Dopamine denervation of frontal cortex or nucleus accumbens does not affect ACTH-induced grooming behaviour

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Super-stereotypy I: enhancement of a complex movement sequence by systemic dopamine D1 agonists

Peripheral administration of D1 dopamine agonists elicits grooming behavior from rodents. The present study examined grooming behavior and the relative probability and stereotypy of a natural sequence of grooming movements (called a syntactic grooming chain) that follows a predictable fixed pattern of serial order. We compared the amount of grooming behavior vs. the stereotypy of sequential patterns after peripheral administration of either a partial D1 agonist (SKF 38393; 2.5, 5.0, 10, 20 mg/kg), a full D1 agonist (SKF 82958; 0.1, 0.2, 0.5, 1.0 mg/kg; i.p.), a D2 agonist (quinpirole; 5.0, 10 mg/kg), or ACTH (2.0, 5.0 mg/kg). There was a dissociation between the elicited grooming amount, the pattern frequency, and the pattern completion or sequential stereotypy after these drugs. Quinpirole and ACTH both reduced the likelihood that the sequential pattern would be completed in the normal pattern (and reduced the overall amount of grooming). Administration of either SKF 38393 or SKF 82958 increased the tendency to engage in complex stereotyped sequential patterns of grooming (even though only the partial D1 agonist increased the total amount of grooming). In addition, SKF 38393 increased the sequential stereotypy of the already-stereotyped pattern itself (as measured by the probability of completing the stereotyped sequence once it began). Thus, dopamine D1 receptor activation appears to contribute to a kind of sequential super-stereotypy in which a complex, stereotyped behavioral sequence is initiated more frequently and more often goes to completion.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

6-Hydroxydopamine lesions of the medial prefrontal cortex of rats do not affect dopamine metabolism in the basal ganglia at short and long postsurgical intervals

Dopamine (DA) in the medial prefrontal cortex (mPFC) has been implicated in the regulation of subcortical DA function. To further characterize the potential interaction between cortical and subcortical DA systems, the short- and long-term neurochemical consequences of 6-hydroxydopamine (6-OHDA) lesions of the mPFC of rats were investigated in the mPFC and in its subcortical target structures. 4 to 5, 10 to 12 and 32 to 36 days after infusion of 6-OHDA, DA was depleted to a larger extent than noradrenaline and serotonin. No lesion-induced changes of DA and its metabolites were detected in subcortical structures. These results show that prefrontal 6-OHDA lesions produce immediate and long lasting depletions of prefrontal monoamines, especially of DA, without increasing basal DA metabolism in the striatum and nucleus accumbens.

6-Hydroxydopamine lesion of the rat prefrontal cortex increases locomotor activity, impairs acquisition of delayed alternation tasks, but does not affect uninterrupted tasks in the radial maze

The role of mesocortical dopamine neurons in locomotion and acquisition of various delayed and uninterrupted maze tasks was investigated in the rat. Dopaminergic terminals of the medial prefrontal cortex were lesioned by stereotaxically guided injections of the selective neurotoxin 6-hydroxydopamine (6-OHDA), while noradrenergic neurons were protected by systemically administered desipramine. 6-OHDA lesions resulted in a selective depletion of dopamine and its metabolite, dihydroxyphenylacetic acid, in the prefrontal cortex but not in subcortical structures. Prefrontal serotonin was not depleted. 6-OHDA-cloned rats performed uninterrupted alternation tasks (spontaneous and reinforced alternation) in the radial maze in the same manner as controls, whereas performance of delayed alternation in the T-maze and the radial maze was impaired in lesioned rats. In addition, locomotor activity during maze performance was increased in lesioned rats. Based on the hypothesis that increased motor activity and impaired delayed alternation performance are due to increased susceptibility to interfering stimuli, we propose tentatively that prefrontal dopamine may function to suppress interference during the delay period of certain cognitive tasks.

CRF-induced excessive grooming behavior in rats and mice

We studied the grooming response to lateral ventricle injection of CRF in both rats and mice under similar conditions. One microgram of CRF ICV induced a pronounced increase (3- to 4-fold) in the frequency of self-grooming in rats, but only a much smaller (less than 20%) increase in mice. The minimum effective dose of CRF in rats was 300 ng. Although ACTH1-24 induced less grooming in mice than in rats, the difference in potency did not appear to be sufficient to explain the differences between the effectiveness of CRF in the two species. Whereas ACTH increased all types of grooming scored. CRF increased all forms of grooming except flank scratching with the hind limb. The major effect of CRF was to increase the number of episodes of grooming, whereas ACTH1-24 tended to prolong the length of individual episodes. The excessive grooming induced by ICV CRF was not affected by prior treatment with dexamethasone, suggesting that the increased grooming was not due to secondary release of ACTH from the pituitary. Nevertheless, ICV CRF might induce grooming by releasing MSH/ACTH from cerebral storage sites. CRF-induced grooming, like ACTH-induced grooming, was inhibited by naloxone pretreatment. Despite the small qualitative differences, CRF-induced grooming could be due to secondary release of ACTH.

