Biomedical subjects

A G Paolini

Publications and source records attributed to A G Paolini.

9 recordsLinked to original sources

PubMed1999

Intracellular responses of onset chopper neurons in the ventral cochlear nucleus to tones: evidence for dual-component processing.

Intracellular responses of onset chopper neurons in the ventral cochlear nucleus to tones: evidence for dual-component processing. The ventral cochlear nucleus (VCN) contains a heterogeneous collection of cell types reflecting the multiple processing tasks undertaken by this nucleus. This in vivo study in the rat used intracellular recordings and dye filling to examine membrane potential changes and firing characteristics of onset chopper (OC) neurons to acoustic stimulation (50 ms pure tones, 5 ms r/f time). Stable impalements were made from 15 OC neurons, 7 identified as multipolar cells. Neurons responded to characteristic frequency (CF) tones with sustained depolarization below spike threshold. With increasing stimulus intensity, the depolarization during the initial 10 ms of the response became peaked, and with further increases in intensity the peak became narrower. Onset spikes were generated during this initial depolarization. Tones presented below CF resulted in a broadening of this initial depolarizing component with high stimulus intensities required to initiate onset spikes. This initial component was followed by a sustained depolarizing component lasting until stimulus cessation. The amplitude of the sustained depolarizing component was greatest when frequencies were presented at high intensities below CF resulting in increased action potential firing during this period when compared with comparable high intensities at CF. During the presentation of tones at or above the high-frequency edge of a cell's response area, hyperpolarization was evident during the sustained component. The presence of hyperpolarization and the differences seen in the level of sustained depolarization during CF and off CF tones suggests that changes in membrane responsiveness between the initial and sustained components may be attributed to polysynaptic inhibitory mechanisms. The dual-component processing resulting from convergent auditory nerve excitation and polysynaptic inhibition enables OC neurons to respond in a unique fashion to intensity and frequency features contained within an acoustic stimulus.

Acoustic Stimulation↗

PubMed1998

Intracellular responses of the rat anteroventral cochlear nucleus to intracochlear electrical stimulation.

The anteroventral cochlear nucleus (AVCN) is the first central processing site for acoustic information. The influence and extent of convergent auditory nerve input to AVCN neurons was investigated using brief (<0.2 ms) intracochlear electrical activation of spiral ganglion cells. In 40 neurons recorded in vivo, the major intracellular response to stimulation was an excitatory postsynaptic potential (EPSP) with short latency (approximately 1 ms) and fast rise time (<1 ms). Graduated EPSP amplitude increases were also seen with increasing stimulation strength resulting in spike generation. Hyperpolarization followed excitation in most neurons, its extent distinguished three response types: Type I showed no hyperpolarization; Type II and Type III displayed short (<10 ms) and long (>19 ms) duration hyperpolarization, respectively. Hyperpolarization was attributed to an inhibitory postsynaptic potential (IPSP) in addition to spike after hyperpolarization. Neurobiotin filling identified Type I and II neurons as stellate and Type III as bushy cells. These results suggests that AVCN neurons receive direct, possibly convergent, excitatory input from auditory nerves emanating from spiral ganglion cells with hyperpolarization resulting from polysynaptic inhibitory input.

Animals↗

PubMed1998

Muscimol suppression of the dorsal cochlear nucleus modifies frequency tuning in rats.

The cochlear nucleus is composed of three sub-nuclei: the dorsal (DCN), anteroventral (AVCN) and posteroventral cochlear nucleus (PVCN). Intrinsic connections from the DCN to the AVCN are inhibitory and organised tonotopically. In this investigation, this pathway and its possible role in frequency tuning was examined using in vivo extracellular recordings. Extracellular recordings were made from 191 units in the AVCN, 69 of which were recorded after suppression of DCN by application of the GABA agonist Muscimol (15 ng, 0.26 mM). Tuning curves were plotted and characteristic frequency (CF) and response threshold (measured in dB SPL) were determined for each unit. Units recorded post-Muscimol showed significantly broader tuning characteristics and lower thresholds. Primary-like and transient chopper neurons contributed to this decrease in threshold suggesting that they receive 'on' CF inhibitory drive from the DCN. Sustained chopper units did not show a significant decrease in response threshold after Muscimol; however, there was a tendency for broader tuning and a significant increase in CF tone evoked maximum discharge rate and chopping frequency suggesting that the DCN may play a role in regulating the temporal firing of these units in addition to providing lateral inhibition. These results suggest that the DCN to AVCN pathway may aid in fine tuning frequency information.

Acoustic Stimulation↗

PubMed1998

Muscimol suppression of the dorsal cochlear nucleus impairs frequency discrimination in rats.

The cochlear nucleus is composed of three sub-nuclei: the dorsal (DCN), anteroventral (AVCN) and posteroventral cochlear nucleus (PVCN). In addition to connections between these sub-nuclei, each nucleus receives frequency specific tonotopically organised input from the cochlea. Evidence suggests that connections from the DCN to the AVCN are inhibitory and organised tonotopically but the functional significance of this pathway has yet to be elucidated. The possible role of this pathway in frequency discrimination using a T-maze behavioural paradigm and DCN suppression was examined. Five rats were trained on a two choice frequency discrimination task. Once frequency difference limens for 10-30% performance above chance were determined, rats had cannulae implanted bilaterally over the DCN. After recovery rats were tested on the behavioural task with nothing, saline and the GABA agonist muscimol injected into the DCN via the cannulae. Muscimol alone significantly reduced the rats ability to perform the task. This performance decrease was attributed to an inability to discriminate high frequency and not low frequency tones suggesting that place and not temporal coding of sound was compromised by DCN suppression. These results are consistent with the hypothesis that inhibitory drive from the DCN to AVCN may be crucial for the fine tuning of frequency information.

