Francis J. White

The discriminative stimulus properties of LSD: Mechanisms of action☆

Abstract The CNS correlates of d -lysergic acid diethylamide (LSD) actions were investigated by means of a sensitive and specific behavioural assay procedure. Rats were trained to discriminate D-LSD from saline in two lever operant chambers. After intraperitoneal injections of 0.08 mg/kg of LSD, reinforcement could be obtained only by responding on one particular lever (left or right); responding on the opposite lever was reinforced only after saline injections. Once the LSD and saline exerted reliable stimulus control over lever choice (85% correct), rats were tested with various agents which alter the functional activity of the transmitter-containing neuronal systems. l -Tryptophan, chlorimipramine and fluoxetine, all of which increase the synaptic availability of serotonin, neither blocked nor, by themselves, mimicked (transferred to) LSD. However, quipazine, a putative serotonin agonist, produced an almost complete (78%) transfer to LSD. The presumed serotonin antagonists: methiothepin, 2-bromo-D-lysergic acid diethylamide, methysergide, cyproheptadine and cinanserin, blocked the LSD cue in a dose-related manner. The potent dopamine antagonists: haloperidol, fluphenazine, trifluoperazine, α-flupenthixol, and (+)-butaclamol, were without effect on the discriminability of LSD as were α- and β-adrenergic, cholinergic and histaminergic antagonists. Apomorphine, a dopamine agonist, produced a partial transfer to LSD. These data indicate that the discriminative stimulus properties of LSD may be mediated by post-synaptic serotonin or LSD receptors. Dopamine receptors did not seem to be involved in the LSD stimulus cue.

A neuropharmacological analysis of the discriminative stimulus properties of fenfluramine

Rats were trained to discriminate fenfluramine (1.0 mg/kg) from saline in a two-lever drug discrimination task. The dose-response curve for this discrimination was orderly with an ED50 of about one-half of the training dose (0.52 mg/kg). In substitution tests, indirect (p-chloroamphetamine) and direct (quipazine, MK-212, lisuride) serotonin (5-HT) agonists substituted for fenfluramine. Since none of these compounds have been reported to be hallucinogenic and the potent hallucinogen LSD did not substitute completely, it was suggested that the discriminative stimulus properties of fenfluramine are not related to its ability to produce hallucinations in humans. The fenfluramine cue, like the quipazine cue, was antagonized by the 5-HT antagonists cyproheptadine and methiothepin. Unlike quipazine, fenfluramine was also partially antagonized by the 5-HT uptake inhibitor, fluoxetine, and the 5-HT synthesis inhibitor, p-chlorophenylalanine. Thus, the fenfluramine cue differs from that of quipazine in that it is mediated via indirect actions on 5-HT receptors. Since the indirect dopamine (DA) agonist d-amphetamine failed to substitute and the DA antagonist haloperidol failed to block the fenfluramine cue, a mediating role for DA was not indicated. Another indirect DA agonist, cocaine, substituted partially for fenfluramine, a result which paralleled that seen with fluoxetine. Both of these partial substututions were reduced by cyproheptadine; therefore, it was concluded that these effects may be due to the common ability of cocaine, fluoxetine, and fenfluramine to inhibit 5-HT uptake.

Discriminative stimulus properties of quipazine: Direct serotonergic mediation

Rats were trained to discriminate intraperitoneal injections of quipazine (1.0 mg/kg) from saline in a two-lever drug discrimination task. After reliable levels of accuracy (> 85%) were attained, various agents which alter the functional activity of serotonergic (5-HT) neuronal systems were tested either alone or in combination with quipazine. Neither the 5-HT precursor, l-tryptophan, nor the 5-HT re-uptake inhibitors, fluoxetine and citalopram (Lu 10-171), either alone or in combination, elicited responding on the quipazine-appropriate lever. However, fenfluramine, a drug which releases endogenous 5-HT from nerve terminals, produced 84% quipazine-like responding. The dopamine (DA) agonists, apomorphine and amphetamine, did not transfer to quipazine. Of the several neurotransmitter antagonists tested, those which block 5-HT were also found to block the quipazine cue. Para-chlorophenylalanine (PCPA), which depletes central 5-HT by inhibiting its synthesis, significantly potentiated a sub-threshold dose (0.25 mg/kg) of quipazine and reduced the transfer of fenfluramine to quipazine, suggesting that quipazine exerts its discriminatory effect by directly stimulating central 5-HT receptors. Since the potent hallucinogen, d′lysergic acid diethylamide (LSD). has been shown to transfer to a relatively high dose of quipazine (2.5 mg/kg), this compound and other known hallucinogens were tested for transfer to the dose of quipazine (1.0 mg/kg) used in the present experiment. Mescaline, LSD, and psilocybin all produced strong transfers to the quipazine cue. These data suggest the possibility that quipazine may possess hallucinogenic properties.

