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The intravenous anaesthetic ketamine, has been demonstrated to inhibit nicotinic acetylcholine receptor (nAChR)-mediated currents in dissociated rat intracardiac ganglion (ICG) neurons (Weber et al., 2005). This effect would be predicted to depress synaptic transmission in the ICG and would account for the inhibitory action of ketamine on vagal transmission to the heart (Inoue and König, 1988). This investigation was designed to examine the activity of ketamine on (i) postsynaptic responses to vagal nerve stimulation, (ii) the membrane potential, and (iii) membrane current responses evoked by exogenous application of ACh and nicotine in ICG neurons in situ. Intracellular recordings were made using sharp intracellular microelectrodes in a whole mount ICG preparation. Preganglionic nerve stimulation and recordings in current- and voltage-clamp modes were used to assess the action of ketamine on ganglionic transmission and nAChR-mediated responses. Ketamine attenuated the postsynaptic responses evoked by nerve stimulation. This reduction was significant at clinically relevant concentrations at high frequencies. The excitatory membrane potential and current responses to focal application of ACh and nicotine were inhibited in a concentration-dependent manner by ketamine. In contrast, ketamine had no effect on either the directly-evoked action potential or excitatory responses evoked by focal application of γ-aminobutyric acid (GABA). Taken together, ketamine inhibits synaptic transmission and nicotine- and ACh-evoked currents in adult rat ICG. Ketamine inhibition of synaptic transmission and nAChR-mediated responses in the ICG contributes significantly to its attenuation of the bradycardia observed in response to vagal stimulation in the mammalian heart.

The expression and properties of ionic channels were investigated in dissociated neurons from neonatal and adult rat intracardiac ganglia. Changes in the hyperpolarization-activated and ATP-sensitive K+ conductances during postnatal development and their role in neuronal excitability were examined. The hyperpolarization-activated nonselective cation current, Ih, was observed in all neurons studied and displayed slow time-dependent rectification. An inwardly rectifying K+ current, IK(IR), was present in a population of neurons from adult but not neonatal rats and was sensitive to block by extracellular Ba2+. Using the perforated-patch recording configuration, an ATP-sensitive K+ (KATP) conductance was identified in ≥50% of intracardiac neurons from adult rats. Levcromakalim evoked membrane hyperpolarization, which was inhibited by the sulphonylurea drugs, glibenclamide and tolbutamide. Exposure to hypoxic conditions also activated a membrane current similar to that induced by levcromakalim and was inhibited by glibenclamide. Changes in the complement of ion channels during postnatal development may underlie observed differences in the function of intracardiac ganglion neurons during maturation. Furthermore, activation of hyperpolarization-activated and KATP channels in mammalian intracardiac neurons may play a role in neural regulation of the mature heart and cardiac function during ischaemia-reperfusion.

The hyperpolarization-activated nonselective cation current, Ih, was investigated in neonatal and adult rat intracardiac neurons. Ih was observed in all neurons studied and displayed slow time-dependent rectification. Ih was isolated by blockade with external Cs+ (2 mM) and was inhibited irreversibly by the bradycardic agent, ZD 7288. Current density of Ih was approximately twofold greater in neurons from neonatal (-4.1 pA/pF at -130 mV) as compared with adult (-2.3 pA/pF) rats; however, the reversal potential and activation parameters were unchanged. The reversal potential and amplitude of Ih was sensitive to changes in external Na+ and K+ concentrations. An inwardly rectifying K+ current, IK(IR), was also present in intracardiac neurons from adult but not neonatal rats and was blocked by extracellular Ba2+. IK(IR) was present in approximately one-third of the adult intracardiac neurons studied, with a current density of -0.6 pA/pF at -130 mV. IK(IR) displayed rapid activation kinetics and no time-dependent rectification consistent with the rapidly activating, inward K+ rectifier described in other mammalian autonomic neurons. IK(IR) was sensitive to changes in external K+, whereby raising the external K+ concentration from 3 to 15 mM shifted the reversal potential by approximately +36 mV. Substitution of external Na+ had no effect on the reversal potential or amplitude of IK(IR).IK(IR) density increases as a function of postnatal development in a population of rat intracardiac neurons, which together with a concomitant decrease in Ih may contribute to changes in the modulation of neuronal excitability in adult versus neonatal rat intracardiac ganglia.

The electrical characteristics of isolated neonatal rat intracardiac neurons were examined at 22 and 37°C using the perforated-patch whole cell recording technique. The mean resting membrane potential was -52.0 mV at 37°C and exhibited no temperature dependence. Lowering the temperature from 37 to 22°C decreased the mean input resistance from 854 to 345 MΩ, respectively, and reduced the membrane time constant approximately threefold yielding a Q10 of 2.1. Hyperpolarizing current pulses induced time- dependent rectification of the voltage response in all neurons at both temperatures. This behavior was previously not observed in dialyzed neurons and was reversibly blocked by external Cs+ (2 mM) but not Ba2+ (1 mM). Voltage-clamp studies of isolated neurons revealed a hyperpolarization- activated inward current. This inwardly rectifying conductance was isolated from other membrane currents using external Cs+. The time and voltage dependence of this current is consistent with I(h) and contributes to the passive electrical properties of rat intracardiac neurons. In >90% of the neurons studied, depolarizing currents evoked firing of multiple, adapting action potentials at 22°C. The number of action potentials increased with current strength producing a mean discharge of 5.1 (+100 pA, 1 s pulse), which was attenuated at 37°C to a mean of 1.4. The amplitude and kinetics of the slow, muscurine-sensitive inward and outward currents (I(M)) were highly temperature dependent. Lowering the temperature from 37 to 22°C reduced the steady-state current amplitude by approximately one-third and the rate of deactivation of I(M) by six- to ninefold at all voltages examined. The average Q10 for the time constant of deactivation of I(M) was 3.7 ± 0.3 (mean ± SE). Acetylcholine (ACh) induced tonic discharges in response to depolarizing currents (+100 pA, 1 s pulse) at both temperatures. This effect of ACh was inhibited by the muscarinic receptor antagonists, pirenzepine (100 nM), and mL-to

The membrane properties of neonatal rat intracardiac neurones were investigated at room temperature (24-26 TC) and at 35-37 °C using the perforated patch technique (Horn & Marty, 1988). Neurones had a mean resting membrane potential (Em) of -51.3 ± 1.0 mV (meanj± S.E.M., n = 12) and -52.7 ± 1.1 mV (n = 7) at 25 and 37 TC, respectively. The mean input resistance was 1302 ± 367 MQ (n = 6) at 25 TC, comparable to values recorded using the whole-cell technique. The input resistance was on average 64 % less at 37 °C, but was not statistically significantly different from that measured at room temperature. Hyperpolarizing pulses induced time-dependent rectification in all neurones at both temperatures. In most neurones, depolarizing currents evoked firing of multiple, adapting, action potentials at 25 TC, the number of which increased with current strength (+50 pA, 1 s pulse producing a median discharge of 6 (range 1-13, n = 6)); this was attenuated at 37 °C to a median of 1 (range 0-2, n = 4, P < 0.03; t test). These results demonstrate marked differences in the discharge activity and response to hyperpolarizing currents from those obtained using the whole-cell, dialysed, patch technique under otherwise comparable recording conditions at room temperature (Xu & Adams, 1992). Moreover, these firing characteristics are altered by temperature.