Tyrosine hydroxylase activation in mesocortical 3,4-dihydroxyphenylethylamine neurons following footshock

Mild electric footshock resulted in activation of tyrosine hydroxylase (TH) in prefrontal cortex of mice and rats. In mice, the activation was also observed following restraint. Shock-evoked activation of prefrontal cortex TH was characterized by a decrease of apparent Km for the pterin cofactor 6-methyl-5,6,7,8-tetrahydropterin and an increase of Vmax. Activation of prefrontal cortical TH was also demonstrated in vitro following preincubation under conditions that activate cyclic AMP-dependent protein kinase. Treatment of mice with the noradrenergic neurotoxin N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP-4) caused a 70% decrease in prefrontal cortex norepinephrine levels but had no significant effect on the activity of TH in that brain region. Footshock resulted in the activation of prefrontal cortex TH of DSP-4-treated mice, suggesting that shock-evoked activation of the enzyme occurs in terminals of mesocortical 3,4-dihydroxyphenylethylamine neurons.

ACTH acts via an anterior ventral third ventricular site to elicit grooming behavior

Intracerebroventricular but not parenteral application of ACTH has been shown to elicit excessive grooming behavior in rats and mice. This behavior is elicited by administration of ACTH into the lateral, third, or fourth ventricles. Plugging of the cerebral aqueduct with cold cream fails to prevent grooming in response to lateral ventricle injection of ACTH. However, cold cream plugs in the third ventricle can prevent the subsequent induction of grooming behavior by lateral ventricle injection of ACTH, but only when the plugs are located in the anterior ventral third ventricle in the region of the organum vasculosum laminae terminalis (OVLT) and median eminence. These data suggest the anterior ventral third ventricle as the periventricular site of action of ACTH in eliciting excessive grooming, although it is possible that peptides taken up in this area are transported to other regions to elicit the behavioral response.

Dopaminergic modulation of ACTH-induced grooming

ACTH-(1-24)-induced grooming was studied after administration of the peptide into the substantia nigra or intracerebroventricularly (i.c.v.). The modulation of dopamine receptors in neostriatum (with haloperidol and apomorphine) and nucleus accumbens (with 3,4-dihydroxyphenylamino-2-imidazoline hydrochloride; DPI and ergometrine) was investigated. In the nucleus accumbens, the modulatory effects of ergometrine and DPI on ACTH-(1-24)-induced grooming were based on their affinity for dopamine receptors and not on their affinity for adrenoceptors. Intrastriatal application of dopaminergic agents inhibited i.c.v. ACTH-(1-24)-induced excessive grooming, whereas the grooming score was enhanced if ACTH-(1-24) was administered into the substantia nigra. The finding of differential effects of dopaminergic agents on ACTH-induced excessive grooming depending on the route of administration indicate that i.c.v. ACTH-induced excessive grooming is not mediated solely through the substantia nigra. The increase in grooming behavior seen after the intrastriatal administration of dopaminergic agents - when ACTH was injected into the substantia nigra - suggests the involvement of the striato-nigral GABAergic pathway. Local injections of ACTH-(1-24) into the periaqueductal gray also induced excessive grooming. Since a second injection of ACTH-(1-24) into the periaqueductal gray did not lead to a grooming response, irrespective of where the first injection of ACTH-(1-24) was given (i.c.v. into the nigra or via the periaqueductal gray) it is suggested that this structure seems to play a primary role in the induction of excessive grooming. Therefore the modulatory effects of the dopaminergic influence on ACTH-(1-24)-induced grooming may be exerted via the striato-nigro-collicular pathway.

Neonatal treatment with monosodium glutamate does not alter grooming behavior induced by novelty or adrenocorticotropic hormone

The increased grooming behavior observed in a novel environment has been attributed to release of peptides derived from proopiomelanocortin (POMC), such as ACTH, alpha-melanocyte-stimulating hormone (alpha-MSH), or beta-endorphin, which themselves can elicit grooming. This is because novelty-induced grooming is attenuated both by hypophysectomy and by antiserum to ACTH injected into the cerebral ventricles. Administration of monosodium glutamate (MSG) to neonatal rats destroys neurons in the arcuate nucleus of the hypothalamus, depleting the brain of POMC peptides, and also hypothalamic dopamine and choline acetyl-transferase activity. Neonatal MSG treatment did not significantly alter the grooming scores of adult rats in either home or novel environments compared to saline-treated animals. There were also no differences between MSG-and saline-treated rats in the grooming scores observed following graded doses of ACTH1-24 (0.2-1.0 micrograms) administered intracerebroventricularly. Thus if increased grooming in the novel environment is due to release into the ventricles of ACTH, alpha-MSH, beta-endorphin, these peptides more likely derive from the pituitary rather than from brain cells, although the failure of the MSG treatment to produce quantitative depletions of cerebral POMC peptides, especially in the brain stem, leaves open the latter possibility.