A subthreshold persistent sodium current mediates bursting in rat subfornical organ neurones

1. It is widely accepted that while release of amino acid neurotransmitters occurs with relatively high fidelity, peptidergic synapses require cluster ed bursts of action potentials for optimal transmitter release. Here we des cribe for the first time the occurrence and mechanisms of bursting by neuro nes in the subfornical organ (SFO), cells that utilize the peptide angioten sin II (ANG) in neurotransmission in autonomic pathways. 2. In current clamp recording of isolated SFO neurones in vitro, 53% (n = 7 4) showed either spontaneous or evoked burst-like discharge patterns. Burst s typically appeared as shifts in bistable membrane potential, with action potentials superimposed on a depolarizing afterpotential (DAP). Similarly, in vivo single unit recordings of identified SFO neurones showed that 9 of 15 neurones fired in bursts. 3. The pattern of bursting, as well as duration of evoked DAPs was strongly dependent upon membrane potential, suggesting that the DAP contributes to burst generation. Based on our previous observation of calcium-sensing rece ptor (CaR)-activated bursts, we investigated the effects of NPS R-467, an a llosteric agonist of the CaR, on evoked DAPs. NPS R-467 (1 muM) potentiated DAP duration throughout the voltage range tested. 4. We observed a dependence of evoked DAPs upon Na+ channels, as shown by s ensitivity to tetrodotoxin (0.5 muM) or reduction of external [Na+] from 14 0 to 40 mM. The duration of DAPs suggested that a persistent Na+ current me diates these events. Voltage-clamp analysis revealed the presence of a subt hreshold sodium current, I-NaP. 5. Pharmacological blockade of I-NaP with 100 muM lidocaine reduced the dur ation of evoked DAPs, and inhibited bursting in SFO neurones. Facilitation of I-NaP with 10 nM anemone toxin (ATX) increased DAP duration and led to m arked excitation of bursting cells. These data indicate that I-NaP is the m ain current underlying bursting in SFO neurones. 6. Our observations of receptor-mediated facilitation of bursting by SFO ne urones represents an intriguing mechanism through which the release of the peptide neurotransmitter ANG may be regulated.