John A. Rosecrans

Antagonism of the effects of the hallucinogen dom and the purported 5-HT agonist quipazine by 5-HT2 antagonists

Rats trained to discriminate 1.0 mg/kg of 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) from saline in a two-lever operant choice task were administered doses of mescaline, LSD, 5-methoxy-N,N-dimethyltryptamine (5-OMe DMT), quipazine, TFMPP and RU-24969. The DOM-stimulus generalized to the three hallucinogenic agents and to quipazine, but not to the purported serotonin agonists TFMPP or RU-24969. Pretreatment of the animals with the 5-HT2 antagonists ketanserin and pirenperone antagonized the effect produced by DOM. Pirenperone also blocked DOM-stimulus generalization to mescaline, LSD, 5-OMe DMT and quipazine. The results of this study suggest that the discriminative stimulus effects of DOM, the three hallucinogenic agents to which DOM-stimulus generalization occurred, and quipazine, may involve those sub-populations of serotonin receptors that are labeled by tritiated ketanserin (i.e. 5-HT2 sites).

Increase in the activity of tryptophan hydroxylase from cortex and midbrain of male Fischer 344 rats in response to acute or repeated sound stress.

Exposure of male Fischer 344 rats to an acute sound stress consisting of 100 dB tones of 2-s duration presented at random 60-s intervals for 2 h, increased cortical and midbrain tryptophan hydroxylase activity, measured in vitro, 50% over that from sham-stressed animals. This increase in enzyme activity was observed when animals were killed immediately, but not 1 h, after termination of the sound stress. It was non-additive with the increase in activity induced by incubation of enzyme under phosphorylating conditions and could be reversed in vitro with alkaline phosphatase. Graded increases in enzyme activity were obtained with increments of sound intensity (90-120 dB). In contrast to acute stress, chronic sound stress (110 dB) repeated over a period of 1, 2 or 6 weeks (3 sessions per week each of 2-h duration) produced a 50% increase in cortical enzyme activity that persisted 24 h after the termination of the stress and was not reversed by alkaline phosphatase. However, a further increase in enzyme activity could be produced if the chronically stressed animals were exposed to an acute 2-h stress (110 dB) immediately before being killed. This additional increase in activity was reversible in vitro by alkaline phosphatase and non-additive with that produced by incubation under phosphorylating conditions. In summary, acute sound stress produced a prompt, reversible activation of tryptophan hydroxylase. Repeated exposure to sound stress induced a persistent increase in enzyme activity that was detected 24 h after the last stress.

Pharmacological effects of epibatidine optical enantiomers

The pharmacology of synthetic D- and L-epibatidine, an alkaloid originally characterized from frog skin, were studied in different behavioral tests in mice and rats. The two enantiomers have potent antinociceptive activity in mice using the tail-flick test, with an ED50 of 6.1 and 6.6 micrograms/kg for L- and D-epibatidine respectively. Epibatidine enantiomers were 200 x more potent than L-nicotine as an antinociceptive agent in mice after s.c. administration. Their analgesic effect was blocked by mecamylamine but not naloxone, an opiate antagonist. Both D- and L-epibatidine have high affinity (Ki 54.7 and 55.0 pM, respectively) for [3H]nicotine binding site in rat brain. In addition, they reduced mice locomotor activity and body temperature in a dose-dependent manner. In rats trained with nicotine (0.4 mg/kg), epibatidine enantiomers engendered nicotine-like responding in a dose-related manner with an ED50 of 1.00 and 0.93 micrograms/kg for D and L, respectively. The discriminative effect of L- and D-epibatidine in rats was blocked by mecamylamine but not by hexamethonium. As in binding results, there was no significant enantioselectivity for these effects in our study.

Studies on the time course and the effect of cholinergic and adrenergic receptor blockers on the stimulus effect of nicotine

This study investigated the stimulus property of nicotine in the rat. The primary objectives of the study were 1. to determine the time course of the nicotine stimulus and its relationship to brain levels of the drug and 2. to determine whether the nicotine stimulus is dependent upon the integrity of specific neurotransmitter systems. A lever choice discrimination was used. After injection of nicotine, depression of one lever in an operant test chamber resulted in food reinforcement according to a variable interval schedule of 15 sec. When saline was administered, the opposite lever was reinforced. A high degree of discriminated responding was observed when either 400 Μg/kg or 200 Μg/kg of nicotine was used as a discriminative stimulus. The degree of discrimination decreased as the length of the time period between the injection of nicotine and the test of discrimination was increased. This decline in discrimination was similar to the decline in brain levels of nicotine suggesting that nicotine discrimination is directly related to the concentration of nicotine in the brain. Atropine, mecamylamine, dibenamine, propranolol and α-methyl-para-tyrosine (AMPT) were all tested, in a range of doses, for effects upon nicotine discrimination. Of these, only mecamylamine antagonized the nicotine stimulus. These results indicate that the stimulus effect of nicotine is mediated specifically through nicotinic-cholinergic receptors and not muscarinic-cholinergic or adrenergic receptors.

