Membrane properties and dendritic arborization of the intermediolateral nucleus neurons in the guinea-pig thoracic spinal cord in vitro

Electrophysiological, pharmacological and molecular profile of the transient outward rectifying conductance in rat sympathetic preganglionic neurons in vitro

Transient outward rectifying conductances or A-like conductances in sympathetic preganglionic neurons (SPN) are prolonged, lasting for hundreds of milliseconds to seconds and are thought to play a key role in the regulation of SPN firing frequency. Here, a multidisciplinary electrophysiological, pharmacological and molecular single-cell rt-PCR approach was used to investigate the kinetics, pharmacological profile and putative K+ channel subunits underlying the transient outward rectifying conductance expressed in SPN. SPN expressed a 4-aminopyridine (4-AP) sensitive transient outward rectification with significantly longer decay kinetics than reported for many other central neurons. The conductance and corresponding current in voltage-clamp conditions was also sensitive to the Kv4.2 and Kv4.3 blocker phrixotoxin-2 (1-10 μM) and the blocker of rapidly inactivating Kv channels, pandinotoxin-Kα (50 nM). The conductance and corresponding current was only weakly sensitive to the Kv1 channel blocker tityustoxin-Kα and insensitive to dendrotoxin I (200 nM) and the Kv3.4 channel blocker BDS-II (1 μM). Single-cell RT-PCR revealed mRNA expression for the α-subunits Kv4.1 and Kv4.3 in the majority and Kv1.5 in less than half of SPN. mRNA for accessory β-subunits was detected for Kvβ2 in all SPN with differential expression of mRNA for KChIP1, Kvβ1 and Kvβ3 and the peptidase homologue DPP6. These data together suggest that the transient outwardly rectifying conductance in SPN is mediated by members of the Kv4 subfamily (Kv4.1 and Kv4.3) in association with the β-subunit Kvβ2. Differential expression of the accessory β subunits, which may act to modulate channel density and kinetics in SPN, may underlie the prolonged and variable time-course of this conductance in these neurons.

Heterogeneity of membrane properties in sympathetic preganglionic neurons of neonatal mice: evidence of four subpopulations in the intermediolateral nucleus

Spinal cord sympathetic preganglionic neurons (SPNs) integrate activity from descending and sensory systems to determine the final central output of the sympathetic nervous system. The intermediolateral column (IML) has the highest number and density of SPNs and, within this region, SPN somas are found in distinct clusters within thoracic and upper lumbar spinal segments. Whereas SPNs exhibit a rostrocaudal gradient of end-target projections, individual clusters contain SPNs with diverse functional roles. Here we explored diversity in the electrophysiological properties observed in Hb9-eGFP-identified SPNs in the IML of neonatal mice. Overall, mouse SPN intrinsic membrane properties were comparable with those seen in other species. A wide range of values was obtained for all measured properties (up to a 10-fold difference), suggesting that IML neurons are highly differentiated. Using linear regression we found strong correlations between many cellular properties, including input resistance, rheobase, time constant, action potential shape, and degree of spike accommodation. The best predictor of cell function was rheobase, which correlated well with firing frequency-injected current (f-I) slopes as well as other passive and active membrane properties. The range in rheobase suggests that IML neurons have a recruitment order with stronger synaptic drives required for maximal recruitment. Using cluster analysis, we identified at least four subpopulations of SPNs, including one with a long time constant, low rheobase, and high f-I gain. We thus propose that the IML contains populations of neurons that are differentiable by their membrane properties and hypothesize they represent diverse functional classes.

Electrophysiological characteristics of vasomotor preganglionic neurons and related neurons in the thoracic spinal cord of the rat: an intracellular study in vivo

Sympathetic preganglionic neurons (SPN) represent the final central neurons in the sympathetic pathways which regulate vasomotor tone; they therefore play a pivotal role in the re-distribution of cardiac output to different vascular beds in response to environmental challenges. While the consensus view is that activity in these neurons is due mainly to supraspinal inputs, the possibility that some activity may be generated intrinsically and modified by synaptic inputs cannot be excluded. Therefore, in order to distinguish between these two possibilities, the electrophysiological properties of cardiovascular-like SPN in the upper thoracic spinal cord of the anesthetized rat were examined and their response to activation of vasodepressor inputs was investigated. Intracellular recordings were made from 22 antidromically identified SPN of which 17 displayed irregular, but maintained, spontaneous activity; no evidence of bursting behavior or pacemaker-like activity was observed. Stimulation of the aortic depressor nerve or a vasodepressor site within the nucleus tractus solitarius (NTS) resulted in a membrane hyperpolarization, decrease in cell input resistance and long-lasting cessation of neuronal firing in SPN including a sub-population which had cardiac-modulated patterns of activity patterns. Recordings were also undertaken from 80 non-antidromically-activated neurons located in the vicinity of SPN; 23% of which fired in phase with the cardiac cycle, with this peak of activity occurring before similar increases in cardiac-modulated SPN. Stimulation of vasodepressor regions of the NTS evoked a membrane hyperpolarization and decrease in cell input resistance in cardiac-modulated but not non-modulated interneurons. These studies show that activity patterns in SPN in vivo are determined principally by synaptic inputs. They also demonstrate that spinal interneurons which exhibit cardiac-modulated patterns of activity are postsynaptically inhibited following activation of baroreceptor pathways. However, the question as to whether these inhibitory pathways and/or disfacilitation of tonic excitatory drive underlies the baroreceptor-mediated inhibition of SPN remains to be determined.

