Differences in sensitivity to neuroleptic blockade: medial forebrain bundle versus frontal cortex self-stimulation

The effects of systemic injections of the dopamine receptor antagonist, cis-flupenthixol were tested on intracranial self-stimulation at electrode sites in the medial forebrain bundle and the medial prefrontal cortex. Changes in the reward effectiveness of the brain stimulation were assessed using a curve-shift paradigm. Low to moderate doses of cis-flupenthixol (0.05, 0.1 and 0.15 mg/kg) consistently produced larger upward shifts in the rate-frequency function for medial forebrain bundle than for medial prefrontal self-stimulation. At the highest doses of cis-flupenthixol (0.15 and 0.2 mg/kg), some of the medial forebrain bundle rats failed to respond, whereas all medial prefrontal rats responded at these doses. These results demonstrate that medial forebrain bundle self-stimulation is much more dependent on dopamine systems than is prefrontal cortex self-stimulation.     

Effects of lesions of various medial forebrain bundle components on lateral hypothalamic self-stimulation.

Unilateral lesions of various medial forebrain bundle components were assessed for their effects on lateral hypothalamic self-stimulation. Damage of areas containig nigrostriatal dopaminergic or ascending noradrenergic neurons had negligible effects on bar pressing, tail moving and alley running for hypothalamic stimulation. Lesions which appeared to destroy most or all of the catecholaminergic fibers in the posterior medial forebrain bundle virtually eliminated reinforced bar pressing and tail moving, but only partially suppressed alley running. The results suggest that brain stimulation reinforcement of the bar press and tail movement tasks depends upon the integrity of neural tissue in the area of the catecholaminergic pathways of the medial forebrain bundle, but not upon specific dopaminergic or noradrenergic systems. The data further suggest that the reinforcement of alley running is at least partially mediated by different neural tissue (possibly non-catecholaminergic) at the level of the posterior medial forebrain bundle lesions.     

Unilaterally activated systems in rats self-stimulating at sites in the medial forebrain bundle, medial prefrontal cortex, or locus coeruleus

Rats with electrodes in either the posterior medial forebrain bundle (MFB), the anterior MFB, the medial prefrontal cortex, or the locus coeruleus self-stimulated during a 45 min period following the injection of [14C]2-deoxyglucose. They were then sacrificed and their brains prepared for autoradiography. The autoradiographs were analyzed for unilaterally activated neural systems, using a computerized image analyzing system to compare the darkness of neural structures on the stimulated side with the darkness of the same structures on the unstimulated side. There was extensive overlap in the neural structures unilaterally activated by stimulation in the anterior and posterior MFB; but there was no overlap between the structures activated by MFB stimulation and the structures activated by stimulation at either of the extradiencephalic sites; nor did the forebrain, diencephalic, and midbrain sites have any readily apparent bilateral effects in common. If there is a substrate common to MFB self-stimulation and extradiencephalic self-stimulation, its activation is not revealed by 2-deoxyglucose autoradiography.     

Basal ganglia neural responses during behaviorally effective deep brain stimulation of the subthalamic nucleus in rats performing a treadmill locomotion test

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective treatment for Parkinson's disease (PD). In spite of proven therapeutic success, the mechanism underlying the benefits of DBS has not been resolved. A multiple-channel single-unit recording technique was used in the present study to investigate basal ganglia (BG) neural responses during behaviorally effective DBS of the STN in a rat model of PD. Rats underwent unilateral dopamine (DA) depletion by injection of 6-hydroxyDA (6-OHDA) into one side of the medial forebrain bundle and subsequently developed a partial akinesia, which was assessed during the treadmill locomotion task. High frequency stimulation (HFS) of the STN restored normal treadmill locomotion behavior. Simultaneous recording of single unit activity in the striatum (STR), globus pallidus (GP), substantia nigra pars reticulata (SNr), and STN revealed a variety of neural responses during behaviorally effective HFS of the STN. Predominant inhibitory responses appeared in the STN stimulation site. Nearly equal numbers of excitatory and inhibitory responses were found in the GP and SNr, whereas more rebound excitatory responses were found in the STR. Mean firing rate did not change significantly in the STR, GP, and SNr, but significantly decreased in both sides of STN during DBS. A decrease in firing rate in the contralateral side of STN provides neural substrate for the clinical observation that unilateral DBS produces bilateral benefits in patients with PD. In addition to the firing rate changes, a decrease in burst firing was observed in the GP and STN. The present study indicates that DBS induces complex modulations of the BG circuit and further suggests that BG network reorganization, rather than a simple excitation or inhibition, may underlie the therapeutic effects of DBS in patients with PD.     

