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R K Shepherd

Publications and source records attributed to R K Shepherd.

At least 55 records · Page 3Linked to original sources
PubMed1996

Adenosine-induced vasoconstriction in vivo. Role of the mast cell and A3 adenosine receptor.

Adenosine, a vasodilator metabolite, is often produced in tissues where the demand for oxygen exceeds the supply. We have recently demonstrated in isolated cannulated arterioles that adenosine and its metabolite, inosine, can also cause vasoconstriction by stimulation of mast cells. Secondary release of histamine and thromboxane is responsible for the inosine-induced constriction in vivo. In the present study, we explored the vasomotor effects of adenosine in vivo and investigated the role of the A3 adenosine receptor in mediating vasoconstriction. In vivo, local application of adenosine (10-6 to 10-4 mol/L) to arterioles consistently caused dose-dependent vasodilation. A fraction of arterioles, however, exhibited a biphasic response, with constriction following dilation. This, too, was dose dependent; 37% of arterioles constricted by 12.7 +/- 4.3% of the initial diameter in response to 10-4 mol/L adenosine. In the presence of 8-(p-sulfophenyl)theophylline (8-SPT), an antagonist of A1 and A2 adenosine receptors, dilation in response to the same dose of adenosine was reduced, and constriction was enhanced; 85% of the tested arterioles constricted by -44.3 +/- 6.0% of the initial diameter. The A3 adenosine receptor has been shown to facilitate mediator release from mast cells, and its role was also examined. N6-(3-Iodo-4-aminobenzyl)adenosine (I-ABA), an agonist of A1 and A3 adenosine receptors, produced dose-dependent vasoconstriction. 1,3-Dipropyl-8-(4-acrylate)phenylxanthine (BW-A1433), an antagonist of A1, A2, and A3 receptors, significantly reduced the vasoconstrictor response to adenosine, which was unmasked during treatment with 8-SPT. In addition, both adenosine and I-ABA stimulated mast cell uptake of ruthenium red, indicating degranulation. The I-ABA-induced constriction was abolished by combined histamine and thromboxane receptor antagonists. We conclude that adenosine can cause vasoconstriction in vivo, which is often masked by A2 receptor-mediated vasodilation. Mast cells are stimulated in the course of the response, and the A3 adenosine receptor is involved in mediating constriction.

Adenosine↗
PubMed1995

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates.

While recent studies have suggested that electrical stimulation of the auditory nerve at high stimulus rates (e.g., 1000 pulses/s) may lead to an improved detection of the fine temporal components in speech among cochlear implant patients, neurophysiological studies have indicated that such stimulation could place metabolic stress on the auditory nerve, which may lead to neural degeneration. To examine this issue we recorded the electrically evoked auditory brainstem response (EABR) of guinea pigs following acute bipolar intracochlear electrical stimulation using charge-balanced biphasic current pulses at stimulus rates varying from 100 to 1000 pulses/s and stimulus intensities ranging from 0.16 to 1.0 microC/phase. Charge density was held constant (approximately 75 microC cm-2 geom/phase) in those experiments. To monitor the recovery in excitability of the auditory nerve following this acute stimulation. EABR thresholds, wave I and III amplitudes and their latencies were determined for periods of up to 12 h following the acute stimulation. Higher stimulus rates and, to a lesser extent, higher intensities led to greater decrements in the post-stimulus EABR amplitude and prolonged the recovery period. While continuous stimulation at 100 pulses/s induced no decrement in the EABR, stimulation at 200 and 400 pulses/s produced an increasingly significant post-stimulus reduction of the EABR amplitude, which showed only partial recovery during the monitoring period. No EABR response could be evoked immediately following stimulation at 1000 pulses/s, using a probe intensity 16-19 dB below the stimulus intensity. However, partial EABR recovery was observed for wave III following stimulation at the lowest stimulus intensity (0.16 microC/phase). These stimulus-induced reductions in the EABR amplitude were also reflected in increased thresholds and latencies. Providing stimulus rate and intensity were held constant, stimulation at different charge densities (37.7, 75.5 and 150.7 microC cm-2 geom/phase) had no influence on the post-stimulus EABR recovery. Significantly, the introduction of a 50% duty cycle into the stimulus pulse train resulted in a more rapid and complete post-stimulus recovery of the EABR compared to continuous stimulation. These data suggest that stimulus rate is a major contributor to the observed reduction in excitability of the electrically stimulated auditory nerve. This reduction may be a result of an activity-induced depletion of neural energy resources required to maintain homeostasis. The present findings have implications for the design of safe speech-processing strategies for use in multichannel cochlear implants.