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

The role of GABA receptors in synaptic transmission to neonatal rat sympathetic preganglionic neurones (SPNs) was investigated utilizing whole-cell patch clamp recording techniques in longitudinal and transverse spinal cord slice preparations. In the presence of glutamate receptor antagonists (NBQX, 5 microm and D-APV, 10 microm), electrical stimulation of the ipsilateral or contralateral lateral funiculi (iLF and cLF, respectively) revealed monosynaptic inhibitory postsynaptic potentials (IPSPs) in 75% and 65% of SPNs, respectively. IPSPs were sensitive to bicuculline (10 microM) in all neurones tested and reversed polarity around -55 mV, the latter indicating mediation via chloride conductances. In three neurones IPSPs evoked by stimulation of the iLF (n = 1) or cLF (n = 2) were partly sensitive to strychnine (2 microM). The expression of postsynaptic GABA(A) and GABA(B) receptors were confirmed by the sensitivity of SPNs to agonists, GABA (2 mm), muscimol (10-100 microM) or baclofen (10-100 microM), in the presence of TTX, each of which produced membrane hyperpolarization in all SPNs tested. Muscimol-induced responses were sensitive to bicuculline (1-10 microM) and SR95531 (10 microM) and baclofen-induced responses were sensitive to 2-hydroxy-saclofen (100-200 microM) and CGP55845 (200 nM). The GABA(C) receptor agonist CACA (200 microM) was without significant effect on SPNs. These results suggest that SPNs possess postsynaptic GABA(A) and GABA(B) receptors and that subsets of SPNs receive bilateral GABAergic inputs which activate GABA(A) receptors, coupled to a chloride conductance. At resting or holding potentials close to threshold either single or bursts (10-100 Hz) of IPSPs gave rise to a rebound excitation and action potential firing at the termination of the burst. This effect was mimicked by injection of small (10-20 pA) rectangular-wave current pulses, which revealed a time-dependent, Cs(+)-sensitive inward rectification and rebound excitation at the termination of the response to current injection. Synaptic activation of a rebound excitation mediated by a time-dependent inward rectification expressed intrinsically by SPNs may provide a novel mechanism enabling SPNs to be entrained to rhythms driven from the brainstem or higher centres.

Vasomotor centre of medulla - morpho-functional and neurochemical organization

The analysis of ventrolateral medulla morpho-functional and neurochemical organization is the aim of this survey. The date on the system of activation and inhibition of the spinal cord vasomotor neurons is represented. In addition, we discuss the role of catecholamines, substance P, glutamate, gamma-aminobutiric acid as neuromediators in the regulation of circulation.

Active and passive membrane properties of rat sympathetic preganglionic neurones innervating the adrenal medulla

