Nicotine modulates the spontaneous synaptic activity in cultured embryonic rat spinal cord interneurons

The nicotine-induced modulation of the synaptic activity was studied in cultured spinal cord neurons from embryonic rats, using the patch-clamp technique, alone or in combination with Ca ²⁺ imaging. Morphologically, neurons could be divided into two populations: multipolar nerve cells and bipolar, spindle-shaped neurons. Neurons were predominantly GABAergic, with ∼70% of bipolar cells and 60% of multipolar cells positive for GABA immunostaining. Nicotine (Nic) did not affect the activity of the spontaneous postsynaptic current (sPSC) in multipolar neurons, whereas bipolar cells responded to Nic applications with an enhancement of both inhibitory and excitatory synaptic activity (threefold for 100 μM Nic). No change in the mean event amplitude was observed. The increase of sPSC frequency was detectable at 1-10 μM Nic, and was prevented by dihydro-β-erythroidine (DHβE) but not by α-bungarotoxin. Choline, a selective α7-nAChR agonist, did not mimic the Nic action. Simultaneous treatment with inhibitors of ionotropic glutamate receptors, CNQX (20 μM) and AP5 (20 μM), completely blocked the excitatory sPSC activity but did not prevent the Nic-induced enhancement of inhibitory sPSC activity. Tetrodotoxin (1 μM) reduced the basal spontaneous activity but did not block the Nic-induced effects on bipolar neurons. In a subset of bipolar neurons (12%) exposed to AP5 and CNQX, Nic activated DHβE-sensitive inward currents, associated with an elevation of cytosolic Ca ²⁺ ([Ca ²⁺] _i). Our results provide the first evidence of modulation of both excitatory and inhibitory neurotransmitter release in embryonic spinal cord interneurons by non-α7-containing nicotinic receptors.