EXCITATION OF SYMPATHETIC PREGANGLIONIC NEURONS VIA METABOTROPIC EXCITATORY AMINO-ACID RECEPTORS

The role of excitatory amino acid metabotropic receptors in the regula tion of excitability of sympathetic preganglionic neurons was investig ated. This study used both conventional intracellular and whole-cell p atch clamp techniques to record from sympathetic preganglionic neurons in transverse spinal cord slices of the rat (9-21 days old) The metab otropic receptor agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) (10-200 mu M, superfused for 2-60 s) and quisqualat e (1-50 mu M, superfused for 2-60 s) induced concentration-dependent d epolarizing responses which did not desensitize. These responses were unaffected by the glutamate ionotropic receptor antagonists 6-cyano-7- nitroquinoxaline-2,3-dione (CNQX, 10-50 mu M), 6,7-dinitroquinoxaline- 2,3-dione (DNQX, 10 mu M), dizocilpine (MK-801, 10-40 mu M), [(R)-2-ca rboxy-piperazin-4-yl]-propyl-1-phosphonic acid (D-CPP, 10-50 mu M) and DL-2-amino-5-phosphonovaleric acid (DL-AP5, 20-100 mu M). Depolarizin g responses to 1S,3R-ACPD and quisqualate were unaffected by L-2-amino -3-phosphonopropionic acid (L-AP3, 30 mu M-1 mM) and L-2-amino-4-phosp honobutanoic acid (L-AP4, 100 mu M-1 mM). The responses to 1S,3R-ACPD and quisqualate were reduced by including the G-protein blocker GDP-be ta-S (400 mu M) in the patch pipette solution by 77 +/- 2% (mean +/- S .E) of control (n = 3), suggesting that these agonists activate a G-pr otein-coupled receptor. Metabotropic receptor-mediated responses were maintained in the presence of tetrodotoxin (500 nM), progressively red uced with increased membrane hyperpolarization to around -95 mV and as sociated with either an increase of 16.5 +/- 2.8% (data from four neur ons) in the majority of neurons (n = 22 of 34) or no measurable change (n = 12) in neuronal input resistance. These data suggest that the ag onists exert a direct action on sympathetic preganglionic neurons invo lving a reduction in one or more membrane conductances. 1S,3R-ACPD and quisqualate had several effects on sympathetic preganglionic neuron m embrane properties including: inhibition of a slow apamin-insensitive component of the afterhyperpolarization; a reduction in spike frequenc y adaptation leading to increases in firing frequency from 6.4 +/- 2.8 Hz in control experiments up to 14.7 +/- 3.0 Hz (n = 6 neurons) in th e presence of a metabotropic receptor agonist; a broadening of the act ion potential by 37.5 +/- 6.4% (n = 6 neurons) of control. These obser vations suggest that the metabotropic receptor-mediated depolarization is due, at least in part, to the reduction of potassium conductances involved in the spike afterhyperpolarisation potential. In quiescent s ympathetic preganglionic neurons, 1S,3R-ACPD and quisqualate induced r hythmic oscillations in membrane potential. This effect could be irrev ersible with oscillations persisting for several hours post-induction. These oscillations could give rise to sustained, rhythmic activity ch aracterized by burst firing or regular single spike discharge. The osc illations were similar to those observed by us to occur spontaneously in some sympathetic preganglionic neurons. The amplitude and frequency of oscillations varied considerably in the same neuron, due to differ ent rhythms being superimposed on top of one another, and between diff erent sympathetic preganglionic neurons. The mean peak amplitude and f requency of oscillations observed with patch electrodes at room temper ature was 9.6 +/- 2.3 mV and 0.31 +/- 0.05 Hz (data pooled from six sy mpathetic preganglionic neurons) respectively. These effects were not sensitive to ionotropic receptor antagonists and were not mimicked by the ionotropic receptor agonists AMPA or kainate, suggesting selective activation of metabotropic receptors is required to induce oscillator y activity in some previously silent sympathetic preganglionic neurons . As oscillations are thought to reflect electrotonic coupling between sympathetic preganglionic neurons these observations suggest metabotr opic receptors may have an important role to play in synchronizing the electrical activity of groups of sympathetic preganglionic neurons. T he results suggest that sympathetic preganglionic neurons possess func tional metabotropic excitatory amino acids receptors whose activation increases the excitability of these neurons by reducing potassium cond uctances. In addition, activation of metabotropic receptors can lead t o the induction of rhythmic oscillations in membrane potential and lon g-term changes in the excitability of these neurons.