Homosynaptic and heterosynaptic modulation of the information processing in the mammalian parasympathetic ganglia.

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Two types of homosynaptic facilitation [1 & 2 (1)] and one type of heterosynaptic depression [2 (2)] were determined in mammalian pelvic parasympathetic ganglia in vitro, utilizing intracellular recording techniques.1. rabbit vesical pelvic ganglia that innervate the urinary bladderMost of synapses in the ganglia function as a high pass filter ; only when the action potentials of preganglionic neurons occur at high frequency such as 10 Hz, the nicotinic fast EPSP can evoke the orthodromic action potentials. Mechanisms of the filter involve the purinergic low-cut-filter and the frequency dependent increase of transmitter release. The former consists of endogenous ATP and presynaptic inhibitory P_2 receptors. The later might be explained by the "residual calcium" hypothesis.2. feline colonic ganglia that innervate colonic effector structures(1) effect of pelvic nerve stimulationRepetitive stimulation (10 Hz, 30 sec) of pelvic nerve (PN) augmented a spontaneous oscillation of membrane potential termed a depolarizing membrane oscillation (DMO) of feline parasympathetic neurons. These augmented DMO triggered an action potential, converting a silent neuron to a spontaneously firing neuron. Effects of PN-stimulation were antagonized by atropine and mimicked by muscarine.(2) effect of hypogastric nerve stimulationContrary repetitive stimulationof hypogastric nerve (HGN) often depressed the DMO,converting the spontaneously firing neuron to the silent neuron. Effects of HGN-stimulation were partially antagonized by yohimbine and mimicked by norepinephrine.These results provide a new type of autoinhibition at nicotinic synapses and interaction between sympathetic and parasympathetic division in the parasympathetic ganglia.