Paul S. Katz

Behavioral variation correlates with differences in single neuron serotonin receptor subtype expression within and across species

G protein-coupled receptors (GPCRs) activated by biogenic amines such as serotonin (5-HT) can modulate neuronal signaling and membrane properties. Expression differences in GPCRs could underlie species or individual differences in behaviors. This study examined correlations between behavior and GPCR gene expression in single identified neurons. In the nudibranch Tritonia diomedea (Mollusca; Gastropoda; Nudipleura), 5-HT plays an essential role in the generation of dorsal-ventral (DV) escape swimming by enhancing the strength of synapses made by neuron C2, which is a member of the swim central pattern generator (CPG). Neurons homologous to C2 are present across Nudipleura. Previous research found that in the nudibranch, Hermissenda crassicornis, which does not swim, C2 synapses are not modulated by 5-HT. The pleurobranchomorph, Pleurobranchaea californica, has variable swimming, which correlates with the extent of modulation of C2 synaptic strength. We tested whether the expression of particular 5-HT receptor genes correlated with species and individual variation in swimming. Using single C2 neuron transcriptome sequencing and absolute quantitative PCR (qPCR), we found that 5-HT2a and 5-HT7 receptor expression was high in C2 neurons isolated from Tritonia and individual Pleurobranchaea that swam, but not Hermissenda or non-swimming Pleurobranchaea. In contrast, C2 neurons isolated from Hermissenda exhibited higher 5-HT4 and 5-HT6 expression, neither of which was expressed in Tritonia C2 neurons. The results suggest that differences in the expression of particular GPCRs in single neurons could account for differences in behavior both across and within species. This finding has implications for our understanding of the evolution of species-typical behaviors.The marine mollusc, Pleurobranchaea californica varies daily in whether it swims and this correlates with whether serotonin (5-HT) enhances the strength of synapses made by the swim central pattern generator neuron, C2. Another species, Tritonia diomedea, reliably swims and does not vary in serotonergic neuromodulation. A third species, Hermissenda crassicornis, never produces this behavior and lacks the neuromodulation. We found that expression of particular 5-HT receptor genes in C2 correlates with swimming. Seven 5-HT receptor subtype genes were identified from whole-brain transcriptomes. We isolated individual C2 neurons and sequenced their RNA or measured 5-HT receptor gene expression using quantitative PCR. C2 neurons isolated from Pleurobranchaea individuals that produced a swim motor pattern just prior to isolation expressed the 5-HT2a and 5-HT7 receptor genes, as did the Tritonia samples. These subtypes were absent from C2 neurons isolated from Pleurobranchaea individuals that did not swim that day and from Hermissenda C2 neurons. Expression of other receptors did not correlate with swimming. This suggests that 5-HT2a and 5-HT7 receptors mediate the modulation of C2 synaptic strength and play an important role in swimming. Furthermore, the results suggest that regulation of receptor expression might underlie daily changes in behavior as well as behavioral evolution.

Single neuron serotonin receptor subtype gene expression correlates with behaviour within and across three molluscan species

The marine mollusc, Pleurobranchaea californica varies daily in whether it swims and this correlates with whether serotonin (5-HT) enhances the strength of synapses made by the swim central pattern generator neuron, A1/C2. Another species, Tritonia diomedea, reliably swims and does not vary in serotonergic neuromodulation. A third species, Hermissenda crassicornis, never produces this behaviour and lacks the neuromodulation. We found that expression of particular 5-HT receptor subtype (5-HTR) genes in single neurons correlates with swimming. Orthologues to seven 5-HTR genes were identified from whole-brain transcriptomes. We isolated individual A1/C2 neurons and sequenced their RNA or measured 5-HTR gene expression using absolute quantitative PCR. A1/C2 neurons isolated from Pleurobranchaea that produced a swim motor pattern just prior to isolation expressed 5-HT2a and 5-HT7 receptor genes, as did all Tritonia samples. These subtypes were absent from A1/C2 isolated from Pleurobranchaea that did not swim on that day and from Hermissenda A1/C2 neurons. Expression of other receptors was not correlated with swimming. This suggests that these 5-HTRs may mediate the modulation of A1/C2 synaptic strength and play an important role in swimming. Furthermore, it suggests that regulation of receptor expression could underlie daily changes in behaviour as well as evolution of behaviour.