Animals↗
PubMed1995

Onset of ototoxicity in the cat is related to onset of auditory function.

Cats are altricial mammals; they are born deaf and undergo rapid maturation of the auditory periphery late in the first and throughout the 2nd week of life. Previous studies, using multiple aminoglycoside administration over several days or weeks, have indicated that there is a reduction in the degree of ototoxicity in young animals provided the drug is administered prior to the onset of auditory function. In order to provide a more precise relationship between the degree of ototoxicity and auditory development, we used a single administration of Kanamycin (KA) and the loop diuretic ethacrynic acid (EA), as the co-administration of these drugs is known to produce a rapid and profound hearing loss in adult animals. Thirty kittens were administered with KA and EA at ages that varied from 2 to 16 days after birth (DAB) using a fixed dose per kilogram body weight sufficient to profoundly deafen adult animals. All animals made an uneventful recovery from the procedure. At 26 DAB, tone-pip-evoked auditory brainstem responses (ABR) were recorded from each animal in order to establish the extent of the hearing loss. The degree of hearing loss was compared with normal ABR audiograms recorded from 6 age-matched control animals. All animals treated with KA/EA at 9 DAB or older had a profound hearing loss similar to adult animals. Animals treated between 2 and 8 DAB exhibited severe high-frequency hearing losses. The extent of the loss was correlated with age (r = 0.63) and body weight (r = 0.72) such that hearing loss tended to spread towards lower frequencies as age and/or weight increased. All animals exhibited bilaterally symmetrical hearing losses which remained relatively stable over monitoring periods of up to 6 months following the drug treatment. These findings imply that the onset of ototoxicity is related, at least in part, to the onset of auditory function in the kitten. The rapid onset of deafness following this procedure makes it a useful technique in the study of both ototoxicity and cochlear development.

Aging↗
PubMed1995

Use of Ruthenium Red staining to detect mast cell degranulation in vivo.

OBJECTIVE: To establish a method of detecting mast cell degranulation in tissues during in vivo microscopy. METHODS: Hamster tissues were prepared for intravital microscopy. Ruthenium red (RR) was superfused over the cheek pouch at concentrations of 0.0001-0.01% to determine the optimal concentration. Mast cells were stimulated with compound 48/80, as well as with vasoactive agents not known to be stimulatory to mast cells, following which, mast cell staining was observed. Mesenteries were stained with Toluidine Blue (TB) or RR and mast cell degranulation was assessed during treatment with compound 48/80, or control. RESULTS: During superfusion with varying concentrations of RR, a dose dependence for background staining of unstimulated cells was observed. A RR concentration of 0.001% was optimal for in vivo detection of mast cell degranulation. Mast cells exposed to 0.001% RR were stained following stimulation with compound 48/80 but not after treatment with KCl or acetylcholine. The latter agents are not known to stimulate mast cells. Thus, arteriolar vasomotor responses, per se, did not appear to play a role in mast cell RR uptake. Comparable results were obtained with RR versus TB in control or 48/80-treated mesenteries. CONCLUSIONS: This RR technique facilitates rapid detection of mast cell degranulation in vivo and provides an opportunity to assess both mast cell and microvascular function simultaneously.

Animals↗
PubMed1995

Electrophysiologic effects following acute intracochlear direct current stimulation of the guinea pig cochlea.

