Cortical sensory gating and reactions to dynamic speech-in-noise in older normal-hearing and hearing-impaired adults

OBJECTIVE

To examine whether cortical sensory gating predicts how older adults with and without hearing loss perform the Tracking of Noise Tolerance (TNT) test.

DESIGN

Single-blind mixed design. TNT performance was defined by average tolerated noise relative to speech levels (TNTAve) and by an average range of noise levels over a two-minute trial (excursion). Sensory gating of P1-N1-P2 components was measured using pairs of 1 kHz tone pips.

STUDY SAMPLE

Twenty-three normal-hearing (NH) and 16 hearing-impaired (HI) older adults with a moderate-to-severe degree of sensorineural hearing loss.

RESULTS

NH listeners tolerated significantly more noise than HI listeners, but the two groups did not differ in their excursion. Both NH and HI listeners exhibited significant gating of P1 amplitudes and N1P2 peak-to-peak amplitudes with no difference in gating magnitudes between listener groups. Sensory gating magnitudes of P1 and N1P2 did not predict TNTAve scores, but N1P2 gating negatively predicted excursion after accounting for listener age and hearing thresholds.

CONCLUSIONS

Listeners' reactivity to a roving noise (excursion), but not their average noise tolerance (TNTAve), was predicted by sensory gating at N1P2 generators. These results suggest that temporal aspects of speech-in-noise processing may be affected by declines in the central inhibition of older adults.

Alpha-Band Dynamics of Hearing Aid Wearers Performing the Repeat-Recall Test (RRT)

This study measured electroencephalographic activity in the alpha band, often associated with task difficulty, to physiologically validate self-reported effort ratings from older hearing-impaired listeners performing the Repeat-Recall Test (RRT)-an integrative multipart assessment of speech-in-noise performance, context use, and auditory working memory. Following a single-blind within-subjects design, 16 older listeners (mean age = 71 years, SD = 13, 9 female) with a moderate-to-severe degree of bilateral sensorineural hearing loss performed the RRT while wearing hearing aids at four fixed signal-to-noise ratios (SNRs) of -5, 0, 5, and 10 dB. Performance and subjective ratings of listening effort were assessed for complementary versions of the RRT materials with high/low availability of semantic context. Listeners were also tested with a version of the RRT that omitted the memory (i.e., recall) component. As expected, results showed alpha power to decrease significantly with increasing SNR from 0 through 10 dB. When tested with high context sentences, alpha was significantly higher in conditions where listeners had to recall the sentence materials compared to conditions where the recall requirement was omitted. When tested with low context sentences, alpha power was relatively high irrespective of the memory component. Within-subjects, alpha power was related to listening effort ratings collected across the different RRT conditions. Overall, these results suggest that the multipart demands of the RRT modulate both neural and behavioral measures of listening effort in directions consistent with the expected/designed difficulty of the RRT conditions.

Characteristics of the quick repeat-recall test (Q-RRT)

OBJECTIVE

To determine the reliability and validity of the Quick Repeat-Recall Test (Q-RRT).

DESIGN

Within-subject repeated measures. Participants completed the Q-RRT at a speech level of 75 dB SPL in 2 noise configurations at signal-to-noise ratios of 5, 10, and 15 dB in a counterbalanced order, along with the full-RRT. The Q-RRT was repeated after 1-3 weeks to estimate within-session and between-session reliability. Participants also completed external validation measures relating speech-in-noise ability (Hearing-In-Noise Test, HINT), memory (Reading Span Test, RST; Forward Digit Span Test, DST-F), and noise acceptance (Tracking of Noise Tolerance, TNT).

STUDY SAMPLE

Twenty-five normal-hearing listeners (mean age = 63.9 years).

