Catecholamines and self-stimulation: evidence suggesting a reinforcing role for noradrenaline and a motivating role for dopamine.

Investigation of the role of noradrenaline (NA) and dopamine (DA) in self-stimulation showed that d-amphetamine (which releases more DA than does l-amphetamine, but not more NA) was much more effective than l-amphetamine in enhancing self-stimulation of NA sites in the locus coeruleus and near-lateral hypothalamus. In DA sites in the substantia nigra and far-lateral hypothalamus the effects of the 2 isomers were confirmed to be more nearly equal. Thymoxamine HCl (10 mg/kg IP), a specific alpha-adrenergic receptor blocker, depressed self-stimulation at all sites, but significantly more severely at DA sites. Thus the drugs most effective in influencing self-stimulation at a particular site were those acting predominantly on the unstimulated system. These findings were interpreted in terms of a hypothesis that DA and NA play complementary roles in self-stimulation and that both are essential; or, more specifically, that DA pathways, implicated in other motivational activites, contribute to a state of drive or arousal necessary for self-stimulation; while response-contingent noradrenergic activity (elicited by the electrodes directly via a transsynaptic route) mediates reinforcement. Further predictions from this hypothesis were tested as follows: (1) Direct pharmacological stimulants of adrenergic alpha-receptors should disrupt self-stimulation by acting randomly on the reinforcement system and disrupting response-reward contingencies; this was confirmed by the finding that the alpha-receptor stimulant clonidine HCl (0.05 mg/kg) depressed self-stimulation at all sites tested. (2) Drect stimulants of DA receptors should enhance self-stimulation of NA sites by augmenting dopaminergic motivational activity; but in rats with DA electrodes, noncontingent stimulation of DA receptors would also impose similar noncontingent activity on the transsynaptic noradrenergic reinforcement pathways and thus depress self-stimulation; this was confirmed by the finding that apomorphine (0.3-1.0 mg/kg) was strongly stimulant for NA electrodes but strongly depressant for DA electrodes, and that the degree and direction of these effects was highly correlated with the differential effects of d- l-amphetamine (rho = .65, p less than 0.01). Neither effect of apomorphine depended on the occurrence of motor stereotypy. These results can be interpreted in terms of 2-component models for self-stimulation, with the predominant transmitter of the drive component being identified as DA and that g the reinforcing component as NA.