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.

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 l-1, while for half-maximal release of SP-LI significantly higher concentrations of Ca2+ (4.2 mmol l-1) would be required.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and chronic studies on functional aspects of coexistence.

Autoinhibition of acetylcholine release by the coexisting peptide galanin in the septal afferents to the hippocampus of the rat was examined in tissue slices from the hippocampus. Galanin inhibits the evoked release of the coexisting neurotransmitter, acetylcholine, in the ventral hippocampus, providing an example of autoinhibition of release of a neurotransmitter by one of the coexisting neurotransmitters. The galanin mediated inhibition of the acetylcholine release is a complement to the well known strong cholinergic autoinhibition. The effects of the coexisting galanin and acetylcholine on several second messenger systems were also examined: acetylcholine acting at muscarinic receptors depresses cyclic adenosine 3',5'-monophosphate and stimulates elevation of cyclic guanosine 3',5'-monophosphate levels, whereas neither cyclic adenosine 3',5'-monophosphate nor cyclic guanosine 3',5'-monophosphate levels were affected by galanin (1 microM). Galanin however inhibited partly the muscarinic stimulation of phosphoinositide breakdown, suggesting that inositol phosphate(s) or diacylglycerol may act as second messenger(s) of the galanin action in the hippocampus. The effects of chronic changes in firing rate on the coexisting neurotransmitters in the rat ventral spinal cord containing serotonin, thyrotropin releasing hormone, substance P and substance K were examined. The tissue levels of the coexisting transmitters were studied in rats chronically treated with imipramine (14 days; 2 x 10 mumoles/kg/day) and zimelidine (14 days; 2 x 10 mumoles/kg/day). Upon treatment with zimelidine the tissue levels of the serotonin metabolite 5-hydroxyindoleacetic acid fall by 32% while thyrotropin releasing hormone levels seem to increase 35% and substance P/substance K levels also increase 48 and 72% respectively. Imipramine treatment resulted in similar although less pronounced changes.(ABSTRACT TRUNCATED AT 250 WORDS)