RESULTS

Group Repeat and Recall scores were similar between the full-RRT and the Q-RRT. However, Listening Effort and Tolerable Time ratings were significantly poorer for the full-RRT than the Q-RRT. The average repeat scores correlated with the HINT scores, while the recall scores correlated with the DST-F and RST scores. Tolerable Time ratings also correlated with the TNT scores. Within-session and between-session test-retest reliability using intraclass correlation coefficients were fair-to-excellent (0.41-0.91) depending on the outcome measures.

CONCLUSIONS

The Q-RRT is a reliable and valid measure of speech-in-noise ability, working memory, listening effort, and noise acceptance.

Measuring objective and subjective intelligibility using speech materials from the Tracking of Noise Tolerance (TNT) test

BACKGROUND

The results of tests measuring objective speech intelligibility are similar to those measuring subjective speech intelligibility using speech materials with minimal context. It is unclear if such is the case with contextual materials.

PURPOSE

To compare objective and subjective intelligibility difference (OSID) between normal-hearing (NH) and hearing-impaired (HI) listeners in the unaided and aided modes using speech materials adapted from the Tracking of Noise Tolerance (TNT) test.

RESEARCH DESIGN

Single-blind within-subjects design.

STUDY SAMPLE

Twenty-four NH and 17 HI older adults.

DATA COLLECTION AND ANALYSIS

Listeners completed the objective and subjective intelligibility measures at 75 dB SPL and 82 dB SPL speech input levels. Five signal-to-noise ratios (SNR) were tested to generate the objective and subjective speech intelligibility performance-intensity (P-I) functions. Both NH and HI listeners were tested in the unaided mode. The HI listeners were also tested using their own hearing aids (HAs). Objective and subjective speech-reception thresholds at a 50% criterion (SRT50s) were estimated from each individual P-I function. The difference between the objective SRT50 and subjective SRT50 was used to estimate OSID.

RESULTS

Objective and subjective SRT50s were significantly better in NH than in HI listeners (χ2(1) = 26.29, p < 0.001) at each speech input level in the unaided mode. However, there was a significant interaction between listener group and intelligibility type (χ2(1) = 9.43, p = 0.002) where SRT50s were lower for subjective than objective P-I functions only in the HI group. The SRT50s of HI listeners were also affected by hearing mode, where both objective and subjective intelligibility was improved when HI listeners were tested while wearing their own HAs. In general, objective and subjective SRT50s showed moderate-to-strong correlations across most combinations of listener groups and test conditions (r = 0.59-0.86, p < 0.01) except for HI listeners tested with their own HAs (r = 0.39, p = 0.128).

CONCLUSIONS

Similar objective and subjective intelligibility was observed in NH listeners but better subjective intelligibility than objective intelligibility was noted in HI listeners when tested in the unaided and aided modes.

Performance of older normal-hearing listeners on the tracking of noise tolerance (TNT) test

OBJECTIVE

To gather preliminary reference data on older normal-hearing (NH) adults for the refined Tracking of Noise Tolerance (TNT) test.

DESIGN

Within-subject repeated measures. Participants were tested on the TNT in the sound-field and under headphones. In the sound-field, speech stimuli were presented at 75 dB SPL and 82 dB SPL from 0° with a speech-shaped noise presented either from 0° or 180° at a level controlled by the participants. The order of signal level, mode of presentation, noise azimuth, and TNT passages were counterbalanced across listeners. Testing was repeated for one condition after 1-3 weeks to estimate within-session and between-session reliability.

STUDY SAMPLE

Twenty-five NH listeners (51-82 yrs of age).

RESULTS

Mean TNT scores (TNT_Ave) were about 4 dB at a speech input of 75 dB SPL and 3 dB at 82 dB SPL. The TNT_Ave was similar between the headphone and sound-field presentations in the co-located noise. TNT_Ave scores measured with noise-back were about 1 dB better than those measured from the front. The 95% confidence intervals of absolute test-retest differences were about 1.2 dB within-session and 2.0 dB between sessions.

CONCLUSIONS

The refined TNT may be a reliable tool to measure noise acceptance and subjective speech intelligibility.

