Agneta Wijkström

Behavioral and biochemical effects of the 5-HT1A receptor agonists flesinoxan and 8-OH-DPAT in the rat

The 5-HT1A receptor agonists flesinoxan (0.2-3.2 mg kg-1 s.c.) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.025-0.4 mg kg-1 s.c.) produced (1) a dose-dependent facilitation of male rat ejaculatory behavior and (2) characteristic, dose-dependent effects on spontaneous motor activity. Thus, total locomotor activity and rearing activity were decreased. However, forward locomotion and peripheral locomotion were increased relative to the total horizontal activity. Furthermore, (3) 5-HTP accumulation, after inhibition of cerebral decarboxylase, was dose dependently decreased by both compounds in the ventral striatum and in the prefrontal cortex. There was a statistically significant decrease in DOPA accumulation in the ventral striatum after administration of a high dose of flesinoxan (3.2 mg kg-1), and a tendency for 8-OH-DPAT to produce the same effect. The efficacy of the compounds to affect male rat sexual behavior, spontaneous motor activity in the open-field and forebrain 5-HT synthesis was approximately the same, whereas flesinoxan was about an order of magnitude less potent than 8-OH-DPAT.

Evidence for selective inhibition of limbic forebrain dopamine synthesis by 8-OH-DPAT in the rat

SummaryRegional dopamine synthesis in the rat striatum was estimated by measuring DOPA accumulation following inhibition of cerebral aromatic l-amino acid decarboxylase by means of NSD-1015, 100 mg kg−1 intraperitoneally. In animals treated with reserpine, 5 mg kg−1 subcutaneously −18 h, there was a statistically significant increase in DOPA accumulation in the nucleus accumbens, the ventro-medial neostriatum, the dorso-lateral neostriatum and in the posterior limb of the neostriatum. This increase in DOPA accumulation was antagonized dose-dependently in the nucleus accumbens and ventro-medial neostriatum, but not in the other two regions, by treatment with the 5-HT1A receptor agonist 8-OH-DPAT, 0.15–2.4 μmol kg−1, whereas the partial dopamine D2 receptor agonist (−)3-PPP, 2.5–10.0 μmol kg−1, or the full dopamine D2 receptor agonist quinpirole, 0.05–0.8 μmol kg−, antagonized the reserpine-induced increase in DOPA accumulation uniformly in all four regions of the striatum. The suppression of DOPA accumulation by 8-OH-DPAT in reserpine-treated animals, was completely antagonized by raclopride, 1 μmol kg−1, but not by (−)pindolol, 8 μmol kg−1. The accumulation of 5-HTP in all regions of the striatum as well as in the neocortex following decarboxylase inhibition and reserpine pretreatment, was also inhibited by 8-OH-DPAT, and this inhibition was unaffected by treatment with raclopride or (−)pindolol. It is concluded that 8-OH-DPAT, in addition to general effects on forebrain 5-hydroxytryptamine synthesis, selectively affects limbic forebrain dopamine synthesis. This latter effect is probably due to direct stimulation of dopamine autoreceptors, since it was obtained in reserpine-treated rats, and was completely antagonized by raclopride, but not (−)pindolol.

Increased dopamine turnover in the ventral striatum by 8-OH-DPAT administration in the rat

Abstract— The administration of the 5‐HT1A agonist 8‐OH‐DPAT (0.8 μmol kg−1 s.c. — 40 min) produced an increase in dopamine (DA) turnover, estimated by the quotient (DOPAC + HVA) DA−1, in the ventral striatum of the rat. No statistically significant effects were obtained in the dorsal striatum. The accumulation of 3‐MT in pargyline‐treated animals (375 μmol kg−1 s.c.— 60 min) was not affected by 8‐OH‐DPAT treatment (0.15‐2.4 μmol kg−1 s.c.‐30 min). These findings indicate that 8‐OH‐DPAT has weak antagonist properties at striatal DA receptors in normal rats. Both the 5‐HT1A agonist flesinoxan (0.06–17.8 μmol kg−1 s.c. — 50 min) and the mixed 5‐HT1 and 5‐HT2 agonist 5‐MeODMT (1.6–26.0 μmol kg−1 s.c. −50 min) produced a decrease in forebrain 5‐HTP accumulation (striatum and neocortex), following decarboxylase inhibition by means of NSD‐1015 in reserpine treated rats, indicating stimulation of central 5‐HT receptors by these two compounds. At the same time, the DOPA accumulation in the ventral striatum was decreased by flesinoxan and increased by 5‐MeODMT treatment. These observations show that, under these conditions, the decrease in DA synthesis is not directly coupled to the decreased 5‐HT synthesis produced by flesinoxan, as previously demonstrated for 8‐OH‐DPAT. Taken together with previous observations, the present results suggest that 8‐OH‐DPAT, depending on the experimental conditions, is an agonist or antagonist at striatal DA receptors, in all probability due to partial DA receptor agonist properties of the compound.

