate regeneration.

Neuroneuronal interconnections in the rat superior cervical ganglion; possible anatomical bases for modulatory interactions revealed by intracellular horseradish peroxidase labelling

Electrophysiologically identified neurons of rat superior cervical ganglion were intracellularly injected with horseradish peroxidase and processed for light and electron microscopic observation. At light microscope level, neurons could be classified according to their dendritic arborization pattern in the vicinity of the soma into radiate, tufted and intermediate types. Upon electrical stimulation of the internal and external carotid nerves it was observed that radiate and intermediate neurons sent their axons into one or the other of these nerve trunks, whereas a majority of tufted neurons gave no response to stimulation of either of these postganglionic nerves. Electron microscopic exploration of horseradish peroxidase-labelled neurons revealed a surprisingly high prevalence of interconnectivity between ganglionic neurons. These contacts were both dendrosomatic and dendrodendritic, and were a universal feature of the labelled neurons explored. Twenty-two of the 23 labelled cells were found to receive direct dendritic appositions on their somata, and 13 of these 23 cells were seen each to send their dendrites into contact with at least one unlabelled neuronal soma. Dendrodendritic contacts were observed for 87% of the labelled neurons, and most of the cells (80%) were seen to form triadic contacts which included two dendrites and a preganglionic nerve ending. All these figures represent minimum incidences. None of the dendrosomatic or dendrodendritic appositions observed was overtly synaptic although several morphological features indicated the possibility of somatic and or dendritic release and uptake at sites of apposition. It is suggested that the observed appositions provide anatomical substrates for modulatory interactions between the ganglionic neurons, possibly involving slow potentials or the switching of metabolic pathways.

Morphology of synapses in the autonomic nervous system

The ultrastructure of synapses in the autonomic nervous system is reviewed. The synaptic organization of the parasympathetic ganglia is relatively simple. Preganglionic axons form synapses either on the soma or on short perikaryal processes of the ganglionic neurons. The presynaptic terminals have a cholinergic morphology and contain mainly small clear vesicles with a few large dense cored vesicles. A few neuropeptides have been localized to the large dense cored vesicles of these terminals. The postganglionic parasympathetic axons ramify within their target tissues where they form close associations, but not true synaptic contacts. Sites of release of transmitter are recognized morphologically as varicosities along the length of the axon that contain clusters of small clear vesicles with a few large dense cored vesicles. The organization of the sympathetic nervous system is somewhat more complex. In addition to acetylcholine, enkephalin also exists in these terminals, probably in the large dense cored vesicles. There are at least three types of ganglion cell neurons in the paravertebral portion of the sympathetic nervous system: those that contain norepinephrine alone, those that contain norepinephrine along with neuropeptide Y, and those that contain acetylcholine and vasoactive intestinal polypeptide. The first type provides innervation to the parenchyma of the target tissues, while the second mainly innervates blood vessels. The third type innervates the sweat glands. In the prevertebral ganglia, a fourth type of neuron exists that contains norepinephrine and somatostatin. This neuron probably innervates the gut. Preganglionic terminals of the cholinergic type form synaptic connections mainly with the dendrites of the sympathetic ganglion neurons. In addition to the types of synapses described for the paravertebral ganglia, neurons in the prevertebral ganglia receive synaptic connections from dorsal root ganglia and from the enteric nervous system. The sympathetic ganglia also contain interneurons that receive preganglionic synapses and form efferent synapses with some of the principal ganglion cells. The interneurons have been shown to contain a variety of transmitters, including norepinephrine, epinephrine, dopamine, serotonin, and a number of neuropeptides. The postganglionic sympathetic axons have a similar morphology to the parasympathetic axons. They form networks in their targets, and the axons display varicosities with concentrations of both small and large vesicles. After appropriate fixation, these vesicles are seen to possess dense cores.(ABSTRACT TRUNCATED AT 400 WORDS)