Cholecystokinin Dependent Long-Term Potentiation in Auditory Thalamocortical Pathway

聽覺丘腦皮層通路中膽囊收縮素依賴的長時程增強

Student thesis: Doctoral Thesis

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Award date30 May 2018

Abstract

Previous studies have demonstrated that thalamocortical synapses remain plastic in adult rodents. However, the underlying mechanism remains elusive. Cholecystokinin (CCK) is the most commonly found neuropeptide in the brain and is widely distributed throughout the central nervous system including the medial geniculate body (MGB) and the auditory cortex (ACx). Our recent findings suggest that CCK plays a crucial role in synaptic plasticity in rodents. In this study, we investigated whether the age-dependent plasticity in the adult auditory thalamocortical pathway is modulated by CCK.

Our first objective was to confirm the auditory thalamocortical plasticity in the young adult rodents. Theta burst stimulation (TBS) in the ventral MGB (MGv) was used to induce long-term potentiation (LTP) in the thalamocortical pathway. Robust LTP was achieved after TBS in MGv. Next, we investigated whether this thalamocortical LTP is also CCK dependent. Immunostaining results revealed that nearly all the thalamocortical projecting neurons in the MGv expressed CCK. Furthermore, TBS failed to induce the thalamocortical LTP in CCK-/- mice, and the CCK-B receptor antagonist, L365, 260 completely blocked the thalamocortical LTP in C57 mice. These results suggest that TBS-induced thalamocortical LTP is CCK dependent. To test the sufficiency of CCK positive MGv neurons activation in the thalamocortical LTP induction, Cre-dependent virus was injected into CCK-Cre mice to express hChR2 in CCK positive neurons of MGv, and high-frequency laser stimulation of these neurons in MGv was applied. This successfully induced LTP in the thalamocortical pathway.

To investigate the mechanism of CCK release in the auditory thalamocortical pathway, N-methyl-D-aspartate (NMDA) receptor, metabotropic glutamate receptor 1 and 5 (mGluR1/5) antagonists were utilized, respectively. The results suggest that TBS-induced thalamocortical LTP is not NMDA receptor but mGluR1/5 dependent. Immunostaining results also showed the existence of mGluR1/5 on the CCK positive thalamocortical presynaptic terminals. These results may suggest that CCK release in the auditory thalamocortical pathway is probably controlled by mGluR1/5.

We further hypothesized that the emergence of plasticity in the thalamocortical pathway of young adult rodents is regulated by CCK expression in the MGB, during development. To test this hypothesis, both LTP induction and CCK expression levels were examined at different postnatal days (P10, P14, P21, P28, and adult). We could not observe LTP in rats at P10 and P14 in vivo. Immunostaining results also demonstrated that CCK was barely detectable in MGB in the first two postnatal weeks. At the end of the third postnatal week, a weak CCK expression was detected in MGB, and minor LTP could be induced by TBS. Both CCK expression level and LTP continued to increase after P21 until adulthood. There was only a sparse CCK expression in MGB of old rats (>18 months), and the auditory thalamocortical pathway lost the plasticity. CCK-8S injection in the auditory cortex of the old rats could partially restore the plasticity of the auditory thalamocortical pathway. Another interesting finding was that the peak latency of the thalamocortical activation became shorter (31.55±0.75ms ~ 7.40±0.16ms) during the developmental stage (P10-P60), which is a sign of a maturation process of the thalamocortical synapses. This finding also supports the subplate neuron model of the thalamocortical development.

Since auditory cortical plasticity is associated with improved tone-frequency discrimination, the effect of CCK-induced plasticity on the frequency discrimination ability of behaving mice was examined by a modified gap startle reflex test. Prepulse inhibition level of modified gap startle reflex was measured as an indicator of the frequency discrimination ability of mice. The animals showed a significant increase in prepulse inhibition 1d after CCK-auditory cortical injection and passive tone exposure, compared to the control group. This result suggests that CCK could significantly enhance the frequency differentiation ability of mice. 

This study is consistent with previous findings that the auditory thalamocortical pathway develops plasticity during the young adulthood and shows for the first time how CCK regulates the thalamocortical plasticity at different ages. Further, these findings may be translated to develop a therapeutic intervention for treating hearing loss and tinnitus.

    Research areas

  • Cholecystokinin, Long-term potentiation, Thalamocortical