Withdrawal from chronic phencyclidine produces a pentylenetetrazol-like discriminative stimulus

Pentylenetetrazol-like stimulus is not produced following naloxone-precipitated mitragynine withdrawal in rats

RATIONALE

Kratom (Mitragyna speciosa Korth), a native medicinal plant of Southeast Asia, is proposed to exhibit potential therapeutic value as an opioid substitute. However, studies of its negative emotional states resulting from withdrawal particularly of its main psychoactive compound, mitragynine (MG), are limited.

OBJECTIVES

Using the pentylenetetrazol (PTZ) discrimination assay, this study aims to investigate the effects of MG in responding to the PTZ stimulus and to assess the generalisation effects of withdrawal from MG to the PTZ stimulus.

METHODS

Rats (n = 20) were trained on a tandem (FR-10, VI-15) schedule of food reinforcement to press one lever after administration of the anxiogenic compound PTZ (16 mg/kg, i.p.) and an alternate lever after vehicle. Following acute tests, training was suspended, and rats were chronically treated with MG or morphine at 8-h intervals for 9 days and withdrawal was precipitated on the tenth day using naloxone (1 mg/kg, i.p.). The rats were tested for generalisation to PTZ at 2, 8 and 24 h after the last dose of MG or morphine administration.

RESULTS

Unlike morphine that produced dose-related PTZ-like stimulus, MG at 3, 10, 30 and 45 mg/kg doses showed no substitution to the PTZ discriminative stimulus. In contrast to morphine which produced a time-dependent generalisation to the PTZ stimulus, naloxone did not precipitate withdrawal effects in MG-treated rats as they selected the vehicle lever at three withdrawal time points.

CONCLUSION

These results demonstrate that MG produces a very different response to morphine withdrawal that is not associated with anxiogenic-like subjective symptoms. These characteristics of MG may provide further support for use as a novel pharmacotherapeutic intervention for managing opioid use disorder.

The discriminative stimulus effects of pentylenetetrazol as a model of anxiety: recent developments

Pentylenetetrazol (PTZ), a GABA(A) receptor antagonist and prototypical anxiogenic drug, has been extensively utilized in animal models of anxiety. PTZ produces a reliable discriminative stimulus which is largely mediated by the GABA(A) receptor. Several classes of compounds can modulate the PTZ discriminative stimulus including 5-HT(1A), 5-HT(3), NMDA, glycine, and L-type calcium channel ligands. Spontaneous PTZ-lever responding is seen in trained rats during withdrawal from GABA(A) receptor compounds such as chlordiazepoxide and diazepam, and also ethanol, morphine, nicotine, cocaine, haloperidol, and phencyclidine. This effect is largely mediated by the GABA(A) receptor, which suggests that anxiety may be part of a generalized withdrawal syndrome mediated by the GABA(A) receptor. There are also important hormonal influences on PTZ. Corticosterone plays some role in mediation of its anxiogenic effects. There is a marked sex difference in response to the discriminative stimulus effects of PTZ, and estrogens appear to protect against its anxiogenic effects. Further work with the PTZ drug discrimination is warranted for characterization of anxiety during withdrawal, and the hormonal mechanisms of anxiety.

Withdrawal from chronic haloperidol substitutes for the pentylenetetrazol discriminative stimulus

The present study was designed to examine withdrawal from a therapeutic, non abused drug, haloperidol. Rats were trained to discriminate the anxiogenic compound pentylenetetrazol (PTZ) from water in a two lever, food reinforced, drug discrimination procedure. Dose effect curves were then determined for PTZ and the antipsychotic drug, haloperidol (0.1-1 mg/kg). Haloperidol did not substitute for PTZ, even at a dose that decreased rates of responding to approximately 15% of control values. Rats were then treated chronically with either 1 or 2 mg/kg/day haloperidol while training was suspended. After 5 days of chronic haloperidol 4/6 animals in the 1 mg/kg/day group and 5/7 in the 2 mg/kg/day group chose the PTZ lever when tested 24-48 hours after the last haloperidol injection. Haloperidol, 1 or 2 mg/kg, did not reverse PTZ-lever responding. After an additional 5 days of chronic haloperidol, 3/6 rats in the 1 mg/kg/day group and 5/7 rats in the 2 mg/kg/day group responded on the PTZ lever 24 hours after the last injection, and this was reversed with the anxiolytic, chlordiazepoxide (3.2-5.6 mg/kg). The current findings indicate that there is an anxiogenic component to withdrawal from haloperidol. In psychotic patients, abrupt discontinuation of haloperidol results in nausea, vomiting and sweating, as well as a "relapse into psychosis" characterized by anxiety, depression and internal chaos (1). Interestingly, the authors caution that the so-called relapse into psychosis may simply be a sign of withdrawal. The current findings support their view and suggest that abrupt discontinuation of psychoactive therapeutic agents may result in anxiety.