ON and OFF unipolar brush cells transform multisensory inputs to the auditory system

Abstract

Unipolar brush cells (UBCs) of the dorsal cochlear nucleus (DCN) and vestibular cerebellar cortex receive glutamatergic mossy fiber input on an elaborate brush-like dendrite. Two subtypes of UBC have been established based on immunohistochemical markers and physiological profiles, but the relation of these subtypes to the response to mossy fiber input is not clear. We examined the synaptic physiology of auditory UBCs in mouse brain slices, identifying two response profiles, and correlated each with a specific UBC subtype. One subtype had a striking biphasic excitatory response mediated by AMPAR and mGluR1α. The second was mGluR1α negative and was dominated by a strongly inhibitory outward K(+) current. These two subtypes upregulated or downregulated spontaneous firing, respectively. By analogy to the retina, we propose that UBCs comprise ON and OFF cells with respect to their response to glutamatergic input and may therefore provide distinct parallel processing of multisensory input to their targets.

Figure 1

UBC localization in the DCN. (A) Location of the DCN (yellow) in the brainstem (top), and the three components of DCN (bottom) comprising the molecular superficial layer (L1), principal cell body cell layer (L2) and deep layer (L3), the latter receiving multisensory input via mossy fibers (MF) and auditory input from the auditory nerve (AN). (B) The cerebellum-like organization of the DCN: AN fibers project to L3 and contact the basal dendrites of fusiform cells (FC), the output neurons of the DCN, located in L2 and projecting to the inferior colliculus (IC). Extrinsic glutamatergic mossy fibers (eMF) relaying multisensory input, terminate in L3, in large presynaptic terminals. eMF contact granule cells (GrC) and unipolar brush cells (UBC). UBCs contact GrCs via large glutamatergic intrinsic mossy fibers (iMF). GrC axons project to the molecular layer as parallel fibers (PF) and contact the apical dendrites of FC, as well as other L1 interneurons. Black circles indicate excitatory contacts. (C) Confocal immunofluorescence images of a coronal section of DCN in a mouse expressing GFP under control of the mGluR2 promoter. Top panel shows GFP labeling with Alexa 488 secondary antibody, middle panel shows mGluR2 labeling with Cy3 secondary antibody, and bottom panel shows overlay of the two channels. UBCs are labeled in the L3. Scale bar: 150 μm. (D) High magnification of the image in panel C, shows GFP and mGluR2 co-localization of a labeled UBC. Scale bar: 10 μm. White arrow indicates the dendritic brush.

Figure 2

ON and OFF responses of UBCs. (A) and (B) Postsynaptic responses to electric stimulation of presynaptic mossy fibers. (C) and (D) Responses to 1 mM glutamate puff application (7 ms, 5 psi). In each panel, left traces are in voltage-clamp and right are in current clamp (no bias current) from the same cells. (A) and (C) Example recordings show an inward biphasic EPSC (fast peak, followed by a sag in the current and a second slow decaying component), which led to prolonged increase in firing. (B) and (D) Example recordings show biphasic EPSC with a small, short inward current followed by a large slow decaying outward component; the latter pauses intrinsic firing. Black arrowheads indicate onset of stimulation in A and B and downward black arrows indicate onset of puff application in C and D. (E) Puff responses correlate with synaptically evoked signals. Diagram of the recording configuration: within the same cell both glutamate puff and subsequent electrical stimulus of presynaptic fibers were performed to confirm the current profiles in ON and OFF UBCs. (F) and (G) Puff responses in black, electrical stimulation in gray. Overlay of traces from an ON UBC in (F) and from an OFF UBC in (G). The white arrows indicate the onset of puff and electrical stimulus (H) Scatter plot comparing the amplitudes of postsynaptic currents in response to synaptic stimulation and amplitude of currents elicited in response to glutamate in both ON and OFF cells acquired in the experiments as the ones shown in panels F and G. N= 8, r²=0.48, P=0.057.

