A novel methodology for simultaneous assessment of the effects of 5-hydroxytryptamine on primary afferent polarisation and synaptic transmission in rat dorsal horn neurones in vitro.

The rat hemisected spinal cord in vitro preparation was used to test simultaneously the effects of 5-hydroxytryptamine (5-HT) on primary afferent polarisation and synaptic transmission onto dorsal horn (DH) neurons. Primary afferent polarisation was measured from the cut end of a transected lumbar dorsal root (DR; L3-L6) using tight suction electrodes coupled to a D.C. amplifier. Conventional sharp microelectrodes were used to record intracellularly the excitatory postsynaptic potential (EPSP) evoked by high intensity electrical stimulation (100 microA, 100 microseconds) of another DR contiguous to that used for the suction electrode recording. Superfusion of 5-HT (5-10 microM) caused primary afferent depolarisations (PAD) of 227.5 +/- 26.5 microV (mean +/- SEM) and 221 +/- 32 microV, respectively, values comparable to the PAD caused by 10-100 microM gamma-aminobutyric acid (GABA) superfusion. 5-HT-induced PAD was tetrodotoxin (TTX) resistant and non-additive to capsaicin-induced PAD suggesting a direct depolarising action of 5-HT on a population of primary afferents which may include a high proportion of unmyelinated fibres. Simultaneous intracellular recordings showed that 5-HT, in addition to generating PAD, depressed primary afferent-evoked synaptic transmission to DH neurons reflected by a significant reduction (p < 0.05) in the amplitude and duration of the EPSP. In contrast, GABA, despite producing a PAD of similar amplitude, failed to depress synaptic transmission. These data suggest that PAD alone may be insufficient to account for the 5-HT-induced depression of synaptic transmission. This novel experimental approach offers a means to explore further the possible causal relationship between pre- and post-synaptic effects of 5-HT in the DH and its ability to modulate somatosensory processing and nociception.