Left Lateralization of the Cortical Auditory-Evoked Potential Reflects Aided Processing and Speech-in-Noise Performance of Older Listeners With a Hearing Loss

OBJECTIVES

We analyzed the lateralization of the cortical auditory-evoked potential recorded previously from aided hearing-impaired listeners as part of a study on noise-mitigating hearing aid technologies. Specifically, we asked whether the degree of leftward lateralization in the magnitudes and latencies of these components was reduced by noise and, conversely, enhanced/restored by hearing aid technology. We further explored if individual differences in lateralization could predict speech-in-noise abilities in listeners when tested in the aided mode.

DESIGN

The study followed a double-blind within-subjects design. Nineteen older adults (8 females; mean age = 73.6 years, range = 56 to 86 years) with moderate to severe hearing loss participated. The cortical auditory-evoked potential was measured over 400 presentations of a synthetic /da/ stimulus which was delivered binaurally in a simulated aided mode using shielded ear-insert transducers. Sequences of the /da/ syllable were presented from the front at 75 dB SPL-C with continuous speech-shaped noise presented from the back at signal-to-noise ratios of 0, 5, and 10 dB. Four hearing aid conditions were tested: (1) omnidirectional microphone (OM) with noise reduction (NR) disabled, (2) OM with NR enabled, (3) directional microphone (DM) with NR disabled, and (4) DM with NR enabled. Lateralization of the P1 component and N1P2 complex was quantified across electrodes spanning the mid-coronal plane. Subsequently, listener speech-in-noise performance was assessed using the Repeat-Recall Test at the same signal-to-noise ratios and hearing aid conditions used to measure cortical activity.

RESULTS

As expected, both the P1 component and the N1P2 complex were of greater magnitude in electrodes over the left compared to the right hemisphere. In addition, N1 and P2 peaks tended to occur earlier over the left hemisphere, although the effect was mediated by an interaction of signal-to-noise ratio and hearing aid technology. At a group level, degrees of lateralization for the P1 component and the N1P2 complex were enhanced in the DM relative to the OM mode. Moreover, linear mixed-effects models suggested that the degree of leftward lateralization in the N1P2 complex, but not the P1 component, accounted for a significant portion of variability in speech-in-noise performance that was not related to age, hearing loss, hearing aid processing, or signal-to-noise ratio.

CONCLUSIONS

A robust leftward lateralization of cortical potentials was observed in older listeners when tested in the aided mode. Moreover, the degree of lateralization was enhanced by hearing aid technologies that improve the signal-to-noise ratio for speech. Accounting for the effects of signal-to-noise ratio, hearing aid technology, semantic context, and audiometric thresholds, individual differences in left-lateralized speech-evoked cortical activity were found to predict listeners' speech-in-noise abilities. Quantifying cortical auditory-evoked potential component lateralization may then be useful for profiling listeners' likelihood of communication success following clinical amplification.

Measuring the effect of adaptive directionality and split processing on noise acceptance at multiple input levels

OBJECTIVE

This study used the multi-level Tracking of Noise Tolerance (TNT) test to compare Augmented Focus (AF) or split processing over non-AF processing with adaptive directionality (non-AF-dirm) and with an omnidirectional microphone (non-AF-omni).

DESIGN

This was a single-blind, within-subject repeated measures design.

STUDY SAMPLE

Nineteen listeners with a mild-to-moderate hearing loss. The listeners' task was to determine their tolerable noise level (TNL) in the sound-field at four fixed speech levels (i.e. 62, 68, 75, and 85 dB SPL) in the unaided condition and the different aided conditions. The speech passages were presented from 0° while a continuous speech-shaped noise was presented from 180°. Each condition was tested twice, each in a different counterbalanced order.

RESULTS

AF improved TNL by an average of 2.9 dB over non-AF-dirm. Adaptive directionality improved the aided TNL by 4.7 dB over the non-AF-omni condition. The unaided TNL was similar to the aided non-AF-omni TNL. Whereas a stable TNL was reached in 20-30 s for non-AF-dirm, it took AF < 15 s to reach a stable TNL at all input levels.

