Effects of trigeminal ganglion stimulation on unit activity of ventral cochlear nucleus neurons

Multisensory integration in the dorsal cochlear nucleus: unit responses to acoustic and trigeminal ganglion stimulation

A necessary requirement for multisensory integration is the convergence of pathways from different senses. The dorsal cochlear nucleus (DCN) receives auditory input directly via the VIIIth nerve and somatosensory input indirectly from the Vth nerve via granule cells. Multisensory integration may occur in DCN cells that receive both trigeminal and auditory nerve input, such as the fusiform cell. We investigated trigeminal system influences on guinea pig DCN cells by stimulating the trigeminal ganglion while recording spontaneous and sound-driven activity from DCN neurons. A bipolar stimulating electrode was placed into the trigeminal ganglion of anesthetized guinea pigs using stereotaxic co-ordinates. Electrical stimuli were applied as bipolar pulses (100 micros per phase) with amplitudes ranging from 10 to 100 microA. Responses from DCN units were obtained using a 16-channel, four-shank electrode. Current pulses were presented alone or preceding 100- or 200-ms broadband noise (BBN) bursts. Thirty percent of DCN units showed either excitatory, inhibitory or excitatory-inhibitory responses to trigeminal ganglion stimulation. When paired with BBN stimulation, trigeminal stimulation suppressed or facilitated the firing rate in response to BBN in 78% of units, reflecting multisensory integration. Pulses preceding the acoustic stimuli by as much as 95 ms were able to alter responses to BBN. Bimodal suppression may play a role in attenuating body-generated sounds, such as vocalization or respiration, whereas bimodal enhancement may serve to direct attention in low signal-to-noise environments.

Use of time differences in normal hearing – cortical processing of promontorial stimuli

To test the hypothesis that ability to discriminate small duration differences is positively correlated with activity in the right temporal lobe, we used positron emission tomography in six normally hearing subjects, stimulated via the promontory in a procedure that mimics the auditory nerve stimulation with a cochlear implant. Stimulus consisted of electrical bursts, and tasks included gap detection and temporal difference limen (TDL). TDL is a measure of discriminatory processing of sound duration in cochlear implant candidates, demonstrated to predict outcome. Good speech perception after cochlear implantation is associated with activity in right temporal areas. Although perceived variably by the subjects, the stimulus itself activated bilateral secondary somatosensory cortex, suggesting differential stimulation of multiple sensory modalities. Only TDL raised blood flow in both posterior middle temporal gyri (MTG) and the right prefrontal cortex. As the right posterior MTG is known to be active during duration discrimination of different modalities and in the perception of words containing manipulated phonemes, we conclude that recruitment of this part of the right hemisphere is important to the comprehension of speech containing mostly temporal cues. The study shows that stimulus-induced activation reflects the goal of the task rather than the nature of the stimulus.

Projections from the spinal trigeminal nucleus to the cochlear nucleus in the rat

The integration of information across sensory modalities enables sound to be processed in the context of position, movement, and object identity. Inputs to the granule cell domain (GCD) of the cochlear nucleus have been shown to arise from somatosensory brain stem structures, but the nature of the projection from the spinal trigeminal nucleus is unknown. In the present study, we labeled spinal trigeminal neurons projecting to the cochlear nucleus using the retrograde tracer, Fast Blue, and mapped their distribution. In a second set of experiments, we injected the anterograde tracer biotinylated dextran amine into the spinal trigeminal nucleus and studied the resulting anterograde projections with light and electron microscopy. Spinal trigeminal neurons were distributed primarily in pars caudalis and interpolaris and provided inputs to the cochlear nucleus. Their axons gave rise to small (1-3 microm in diameter) en passant swellings and terminal boutons in the GCD and deep layers of the dorsal cochlear nucleus. Less frequently, larger (3-15 microm in diameter) lobulated endings known as mossy fibers were distributed within the GCD. Ventrally placed injections had an additional projection into the anteroventral cochlear nucleus, whereas dorsally placed injections had an additional projection into the posteroventral cochlear nucleus. All endings were filled with round synaptic vesicles and formed asymmetric specializations with postsynaptic targets, implying that they are excitatory in nature. The postsynaptic targets of these terminals included dendrites of granule cells. These projections provide a structural substrate for somatosensory information to influence auditory processing at the earliest level of the central auditory pathways.

Projections from the trigeminal nuclear complex to the cochlear nuclei: a retrograde and anterograde tracing study in the guinea pig

In addition to input from auditory centers, the cochlear nucleus (CN) receives inputs from nonauditory centers, including the trigeminal sensory complex. The detailed anatomy, however, and the functional implications of the nonauditory innervation of the auditory system are not fully understood. We demonstrated previously that the trigeminal ganglion projects to CN, with terminal labeling most dense in the marginal cell area and secondarily in the magnocellular area of the ventral cochlear nucleus (VCN). We continue this line of study by investigating the projection from the spinal trigeminal nucleus to CN in guinea pig. After injections of the retrograde tracers FluoroGold or biotinylated dextran amine (BDA) in VCN, labeled cells were found in the spinal trigeminal nuclei, most densely in the pars interpolaris and pars caudalis with ipsilateral dominance. The anterograde tracers Fluoro-Ruby or BDA were stereotaxically injected into the spinal trigeminal nucleus. Most labeled puncta were found in the marginal area of VCN and the fusiform cell layer of dorsal cochlear nucleus (DCN). A smaller number of labeled puncta was located in the molecular and deep layers of DCN and the magnocellular area of VCN. The trigeminal projection to CN may provide somatosensory information necessary for pursuing a sound source or for vocal production. These projections may have a role in the generation and modulation of tinnitus.

