Mary C. Wolff

Selective serotonin reuptake inhibitors decrease impulsive behavior as measured by an adjusting delay procedure in the pigeon.

The inability to delay gratification (reinforcement or reward) is one index of impulsive behavior. In order to measure the willingness of pigeons to delay reinforcement, an adjustable delay schedule was developed that allowed daily approximations of an indifference point between immediate brief access to reinforcer and delayed, longer access to reinforcer. Acute administration of the anxiolytic alprazolam (5 mg/kg) decreased the length of delay tolerated before a larger reinforcement. Likewise, acute administration of the anxiolytic chlordiazepoxide (10 mg/kg) produced a similar, although not significant, effect. Neither acute nor five daily injections of 8-OH-DPAT, a 5-HT1A agonist, or WAY100635, a 5-HT1A antagonist, affected the length of the delay period. Chronic (17 day), but not acute injections of the selective serotonin reuptake inhibitors (SSRIs) fluoxetine (10 mg/kg), citalopram (10 mg/kg) and paroxetine (3 mg/kg) increased the delay period. When given in addition to 1 mg/kg of 8-OH-DPAT, but not 1 mg/kg WAY100635, the effect of fluoxetine was accelerated in that the increase in delay was observed earlier in the treatment. These data support the use of SSRIs to decrease impulsive behavior. Addition of a 5-HT1A agonist, but not a 5-HT1A antagonist, to the SSRI may hasten the therapeutic activity of the SSRI in treating impulsivity.

Discovery of Novel Hippocampal Neurogenic Agents by Using an in Vivo Stable Isotope Labeling Technique

Neurogenesis occurs in discrete regions of adult mammalian brain, including the subgranular zone of the hippocampus. Hippocampal neurogenesis is enhanced by different classes of antidepressants, but screening for neurogenic actions of novel antidepressants has been inefficient because of limitations of 5-bromo-2′-deoxyuridine labeling techniques. We describe an efficient in vivo method for measuring hippocampal neurogenesis involving incorporation of the stable isotope, 2H, into genomic DNA during labeling with 2H2O (heavy water). Male rodents received 8 to 10% 2H2O in drinking water; DNA was isolated from hippocampal progenitor cells or neurons. Label incorporation into progenitor cells of Swiss-Webster mice revealed subpopulation kinetics: 16% divided with t1/2 of 2.7 weeks; the remainder did not divide over 1 year. Progenitor cell proliferation rates in mice were strain-dependent. Chronic antidepressant treatment for 3 weeks, with 2H2O administered during the final week, increased progenitor cell proliferation across all the strains tested. Fluoxetine treatment increased 2H incorporation into DNA of gradient-enriched neurons or flow-sorted neuronal nuclei 4 weeks after 2H2O labeling, representing the survival and differentiation of newly divided cells into neurons. By screening 11 approved drugs for effects on progenitor cell proliferation, we detected previously unrecognized, dose-dependent enhancement of hippocampal progenitor cell proliferation by two statins and the anticonvulsant topiramate. We also confirmed stimulatory activity of other anticonvulsants and showed inhibition of progenitor cell proliferation by isotretinoin and prednisolone. In conclusion, stable isotope labeling is an efficient, high-throughput in vivo method for measuring hippocampal progenitor cell proliferation that can be used to screen for novel neurogenic drugs.

Antiemetic effects of 5-HT1A agonists in the pigeon.

Ditolyguanidine (DTG) induced a dose-dependent emetic response in pigeons, with 100% of the birds vomiting after 5.6 mg/kg. Retching and vomiting originally induced by DTG could be conditioned to the test situation. Both the unconditioned and conditioned emetic responses were dose-dependently blocked by 8-hydroxy-(di-n-propylamino)tetralin (8-OH-DPAT) and LY228729, agonists at the 5-HT1A subtype of serotonin receptor, but not by the 5-HT3, antagonist tropisetron. Higher doses (0.25-0.5 mg/kg) of tropisetron exhibited intrinsic emetic activity which could also be prevented by 8-OH-DPAT. NAN-190, a putative 5-HT1A partial agonist, produced both an antiemetic response when administered before DTG and also attenuated the antiemetic effects of 8-OH-DPAT. Pentobarbital blocked the conditioned, but not the unconditioned DTG-induced emesis. These results support the possibility that 5-HT1A agonists exhibit antiemetic activity against a broad range of emetic stimuli, including conditioned vomiting which is usually resistant to pharmacological attenuation.

