6-OHDA-treated weanling rats show normal neuroleptic sensitivity as adults on LHSS

Brain reward circuitry beyond the mesolimbic dopamine system: a neurobiological theory

Reductionist attempts to dissect complex mechanisms into simpler elements are necessary, but not sufficient for understanding how biological properties like reward emerge out of neuronal activity. Recent studies on intracranial self-administration of neurochemicals (drugs) found that rats learn to self-administer various drugs into the mesolimbic dopamine structures-the posterior ventral tegmental area, medial shell nucleus accumbens and medial olfactory tubercle. In addition, studies found roles of non-dopaminergic mechanisms of the supramammillary, rostromedial tegmental and midbrain raphe nuclei in reward. To explain intracranial self-administration and related effects of various drug manipulations, I outlined a neurobiological theory claiming that there is an intrinsic central process that coordinates various selective functions (including perceptual, visceral, and reinforcement processes) into a global function of approach. Further, this coordinating process for approach arises from interactions between brain structures including those structures mentioned above and their closely linked regions: the medial prefrontal cortex, septal area, ventral pallidum, bed nucleus of stria terminalis, preoptic area, lateral hypothalamic areas, lateral habenula, periaqueductal gray, laterodorsal tegmental nucleus and parabrachical area.

Impaired acquisition and operant responding after neonatal dopamine depletion in rats

The effect of neonatal dopamine depletion in rats was examined using operant conditioning. Rat pups were given 6-hydroxydopamine (6-OHDA) or sham lesions at 3 days of age. When tested as adults, 6-OHDA treated subjects were impaired in the acquisition of lever pressing for reward and displayed stereotyped sniffing patterns not observed in the control subjects. In addition, significantly lower rates of responding were measured for the lesion group during continuous reinforcement (CRF), dilution of reinforcer efficacy, and with progressively increasing fixed ratio requirements. Alpha-methyl-p-tyrosine (AMPT), given before one CRF session, attenuated responding in over half the lesion animals and in none of the controls. Dopamine content in caudate nucleus was found to significantly correlate with number of trials to acquisition and rates of responding following AMPT in the lesion group, but not in the control group. Overall, the results of this experiment showed that neonatal dopamine depletion does not lead to severe motor impairment or the inability to learn, but does disrupt the normal patterns of behavior associated with operant conditioning.