Electrophysiological characteristics of neurons in forebrain regions implicated in self-stimulation of the medial forebrain bundle in the rat

In an attempt to identify neurons likely to play a role in self-stimulation of the medial forebrain bundle (MFB), action potentials of single neurons in the septum and basal forebrain of anesthetized rats were recorded by means of extracellular electrodes. Refractory period estimates were obtained from cells antidromically activated by stimulation of the lateral hypothalamus or ventral tegmental area, and estimates of interelectrode conduction time were obtained from cells that were driven by stimulation of both sites. The results show that some descending MFB axons arising in the medial septum, diagonal band of Broca and neighboring forebrain structures have characteristics comparable to properties of MFB reward neurons inferred from behavioral experiments.     

The substrate for brain-stimulation reward in the lateral preoptic area. I. Anatomical mapping of its boundaries

Given the putative role of the lateral preoptic area as a primary contributor of the cell bodies of origin of the descending pathway linking a subset of lateral hypothalamic and ventral tegmental area reward neurons, the distribution of self-stimulation sites in this structure was mapped in 22 animals using moveable electrodes and threshold procedures. Ninety-seven electrode sites were evaluated with placements ranging from just rostral to the midline convergence of the anterior commissure back to the transition zone between the lateral preoptic and lateral hypothalamic areas; of these, roughly 2/3 supported self-stimulation which was widely observed throughout the lateral preoptic area and medial forebrain bundle. In general, self-stimulation thresholds obtained from lateral sites were most stable, and progressively so approaching more caudal regions. Examination of the slopes of the period/current trade-off functions revealed a tendency for higher values in lateral and caudal sites; in contrast, dorsoventral excursions did not influence these estimates. Taken together, these data provide support for the notion that the substrate for brain-stimulation reward in the lateral preoptic area has a relatively homogeneous distribution that is more diffusely organized than that found in reward sites activated further caudally in the medial forebrain bundle.     

Mechanics of self-stimulation and dopamine release in the nucleus accumbens

Robust self-stimulation can be obtained from electrodes implanted in the medial forebrain bundle. We used in-vivo voltammetry to monitor stimulated dopamine release in the mouse nucleus accumbens during implantation of the stimulating electrodes. The higher the level of stimulated dopamine release during electrode implantation, the lower was the threshold for self-stimulation and the shorter the duration of the stimulation train when it was controlled by animal. We suggest that dopamine release is a reliable indicator of the proximity of the stimulating electrode to the brain reward sites. Inclusion of this indicator solves the problem of large interindividual variation in self-stimulation currents and permits a new approach to studies on mechanisms and pathways involved in brain reward.     

Convergence of projections from the rat hippocampal formation, medial geniculate and basal forebrain onto single amygdaloid neurons: an in vivo extra- and intracellular electrophysiological study.

