Differential release of coexisting neurotransmitters: frequency dependence of the efflux of substance P, thyrotropin releasing hormone and [3H]serotonin from tissue slices of rat ventral spinal cord

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Abstract
In few systems has the release of coexisting classical and peptide neurotransmitters been studied. Here the release of substance P‐like immunoreactivity (SP‐LI), thyrotropin releasing hormone‐like immunoreactivity (TRH‐LI) and [3H]serotonin ([3H]5‐HT) from tissue slices of rat ventral spinal cord was investigated in a superfusion system. The slices were stimulated electrically with field stimulation (900 pulses, 2 ms duration, 36 V) at frequencies between 0.25 Hz and 40 Hz. The evoked fractional release of SP‐LI increased significantly from 0.46 to 1.24% of the total tissue store when the frequency of stimulation was changed from 3 to 10 Hz, while the evoked fractional release of TRH‐LI increased significantly from 0.28 to 0.71% of the total tissue store with increasing frequency of stimulation between 0.5 and 3 Hz. The evoked fractional release of [3H]5‐HT did not show any significant change when the frequency of stimulation was changed in the frequency range of 0.25–40 Hz but remained between 5.6 and 7.2% of the total tissue store. It appears that at frequencies lower than 0.5–1 Hz these 5‐HT/SP/TRH neurons may function predominantly as serotonergic neurons. At 3 Hz stimulation with 900 pulses the extracellular Ca2+ concentration required for half‐maximal release of [3H]5‐HT was 1.2 mmol 1‐‐1, while for half‐maximal release of SP‐LI significantly higher concentrations of Ca2+ (4.2 mmol 1‐‐1) would be required. The results demonstrate that the coexisting neurotransmitters substance P, thyrotropin releasing hormone and [3H]serotonin show different dependence on frequency of stimulation for the electrically evoked in‐vitro release and suggest that the mechanism of release of these coexisting neurotransmitters is at least partly different.