Dopaminergic Actions of d-Amphetamine on Schedule-Induced Polydipsia in Rats

Repeated Δ-9-Tetrahydrocannabinol administration dose dependently increases stablished schedule-induced drinking

RATIONALE

Schedule-induced drinking (SID) reproduces an excessive and repetitive behavioural pattern that has led to propose this procedure as an animal model to study compulsive behaviours. Although it is known that cannabis can cause several adverse effects, in recent years there has been great interest in the medical application of cannabis derivatives for obsessive-compulsive related disorders.

OBJECTIVES

The present study investigated the effects of repeated THC administration on rates of previously acquired SID, as well as the possible alteration of its temporal distribution along inter-food intervals.

METHODS

Male Wistar rats acquired SID under a 30 min fixed-time 30-sec food delivery schedule (from 30 to 43 sessions to reach a stable level). Thereafter, 5 or 10 mg/kg daily i.p. injections of THC or vehicle were repeatedly administered for 7 days to evaluate the effects on SID.

RESULTS

Repeated THC administration at a dose of 5 mg/kg resulted in an increase on licking. Surprisingly, no effects on SID were observed with the 10 mg/kg dose. However, magazine entries were reduced with both THC doses. THC also modified the temporal distributions of licking and magazine entries during inter-food intervals.

CONCLUSIONS

The present results show that repeated THC administration may (i) increase induced licking at moderate doses, (ii) reduce magazine entries, and (iii) affect the temporal pattern of SID. These findings suggest that THC does not appear to be beneficial to reduce compulsive behaviour in this animal model, while another collateral effect of THC -such as a greater habitual-like behaviour- needs to be considered.

Effects of dopamine agents on a schedule-induced polydipsia procedure in the spontaneously hypertensive rat and in Wistar control rats

The spontaneously hypertensive rat (SHR) has been proposed as an animal model for attention deficit hyperactivity disorder (ADHD), and typically develops excessive patterns of response under most behavioural protocols. Schedule-induced polydipsia (SIP) is the excessive water consumption that occurs as a schedule effect when food is intermittently delivered and animals are partially food- but not water-deprived. SIP has been used as a model of excessive behaviour, and considerable evidence has involved the dopaminergic system in its development and maintenance. The aim of this study was to evaluate the effects of the most common psychostimulants used in ADHD treatment on SIP, comparing their effects in SHRs with rats from control populations. SHR, Wistar Kyoto (WKY) and Wistar rats were submitted to a multiple fixed time (FT) food schedule with two components: 30 s and 90 s. The acute effects of different dopaminergic compounds were evaluated after 40 sessions of SIP acquisition. All animals showed higher adjunctive drinking under FT 30 s than FT 90 s, and SHRs displayed higher asymptotic SIP levels in FT 90 s compared to WKY and Wistar rats. SHRs were less sensitive to dopaminergic agents than control rats in terms of affecting rates of adjunctive drinking. These differences point to an altered dopaminergic system in the SHR and provide new insights into the neurobiological basis of ADHD pharmacological treatments.