We recorded extra- and intracellular responses from rat amygdaloid neurons in vivo after electrical stimulation of the hippocampal formation (dentate gyrus, hippocampal fields CA3 and CA4, entorhinal cortex, subicular complex); medial geniculate; and basal forebrain (diagonal band, ventral pallidum, olfactory tubercle, nucleus accumbens, bed nucleus of stria terminalis, lateral preoptic area, substantia innominata). Stimulation of hippocampal formation structures evoked IPSPs or EPSP-IPSP sequences in which the IPSP had a lower threshold than the EPSP. Recordings from candidate inhibitory neurons in the amygdala indicated that excitatory afferents from the hippocampal formation contact both amygdaloid inhibitory and principal neurons (feedforward inhibition), and that the inhibitory neurons have a lower threshold of activation. Medial geniculate stimulation also evoked EPSP-IPSP sequences. In marked contrast to these results, stimulation of basal forebrain structures evoked short latency IPSPs in amygdaloid neurons. This provides the first physiological evidence for direct inhibition of the amygdala by the basal forebrain. Basal forebrain stimulation also evoked EPSP-IPSP sequences in amygdaloid neurons. Individual amygdaloid neurons could show responses to stimulation of the hippocampal formation, basal forebrain, and medial geniculate, indicating that synaptic input from these areas converges onto single amygdaloid cells. The findings provide further information about the synaptic organization of afferents to the amygdala, and indicate that single amygdaloid neurons play a role in the synaptic integration of input from these diverse sources.     

Conditioning-related single unit activity in the frontal cortex of urethane anesthetized rats

Responses of frontal cortex single units to a tone preceding medial forebrain bundle (MFB) stimulation were recorded in urethane anesthetized rats. The animals were implanted with monopolar electrodes for MFB stimulation and, following recovery, stimulation parameters which supported self-stimulation were determined for each rat. Prior to the unit recording experiment, the animals were trained to associate a 2-sec tone with MFB stimulation. Trials were presented at variable intervals. Under urethane anesthesia, single units were isolated and the responses of units to paired and unpaired tones were determined. The results indicate that conditioning-related responses of frontal cortex single units can be recorded in urethane anesthetized rats.     

4-Ethoxyamphetamine: effects on intracranial self-stimulation and in vitro uptake and release of 3H-dopamine and 3H-serotonin in the brains of rats.

Experiments were conducted to compare the effects of 4-ethoxyamphetamine, a novel "designer" amphetamine, with (+)-amphetamine and an earlier "designer" amphetamine, 4-methoxyamphetamine, on rats. (+)-Amphetamine significantly decreased frequency threshold measures in an intracranial self-stimulation (ICSS) procedure using medial forebrain bundle electrodes, while 4-methoxyamphetamine and 4-ethoxyamphetamine increased these ICSS frequency thresholds. 4-Methoxyamphetamine and 4-ethoxyamphetamine had more potent effects on inhibition of uptake and stimulation of spontaneous release of 5-hydroxytryptamine (serotonin) than of dopamine. It is concluded that the neuropsychopharmacological profile of 4-ethoxyamphetamine is unlike that of (+)-amphetamine, but similar to that of 4-methoxyamphetamine, a potent hallucinogen in humans.     

Muscimol inactivation of the septo-preoptic complex affects medial forebrain bundle self-stimulation only when directed at the complex's ventrolateral components

Elements of the septo-preoptic basal forebrain complex, particularly the lateral and medial septum, the diagonal band of Broca, and the magnocellular preoptic area, have been linked to medial forebrain bundle (MFB) self-stimulation. This study examines the roles of these areas in MFB self-stimulation by temporarily inactivating them with 25 and 50ng doses of the GABA(A) receptor agonist muscimol. Changes in performance capacity and stimulation reward effectiveness were evaluated with the rate-frequency curve shift paradigm. When infused into the lateral and medial septum and the vertical limb of the diagonal band of Broca, both doses of muscimol were as ineffective as saline in altering either the rats' maximum rate of response for stimulation or the frequency required to maintain half-maximal response rate (required frequency). However, when infused into the horizontal limb of the diagonal band of Broca or the magnocellular preoptic area, muscimol substantially decreased maximal response rate and modestly increased required frequency. Changes in maximum rate were dose-dependent, but changes in required frequency were not. Muscimol infusions contralateral to the stimulated hemisphere were as effective as ipsilateral infusions; bilateral infusions tended to so suppress responding that resulting rate-frequency curves were often invalid. These results suggest a role in MFB self-stimulation for only the ventrolateral components of the septo-preoptic complex, and support past observations of considerable bilaterality in the neural systems that support self-stimulation.     

