Evidence for a 5-HT6 receptor-mediated control of midbrain 5-HT neurons

The dorsal raphe nucleus (DRN) contains the majority of 5-hydroxytryptamine (5-HT)-producing neurons, whose firing is influenced by homeostatic feedback pathways. Dysfunction of 5-HT neurons is implicated in the pathophysiology of diseases such as depression. Thus, pathways regulating 5-HT neuron activity represent potential therapeutic targets. Negative feedback pathways influencing 5-HT neurons include 5-HT1A autoreceptors located in the DRN, and feedback from 5-HT1A, 5-HT2A and 5-HT2C receptors on postsynaptic neurons in forebrain regions. One pathway, mediated by 5-HT4 receptors on medial prefrontal cortex (mPFC) neurons, positively regulates 5-HT neuron activity. Preliminary findings in the Sharp Laboratory suggested that 5-HT6 receptors may also positively regulate 5-HT neuron activity. Using in vivo electrophysiological recordings in combination with drug tools, this thesis investigated the 5-HT6 receptor control of 5-HT neurons, a proportion of which were identified as 5-HT-containing using a juxtacellular labelling technique. Electrophysiological recordings in anaesthetised rats found that 5-HT6 receptor agonists WAY-181187 and WAY-208466 increased 5-HT neuron firing to 58% and 47% above pre-drug firing, respectively. By contrast 5-HT6 receptor antagonist SB-399886 reduced 5-HT neuron firing to 56% below pre-drug firing. Four other 5-HT6 ligands, agonist ST-1936, and antagonists AE-58054, SB-258585, and SB-271046, had no significant effect on 5-HT neuron firing. Simultaneous electroencephalogram (EEG) recordings found that WAY-181187 and WAY-208466 reduced frontal cortex slow wave (SW) oscillation power to 62% and 60% below pre-drug values, respectively. It was considered that the frontal cortex may be involved in the effect of these agonists on 5-HT neuron firing. Subsequent experiments investigated the influence of 5-HT6 receptors over mPFC neural activity. WAY-181187 reduced mPFC SW oscillation power maximally to 54% below pre-drug values. This occurred in a putatively 5-HT6 receptor-dependent manner since the effect was blocked by SB-399885 and AE-58054. WAY-181187 also modulated the firing of mPFC pyramidal neurons, including those projecting to the DRN. Specifically, one population of neurons was excited by WAY-181187, with a maximum increase in firing of 161% above pre-drug firing, whereas another population was inhibited by WAY-181187, with a maximum decrease to 73% below pre-drug firing. In a final set of experiments neurotoxic mPFC lesion attenuated the excitatory effect of WAY-181187 on 5-HT neuron firing. Thus the effect of WAY-181187 at 2mg/kg was significantly reduced in lesioned rats compared to sham controls. Experiments in this thesis provide evidence to support a role of 5-HT6 receptor-mediated excitatory control of DRN 5-HT neurons via an mPFC-dependent mechanism. This control may act with previously reported feedback mechanisms to balance inhibitory and excitatory input to 5-HT neurons. Finally, 5-HT6 receptor-mediated feedback may be a useful target for modulating the 5-HT-system, such as in depression therapy.