Activation and integration of bilateral GABA-mediated synaptic inputs in neonatal rat sympathetic preganglionic neurones in vitro

Rebound from Inhibition: Self-Correction against Neurodegeneration?

Neural networks play a critical role in establishing constraints on excitability in the central nervous system. Several recent studies have suggested that network dysfunction in the brain and spinal cord are compromised following insult by a neurodegenerative trigger and might precede eventual neuronal loss and neurological impairment. Early intervention of network excitability and plasticity might therefore be critical in resetting hyperexcitability and preventing later neuronal damage. Here, the behavior of neurons that generate burst firing upon recovery from inhibitory input or intrinsic membrane hyperpolarization (rebound neurons) is examined in the context of neural networks that underlie rhythmic activity observed in areas of the brain and spinal cord that are vulnerable to neurodegeneration. In a non-inflammatory rodent model of spongiform neurodegenerative disease triggered by retrovirus infection of glia, rebound neurons are particularly vulnerable to neurodegeneration, likely due to an inherently low calcium buffering capacity. The dysfunction of rebound neurons translates into a dysfunction of rhythmic neural circuits, compromising normal neurological function and leading to eventual morbidity. Understanding how virus infection of glia can mediate dysfunction of rebound neurons, induce hyperexcitability and loss of rhythmic function, pathologic features observed in neurodegenerative disorders ranging from epilepsy to motor neuron disease, might therefore suggest a common pathway for early therapeutic intervention.

Modulation of synchronous sympathetic firing behaviors by endogenous GABA(A) and glycine receptor-mediated activities in the neonatal rat spinal cord in vitro

Delivering effective commands in the nervous systems require a temporal integration of neural activities such as synchronous firing. Although sympathetic nerve discharges are characterized by synchronous firing, its temporal structures and how it is modulated are largely unknown. This study used a collagenase-dissociated splanchnic sympathetic nerve-thoracic spinal cord preparation of neonatal rats in vitro as an experimental model. Several single-fiber activities were recorded simultaneously and verified by rigorous computational algorithms. Among 3763 fiber pairs that had spontaneous fiber activities, 382 fiber pairs had firing positively correlated. Their temporal relationship was quantitatively evaluated by cross-correlogram. On average, correlated firing in a fiber pair occurred in scales of ∼40ms lasting for ∼11ms. The relative frequency distribution curves of correlogram parametrical values pertinent to the temporal features were best described by trimodal Gaussians, suggesting a correlated firing originated from three or less sources. Applications of bicuculline or gabazine (noncompetitive or competitive GABA(A) receptor antagonist) and/or strychnine (noncompetitive glycine receptor antagonist) increased, decreased, or did not change individual fiber activities. Antagonist-induced enhancement and attenuation of correlated firing were demonstrated by a respective increase and decrease of the peak probability of the cross-correlograms. Heterogeneity in antagonistic responses suggests that the inhibitory neurotransmission mediated by GABA(A) and glycine receptors is not essential for but can serve as a neural substrate to modulate synchronous firing behaviors. Plausible neural mechanisms were proposed to explain the temporal structures of correlated firing between sympathetic fibers.

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.

A horizontal slice preparation for examining the functional connectivity of dorsal column fibres in mouse spinal cord

In spinal cord injury (SCI) research, axon regeneration across spinal lesions is most often assessed using anatomical methods. It would be extremely advantageous, however, to examine the functional synaptic connectivity of regenerating fibres, using high-resolution electrophysiological methods. We have therefore developed a mouse horizontal spinal cord slice preparation that permits detailed analysis of evoked dorsal column (DCol) synaptic inputs on spinal neurons, using whole-cell patch clamp electrophysiology. This preparation allows us to characterise postsynaptic currents and potentials in response to electrical stimulation of DCol fibres, along with the intrinsic properties of spinal neurons. In addition, we demonstrate that low magnification calcium imaging can be used effectively to survey the spread of excitation from DCol stimulation in horizontal slices. This preparation is a potentially valuable tool for SCI research where confirmation of regenerated, functional synapses across a spinal lesion is critical.

