Evidence for a similar compartmentation of recaptured and endogenously synthesized dopamine in striatal synaptosomes

5HT2-receptors and serotonin release: their role in human platelet aggregation

Involvement of 5HT2 receptors in human platelet aggregation was assessed by studying the effect of ADP, epinephrine and thrombin on 3H-5HT release from platelets. The release experiments were made with a perfusion method to preserve any compound, released or formed by platelet, from interacting with platelet itself. In these conditions, aggregation does not occur, as confirmed by Scanning Electron Microscopy. These release experiments showed that the platelet activation by such agents is coupled with 5-HT release. The aggregation experiments, made on different aliquots of the same platelet-rich plasma (PRP), showed that the released 5-HT, interacting with its own receptors on platelet activated surface, determines aggregation. In fact, although it is known that 5-HT added to PRP was only able to induce a moderate platelet aggregation, the 5-HT2 antagonist ketanserin counteracted the aggregation induced by ADP, epinephrine and thrombin. These results suggest that a 5HT2 antagonist could be therapeutically important in those pathological states in which serotonin, released by activated platelets, may increase aggregation.

Factors influencing the calculation of results from studies of the release of tritiated neurotransmitters from superfused slices of guinea pig striata

Slices of guinea pig striata were incubated with tritiated choline, dopamine, or serotonin and the release of radioactive transmitter was studied in a superfusion system. Some experiments were also done on the release of [3H]acetylcholine from rat striatal slices. For analysis of the results, various parameters of the system were determined in order to establish the most reliable method of assessing drug effects on transmitter release. Peaks of radioactivity (S1 and S2) above basal release of radioactivity (B1 and B2) were observed after two depolarizations of the nerve endings with high K+ buffer. Each stimulation was for 2 min, and S2 occurred 20 min after S1. There was a highly significant correlation between S1 and the protein content of the slices for acetylcholine release from guinea pig striata. Basal release of radioactivity, and the ratio S2/S1, were not sensitive to minor changes in the experimental conditions. It was concluded that S2/S1, rather than S1 or S1/B1, should be used as a measure of drug effects on release. The experiments also demonstrated that re-uptake of released neurotransmitter is operative in the superfusion system for dopamine, but not for serotonin. Differences were observed between the rat and the guinea pig with respect to the release of [3H]acetylcholine.

Effect of chronic desipramine treatment on adrenoceptor modulation of [3H]dopamine release from rat nucleus accumbens slices

The effects of alpha 2- and beta-adrenoceptor agonists on the 25 mM K+-induced release of [3H]dopamine [( 3H]DA) from the nucleus accumbens slices of chronic desipramine (DMI)- and saline-treated rats were investigated using a superfusion technique. The K+-induced release of [3H]DA from nucleus accumbens slices was shown to be Ca2+ dependent and to be enhanced by ascorbic acid. In experiments with isoproterenol, ascorbic acid was added to the superfusion media in order to prevent the otherwise rapid oxidation of the drug. The K+-induced release of [3H]DA from nucleus accumbens slices of saline-treated rats was significantly decreased by the alpha 2-adrenoceptor agonist, clonidine (10 microM; 89 +/- 2.4% of control values; P less than 0.002), and significantly enhanced by the beta-adrenoceptor agonist, isoproterenol (1 and 10 microM; 122 +/- 4.3 and 171 +/- 2.9% of control values, P less than 0.002 and P less than 0.001, respectively). The basal release of [3H]DA was strongly enhanced by 10 microM but not 1 microM isoproterenol. Chronic DMI pretreatment (10 mg/kg i.p. for 28 days) did not significantly alter the K+-induced release of [3H]DA. Chronic DMI treatment attenuated the alpha 2-adrenoceptor-mediated inhibition of [3H]DA release, while the beta-adrenoceptor-mediated stimulation remained unchanged. The net effect of chronic DMI treatment therefore would appear to be a facilitation of dopaminergic neurotransmission in the mesolimbic system. This is consistent with behavioural evidence which suggests that the function of the mesolimbic dopaminergic reward system is facilitated by chronic treatment with antidepressant drugs.

Alpha 2 and beta-adrenoceptor agonists modulate [3H]dopamine release from rat nucleus accumbens slices: implications for research into depression

The modulatory effects of noradrenergic agonists on the 25 mM K+-induced release of [3H]dopamine (3H-DA) from rat brain nucleus accumbens slices was investigated, using a superfusion technique. The K+-induced release of 3H-DA was Ca2+ dependent, significantly enhanced (25-32%; p less than 0.02) by the beta-adrenoceptor agonist isoproterenol (10 microM), and significantly decreased (13-25%; p less than 0.05) by the alpha 2-adrenoceptor agonist clonidine (10 microM). At these concentrations neither drug affected basal release of 3H-DA. Clonidine (100 microM) increased the basal release of 3H-DA, while decreasing the K+-induced release by 19% (p less than 0.01). The inclusion of desipramine in the incubation medium, to prevent accumulation of 3H-DA into noradrenergic neurons, did not alter the inhibitory effect of clonidine (10 microM) on 3H-DA release. This study provides direct evidence that noradrenergic neurons can modulate dopaminergic neurotransmission in the mesolimbic system.

Glutamate as a putative transmitter in the cerebellum: stimulation by GABA of glutamic acid release from specific pools

The aim of the present paper was to determine whether the release of glutamate from putative "glutamergic" terminals in the cerebellum is influenced by gamma-aminobutyric acid (GABA). In a group of preliminary experiments, we present biochemical evidence in favour of a neurotransmitter role of glutamate in the cerebellum: (1) endogenous glutamate was released from depolarized cerebellar synaptosomal preparations in a Ca2+-dependent away; (2) [14C]glutamate was synthesized from [14C]glutamine in cerebellar synaptosomes, and the newly synthesized [14C]glutamate was released released in a Ca2+-dependent way; (3) the elevation of cyclic GMP elicited by depolarization of cerebellar slices in the presence of Ca2+ was partly reversed by the glutamate antagonist glutamic acid diethyl ester, which probably prevented the interaction of endogenously released glutamate with postsynaptic receptors. GABA and muscimol at low concentrations (2--20 micrometers) potentiated the depolarization-induced release of D-[3H]aspartate (a glutamate analogue which labels the glutamate "reuptake pool") from cerebellar synaptosomes. The effect was concentration dependent and was largely prevented by two GABA antagonists, bicuculline and picrotoxin. The stimulation of D-[3H]aspartate release evoked by muscimol was linearly related to the logarithm of K+ concentration in the depolarizing medium. GABA did not affect the overall release of endogenous glutamate, but potentiated, in a picrotoxin-sensitive manner, the depolarization-evoked release of [14C]glutamate previously synthesized from [14C]glutamine. Since nerve endings are the major site of glutamate synthesis from glutamine, GABA and muscimol appear to exert their stimulatory effect at the level of "glutamergic" nerve terminals, probably after interacting with presynaptic GABA receptors. The possible functional significance of these findings is briefly discussed.