CONCLUSIONS

This study shows that AF allowed 2.9 dB of signal-to-noise ratio (SNR) improvement over that of non-AF-dirm and 7.6 dB over the aided non-AF-omni condition.

Use of Emotional and Neutral Speech in Evaluating Compression Speeds

BACKGROUND

 Emotional speech differs from neutral speech in its envelope characteristics. Use of emotional speech materials may be more sensitive for evaluating signal processing algorithms that affect the temporal envelope.

PURPOSE

 Subjective listener preference was compared between variable speed compression (VSC) and fast acting compression (FAC) amplitude compression algorithms using neutral and emotional speech.

RESEARCH DESIGN

 The study used a single-blinded, repeated measures design.

STUDY SAMPLE

 Twenty hearing-impaired (HI) listeners with a bilaterally symmetrical, mild- to-moderately severe sensorineural hearing loss and 21 listeners with normal hearing (NH) participated.

INTERVENTION

 Speech was processed using FAC and VSC algorithms.

DATA COLLECTION AND ANALYSIS

 A paired-comparison paradigm assessed subjective preference for FAC versus VSC using emotional and neutral speech materials. The significance of subjective preference for compression algorithm (FAC or VSC) was evaluated using a linear mixed effects model at each combination of stimulus type (emotional or neutral speech) and hearing group (NH or HI).

RESULTS

 HI listeners showed a preference for VSC over FAC when listening to emotional speech. The same listeners showed a nonsignificant, preference for VSC over FAC when listening to neutral speech. NH listeners showed preference for VSC over FAC for both neutral and emotional speech materials.

CONCLUSION

 These results suggest that the subjective sound quality of emotional speech is more susceptible than neutral speech to changes in the signal introduced by FAC. Clinicians should consider including emotional speech materials when evaluating listener preference for different compression speeds in the clinic.

Using the Repeat-Recall Test to Examine Factors Affecting Context Use

BACKGROUND

The effect of context on speech processing has been studied using different speech materials and response criteria. The Repeat-Recall Test (RRT) evaluates listener performance using high context (HC) and low context (LC) sentences; this may offer another platform for studying context use (CU).

OBJECTIVE

This article aims to evaluate if the RRT may be used to study how different signal-to-noise ratios (SNRs), hearing aid technologies (directional microphone and noise reduction), and listener working memory capacities (WMCs) interact to affect CU on the different measures of the RRT.

DESIGN

Double-blind, within-subject repeated measures design.

STUDY SAMPLE

Nineteen listeners with a mild-to-moderately severe hearing loss.

DATA COLLECTION

The RRT was administered with participants wearing the study hearing aids under two microphone (omnidirectional vs. directional) by two noise reduction (on vs. off) conditions. Speech was presented from 0 degree at 75 dB sound pressure level and a continuous speech-shaped noise from 180 degrees at SNRs of 0, 5, 10, and 15 dB. The order of SNR and hearing aid conditions was counterbalanced across listeners. Each test condition was completed twice in two 2-hour sessions separated by 1 month.

RESULTS

CU was calculated as the difference between HC and LC sentence scores for each outcome measure (i.e., repeat, recall, listening effort, and tolerable time). For all outcome measures, repeated measures analyses of variance revealed that CU was significantly affected by the SNR of the test conditions. For repeat, recall, and listening effort measures, these effects were qualified by significant two-way interactions between SNR and microphone mode. In addition, the WMC group significantly affected CU during recall and rating of listening effort, the latter of which was qualified by an interaction between the WMC group and SNR. Listener WMC affected CU on estimates of tolerable time as qualified by significant two-way interactions between SNR and microphone mode.

CONCLUSION

The study supports use of the RRT as a tool for measuring how listeners use sentence context to aid in speech processing. The degree to which context influenced scores on each outcome measure of the RRT was found to depend on complex interactions between the SNR of the listening environment, hearing aid features, and the WMC of the listeners.

