Cerebral lateralization has been a widely observed phenomenon from fishes (Watkins et al 2004) to mammals (Hosaka et al 2015) though it had been thought to be a unique trait to human at first. The most perceptible traits in human cerebral lateralization including different anatomical brain structure, dominant hemisphere for distinct functions such as language and facial recognition (Keller et al 2011, Okazaki et al 2014), and preferential use of hands (Riolo-Quinn 1991, Moura 2015).
Increasing studies indicated the functional lateralization by lesion on unilateral hemisphere, which caused impairment on different aspects including learning and memory (Floel et al 2004, Glikmann-Johnston et al 2008). Given the previous studies are exclusively focused on human patients and mammalian models, with the advantage of easy maintain and molecular manipulation, it is imperative to study the possible cerebral lateralization phenomena in zebrafish models.
In the present study, we used electrophysiological approaches and zebrafish model to characterize the synaptic plasticity of contralateral dorsal lateral and dorsal medial pathway within dorsal telencephalon. It is a nice extension of our previous results showed that after given electrical stimulation in the Dl division can evoked a field potential in the ipsilateral Dm division. In addition, both LTP and LTD were induced by applying HFS and LFS, respectively. Our previous results also demonstrated that a glutamatergic neurotransmission is involved in both
LTP and LTD formation. Co-administration of NMDA receptor agonist AP5 blocked the formation of HFS induced LTP in Dm division. Either LFS or suprafusion of metabotropic glutamate receptor agonist DHPG induced LTD in the Dm division (Ng et al 2012).
In the present study, we expand our research interest on the bilateral neuroplasticity in the Dm region. By giving HFS in either left or right Dl, we induced stable, robust LTP in both Dm division of ipsilateral and contralateral side following stimulation from either left or right Dl.
Similar results of LTP induction in contralateral hippocampus of other species were observed including mouse (El-Gaby et al 2015) and guinea pigs (Chirwa et al 2001). Which implies a highly evolutional conservation on the neural mechanism of neuroplasticity is existed among different species. Many of the studies regarding neuronal transduction between two hemispheres use in vivo experiments due to the limitation of brain slice preparation (Shipton et al 2014, El-Gaby et al 2015). The complex structure and neuronal circuit of mammalian brain might be more vulnerable during the process of brain slice preparation due to larger size of brain.
Several studies indicated the LTP induction is NMDA-dependent in CA1-CA3 connection of hippocampus (Kanterewicz et al 2000, Otmakhov et al 2004) and zebrafish telencephalon (Nam et al 2004, Ng et al 2012), which is consistent with our data that application of NMDA
receptor antagonist, D-AP5 blocked the induction of LTP, while upon washout of D-AP5, LTP could be restored. The similar results are also discovered in LTP induction from Dl division to contralateral Dm division of the telencephalon in zebrafish. Which suggest zebrafish can be used as an alternative model for studying the NMDA dependent neuroplasticity.
It had been proven that activation of NMDA receptors triggered long-term potentiation in hippocampal slices of both ipsilateral and contralateral side and is related to the cellular mechanism responsible for synaptic plasticity and resulted in the formation of learning and memory (Beck et al 2000, Knafo et al 2012). As reported in previous studies, by applying specific antagonist of NMDA receptors has demonstrated to cause impairment in spatial learning (Morris 1989) and memory retention (Kim et al 1991) in rodents. In teleost fish, NMDA receptor plays an important role in avoidance learning (Xu et al 2003) and spatial learning (Gomez et al 2006) as well. Interestingly, several studies indicated the allocation of NMDA receptor subunits is asymmetrical in hippocampal CA1-CA3 circuitry, which may produce unequal numbers of NMDA receptor and therefore resulting in distinct ability to express synaptic plasticity (Kawakami et al 2003, Shinohara & Hirase 2009). In present studies, we divided the position of stimulus into left and right hemisphere and discovered that PS amplitude of Dm division in both ipsilateral and contralateral side exhibited stimulus from the left caused greater PS
amplitude than that from the right in early stage LTP. While applying a stimulus from the right, PS amplitude of Dm of contralateral side will be larger than Dm of ipsilateral side. Both results indicated that telencephalon of the left hemisphere is dominant than that of right hemisphere in the synaptic plasticity. We speculate the similar asymmetric distribution of NMDA receptors may also occur in the Dl-Dm circuitry of zebrafish telencephalon and therefore caused different ability to express synaptic plasticity in left and right hemisphere of telencephalon. Therefore, we conducted real-time PCR to verify the distribution of NMDAR subtype including NMDAR1a and NMDAR1b.
