EFFECTS OF SOMATOSENSORY AND PARALLEL STIMULATION ON NEURONS IN DORSAL COCHLEAR NUCLEUS

1. Single units and evoked potentials were recorded in the dorsal coch lear nucleus (DCN) of paralyzed decerebrate cats in response to electr ical stimulation at two sites: 1) in the somatosensory dorsal column n uclei (together called MSN below for medullary somatosensory nuclei), which activates mossy-fiber inputs to granule cells in superficial DCN , and 2) on the free surface of the DCN, which activates granule cell axons (parallel fibers) directly. The goal was to evaluate hypotheses about synaptic interactions in the cerebellum-like circuitry of the su perficial DCN. A four-pulse facilitation paradigm was used (50-ms inte rpulse interval); this allows identification of three components of th e responses of DCN principal cells (type IV units) to these stimuli. T he latencies of the response components were compared with the latency of the evoked potential in DCN, which signals the arrival of the para llel fiber volley at the recording site. 2. The first component is a s hort-latency inhibitory response; this component is seen only with MSN stimulation and is seen almost exclusively in units also showing the second component, the transient excitatory response. The short-latency inhibitory component precedes the evoked potential. No satisfactory e xplanation for the short-latency component can be given at present; it most likely reflects a fast-conducting inhibitory input that arrives at the type IV unit before the slowly conducting parallel fibers. 3. T he second component is a transient excitatory response; this component is seen with both MSN and parallel fiber stimulation; it is weak and appears to be masked easily by the inhibitory response components. The excitatory component occurs at the same latency as the evoked potenti al and probably reflects direct excitation of principal cells by granu le cell axons. The excitatory component is seen in about half the type IV units for both stimulating sites. With MSN stimulation, the lack o f excitation in some units suggests a heterogeneity of cochlear granul e cells, with some carrying somatosensory information and some not car rying this information; with parallel fiber stimulation, excitation pr obably requires the stimulating and recording electrodes to be lined u p on the same ''beam'' of parallel fibers. 4. The third component is a long-lasting inhibitory response that is observed in virtually all ty pe IV units with both MSN and parallel-fiber stimulation; its latency is longer than the evoked potential. Evidence suggests that is propose d by inhibitory input from cartwheel cells. The appearance of this inh ibitory component in almost all type IV units can be accounted for by the considerable spread of cartwheel-cell axons in the direction perpe ndicular to the parallel fibers. 5. The evoked potential and all three components of the unit response vary systematically in size over the four pulses of the electrical stimulus These results can be accounted for by two phenomena; 1) a facilitation of the granule cell synapses o n all cell types that produces a steadily growing response through the four pulses, resembles presynaptic facilitation, and is seen with bot h MSN and parallel-fiber stimulation; and 2) a strong reduction in the granule cell response between the first and second pulse for MSN stim ulation only. This reduction probably occurs presynaptically in the gl omerulus or in the granule cell itself and could reflect inhibitory in puts. 6. The response components described above are seen in type IV u nits recorded in both the fusiform-cell and deep layers of the DCN; th is suggests that both pyramidal and giant cells are activated similarl y. The simplest interpretation is that both principal cell types are a ctivated by the cerebellum-like circuitry in superficial DCN. Alternat ively, because giant cells appear to make limited contact with granule -cell circuits of superficial DCN, this finding may suggest the existe nce of currently undescribed granule cell circuits in deep DCN that ar e similar in function to those in superficial DCN.