Responses of normal and denervated cat superior cervical ganglia to some stimulant compounds

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.

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.

Subcellular innervation patterns of the calcitonin gene-related peptidergic efferent terminals in the chinchilla vestibular periphery

We examined the ultrastructural distribution of calcitonin gene-related peptide immunoreactivity in the peripheral vestibular system of the chinchilla to study the innervation patterns of this efferent neuropeptide. Immunoelectron microscopic localization of calcitonin gene-related peptide immunoreactive terminals in the maculae and cristae revealed an extensive innervation pattern on the afferent vestibular pathway. Calcitonin gene-related peptide immuno-reactive terminals made synaptic contacts with the unmyelinated portions of the primary afferent vestibular dendrites innervating both type I and type II hair cells. Abundant synaptic contact between calcitonin gene-related peptide immunoreactive terminals and the chalices surrounding type I hair cells was observed. Direct contact between calcitonin gene-related peptide immunoreactive terminals and type II hair cells was observed. In addition, vesiculated efferent terminals without calcitonin gene-related peptide immunoreactivity were seen synapsing on the chalices of type II hair cells and on the surrounding type I hair cells. The primary afferent somata in the vestibular ganglion of Scarpa did not contain calcitonin gene-related peptide immunoreactivity. Unmyelinated calcitonin gene-related peptide immunoreactive axons passed among the primary afferent fibers in Scarpa's ganglion, and these fibers continued through the subepithelial regions of the vestibular end-organs. The calcitonin gene-related peptide immunoreactive axons ramified to produce numerous calcitonin gene-related peptide immunoreactive terminals throughout the neurosensory epithelium of the maculae and cristae. These data suggest that calcitonin gene-related peptide-mediated modulation of the afferent vestibular system is functionally important.

The role of acetylcholinesterase in denervation supersensitivity in the frog cardiac ganglion

1. The sensitivity of normal and denervated cardiac ganglion cells to the cholinergic agonists acetylcholine and carbamylcholine (carbachol) were compared in the frog, Rana pipiens. Acetylcholine and carbachol bind to the same acetylcholine receptors, but, unlike acetylcholine, carbachol is resistant to hydrolysis by acetylcholinesterase. 2. Sensitivity was assessed by the peak depolarization elicited in response to a sustained pulse of ligand emitted from a pipette positioned 10 microns from the ganglion cell surface. This technique allows the sensitivity of the entire cell to be recorded with a single measurement. 3. The acetylcholine sensitivity of normal cardiac ganglion cells was increased by inhibiting extracellular acetylcholinesterase with echothiophate. 4. Denervation increased the sensitivity of cardiac ganglion cells to acetylcholine but not to carbachol. 5. Following the inhibition of extracellular acetylcholinesterase with echothiophate, sensitivity to acetylcholine was similar in normal and in denervated ganglion cells. 6. The increased sensitivity to acetylcholine of cardiac ganglion cells following denervation is caused by a reduction in the hydrolysis of the transmitter by acetylcholinesterase rather than by changes in the number and/or properties of acetylcholine receptors.

Organization and plasticity of GABA neurons and receptors in monkey visual cortex

The GABA neurons of monkey area 17 are a morphologically and chemically heterogeneous population of interneurons that are normally distributed most densely within the geniculocortical recipient zones of the visual cortex. In adult monkeys deprived of visual input from one eye, the levels of immunoreactivity for GABA and GAD within neurons of these geniculocortical zones is reduced. Similar changes are seen in the levels of proteins that make up the GABAA receptor sub-type. The effects of monocular deprivation on other substances suggest that specific types of GABA neurons, such as those in which the tachykinin neuropeptide family and parvalbumin coexist with GABA, are greatly influenced by changes in visual input. That some proteins remain normal within deprived-eye neurons and that other proteins are increased indicates the changes in the GABA cells of the cortex are not the result of a general reduction in protein synthesis. Comparisons of what is known about the morphological and synaptic features of GABA cells in area 17 and the characteristics of cells affected by monocular deprivation suggests that certain classes, such as the clutch cell, may be preferential targets of deprivation. Such a selective loss of certain GABA neurons would have broad implications for the possible physiological plasticity of cortical cells, for if ongoing studies determine that specific receptive field properties are affected by monocular deprivation in adults, the correlation of functional properties and classes of GABA cells would be possible.

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.

Tetramine: occurrence in marine organisms and pharmacology

The occurrence of tetramethylammonium ion (TMA) in marine organisms is reviewed. The pharmacological action of TMA is also discussed, with special emphasis on the sign and symptoms experienced by oral poisonings. It is concluded that the major manifestations of TMA poisonings may be attributed to interactions with the autonomous nervous system. Fatal intoxications are due to neuromuscular blockade.

