Single-unit responses to 22 kHz ultrasonic vocalizations in rat perirhinal cortex

Abstract

Rats emit ultrasonic vocalizations (USVs) as social signals in several situations. Lesion studies have shown that rat perirhinal cortex (PR), a polymodal sensory region that is reciprocally connected with the amygdala, is critical for normal fear conditioning to so-called "22 kHz USVs". Here we evaluated single-unit responses in rat PR to 22 kHz USVs and other acoustic stimuli. One question was whether PR circuits are specifically and preferentially tuned, prior to fear conditioning, to respond to USVs and USV-like stimuli. Two 22 kHz USVs were pre-recorded from different conspecifics. Each USV consisted of a "bout" of several discrete calls. Using experimentally naïve rats, single-unit responses to the USVs were compared with responses to continuous or discontinuous tones that had the same root frequency as the USVs (19 or 22 kHz). The on/off patterns of the discontinuous tones were temporally matched to the call structure in the corresponding USVs. Compared to continuous tones, the USVs were no more likely to elicit single-unit firing changes in PR. On the other hand, the continuous tones and USVs clearly did elicit different firing patterns in many units. More specifically, the USVs sometimes elicited a transient increase in discharge frequency to each call in a bout of calls. Interestingly, the USVs and the temporally matched tone segments usually elicited similar firing patterns. The USV-elicited firing pattern in PR thus appears to be controlled by the on/off temporal structure of the calls rather than by the frequency or amplitude modulations associated with each call in a bout of calls.

Figure 1

Spectrograms of six of the seven auditory stimuli. (A) A 19 kHz continuous tone. (B) A 19 kHz discontinuous tone that was temporally- and frequency-matched to the USV depicted in part C. (C) A bout of 11 calls centered at ~19 kHz (range: 19 22 kHz). The mean (± SE) call duration was 575 ± 44 ms and the mean inter-call interval was 132 ± 2 ms. (D) An oscilloscope trace of the amplitude modulations (voltage against time) of the USV depicted in part C. (E) A 22 kHz continuous tone. (F) A 22 kHz discontinuous tone that was temporally- and frequency-matched to the USV depicted in part G. (G) A bout composed of 4 calls centered at ~22 kHz (range: 22 – 24 kHz). The mean (± SE) call duration was 1853 ± 373 ms and the mean inter-call interval was 213 ± 2 ms. (H) An oscilloscope trace of the amplitude modulations (voltage against time) of the USV depicted in part G.

Figure 2

An example of a PR single-unit recording and horizontal sections showing the location of recording sites within PR. (A) Twenty-five electrode placements are mapped onto stereotaxic plates modified from the Paxinos and Watson atlas [20]. Results are combined from Experiment 1 (marker = ○) and Experiment 2 (marker = ●). Dorso-ventral locations of plates 98–100 showing depth relative to Bregma. (B) A 70 μm horizontal section corresponding to plate 98 of the Paxinos and Watson atlas [20]. A representative marking lesion, indicated by the arrow, was made at the tip of the electrode bundle. (C) An example of a recording from a single unit in PR in a freely-behaving rat. Abbreviations: CA1, field CA1 of the hippocampus; CA3, field CA3 of the hippocampus; LA, lateral nucleus of the amygdala; LEnt, lateral entorhinal cortex; PR, perirhinal cortex.

Figure 3

Four common firing patterns elicited by continuous tones in Experiment 1 (presented for 10 s, shown by gray shading). Time bins are 50 ms wide. (A) A phasic onset (+) response. (B) A phasic offset (+) response. (C) A phasic onset (+)/offset (+) response. (D) A tonic (+) response.

Figure 4

An example of a PR single-unit recorded in Experiment 1 that exhibited a “call-related” firing pattern by a 22 kHz USV (shown in Fig. 1C, D). The time bins are 50 ms wide. (A) A perievent histogram and raster plot display of a single-unit response to the 11-call USV. The USV is present from 0 to 7.8 s (shaded area). The histogram suggests a phasic onset (+) and a tonic (+) response. There is also a hint of periodicity in the firing rate across the course of the stimulus. (B) A 3-D contour plot reveals that the apparent periodicity is associated with the onset of individual calls. The axis labeled “Time” aligns firing to the onset of each individual call. The axis labeled “Call Segment” shows the responses to each of the successive calls. The left edge of the contour begins 100 ms before the stimulus onset. Lines that are more or less perpendicular to the Time axis represent the beginning and ends of successive time bins. The first two time ribbons (first 100 ms) show firing levels before the stimulus onset (arrow). In addition to the large phasic increase in the firing rate to the initial call, there is also a phasic increase in the firing rate to each of the subsequent calls. This repeating increase in the firing rate has been termed a “call-related” firing pattern. The ridge of the initial part of the contour occurs at a latency (from the call onsets) in the bin that spans the interval from 50 to 100 ms.

Figure 5

An example of phasic onset (+) firing patterns elicited by 6 stimuli in a single unit from Experiment 2. (A) Perievent raster and histogram plots (bin size = 50 ms) are depicted with the spectrograms of the eliciting 11-call USV (shown in Fig. 1 C, D) directly underneath. The 3-D contour plot (bin size = 25 ms) reveals a “call-related” firing pattern. The ridge of the contour occurred at a latency (from the call onsets) of 25 – 50 ms. (B) The same plots are shown for the temporally- and frequency-matched discontinuous tone (shown in Fig. 1 B). The ridge of the contour occurred at a latency of 25 – 50 ms. (C) The analogous plots are shown for the frequency-matched continuous tone (shown in Fig. 1 E). The peak occurred at a latency of 50 – 75 ms. The histogram suggests that the phasic onset (+) response might be followed by a tonic (−) response. However, the apparent tonic (−) component was not statistically significant (p > 0.05). (D) Single-unit responses to a 4-call USV (shown in Fig. 1 G, H) plotted as in part A. The ridge of the contour occurred at a latency of 25 – 50 ms. (E) The same plots are shown for the matched discontinuous tone (shown in Fig. 1 F). The ridge of the contour occurred at a latency of 25 – 50 ms. (F) The analogous plots are shown for the frequency-matched continuous tone (shown in Fig. 1 E). The peak occurred at a latency of 25 – 50 ms.

Figure 6

An example of phasic offset (+) firing patterns elicited by 6 stimuli in a single unit from Experiment 2. (A) Perievent raster and histogram plots (bin size = 50 ms) are depicted with the spectrograms of the eliciting 11-call USV (shown in Fig. 1 C, D) directly underneath. The 3-D contour plot (bin size = 25 ms) reveals a “call-related” firing pattern. The ridge of the contour occurred at a latency (from the call offsets) of 75 – 100 ms. (B) The same plots are shown for the temporally- and frequency-matched discontinuous tone. The ridge of the contour occurred at a latency of 75 – 100 ms. (C) The analogous plots are shown for the frequency-matched continuous tone. The peak response occurred at a latency of 75 – 100 ms. (D) Single-unit responses to a 4-call USV (shown in Fig. 1 G, H) plotted as in part A. The ridge of the contour occurred at a latency of 50 – 75 ms. (E) The same plots are shown for the matched discontinuous tone (shown in Fig. 1 F). The ridge of the contour occurred at a latency of 75 – 100 ms. (F) The analogous plots are shown for the frequency-matched continuous tone (shown in Fig. 1 E). The peak occurred at a latency of 75 – 100 ms.