Clifton W. Callaway

The carbachol-induced enhancement of desynchronized sleep signs is dose dependent and antagonized by centrally administered atropine.

Considerable data show that microinjection of carbachol into the pontine reticular formation produces a desynchronized (D) sleep-like state. The present study examined the hypothesis that this carbachol-induced enhancement of D sleep signs is mediated by muscarinic, cholinergic receptors. This hypothesis was tested by quantifying the dose-dependent effects of centrally administered carbachol on the D sleep-like state and by pretreating the animals with centrally administered atropine. Six dosages of carbachol were microinjected into the pontine reticular formation of conscious cats and polygraphic measures of behavioral state were recorded. The percentage, latency, duration, frequency, and time course of the carbachol-induced D sleep-like state were dose dependent. Centrally administered atropine competitively antagonized the ability of carbachol to induce the D sleep-like state, whereas pontine administration of L-glutamate did not significantly alter D sleep. These data demonstrate that muscarinic, cholinergic receptors within the pontine reticular formation mediate the phenomenon of cholinoceptive D sleep sign enhancement.

Serotonin1B receptor activation mimics behavioral effects of presynaptic serotonin release

The locomotor hyperactivity induced by 3,4-methylenedioxymethamphetamine (MDMA) and related drugs in rats appears to be due to the drug-induced release of presynaptic serotonin (5-HT). Thus, these drugs increase locomotor activity by acting as indirect 5-HT agonists. The subtype of 5-HT receptor upon which this released 5-HT acts postsynaptically to produce the activating effect of MDMA-like drugs is not known. When tested under conditions in which MDMA increases locomotion, direct agonists at both 5-HT1A and 5-HT1C12 receptors consistently decrease locomotion. Hence, the present experiments tested the hypothesis that the hyperactivity produced by the release of endogenous 5-HT is due to the activation of 5-HT1B receptors. Using the Behavioral Pattern Monitor (BPM), the profile of behavioral effects of a 5-HT1B agonist, 5-methoxy-3(1,2,3,6)tetrahydropyridin- 4yl)-1H-indole (RU 24969), was compared to that previously described for MDMA and related indirect 5-HT agonists. The BPM provided detailed information regarding the amount and qualitative patterning of locomotor activity and investigatory responses in rats. Various doses of RU 24969 (1.25 to 5 mg/kg) were administered to naive male rats 10 minutes prior to placement in the test chambers. As previously reported for MDMA, locomotor activity increased with dose, and investigatory rearings and holepokes decreased. The hyperactivity was characterized by repetitive spatial patterns of locomotion that were qualitatively similar to those produced by indirect 5-HT agonists such as MDMA and dissimilar to those produced by indirect dopamine (DA) agonists such as amphetamine. Pretreatment with racemic propranolol but not (+)propranolol antagonized the hyperactivity induced by RU 24959. Fluoxetine, a 5-HT reuptake inhibitor, failed to block the locomotor activating effects of RU 24969. These findings confirm the similarity between the behavioral effects of RU 24969 and indirect 5-HT agonists and suggest that the locomotor hyperactivity produced by both RU 24969 and MDMA is mediated by the activation of 5-HT1B receptors. Although the effects of MDMA on 5-HT1B receptors are secondary to its ability to release presynaptic 5-HT, the activation produced by RU 24969 appears to be a consequence of its direct agonist effects.