GABA(B) Mediated Regulation of Sympathetic Preganglionic Neurons: Pre- and Postsynaptic Sites of Action

Modulatory influences on sympathetic nervous system activity are diverse and far reaching, acting at select points in the complex pathways controlling sympathetic outflow to enable subtle changes or more global effects. Changes in the degree of sympathetic neuromodulation can have serious consequences on homeostatic variables such as heart rate, blood pressure and gut motility. At the level of the spinal cord, the sympathetic preganglionic neurons (SPNs) can be modulated by activation of presynaptic GABA(B) heteroreceptors on glutamatergic terminals and by postsynaptic GABA(B) receptors. Here we show that a low concentration of the GABA(B) agonist baclofen (1 μM) attenuated GABAergic inhibitory postsynaptic potentials in SPNs elicited from stimulation of either the central autonomic area or descending fibers in the lateral funiculus. This low baclofen concentration also elicited three categories of postsynaptic response: a large hyperpolarization with a decrease in input resistance, a moderate hyperpolarization with no change in input resistance and no response. Using cesium-loaded, tetraethylammonium chloride containing electrodes (to block potassium conductance), baclofen elicited moderate hyperpolarizations with no change in input resistance in 50% of SPNs; the remainder were unaffected. These modest hyperpolarizations were reduced in Ca(2+) free solution or cadmium. Hyperpolarizing responses were also observed in interneurons in the vicinity of SPNs. These studies provide the first evidence for GABA(B) autoreceptors involved in inhibitory GABAergic transmission onto SPNs and for postsynaptic GABA(B) receptors on interneurons. The data also indicate that there is heterogeneity in the postsynaptic responses of SPNs.

Tonic GABAergic inhibition of sympathetic preganglionic neurons: a novel substrate for sympathetic control

The sympathetic tone is primarily defined by the level of activity of the sympathetic preganglionic neurons. We report a novel inhibitory influence on sympathetic activity, that of tonic GABAergic inhibition which could have a profound global effect on sympathetic outflow. Recording from identified SPNs in the intermediolateral cell column (IML) of rat spinal cord slices, application of the GABA receptor antagonist bicuculline, but not gabazine, elicited a change in voltage that lasted for the duration of application. This response was mediated by a direct effect on SPNs since it persisted in tetrodotoxin and low Ca(2+)/high Mg(2+) and the amplitude of responses were related to Cl(-) concentration in patch solutions. Such tonic inhibitory responses were not observed in interneurons, the other neuronal type in the IML, although ongoing IPSPs were antagonized in these neurons. The effects of bicuculline were enhanced by diazepam but not zolpidem or the GABA modulators THIP and THDOC suggesting a role for alpha5 subunits. PCR using primers for the alpha5 and delta subunits indicated the presence of alpha5, but not delta subunits in the IML. Firing rates of SPNs were enhanced by bicuculline and decreased by diazepam indicating that this tonic inhibition has a profound effect on the excitability of SPNs. These data indicate a novel influence for controlling the activity of SPNs regardless of their function.

Endogenous activation of nicotinic receptors underlies sympathetic tone generation in neonatal rat spinal cord in vitro

Without the brainstem, thoracic spinal cords of neonatal rats in vitro spontaneously generate tonic sympathetic nerve discharge (SND) in the splanchnic nerves. Activation of nicotinic receptors in cords is known to alter a repertoire of neurotransmitter releases to sympathetic preganglionic neurons (SPNs). Using in vitro nerve-cord preparations, we investigated whether endogenous nicotinic receptor activity is essential for SND genesis. Application of mecamylamine, an open-channel nicotinic receptor blocker, reduced SND in a progressive manner. Exogenous activation of nicotinic receptors by application of various nicotinic agonists generally excited SND at low agonistic concentrations. At higher concentrations, however, agonists induced biphasic responses characterized by an initial excitation followed by prolonged SND suppression. Whether ionotropic glutamate, GABA(A), or glycine receptors are downstream signals of nicotinic receptor activation was explored by pretreatment of cords with selective antagonists. The initial excitation of SND persisted in the presence of ionotropic glutamate receptor antagonists. In contrast, the SND suppression was partially reversed by glycine or GABA(A) receptor antagonists. Incubation of the cord in a low Ca(2+)/high Mg(2+) bath solution to block Ca(2+)-dependent synaptic transmission did not affect SND excitation induced by nicotinic agonists, confirming direct activation of postsynaptic nicotinic receptors on SPNs. In conclusion, the endogenous activity of nicotinic receptors is essential for SND genesis in the thoracic spinal cord. Nicotinic activation of glycinergic and GABAergic interneurons may provide a recurrent inhibition of SPNs for homeostatic regulation of sympathetic outflow.

