Amine modulation of Ih in a small neural network

We studied the functional role and modulation of the hyperpolarization-activated inward current (I-h) in the pyloric network of the lobster stomatogastric ganglion. In isolated neurons, I-h is a small current with a hyperpolarized voltage of half-activation (V-Act) and a slow time constant of activation (tau(Act)). Bath application of dopamine (DA), octopamine (OCT), or serotonin (5HT) modified I-h in selected synaptically isolated pyloric neurons. DA significantly enhanced I-h in the anterior burster ( AB) neuron by depolarizing its V-Act, accelerating its tau(Act), and enhancing its maximal conductance (g(max)). DA more weakly enhanced I-h in the pyloric constrictor (PY) and ventricular dilator (VD) neurons. OCT weakly depolarized V-Act and accelerated tau(Act) in the VD and inferior cardiac (IC) neurons. 5HT depolarized V-Act in the IC neuron. Under control conditions with intact modulatory inputs from other ganglia, the pyloric rhythm cycles strongly at about 1-2 Hz. Bath application of the I-h blocker cesium (Cs+) caused a mean increase in the period of 8%, although this effect was highly variable. When Cs+ was applied to an isolated ganglion where the pyloric rhythm had been activated only by DA, the cycle period was consistently increased by 13.5%, with no other strong changes in rhythm parameters. These results suggest that I-h regulates the pyloric rhythm by accelerating AB pacemaker frequency, but that this effect can vary with the modulatory conditions.