Figure 3

Characteristics of responses of ON and OFF UBCs to mossy fiber input. (A) and (B), examples of postsynaptic responses to synaptic stimulation of ON (n=7) and OFF (n=6) UBCs, respectively. A train with 10 stimuli was applied at 100 Hz, 50 Hz, 20 Hz and 10 Hz. Black arrowhead indicates the onset of the stimulus. (C) and (D) Graphic representation of the change in amplitude and charge of both ON (black) and OFF (blue) UBCs with the change of frequency. (E) and (F) Example of postsynaptic responses to changing synaptic stimulation number for ON (n=7) and OFF (n=5) UBCs respectively. A train at 100 Hz was applied with 1 (red), 3 (yellow), 5 (green), 10 (blue), 20 (gray) and 50 (black) stimuli. (G) Graphic representation of the change in charge of the postsynaptic response with the increase in number of stimuli, as shown in E and F for ON (black) and OFF (blue) UBCs. Black arrowheads indicated the onset of the stimulus. (H) and (I) Example of postsynaptic responses to Poisson stimulation trains at 10 Hz and 50 Hz for ON and OFF UBCs. The stimulus duration is indicated by the black bar and lasted 5 s. (J) Comparison of the ratio of the first charge measurement (Q1) taken at the time-point of the last stimulus and the second charge measurement (Q2) taken 4 seconds after the last stimulus. Data for ON (black) and OFF (blue) cells at 10 Hz (darker colors) and 50 Hz (lighter colors) poisson stimulation. (K) Mean charge elicited at Q2 with Poisson stimulation for OFF (blue) and ON (black) UBCs respectively. For 10 Hz OFF UBC n=6, ON UBC n=7 and for 50 Hz OFF UBC n=5 and ON UBC n=6. Error bars indicate ±SEM and significance level symbols are ns (non-significant, p>0.05), (*) (p≤0.05), (**) (p≤0.01).

Figure 4

ON UBCs are mGluR1α⁺. (A) Diagram of the recording configuration: whole-cell recording from an UBC: (1) Puff application of glutamate was done to identify the UBC subtype; (2) Subsequently, a puff pipette was exchanged for a pipette containing 200 μM (S)-DHPG. (B) ON UBC recordings. Top panel: voltage-clamp recordings and an inward current elicited in response to (S)-DHPG puff (black trace). This current is completely blocked by 150 μM LY367385 (gray trace). Bottom panel: the pronounced increase in intrinsic firing in response to the (S)-DHPG current. (C) OFF UBC recordings. Top panel: complete absence of currents elicited by (S)-DHPG puff, suggesting this subtype is mGluR1α negative. Bottom panel: no change in intrinsic firing in response to the puff application of the agonist. Black arrows in B and C indicate puff onset. Puff duration was of 10 to 35 ms depending on the location of the pipette. (D) ON UBC response to a single synaptic stimulus (arrowhead) in control (black), in the presence of the mGluR1α antagonist LY367385 (dark gray) and in the presence of LY367385 plus 5 μM NBQX (light gray). LY367385 had no effect on the fast EPSC peak amplitude (inset) and it did not affect the decay time. (E) ON UBC response to 20 stimuli at 100 Hz (arrowhead shows onset of the stimulus) in control (black), in LY367385 (dark gray) and in LY367385 plus 5 μM NBQX (light gray). LY367385 had a mild affect in amplitude and the decay time of the slow EPSC. (F) ON UBC response to 7-ms puff application of 1 mM glutamate (black arrow shows onset of the puff) in control (black), in LY367385 (dark gray) and in LY367385 plus 5 μM NBQX (light gray). LY367385 had a clear affect in amplitude and the decay time of the slow EPSC. (G) Histogram of the charge difference between control, LY367385, NBQX and GYKI53655 blocked currents, in ON UBC currents elicited by electrical stimulation, single pulse as in D or train as in E, and by glutamate puff, as in F. Responses were normalized to control of each cell control recording. Error bars show ±SEM and significance level symbols are: ns, non-significant or (p>0.05), (*) (p≤0.05), (**) (p≤0.01), (***) (p≤0.001), (****) (p≤0.0001).

