Glycine-like immunoreactive input to sympathetic preganglionic neurons

Impaired Verbal Memory Recall in Patients With Axonal Degeneration and Serum Glycine-Receptor Autoantibodies-Case Series

BACKGROUND

Glycine receptor antibody-associated neuropsychiatric disease is currently known to be dominated by the phenotypes stiff-person syndrome and progressive encephalomyelitis entailing rigidity and myoclonus. In our case series we aim to depict the less-often reported feature of cognitive impairment associated with glycine receptor antibodies.

METHODS

We investigated five patients with cognitive impairment varying from mild cognitive impairment to dementia associated with serum glycine receptor antibodies. Mild and major neurocognitive disorders were diagnosed according to the DSM-5 (fifth edition of the Diagnostic and Statistical Manual of Mental Disorders). Neuropsychology via Consortium to Establish a Registry for Alzheimer's Disease (CERAD) testing results, psychopathology data via the Manual for the Assessment and Documentation of Psychopathology in Psychiatry (AMDP), cerebrospinal fluid analysis and magnetic resonance imaging data were retrospectively analyzed from patient files.

RESULTS

We identified five patients with cognitive impairment as the main neuropsychiatric feature associated with serum glycine receptor antibodies. One patient also presented akinetic rigidity syndrome. The psychopathology comprised disorders of attention and memory, orientation, formal thought, and affect. In addition to suffering deficits in verbal memory function, figural recall, phonematic fluency, and globally deteriorated cognitive function, these patients presented seriously impaired memory recall in particular. Tau protein and phosphorylated tau protein 181 were elevated in 75% of patients.

CONCLUSIONS

Our results suggest that axonal neurodegeneration and especially impaired verbal memory recall in addition to deficits in verbal and figural memory characterize patients with progressive cognitive impairment associated with glycine receptor antibodies. This unresolved issue should be clarified by researchers to discover whether axonal degeneration is merely an age-related phenomenon or one related to glycine-receptor autoantibodies in old age. Cognitive impairment as a neuropsychiatric syndrome of glycine-receptor antibody disease is a potential, conceivable, but so far unproven additional feature requiring deeper large-scale investigations and consideration during differential diagnosis in memory clinics.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

Autoimmune channelopathies in paraneoplastic neurological syndromes

Paraneoplastic neurological syndromes and autoimmune encephalitides are immune neurological disorders occurring or not in association with a cancer. They are thought to be due to an autoimmune reaction against neuronal antigens ectopically expressed by the underlying tumour or by cross-reaction with an unknown infectious agent. In some instances, paraneoplastic neurological syndromes and autoimmune encephalitides are related to an antibody-induced dysfunction of ion channels, a situation that can be labelled as autoimmune channelopathies. Such functional alterations of ion channels are caused by the specific fixation of an autoantibody upon its target, implying that autoimmune channelopathies are usually highly responsive to immuno-modulatory treatments. Over the recent years, numerous autoantibodies corresponding to various neurological syndromes have been discovered and their mechanisms of action partially deciphered. Autoantibodies in neurological autoimmune channelopathies may target either directly ion channels or proteins associated to ion channels and induce channel dysfunction by various mechanisms generally leading to the reduction of synaptic expression of the considered channel. The discovery of those mechanisms of action has provided insights on the regulation of the synaptic expression of the altered channels as well as the putative roles of some of their functional subdomains. Interestingly, patients' autoantibodies themselves can be used as specific tools in order to study the functions of ion channels. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Glycine receptor antibodies in PERM and related syndromes: characteristics, clinical features and outcomes

See Martinez-Martinez et al. (doi:10.1093/brain/awu153) for a scientific commentary on this article.

Carvajal-González et al. describe the first prospective cohort of patients with glycine receptor antibodies. The majority have progressive encephalomyelitis with rigidity and myoclonus. The antibodies bind to extracellular determinants on glycine receptor-α1 and to glycine receptors on spinal cord and brainstem neurons. The patients make a good recovery with immunotherapies.