Performance of normal-hearing listeners on the Repeat-Recall test in different noise configurations

OBJECTIVE

This study measured the performance of normal-hearing listeners on the Repeat-Recall Test (RRT) in two noise types (2-talker babble [2TBN] and continuous speech-shaped noise [SSN]) by two noise azimuths (0° and 180°) configurations at signal-to-noise ratios (SNRs) of 0, 5, 10, and 15 dB and quiet.

DESIGN

Within-subject repeated measures.

STUDY SAMPLE

Twenty-one listeners with normal hearing who also passed cognitive screening were tested in the sound-field with the speech stimulus presented from 0° at 75 dB SPL in 4 noise configurations. The order of SNRs, noise configurations, and RRT topic conditions was counterbalanced across listeners.

RESULTS

Analysis revealed that repeat scores were significantly better for 2TBN, for noise at 180°, and for high context (HC) sentences. Recall performance was significantly better for SSN and HC sentences. Listening effort ratings were higher for SSN and for noise front condition at SNR ≤ 10 dB. The 2TBN noise was tolerated longer than SSN. Performance on all measures improved with SNRs.

CONCLUSIONS

These data showed performance differences among noise configurations and provided a preliminary basis for comparison with hearing-impaired listeners' performance on the RRT.

An Integrative Evaluation of the Efficacy of a Directional Microphone and Noise-Reduction Algorithm under Realistic Signal-to-Noise Ratios

Background Many studies on the efficacy of directional microphones (DIRMs) and noise-reduction (NR) algorithms were not conducted under realistic signal-to-noise ratio (SNR) conditions. A Repeat-Recall Test (RRT) was developed previously to partially address this issue.

Purpose This study evaluated whether the RRT could provide a more comprehensive understanding of the efficacy of a DIRM and NR algorithm under realistic SNRs. Possible interaction with listener working memory capacity (WMC) was assessed.

Research Design This study uses a double-blind, within-subject repeated measures design.

Study Sample Nineteen listeners with a moderate degree of hearing loss participated.

Data Collection and Analysis The RRT was administered with participants wearing the study hearing aids (HAs) under two microphones (omnidirectional versus directional) by two NR (on versus off) conditions. Speech was presented from 0° at 75 dB SPL and a continuous noise from 180° at SNRs of 0, 5, 10, and 15 dB. The order of SNR and HA conditions was counterbalanced across listeners. Each test condition was completed twice in two 2-hour sessions separated by one month.

Results The recall scores of listeners were used to group listeners into good and poor WMC groups. Analysis using linear mixed-effects models revealed significant effects of context, SNR, and microphone for all four measures (repeat, recall, listening effort, and tolerable time). NR was only significant on the listening effort scale in the DIRM mode at an SNR of 5 dB. Listeners with good WMC performed better on all measures of the RRT and benefitted more from context. Although DIRM benefitted listeners with good and poor WMC, the benefits differed by context and SNR.

Conclusions The RRT confirmed the efficacy of DIRM and NR on several outcome measures under realistic SNRs. It also highlighted interactions between WMC and sentence context on feature efficacy.

An Integrative Evaluation of the Efficacy of a Directional Microphone and Noise Reduction Algorithm Under Realistic Signal-to-Noise Ratios

