An intracellular analysis of dendrodendritic inhibition in the turtle in vitro olfactory bulb

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Abstract
Intracellular recordings from an in vitro preparation of turtle [P. scripta elegans] olfactory bulb were used to determine the pathway responsible for producing synaptic inhibition of mitral cells. Inhibitory post-synaptic potentials (ipsp) could be elicited in mitral cells by both olfactory nerve (orthodromic) and mitral cell axon (antidromic) stimulation and by suprathreshold depolarizing current pulses injected intracellularly through the recording micro-electrode. Reversing the Cl gradient by either intracellular injection of Cl or lowering the external Cl concentration reversed the ipsp into depolarizing potentials. The GABA [.gamma.-aminobutyric acid] antagonists, bicuculline and picrotoxin, blocked the ipsp. A large increment in the size of the orthodromic and antidromic ipsp was associated with an action potential. Grading the stimulus intensity on either side of threshold resulted in graded changes in the size of the ipsp. The increment associated with an action potential and the ability to evoke an ipsp by direct stimulation of a mitral cell suggested that these phenomena were due to activation of the dendrodendritic reciprocal synapses between mitral and granule cells. Orthodromic, antidromic and directly produced action potentials could be fractionated such that regenerative activation of the soma-dendritic membrane could be blocked. Only when this membrane was allowed to reach threshold was a large ipsp recorded. Apparently, the increment in the ipsp was due to activation of a synaptic pathway involving the soma-dendritic membrane. When spike propagation in the mitral cell axons was blocked by tetrodotoxin (TTX), an ipsp could still be produced by direct stimulation, indicating that the mitral cell soma-dendritic membrane is functionally both pre- and post-synaptic. TTX blocked the fast, high amplitude somatic spikes and revealed higher threshold, broader spikes of lower amplitude that were blocked by Co and C-free Ringer. Tetraethylammonium (TEA) increased the duration and the amplitude of the Ca spike. The amplitude was also increased by Ba which prolonged the spike only if TEA was present. In the presence of TEA, bicuculline also prolonged the Ca spike. Apparently, 3 ionic conductances limit the duration of the Ca action potential: a voltage-dependent K conductance, a Ca-dependent K conductance, and the Cl conductance associated with the ipsp. Spontaneous, bicuculline-sensitive, depolarizing potentials were recorded in mitral cells impaled with KCl-filled electrodes. Orthodromic or antidromic stimulation increased the frequency of these small potentials for the duration of the ipsp, indicating prolonged GABA release. Stimulation of the olfactory nerves, the mitral cell axons, and direct stimulation could elicit action potentials in granule layer cells. Orthodromic and antidromic activation was followed by a hyperpolarization of about the same duration as the mitral cells ipsp and was probably the result of dysfacilitation. Paired stimulation of the mitral cell axons resulted in the diminution of the granule cell ipsp evoked by the 2nd shock, indicating that the predominant excitatory input to the granule cells is through the mitral cell dendrites. Apparently, both synaptic inhibition of mitral cells and excitation of granule cells is mediated primarily by the dendrodendritic reciprocal pathway.