The clinical associations of glycine receptor antibodies have not yet been described fully. We identified prospectively 52 antibody-positive patients and collated their clinical features, investigations and immunotherapy responses. Serum glycine receptor antibody endpoint titres ranged from 1:20 to 1:60 000. In 11 paired samples, serum levels were higher than (n = 10) or equal to (n = 1) cerebrospinal fluid levels; there was intrathecal synthesis of glycine receptor antibodies in each of the six pairs available for detailed study. Four patients also had high glutamic acid decarboxylase antibodies (>1000 U/ml), and one had high voltage-gated potassium channel-complex antibody (2442 pM). Seven patients with very low titres (<1:50) and unknown or alternative diagnoses were excluded from further study. Three of the remaining 45 patients had newly-identified thymomas and one had a lymphoma. Thirty-three patients were classified as progressive encephalomyelitis with rigidity and myoclonus, and two as stiff person syndrome; five had a limbic encephalitis or epileptic encephalopathy, two had brainstem features mainly, two had demyelinating optic neuropathies and one had an unclear diagnosis. Four patients (9%) died during the acute disease, but most showed marked improvement with immunotherapies. At most recent follow-up, (2–7 years, median 3 years, since first antibody detection), the median modified Rankin scale scores (excluding the four deaths) decreased from 5 at maximal severity to 1 (P < 0.0001), but relapses have occurred in five patients and a proportion are on reducing steroids or other maintenance immunotherapies as well as symptomatic treatments. The glycine receptor antibodies activated complement on glycine receptor-transfected human embryonic kidney cells at room temperature, and caused internalization and lysosomal degradation of the glycine receptors at 37°C. Immunoglobulin G antibodies bound to rodent spinal cord and brainstem co-localizing with monoclonal antibodies to glycine receptor-α1. Ten glycine receptor antibody positive samples were also identified in a retrospective cohort of 56 patients with stiff person syndrome and related syndromes. Glycine receptor antibodies are strongly associated with spinal and brainstem disorders, and the majority of patients have progressive encephalomyelitis with rigidity and myoclonus. The antibodies demonstrate in vitro evidence of pathogenicity and the patients respond well to immunotherapies, contrasting with earlier studies of this syndrome, which indicated a poor prognosis. The presence of glycine receptor antibodies should help to identify a disease that responds to immunotherapies, but these treatments may need to be sustained, relapses can occur and maintenance immunosuppression may be required.

Co-localisation of markers for glycinergic and GABAergic neurones in rat nucleus of the solitary tract: implications for co-transmission

Immunoreactive structures visualised with antibodies to glycine were prominent in areas of the nucleus of the solitary tract (NTS) surrounding the tractus solitarius, but scarcer in medial and ventral areas of the nucleus. This contrasted with a higher density, more homogenous distribution of structures labelled for gamma-aminobutyric acid (GABA). Immunolabelling of adjacent semi-thin sections nonetheless indicated a close correspondence between cells and puncta labelled by glycine and GABA antisera in certain NTS areas. With post-embedding electron microscopic immunolabelling, synaptic terminals with high, presumed transmitter levels of glycine were discriminated from terminals containing low, metabolic levels by quantitative analysis of gold particle labelling densities. In a random sample of terminals, 28.5% qualified on this basis as glycinergic (compared to 44.4% GABAergic); these glycinergic terminals targeted mainly dendritic structures and contained pleomorphic vesicles and symmetrical synapses. Serial section analysis revealed few terminals (5.2%) immunoreactive for glycine alone, with 82% of glycinergic terminals also containing high levels of GABA immunoreactivity. No evidence for co-localisation of glycine and glutamate was found. Light, confocal and electron microscopic labelling with antibodies to proteins specific for glycine and GABA synthesis, release and uptake confirmed that glycinergic terminals also containing GABA are found predominantly in more lateral areas of NTS, despite glycine receptors and the 'glial' glycine transporter (GLYT1) being expressed throughout all areas of the nucleus. The data suggest that synaptic terminals in certain functionally distinct areas of NTS co-release both inhibitory amino acids, which may account for the previously reported differential inhibitory effects of glycine and GABA on NTS neurones.

Effects of spinal cord injury on synaptic inputs to sympathetic preganglionic neurons

Spinal cord injuries often lead to disorders in the control of autonomic function, including problems with blood pressure regulation, voiding, defecation and reproduction. The root cause of all these problems is the destruction of brain pathways that control spinal autonomic neurons lying caudal to the lesion. Changes induced by spinal cord injuries have been most extensively studied in sympathetic preganglionic neurons, cholinergic autonomic neurons with cell bodies in the lateral horn of thoracic and upper lumbar spinal cord that are the sources of sympathetic outflow. After an injury, sympathetic preganglionic neurons in mid-thoracic cord show plastic changes in their morphology. There is also extensive loss of synaptic input from the brain, leaving these neurons profoundly denervated in the acute phase of injury. Our recent studies on sympathetic preganglionic neurons in lower thoracic and upper lumbar cord that regulate the pelvic viscera suggest that these neurons are not so severely affected by spinal cord injury. Spinal interneurons appear to contribute most of the synaptic input to these neurons so that injury does not result in extensive denervation. Since intraspinal circuitry remains intact after injury, drug treatments targeting these neurons should help to normalize sympathetically mediated pelvic visceral reflexes. Furthermore, sympathetic pelvic visceral control may be more easily restored after an injury because it is less dependent on the re-establishment of direct synaptic input from regrowing brain axons.

GABAergic and glycinergic presympathetic neurons of rat medulla oblongata identified by retrograde transport of pseudorabies virus and in situ hybridization