Background: Many studies on the efficacy of directional microphones (DIRMs) and noise reduction (NR)algorithms were not conducted under realistic signal-to-noise ratio (SNR) conditions. A Repeat–RecallTest (RRT) was developed previously to partially address this issue.<br />Purpose: This study evaluated whether the RRT could provide a more comprehensive understanding ofthe efficacy of a DIRM and NR algorithm under realistic SNRs. Possible interaction with listener workingmemory capacity (WMC) was assessed.<br />Research Design: This study uses a double-blind, within-subject repeated measures design.<br />Study Sample: Nineteen listeners with a moderate degree of hearing loss participated.<br />Data Collection and Analysis: The RRT was administered with participants wearing the study hearingaids (HAs) under two microphones (omnidirectional versus directional) by two NR (on versus off) conditions.Speech was presented from 0° at 75 dB SPL and a continuous noise from 180° at SNRs of 0, 5,10, and 15 dB. The order of SNR and HA conditions was counterbalanced across listeners. Each testcondition was completed twice in two 2-hour sessions separated by one month.<br />Results: The recall scores of listeners were used to group listeners into good and poor WMC groups.Analysis using linear mixed-effects models revealed significant effects of context, SNR, and microphonefor all four measures (repeat, recall, listening effort, and tolerable time). NR was only significant on thelistening effort scale in the DIRM mode at an SNR of 5 dB. Listeners with good WMC performed better onall measures of the RRT and benefitted more from context. Although DIRM benefitted listeners with goodand poor WMC, the benefits differed by context and SNR.<br />Conclusions: The RRT confirmed the efficacy of DIRM and NR on several outcome measures underrealistic SNRs. It also highlighted interactions between WMC and sentence context on feature efficacy.<br />

Tracking of Noise Tolerance to Predict Hearing Aid Satisfaction in Loud Noisy Environments

BACKGROUND

A method that tracked tolerable noise level (TNL) over time while maintaining subjective speech intelligibility was reported previously. Although this method was reliable and efficacious as a research tool, its clinical efficacy and predictive ability of real-life hearing aid satisfaction were not measured.

PURPOSE

The study evaluated an adaptive method to estimate TNL using slope and variance of tracked noise level as criteria in a clinical setting. The relationship between TNL and subjective hearing aid satisfaction in noisy environments was also investigated.

RESEARCH DESIGN

A single-blinded, repeated-measures design.

STUDY SAMPLE

Seventeen experienced hearing aid wearers with bilateral mild-to-moderately-severe sensorineural hearing loss.

DATA COLLECTION AND ANALYSIS

Participants listened to 82-dB SPL continuous speech and tracked the background noise level that they could "put up with" while subjectively understanding >90% of the speech material. Two trials with each babble noise and continuous speech-shaped noise were measured in a single session. All four trials were completed aided using the participants' own hearing aids. The stimuli were presented in the sound field with speech from 0° and noise from the 180° azimuth. The instantaneous tolerable noise level was measured using a custom program and scored in two ways; the averaged TNL (aTNL) over the 2-min trial and the estimated TNL (eTNL) as soon as the listeners reached a stable noise estimate. Correlation between TNL and proportion of satisfied noisy environments was examined using the MarkeTrak questionnaire.

RESULTS

All listeners completed the tracking of noise tolerance procedure within 2 min with good reliability. Sixty-five percent of the listeners yielded a stable noise estimate after 59.9 sec of actual test time. The eTNL for all trials was 78.6 dB SPL (standard deviation [SD] = 4.4 dB). The aTNL for all trials was 78.0 dB SPL (SD = 3.3 dB) after 120 sec. The aTNL was 79.2 dB SPL (SD = 5.4 dB) for babble noise and 77.0 dB SPL (SD = 5.9 dB) for speech-shaped noise. High within-session test-retest reliability was evident. The 95% confidence interval was 1.5 dB for babble noise and 2.8 dB for continuous speech-shaped noise. No significant correlation was measured between overall hearing aid satisfaction and the aTNL (ρ = 0.20 for both noises); however, a significant relationship between aTNL and proportion of satisfied noisy situations was evident (ρ = 0.48 for babble noise and ρ = 0.55 for speech-shaped noise).

CONCLUSION

The eTNL scoring method yielded similar results as the aTNL method although requiring only half the time for 65% of the listeners. This time efficiency, along with its reliability and the potential relationship between TNL and hearing aid satisfaction in noisy listening situations suggests that this procedure may be a good clinical tool to evaluate whether specific features on a hearing aid would improve noise tolerance and predict wearer satisfaction with the selected hearing aid in real-life loud noisy situations. A larger sample of hearing aid wearers is needed to further validate these potential uses.