Discriminative stimulus properties of quipazine

Abstract Rats were trained to discriminate quipazine (2.5 mg/kg) from saline in a two-lever, drug discrimination task. After high levels of accuracy (i.e. 90% correct choice responding) were attained, dose and time parameters of the quipazine cue were studied during extinction test sessions. Quipazine was highly discriminable at doses of 1.25 and 0.94 mg/kg. With decreasing doses, the percentage of responding on the quipazine lever declined in a dose-related fashion. By varying the normal 30-min time interval between injection and testing, the onset and duration of action of quipazine were determined to be about 5–15 min and 60–90 min respectively. After injections with novel drugs, animals were tested during extinction sessions. Lysergic acid diethylamide, (LSD), at doses of 0.08 mg/kg and 0.10 mg/kg, transferred completely to the quipazine cue (i.e. above 95% quipazine responding); neither d -amphetamine (1.0 and 2.0 mg/kg) nor apomorphine (0.5 and 1.0 mg/kg) showed any transfer to the quipazine cue. By administering various neurotransmitter blockers in conjunction with quipazine and testing during extinction sessions, it was found that the putative central 5-HT antagonists cyproheptadine (1.0 mg/kg), methysergide (1.0 mg/kg), and methiothepin (1.0 mg/kg) produced significant decreases in the discriminability of quipazine. The dopamine antagonists haloperidol (0.05 and 0.10 mg/kg) and fluphenazine (1.0 mg/kg) were without effect. It was concluded that LSD and quipazine have a common mechanism of action which may (or may not) involve 5-HT; a role for DA was not indicated.

Dopaminergic and serotonergic mediation of the discriminable effects of ergot alkaloids.

The involvement of dopamine (DA) and serotonin (5-HT) neuronal systems in the discriminative stimulus effects of various ergot derivatives was assessed by administering four ergots to 36 rats which had been trained to discriminate either apomorphine (APO) or d-lysergic acid diethylamide (LSD) from saline. Lergotrile, lisuride and LSD substituted for APO (0.25 mg/kg) while bromocriptine and ergonovine (ergometrine) did not; only lisuride mimicked LSD (0.08 mg/kg). Antagonism tests showed that the DA antagonist haloperidol but not the 5-HT antagonist BC-105 (pizotifen) blocked the APO cue; both the LSD cue and the substitution of LSD for APO were blocked by BC-105 but not by haloperidol. It was concluded that DA receptor activation plays a prominent role in the discriminative stimulus effects of lergotrile and lisuride as well as APO and a secondary role in the LSD cue; 5-HT seems to be of major importance in the mediation of the effects of LSD and, to a lesser extent, lisuride. The functions of the two monoamines in the discriminable effects of bromocriptine and, particularly, ergonovine are less clear.

Lack of specificity of an animal behavior model for hallucinogenic drug action

It has been proposed recently that the occurrence of drug-induced limb-flicking (LF) and abortive grooming (AG) in cats can serve as a viable animal behavior model for the actions of hallucinogens in humans. If this is the case, such behaviors should occur reliably following the administration of drugs that produce hallucinations in humans and should not occur after administration of other, non-hallucinogenic drugs--a hypothesis that was examined in the present experiment. The frequency of LF and AG were observed in 12 cats which were given a wide range of doses of the potent hallucinogen, d-LSD (0.01-0.16 mg/kg), as well as several other compounds. The results showed that three non-hallucinogenic agents which are related to LSD in various ways, the ergot derivative lisuride, the serotonin agonist, quipazine, and the dopamine agonist, apomorphine, significantly increased LF frequency. Lisuride and quipazine also caused AG. Cocaine did not elicit either behavior. Thus, it was concluded that the proposed model cannot be regarded as specific to hallucinogenic drugs. In addition, the frequency of these behaviors, as well as their reliability and robustness, were shown to be party dependent on the environment in which observation occurs.