Electron microscopy suggests that up to half the synaptic input to sympathetic preganglionic neurons (SPGNs) is GABAergic or glycinergic. A proportion of this input is suspected to originate from neurons located within the medulla oblongata. The present study provides definitive evidence for the existence of these supraspinal presympathetic (PS) neurons with inhibitory phenotypes. PS neurons were identified by retrograde trans-synaptic migration of pseudorabies virus (PRV) injected into the adrenal gland. GABAergic or glycinergic cell bodies were identified by the presence of glutamate decarboxylase (GAD)-67 mRNA or glycine transporter (GlyT)-2 mRNA detected with in situ hybridization (ISH). Neither GABAergic nor glycinergic PS neurons were tyrosine hydroxylase (TH)-immunoreactive (ir). GABAergic PS neurons were located within the ventral gigantocellular nucleus, gigantocellular nucleus alpha, and medial reticular formation, mostly medial to the TH-ir PS neurons. About 30% of GABAergic PS neurons were serotonergic cells located in the raphe pallidus (RPa) and parapyramidal region (PPyr). Glycinergic PS neurons had the same general distribution as the GABAergic cells, except that no glycinergic neurons were located in the RPa or PPyr and none were serotonergic. PRV immunohistochemistry combined with ISH for both GlyT2 and GAD-67 mRNAs showed that at least 63% of midline medulla GABAergic PS neurons were also glycinergic and 76% of glycinergic PS neurons were GABAergic. In conclusion, the rostral ventromedial medulla contains large numbers of GABAergic and glycinergic neurons that innervate adrenal gland SPGNs. Over half of these PS neurons may release both transmitters. The physiological role of this medullary inhibitory input remains to be explored.

Nicotinic receptors regulate the release of glycine onto lamina X neurones of the rat spinal cord

Whole-cell patch clamp recordings were performed on neurones in the lamina X of rat spinal cord slices in order to characterize glycinergic synaptic currents and their modulation by nicotinic acetylcholine receptors. In the presence of TTX, bicuculline and kynurenic acid, glycine-induced currents and miniature glycinergic postsynaptic currents (mIPSCs) were recorded. These currents reversed near the chloride ion equilibrium potential and were blocked by strychnine (1 microM). A selective nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP), increased the frequency of glycinergic mIPSCs without altering significantly their amplitude distributions or their kinetic properties. The effects of DMPP were mimicked by different nAChRs agonists with the following apparent order of potency: ACh > DMPP > nicotine > cytisine. The effect of DMPP on mIPSCs was blocked by both d-tubocurarine and hexamethonium, and was reduced by dihydro-beta-erythroidine and methyllycaconitine (MLA), antagonists of non alpha7- and alpha7-containing nAChRs, respectively. In the absence of TTX, strychnine-sensitive glycinergic electrically evoked postsynaptic currents (eIPSCs) could be recorded. DMPP blocked the appearance of electrically evoked IPSCs while still inducing the appearance of spontaneous glycine IPSCs. These data demonstrate that neurones surrounding the central canal of the spinal cord present a glycinergic synaptic transmission which is modulated by terminal nAChRs.

GABA in the control of sympathetic preganglionic neurons

1. Amino acid neurotransmitters are critical for controlling the activity of most central neurons, including sympathetic preganglionic neurons (SPN), the spinal cord neurons involved in controlling blood pressure and other autonomic functions. 2. In studies reviewed here, SPN were identified either by retrograde tracing from a peripheral target (superior cervical ganglion or adrenal medulla) or by detection of immunoreactivity for choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme that is a marker for all SPN, in intact or completely transected rat spinal cord. 3. Postembedding immunogold labelling on ultrathin sections was then used to detect GABA and sometimes glutamate in nerve terminals on SPN or near them in the neuropil of the lateral horn. 4. In some cases, the terminals were prelabelled to show an anterograde tracer or immunoreactivity for ChAT or neuropeptide Y. 5. This anatomical work has provided information that is helpful in understanding how SPN are influenced by their GABAergic innervation. 6. Immunogold studies showed that the proportion of input provided by GABAergic terminals varies between different groups of SPN. For some groups, this input may be preferentially targeted to cell bodies. 7. Anterograde tracing demonstrated that supraspinal as well as intraspinal GABAergic neurons innervate SPN and investigations on completely transected cord suggested that supraspinal neurons may provide a surprisingly large proportion of the GABAergic terminals that contact SPN. 8. The double-labelling studies in which other amino acids, ChAT or neuropeptide Y were localized along with GABA indicate that GABAergic terminals contain other neurochemicals that could modulate the actions of GABA, depending on the complement of receptors that are present pre- and post-synaptically. 9. Taken together, these data indicate that GABAergic transmission to SPN may be much more complicated than suggested by the currently available electrophysiological studies.

Intraspinal amino acid neurotransmitter activities are involved in the generation of rhythmic sympathetic nerve discharge in newborn rat spinal cord

Endogenous neurotransmitter activities underlying the sympathetic nerve discharge (SND) generated by newborn rat spinal cord in vitro were investigated using glutamatergic, glycinergic, and GABAergic antagonists. Under control conditions, the SND power spectrum had two major frequency components: synchronous bursting SND (bSND) with power dominant at < 0.1 Hz and quasiperiodic SND (qSND) oscillating at 1-2 Hz. Using high Mg2+ solution (12-24 mM) to block Ca2+-dependent synaptic transmission reversibly abolished SND. An interruption of glutamatergic neurotransmission by CNQX (non-NMDA receptor blocker) or L-AP4 (reducing the synaptic release of glutamate) failed to affect qSND, but consistently reduced bSND. Application of kynurenate, a broad-spectrum ionotropic glutamate receptor blocker, only caused an unstable SND but did not reduce SND. In contrast, strychnine (Stry, glycine receptor antagonist) consistently reduced qSND in a dose-dependent manner. Bicuculline (Bic, GABA(A) receptor antagonist) induced a synchronous bSND of irregular rhythm, which could be further regularized by adding Stry. Bic-induced bSND was reversibly abolished by CNQX or L-AP4. In conclusion, intraspinal glycinergic, GABAergic, and glutamatergic activities are involved in the generation of the spinal cord-derived SND in newborn rats. Intraspinal GABAergic interneurons may tonically inhibit the glutamatergic bursting neurons that generate a synchronous bSND. Activities of these glutamatergic bursting neurons may also be modulated by intraspinal glycinergic interneurons.