Habituation of the acoustic and the tactile startle responses in mice: two independent sensory processes

To test whether habituation is specific to the stimulus modality, the authors analyzed cross-habituation between the tactile startle response' (TSR) and the acoustic startle response (ASR). The acoustic artifacts of airpuffs used to elicit the TSR were reduced by using a silencer and were effectively masked by background noise of 90-100 dB sound-pressure level. ASR was elicited by 14-kHz tones. TSR and ASR habituated in DBA and BALB mice: both the TSR and ASR habituated to a greater extent in DBA mice than in BALB mice. In both strains, habituation of the TSR did not generalize to the ASR, and vice versa. From this, the authors concluded that habituation of startle is located in the sensory afferent branches of the pathway.

The neuroscience of tinnitus

Tinnitus is an auditory phantom sensation (ringing of the ears) experienced when no external sound is present. Most but not all cases are associated with hearing loss induced by noise exposure or aging. Neuroscience research has begun to reveal how tinnitus is generated by the brain when hearing loss occurs, and to suggest new avenues for management and prevention of tinnitus following hearing injuries. Downregulation of intracortical inhibition induced by damage to the cochlea or to auditory projection pathways highlights neural processes that underlie the sensation of phantom sound.

Mecanismos fisiopatológicos en la génesis y cronificación del acúfeno

Progress in neuroscience research has given birth to new theories for tinnitus generation. From a point of view where cochlear dysfunctions would be considered as the origin and maintenance mechanisms, it has been introduced the important role of compensation systems from the central auditory pathways. They could act as the most relevant factor for chronic persistent tinnitus after a peripheral aggression. Unmasking of silent synapses or sprouting of new ones activate cortical reorganization for frecuencial areas nearby the non-stimulated ones through brain plasticity. Connections to associative cortex and limbic-amigdala area using the non-classical auditory system explain the presence of hyperacusis, anxiety or depression, factors that increase the severity of tinnitus. Implementation of these physiopathological theories reinforces the tinnitus neurophysiological model. The development of an aversive response through the survival reflex and the participation of negative emotional response are the responsible for signal persistence and vegetative reactions from the autonomous nervous system. Implications of this knowledge for tinnitus treatment involve the central auditory system approach through the combination of medical counselling for reduction of the aversive reaction and sound therapy to diminish its perception.

Mechanisms of tinnitus generation

PURPOSE OF REVIEW

The current understanding of mechanisms of tinnitus generation is continuing to advance. This review is intended to outline new knowledge in the areas of neuroanatomy, physiology, psychophysics, and brain imaging that are revealing novel mechanisms of tinnitus development. Advances in these areas will open new avenues for effective treatment of tinnitus.

RECENT FINDINGS

Application of high-pulse train electrical stimulation to the cochlea may be effective in restoring the normal pattern of spontaneous activity from the periphery that is interpreted by the auditory brainstem as coding for silence. Clinical and laboratory evidence for a significant interaction between the somatosensory and auditory systems has important implications for understanding and treating tinnitus. Application of principles of neuroplasticity and novel imaging techniques has expanded our understanding of tinnitus through analogous approaches to phantom limb pain. Finally, a novel receptor type recently located in auditory neurovascular structures has opened a new field of study of inflammatory mechanisms contributing to tinnitus.

SUMMARY

Our understanding of the mechanisms that lead to a phantom auditory perception, and the associated debilitating consequences of this sensory experience, is continuing to improve. Tinnitus appears to be significantly affected in complex ways by somatosensory, limbic, and motor influences. Effective treatments will certainly emerge from these new areas of research.

Effects of trigeminal ganglion stimulation on the central auditory system

A projection from the trigeminal ganglion to the ventral cochlear nucleus (VCN) of the guinea pig was recently described. The synaptic terminals of this projection terminate in the granule and magnocellular regions of the VCN. Stimulation of this projection has been shown to result in activation of neurons of the ventral cochlear nucleus. We investigated the effect of electrically stimulating the trigeminal ganglion on the central auditory system activity using 2-deoxyglucose (2-DG) autoradiographic techniques. Electrical stimuli were applied to the left trigeminal ganglion as bipolar pulses, 100 micros per phase, at intervals of 200 ms and an amplitude of 100 microA. Negative control animals were not stimulated. A positive control animal was stimulated in the left ear using a 1 kHz tone burst with 200 ms duration and an amplitude of 80 dB SPL. 2-DG was administered by intramuscular injection. Following a 1 h incorporation period, animals were sacrificed, the brains rapidly harvested, and prepared for autoradiography using standard techniques. Autoradiographs were analyzed using computer-assisted video densitometry to determine film optical density in the central auditory regions of interest. The cerebellum was also sampled as a gray matter indifferent intra-brain control region. Results showed systematic and significant differences between 2-DG uptake in the cochlear nucleus and higher auditory centers between control and stimulated animals. Trigeminally stimulated animals showed significantly higher uptake than unstimulated animals in all auditory centers examined, especially ipsilateral to the stimulation site. The activation pattern differs qualitatively from that seen with sound stimulation in that mainly contralateral pathways are activated with sound stimulation. These results demonstrate that a projection from the predominantly somatosensory trigeminal ganglion can influence the activity of central auditory neurons in a manner distinct from acoustic stimulation, suggesting activation of non-classical auditory pathways.