Using a rate-frequency curve method to assess the rewarding properties of morphine in the intracranial self-stimulation paradigm in rats

Abstract

Intracranial self-stimulation (ICSS) is a sophisticated paradigm that can be used to assess the rewarding properties of drugs of abuse such as cocaine and amphetamine. Initial studies using this method to assess brain stimulation reward (BSR) examined the enhancement of response rates after drug administration. In the mid to late 1980s several groups began implementing paradigms to assess the direct rewarding effects of drugs of abuse that, for the most part, are independent of rate (i.e., progressive-ratio, auto-titration, rate-frequency curve), providing a representative assessment of a drugs rewarding effects. However, some drugs such as morphine and ethanol, which are known to have abuse potential, have sedative effects that may impede the ability to accurately quantify rewarding effects, even in these rate-independent paradigms. Few studies to date report effects of morphine in the ICSS paradigm and those that do appear to be inconsistent, lack robustness, have not been reproducible by other groups, or require inconvenient experimental designs. Here, we demonstrate a reliable and robust method to assess the rewarding effects of morphine using the rate-frequency curve paradigm.