Ultrastructural localization of the binding fragment of tetanus toxin in putative gamma-aminobutyric acidergic terminals in the intermediolateral cell column: a potential basis for sympathetic dysfunction in generalized tetanus

Tetanus toxin (TeTx) causes sympathetic hyperactivity, a major cause of mortality in generalized tetanus, apparently by obstructing the inhibition of sympathetic preganglionic neurons (SPNs). Neuroanatomic tracing and immunohistochemistry were used to investigate whether axon terminals in the intermediolateral cell column (IML) that synapse on SPNs and use the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) may be infected transsynaptically with TeTx. The binding fragment of TeTx (TTC; an atoxic surrogate of TeTx) and the cholera toxin B subunit (CTB; a retrograde tracer) were injected into the rat superior cervical ganglion and, over 16-48 hours, were transported to the ipsilateral IML in the caudal half of the last cervical and first three thoracic spinal cord segments. With light microscopy, diffuse CTB immunolabeling extended throughout SPN perikarya and dendrites. Punctate TTC and GABA immunolabeling were accumulated densely in the neuropil between and surrounding SPN processes. With electron microscopy, 54% of the axon terminals in the IML (n = 1,337 terminals) were TTC immunolabeled (TTC(+)), and 25% contained putative neurotransmitter levels of GABA immunolabeling (GABA(+)). On average, GABA(+) terminals had a 76% chance of also being TTC(+) and a 62% greater chance of being TTC(+) than GABA(-) terminals (P < 0.000001). Axon terminals were just as likely to be TTC(+) and/or GABA(+) regardless of whether the dendrites they synapsed on were large (>1 microM) or small in cross-sectional area or were labeled retrogradely. Sympathetic hyperactivity in tetanus may involve 1) retrograde and transsynaptic transport of TeTx by SPNs and 2) at least in part, an infection of GABAergic terminals in the IML.

Glycine-immunoreactive synaptic terminals in the nucleus tractus solitarii of the cat: ultrastructure and relationship to GABA-immunoreactive terminals

Postembedding immunogold labeling methods applied to ultrathin and semithin sections of cat dorsomedial medulla showed that neuronal perikarya, dendrites, myelinated and nonmyelinated axons, and axon terminals in the nucleus tractus solitarii contain glycine immunoreactivity. Light microscopic observations on semithin sections revealed that these immunoreactive structures were unevenly distributed throughout the entire nucleus. At the electron microscopic level, synaptic terminals with high levels of glycine-immunoreactivity, assumed to represent those releasing glycine as a neurotransmitter, were discriminated from terminals containing low, probably metabolic levels of glycine-immunoreactivity, by a quantitative analysis method. This compared the immunolabeling of randomly sampled terminals with a reference level of labeling derived from sampling the perikarya of dorsal vagal neurones. The vast majority of these "glycinergic" terminals contained pleomorphic vesicles, formed symmetrical synaptic active zones, and targeted dendrites. They appeared to be more numerous in areas of the nucleus tractus solitarii adjoining the tractus solitarius, but rather scarce caudally, medially, ventrally, and in the dorsal motor vagal nucleus. In a random analysis of the entire nucleus tractus solitarii, 26.2% of sampled terminals were found to qualify as glycine-immunoreactive. In contrast, boutons immunoreactive for gamma-aminobutyric acid (GABA) were more evenly distributed throughout the dorsal vagal complex and accounted for 33.7% of the synaptic terminals sampled. A comparison of serial ultrathin sections suggested three subpopulations of synaptic terminals: one containing high levels of both GABA- and glycine-immunoreactivities (21% of all terminals sampled), one containing only GABA-immunoreactivity (12.7%), and relatively few terminals (5.2%) that were immunoreactive for glycine alone. These results were confirmed by dual labeling of sections using gold particles of different sizes. This study reports the first analysis of the ultrastructure of glycinergic nerve terminals in the cat dorsal vagal complex, and the pattern of coexistence of glycine and GABA observed provides an anatomical explanation for our previously reported inhibitory effects of glycine and GABA on neurones with cardiovascular and respiratory functions in the nucleus tractus solitarii.

