Neurochemical mediators of anxiety have inconsistent effects on hypothalamic self-stimulation in rats

5-HT receptors and reward-related behaviour: a review

The brain's serotonin (5-HT) system is key in the regulation of reward-related behaviours, from eating and drinking to sexual activity. The complexity of studying this system is due, in part, to the fact that 5-HT acts at many receptor subtypes throughout the brain. The recent development of drugs with greater selectivity for individual receptor subtypes has allowed for rapid advancements in our understanding of this system. Use of these drugs in combination with animal models entailing selective reward measures (i.e. intracranial self-stimulation, drug self-administration, conditioned place preference) have resulted in a greater understanding of the pharmacology of reward-related processing and behaviour (particularly regarding drugs of abuse). The putative roles of each 5-HT receptor subtype in the pharmacology of reward are outlined and discussed here. It is concluded that the actions of 5-HT in reward are receptor subtype-dependent (and thus should not be generalized) and that all studied subtypes appear to have a unique profile which is determined by content (e.g. receptor function, localization - both throughout the brain and within the synapse) and context (e.g. type of behavioural paradigm, type of drug). Given evidence of altered reward-related processing and serotonergic function in numerous neuropsychiatric disorders, such as depression, schizophrenia, and addiction, a clearer understanding of the role of 5-HT receptor subtypes in this context may lead to improved drug development and therapeutic approaches.

Reinforcing versus anticonvulsant drugs: effects on intracranial self-stimulation rate-frequency M50 indices

Drugs of abuse, such as amphetamine and morphine, produce reward-related shifts on intracranial self-stimulation (ICSS) thresholds. The facilitatory effects on ICSS thresholds of drugs that act through the GABAergic system, however, are reported to be attributed to their antiseizure and anticonvulsant effects, rather than their reinforcing effects. Using a rate-frequency ICSS paradigm, we examined the effects of amphetamine (a reinforcing drug of abuse that acts via the catecholaminergic system), pentobarbital (a GABA(A) receptor agonist and reinforcing barbiturate with anticonvulsant properties), and gabapentin (a nonspecific GABAergic agonist and anticonvulsant with low abuse potential) on ICSS M(50) indices. All three doses of amphetamine (0.5, 1.0, and 2.0 mg/kg) and pentobarbital (2.5, 5.0, and 10.0 mg/kg) significantly lowered rate-frequency M(50) values. Gabapentin, on the other hand, significantly raised rate-frequency M(50) values, albeit only at the highest dose administered (30 mg/kg). Our results indicate that shifts in ICSS M(50) values produced by pentobarbital are associated with the reinforcing, not the anticonvulsant, effect of pentobarbital. These results are consistent with the view that there is a common system underlying the reinforcing effects of drugs and ICSS reinforcement, and suggest that the reinforcing and anticonvulsant effects of GABA agonists are dissociable.

The 5-HT3 antagonist Y-25130 blocks cocaine-induced lowering of ICSS reward thresholds in the rat

Serotonin-3 (5-HT(3)) receptor antagonists have been shown to attenuate drug-induced increases in mesolimbic dopamine (DA), locomotor activation, and drug self-administration. In the present study, we tested whether the selective 5-HT(3) antagonist Y-25130 would attenuate cocaine-induced lowering of intracranial self-stimulation (ICSS) reward thresholds. Rats (n=6) were surgically prepared with bipolar stimulation electrodes and trained to self-administer electrical stimulation delivered to the medial forebrain bundle-lateral hypothalamus (MFB-LH). A discrete-trial, rate-free threshold determination procedure was used to detect pharmacologically induced changes from baseline reward thresholds. Four doses of Y-25130 (0.0, 0.03, 0.3, and 3.0 mg/kg ip) were given alone and in combination with cocaine (4.0 mg/kg ip). Y-25130 did not significantly alter reward thresholds or response latencies when given alone as compared to baseline measures. While there were no significant effects at lower doses, the middle and highest doses of Y-25130 (0.3 and 3.0 mg/kg) did attenuate the threshold-lowering effect of cocaine. These findings suggest that the rewarding effects of cocaine are mediated through 5-HT(3) receptor activity.

The orphanin receptor agonist RO 64-6198 does not induce place conditioning in rats

The abuse liability of Ro 64-6198, an orphanin FQ (OFQ) receptor full agonist that exhibits anxiolytic properties, was evaluated using an unbiased conditioned place preference (CPP) paradigm in rats. As OFQ is structurally related to opioid peptides and also exhibits anxiolytic-like properties, the effect of Ro 64-6198 on CPP was compared with those of morphine and alprazolam. We show here that neither Ro 64-6198 nor alprazolam exhibited rewarding or aversive properties, whereas morphine induced a pronounced CPP. These results strengthen the previous finding that Ro 64-6198 lacked abuse liability in a self-stimulation paradigm, suggesting that this new class of anxiolytic drugs is devoid of the risk for potential non-medical use and dependence.

Control of motor seizures by brotizolam with maintenance of stable refractory periods for self-stimulation

In recent years, we have been pursuing our mapping investigations of the substrate for brain-stimulation reward in regions of the anterior hypothalamic and lateral preoptic areas. However, one problem is that stimulation of these sites often generates overt seizures so that their suppression via a pharmacological means would be very useful. The sedative-hypnotic benzodiazepine, brotizolam, is reportedly a long-lasting anticonvulsant. Hence, its effects on motor seizures elicited from stimulation of the lateral preoptic area were evaluated in the first experiment. Both tested doses (5.0 and 7.5 mg/kg) of the drug were shown to significantly decrease the number, and marginally, the severity of stimulation-induced seizures; furthermore, this effect was relatively long lasting, up to about 3 h. The higher dose of brotizolam did not alter the single-pulse thresholds for self-stimulation, a requirement for evaluations of poststimulation excitability, the purpose of the second experiment. Here, our interest was in documenting whether the membrane properties of the stimulated neurons, as assessed by refractory periods, were altered by brotizolam. No differences in the time course of recovery were observed; refractoriness began between 0.4 and 0.8 ms, and reached 50% recovery by 2.0 ms, which is consistent with the pattern of poststimulation excitability typically measured at these sites. Thus, in addition to its long-lasting suppression of motor seizures in rats, brotizolam does not alter the time course of recovery from refractoriness of the neurons that mediate brain-stimulation reward in the lateral preoptic area.