Both subtypes were reported expressed mainly in brains and nervous system (Cox et al 2005, Tzeng et al 2007). Our results suggested that the NMDAR1a mRNA expression of left hemisphere showed the tendency of higher expression than right hemisphere, while NMDAR1b expressed no difference or tendency. The results were undetermined possibly due to insufficient sample number or the dilution of exact NMDA mRNA caused by numerous telencephalons mixed as one sample. Further experiments such as immunohistochemistry and comparison of NMDA-dependent LTP on D-AP5 threshold dose are worth for determining this possibility.
Besides bilateral LTP formation, we demonstrated that application of mGluR agonist, DHPG could also triggered a stable, robust LTD in both Dm division of ipsilateral and contralateral side, which was
consistent with previous results of DHPG-induced LTD in unilateral hippocampal CA1 region (Palmer et al 1997, Fitzjohn et al 1999, Schnabel et al 1999). We also discovered that the PS amplitude of DHPG-induced LTD of contralateral telencephalon was smaller than that of ipsilateral telencephalon in early phase. However, Studies related to DHPG-induced LTD are scarce and the mechanism underlying LTD asymmetry remains unclear. Future studies related to application of mGluR antagonist or separation of stimulus into different hemispheres would help to clarify the mechanism of LTD induction in different hemispheres. It is well known that the mGluR regulates activity of neuronal ion channels, such as AMPA receptor. A previous study indicated that DHPG-induced LTD was related to the internalization of AMPA receptors, causing the removal of AMPA receptors from the membrane (Xiao et al 2001). Hence, from our results we speculated the molecular mechanism that regulates LTD formation is similar between adult zebrafish and mammals. Further experiments such as comparison of the AMPA receptor expression among the bilateral Dm division will be helpful for testing our hypothesis.
Another interesting result from our results is the difference on the latency of initial positive deflection between Dm field potential of contralateral and ipsilateral side, the latency of initial positive deflection of contralateral side lasted longer than that of ipsilateral side, which might be caused by the different distance of stimuli through biological
tissue towards recording cathode between contralateral and ipsilateral side. According to our lab’s previous studies, the field potential is composed of non-synaptic (P1) and synaptic (N2) components according to the variance of potential amplitude in intensity-response curve (IO-curve), paired pulse facilitation and application of tetrodotoxin (TTX), a strong inhibitor of voltage gated sodium channels (Ng et al 2012). P1 remained the same in paired pulse facilitation and variant in IO-curve. These results correspond to the observation that a positive deflection fiber volley (FV) waveform was evoked in CA3 division following a stimulation in hippocampal mossy fibers (Henze et al 1997) and so P1 component were identified as fiber volley. In present studies, the field potential recorded in Dm of contralateral and ipsilateral side both exhibited the same phenomenon, which the amplitude of P1 of both sides increased following the increase stimulus intensity in IO-curve while the amplitude of P1 of both sides maintained the same in paired pulse stimulation, suggesting the same results with Ng et al. that P1 component is non-synaptic and N2 is synaptic. An elaboration of this point can be studied by applying Ca2+ and Mg2+ solution to assess ionic channel underlying field potential transduction.
A recent study indicated that bilateral hippocampal structure is functionally unequal. Left hippocampus plays a dominant role in spatial short-term and long-term memory, whereas asymmetric synaptic plasticity was discovered in mouse hippocampus as well (Shipton et al
2014). Also, from our lab’s previous studies, unilateral side of telencephalic ablation in zebrafish impaired spatial short-term memory and moreover indicating left side telencephalon is dominant in learning and identifying new objects while right side telencephalon is dominant in long-term memory acquisition, retention and retrieval (Wu 2008). The results provided an evidence of functional lateralization and were correspondent with those of our electrophysiological studies, which implies left hemisphere telencephalon is dominant in learning than left hemisphere.
With the progressive medical imaging equipment, increasing numbers of human anatomical and functional evidences involving lesion on unilateral hippocampi caused impairment on spatial memory have shown functional asymmetry between the two hemispheres (Abrahams et al 1997, Glikmann-Johnston et al 2008, Duecker et al 2013). Evidences from cellular mechanism to behavioral aspect are implying the notion of functional lateralization of the hippocampus.
In conclusion, our results demonstrated that stimulus in Dl division can evoked extracellular potential in Dm division of both ipsilateral and contralateral side. Dm division of bilateral hemispheres exhibits paired pulse facilitation, long-term potentiation and long-term depression. Most importantly, we discovered the different ability to express long-term potentiation between the two hemispheres of zebrafish telencephalon.
Our findings present new electrophysiological evidence in cerebral lateralization and facilitate the use of zebrafish as a model for electrophysiological researches.