Characterization of egg white antibacterial properties during the first half of incubation: A comparative study between embryonated and unfertilized eggs

Egg white is an important contributor to the protection of eggs against bacterial contaminations during the first half of incubation (day zero to 12), prior to the egg white transfer into the amniotic fluid to be orally absorbed by the embryo. This protective system relies on an arsenal of antimicrobial proteins and on intrinsic physicochemical properties that are generally unfavorable for bacterial multiplication and dissemination. Some changes in these parameters can be observed in egg white during egg storage and incubation. The aim of this work was to characterize changes in the antibacterial potential of egg white in embryonated eggs (FE) during the first half of incubation using unfertilized eggs (UF) as controls. Egg white samples were collected at day zero, 4, 8, and 12 and analyzed for pH, protein concentration, and protein profile. Antibacterial properties of egg white proteins were evaluated against Listeria monocytogenes, Streptococcus uberis, Staphylococcus aureus, Escherichia coli, and Salmonella Enteritidis. During incubation, differential variations of egg white pH and protein concentrations were observed between UF and FE. At equal protein concentrations, similar activities against L. monocytogenes and S. uberis were observed for FE and UF egg white proteins. A progressive decline in these activities, however, was observed over incubation time, regardless of the egg group (UF or FE). SDS-PAGE analysis of egg white proteins during incubation revealed discrete changes in the profile of major proteins, whereas the stability of some less abundant antimicrobial proteins seemed more affected. To conclude, the antibacterial activity of egg white proteins progressively decreased during the first half of egg incubation, possibly resulting from the alteration of specific antimicrobial proteins. This apparent decline may be partly counterbalanced in embryonated eggs by the increase in egg white protein concentration. The antibacterial potential of egg white is very effective during early stages of embryonic development but its alteration during incubation suggests that extra-embryonic structures could then progressively ensure protective functions.

The end of a myth: cloning and characterization of the ovine melatonin MT(2) receptor

BACKGROUND AND PURPOSE

For many years, it was suspected that sheep expressed only one melatonin receptor (closely resembling MT(1) from other mammal species). Here we report the cloning of another melatonin receptor, MT(2), from sheep.

EXPERIMENTAL APPROACH

Using a thermo-resistant reverse transcriptase and polymerase chain reaction primer set homologous to the bovine MT(2) mRNA sequence, we have cloned and characterized MT(2) receptors from sheep retina.

KEY RESULTS

The ovine MT(2) receptor presents 96%, 72% and 67% identity with cattle, human and rat respectively. This MT(2) receptor stably expressed in CHO-K1 cells showed high-affinity 2[(125)I]-iodomelatonin binding (K(D)= 0.04 nM). The rank order of inhibition of 2[(125)I]-iodomelatonin binding by melatonin, 4-phenyl-2-propionamidotetralin and luzindole was similar to that exhibited by MT(2) receptors of other species (melatonin > 4-phenyl-2-propionamidotetralin > luzindole). However, its pharmacological profile was closer to that of rat, rather than human MT(2) receptors. Functionally, the ovine MT(2) receptors were coupled to G(i) proteins leading to inhibition of adenylyl cyclase, as the other melatonin receptors. In sheep brain, MT(2) mRNA was expressed in pars tuberalis, choroid plexus and retina, and moderately in mammillary bodies. Real-time polymerase chain reaction showed that in sheep pars tuberalis, premammillary hypothalamus and mammillary bodies, the temporal pattern of expression of MT(1) and MT(2) mRNA was not parallel in the three tissues.

CONCLUSION AND IMPLICATIONS

Co-expression of MT(1) and MT(2) receptors in all analysed sheep brain tissues suggests that MT(2) receptors may participate in melatonin regulation of seasonal anovulatory activity in ewes by modulating MT(1) receptor action.