Self-stimulation of circuitry that courses between the ventromedial mesencephalic area known as the ventral tegmental area (VTA) and the nucleus accumbens has long been recognized as the fundamental reinforcement or reward circuit of the brain (for overview, see Ikemoto and Panksepp, 1999). As we have come to appreciate the power and nature of instinctual systems of the brain, certain dubious ideas that came down to us from behavioral psychology have been recently recast into an ethological view of animal nature. This so-called reward circuitry is, in fact, more critical for arousing exploratory urges and energetic foraging as animals seek rewards (Panksepp, 1998a). The system is especially responsive when there is an element of unpredictability in forthcoming rewards (Schultz, 2000, 2002), for these are times when animals begin to exhibit especially vigorous curiosity and exploratory responses.
This system allows animals to search, find, and eventually eagerly anticipate the many things needed for survival. The system is not as concerned about the nature of specific rewards; it works equally well in seeking food, water, warmth as well as social goals, including sexual gratification, maternal engagement, and probably playful urges. In short, this system promotes interest, curiosity, and desire for engagement with a host of life activities, and in this capacity it helps animals learn about the reward contingencies in their environments (Berridge and Robinson, 1998; Ikemoto and Panksepp, 1999). This appetitive urge has now been imaged in humans (Breiter et al., 2001; Knutson et al., 2001a,b).
Underactivity in these circuits can promote depression and dysphoria—a generalized failure of "libido." As it facilitates the fulfillment of many goals, this system may be the closest we have yet come to envisioning neural underpinning for the generalized Freudian concept of drive. Overactivity is generally regarded to have important implications for understanding paranoid schizophrenia, as well as mania and various cravings, from food and drugs to sex and gambling. Every addictive drug converges on this system (Wise, 2002) and tends to amplify desire as a trait characteristic of an organism (Nocjar and Panksepp, 2002).
When this system is poorly regulated or overactive for extended periods, as indexed by elevated D2 receptor populations, schizophrenic tendencies ensue—especially positive "functional" symptoms such as delusions and hyperemotionality (Kapur, 2003), which can be ameliorated with most existing antipsychotic medications (Chapter 10). Negative symptoms of social withdrawal and psychomotor retardation are promoted when this system is underactive. A key neurochemical in the SEEKING system is dopamine, especially the dopaminergic mesolimbic and mesocortical dopamine circuits arising from the VTA (see Fig. 1.1), but there are an enormous number of converging chemistries on this circuitry, and little is known about the specific types of information that are harvested. Nonetheless, we do know that this convergence does not simply yield "information" as an output but rather, an insistent urge to act in certain ways. We know this because all the dopamine neurons behave essentially in the same way, with no indication that they are parsing differences that reflect the many distinct aspects of the world. In other words, there appears to be a mass-action effect of this system that increases an organic pressure for action—a process that has often been called metaphorically "psychic energy."
A diversity of neuropeptide-containing circuits converge on the SEEKING system, including neurotensin, opioids, cholecystokinin (CCK), substance P, orexin, and others, allowing diverse neuropsychic influences to control exploration and anticipatory eagerness. Many of these chemistries are targets for antipsychotic drug development (Chapter 21). Psychostimulant drugs derive their affective appeal and potential to produce craving and psychosis by overarousing this emotional system. Other drugs of addiction, such as opiates, nicotine, and alcohol, also derive at least part of their addictive edge by interacting with this system (Wise, 2002). Among the interesting properties of this system are sensitization effects that emerge from stress as well as periodic experiences with neuropharmacological activators of the system such as amphetamines and cocaine (Robinson and Berridge, 2003). Sensitization reflects an elevated responsivity of the system to both internal and external stimuli.
Dopamine circuits tend to energize many basic appetitive behavioral tendencies as well as higher brain areas that mediate planning and foresight (such as the executive functions of the frontal cortex) promoting, presumably, psychic states of eagerness and hopefulness that help mold purposive behaviors by interacting with higher cortico-cognitive structures such as the working memory systems of frontal lobes. Only recently have we started to grasp the importance of such state-control systems of the brain, and many fit as well or better with the instinctive emotional conceptions of brain functions than currently popular theories of information processing. Indeed, we can generate a remarkable number of compelling working hypotheses when we consider this system from several different vantages:
1. Dopamine cells exhibit a rhythmic firing that resembles the second hand of a clock, and it has been found that this brain system elaborates behavioral eagerness on fixed-interval schedules of reinforcement (see Panksepp, 1981, 1998a), and they show bursting when animals are behaviorally excited and very regular firing when they are not, which is suggestive of some type of background clocking function (Hyland et al., 2002). When one gets tired and bored, subjective time is experienced as slowing down. This is especially evident during physical fatigue (e.g., presumably the internal clock is "ticking" very slowly, as at the end of an exhausting exercise program). Thus, a diminution in the rate of dopamine cell firing may contribute to feelings of fatigue. A related prediction would be that as we pharmacologically reduce dopamine firing, a psychological sense of fatigue would begin to emerge. If so, neuropeptides such as neurotensin and orexin, which facilitate dopamine activity (Chapter 21), might be developed into mild antifatigue agents.
2. The dopamine system seems to facilitate the transition from the perception of temporally correlated events to the conviction that there is causality among those events. There are many relevant examples from animal brain research, for instance, schedule-induced polydipsia and adjunctive (i.e., superstitious) behaviors that depend on dopamine systems (for reviews, see Panksepp, 1981, 1998a). Might delusions be facilitated by activity in this system? It is well known that paranoia tends to be increased by psychostimulants that promote dopamine transmission while being diminished by antidopaminergics (Kapur, 2003).
3. Remarkable relationships have been demonstrated between the psychic energy of the SEEKING system and the dreams of rapid eye movement (REM) sleep (see Chapters 7 and 8; Panksepp, 1998a). On the basis of such relationships, Solms (2000) has argued that dream "energies" can be disassociated from those that promote REM sleep, and that the former is more closely linked to dopamine arousal than to the pontine REM-sleep generators. On the basis of this, tight relations would be predicted between antipsychotic doses of dopamine receptor blocking agents and the vividness, and perhaps the frequency, of dreams. Predictions similar to these could be generated for all of the basic emotional systems of the brain and thereby guide forward-looking thought in biological psychiatry and depth psychology.
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