Comparison of the effects of haloperidol, remoxipride and raclopride on pre- and postsynaptic dopamine receptors in the rat brain

SummaryThe ability of the dopamine receptor antagonists haloperidol, raclopride and remoxipride to prevent the B-HT 920-induced decrease in striatal and limbic L-DOPA accumulation in gamma-butyrolactone (GBL)- and NSD 1015-treated rats (termed ‘GBL-reversal’) was used to define the effects of these compounds on “presynaptic” dopamine receptors. The doses of the dopamine antagonists producing antagonism of GBL-reversal were in each case roughly similar to the doses required to increase dopamine turnover in striatal and limbic areas. The potencies of haloperidol, raclopride and remoxipride in the GBL model were compared with their potencies in behavioural models for postsynaptic dopamine receptors. Haloperidol produced antagonism of GBL-reversal over a similar dose range to that required for antagonism of apomorphine-induced hyperactivity and stereotypy syndromes. Raclopride was effective in the order of potency: antagonism of apomorphine-induced hyperactivity > antagonism of GBL-reversal > antagonism of apomorphine-induced stereotypy. For remoxipride, the dose-response curve for antagonism of GBL-reversal was superimposable over that for antagonism of apomorphine-induced stereotypies, with an ED50 value about 12 times higher than that for antagonism of apomorphine-induced hyperactivity. Thus, the relative potencies of dopamine receptor antagonists at “pre-” and postsynaptic dopamine receptors vary considerably from compound to compound.

Plasma protein binding of sulphadiazine, sulphamethoxazole and trimethoprim determined by ultrafiltration

The plasma protein binding of trimethoprim, sulphadiazine and sulphamethoxazole was studied at 37 degrees C by ultrafiltration. Plasma samples contained steady state levels of the drugs from ten volunteers from a cross-over comparative pharmacokinetic study on co-trimazine and co-trimoxazole. The three compounds were determined in plasma and ultrafiltrate by HPLC, the recoveries being close to 100% in each case. Freezing of spiked samples had no influence on the binding. Trimethoprim was 48.5-52.2% bound (mean 50.0%); sulphadiazine was 50.9-60.6% bound (mean 56.2%); and sulphamethoxazole was 74.3-80.8% bound (mean 76.9%). The significantly lower protein binding of sulphadiazine compared to sulphamethoxazole means that equivalent non-protein bound plasma levels of the two sulphonamides are achieved from smaller doses of co-trimazine than co-trimoxazole. Use of co-trimazine may thus minimize the risk of adverse reactions.

Partial dopamine receptor agonists with different degrees of intrinsic activity within a series of 2-(4-aminophenyl)-N,N-dipropylethylamine derivatives: synthetic chemistry and structure-activity relationships

Summary A series of 2-(4-aminophenyl)- N,N -dipropylethylamine derivatives were synthesized and tested for in vivo intrinsic activity at brain dopamine receptors in the rat. Differences in the sensitivity of dopamine receptors pre- and post-synaptically in the reserpine-treated rat were used to estimate the intrinsic activity of the various compounds as dopamine receptor agonists. Thus, the ability of the compounds to antagonize reserpine-induced increase in neostriatal dopamine synthesis and the suppression of spontaneous locomotor activity were taken as pre-and post-synaptic indices, respectively. The compounds in the present series display a gradient of intrinsic activity depending on the substituents in the aromatic ring. The presence of an amino group or an appropriate acylamino group in the 4-position was found to be critical for the biological activity of these compounds as agonists or antagonists. The introduction of halogen or a trifluoromethyl group in the 3-position resulted in high intrinsic activity (ie, agonist activity). The incorporation of a methyl group in the 3-position or halogens in the 3,5-positions resulted in a gradual decrease in intrinsic activity at rat brain dopamine receptors resulting in a series of compounds ranging from a full agonist to dopamine receptor blockade.