Comparison of the antiemetic effects of a 5-HT1A agonist, LY228729, and 5-HT3 antagonists in the pigeon

Vomiting may be induced by a variety of agents such as drugs, oncolytics, and provocative motion, as well as being conditioned to occur to environmental stimuli. Such emesis has recently been shown to be blocked by agonists at the 5-HT1A subtype of serotonin receptor. The antiemetic effects of LY228729 [(-)-4-(dipropylamine)-1,3,4,5-tetrahydrobenz-(c,d)indole-6- carboxamide], a 5-HT1A receptor agonist, were tested and compared to the antiemetic effects of the 5-HT3 receptor antagonists ondansetron, tropisetron, and MDL 72222 (3-tropanyl-3,5-dichlorobenzoate). The emetic stimuli tested are known to be blocked by 5-HT3 antagonists in species other than the pigeon. In the pigeon, LY228729 totally abolished vomiting induced by fully emetic doses of cisplatin (10 mg/kg), ipecac (3 ml/kg), emetine (10 mg/kg), and a 5-HT3 agonist, m-(chlorophenyl)-biguanide (1.25 mg/kg). MDL 72222 blocked ipecac-induced vomiting in a dose-related manner and was partially effective in attenuating cisplatin-induced emesis. Ondansetron and tropisetron were partially effective in blocking emetine- and mCPBG-induced vomiting. Ondansetron exhibited an intrinsic emetic response that could not be blocked by MDL 7222, but which was eliminated by LY228729. It was concluded that 5-HT1A agonists are more effective in the pigeon than are 5-HT3 antagonists against these types of emetic stimuli. These results broaden the range of emetic stimuli that are blocked by 5-HT1A agonists in the pigeon.

Differentiation of 5-HT1A receptor ligands by drug discrimination.

Pigeons were trained to discriminate 0.64 mg/kg (high dose) of 8-OH-DPAT (8-hydroxy-(2-di-n-propylamino)tetralin) from saline or were retrained to discriminate 0.16 mg/kg (low dose) of 8-OH-DPAT from saline. This resulted in a decrease of the ED50 for recognition of the 8-OH-DPAT cue from 0.14 to 0.04 mg/kg. Partial agonists for the 5-HT1A receptor (e.g., buspirone) were generalized fully in the low dose condition, but only partially in the high dose condition. Full antagonists, such as N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (WAY-100635), antagonized the 8-OH-DPAT cue in both groups without producing generalization in either group. (-)-Pindolol produced full generalization in the low dose group, but antagonized the high dose stimulus cue. The behavioral effects of other compounds with 5-HT1A receptor activities (4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-pyridinyl-benz ami de hydrochloride (p-MPPI): (-)-1-(1H-indol-4-yloxy)-3-(cyclohexylamino)-2-propanol maleate ((-)-LY206130); racemic pindolol and idazoxan) also differed between groups. Comparing results obtained using differing training doses in the drug discrimination paradigm simplifies determination of the full agonist, partial agonist, or antagonist properties of compounds.

Effects of a 5-HT1A receptor agonist on acute and delayed cyclophosphamide-induced vomiting

LY228729 [(-)-4(dipropylamino)-1,3,4,5-tetrahydrobenz-[c,d]indole-6-carboxa mide]], an agonist at the 5-HT1A subtype of 5-HT receptor, was studied as an antiemetic in pigeons dosed with a highly emetic oncolytic agent, cyclophosphamide. An intramuscular injection of 0.32 mg/kg of LY228729 administered 15 min prior to the intravenous injection of 200 mg/kg of cyclophosphamide totally prevented the acute emetic response induced by cyclophosphamide. When used as a rescue therapy in a separate group of pigeons, LY228729 (0.32 mg/kg, i.m.) prevented further emetic episodes when it was administered after vomiting had already been induced by cyclophosphamide. Injections of LY228729 given at intervals over the next 2 d also attenuated the delayed emetic response induced by cyclophosphamide. LY228729 appears to be a broad spectrum antiemetic agent that is effective against the anticipatory, the acute and the delayed stages of emesis induced by oncolytic agents.