Figure 5

Cerebellar UBCs also show either ON or OFF subtypes. Left panels show voltage-clamp recordings and right panels show current-clamp recordings. Arrowheads show onset of puff application. (A) Responses of a cerebellar ON UBC. (Ai) Biphasic inward response to 1 mM glutamate puff application (7 ms at 5 psi) and the corresponding increase in intrinsic firing frequency. (Aii) Example of a cell with inward current elicited in response to (S)-DHPG puff application and the corresponding increase in firing frequency. (B) Responses of a cerebellar OFF UBC). (Bi) Example of a cell with pronounced outward current in response to 1 mM glutamate puff application and the corresponding pause in firing. (Bii) Absence of any current elicited by (S)-DHPG and no change in intrinsic firing frequency after the puff. (C) Histogram showing the distribution of peak amplitudes of puff responses for 182 UBCs from both cerebellum and DCN.

Figure 6

Synaptic activation of mGluR2 receptors in OFF UBCs. (A) voltage-clamp recording of an OFF UBC in response to 20x 100 Hz stimuli (arrowhead indicates onset of the stimulus) in control (black), in the presence of LY341495 (dark gray) and in LY341495 plus NBQX (light gray). LY341495 completely blocked the IPSC and revealed an occluded inward current that was blocked by NBQX. (B) Response of the same cell to electrical stimuli with absence of the pause in firing in the presence of LY341495. (C) Top panel: OFF UBC response to a single synaptic stimulus and bottom panel: OFF UBC response to a train of stimuli (arrowhead indicates onset of the stimulus). Both panels show the current in control (black) and in the presence of group II mGluR antagonist LY341495 (dark gray). The subtraction of the response in the antagonist from control is show in light gray, making evident the current blocked by the antagonist. (D) I/V curve obtained with a current ramp from −30 mV to −140 mV, showing inward rectification and a reversal potential of −81.4±1.8 mV, close to the calculated reversal potential of K⁺ under our recording conditions. Gray shading shows ±S.E.M. (n=8) (E) Histogram of the charge difference between control, plus LY341495 and NBQX blocked currents in OFF UBC. Unsubtracted values show the over all charge measurements after a train of stimuli, measured after the last stimulus artifact. Subtracted values show the charge values of the subtraction of LY341495 from control after a single stimulus or after a train, respectively, without the stimulus artifact. The last bar shows the absolute NBQX blocked current charge, as the subtraction of the NBQX response from the response in LY341495. Error bars show ±SEM and significance symbols are: ns, non-significant or (p>0.05), (*) (p≤0.05), (**) (p≤0.01). (F) Diagram of recording configuration for G and H. Glutamate was applied first for subtype identification and switched to LY354740. (G) and (H) Responses of OFF and ON UBCs to the puff application of the group II mGluR agonist LY354740 in both voltage clamp (top panels) and current clamp (bottom panels). Both UBC subtypes showed an outward current in response to a 100-ms puff and a pause in intrinsic firing. However ON UBCs showed smaller amplitude currents and significantly shorter pause in firing under the same conditions.

Figure 7

Correlation of physiology and UBC subtype with immunohistochemistry. For each (A) and (B) Top left panels shows confocal image of an ON or OFF UBC, respectively, identified by puff application of glutamate shown in the bottom left panel, patched with biocytin added to the pipette solution. Biocytin was detected with streptavidin Alexa Fluor 488 Conjugated. Right panels show confocal immunofluorescence images of the same UBC identified with glutamate puff after triple labeling with streptavidin-Alexa 488 (green), mGluR1α with Cy3 secondary antibody (red) and calretinin with Alexa 647 secondary antibody (magenta). Scale bar: 10 μm. Solid white arrowhead indicates ON UBC and open white arrowhead indicates OFF UBC. Black arrowhead indicates onset of the application of glutamate for both ON and OFF UBCs.

Figure 8

Intrinsic firing of ON and OFF UBCs. (A) Example traces of ON and OFF UBCs firing intrinsically with no bias current injected. (B) Change in firing frequency of each UBC subtype in response to 2-pA increment changes in bias current. Samples were taken from 20-second long sweeps. ON UBC in gray, OFF UBC in black. (C) Top panels: corresponding responses to puff application of glutamate (arrowhead) for subtype identification as ON (Ci) or OFF (Cii) UBCs. A step-wise current-ramp was applied to each subtype with the same protocol of 200-ms rise time, 2000-ms decay time and steps down to −10 pA and −30 pA. The two bottom panels of Ci and Cii show the response to the current-ramp for each subtype. Both UBCs had identical pause in firing and rebound excitation in response to the hyperpolarizing steps.