The effect of serotonin depletion on the discriminability of LSD.

Nine groups of rats were trained to discriminate LSD (0.12 mg/kg) from saline in a two-lever, water-reinforced, drug discrimination procedure. After stable discriminative performance was obtained (>95% correct), groups were administered one of several treatments which lower the concentration of serotonin (5-HT) in brain: (1) 12.5, 25, 50, 100 or 200 microgram of 5,7-dihydroxytryptamine (5,7-DHT) intraventricularly (IVT); (2) 3 X 100 mg/kg of p-chlorophenylalanine (PCPA) intraperitoneally (IP); or (3) 20 mg/kg of p-chloroamphetamine (PCA) IP. Control rats received either IVT injections of 5,7-DHT vehicle or IP injections of PCA or PCPA vehicles. Beginning 12 days after treatment, lever preference following various doses of LSD was determined. The results indicated that only the 200 microgram dose of 5,7-DHT and PCPA caused a significant potentiation of LSD-lever responding at the 0.03 mg/kg dose of LSD while all treatments except 12.5 and 25 microgram of 5,7-DHT resulted in significant depletion of 5-HT. Moreover, amount of 5-HT and percent LSD responding following 0.03 mg/kg LSD were not significantly correlated. It was concluded that 5-HT depletion, per se, cannot account for supersensitivity to the behavioral effects of LSD.

Discriminative Stimulus Properties of Hallucinogens: Behavioral Assay of Drug Action

In this chapter we will review existing literature on the discriminative stimulus properties of several drugs which are often classified as hallucinogens. At the same time we ‘will try to answer several questions about the usefulness of this procedure. For example, can it be used as a specific behavioral assay or screening technique for hallucinogens? Can the mechanism of action of hallucinogenic drugs be effectively studied and, if so, can it tell us anything about the nature of hallucinations?

Behavioral and biochemical evidence for serotonergic actions of tetrahydro-β-carbolines

Abstract In two groups of rats trained to discriminate LSD (0.1 mg/kg or 0.24 mg/kg) from saline, tetrahydro-β-carboline (THBC; 1–12 mg/kg as free base) and its derivative 6-methoxy-THBC (1–12 mg/kg as free base) substituted partially for LSD. The substitution of THBC for 0.1 mg/kg of LSD was analyzed further with antagonism tests in 16 animals and was attenuated by the serotonin (5-HT) antagonist BC-105 (pizotifen; 3 mg/kg) but not by the dopamine (DA) antagonist haloperidol (0.1 mg/kg). It was abolished by pre-treatment with the 5-HT synthesis inhibitor p-chlorophenylalanine (100 mg/kg/day for 3 days). In addition, THBC was found to inhibit 3 H-LSD binding to homogenates of rat frontal cortex with an IC 50 value of 4 μM which is similar to that previously reported for other 5-HT agonists. These data indicate that THBCs exert potent 5-HT agonist actions. Since THBCs have recently been found in mammalian brain and other tissues, the present results are of interest in relation to a possible role of these substances in endogenous psychosis.

Antagonism of a behavioral effect of LSD and lisuride in the cat

These experiments investigated the role of serotonin (5-HT) and dopamine (DA) receptors in the limb-flick (LF) response elicited by the hallucinogenic ergot LSD and its nonhallucinogenic structural congener lisuride. Pretreatment with either the 5-HT antagonist pizotifen (BC-105) or the DA antagonist haloperidol significantly attenuated LF elicited by either LSD or lisuride. Thus, the LF model failed to differentiate the neuropharmacological actions of LSD and lisuride. Cocaine also prevented LSD- and lisuride-elicited LF simply by reducing the activity of cats (response competition) suggesting the need for caution in interpreting ‘antagonism’ of the LF response.