Morphine and ?9-tetrahydrocannabinol: Tolerance to the stimulus effects

Morphine and saline served as discriminative Stimuli for one group of rats in a 2-lever discrimination task. δ9-THC and vehicle were discriminative stimuli for a second group. Depression of one lever resulted in reinforcement following the administration of morphine or δ9-THC and the opposite lever was reinforced after vehicle. A high degree of discriminated responding occurred with both drugs. During daily supplemental injections of morphine or δ9-THC up to several times the training dose for a period of 2 months, subjects still discriminated morphine or δ9-THC from vehicle. However, the degree of discrimination was reduced indicating a limited tolerance to either drug. Naloxone precipitated narcoticlike withdrawal symptoms in rats chronically treated with either morphine or δ9-THC, indicating that there is an interaction between chronic administration of δ9-THC and the narcotic antagonist, naloxone.

D-amphetamine as a discriminative cue: drugs with similar stimulus properties.

Abstract Rats were trained to choose between the side compartments of a 3-chambered shock-escape apparatus according to whether they were injected (i.p.) with 4.0 mg/kg d-amphetamine or 0.9% saline. The d-amphetamine drug state acquired all the properties of a discriminative stimulus by producing interoceptive cues. Doses of 0.4 mg/kg nicotine, 7.4–29.7 mg/kg mescaline, 4.0–8.0 mg/kg fenfluramine and 0.048 mg/kg LSD failed to produce an amphetamine-like cue. However, l-amphetamine, 8.0 mg/kg, produced responses shown to be statistically similar to the 4.0 mg/kg training dose of d-amphetamine.

Effects of medial raphe and raphe magnus lesions on the analgesic activity of morphine and methadone

The effects of lesions of the raphe nuclei on opiate-induced antinociception and brain serotonin (5-HT) levels were investigated. Lesions of the medial raphe nucleus effectively antagonized the analgesic effects of morphine, but not methadone, and lowered brain 5-HT. The decrement in analgesic activity of morphine was reversed by pretreatment with 5-hydroxytryptophan. Lesions of the raphe magnus, a descending 5-HT system, antagonized the analgesic potency of both morphine and methadone. These experiments indicate a differential effect of 5-HT manipulation on opiate-induced analgesia, suggesting a different mechanism of analgesic action for morphine and methadone.

Discriminative stimulus properties of buspirone compared to central nervous system depressants in rats.

Two groups of rats were trained to discriminate either IP buspirone from vehicle or IP oxazepam from vehicle using a two-lever FR-10 schedule of sweetened milk presentation. The discrimination in the buspirone group was difficult to establish due to potent response rate decreasing effects. Oxazepam was a very effective stimulus and the discrimination in the oxazepam group was readily established. Tests with oxazepam and pentobarbital in the buspirone group provided no evidence for generalization of the stimulus properties of buspirone to either drug. Tests with buspirone in the oxazepam group also provided no evidence of generalization to buspirone, although tests with pentobarbital indicated almost complete generalization. Finally buspirone and oxazepam were tested in a group of rats previously trained to discriminate pentobarbital from vehicle. The findings supported the data obtained in the buspirone and oxazepam groups, with no indication of generalization from pentobarbital to buspirone, but with complete generalization to oxazepam. These data suggest that buspirone does not share discriminative stimulus properties which are common to other CNS depressants.

Lack of effect of naloxone on autoanalgesia.

Autoanalgesia (behaviorally-induced antinociception) may be elicited by acute stress or clasically conditioned fear. Antinociception within both of these paradigms is reportedly associated with increased CNS opioid peptide activity. Large doses of naloxone (20 mg/kg) failed to modify antinociception elicited by acute footshock or conditioned fear in rats. Naloxone (4 mg/kg) was also ineffective against antinociception following footshock in mice. These data suggest that if an endorphin does mediate autoanalgesia, the affinity of its receptor for naloxone is very low. Alternatively, parallel opioid and non-opioid systems may be activated by autoanalgesic procedures, with antagonism of the opioid component being insufficient to reduce the antinociception.

Discriminative stimulus properties of the optical isomers of nicotine

Rats were trained to discriminate 200 or 400 μg/kg (-)nicotine from saline in a two-bar operant paradigm. Dose-response relationships for optically pure (-)- and (+)nicotine as well as antagonistic effects were examined in both groups of rats. The natural isomer (-)nicotine was approximately nine-times more potent than (+)nicotine. Mecamylamine produced equal blockade of the (-)- and (+)nicotine cues. Hexamethonium and atropine were without effect. These data demonstrate the possible stereospecificity of the central nicotinic receptor that mediates the stimulus effect produced by nicotine.