Electrophysiological evidence for multiple glycinergic inputs to neonatal rat sympathetic preganglionic neurons in vitro

The time pattern of glycinergic inhibitory postsynaptic currents (IPSCs) in sympathetic preganglionic neurons was studied in thin transverse spinal cord slices of neonatal (1-10 days postnatal) rats by means of the patchclamp technique. Three time patterns could be distinguished: (i) large events [mostly > 400 pA (30-36 degrees C)] occurring at regular intervals, (ii) small events occurring at irregular intervals, and (iii) small events occurring in transient (1.5-10 s), high-frequency (> 15 Hz) bursts of synaptic activity. The large regular events had uniform kinetics which was consistent with the idea of a proximal site of origin for all of these events. They were reversibly inhibited in amplitude and frequency by extracellular application of a high concentration of acetylcholine (200 microM) or the specific nicotinic acetylcholine receptor agonist dimethylphenylpiperazinium iodide (DMPP; 1 mM), but unaffected by glutamate (100 microM). IPSCs occurring in bursts had slower and less uniform kinetics, suggesting a more diverse site of origin. The frequency of events decreased during a burst. Similar bursts could be induced by extracellular application of glutamate receptor agonists. These results indicate that sympathetic pregnanglionic neurons in a thin, transverse spinal cord slice receive at least two different glycinergic inputs.

Glycine-immunoreactive terminals in the rat trigeminal motor nucleus: light- and electron-microscopic analysis of their relationships with motoneurones and with GABA-immunoreactive terminals

Post-embedding immunolabelling methods were applied to semi-thin and ultrathin resin sections to examine the relationships between glycine- and gamma-aminobutyric acid (GABA)-immunoreactive terminals on trigeminal motoneurones, which were identified by the retrograde transport of horseradish peroxidase injected into the jaw-closer muscles. Serial sections were cut through boutons and alternate sections were incubated with antibodies to glycine and GABA. Light-microscopic analysis of semi-thin sections revealed a similar pattern of glycine and GABA-immunoreactive boutons along the motoneurone soma and proximal dendrites, and of immunoreactive cell bodies in the parvocellular reticular and peritrigeminal areas surrounding the motor nucleus. Immunoreactive synaptic terminals on motoneurones were identified on serial ultrathin sections at electron-microscopic level using a quantitative immunogold method. Three populations of immunolabelled boutons were recognized: boutons immunoreactive for glycine alone (32%), boutons immunoreactive for GABA alone (22%), and boutons showing co-existence of glycine and GABA immunoreactivities (46%). Terminals which were immunoreactive for glycine only contained a higher proportion of flattened synaptic vesicles than those which were immunoreactive for GABA only, which contained predominantly spherical vesicles. Terminals which exhibited both immunoreactivities contained a mixture of vesicle types. All three classes of terminal formed axo-dendritic and axo-somatic contacts onto retrogradely labelled motoneurones. A relatively high proportion (25%) of boutons that were immunoreactive for both transmitters formed synapses on somatic spines. However, only GABA-immunoreactive boutons formed the presynaptic elements at axo-axonic contacts: none of these were found to contain glycine immunoreactivity. These data provide ultrastructural evidence for the role of glycine and GABA as inhibitory neurotransmitters at synapses onto jaw-closer motoneurones, but suggest that presynaptic control of transmission at excitatory (glutamatergic) synapses on motoneurones involves GABAergic, but not glycinergic inhibition.

Monosynaptic projections from the medullary gigantocellular reticular formation to sympathetic preganglionic neurons in the thoracic spinal cord

Microinjection of L-glutamate into a restricted area of the medullary gigantocellular reticular formation, the gigantocellular depressor area (GiDA), lowers arterial pressure. Unlike the nuclei tractus solitarii and the caudal ventrolateral medulla, the two principle medullary vasodepressor areas, the GiDA projects directly to the spinal cord and not to the rostral ventrolateral medulla (Aicher et al. [1994] Neuroscience 60:761-779). We investigated whether neurons within GiDA directly innervate autonomic areas of the thoracic spinal cord. Fluoro-Gold injected into the thoracic spinal cord labeled neurons within functionally defined vasodepressor sites in the GiDA in the same animal. To examine the morphology of GiDA efferents to the spinal cord, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoresed into the GiDA, and efferent processes in the intermediolateral cell column and nucleus intercalatus spinalis were examined by electron microscopy. Labeling was confined to axons and axon terminals (n = 144) that usually contained primarily small clear vesicles, contacted large and small dendrites, and formed symmetric (inhibitory) synapses. To determine whether some of the postsynaptic targets of GiDA efferent terminals in the thoracic spinal cord were sympathoadrenal preganglionic neurons, these neurons were retrogradely labeled from the adrenal gland with Fluoro-Gold in rats that had deposits of the anterograde tracer, biotinylated dextran amine (BDA), in the GiDA. Some BDA-containing terminals formed symmetric synapses with dendrites containing Fluoro-Gold. We conclude that a population of neurons in the GiDA monosynaptically innervates some sympathetic preganglionic neurons. The findings suggest the presence of a novel reticulospinal sympathoinhibitory projection originating in the GiDA.

