Neural Pathways Involved In The Projections From The Spinal Trigeminal Nucleus And The Dorsal Column Nucleus To The Nucleus Of The Lateral Lemniscus Of The Rat

The somatosensory system is an important source of non-auditory inputs into various nuclei related to auditory.It coveys touch,temperature,pain from the body surface and proprioception from proprioceptive receptors of the body.The trigeminal sensory complex and the dorsal column nucleus are the sites of the secondary neurons receiving the somatosensory primary afferents from head,face,body and limbs,and regulating the acoustic activity through the efferent projections.Somatosensory and auditory signals are integrated by auditory neurons in different auditory nuclei.It has been established that somatosensory signals input to auditory neurons in the inferior colliculus(IC),auditory cortex(AC),superior olive complex(SOC) and cochlear nucleus(CN).As recently studies supported,tinnitus arises after the damage of the inner ear hair cells by noise exposure or application of drugs.These imbalance inputs are transferred from the cochlea to auditory nucleus,and then the hyperactivity of neurons were induced in the IC or other acoustic nuclei.Clinical evidence and animal experiments demonstrated that tinnitus can be suppressed by somatosensory electrical stimulation or acupuncture point stimulation. Therefore,it has become a focus on the relationship of the somatosensory system in modulating auditory activity,especially the mechanism of tinnitus suppressed by somatosensory stimulation.These illuminations of neurochemical pathways and physiological mechanisms have important academic and practical significance which to suppress tinnitus, and to further develop treatment for tinnitus.Several lines of evidence suggest that the increase in firing rate of neurons by reducing of GABA-release in IC induced tinnitus.The loudness of tinnitus could be markedly suppressed by microinjection of GABA receptor agonist into IC.In 2006,Jain and Shore found that electrical stimulation of the trigeminal nerve produced suppression of sound-evoked responses in IC.Thus,there is an assumption whether its somatosensory inputs increased GABA-release within IC by certain neural pathway,and the discharge of neurons will be inhibited in the IC.It may be one of the mechanisms underlying of alleviation for tinnitus by somatosensory stimulation.Up to now,numerous physiological and anatomical studies have been conducted on the integration between somatosensory and auditory in the level of CN and IC.However,it still remains unclear that what is the relationship between the somatosensory and auditory in the level of nucleus of the lateral lemniscus(NLL).Immunocytochemical studies have shown that three subnucleus of the lateral lemniscus contain GABAergic neurons,respectively.In addition,a GABAergic projection to the IC from the NLL has been confirmed,which is known to be a major source of inhibitory inputs to the IC.Previous studies have demonstrated that the effects of electrical stimulation within the DNLL can increase GABA-release in IC, and change the discharge characteristics of IC neuron.It is considered that the roles of inhibitory inputs were related to shaping binaural and monaural responses,and to pursuing sound source.It is unknown;however,whether there are neural pathways from Sp5,dorsal column nucleus to IC relayed by GABAergic neurons in NLL,by which conveyed the somatosensory signal to NLL and the GABAergic neurons projected to the IC were activated. The neural pathway may play an important role in effects of somatosensory signals on the discharge of IC neuron.Based on those,the following experiments were processed by using anterograde and retrograde neural tracing technique,combined with immunofluorescence in Sprague-Dawley rat.The direct projection from Sp5I and dorsal column nucleus to GABAergic neurons in NLL was studied in the following experiments.Those included three parts:(1) whether there were the direct projections from the Sp5I to the NLL;(2) whether there were the terminal endings from the Sp5I making contacts with GABAergic neurons in NLL;(3) whether there were direct projections from the dorsal column nucleus to the NLL.The results were as followed:1.The direct projection from the Sp5I to the NLLWith the respective injection of anterograde trace dextran-Texas Red into the Sp5I and retrograde tracer FG into DNLL,1NLL and VNLL,the distribution of dextran-Texas Red-labeled fibers and terminals were observed in the NLL,and the FG-labeled neurons were observed in the Sp5I.The following were the results:①The FG-labeled neurons were located in dorsal,ventral and ventrolateral of contralateral the Sp5I.②The fibers and terminals of the Sp5I-NLL projection were located in the DNLL,INLL and VNLL.③The anterogradely labeled fibers and terminals were observed in contralaterally external cortex of the inferior colliculus,superior colliculus,pontine reticular nucleus,lateral pontine nucleus,medial pontine nucleus and bilateral CN.2.Relationship between fibers projected from the Sp5I and GABAergic neurons in the NLLAfter anterograde tracer dextran-Texas Red was injected into the Sp5I,and then frozen sections of rat's brainstem were immunostained by fluorescence with glutamic acid decarboxylase(GAD) antibody to reveal the GABAergic neurons in the NLL.The overlap distribution of anterograde labeled fibers and terminals with GABAergic neurons were found in the contralaterally DNLL,INLL,VNLL and external cortex of the inferior colliculus.The direct contacts between the dextran-Texas Red-labeled Sp5I terminal endings and GABAergic neurons in the DNLL,INLL and VNLL were found under the laser scanning confocal microscope(LSCM).3.The direct projection from the dorsal column nucleus to the NLLFollowing dextran-Texas Red was injected into the dorsal column nucleus,the anterograde labeled fibers and terminals were seen in contralaterally DNLL,INLL and VNLL. Moderate terminals were observed in paralemniscal nucleus,lateral border region of the IC, inferior olive,intercollicular nucleus,lateral pontine nucleus and ventral pontine nucleus. After FG was injected into the DNLL,INLL and VNLL,the FG-retrograde labeled neurons were found in contralateral gracile nucleus and cuneate nucleus.After anterograde trace dextran-Texas Red was injected into the dorsal column nucleus,and the GABAergic neurons were visualized by immunofluorescence,the direct contacts between the anterograde terminals and GABAergic neurons were not observed by LSCM.The above results show that:(1) There were direct projections from the Sp5I and the dorsal column nucleus to the NLL;and the terminal endings from the Sp5I made contacts with GABAergic neurons within the DNLL,INLL and VNLL,the possible neural pathway was Sp5→NLL(GABAergic)→IC.(2) The dorsal column nucleus was found to project directly to the NLL;and there was not any contact between terminal from the Sp5I and GABAergic neuron in the NLL.To confirm this possible pathway and physiological function, further researches are still needed by electrophysiology and immunoelectron microscopy.The results of present study were not the same report in the world ground on the indagator of Hebei Science and Technology Information Institute recently.The results provided morphological foundation for investigating the integrated mechanism of somatosensory with auditory in auditory nuclei and modulation mechanism of suppressing tinnitus by somatosensory stimulation in future.