Pharmacological specialization of learned auditory responses in the inferior colliculus of the barn owl

Abstract

Neural tuning for interaural time difference (ITD) in the optic tectum of the owl is calibrated by experience-dependent plasticity occurring in the external nucleus of the inferior colliculus (ICX). When juvenile owls are subjected to a sustained lateral displacement of the visual field by wearing prismatic spectacles, the ITD tuning of ICX neurons becomes systematically altered; ICX neurons acquire novel auditory responses, termed "learned responses," to ITD values outside their normal, pre-existing tuning range. In this study, we compared the glutamatergic pharmacology of learned responses with that of normal responses expressed by the same ICX neurons. Measurements were made in the ICX using iontophoretic application of glutamate receptor antagonists. We found that in early stages of ITD tuning adjustment, soon after learned responses had been induced by experience-dependent processes, the NMDA receptor antagonist D, L-2-amino-5-phosphonopentanoic acid (AP-5) preferentially blocked the expression of learned responses of many ICX neurons compared with that of normal responses of the same neurons. In contrast, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked learned and normal responses equally. After long periods of prism experience, preferential blockade of learned responses by AP-5 was no longer observed. These results indicate that NMDA receptors play a preferential role in the expression of learned responses soon after these responses have been induced by experience-dependent processes, whereas later in development or with additional prism experience (we cannot distinguish which), the differential NMDA receptor-mediated component of these responses disappears. This pharmacological progression resembles the changes that occur during maturation of glutamatergic synaptic currents during early development.

Fig. 1.

ITD pathway to the optic tectum and modification of ITD tuning by prism experience. A, ITD pathway to the optic tectum. The ICCls projects topographically to the ICX (Wagner et al., 1987), which in turn projects topographically to the optic tectum (Knudsen and Knudsen, 1983). Horizontal tick marksindicate tonotopic organization, used to distinguish the ICCls from the ICX. B, Schematic stages of ITD tuning modification in the ICX and tectum during prism experience. Before prism attachment, ITD tuning for each recording site is centered around a best ITD value (Normal) that is systematically related to anatomical location in the ICX and to the visual receptive field azimuth for sites in the tectum (Feldman and Knudsen, 1997). After a few weeks of prism experience, ICX and tectal neurons acquire novel responses to ITD values displaced from the normal value by an amount corresponding to the optical displacement of the prisms (Learned). Learned and normal responses are coexpressed at many sites to create transition state ITD tuning, which can be broad or even double-peaked (Brainard and Knudsen, 1995). With additional prism experience, normal responses are greatly reduced.

Fig. 2.

Method for identification of transition state ITD tuning in the ICX. A, Iso-ITD transects (dashed lines) superimposed on the maps of ITD in the ICCls, ICX, and optic tectum in a schematic horizontal section through the optic lobe of a normal owl. Ovals denote locations of neurons tuned to 0, 45, and 100 μsec contralateral ear leading ITD in each nucleus, based on previous mapping studies (Brainard and Knudsen, 1993; Feldman and Knudsen, 1997). Dark lines indicate axes of varying best ITD in each nucleus. The map of visual azimuth in the tectum is also indicated. c, Contralateral; i, ipsilateral; OT, optic tectum. B, Representative ITD tuning curves from sites in the lateral half of the ICX on the 0 transect in a juvenile (circles) and a normal adult (squares) owl. Trianglesindicate best ITDs. Inset, Distribution of best ITD values measured in the lateral ICX on the 0 transect for 90 sites in six normal owls. Ninety-five percent of all units had best ITDs within ± 9 of 0 μsec (indicated by the gray region in all panels). C, Three representative transition state ITD tuning curves (square,circle, and small triangle) recorded in the lateral half of the ICX on the 0 transect in owls with 20–60 d of prism experience. Best ITDs (large triangles) are shifted in the adaptive direction determined by the direction of the prismatic displacement: toward left ear leading ITD values in owls wearing R23° prisms and toward right ear leading ITD values in owls wearing L23° prisms. For all figures in this paper, the ITD axis has been adjusted so that the adaptive direction is toward theright side of the page. NML andLND, Normal and learned ranges of ITDs for transition state tuning curves, defined in Materials and Methods.D, Typical ITD tuning curves recorded after >60 d of prism experience.

Fig. 3.