Chemical mediators of spinal inhibition of rat sympathetic neurones on stimulation in the nucleus tractus solitarii

1. This study was undertaken to gain more direct evidence of the pathways and neurochemical mediators of a spinally mediated baroreceptor inhibition of sympathetic preganglionic neurones (SPNs). 2. For this purpose, single-pulse electrical stimulation within identified vasodepressor regions of the nucleus tractus solitarii (NTS) was used together with extracellular recordings of single antidromically identified SPNs in the T2 segment of the spinal cord of anaesthetized rats. 3. The actions of agonists and antagonists of inhibitory amino acids on the NTS-induced inhibitions were determined, when they were iontophoretically applied in the vicinity of SPNs via a multibarrel micropipette assembly. 4. Extracellular recordings were made from sixty-nine SPNs. In forty-four SPNs, NTS stimulation elicited a period of inhibition of activity in both spontaneous and 'D,L-homocysteic acid-driven' SPNs with a latency to onset of 60 +/- 6 ms and a magnitude of 80 +/- 3%. 5. In six out of eight neurones, the NTS-induced inhibition was reduced by 74 +/- 16% during the application of the glycine antagonist strychnine (0-10 nA, 5-10 min) with doses that selectively blocked the inhibitory effect of iontophoretically applied glycine. 6. In nine out of nine neurones, the NTS-induced inhibition was reduced by 38 +/- 6% during the application of the GABAA antagonist bicuculline (5-15 nA, 4-14 min) with doses that selectively blocked the inhibitory effect of iontophoretically applied GABA. 7. In two SPNs, the actions of strychnine and bicuculline were shown to be additive in blocking the NTS inhibition. 8. The selective GABAB antagonists, CGP 35348 (20-50 nA, 6-25 min) and CGP 55845A (10 nA, 11 min) did not antagonize the NTS-induced inhibition. 9. It is suggested that GABA and glycine interneurones are activated by a baroreceptor bulbospinal pathway to inhibit sympathetic preganglionic neurones in the spinal cord.

Postsynaptic gephyrin immunoreactivity exhibits a nearly one-to-one correspondence with gamma-aminobutyric acid-like immunogold-labeled synaptic inputs to sympathetic preganglionic neurons

Peripheral regulation of cardiovascular function is fundamentally influenced by central excitation and inhibition of sympathetic preganglionic neurons in thoracic spinal cord. This electron microscopy study investigated whether the gamma-aminobutyric acid (GABA)-ergic and glycinergic inhibitory innervation of sympathetic preganglionic neurons arises from mutually exclusive afferent populations. Sympathetic preganglionic neurons were retrogradely labeled with cholera beta subunit. GABAergic terminals were identified using strict quantitative statistical analyses as those boutons containing significantly elevated levels of GABA-like immunogold labeling (GABA+). Glycinergic terminals were classified as those boutons opposite postsynaptic gephyrin immunostaining containing background levels of GABA-like immunogold labeling (gephyrin+/GABA- association). Approximately 43% of the synaptic terminals that contacted sympathetic preganglionic somata and proximal dendrites and that were opposite gephyrin were GABA-; the remaining 57% were GABA+. Only two GABA+ boutons (4%) that synapsed on identified sympathetic preganglionic neuron (SPN) processes were not opposite gephyrin immunostaining (GABA+/gephyrin- association). GABA-/gephyrin+ associations were anticipated given prior anatomical, physiological, and pharmacological data. The observed nearly one-to-one correspondence between postsynaptic gephyrin immunoreactivity and GABA+ boutons was unexpected. Prior physiological and pharmacological experiments suggest that the postsynaptic effects of GABAergic inputs to sympathetic preganglionic neurons are mediated by activation of GABAA receptors. Those data, the present results, and other molecular, biochemical, and anatomical studies of gephyrin in the central nervous system (CNS) are consistent with two hypotheses: 1) Postsynaptic gephyrin is associated with GABAA receptors in the membranes of sympathetic preganglionic neurons, and 2) GABA+/gephyrin+ associations do not necessarily predict colocalization of GABA and glycine within single boutons synapsing on sympathetic preganglionic somata and dendrites.

Synapses on axons of sympathetic preganglionic neurons in rat and rabbit thoracic spinal cord

Axosomatic and axodendritic synapses occur on sympathetic preganglionic neurons, but it is not yet known whether their axons receive synaptic input, which could be particularly effective at regulating sympathetic outflow. Here, we examined retrogradely labelled sympathetic preganglionic axons to see if they received synapses. Cholera toxin B subunit (CTB) or CTB conjugated to horseradish peroxidase (CTB-HRP) was used to label neurons projecting to the rat or rabbit superior cervical ganglion, the rat adrenal medulla, or the rabbit stellate ganglion. At the light microscopic level, small groups of CTB-immunoreactive axons travelled through the ventral horn near its lateral boundary, with occasional axons taking a more medial course. The axons passed through the ventrolateral funiculus to exit at the ventral roots. In parasagittal section, a few axons branched within the ventral horn, sending processes rostrally and caudally for short distances before they turned ventrally to exit the spinal cord. At the ultrastructural level, CTB-immunoreactive rat and rabbit sympathetic preganglionic axons were almost exclusively unmyelinated. In contrast, labelling with CTB-HRP revealed both myelinated and unmyelinated axons in the ventral horn, the ventrolateral white matter, and the ventral roots. CTB-HRP also allowed the detection of the initial segment of a sympathetic preganglionic axon. Synapses, with vesicles clustered presynaptically and membrane specializations postsynaptically, were found on some unmyelinated CTB-immunoreactive axons. Occasional axons received several synapses. Synapses were most common on CTB-containing axons just ventral to the intermediolateral cell column. One synapse was found on an axon within 2 microns of its origin from a proximal dendrite. Rare synapses were found several hundred micrometers ventral to the intermediolateral cell column. One branching axon had synapses just below the branch point on both the main axon and the axonal branch. These findings indicate an extensive synaptic input to the axons of at least some sympathetic preganglionic neurons. These axoaxonic synapses could have a profound effect on sympathetic activity.

