Heterogeneous effects of norepinephrine on spontaneous and stimulus-driven activity in the male accessory olfactory bulb

Abstract

Norepinephrine (NE) release has been linked to experience-dependent plasticity in many model systems and brain regions. Among these is the rodent accessory olfactory system (AOS), which is crucial for detecting and processing socially relevant environmental cues. The accessory olfactory bulb (AOB), the first site of chemosensory information processing in the AOS, receives dense centrifugal innervation by noradrenergic fibers originating in the locus coeruleus. Although NE release has been linked to behavioral plasticity through its actions in the AOB, the impacts of noradrenergic modulation on AOB information processing have not been thoroughly studied. We made extracellular single-unit recordings of AOB principal neurons in ex vivo preparations of the early AOS taken from adult male mice. We analyzed the impacts of bath-applied NE (10 μM) on spontaneous and stimulus-driven activity. In the presence of NE, we observed overall suppression of stimulus-driven neuronal activity with limited impact on spontaneous activity. NE-associated response suppression in the AOB came in two forms: one that was strong and immediate (21%) and one other that involved gradual, stimulus-dependent monotonic response suppression (47%). NE-associated changes in spontaneous activity were more modest, with an overall increase in spontaneous spike frequency observed in 25% of neurons. Neurons with increased spontaneous activity demonstrated a net decrease in chemosensory discriminability. These results reveal that noradrenergic signaling in the AOB causes cell-specific changes in chemosensory tuning, even among similar projection neurons.NEW & NOTEWORTHY Norepinephrine (NE) is released throughout the brain in many behavioral contexts, but its impacts on information processing are not well understood. We studied the impact of NE on chemosensory tuning in the mouse accessory olfactory bulb (AOB). Electrophysiological recordings from AOB neurons in ex vivo preparations revealed that NE, on balance, inhibited mitral cell responses to chemosensory cues. However, NE's effects were heterogeneous, indicating that NE signaling reshapes AOB output in a cell- and stimulus-specific manner.

Keywords: accessory olfactory bulb; accessory olfactory system; chemical senses; information processing; norepinephrine.

Fig. 1.

Studying stimulus tuning and modulation in the AOS ex vivo preparation. A: diagram of the ex vivo preparation of the mouse AOS. Stimuli were delivered to the VNO while single-unit electrophysiological activity was recorded in the AOB. B: experimental overview. Three rounds of randomized stimulus trials were conducted before (baseline) and during (test) NE application. C: raster plot of stimulus-driven single-unit responses in the AOB in baseline and test periods in a control experiment (not exposed to NE). Response magnitude and stimulus selectivity remained constant over time. D: changes in firing rate (ΔR) across stimulus responses for the same cell shown in C. Numbers on x-axis refer to the stimulus repeat within each stimulus battery. E: average ΔR per stimulus battery for the cell represented in C and D. Error bars reflect SE. F: heat map representation of stimulus responsiveness of 23 cells. Each row is a different stimulus; each column is a different cell. Black indicates stimuli that were not delivered to that particular cell. Asterisk indicates the cell shown in C–E. Norm. ΔR, normalized ΔR.

Fig. 2.

Spontaneous activity is increased in a subpopulation of AOB neurons. A: scatter plot of spontaneous activity in an AOB neuron that showed increased spontaneous activity (Spont. Rate) in the NE period (****P = 1.25 × 10⁻¹⁶). Red circles indicate the unstimulated firing rate (measured just before a stimulus onset). Black circles indicate firing rate measurements that happened to follow strong stimulus responses (which may not fully recover to baseline before the subsequent trial). Inset shows the mean and SD of the measured spontaneous rate for this cell. B: the number of cells in control (n = 11) and NE-treated (n = 12) conditions that exhibited significantly changed or unchanged spontaneous activity (Num., number; Inc., increased; Dec., decreased; Not Sig., no change). There were significantly more cells that exhibited increased spontaneous activity (P = 0.0185 compared with a shuffled population). At right are histograms relating the observed prevalence (% Obs.) of spontaneous rate increases (red arrows) to shuffle test expectations (which are based on control recordings). C: scatter plot of stimulus-evoked ΔR, with changes in spontaneous activity indicated by color (blue, decreased; red, increased; black, no change). Error bars represent SE. D: box plots showing mean stimulus responses (Resp.) for control and NE-exposed cells (left 2 graphs), and breakdowns based on spontaneous activity changes (right 2 graphs). *P = 0.01 (paired Student’s t-test). E: scatter plots of the sensitivity index (d′) for control (n = 16) and NE-treated (n = 19) conditions. Colors reflect the same parameters as C. F: box plots showing d′ values for all stimuli in control and NE-treated conditions, broken down by categories as in D. *P = 0.0133 (paired Student’s t-test).

