Neurokinin-1 And -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input
Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK3R). This study investigated a similar role for neurokinin-1 receptors (NK1R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK3R agonist. [Sar9,Met(O2)11]-substance P, an NK1R agonist, depolarised a smaller number of neurons. SR142801, a selective NK3R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK1R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar9,Met(O2)11]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK1R had significantly shorter latencies than those due to activation of NK3R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine1A receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]- piperazine (NAN-190), but unaffected by phentolamine, an a- adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK1R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK1R and NK3R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.