The role of serotonin in synaptic plasticity: detailed characterisation of the effect of serotonin on long-term potentiation across the hippocampus

Serotonin (5HT) is one of the chief modulators of synaptic transmission in the brain. As such, 5HT has been suggested to be capable of modulating neuronal plasticity in the hippocampus, thereby regulating the processes of learning and memory.

In this thesis, I extend the investigation of the role of broader 5HT signalling as well as the 5HT1A receptor signalling on the CA3—CA1 synaptic plasticity in the hippocampus. A systematic review and meta-analysis is performed that provides a comprehensive overview of the reported effects of 5HT signalling on LTP at the CA3—CA1 synapse. The systematic review of the existing literature shows that, overall, an increase in hippocampal 5HT levels can suppress CA3—CA1 LTP but a reduction in 5HT signalling appears to have little effect on LTP. This indicates that 5HT may not be essential for LTP induction and maintenance but that enhanced 5HT signalling can interfere with its expression.

The systematic review, however, also revealed some contradictions in the reported effects of 5HT on CA3—CA1 LTP. Interestingly, while most existing reports of CA3—CA1 LTP treat the hippocampus as a homogenous structure, it has been suggested that the dorsal hippocampus is primarily involved in cognitive functions, while the ventral hippocampal functions have been related to stress, emotion and affect. In light of such findings, the effects of 5HT signalling on CA3—CA1 synaptic transmission and plasticity were investigated by measuring local field potential from mouse hippocampal slices using a multielectrode array (MEA). The effects of exogenous 5HT and a 5HT1A receptor agonist, NLX101, on basal transmission and LTP induction at the CA3—CA1 synapse were examined at different CA1 layers in slices taken from the dorsal or the ventral hippocampus.

The presented data provides evidence, for the first time, that 5HT suppresses basal signalling in ventral layers stratum pyramidale (SP) stratum radiatum (SR) and stratum lacunosum-moleculare (SLM), but not in the dorsal layers. In contrast, the 5HT1A receptor selective agonist, NLX101, reduced basal transmission in layers SR and SLM, but the reduction was no longer statistically significant when sorted into dorsal or ventral slices. Nevertheless, following a high frequency stimulation (HFS), while 5HT was only able to suppress LTP in the dorsal layer SLM, the selective activation of the 5HT1A receptor suppressed LTP in all layers of the dorsal CA1. No significant change was observed in the ventral slices following an HFS in either drug group. The role of the serotonergic system on basal transmission and LTP induction appear to, primarily, involve postsynaptic mechanisms as paired pulse facilitation at the CA3—CA1 synapse remained unaffected by 5HT or NLX101.

Finally, immunohistochemistry protocol was optimised with the aim of investigating the 5HT1A receptor expression in different areas of the hippocampus. The goal, guided by the principles of the 3Rs, was to use the same slices used for electrophysiological recordings so as to directly correlate the receptor expression study with the LTP study. Different protocols have been explored whereby the fixation, permeabilisation and antibody incubation steps were reviewed. We found that PFA fixation for 2 hours at 4°C, Triton X-100 at 1% w/w and primary and secondary antibody incubation for 2 nights each improved staining. Although there is scope for further optimisation such as antigen retrieval, the revised protocol showed that the 5HT1A receptors are expressed mostly in the pyramidal cell soma.

The findings from this thesis suggest that while 5HT, in general, suppresses hippocampal LTP, different hippocampal sub-regions have different sensitivity to 5HT, and some, but not all, of the effects are mimicked by 5HT1A receptors. Such differential effects on LTP expression also provides support to the functional difference that is associated with the dorsoventral axis of the hippocampus.