Stimulation of brain dopamine autoreceptors by remoxipride administration in reserpine‐treated male rats

Abstract— Male Sprague‐Dawley rats were treated subcutaneously with reserpine (5 mg kg−1, — 18 h) and with the aromatic amino acid decarboxylase inhibitor, NSD‐1015 (3‐hydroxybenzylhydrazine) (100 mg kg−1 ‐30 min). Remoxipride 0 ‐8, 3 ‐2 or 12 ‐8 mg kg−1 was administered subcutaneously at ‐50 min. Immediately following decapitation (0 h), the ventral striatum and the anterior neocortex were dissected. Dopa and 5‐hydroxytryptophan accumulation in these brain areas were analysed by HPLC with electrochemical detection. Reserpine produced a marked increase in striatal and neocortical dopa accumulation, in comparison with glucose vehicle + NSD‐1015‐treated controls, and this increase was dose‐dependently antagonized by remoxipride treatment. Thus, together with demonstrated dopamine receptor antagonist actions in intact animals, remoxipride behaves as a mixed dopamine receptor agonist‐antagonist. Such properties could contribute to the favourable endocrine and extrapyramidal side effect profile of remoxipride as an antipsychotic agent.

In vivo characterization of novel full and partial 2-(4-aminophenyl)-N, N-dipropylethylamine dopamine D2 receptor agonists

Behavioral and biochemical techniques were used to compare the in vivo intrinsic efficacy of two new 2-(4-aminophenyl)-N, N-dipropylethylamine dopamine D(2) receptor agonists, 2-(4-amino-3-trifluoromethylphenyl)-N-N-dipropyl-ethylamine (NBF-203) and 2-(4-amino-3-bromo-5-trifluoromethylphenyl)-N-N-dipropylethylamine (NBF-234). Adult male Sprague-Dawley rats were used as experimental animals. NBF-203 was characterized as a full dopamine D(2) receptor agonist, whereas NBF-234 displayed properties of a partial agonist, or antagonist, at dopamine D(2) receptors. Thus, NBF-203 produced effects similar to those of apomorphine in models for dopamine synthesis, release and turnover. As a strong indication of markedly less intrinsic efficacy, the administration of NBF-234 did not result in antagonism of reserpine-induced suppression of locomotor activity in the presence of (+/-)-1-phenyl-2,3,4,5, -tetrahydro-(1H)-3-benzazepine-7,8-diol HCl (SKF-38393)-induced dopamine D(1) receptor activation. The present series of compounds offer the possibility of adjusting intrinsic efficacy at dopamine D(2) receptors, and such fine-tuning could be an important strategy in the search for optimal antipsychotic or antiparkinson drugs within the partial dopamine D(2) receptor agonist concept.

Quinpirole — A 5-HT receptor antagonist?

The rate of brain monoamine synthesis was estimated in the rat by measuring the accumulation of dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) following inhibition of cerebral aromatic amino acid decarboxylase by NSD-1015 (475 mumol/kg, i.p., 30 min before decapitation). As expected, pretreatment with reserpine, (8.2 mumol/kg, s.c., -18 h) produced a marked and statistically significant increase in the DOPA accumulation in the ventral striatum and in the neocortex, whereas only minor changes were noted in 5-HTP accumulation in the same brain areas. The administration of terguride or quinpirole (30 mumol/kg, s.c., -65 min) resulted in both cases in an antagonism of the reserpine-induced increase in the DOPA accumulation. The effect was less marked in the neocortex than in the ventral striatum, but there was no difference between the effects produced by either compound. In contrast, the two drugs produced opposite effects on the 5-HTP accumulation in the ventral striatum as well as in the neocortex. Thus, there was a decrease and an increase in the 5-HTP accumulation by terguride and quinpirole administration, respectively. Together the results suggest that, in the reserpine treated rat, both terguride and quinpirole to the same degree stimulate dopamine receptors in the ventral striatum and noradrenaline receptors in the neocortex. To the extent that serotonin receptors in these two brain areas mediate the effects on 5-HTP accumulation of terguride and quinpirole, respectively, these receptors appear differently affected by the two compounds: stimulation by terguride and blockade by quinpirole.