The discriminative stimulus properties of LY233708, a selective serotonin reuptake inhibitor, in the pigeon

Abstract  Rationale: Understanding the contribution of the various serotonin (5-HT) receptor subtypes to the behavioral effects of selective serotonin reuptake inhibitors (SSRIs) may contribute to the discovery of increasingly effective drugs for the treatment of conditions such as depression, panic and obsessive-compulsive disorders. Objectives: A drug discrimination procedure was used to determine whether the administration of an SSRI was associated with a specific interoceptive stimulus cue and to what extent that cue was related to activation of the 5-HT1A receptor. Method: Pigeons were trained to discriminate 20 mg/kg of the short acting, SSRI, LY233708 dihydrochloride dihydrate [(–)-cis-1-(6-chloro-1,2,3,4- tetrahydro-1-methyl-2-naphthalenyl)piperazine] from saline. Results: LY233708 induced a specific, dose-related stimulus cue. The SSRIs, fluoxetine and citalopram, induced dose-related responding on the LY233708-associated key. In contrast, nisoxetine, a selective norepinephrine uptake inhibitor, induced responding on the saline-associated key. The prototypical 5-HT1A agonist, 8-OH-DPAT [8-hydroxy-(2-di-n-propylamino)tetralin] substituted for LY233708. This generalization was blocked by the selective 5-HT1A antagonist, WAY-100635 [N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl) cyclohexanecarboxamide]. Conclusion: These data demonstrate that an SSRI can induce a specific, stable discriminative stimulus that appears to involve activation of the 5-HT1A receptor in the pigeon.

Pharmacological profile of LY301317, a potent and selective 5-HT1A agonist

LY301317 ((4r)‐(−)‐4‐(dipropylamino)‐6‐(5‐oxazolinyl)‐1,3,4,5‐tetrahydrobenz[c,d]indole) has high affinity for the 5‐HT1A receptor and weak affinity for the 5‐HT1D and histamine‐H1 receptors. No significant affinity was found for the other amine receptors studied. In rats, LY301317 produced potent in vivo effects that are characteristic of compounds with agonist activity at the 5‐HT1A receptor, such as an increase in serum corticosterone concentration, a reduction in 5‐HIAA concentration in brain tissue, induction of flat body posture and lower lip retraction (components of a serotonin syndrome), and a decrease of core body temperature. In pigeons trained to discriminate 8‐hydroxy‐2‐(di‐n‐propylamino)‐tetralin (8‐OH‐DPAT) from saline, full generalization to LY301317 was observed. LY301317 increased punished responding in both the pigeon and rat conflict tests for anxiolytic activity. LY301317 reduced immobility in the rat forced swim model used to indicate potential antidepressant activity. In pigeons, LY301317 blocked emesis induced by a chemical (ditolyguanidine), by a 5‐HT3 agonist (m‐(chlorophenyl)‐biguanide), and by an oncolytic agent (cisplatin), as well as vomiting induced by conditioning to environmental stimuli (a model of anticipatory nausea and vomiting). In addition, LY301317 blocked cisplatin‐ and ipecac‐induced vomiting in the dog and motion‐induced emesis in the cat. It was concluded that LY301317 is an orally active, potent, and selective agonist for the 5‐HT1A receptor with potential clinical utility as an anxiolytic, an antidepressant, and a broad‐spectrum antiemetic. Drug Dev. Res. 40:17–34, 1997.

Selective serotonin reuptake inhibitors potentiate 8-OH-DPAT-induced stimulus control in the pigeon

The effects of two selective serotonin reuptake inhibitors, fluoxetine and citalopram, and a nonselective monoamine reuptake inhibitor, imipramine, were characterized in pigeons that had been trained to discriminate 0.64 mg/kg of 8-hydroxy-(2-di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), a 5-HT1A receptor agonist, from saline. Neither fluoxetine, citalopram, nor imipramine generalized to the 8-OH-DPAT-induced stimulus cue. However, when administered in addition to 8-OH-DPAT, both fluoxetine (10 mg/kg) and citalopram (10 mg/kg) lowered the ED50 for generalization of 8-OH-DPAT from 0.16 mg/kg (8-OH-DPAT by itself) to 0.05 mg/kg (fluoxetine + 8-OH-DPAT) and 0.06 mg/kg (citalopram + 8-OH-DPAT). Under similar conditions, imipramine (1 mg/kg) had no effect on the generalization curve for 8-OH-DPAT. The data support the hypothesis that activation of the 5-HT1A receptor may be relevant to the mechanism of action of serotonin reuptake inhibitors.