Background

Serotonergic receptors of the 5-HT₇type (5-HT₇Rs) are widely expressed in the central nervous system, where they modulate several functions, such as sleep induction, learning, mood, and vegetative behaviours. Along the pain axis, 5-HT₇Rs are expressed on nociceptive primary afferent fibers and in the dorsal horn, both on neurons and astrocytes. [1]. Behavioural experiments have produced controversial results about anti- and pro-nociceptive actions of 5-HT₇Rs. The low agonist selectivity and the different pain animal models used have likely contributed to the heterogeneity of the results [2].

To investigate the effects of 5-HT₇Rs on spinal pain, we have performed an electrophysiological study on mouse spinal cord slices, using the selective agonist LP-211 [3]. The recorded neurons have been functionally characterized, in order to identify the neural circuits involved in the serotonergic modulation.

Methods

Patch-clamp recording was performed on lamina II neurons in spinal cord slices obtained from postnatal CD1 mice (P15-P25) [4]. Excitatory postsynaptic currents (EPSCs) were recorded in voltage clamp; evoked EPSCs were elicited by stimulating the dorsal root with a suction electrode in the Adand C fibers range.

Results.

Application of 1 mM LP-211 to the spinal cord slice induced a facilitation of glutamatergic transmission: the frequency of spontaneous EPSCs was significantly increased in a subpopulation of neurons (control: 0.9±0.2 Hz; LP-211: 1.8±0.6 Hz; 5 responsive neurons out of 8). The recorded neurons were characterized from their firing pattern: significant effects of LP-211 were observed in both tonic and delayed firing neurons, corresponding to inhibitory and excitatory interneurons, respectively.

Application of 1 mM LP-211 in the presence of 10 mM SB269970 (a 5-HT₇R antagonist) did not alter spontaneous EPSC frequency in 11 lamina II neurons, confirming the involvement of  5-HT₇Rs in glutamate release facilitation.

EPSCs evoked by dorsal root stimulation were also tested with LP-211. The currents, evoked by paired pulse protocol, were significantly potentiated by the compound (mean potentiation: 19±4.2%; 4 responsive neurons out 7). The second EPSC was less potentiated than the first and the paired pulse ratio decreased in 3 neurons.

Conclusion

The compound LP-211 is able to selectively activate 5-HT₇Rs in the dorsal horn, causing a facilitatory effect of both spontaneous and evoked EPSCs. The decrease of paired-pulse ratio suggests that LP-211 activates presynaptic 5-HT₇Rs, increasing glutamate release. The study of specific effects of these receptors on the different neuron populations will be critical to determine whether 5-HT₇Rs exert anti- or pro-nociceptive effects at the spinal level.
 

References

1.  Cortes-Altamirano JL et al., Curr. Neuropharm, 2018, 16:210-221

2. Bardoni R, Curr. Neuropharm, 2019, 17:1133-1145

3. Hedlund PB et al, Neurosci Lett. 2010 481:12-6.

4. Betelli C et al., Mol. Pain, 2015, 11:6