Auditory brain stem responses to both acoustic (auditory brain stem response [ABR]) and electrical (electrically evoked auditory brain stem response [EABR]) stimuli, as well as the frequency-specific compound action potential (CAP), were recorded before and periodically following continuous intracochlear DC stimulation (2, 7, and 12 microA) for 2 hours in normal-hearing guinea pigs, by means of a banded intracochlear electrode array. Click-evoked ABR, frequency-specific CAP, and the EABR input-output function remained generally unchanged following stimulation at 2 microA DC. However, following stimulation at 7 and 12 microA, a significant decrement of the amplitude of the click-evoked ABR, frequency-specific CAP, and electrophonic component of the EABR was observed, while there was an increase in the amplitude of the EABR, associated with direct electrical stimulation of the auditory nerve.

Acoustic Stimulation↗
PubMed1995

Cochlear pathology following reimplantation of a multichannel scala tympani electrode array in the macaque.

The histopathologic consequence of removing and reimplanting intracochlear electrode arrays on residual auditory nerve fibers is an important issue when evaluating the safety of cochlear prostheses. The authors have examined this issue by implanting multichannel intracochlear electrodes in macaque monkeys. Macaques were selected because of the similarity of the surgical technique used to insert electrodes into the cochlea compared to that in humans, in particular the ability to insert the arrays into the upper basal turn. Five macaques were bilaterally implanted with the Melbourne/Cochlear banded electrode array. Following a minimum implant period of 5 months, the electrode array on one side of each animal was removed and another immediately implanted. The animals were sacrificed a minimum of 5 months following the reinsertion procedure, and the cochleas prepared for histopathologic analysis. Long-term implantation of the electrode resulted in a relatively mild tissue response within the cochlea. Results also showed that inner and outer hair cell survival, although significantly reduced adjacent to the array, was normal in 8 of the 10 cochleas apicalward. Moreover, the electrode reinsertion procedure did not appear to adversely affect this apical hair cell population. Significant new bone formation was frequently observed in both control and reimplanted cochleas close to the electrode fenestration site and was associated with trauma to the endosteum and/or the introduction of bone chips into the cochlea at the time of surgery. Electrode insertion trauma, involving the osseous spiral lamina or basilar membrane, was more commonly observed in reimplanted cochleas. This damage was usually restricted to the lower basal turn and resulted in a more extensive ganglion cell loss. Finally, in a number of cochleas part of the electrode array was located within the scala media or scala vestibuli. These electrodes did not appear to evoke a more extensive tissue response or result in more extensive neural degeneration compared with electrodes located within the scala tympani. In conclusion, the present study has shown that the reimplantation of a multichannel scala, tympani electrode array can be achieved with minimal damage to the majority of cochlear structures. Increased insertion trauma, resulting in new bone formation and spiral ganglion cell loss, can occur in the lower basal turn in cases where the electrode entry point is difficult to identify due to proliferation of granulation and fibrous tissue.

Animals↗
PubMed1994

Hearing, vocalization and the external ear of a marsupial, the northern Quoll, Dasyurus hallucatus.

As part of a continuing study of the development of the marsupial auditory system, auditory brainstem responses (ABR) were recorded and an ABR audiogram was constructed for five female Northern Quolls (Dasyurus hallucatus), which are nocturnal carnivores. The best frequency for hearing lies between 8 and 10 kHz, and at 50 dB SPL there is a range from about 0.5 to 40 kHz. Vocalizations of adult quolls and pouch-young were recorded with a digital audio tape recorder, and the power spectra of representative calls were compared with the ABR audiogram. The common adult vocalizations have most energy at the lower end of the hearing range, whereas frequencies that are dominant in the isolation calls of the pouch-young lie close to the best frequency of hearing. Samples of nocturnal sounds of the habitat of the quoll were also recorded and analyzed. Power spectra have peak energy at frequencies between 2 and 5 kHz, with a smaller contribution above 10 kHz. The spectrum contains relatively little power at the best frequency of hearing. Measurements of the sound pressure level at the external ear canal as a function of stimulus frequency and location in space suggest that the directional amplifying properties of the pinna will operate most effectively on sound frequencies at the upper end of the quoll's hearing range, a region that may be important in prey detection. Comparisons are made with other mammalian nocturnal carnivores and with other marsupials. We speculate that, for nocturnal carnivores, one role of the low-frequency part of the hearing range concerns the recognition of adult conspecifics, the mid-frequency range is important for the detection of pouch-young, and the upper range may be particularly concerned with prey/predator detection.