Neural regulation of acetylcholine sensitivity in embryonic sympathetic neurons

The development of transmitter sensitivity is an important component of synaptic differentiation. Despite a wealth of information about the appearance of acetylcholine (AcCho) sensitivity at the neuromuscular junction, the onset and regulation of this critical aspect of synaptogenesis has not previously been examined for synapse formation between neurons. To determine whether there is a role of presynaptic input in the induction of AcCho sensitivity at interneuronal synapses, AcCho-induced currents were measured in embryonic sympathetic neurons before and after synapse formation in vitro. The total AcCho sensitivity of postsynaptic neurons was increased nearly 10-fold after innervation. The effects of innervation are mimicked by medium conditioned by preganglionic neurons, suggesting that presynaptic neurons regulate postsynaptic AcCho sensitivity by release of a soluble factor. These observations provide evidence that presynaptic input regulates neuronal sensitivity to an identified synaptic transmitter.

Lack of nicotinic supersensitivity in frog sympathetic neurones following denervation

The sensitivity of bull-frog sympathetic neurones to nicotinic, cholinergic agonists has been studied in both normal (control) and surgically denervated ganglia. Using gross extracellular recording, the sensitivity to acetylcholine (ACh) increased 18-fold following denervation, while that to carbachol (CCh) was unchanged. Normal ganglia showed a similar sensitivity increase after inhibition of cholinesterase. This suggests that the rise in ACh sensitivity is due to reduced cholinesterase activity, not to true supersensitivity. There was no significant difference in resting membrane potential or input resistance between normal and denervated neurones. Neurones denervated for 7-50 days showed no significant change in sensitivity to ACh or CCh applied iontophoretically at a distance of 10 micron from the cell surface. In control ganglia, localized iontophoretic application of ACh revealed an uneven distribution of sensitivity which is attributed to the localization of receptors to synaptic areas. Fourteen days after denervation, the geometric mean sensitivity to focally applied ACh was not significantly different from that found in control ganglia. The variation in sensitivity to focally applied ACh at randomly chosen sites on denervated neurones was as great as that found in control ganglia. It is concluded that denervation does not cause frog sympathetic neurones to become supersensitive to ACh. The apparent increase in nicotinic ACh sensitivity observed using extracellular recording from whole ganglia is due not to a change in the number or distribution of ACh receptors, but to a decrease in cholinesterase activity.

Specific binding of alpha-bungarotoxin to synaptic membranes in rat sympathetic ganglion: computer best-fit analysis of electron microscope radioautographs

In the rat superior cervical sympathetic ganglion (SCG), alpha-bungarotoxin (alpha BT) demonstrates binding that is saturable and inhibited by nicotinic ligands. However, alpha BT does not inhibit the physiological response of ganglionic neurons to preganglionic stimulation or to exogenously applied acetylcholine. Thus the specificity of alpha BT for ganglionic nicotinic cholinergic receptors has been questioned. The present study provides a morphological localization of the binding sites of 125I-labelled alpha BT in the rat SCG using the method of Blackett and Parry on electron microscopic radioautographs. The distribution of grains resulting from specific binding was calculated by subtracting the nonspecific distribution (alpha BT in the presence of D-tubocurarine, a known nicotinic ligand) from the total grain distribution (alpha BT alone). A hypothetical grain distribution was obtained based on the geometrical properties of the tissue sections. A computer minimizing routine was employed to adjust the relative weights of each of the potential sources of hypothetical grains until a 'best-fit' with the real grain distributions occurred. The nonspecific binding of alpha BT was uniform across all tissue components, with the exception of a significant concentration on the membrane of the ganglion cell body. By contrast, the specific binding of alpha BT was highly localized to synaptic membranes, and to a lesser extent, to dendritic membranes.

