Age-related hearing loss frequently results in a loss in the ability to discriminate speech signals, especially in noise. This is attributable, in part, to a loss in temporal resolving power and ability to adjust dynamic range. Circuits in the adult dorsal cochlear nucleus (DCN) have been shown to preserve signal in background noise. Fusiform cells, major DCN output neurons, receive focused glycinergic inputs from tonotopically aligned vertical cells that also project to the ventral cochlear nucleus. Glycine-mediated inhibition onto fusiform cells results in decreased tone-evoked activity as intensity is increased at frequencies adjacent to characteristic frequency (CF). DCN output is thus shaped by glycinergic inhibition, which can be readily assessed in recordings from fusiform cells. Previous DCN studies suggest an age-related loss of markers for glycinergic neurotransmission. The present study postulated that response properties of aged fusiform cells would show a loss of inhibition, resembling conditions observed with glycine receptor blockade. The functional impact of aging was examined by comparing response properties from units meeting fusiform-cell criteria in young and aged rats. Fusiform cells in aged animals displayed significantly higher maximum discharge rates to CF tones than those recorded from young-adult animals. Fusiform cells of aged rats displayed significantly fewer nonmonotonic CF rate-level functions and an age-related change in temporal response properties. These findings are consistent with an age-related loss of glycinergic input, likely from vertical cells, and with findings from other sensory aging studies suggesting a selective age-related decrement in inhibitory amino acid neurotransmitter function.

Loss of temporal processing is characteristic of age-related loss of speech understanding observed in the elderly. Inhibitory glycinergic circuits provide input onto dorsal cochlear nucleus (DCN) projection neurons which likely serve to modulate excitatory responses to time-varying complex acoustic signals. The present study sought to test the hypothesis that age-related loss of inhibition would compromise the ability of output neurons to encode sinusoidally amplitude modulated (SAM) tones. Extracellular recordings were obtained from young and aged FBN rat DCN putative fusiform cells. Stimuli were SAM tones at 3 modulation depths (100, 50, and 20%) at 30 dB HL with the carrier frequency set to the unit's characteristic frequency. Discharge rate and synchrony were calculated to describe SAM responses. There were significant age-related changes in the shape and peak vector strength [best modulation frequency (BMF)] of temporal modulation transfer functions (tMTFs), with no significant age-related changes in rate modulation transfer functions (rMTFs) at BMF. Young neurons exhibited band-pass tMTFs for most SAM conditions while aged fusiform cells exhibited significantly more low-pass or double-peaked tMTFs. There were significant differences in tMTFs between buildup, pauser-buildup, and wide-chopper temporal response types. Young and aged wide-choppers displayed significantly lower vector strength values than the other two temporal DCN response types. Age-related decreases in the number of pauser-buildup response types and increases in wide-chopper types with age reported previously, could account, in part, for the observed loss of temporal coding of the aged fusiform cell. Age-related changes in SAM coding were similar to changes observed with receptor blockade of glycinergic inhibition onto fusiform cells and consistent with previously observed age-related loss of endogenous glycine levels and changes in normal adult glycine receptor function. DCN changes in SAM coding could, in part, underpin temporal processing deficits observed in the elderly. KeywordsDorsal Cochlear Nucleus; Aging; Auditory; Glycine; Amplitude Modulation; Temporal Processing Presbycusis, age-related hearing loss, is a complex disorder that may result in a slow deterioration of peripheral auditory input to auditory regions of the brain (see Willott, 1991; Corresponding Author: Dr. Donald M. Caspary, Department of Pharmacology, Southern Illinois University School of Medicine, P.O. Box 19629, Springfield, IL 62794-9629. Phone: (217) 545-2195, Fax: (217) 545-0145, email: dcaspary@siumed.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal...

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For many grassland songbird species, pastures represent some of the best available breeding habitat in the Upper Midwest. Increasing interest in intensive rotational grazing (lRG) among midwestem livestock farmers may result in an expansion of pasture hectares in the region. We evaluated the effects of several cattle stocking densities on ground nest survival in rotationally graz.ed cool-season pastures in southwestern Wisconsin. Ground nests were simulated with clutches of 3 unwashed pheasant eggs. We tested 3 rotational grazing systems: a l-day dab-y rotation stocked at 60 bead ha-'; a 4-day beef rotation at 15 bead by?; and a traditional, non-intensive 7-day rotation at 8 bead ti*. Paddock size (1.2 ha) and nest density (15 nests paddock') were held constant. The simulated nests were observed 4 times day-' to document trampling patterns during the herds' diurnal graz*mg and nunination cycles. Trampling damaged a mean of 75% (* 3.1%) of the nests for all 3 treatments during 8 consecutive replications. While the 7-day treatment exhibited a pattern of greater nest trampling during cattle grazing periods than during rumination periods, this pattern was less evident in the 4day treatment and absent in the l-day treatment. Increasing vegetation beigbtdensity and percent vegetation cover were associated witb reduced nest trampling rates, but pasture forage production and removal were not associated with nest damage.

Ornithologists often use simulated nests consisting of game bird or domestic poultry eggs to study nest survival. Researchers investigating cattle trampling of ground nests have sometimes used clay targets instead of actual eggs to avoid the confounding effects of nest depredation. To determine whether livestock respond similarly to clay targets and egg nests, we compared inadvertent trampling and intentional disturbance of clay targets versus clutches of 3 pheasant eggs by Angus x Holstein heifers. Overall trampling levels for clay target-and egg-nests were shnllar (35 and 36%, respectively). Cattle noticed and responded to both types of nests. When noticed, simulated nests were kicked, s&fed, licked, or picked up in the mouth. Cattle disturbed an average of 25% of the clay targets and 8% of the egg nests during 4 trials. Our results suggest that cattle are as likely to hmdvertently trample egg nests as they are clay targets, but targets are more Likely tu attract attention and are therefore dllurbed more often tban egg nests. The greater likelihood of intentional disturbance of clay targets by cattle reduces the confidence of extrapolating the fate of this type of simulated nest to that of actual nests.