Effects of Narcotic Analgesics on the Uptake and Release of 5‐hydroxytryptamine in Rat Synaptosomal Preparations

Abstract

1 The effects of various narcotic analgesics on the uptake and release of labelled 5-hydroxytryptamine (5-HT) in brain and spinal cord synaptosomes were investigated.2 Methadone was most active in inhibiting 5-HT uptake (IC50 2.5 x 10-M). Levorphanol also inhibited 5-HT uptake to a large extent (IC5o 8.8 x 10-7 M) while dextrorphan, pethidine and pentazocine showed much less activity. Etorphine and morphine had virtually no such activity, with IC50s higher than 10-' and 1O-M respectively.3 The same order of potency as '5-HT releasers' was found when radioactivity was measured in [3H]-5-HT preloaded synaptosomal pellets incubated for 20min with the various narcotics. Methadone, like chlorimipramine, showed a significant effect at a concentration of 10-7M while morphine, at a concentration of 10-4 M, had no effect.4 When 5-HT release was studied by a perfusion technique, which largely prevents reuptake of the released amine, only fenfluramine, an anorectic agent proposed as a 5-HT releaser, significantly increased spontaneous 5-HT release. These data suggest that the apparent 5-HT release induced by various narcotics in traditional incubation techniques may largely depend on their ability to interfere with neurotransmitter reuptake mechanisms. 5 The effects of the various narcotics on 5-HT uptake have no relationship to their relative potency as analgesics in the rat. In the light of their poor effectiveness as 5-HT releasers, it can be concluded that mechanisms other than 5-HT uptake inhibition and release are probably involved in the analgesic effects of these compounds in intact animals.

Increased Extracellular Serotonin in Rat Brain After Systemic or Intraraphe Administration of Morphine

“…Morphine perfusion into the site of 5-HT nerve terminals in the diencephalon had no effect on extracellular 5-HT or 5-HIAA . This is consistent with other reports that morphine did not enhance 5-HT release from synaptosomes or brain slices lacking 5-HT cell bodies (Mennini et al, 1978 ;Bineau-Thurotte et al, 1984;Monroe et al ., 1986) . We were unable to test higher morphine concentrations because of difficulties in separating 5-HT from the very large chromatography peak produced by 100 /tM morphine .…”

“…The site where opioids act to enhance 5-HT turnover and release is a second unresolved issue. In vitro studies found no effect of morphine on 5-HT released from synaptosomes or brain slices (Mennini et al ., 1978) . This suggests that morphine does not act directly on 5-HT nerve terminals .…”

Effect of long term amineptine treatment on pre‐ and postsynaptic mechanisms in rat brain

“…Data were calculated as % of radioactivity in each sample over total radioactivity present at the beginning of superfusion. More detailed information about the methods used in studies of monamine uptake and release is given elsewhere (Mennini et al, 1978;1981) (Table 2). However, in superfusion experiments, a tendency to Table 4 shows that chronic amineptine treatment significantly reduced the maximum number of binding sites for [3H]-spiperone in striatum, as well as those for and a2-and a-NA ligands in rat cortex.…”

Differences Between d‐Fenfluramine and d‐Norfenfluramine in Serotonin Presynaptic Mechanisms

“…[3H]5-HT accumula-in accumulation studies does not seem to play an important role for d-F, suggesting that release from the granular pool and true uptake inhibition are two different mechanisms by which d-F affects serotonin neurons in vitro. tion and [$HH]S-HT release were determined as described by Mennini et al (1978;1981b) in synaptosomes obtained from whole brains, excluding cerebellum, of normal rats or rats treated 24 h before the experiment with reserpine (10 mg/kg i.p.). [3H]5-HT (NEN, S.A. 30 Ci/mmol) was used, at a final concentration of 0.1 p M .…”

“…The reaction was stopped by rapid filtration under vacuum through Sartorius membrane filters (0.65 pm). SeeMennini et al (1978) for details. IC," are means of 4-6 separate experiments, calculated from log-dose effect plots of 3 -4 drug concentrations.…”