Inhibition of serotonin release by bombesin-like peptides in rat hypothalamus in vitro.

We investigated the activity of bombesin (BN), neuromedin-C (NM-C) and neuromedin-B (NM-B) on serotonin (5-HT) release and reuptake in rat hypothalamus (HYP) in vitro. BN and NM-C but not NM-B (all 1 microM) decreased K+ evoked 3H-5-HT release from superfused HYP slices by 25%. Bacitracin (BCN, 2 micrograms/ml), a nonspecific peptidase inhibitor, reversed the inhibitory effect of BN on K+ evoked 3H-5-HT release. Phosphoramidon (PAN, 10 microM) an endopeptidase 24.11 inhibitor, abolished the inhibitory effect of BN, but not NM-C, on K+ evoked 3H-5-HT release. The peptidyl dipeptidase A inhibitor enalaprilat (ENP, 10 microM), enhanced both BN and NM-C inhibition of 3H-5-HT release. Bestatin (BST, 10 microM) had no effect on BN or NM-C inhibitory activity on 3H-5-HT release. Neither BN, NM-C nor NM-B affected reuptake of 3H-5-HT into HYP synaptosomes alone or in combination with any of the peptidase inhibitors, nor did these peptides alter the ability of fluoxetine to inhibit 3H-5-HT uptake. These data suggest: a) that BN-like peptides may alter neurotransmission in the HYP by acting presynaptically on the 5-HT release mechanism; b) a similarity in the structural requirements for the BN induced inhibition of 5-HT release and BN evoked thermoregulatory disturbances; and c) that peptidases may selectively augment or reduce pharmacologic activity of BN-like peptides upon CNS administration.

Differential effects of nialamide and clomipramine on serotonin efflux and autoreceptors.

Serotonin (5-HT) activity in vivo and in vitro was evaluated in rats following acute and chronic administration of the antidepressants nialamide (NMD) and clomipramine (CMI). The 5-HT motor syndrome was used as an index of in vivo serotonergic function. In vitro, 3H-5-HT uptake, potassium-evoked 3H-5-HT release and 5-HT autoreceptor activity were evaluated as measures of presynaptic function. Repeated injections of NMD abolished the 5-methoxy-N, N-dimethyltryptamine (5-MeODMT)-induced motor syndrome and the ability of 5-methoxytryptamine (5-MEOT) to attenuate the potassium-evoked release of 3H-5HT. Autoreceptor subsensitivity was associated with a marked increase in basal and potassium-evoked 3H-5-HT release. In contrast, acute NMD, and acute and chronic CMI did not affect the expression of the motor syndrome or alter 3H-HT release or autoreceptor activity. Acute and chronic injections of NMD enhanced 3H-5-HT uptake. The results suggest that the antidepressant efficacy of monoamine oxidase inhibitor (MAOI) antidepressants may be related to their ability to increase endogenous levels of 5-HT and thereby produce a subsensitivity of 5-HT1 type receptors. This subsensitivity is reflected both by attenuation of the motor syndrome and enhanced 5-HT neurotransmission resulting in part from autoreceptor down-regulation.

Altered distribution and toxicity of digitoxigenin in fasted mice.

Intravenous administration of digitoxigenin (DTXGN) evokes seizure episodes in mice which may be dependent on brain biogenic amines such as serotonin (5-HT). Fasting is known to have effects on both drug toxicity and brain 5-HT synthesis. The purpose of this study was to assess the effects of overnight fasting on DTXGN toxicity. The i.v. LD-50 of DTXGN was increased by 61% in fasted mice. Adjustment of DTXGN dose for the decrease in body weight of fasted mice did not alter the fasting induced protection. A loading dose of 1-tryptophan (25 mg/kg, i.p.) did not alter mortality rates in either fed or fasted mice. Cortical levels of 3H-DTXGN were decreased significantly by 25% in fasted mice. Liver and blood levels were elevated significantly. These data suggest that decreased DTXGN toxicity is associated with a decrease in its distribution to the cerebral cortex and emphasize the importance of acute dietary status in the expression of drug toxicity.

Ketanserin alters [3H]serotonin uptake and release in rat hypothalamus.

Ketanserin, a serotonin2 receptor antagonist, evoked a dose-related non-calcium dependent release of tritium from slices of rat hypothalamus preloaded with [3H]serotonin. Concentrations of 1, 5 and 10 microM ketanserin caused a 0.5%, 64% and 110% increase, respectively, in tritium release. Chlorimipramine, a potent inhibitor of the serotonin uptake mechanism, did not alter the spontaneous release of tritium. In addition, 5 microM ketanserin reduced high affinity uptake of 0.1 microM [3H]serotonin into rat hypothalamus slices by 36%. These data suggest ketanserin possesses 'amphetamine-like' releasing activity and an uptake inhibitory action on serotonin nerve terminals. Both of these properties could mask 5-HT receptor antagonism in vitro and in vivo.

Effect of methadone and morphine on serotonin uptake in rat periaqueductal gray slices.

In vitro effects of d,l-methadone and morphine on [3H]serotonin (3H-5-HT) uptake in rat periaqueductal gray (PAG) slices were investigated. Only methadone had a significant inhibitory effect on 3H-5-HT uptake which was significantly enhanced by naloxone. The systemic administration of methadone did not affect 3H-5-HT uptake in vitro. These data further weaken the possible relationship between narcotic analgesia and blockade of the 5-HT reuptake mechanism.

Effect of acute and chronic morphine treatment on serotonin uptake into rat hypothalamic synaptosomes.

The effect of morphine on uptake of serotonin into hypothalamic synaptosomes was determined following acute and chronic morphine treatment in the rat. The uptake of serotonin was noncompetitively inhibited by in vitro morphine (IC50 = 7 X 10(-4) M) but not by an acute morphine treatment (30 mg/kg s.c.) which produces hypothermia. Uptake of serotonin into hypothalamic synaptosomes derived from rats rendered tolerant to the hypothermic effect of morphine or rats withdrawn from morphine was not altered either in the absence or presence of in vitro morphine as compared to control rats. These data indicate that alterations in thermoregulatory responses of the rat to either acute or chronic morphine treatment do not result from morphine-induced modification of serotonin uptake into hypothalamic neurons.

Studies relating morphine hypothermia with serotonin reuptake in the rat hypothalamus.

Experiments were conducted to determine if the acute hypothermia elicited by morphine in the rat resulted from morphine-induced inhibition of serotonin reuptake into hypothalamic nerve-endings. The acute hypothermia elicited by morphine (30 mg/kg, s.c.) was reversed by pretreatment with naloxone (10 mg/kg, s.c.), a narcotic antagonist, which alone did not alter body temperature. In vitro uptake of serotonin into synaptosomes prepared from rat hypothalamus was inhibited 53% by the addition of morphine (7 X 10(-4) M) and 53.9% by naloxone (1 X 10(-3) M). Simultaneous addition of both drugs in these same concentrations further inhibited synaptosomal serotonin uptake 77.8%. Uptake of serotonin uptake into synaptosomes isolated from rats injected with morphine (30 mg/kg, s.c.) was not altered as compared to controls. These data suggest that the acute hypothermic acition in morphine in the rat is not related to an inhibition of the serotonin reuptake mechanism in hypothalamic nerve-endings.