Self-stimulation of the MFB or VTA after microinjection of haloperidol into the prefrontal cortex of the rat

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.

The acquisition of self-stimulation of the medical prefrontal cortex following exposure to escapable or inescapable footshock

The effect of acute stress on the acquisition of an instrumental action reinforced by electrical stimulation of the medial prefrontal cortex (MPC) was investigated by exposing rats to either escapable, inescapable or no footshock prior to daily self-stimulation training sessions. Treatment with inescapable footshock did not affect the number of sessions required for acquisition of MPC self-stimulation but did increase the rate of responding over acquisition sessions compared with the no-shock group. When the treatment footshock was escapable, however, both a facilitation in acquisition, as indexed by a reduction in the number of sessions to criterion, and an increase in the rate of MPC self-stimulation was found. These data were interpreted as offering evidence for the operation of a dopaminergic mechanism in the acquisition of MPC self-stimulation. Further, they indicate, contrary to the reported effects of footshock on self-stimulation of other brain areas, that exposure to acute stress has a facilitatory effect on the rate of self stimulation of the MPC.

The dopamine innervation of the visceral cortex mediates the aversive effects of opiates

We have previously reported that opiates acting on peripheral receptors produce aversive effects whereas opiates acting on central brain receptors produce rewarding effects. The neurotransmitter dopamine (DA) has previously been implicated in both opiate reinforcing (positive) and aversive (negative) effects. We, therefore, chose to investigate the effects of disruption of DA systems on these two motivational properties of the opiate, morphine. Moreover, we sought to determine the brain site where dopamine might act as a mediator of these motivational effects. One group of rats received 6-hydroxydopamine (6-OHDA) lesions of the visceral (agranular insular) cortex to destroy dopaminergic innervation to this area. A separate group of animals were pretreated with intraperitoneal (IP) injections of the DA receptor blocker, alpha-flupenthixol (0.8 mg/kg), followed in both groups by 15 mg/kg (IP) morphine. Both 6-OHDA-lesioned and alpha-flupenthixol-pretreated subjects failed to develop the normal aversion to saccharin seen in control groups following conditioned taste aversion training with morphine. In a place conditioning paradigm, the aversive effects produced by low IP injections of morphine (acting on peripheral receptors) were blocked by 6-OHDA lesions of the visceral cortex. However, DA depletion of the visceral cortex did not disrupt the ability of animals to acquire a morphine place preference. Taken together, these results indicate that DA innervation of the visceral cortex mediates the aversive, but not the rewarding, properties of opiates.

Cortical and ventral tegmental systems exert opposing influences on self-stimulation from the prefrontal cortex

Intracranial self-stimulation (ICSS) was obtained from 3 areas of anteromedial cortex: the prelimbic area (Brodman's area 32), the anterior cingulate area and the posterior cingulate area. Electrical stimulation in the prelimbic and anterior cingulate areas also produces a behavioral inhibition which is most pronounced at anterior sites (i.e. prelimbic) and declines at increasingly more posterior sites. It was found that the acquisition of responding for ICSS and the magnitude of amphetamine's facilitation of ICSS were inversely related to the degree of behavioral inhibition. These data and the ability of amphetamine to reverse prefrontal stimulation-induced inhibition suggest an important interaction between the prefrontal cortex and the mesolimbic dopamine systems in the control of goal-directed behavior. A model involving cortical suppression of mesolimbic dopamine function is discussed.

Brain reward circuitry: four circuit elements "wired" in apparent series

Activation of a variety of anatomically distinct sites in the central nervous system can produce rewarding states. Four central reward phenomena are amphetamine injections into nucleus accumbens, morphine injections into the ventral tegmental area, electrical stimulation of the ventral tegmental area, and electrical stimulation of the lateral hypothalamic medial forebrain bundle. Current evidence suggests that these four rewarding events trigger activity in elements of a common reward circuit and that the elements are connected in series. The four partially identified elements in this circuit are (1) descending, fast-recovering, short refractory period fibers of the medial forebrain bundle, (2) separate, opioid peptide-containing afferents to the ventral tegmental area, (3) the dopaminergic cells projecting from the ventral tegmental area to nucleus accumbens, and (4) the dopaminoceptive cells of nucleus accumbens.

Effects of agonists and antagonists of D1 and D2 dopamine receptors on self-stimulation of the medial prefrontal cortex in the rat

The possible participation of D1 versus D2 dopamine receptors in mediating dopaminergic neurotransmission of self-stimulation (SS) in the medial prefrontal cortex (MPC) of the rat was studied neuropharmacologically. Intracerebral as well as intraperitoneal injections of agonists and antagonists of dopamine receptors were used in this study. In all experiments performed with systemic injections, spontaneous motor activity (SM) was measured parallel to self-stimulation behavior as control for non specific effects of the drugs. Intracranial injections were done unilaterally serving SS of the contralateral side (not injected or injected with 0.9% NaCl) as control in the same animals. Spiroperidol and pimozide were used as D1-D2 dopamine antagonists, while sulpiride was used as a specific D2 antagonist. Apomorphine was used as D1-D2 agonist, while bromocriptine and lergotrile were used at doses in which these ergot drugs are considered predominantly D2 agonists. Sulpiride, intraperitoneally or intracerebrally injected at the same locus at which the stimulating electrode was located produced no effect on SS. On the contrary, the D1-D2 antagonists, spiroperidol and pimozide intraperitoneally or intracerebrally injected produced a dose-dependent decrease on SS. On the basis of these data it is suggested, that the dopamine neurotransmission involved in SS of the MPC is mediated via D1 dopamine receptors. This suggestion is further emphasized by the results obtained with the agonists, apomorphine, bromocriptine and lergotrile. Apomorphine produced a dose-related decrease on SS and a decrease at lower doses and an increase at higher doses on SM. Bromocriptine and lergotrile had, on the contrary, no effect on SS and a dose-related decrease on SM.

