Physiology of neural networks in the dorsal raphe nucleus
Keywords: serotonin
neuronal interactions
neuronal decoding
neuronal correlations
electrophysiology
SSRIs
silicon probes
DRN
dorsal raphe nucleus
5-HT

5-HT (5-Hydroxy-tryptamine; serotonin) neurons located in the midbrain dorsal raphe nucleus (DRN) are implicated in many brain functions in health including aversive processing and are the target of the first line drug treatment for anxiety and depression, selective serotonin reuptake inhibitors (SSRIs). The research presented in this thesis employs silicon probe electrodes to investigate (1) the diversity of firing properties of both 5-HT and non-5-HT neurons in the DRN, (2) potential emergent population dynamics in the DRN network and (3) the effect on these parameters of an aversive foot-shock stimulus and treatment with the SSRI citalopram.
A first set of experiments established the silicon probe recording method. Histological analysis of electrode tracks and spike sorting of electrode signals confirmed the capacity of silicon probes to generate large-scale simultaneous recordings of DRN neurons. Through clustering of neuronal electrophysiological properties, three putative neuron types could be identified: slow regular firing neurons, fast firing neurons, and slow irregular firing neurons. The properties of these neurons were consistent with 5-HT, GABA, and generic non-5-HT neurons previously identified using cell labelling methods in the literature.
Next, the silicon probe recording method was applied to quantify the effects of an aversive foot-shock stimulus on the firing of DRN neurons. Widespread and diverse responses were observed among both putative 5-HT and non-5-HT neurons. Furthermore, analysis of the performance of stimulus decoders as a function of the number of DRN neurons used showed a positive relationship between decoder performance and neuron number, indicating DRN neurons may encode the foot-shock stimulus using a population code.
Silicon probe recordings were then applied to quantify interactions between simultaneously recorded neurons in the DRN network. Spike count correlations between pairs of putative 5-HT neurons were heterogeneous, with some neuron pairs being weakly correlated, others negatively correlated and yet others positively correlated. Groups of positively correlated putative 5-HT neurons formed neuronal ensembles. Spike count correlations between putative 5-HT and non-5-HT neurons were also heterogeneous: putative non-5-HT neurons could be positively correlated with putative 5-HT neurons, even being members of the same neuronal ensemble but, particularly for putative GABA neurons, were often negatively correlated with them when they were not.
In a final set of experiments, the effect on the DRN network of continued treatment with the SSRI citalopram was assessed. Citalopram treatment was associated with: (1) the generation of two populations of broad waveform putative 5-HT neurons with distinct firing patterns, consistent with citalopram having diverse effects on 5-HT neurons, (2) an attenuation of negative spike count correlations between putative 5-HT neurons, (3) an enhanced excitation of putative 5-HT neurons to aversive foot-shocks, (4) DRN putative GABA neurons having a reduced prevalence and lower firing rate than saline controls. Thus, these experiments revealed potential diverse effects of citalopram on 5-HT and non-5-HT DRN neurons and demonstrated that citalopram affects the sensitivity of putative 5-HT neurons to physiological stimuli.
These findings extend existing knowledge on 5-HT neuron diversity, the functioning of non-5-HT neurons within the DRN, and the ways by which 5-HT neurons function as part of the DRN network.