Animals↗
PubMed1994

Cochlear pathology following chronic electrical stimulation of the auditory nerve: II. Deafened kittens.

The present study examines the effects of long-term electrical stimulation of the auditory nerve on cochlear histopathology and spiral ganglion cell survival in young sensorineural deafened cats. Eight kittens were deafened using kanamycin and ethacrynic acid, and implanted with bipolar or monopolar scala tympani electrodes. Following recovery from surgery the animals were unilaterally stimulated using charge balanced biphasic current pulses for 450-1730 hours over implant periods of up to four months. Charge densities varied from 0.6-0.9 microC.cm-2 geom. per phase for monopolar electrodes to 12-26 microC.cm-2 geom. per phase for the bipolar electrodes. Electrically-evoked auditory brainstem responses (EABRs) were periodically monitored during stimulation to confirm that the stimulus levels were above threshold, and to monitor any change in the response of the auditory nerve. Following completion of the stimulation program cochleae were prepared for histological examination. EABRs exhibited relatively stable thresholds for both stimulated and implanted, unstimulated control cochleae for the stimulus duration. While the growth in response amplitude as a function of stimulus current remained stable for the bipolar control and monopolar stimulated cochleae, the five cochleae chronically stimulated using bipolar electrodes exhibited a moderate to large increase in response amplitude. These increases were associated with a more widespread fibrous tissue response which may have altered the current distribution within these cochleae. Implanted control cochleae exhibited significantly less tissue response within the scala tympani. Importantly, we observed no statistically significant difference in the spiral ganglion cell density associated with chronic electrical stimulation when compared with unstimulated control cochleae. While the present study supports the safe application of cochlear implants in young profoundly deafened children, it does not corroborate previous studies that have reported electrical stimulation providing a trophic effect on degenerating auditory nerve fibres.

Analysis of Variance↗
PubMed1994

Partial hearing loss in the macaque following the co-administration of kanamycin and ethacrynic acid.

Co-administration of kanamycin (KA) with the loop diuretic ethacrynic acid (EA) rapidly produces a profound hearing loss in the cat while maintaining normal renal function [Xu et al., Hear. Res. 70, 205-215 (1993)]. In the present paper we have applied this deafening procedure to the old world monkey Macaca fascicularis (macaque). Following the co-administration of KA and EA, the hearing loss in the macaque developed far slower than we observed in the cat. Moreover, unlike the cat, there was evidence of a partial recovery in the animal's hearing, resulting in a bilaterally symmetrical high frequency hearing loss. The extent of this hearing loss was dependent on the dose of the EA administered. Finally, the most unexpected result of the present study was the degree of acute nephrotoxicity experienced by these animals following the drug administration. The sensitivity of this species to renal failure restricted the dose of EA that could be safely administered. In conclusion, the co-administration of KA and EA cannot reliably produce a profound hearing loss in the macaque. While it can produce a dose dependent high frequency hearing loss the animal will also experience acute renal failure that requires careful management.

Animals↗
PubMed1994

Cochlear implantation in young children: histological studies on head growth, leadwire design, and electrode fixation in the monkey model.

For safe cochlear implantation in children under 2 years of age, the implant assembly must not adversely affect adjacent tissues or compromise head growth. Furthermore, growth changes and tissue responses should not impair the function of the device. Dummy receiver-stimulators, interconnect plugs, and leadwire-lengthening systems were implanted for periods of 36 months in the young monkey to effectively model the implantation of the young child. The results show that implanting a receiver-stimulator package has no adverse effects on skull growth or the underlying central nervous system. The system for fixing the electrode at the fossa incudis proved effective. There was marked osteoneogenesis in the mastoid cavity, resulting in the fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young subjects.