Muscarinic receptors in the central nervous system of the rat. IV. A comparison of the effects of axotomy and deafferentation on the binding of [3H]propylbenzilylcholine mustard and associated synaptic changes in the hypoglossal and pontine nuclei

The reaction of axotomy has been studied in the rat hypoglossal nucleus by quantitative electron microscopical counts of numbers of synapses and by changes in muscarinic receptors assessed by counting silver grains in light microscope autoradiographs of the specific (atropine-sensitive) binding of [3H]propylbenzilylcholine mustard in cryostat sections. For the first 5 days after unilateral peripheral hypoglossal nerve axotomy the muscarinic ligand binding falls to 50% of control levels and then shows no further fall for up to 30 days. Synapse numbers decrease progressively over the first 10 days after operation, by which time they reach 50% of normal. Thus receptor changes reach completion at a time when synapse loss is still continuing. Later, both muscarinic ligand binding and synapse numbers recover to an extent which depends at least in part on the effectiveness of the peripheral nerve regeneration, suggesting that both the receptor and synapse changes may be dependent upon neuromuscular contacts. The reactions of muscarinic receptors to axotomy and deafferentation have been studied in the rat basilar pontine nuclei. Cerebellectomy, which causes axotomy of the pontine neurones and also removes their postsynaptic targets (the granule cells), causes no change in pontine muscarinic receptor over the first week after operation. This differs from the rapid fall in hypoglossal muscarinic receptors induced by axotomy. At longer survivals after cerebellectomy there is a partial loss of pontine muscarinic receptors associated with atrophy of the pontine neurones. Destruction of the neocortical afferents causes a loss of at least half of the synapses in the pontine neuropil. However, the light microscopic autoradiographic study revealed no obvious changes in the dentisy or distribution of the pontine muscarinic receptors from 4 days to more than 6 months after operation.

Denervation supersensitivity in the cholinergic septo-hippocampal pathway: a microiontophoretic study

The purpose of this study was to determine whether denervation supersensitivity could be produced in an identified cholinergic pathway in the CNS of the rat. The mechanism for the development of this phenomenon was also explored. Cholinergic denervation of the hippocampus was accomplished by lesions of the medial septum. The response of hippocampal pyramidal cells to microiontophoretic application of acetylcholine (ACh) and carbachol in lesioned and unlesioned animals was determined by extracellular recording. There was a marked increase (6 X) in sensitivity to ACh 2-43 days following lesions. However, there was no increase in sensitivity to carbachol or glutamate. Other workers have shown that septal lesions cause a large decrease in hippocampal acetylcholinesterase (AChE) which is located mainly presynaptically. The absence of increase in sensitivity to carbachol, a cholinomimetic resistant to hydrolysis by AChE, suggests that the postlesion increase in sensitivity to ACh results from a decrease in its inactivation by AChE. A time course for the development of ACh supersensitivity was found to be similar to the time course of AChE loss previosly reported. Experiments using physostigmine, an AChE inhibitor, demonstrated that inhibition of AChE can potentiate the effects of ACh in unlesioned preparations, but not in lesioned preparations. We conclude: (1) denervation supersensitivity to ACh occurs in the septo-hippocampal pathway; and (2) the supersensitivity is probably due to decreased inactivation of ACh by AChE. The results suggest that presynaptic AChE plays a significant role in modulating the neurotransmitter function of ACh in the septo-hippocampal pathway.

Cholinergic sensitivity: normal variability as a function of stimulus background

The sensitivity of the normally innervated iris sphincter to its neuro-transmitter, acetylchloline, and to relatd agents varies inversely with the preexisting physiological stimulus background, that is, the environmental light intnsity. This normal variability suggests the existence of a negative feedback mechahnism whereby sensitivity of the effector cell is modutlated by a product of neuronal activity.

The release of acetylcholine by acetylcholine in the cat's superior cervical ganglion

1. The experiments described in this paper tested the effect of acetylcholine (ACh), carbachol or preganglionic nerve stimulation on the release of ACh from the cat's perfused superior cervical ganglion; radioactive tracer methods were used.2. When the ganglion's transmitter store of ACh had been labelled, radioactive ACh was released by nerve stimulation (5 Hz for 2 min), but there was no release by ACh (0.15-15 mug) or by carbachol (1-10 mug) when these drugs were injected close to the ganglion. Perfusion with low or moderate concentrations of ACh (0.15-5 mug/ml) also failed to release ACh, but high concentrations (15-50 mug/ml) released a small amount of labelled material. There was no correlation between ganglion stimulation by ACh and release of radioactivity.3. Ganglion-blocking concentrations of ACh did not reduce the release of ACh during continuous nerve stimulation.4. When resting (unstimulated) ganglia were perfused with (3)H-choline and eserine, the extra ACh synthesized and stored by such ganglia (surplus ACh) was labelled. Preganglionic nerve stimulation (5 Hz for 2 min) did not release surplus ACh, but perfusion with ACh (0.5-15 mug/ml), or injection of carbachol (0.5-2.5 mug) did.5. Surplus ACh released by ACh or by carbachol did not contribute to the ganglion stimulating effect of either drug.6. It is concluded that the presynaptic effects of ACh are not of physiological importance.