The intravascular release of adrenal catecholamines is a fundamental homeostatic process mediated via thoracolumbar spinal sympathetic preganglionic neurones (AD-SPN). To understand mechanisms regulating their excitability, whole-cell patch-clamp recordings were obtained from 54 retrogradely labelled neonatal rat AD-SPN. Passive membrane properties included a mean resting membrane potential, input resistance and time constant of -62 +/- 6 mV, 410 +/- 241 MOmega and 104 +/- 53 ms, respectively. AD-SPN were homogeneous with respect to their active membrane properties. These active conductances included transient outward rectification, observed as a delayed return to rest at the offset of the membrane response to hyperpolarising current pulses, with two components: a fast 4-AP-sensitive component (A-type conductance), contributing to the after-hyperpolarisation (AHP) and spike repolarisation; a slower prolonged Ba(2+)-sensitive component (D-like conductance). All AD-SPN expressed a Ba(2+)-sensitive instantaneous inwardly rectifying conductance activated at membrane potentials more negative than around -80 mV. A potassium-mediated, voltage-dependent sustained outward rectification activated at membrane potentials between -35 and -15 mV featured an atypical pharmacology with a component blocked by quinine, reduced by low extracellular pH and arachidonic acid, but lacking sensitivity to Ba(2+), TEA and intracellular Cs(+). This quinine-sensitive outward rectification contributes to spike repolarisation. Following block of potassium conductances by Cs(+) loading, AD-SPN revealed the capability for autorhythmicity and burst firing, mediated by a T-type Ca(2+) conductance. These data suggest the output capability is dynamic and diverse, and that the range of intrinsic membrane conductances expressed endow AD-SPN with the ability to generate differential and complex patterns of activity. The diversity of intrinsic membrane properties expressed by AD-SPN may be key determinants of neurotransmitter release from SPN innervating the adrenal medulla. However, factors other than active membrane conductances of AD-SPN must ultimately regulate the differential ratio of noradrenaline (NA) versus adrenaline (A) release secreted in response to various physiological and environmental demands.

Dynamics of intrinsic electrophysiological properties in spinal cord neurones

The spinal cord is engaged in a wide variety of functions including generation of motor acts, coding of sensory information and autonomic control. The intrinsic electrophysiological properties of spinal neurones represent a fundamental building block of the spinal circuits executing these tasks. The intrinsic response properties of spinal neurones--determined by the particular set and distribution of voltage sensitive channels and their dynamic non-linear interactions--show a high degree of functional specialisation as reflected by the differences of intrinsic response patterns in different cell types. Specialised, cell specific electrophysiological phenotypes gradually differentiate during development and are continuously adjusted in the adult animal by metabotropic synaptic interactions and activity-dependent plasticity to meet a broad range of functional demands.

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.

Voltage-dependent potassium currents of sympathetic preganglionic neurons in neonatal rat spinal cord thin slices

Voltage-dependent potassium currents were analyzed in the visually identified sympathetic preganglionic neurons (SPNs) of neonatal rat spinal cord thin slices by the whole-cell patch-clamp technique. Some of the SPNs were identified by the presence of retrogradely transported fluorescent dye, DiI, injected into the superior cervical ganglion several days prior to experimentation. In a tetrodotoxin (TTX)-containing solution, a step depolarization from the holding potential of -72 mV generated a slow outward current that was suppressed by tetraethylammonium (TEA) and by Ca(2+)-free/2.5 ImM Co2+ solution. Ca(2+)-dependent current consisted of a transient and a sustained components. In a Ca(2+)-free (substituted with Mg2+) solution with TTX and TEA, a step depolarization from a hyperpolarized potential evoked a transient outward current that was blocked by 4-aminopyridine (4-AP). A step hyperpolarization evoked a voltage-dependent inward current, the conductance of which was dependent not only on the membrane potential, but also on the extracellular K+ concentration. Tail current analyses revealed that all of these currents were carried by K+ ions. These results indicate that SPN possesses at least five types of voltage-dependent K+ current, including the delayed rectifier current (IK), Ca(2+)-dependent transient current (IC), Ca(2+)-dependent sustained current (IAHP), A-current (IA) and inward rectifying current (Iu), which may be targets of putative transmitters released from various descending and segmental inputs impinging upon the SPN.

Nicotinic actions on neurones of the central autonomic area in rat spinal cord slices