Modulation of GABAergic synaptic transmission by terminal nicotinic acetylcholine receptors in the central autonomic nucleus of the neonatal rat spinal cord

Using patch clamp recordings from an in vitro spinal cord slice preparation of neonatal rats (9-15days old), we characterized the GABAergic synaptic transmission in sympathetic preganglionic neurones (SPN) of the central autonomic nucleus (CA) of lamina X. Local applications of isoguvacine (100microM), a selective agonist at GABA(A) receptors, induced in all cells tested a chloride current which was abolished by bicuculline, a competitive antagonist at GABA(A) receptors. In addition, 25% of the recorded cells displayed spontaneous tetrodotoxin-insensitive and bicuculline-sensitive chloride miniature inhibitory postsynaptic currents (mIPSCs). Acetylcholine (100microM) increased the frequency of GABAergic mIPSCs without affecting their amplitudes or their kinetic properties indicating a presynaptic site of action. The presynaptic effect of ACh was restricted to GABAergic neurones synapsing onto sympathetic preganglionic neurones. The facilitatory effect of ACh was abolished in the absence of external calcium or in the presence of 100microM cadmium added to the bath solution. Choline 10mM, an agonist at alpha7 nicotinic acetylcholine receptors (nAChRs) or muscarine (10microM), a muscarinic receptor agonist, did not reproduce the presynaptic effect of ACh. The presynaptic effect of ACh was blocked by 1microM of dihydro-beta-erythroidine (DHbetaE), an antagonist of non-alpha7 nAChRs but was insensitive to alpha7 nAChRs antagonists (strychnine, alpha-bungarotoxin and methyllycaconitine) or to the muscarinic receptor antagonist atropine (10microM). It was concluded that SPNs of the central autonomic nucleus displayed a functional GABAergic transmission which is facilitated by terminal non alpha7 nAChRs.

Integration of metabolic stimuli in the hypothalamic arcuate nucleus

Integration of peripheral and central anabolic and catabolic inputs within the hypothalamic arcuate nucleus (ARC) is believed to be central to the maintenance of energy balance. In order to perform this complex task, neurons in the ARC express receptors for all major humoral and central transmitters involved in the maintenance of energy homeostasis. The integration of these inputs occurs at the cellular and circuit level and the resulting electrical output forms the origins for the activation of feeding and energy balance-related networks. Here, we discuss the role that active intrinsic membrane conductances, K(ATP) channels and intracellular second messenger systems play in the integration of metabolic stimuli at the cellular level in the ARC. We conclude that the research into the integration of hunger and satiety signals in the ARC has made substantial progress in the last decade, but we are far from unraveling the complex neuronal networks involved in the maintenance of energy homeostasis. The diverse range of inputs, neuronal integrative properties, targets, output signals and how these signals relate to the physiological output provides us with a colossal challenge for years to come. However, to battle the current obesity epidemic, target-specific drugs need to be developed for which the knowledge of neuronal pathways involved in the maintenance of energy homeostasis will be crucial.

GABAB-receptor-mediated suppression of sympathetic outflow from the spinal cord of neonatal rats

Using a splanchnic nerve-spinal cord preparation in vitro that could spontaneously generate sympathetic nerve discharge (SND), we investigated the roles of intraspinal GABAB receptors in the regulation of SND. Despite an age-dependent difference in sensitivity, bath applications of baclofen (Bac; GABAB-receptor agonist) consistently reduced SND in a concentration-dependent manner. The drug specificity of Bac in activation of GABAB receptors was verified by application of its antagonist saclofen (Sac) or CGP-46381 (CGP). Sac or CGP alone did not change SND. However, in the presence of Sac or CGP, the effects of Bac on SND inhibition were reversibly attenuated. The splanchnic sympathetic preganglionic neuron (SPN) was recorded by blind whole cell, patch-clamp techniques. We examined Bac effects on electrical membrane properties of SPNs. Applications of Bac reduced excitatory synaptic events, induced membrane hyperpolarizations, and inhibited SPN firing. In the presence of 12 mM Mg2+ or 0.5 μM TTX to block Ca2+- or action potential-dependent synaptic transmissions, applications of Bac induced an outward baseline current that reversed at −29 ± 6 mV. Because the K+ equilibrium potential in our experimental conditions was −100 mV, the Bac-induced currents could not simply be attributed to an alteration of K+ conductance. On the other hand, applications of Bac to Cs+-loaded SPNs reduced Cd2+-sensitive and high-voltage-activated inward currents, indicating an inhibition of voltage-gated Ca2+ currents. Our results suggest that the activation of intraspinal GABAB receptors suppresses SND via a mixture of ion events that may link to a change in Ca2+ conductance.