Acoustic Stimulation↗

PubMed1997

Intracellular responses of the rat cochlear nucleus to sound and its role in temporal coding.

The anteroventral cochlear nucleus (AVCN), the first centre of the central auditory pathway, contains globular bushy cells, which are unique in their ability to produce fast excitatory post-synaptic potentials (EPSPs). Using in vivo intracellular recordings in the rat AVCN we examined these fast EPSPs in relation to temporal coding. At frequencies up to 2.5 kHz, EPSPs were evoked on successive sine waves of the stimulus with EPSP summation limited. This one-to-one relationship between the EPSPs and the sound wave period was present at higher frequencies and over a greater intensity range than for action potentials. These results suggest that temporal coding is possible in globular bushy neurones by their ability to extract temporal information through fast processing of convergent presynaptic input.

Acoustic Stimulation↗

PubMed1997

Effects of inactivation of the magnocellular preoptic nucleus of olfactory bulb processing.

The magnocellular nucleus (MCPO) was inactivated in anaesthetized rats, using muscimol, a gamma-amino butyric acid ergic agonist, in order to examine the effect of suppression of its tonic activity on extracellular unit firing in the granular (GRL), mitral (MCL) and external plexiform (EPL) layers of the olfactory bulb (OB). In GRL there was a significant increase in unit activity during the first hour after muscimol injection (30 ng), followed by a significant decrease in activity during the following hour. No effect on activity in MCL was seen after muscimol injection into the MCPO. Unit activity in EPL increased during the second hour post-injection. It was concluded that MCPO plays an important part in regulating the balance between granule and tufted cell activity.

Animals↗

PubMed1997

Intracellular recording of magnocellular preoptic neuron responses to olfactory brain.

The magnocellular preoptic nucleus of the rat supplies centrifugal input to the olfactory bulb as well as projecting to other olfactory-related areas. The extent to which the piriform and entorhinal cortices can influence the activity of magnocellular preoptic neurons and hence that of the olfactory bulb were examined using intracellular in vivo recording. Stable recordings were obtained in 58 neurons impaled in the magnocellular preoptic nucleus. Antidromic responses occurred on stimulating olfactory bulb (15), piriform cortex (14), or entorhinal area (eight). Monosynaptic excitation was evoked by piriform (27 of 37 tested) and entorhinal cortex (15 of 32 tested) stimulation with polysynaptic inhibition occurring in seven and five neurons, respectively. Polysynaptic as well as antidromic excitation by olfactory bulb stimulation occurred in four; a further 28 tested responded polysynaptically. No response to olfactory bulb stimulation was monosynaptic. In stable impalements, 29 neurons discharged spontaneously in the absence of applied current. Lucifer Yellow and Neurobiotin were used to label 16 cells. All but one had smooth dendrites with soma diameters ranging from 8 to 24 microm. These results provide a framework in which magnocellular preoptic neurons can influence olfactory processing by direct action on the olfactory bulb, which action can be boosted by positive feedback from the bulb through the olfactory piriform and entorhinal cortices.

Animals↗

PubMed1996

Lesions in the magnocellular preoptic nucleus decrease olfactory investigation in rats.

The nuclear complex of the horizontal limb of the diagonal band and the magnocellular preoptic nucleus, components of the basal forebrain magnocellular system affected in Alzheimer-type dementia, supply centrifugal innervation to the olfactory bulb. The lateral magnocellular preoptic nucleus provides significant GABAergic input. Since its stimulation may facilitate olfactory bulb mitral cells, we have investigated the effect of sub-total electrolytic lesions in this nucleus on performance in a simple test of olfactory investigation and its habituation. Two groups of rats used with lesions which occupied restricted volumes, approximately 30 and 15% of the magnocellular preoptic nucleus. Behaviorally, there was interference with olfactory investigation, with increased investigation latency and decreased investigation times, the group with larger lesions at 6 and 16 days after operation. There was no significant effect of the smaller lesions. No effects on patterns of olfactory habituation or discrimination were seen. The impairment of olfactory investigation could not be explained by interruption of medial forebrain bundle fibres traversing the nucleus. It is suggested that bilateral partial destruction of magnocellular preoptic neurones may produce significant deficits in either olfactory sensitivity or olfactory motivation.

Animals↗

PubMed1993

Effects of lesions in the horizontal diagonal band nucleus on olfactory habituation in the rat.

The nucleus of the horizontal limb of the diagonal band, a component of the basal forebrain magnocellular complex affected in Alzheimer type dementia, supplies centrifugal innervation to the olfactory bulb. We have tested the hypothesis that horizontal limb of the diagonal band lesions will interfere with olfactory memory in a simple olfactory test paradigm. Lesions occupied a restricted volume, approximately 20%, of medial horizontal limb of the diagonal band. There was interference with habituation of investigation latency and duration, six and 16 days after lesioning. It is concluded that bilateral partial lesions of the medial nucleus of the horizontal limb of the diagonal band interfere with habituation memory for odours.

Animals↗