Effects of AP-5 and CNQX on ITD tuning in the ICCls and ICX of normal adult owls. A, ITD tuning at a representative ICCls site during control and recovery periods (open circles) and during periods of AP-5 (filled circles; 25 nA) and CNQX (squares; 5 nA) iontophoresis. Error bars indicate SEM across multiple 10 repetition tuning curves collected during the different periods. Verticalline, Best ITD; boxes, ITD ranges used to quantify blockade on ipsilateral and contralateral ear leading flanks of the tuning curve.B, Symmetry of drug effects for all ICCls units.Circles, AP-5 (n = 10 units; all 25 nA); squares, CNQX (n = 11 units; 5–25 nA). The diagonal line denotes equal blockade of both flanks. C, Effects of AP-5 (filled circles; 40 nA) and CNQX (squares; 15 nA) at a representative site in the lateral half of the ICX. Vertical line, The predicted best ITD for the transect.D, Symmetry of drug effects for all ICX units.Open symbols, Lateral half of ICX;half-filled symbols, medial half;circles, AP-5 (n = 25 units; 10–40 nA); squares, CNQX (n = 18 units; 5–40 nA). The gray region indicates the normal range of blockade asymmetry produced by AP-5, calculated as the mean ± 2 SD of the difference between blockade of contralateral and ipsilateral ear leading flanks of the tuning curve for all ICX units.

Fig. 4.

Effects of AP-5 and CNQX on ITD tuning in the ICCls of prism-reared owls. A, Effects of AP-5 at a representative site. Open circles, Mean responses during control and recovery periods; large and small filled circles, mean responses during application of 10 and 25 nA AP-5, respectively; arrow, direction of ITD tuning modification observed in the ICX of the same owl; boxes, ITD ranges for quantification of blockade on the adaptive (Adapt) and nonadaptive (Non-) flanks of the tuning curve. B, Blockade of responses by AP-5 on nonadaptive versus adaptive flanks of the tuning curve for all ICCls sites tested. Each point corresponds to the application of one ejection current, of which there were more than one for some sites. Range of ejection currents, 2–40 nA. C, Effects of CNQX at the same site shown in A. Increasing square sizes, 20, 25, and 35 nA CNQX, respectively.D, Effects of CNQX at all sites tested. Range of ejection currents, 7–40 nA.

Fig. 5.

Effects of AP-5 on transition state ITD tuning in the ICX. A, Effect of AP-5 at a representative multiunit site. Vertical line, Normal best ITD for the transect. Best ITD under control conditions was shifted 13 μsec from transect normal. Open circles, Tuning during control and recovery periods; closed circles, tuning during AP-5 applications (increasing closed circle size, 10, 25, and 40 nA, respectively); arrow, direction of ITD tuning modification. B, PSTHs for responses measured at the site in A to stimuli of 0 μsec (Normal) and 30 μsec right ear leading (Learned) ITD. Asterisks denote blockade of learned responses. Vertical lines indicate the onset of the 50 msec noise bursts (horizontal gray bars).C, Effects of AP-5 for different ejection currents (increasing closed circle size: 10, 20, and 40 nA, respectively) at another multiunit site. The asymmetric effect of AP-5 was apparent at all levels of response blockade.

Fig. 6.

Quantification of the effect of AP-5 on transition state ITD tuning in the ICX. All units are from owls with 30–154 d of prism experience. A, Effect of AP-5 (32 nA) at a representative multiunit site. Vertical line, Normal best ITD for the transect; boxes, ITD ranges for quantification of normal (NML) and learned (LND) responses. Bottom panel, Percent control response remaining during AP-5 application for ITDs with control responses >15% of maximum; line, linear regression (R² = 0.95;p < 0.0001). B, Similar effect at a representative single-unit site (25 nA AP-5). Line, Linear regression (R² = 0.77;p < 0.02). At this site, the normal response range was defined as 0–10 μsec right ear leading ITD because significant control responses to 10 μsec left ear leading ITD were not present.C, Effect of AP-5 on normal and learned responses for all sites in the lateral half of the ICX with transition state ITD tuning (range of ejection currents, 6–40 nA). Thediagonal indicates equal blockade of normal and learned responses. Filled circles, Sites with significant asymmetric effects of AP-5 by the linear regression test;concentric circles, single units; gray region, normal range of asymmetry observed with AP-5 in the ICX of normal owls (from Fig. 3D). For sites at which multiple AP-5 current levels were tested (Fig.5C), results of the median current application were used for this analysis. Points corresponding to selected figure panels are indicated.

Fig. 7.

