Frequency dependence of synaptic transmission in nucleus of the solitary tract in vitro

Afferent fibers from visceral sensory receptors enter the medulla oblongata, form the solitary tract, and synapse with neurons in the nucleus of the solitary tract. In the present study longitudinal slices were prepared from guinea pig medulla in order to examine the properties of transmission at these synapses in vitro. Synaptic responses to selective stimulation of solitary tract fibers were recorded intracellularly from neurons in an area, close to the obex and immediately medial and lateral to the tract, where arterial baroreceptor fibers are known to terminate. The amplitude of maximally evoked postsynaptic potentials (PSPs) in solitary tract neurons was strongly dependent on stimulus frequency. On increasing frequency from 0.5 to 20 Hz, a PSP depression of 80% was reached in 4-8 s. The mean depression was 35% at 5 Hz and 60% at 10 Hz. Sufficient local connections were retained in vitro that solitary tract stimulation evoked disynaptic inhibitory potentials and long latency, possibly polysynaptic, excitatory potentials in some neurons. The possibility that frequency-dependent changes in the efficacy of these local synaptic circuits contributed to PSP depression was examined. The role of postsynaptic inhibition in synaptic depression was tested by examining the frequency dependence of PSPs at membrane potentials close to the reversal of their excitatory component. The resulting hyperpolarizing PSPs were also depressed suggesting that a facilitation of postsynaptic inhibition at high frequency does not underlie the depression. The contribution of depression in multisynaptic excitatory pathways to PSP depression was assessed by exclusion. At low stimulus intensities, excitatory synaptic events with no long latency components were evoked. These events exhibited a similar frequency dependence to that of maximal PSPs. These results suggest that mechanisms operating at synapses made by solitary tract fibers are responsible for the frequency dependence of PSPs recorded in solitary tract neurons. Such mechanisms might contribute to the adaptation of some cardiovascular reflexes initiated by baroreceptors.