Discriminating between reward and performance: a critical review of intracranial self-stimulation methodology

Despite numerous pharmacological investigations of intracranial self-stimulation (ICSS), the substrates of this behavior have yet to be completely understood. In view of the likelihood that inadequate methodology has hindered the quest for these substrates, the present review was undertaken. Criteria for ICSS methodology should include not only the ability to discriminate reward from gross performance deficit, but also adequate capacity (ability to generate experimental data at a reasonable rate). For numerous reasons, bar-pressing on a continuous reinforcement schedule fails the first criterion despite its ease and rapidity. The use of partial reinforcement schedules may alleviate some of these shortcomings. Analysis of drug-induced response decrement patterns can discriminate gross motoric incapacity from other variables, although the question of subtle response maintenance deficits remains to be answered. Measurements of response rates using alternative operants do not differentiate reward and performance adequately. More promising, "rate-free" measures using locomotion as an operant include the two-platform method of Valenstein and the "locus of rise" method. Comparison of drug effects on ICSS with those on alternate tasks are fraught with pitfalls including the problems of assuring equivalent rates and patterns of responding. The use of differential electrode placements is ideally suited for neurochemically well-characterized drugs, particularly if "double dissociations" can be established during studies of multiple placements. Presentation of different current intensities or frequencies permits the compilation of rate-intensity functions, and drug-induced shifts in these functions have considerable analytical power. Self-regulation of current intensity constitutes a powerful tool that has yet to realize its full potential in the pharmacological study of ICSS. Extensive studies involving self-regulation of stimulation duration ("shuttlebox" studies) suggest that this method may be highly versatile despite several practical difficulties. It is concluded that at least six of these methods appear to do a reasonable job of excluding gross performance deficit. However, the possible influences of other factors, such as subtle response maintenance deficit, incentive or arousal, remain to be resolved in view of the multifactorial nature of ICSS. Multiple tests for ICSS drug or lesion studies are advocated whenever feasible, as no single test appears capable of resolving all theoretical complexities.

Self-stimulation of the nucleus accumbens and ventral tegmental area of Tsai attenuated by microinjections of spiroperidol into the nucleus accumbens

The contribution of dopaminergic neurons to self-stimulation of the ventral tegmental area, nucleus accumbens and prefrontal cortex was investigated. The ventral tegmental area is the site of non-striatal dopaminergic neurons and their axons project to the nucleus accumbens and prefrontal cortex. Injections of spiroperidol, a dopamine antagonist, into the nucleus accumbens significantly reduced self-stimulation of the ipsilateral ventral tegmental area but did not influence self-stimulation of the contralateral ventral tegmental area. Injections of spiroperidol into the prefrontal cortex did not reduce self-stimulation of the ipsilateral or contralateral ventral tegmental area. Electrical stimulation of sites in the nucleus accumbens positive for self-stimulation antidromically activated neurons of the ventral tegmental area, and a reduction of discharge of these neurons following administration of apomorphine suggested that they were dopaminergic neurons. These observations provide additional evidence implicating dopaminergic neurons in brain-stimulation reward and suggest that dopaminergic neurons contribute to self-stimulation of the nucleus accumbens but not the prefrontal cortex.

Self-stimulation of the sulcal prefrontal cortex in the rat: direct evidence for ascending dopaminergic mediation

Intracranial self-stimulation (ICSS) of the prefrontal cortex dorsal to the rhinal sulcus in rats has been abolished by means of injections of 6-hydroxydopamine (6-OHDA) (4 micrograms/2 microliters) into the ascending trajectory of the A10 mesocortical dopaminergic fibers ipsilateral to the stimulation electrodes. Similar injections made contralateral to the stimulation electrodes produced a transient attenuation of this self-stimulation. All cases of such lesions were confirmed with the use of Vibratome histochemical fluorescence examination of each of the dopamine projection areas in the forebrain.

In vivo neurochemical analysis, by push-pull perfusion, of the mesocortical dopaminergic system of the rat during self-stimulation

The region immediately adjacent to a self-stimulation site in the medial prefrontal cortex of the unanesthetized rat was prelabeled with 0.5 mu Ci 14C-dopamine (DA) injected through an indwelling guide cannula. Then successive 5 min push-pull perfusions of the site with an artificial CSF were carried out at a rate of 25 microliter/min so that a washout curve of declining radioactivity was generated under control conditions. When square wave 100 Hz pulses were delivered to the contiguous self-stimulation site, the release of 14C-DA was enhanced either during the actual interval of electrical stimulation or in the perfusion sample collected immediately thereafter. In parallel experiments, however, self-stimulation by the rat of its ventral tegmental area failed to alter the kinetics of 14C-DA release from the cortex when homologous loci were perfused. Analyses by thin-layer chromatography of the perfusates for their content of catechol metabolities revealed that the homovanillic acid fraction declined during stimulation, whereas the level of DOPAC remained relatively elevated. Evidence was also obtained for the new synthesis and subsequent release of norepinephrine during the stimulation of the cortex of the rat. These results suggest that endogenous dopamine, because of the notable alterations in its release and metabolism, plays an important synaptic role in the mediation of self-stimulation behavior at the level of the cerebral cortex.