Correlation of response blockade by AP-5 with ITD in the adaptive direction, demonstrating a preferential blockade of learned responses. A, Relationship between response blockade by AP-5 (normalized to the mean blockade for normal responses at each site) and ITD relative to transect normal for all sites at which response blockade by AP-5 was significantly asymmetric. On they-axis, 0 indicates complete response blockade. On theleft are sites from owls wearing R23° prisms, for which learned responses are to ITD values that are more left ear leading than normal (i.e., the adaptive direction is left ear leading). On the right are sites from owls wearing L23° prisms, for which the adaptive direction is toward right ear leading ITDs. AP-5 sensitivity increases with ITD in the adaptive direction in both cases.B, Lack of correlation of the magnitude of response blockade by AP-5 with the magnitude of control responses for the same sites shown in A. Response magnitude is normalized to the maximum response observed within the control tuning curve of each site. C, Example of preferential blockade of learned responses by AP-5 (10 nA) when learned responses were at 0 μsec ITD.Horizontal arrow shows the adaptive direction.D, Example of the uniform effect of AP-5 (15 nA) at an unshifted site in the lateral half of the ICX in a prism-reared owl (93 d of prism experience). Arrow shows the adaptive direction. E, Effects of AP-5 (circles; 15–25 nA) and CNQX (squares; all 25 nA) on adaptive and nonadaptive flanks of unshifted tuning curves (n = 7 lateral ICX sites; all < 175 d of prism experience).Gray region, Normal range of blockade asymmetry observed with AP-5 in normal owls.

Fig. 8.

Effect of CNQX on transition state ITD tuning in the ICX. The units were from owls with 30–154 d of prism experience.A, Effect of increasing ejection currents of CNQX (increasing square size, 4, 5, and 10 nA, respectively) at a representative site. Vertical line, Normal best ITD for the transect. The effect of AP-5 (increasing closed circle size, 10, 25, and 40 nA, respectively) at this same site is shown below for comparison (same site shown in Fig. 5A).Bottom panel, Blockade produced by the median ejection currents of CNQX (squares) and AP-5 (circles and regression line) as a function of ITD. AP-5, R² = 0.705 andp = 0.0008; CNQX, no significant regression (p = 0.80). B, Another example. Vertical line, Normal ITD for the transect (R10 μsec). AP-5, R² = 0.946 andp < 0.0001; CNQX, no significant regression (p = 0.37). C, Response blockade produced by CNQX for normal and learned responses for all sites with transition state ITD tuning. Gray region, Normal range of blockade asymmetry observed with CNQX in normal owls, calculated as the mean ± 2 SD of the difference between the response blockade on the two flanks of the ITD tuning curve (from CNQX data in Fig. 3D); squares withX, sites showing significant AP-5 asymmetry;filled square, the single asymmetric effect of CNQX.

Fig. 9.

Effect of AP-5 on ITD tuning curves after >175 d of prism experience. A, Effect of AP-5 (closed circle, 25 nA) at a site in the lateral ICX with ITD tuning typical of that observed after >175 d of prism experience. The owl had 259 d of prism experience. B, Another example of the effect of AP-5 at a site in the lateral ICX in a different owl after 386 d of prism experience. C, Effect of AP-5 on normal and learned responses for all sites tested in owls with >175 d of prism experience. All sites had mean responses to the normal ITD range that were >20% of the maximum response. Gray region, Normal range of blockade asymmetry observed with AP-5 in normal owls (from Fig. 3D).

Fig. 10.

Effect of AP-5 applied in the ICX on transition state ITD tuning recorded in the optic tectum after long and short periods of prism experience. A, Effect of AP-5 at a site in a 336-d-old owl after 274 d of experience with L23° prisms. The visual receptive field was located at R1° azimuth, +2° elevation. Normal and Learned response peaks, inferred from the location of the visual receptive field, are indicated. The double-peaked shape is often observed for transition state ITD tuning curves in the tectum (Brainard and Knudsen, 1995;Feldman, 1997). Open circles, Control and recovery periods; filled circles, responses at the tectal site measured during periods of AP-5 iontophoresis (25 nA) at an ICX site on the 0 transect with matched ITD tuning. Error bars indicate SEM across multiple 20 repetition tuning curves. Note that AP-5 applied in the ICX produced equal blockade of normal and learned responses at this tectal site. B, Effect of AP-5 at a representative tectal site in a 100-d-old owl after 37 d of experience with L23° prisms. The visual receptive field was located at 0° azimuth, +3° elevation. AP-5, 15–25 nA. Data are from Feldman et al. (1996).

Fig. 11.

Asymmetry of blockade produced by AP-5 as a function of age and days of prism experience. The y-axis shows the percent of responses remaining during AP-5 application for normal responses minus that for learned responses; 100% indicates selective blockade of learned responses, and 0% indicates equal blockade of normal and learned responses. Gray region, Normal range of blockade asymmetry observed with AP-5 in the ICX of normal owls (Fig. 3); squares, effects of AP-5 applied in the ICX on transition state ITD tuning recorded in the tectum (Feldman et al., 1996); 10A, site shown in Figure10A; circles, effects of AP-5 observed by recording locally in the ICX; filled circles, sites with significantly asymmetric AP-5 effects;open circles, sites with no significant asymmetry.