Animals↗
PubMed1994

Cochlear implantation in children: labyrinthitis following pneumococcal otitis media in unimplanted and implanted cat cochleas.

Pneumococcal otitis media is frequent in young children and could lead to labyrinthitis post-implantation. To assess the risk, and methods of minimizing it by a graft to the round window around the electrode entry point, we have used a cat animal model of pneumococcal otitis media. Twenty-one kittens were used in the study. Thirty-two cochleas were implanted when the kittens were 2 months of age. Fourteen cochleas were implanted without using a graft (12 were available for study); 9 had a fascial graft, and 9 a Gelfoam graft (7 were available for study). The implanted kittens had their bullae inoculated with Streptococcus pneumoniae 2 months after implantation and were sacrificed 1 week later. There were also 9 unimplanted control ears which were inoculated when the animals were 4 months of age. Labyrinthitis occurred in 44% of unimplanted control, 50% of implanted ungrafted, and 6% of implanted grafted (fascia and Gelfoam) cochleas. There was no statistically significant difference between the unimplanted control and the implanted cochleas (p < 0.05). There was, however, a difference between the implanted-ungrafted and implanted grafted cochleas, but not between the use of fascia and Gelfoam to graft the round window entry point. As a result, the data indicates that cochlear implantation does not increase the risk of labyrinthitis following pneumococcal otitis media, but it is desirable to use fascia as a graft to the round window around the electrode entry point.

Animals↗
PubMed1993

Pediatric cochlear implantation. Radiologic observations of skull growth.

We investigated the effects of long-term implantation of auditory prostheses on skull growth in young animals. Four monkeys were implanted with dummy cochlear implants at 6 months of age. To simulate implantation in children, the bed for the receiver-stimulator or interconnecting plug was drilled across a calvarial suture down to the underlying dura. Plain skull roentgenograms were periodically taken to monitor head growth for up to 3 years after implantation. These longitudinal measurements revealed no significant asymmetric skull growth. Postmortem measurements using computed tomographic scans confirmed these results and showed no significant difference in the intracranial volumes between the implanted and control sides of each animal or between experimental and nonimplanted control monkeys. These results suggest that long-term cochlear implantation in very young children will not cause any significant deformity of the skull.

Animals↗
PubMed1993

Effects on reproduction of estrous cycle variations, rectal temperatures and liveweights in mated Brahman cross heifers.

Estrous cycle variations and the association of rectal temperature with reproductive measurements and liveweight were examined in 25-month-old 1/2 and 3/4 Brahman heifers (n = 88). The mean cycle length was longer in the 1/2 Brahmari (24.3 days) than in the 3 4 Brahman heifers (21.3 days) due to the length of estrus-metestrus, but the overall difference was not statistically significant. Cycle length was not influenced by cycle number or liveweight. Cycles were classified into 6 types: normal, short, long, anovulatory and those involving embryonic mortality and prolonged diestrus. Only 33.6% of 1/2 Brahman cycles and 36.1% of 3/4 Brahman cycles were of normal duration (18 to 24 days), and 13.3% of 1/2 Brahman and 11.6% of 3/4 Brahman cycles were classified as embryonic mortality cycles. On an individual animal basis, 25.0% and 31.8% of 3 4 Brahman heifers, respectively, had cycles in which embryonic mortality was suspected. Heifers that became pregnant were significantly (P < 0.01) heavier throughout mating and had significantly (P < 0.05) lower mean rectal temperatures. Heifers in which embryonic mortality had occurred were lighter and had significantly (P < 0.01) higher rectal temperatures than heifers in which embryonic mortality had not occurred. Correlations between rectal temperature and ambient temperature were nonsignificant after eliminating the effect of genotype, but rectal temperature was significantly (P < 0.01) negatively correlated with liveweight.

Journal Article↗