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

Abstract

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.