Identification of spinal interneurons antecedent to adrenal sympathetic preganglionic neurons using trans-synaptic transport of herpes simplex virus type 1

Control of sympathetic preganglionic neurons appears to be mediated, in part, through polysynaptic pathways using spinal interneurons. To identify spinal interneurons antecedent to adrenal sympathetic preganglionic neurons, we injected herpes simplex virus type 1 into the adrenal gland of hamsters as this virus is an effective trans-synaptic tracer of neural pathways. After a three day survival period, immunocytochemistry was used to visualize virus-infected spinal cord cells. Infected sympathetic preganglionic neurons with somata that were either kite-shaped, elliptical or fusiform and that had extensive dendrite arbors were identified as well as a group of smaller round cells with finer processes. For comparison, in additional hamsters, labelling with the retrograde tracer Fluoro-Gold and histochemical reactions for the enzyme nicotinamide adenine dinucleotide phosphate-diaphorase were used to identify sympathetic preganglionic neurons. Sympathetic preganglionic neurons identified with Fluoro-Gold or herpes virus were present mostly in the nucleus intermediolateralis, pars intermediolateralis and nucleus intermediolateralis, pars funicularis of the spinal cord. The smaller herpes virus-infected cells were found mostly medial to the preganglionic neurons in lamina VII and also dorsally in lamina V of the spinal cord. Assessing immunoreactivity for glial fibrillary acidic protein demonstrated that the smaller herpes virus-infected cells were not reactive astrocytes. Furthermore, these cells were immunoreactive for two neuronal markers, neuron-specific enolase and for microtubule-associated protein 2. These findings suggest that these smaller round cells with finer processes are distinct from sympathetic preganglionic neurons and astrocytes and may be interneurons antecedent to the sympathetic preganglionic neurons.

The one hundred percent hypothesis: glutamate or GABA in synapses on sympathetic preganglionic neurons

The synaptic input from glutamate- or gamma-aminobutyric acid (GABA)-immunoreactive nerve fibers to sympathetic preganglionic neurons projecting to the superior cervical ganglion was assessed using retrograde tracing with cholera toxin B subunit and post-embedding immunogold labelling. Half the boutons that synapsed on or directly contacted this group of neurons were immunoreactive for glutamate and half were immunoreactive for GABA. In previous studies, about 2/3 of the synapses on sympathoadrenal neurons were found to be glutamate-immunoreactive and about 1/3, GABA immunoreactive and these two amino acids were not co-localized. Thus, 100% of the boutons that synapse on sympathetic preganglionic neurons supplying the superior cervical ganglion or the adrenal medulla are likely to contain either glutamate or GABA. Anterograde tracing combined with amino acid immunocytochemistry indicated that at least some glutamate-containing and some GABA-containing nerve fibers in the intermediolateral cell column could come from the rostral ventral medulla.

The spinal organisation of the baroreceptor reflex

Baroreceptor inhibition of a spinally evoked response in a renal nerve was studied following removal of excitatory drive from the rostral ventrolateral medulla (RVLM), by microinjecting glycine into this region (RVLM block). Activation of arterial baroreceptors was still able to inhibit a spinally evoked response after RVLM block and this effect was abolished by intrathecal strychnine. Intrathecal bicuculline also was shown to reduce the magnitude of the baroreceptor inhibition but only when the RVLM was intact indicating that bicuculline was removing a facilitation. Both strychnine and bicuculline antagonised an NTS induced inhibition of activity in single sympathetic preganglionic neurones. It is concluded that arterial baroreceptor reflex regulation of vasomotor activity occurs at a spinal as well as a supraspinal site and GABA and glycine are the likely inhibitory mediators at both sites.

Inhibition of sympathetic preganglionic neurons by spinal glycinergic interneurons