AMPHETAMINE DERIVATIVES INDUCE LOCOMOTOR HYPERACTIVITY BY ACTING AS INDIRECT SEROTONIN AGONISTS

Derivatives of amphetamine are potent releasers of both dopamine (DA) and serotonin (5-HT), but the relative contributions of DA and 5-HT release to the behavioral effects of these drugs have not been established. Previously, S-(+)3,4-methylendioxymethamphetamine (S-(+)MDMA) was found to produce locomotor hyperactivity in rats which was dependent on 5-HT release. The present study found that MBDB (1.25, 2.5, 5.0 or 10.0 mg/kg), the alpha-ethyl derivative of MDMA that produces little or no direct DA release, also induced locomotor hyperactivity that lasted for greater than 60 min after the 5.0 and 10.0 mg/kg doses. MBDB produced spatial patterns of locomotor hyperactivity and suppression of exploratory activity (holepokes and rearings) very similar to the behavioral syndrome produced by MDMA. MBDB-induced hyperactivity was blocked by pretreatment with the selective 5-HT uptake inhibitor fluoxetine (2.5 or 10 mg/kg), suggesting that MBDB produced behavioral effects via uptake-carrier mediated release of 5-HT. Similarly, fluoxetine pretreatment blocked the locomotor hyperactivity produced byS-(+)3,4-methylenedioxyamphetamine (3.0 mg/kg) orp-chloroamphetamine (2.5 mg/kg), supporting a serotonergic basis for the action of these drugs. Tissue levels of 5-HT and its metabolite 5-HIAA were decreased 40 min after administration ofS-(+)MDMA (3.0 mg/kg) or MBDB (5.0 mg/kg), and these decreases were prevented by fluoxetine pretreatment.S-(+)MDMA also produced a fluoxetine-sensitive increase of tissue DA levels, suggesting that 5-HT release may indirectly result in increased DA release, although MBDB did not significantly increase DA levels. These results point to a central role for 5-HT release in the stimulant-like behavioral effects of substituted derivatives of amphetamine.

Reserpine enhances amphetamine stereotypies without increasing amphetamine-induced changes in striatal dialysate dopamine

Indirect evidence suggests that amphetamine (AMPH) releases dopamine (DA) from an extravesicular, cytoplasmic pool. Disruption of vesicular DA storage by reserpine has been hypothesized to increase the concentration of extravesicular DA available for release by AMPH, which is consistent with the observation that reserpine does not prevent but augments the behavioral response to AMPH. In order to more directly test this hypothesis, the in vivo microdialysis technique was used to concurrently examine the behavioral and striatal dopaminergic response to AMPH (1.25 or 2.5 mg/kg) 24 h following reserpine pretreatment (2.5 mg/kg). Reserpine decreased tissue levels of DA by approximately 90% and reduced baseline dialysate DA concentrations by approximately 80%. Reserpine augmented the behavioural effects of AMPH, particularly increasing the occurrence and intensity of stereotypies. In contrast, reserpine did not alter the amount or duration of AMPH-induced DA release. This observation confirms that DA release by AMPH does not depend on vesicular stores but is inconsistent with the hypothesis that augmentation or behaviour by reserpine results from increased striatal DA release.

Quantitative assessment of the microstructure of rat behavior: II. Distinctive effects of dopamine releasers and uptake inhibitors.

The effects of four indirect dopamine agonists,d-amphetamine (0.25–4.0 mg/kg), cocaine (2.5–40.0 mg/kg), GBR 12909 (10.0–30.0 mg/kg), and nomifensine (5.0–20.0 mg/kg), on the behavioral organization of movements in an unconditioned motor paradigm were investigated in rats. The extended scaling hypothesis using the fluctuation spectrum of local spatial scaling exponents was used to quantify the geometrical characteristics of movements. The results reveal a qualitatively similar disruption of behavioral organization by lower doses of these drugs. Specifically, rats treated withd-amphetamine (<2.0 mg/kg), cocaine (<20.0 mg/kg), GBR 12909 (<20.0 mg/kg), or nomifensine (<10.0 mg/kg) exhibited a reduced range in the fluctuation spectrum, reflecting a predominance of meandering movements with local spatial scaling exponents between 1.3 and 1.7. This reduction was accompanied dynamically by a reduced predictability of movement sequences as measured by the dynamical entropy,h. By contrast, higher doses of these drugs produced distinctly different changes in behavioral organization. In particular, 4.0 mg/kgd-amphetamine and 40.0 mg/kg cocaine increased the fluctuation range, reflecting relative increases in both straight and circumscribed movements that are interpreted as a combination of spatially extended and local perseveration. In contrast, high doses of 30.0 mg/kg GBR 12909 and 20.0 mg/kg nomifensine induced only local perseveration. High doses ofd-amphetamine, cocaine, GBR 12909 and nomifensine reduced the dynamical entropy,h, indicating an increased predictability of the movement sequences. These results suggest that the generic behavioral change induced by low doses of dopamine agonists is characterized by a reduced variety of path patterns coupled with an increased variability in sequential movement sequences. The differential effects of higher doses of these drugs may be due to their influences on other neurotransmitter systems or differential affinities for different dopamine subsystems.