Fig. 3.

NE elicits immediate, nonmonotonic suppression in a small fraction of AOB neurons. A: raster plot showing stimulus-evoked spiking responses in a cell demonstrating immediate, nonmonotonic response suppression in the presence of NE. B: per-trial ΔR values for the cell shown in A. C: average ΔR for the cell in A and B. *P = 0.027 (unpaired, 2-tailed Student’s t-test). D: heat map of average stimulus responses in control cells (left; n = 11) and NE-exposed cells (right, n = 12). Nonmonotonic enhancement is shown in red and suppression in blue. White pixels indicate no change, and black indicates that the cell did not respond to the stimulus in either the baseline or test period. Asterisk marks the column showing the response of the cell represented in A–C. E: per-stimulus counts of significant nonmonotonic enhancement (Enh.), suppression (Supp.), or no change (n = 16 for control, n = 19 for NE). The number of responses exhibiting nonmonotonic enhancement was significantly decreased during NE exposure (*P = 0.032 shuffle test; P = 0.03 binomial test). “Other” indicates responses that did not show nonmonotonic changes. F: per-cell count of enhancement, suppression, or a mix of both (n = 11 cells for control, n = 12 for NE). “Other” indicates cells that did not demonstrate any nonmonotonic changes.

Fig. 4.

NE-associated monotonic response suppression. A: raster plot showing stimulus-evoked spiking responses in a cell demonstrating monotonic suppression in the presence of NE. B: per-trial ΔR values for the cell shown in A. C: average ΔR for the cell in A and B. D: across-trial ΔR values for responses that either exhibited monotonic suppression during NE delivery (left; n = 9) or did not (right; n = 10). E: heat map of stimulus responses in control (left; n = 11) and NE-exposed cells (right; n = 12) that exhibited monotonic enhancement (red), suppression (blue), or neither (white). Hues indicate the net difference in ΔR at the third (final) stimulus delivery in test period compared with final stimulus delivery in the baseline period. F: per-stimulus counts of monotonic enhancement, suppression, or no monotonic change in control and NE-exposed conditions. The number of responses exhibiting monotonic suppression was significantly increased (**P = 0.004 shuffle test; P = 0.0009 binomial test), whereas the number of unchanged responses was significantly decreased during NE exposure (**P = 0.001 shuffle test; P = 0.0009 binomial test; n = 16 for control, n = 19 for NE). G: per-cell counts of monotonic enhancement or suppression. There were significantly more cells with a suppressed response (**P = 0.005 shuffle test; P = 0.0008 binomial test) and significantly fewer cells with no significant changes (**P = 0.0008 shuffle test; P = 0.0008 binomial test; n = 11 for control, n = 12 for NE).

Fig. 5.

Combined nonmonotonic and monotonic suppression during NE exposure. A: heat map of stimulus responses that showed monotonic or nonmonotonic suppression (blue), enhancement (red), or no change (No Chng.) in control (left; n = 11) or NE-exposed cells (right; n = 12). B: per-stimulus counts of suppression, enhancement, or no change. The overall number of suppressed stimuli was significantly increased (***P = 0.0009 shuffle test; P = 0.0002 binomial test), and the number of unchanged responses was significantly decreased (*P = 0.035 shuffle test; P = 0.04 binomial test; n = 19 for NE, n = 16 for control). C: per-cell counts of enhancement, suppression, a mixture of enhancement and suppression, or no change. The number of cells with enhanced responses was significantly decreased (*P = 0.021 shuffle test; P = 0.02 binomial test), and the number with suppressed responses was significantly increased (***P = 0.008 shuffle test; P = 0.001 binomial tests).