Intracellular and whole-cell patch-clamp recordings were obtained from sympathetic preganglionic neurons in rat spinal cord slices. Perfusion of selective ionotropic and metabotropic excitatory amino acid agonists induced depolarizing responses in all neurons. In approximately 20% of neurons the application of these agonists also evoked inhibitory postsynaptic potentials. The application of the ionotropic receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (5-40 microM) blocked the inhibitory postsynaptic potential discharges induced by (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (0.5-50 microM) and N-methyl-D-aspartate (0.5-50 microM), but failed to block the inhibitory postsynaptic potentials induced by quisqualate (0.5-50 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (10-200 microM). Similar inhibitory postsynaptic potentials were seen to occur spontaneously or could be evoked by electrical stimulation of the dorsal horn. The application of tetrodotoxin blocked the spontaneous and evoked inhibitory postsynaptic potential, indicating that they result from activity-dependent release of neurotransmitter. Strychnine antagonized all inhibitory postsynaptic potentials suggesting that they were mediated via glycine receptors. The reversal potential of the inhibitory postsynaptic potentials was -65 mV for intracellular and -55 mV for whole-cell recordings. This latter value is close to the reversal potential for chloride, suggesting that the inhibitory postsynaptic potentials were mediated by a chloride conductance. Perfusion of glycine (0.1-1 mM) induced inhibitory hyperpolarizing responses in the majority of neurons. This hyperpolarizing response was associated with a reduction in neuronal input resistance, persisted in the presence of tetrodotoxin, was blocked by strychnine and reversed at -55 mV. In some neurons, glycine induced a membrane depolarization and increased the rate of spontaneous action potential firing. This excitatory effect of glycine was blocked by tetrodotoxin, showed voltage dependency and was less sensitive to strychnine than the glycine-induced inhibitory response. We conclude from these data that spinal interneurons which synapse with sympathetic preganglionic neurons can be activated through multiple subtypes of excitatory amino acid receptor, including both ionotropic and metabotropic receptors. These interneurons release glycine to evoke inhibitory postsynaptic potentials which are mediated via a strychnine-sensitive glycine receptor coupled to a chloride conductance.

Cardiovascular changes elicited by microinjection of glycine or GABA into the spinal intermediolateral nucleus in urethane-anesthetized rats

Sympathetic preganglionic neurons (SPNs), located in the intermediolateral cell column (IML) of the thoracolumbar spinal cord, contribute to the maintenance of arterial pressure (AP) and heart rate (HR) within normal limits under different physiological conditions. The purpose of this study was to examine the effects of microinjecting the putative inhibitory transmitters glycine (GLY) or gamma-amino-butyric acid (GABA) into functionally identified cardioacceleratory and vasoconstrictor IML sites at T1-T3. Rats were anesthetized (1.4 g/kg urethane i.p.), paralysed with decamethonium bromide (3.3 mg/kg i.v.) and artificially ventilated. Glutamate (GLU) microinjection (10-20 nl, pH = 7.4, 0.15 M in phosphate-buffered saline (PBS)) was used to identify 29 vasoconstrictor sites, of which 23 were also cardioacceleratory, in the right side of the T2 segment. GLY microinjection (10-20 nl, pH = 7.4, 0.5 M in PBS) into these sites resulted in significant decreases in both AP (n = 18) and HR (n = 14). The AP and HR responses to GLY were brief in duration and were attenuated by the specific GLY antagonist strychnine (STR). Microinjection of GABA (10-20 nl, pH = 7.4, 0.15-0.84 M in PBS, n = 6) and its agonist muscimol (10-20 nl, pH = 7.4, 0.9 mM in PBS, n = 6) into GLU-identified sites in the IML caused no changes in AP or HR. However, after the application of either GABA or muscimol, the changes in AP or HR elicited by GLU were eliminated, suggesting that GABA and muscimol decrease the excitability of SPNs.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic- and agonist-induced chloride currents in neonatal rat sympathetic preganglionic neurones in vitro

1. By using the whole-cell recording configuration of the patch-clamp technique in a spinal cord slice preparation, we have made recordings from visually identified neurones in the lateral horn of the thoracic and lumbar spinal cord of neonatal rats (newborn to 14 days postnatal). 2. Some of the recorded neurones were labelled with the fluorescent dye Lucifer Yellow (n = 27). Their morphology was typical for sympathetic preganglionic neurones (SPNs). Based on the size of the cell soma and the electrophysiological properties, unlabelled neurones were also regarded as SPNs. 3. Spontaneous synaptic activity of different patterns could be observed in 73% of the recorded neurones (n = 106). It reversed at the chloride equilibrium potential (ECl) and could be reversibly blocked by strychnine (1-10 microM), but not by bicuculline (10 microM) or SR95531 (5-10 microM). 4. Synaptic activity could be elicited by focal electrical stimulation in the vicinity of the recorded neurone. These evoked synaptic events exhibited features similar to the spontaneous synaptic activity. 5. Application of glycine (100 microM-1 mM) by a fast microperfusion system induced a chloride current in twenty-seven out of thirty cells tested. The currents were reversibly blocked by strychnine (1-10 microM), but were only weakly sensitive to bicuculline (10 microM). Stability of current responses to glycine was increased by inclusion of ATP (4 mM) in the intracellular medium. 6. Application of gamma-aminobutyric acid (GABA; 100 microM-1 mM) by the fast microperfusion system induced a chloride current in all twenty neurones tested. These currents were reversibly blocked by bicuculline (10 microM). Strychnine (1-10 microM) blocked this current only weakly. Run-down of GABA-induced currents was prevented to a great extent by inclusion of ATP (4 mM) in the pipette. 7. These results suggest that the inhibitory synaptic activity recorded from SPNs in thin, transverse slices of neonatal rat spinal cord is mediated by glycine receptor-gated Cl- channels. GABAA receptor-gated Cl- channels might be activated by inputs from other spinal segments and/or descending pathways from higher brain regions.