Stimulant effects of 3,4-methylenedioxymethamphetamine in the nucleus accumbens of rat

This study examined the behavioral effects in rats of intracerebral administration of S(+)-3,4-methylenedioxymethamphetamine (S-MDMA) using an automated holeboard and open-field apparatus. Administration of S-MDMA into the nucleus accumbens septi produced locomotor hyperactivity. Although the stimulant effects of S-MDMA administered systemically are antagonized by fluoxetine pretreatment, the activating effects of S-MDMA administered into the nucleus accumbens were not antagonized by fluoxetine. A similar increase in locomotor activity was observed after S-amphetamine administration into the nucleus accumbens. In contrast, the selective 5-HT-releasing drug and S-MDMA congener N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine (MBDB) produces MDMA-like locomotor hyperactivity when administered systemically but did not alter locomotor activity when injected into the nucleus accumbens. These data indicate that S-MDMA actions in the nucleus accumbens are pharmacologically distinct from the primary effects of systemically administered S-MDMA. Behavioral effects of S-MDMA in the nucleus accumbens may result from the catecholamine-releasing properties that S-MDMA shares with S-amphetamine and not via the 5-HT-releasing properties that it shares with MBDB.

Suppression of behavioral activity by norfenfluramine and related drugs in rats is not mediated by serotonin release.

Fenfluramine, a phenalkylamine with serotonin (5-HT) releasing properties, decreases motor activity in rats. The following studies assessed the contribution of 5-HT release to the behavioral effects of fenfluramine and norfenfluramine using a behavioral pattern monitor that simultaneously assesses locomotor and investigatory behavior. First, both fenfluramine and its active metabolited-norfenfluramine dose-dependently reduced locomotor and investigatory activity. The norfenfluramine-induced reduction in activity was not antagonized by pretreatment with the 5-HT uptake inhibitor fluoxetine or the 5-HT synthesis inhibitorp-chlorophenylalanine, drugs that reduce drug-induced 5-HT release. Second, thed- andl-enantiomers of norfenfluramine were nearly equipotent at reducing behavioral activity, althoughd-norfenfluramine is more potent as a 5-HT releasing agent. Third,p-chloroamphetamine, a drug that shares the 5-HT releasing properties of fenfluramine produced locomotor hyperactivity in the same paradigm. Previous studies indicate that other 5-HT releasing phenalkylamines have behavioral effects resembling those ofp-chloroamphetamine rather than those of fenfluramine. Finally, a structurally related drug, 4-methoxy-5-methyl-aminoindan (MMAI), produced dose-dependent reductions in behavioral activity that are similar to the effects of fenfluramine. The behavioral effects of MMAI were not antagonized by fluoxetine or by the 5-HT receptor antagonist methiothepin. These data suggest that the decrease in activity induced by fenfluramine, norfenfluramine and the related drug MMAI is not related to 5-HT release.

Serotonin Release is Responsible for the Locomotor Hyperactivity in Rats Induced by Derivatives of Amphetamine Related to MDMA

A structural derivative of amphetamine, 3,4-methylenedioxymethamphetamine (MDMA), is a potent serotonin (5-HT) releasing drug. When administered to humans, MDMA produces psychological effects that are distinct from the effects of classical psycho-stimulants and hallucinogens. In rats, MDMA produces locomotor hyperactivity, but the spatial pattern of locomotion and the suppression of investigatory behaviors in MDMA-treated rats differs qualitatively from the pattern of exploration produced by other psycho-stimulants. Previous experiments have indicated that the motor activating effects of MDMA in rats depend upon 5-HT release. Antagonism of MDMA-induced norepinephrine release by pretreatment with nisoxetine was less effective than serotonergic manipulations at reducing the behavioral effects of MDMA, suggesting a minor contribution of norepinephrine release to the behavioral effects of MDMA. A derivative of MDMA with greater selectivity for serotonergic systems over catecholaminergic systems, 5,6-(methylenedioxy)-2-aminoindan (MDAI), produced a biphasic effect in rats consisting of an early suppression of exploratory activity and a later motor activation. Only the motor activation by MDAI is antagonized by treatments designed to prevent drug-induced 5-HT release, consistent with the importance of 5-HT release for the activating effects of MDMA. Finally, the locomotor activation, suppression of investigatory responding and detailed spatial patterning of activity produced by MDMA was not diminished in a familiar environment. Similar effects produced by hallucinogens are attenuated in a familiar testing environment, suggesting that these effects of MDMA do not reflect hallucinogenic properties of this drug. These results demonstrate the potential utility of 5-HT releasing drugs in future studies of the influence of endogenous 5-HT on unconditioned exploratory activity.