1. Nicotinic responses and actions on excitatory synaptic activity were studied in eighty-four neurones in the region dorsal to the central canal (lamina X) in transverse thoracolumbar spinal cord slices of neonate (P2-P10) rats by using the whole-cell patch-clamp technique. 2. Neurones (n = 15) labelled with Lucifer Yellow, showed the typical morphology of sympathetic preganglionic neurones (SPNs) in the central autonomic area (CA). Unlabelled neurones of comparable morphology were visually identified and recorded. 3. All neurones recorded responded to the nicotinic acetylcholine receptor (nAChR) agonist, DMPP. Under current-clamp conditions, pressure ejections of DMPP depolarized cells and induced the discharge of action potentials. Tetrodotoxin suppressed action potentials but not DMPP-induced depolarization. 4. Under voltage-clamp conditions at a holding potential (Vh) of -50 mV, DMPP induced a transient inward current (which reversed around 0 mV) and an increase in membrane current noise in 50% of the recorded neurones. In the others, DMPP increased membrane current noise without measurable inward current. The current-voltage relationship showed strong inward rectification at holding potentials more positive than 0 mV. 5. In neurones displaying a detectable current response to DMPP, the following agonist rank order potency could be established: DMPP = nicotine > cytisine > ACh. The DMPP response could be blocked by mecamylamine but was insensitive to methyllycaconite. 6. Pressure application of glutamate induced inward currents in all cells tested at a Vh of -50 mV. This response reversed at 10 mV, displayed a region of negative slope conductance at Vh more negative than -30 mV and was partially blocked by CNQX. Pressure application of DMPP transiently increased the amplitude of the glutamate-induced current in six out of nine cells tested. This potentiation persisted in the presence of tetrodotoxin. 7. Forty per cent of the recorded neurones displayed spontaneous excitatory postsynaptic currents (sEPSCs). At a Vh of -50 mV the sEPSCs had a mean amplitude of -19.3 pA and occurred at a frequency below 0.5 Hz. sEPSCs were blocked by CNQX and inverted around 0 mV. Brief application of DMPP increased the discharge frequency of sEPSCs without affecting their kinetics. Additionally, in some cells DMPP increased mean sEPSC amplitude. 8. Focal electrically evoked EPSCs reversed close to 10 mV and were sensitive to CNQX. They occurred with a constant latency, rise time and a mono-exponential decay time. Application of DMPP decreased the percentage of stimulation failures and increased the amplitude of evoked EPSCs, in all cells tested. 9. It is concluded that neurones in the CA, presumed to be SPNs, have functional nAChRs with activation having two distinct effects: firstly, a direct depolarization of the postsynaptic membrane; and secondly, a facilitation of the excitatory transmission onto these cells. This second effect is achieved by an increase of the size of the glutamate-induced current at the postsynaptic level as well as by an enhancement of the presynaptic release of glutamate.

Membrane properties and synaptic potentials in rat sympathetic preganglionic neurons studied in horizontal spinal cord slices in vitro

Intracellular recordings were made from neurons in the intermediolateral column and adjacent white matter in horizontal slices of upper thoracic spinal cord from rats aged 21-28 days. Membrane properties were studied in the presence of picrotoxin (100 microM) to block ongoing inhibitory synaptic potentials. 37 neurons were identified as sympathetic preganglionic neurons (SPNs) by their electrical behaviour, anatomical location and/or morphology. SPNs had resting potentials of -57 +/- 2 mV and input resistances of 254 +/- 31 M omega (n = 14). Following a hyperpolarising voltage step, a transient outward current was activated which had a time constant of decay of approx. 400 ms. The inflection in the repolarising phase of the action potential and the following prolonged AHP were both abolished by Cd2+ (50 microM). The current underlying the AHP had two components with kinetic properties similar to the two calcium-activated potassium conductances, gKCa1, and gKCa2, characterized in other autonomic neurons. Noradrenaline (10-100 microM) caused a small depolarization and blocked the calcium component of the action potential suppressing the AHP. This revealed an afterdepolarization (ADP) with an underlying inward current with a decay time constant of approx. 150 ms. All effects of noradrenaline were blocked by phentolamine (10 microM). Graded stimulation of the lateral funiculus 0.5-1 mm rostral to the recording site evoked in all cells monosynaptic fast excitatory synaptic potentials (fEPSPs) which were graded in amplitude. fEPSPs decayed with a time constant identical to the cell input time constant and were reduced in amplitude by CNQX (10-20 microM). In 7 cells, higher stimulus voltages elicited slow EPSPs with a time to peak of 1.1 +/- 0.1 s and a half decay of 2.8 +/- 0.3 s (n = 7) which were not reduced by alpha-adrenoceptor antagonists. The AHP was not blocked when the action potential was initiated during the slow EPSP. We conclude that excitatory bulbospinal inputs to SPNs involve at least one fast transmitter which is likely to be glutamate and one slow transmitter which is not noradrenaline.

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.

Properties of preganglionic and postganglionic neurones in vasoconstrictor pathways of rats and guinea pigs

The electrophysiological properties of pre- and postganglionic neurones and their synaptic inputs have been examined in both in vivo and in vitro preparations. Electrically, both neurone types have similar low resting conductance and compact dendritic trees. In preganglionic vasoconstrictor neurones, both slow and fast excitatory and fast inhibitory potentials are probably involved in baroreceptor reflexes, discharge being initiated after summation. In contrast, postganglionic vasoconstrictor neurones receive only one type of fast excitatory input. One of the converging preganglionic inputs has a very high safety factor and always fires the postganglionic neurone ensuring that the centrally-derived pattern of discharge reaches the neurovascular junctions. We do not know if the other subthreshold inputs summate during natural activity in vivo, as it is not known whether functionally distinct preganglionic inputs converge on vasoconstrictor neurones in ganglia.