Evidence of a supraspinal opioid analgesic mechanism engaged by lateral hypothalamic electrical stimulation

Brief trains of electrical stimulation were applied to the tails of rats to evaluate pain thresholds in the presence and absence of concurrent brain stimulation. Lateral hypothalamic (LH) stimulation, particularly in the dorsolateral medial forebrain bundle, elevated threshold for eliciting a post-stimulus vocalization response. Thresholds for eliciting a simple vocalization and motor response--both of which are organized at lower levels of the CNS than post-stimulus vocalization--were not significantly affected. This restricted form of analgesia was reduced by the opioid antagonist, naltrexone. Rewarding effects of stimulation in these LH sites, as evaluated in tests of self-stimulation threshold, were not affected by naltrexone. These results suggest that LH stimulation activates an opioid analgesic mechanism that is selectively active at a supraspinal level and diminishes the affective consequences of otherwise noxious stimuli.     

Behaviourally derived estimates of excitability in striatal and medial prefrontal cortical self-stimulation sites.

The refractory periods of the substrate underlying brain-stimulation reward were investigated in three rats with moveable electrodes implanted in the rostral caudate-putamen and the medial prefrontal cortex. Acquisition of caudate-putamen self-stimulation occurred within the first session, while self-stimulation for medial prefrontal cortex was observed only after three sessions of caudate-putamen stimulation. The currents required for self-stimulation ranged from 300 to 800 microA (0.1 ms pulse duration) across animals; the maximum response rates averaged roughly 40 bar presses per minute for both structures. Refractory period estimates were obtained from ten caudate-putamen and four medial prefrontal cortex sites. The time course of recovery had the following profile: the curves began to rise at 0.65 ms and 0.95 ms for caudate-putamen and medial prefrontal cortex stimulation, respectively, thereafter increasing to approach an asymptote at 6.00 ms for the caudate-putamen and 6.25 ms for the medial prefrontal cortex. The mean effectiveness value corresponding to the asymptotic portion of the curves was 73% for the caudate-putamen and 69% for the medial prefrontal cortex. Like other forebrain structures, the behaviourally derived refractory periods underlying caudate-putamen and medial prefrontal cortex stimulation, at least at these particular sites, are significantly longer than those observed in most medial forebrain bundle areas, both beginning and ending later. One interpretation for the similarity in their refractory period profiles and the apparent facilitating effect of caudate-putamen stimulation on acquisition of medial prefrontal cortex self-stimulation is that these two regions form part of the same reward substrate.     

The role of intrinsic neurons in lateral hypothalamic self-stimulation

In this brief review, we summarize some of our recent work concerning the effect of a specific lesion of the intrinsic neurons located in the middle part of the lateral hypothalamus on electrical self-stimulation of this structure by electrodes implanted along the medial forebrain bundle. In a first experiment the neurons of the lateral hypothalamus were destroyed unilaterally by local injection of ibotenic acid (4 micrograms in 0.5 microliter). The contralateral side served as the sham-lesion control. Between 10 and 20 days later, electrodes were bilaterally implanted, one in the lesioned area, the other in the contralateral hypothalamus. Intracranial self-stimulation (ICSS) was obtained separately for each electrode, at various current intensities, using a nose-poke response. ICSS from electrodes implanted in the lesioned area was decreased in all cases, whereas ICSS of the sham-lesioned side was normal. In a second experiment, two groups of rats lesioned and implanted as above, received two additional electrodes either in the anterior hypothalamus or in the posterior hypothalamus. In rats with electrodes in the anterior hypothalamus, the lesion produced a large deficit in self-stimulation when stimulation was applied to the anterior electrode ipsilateral to the lesion. Only 3 of 6 rats showed a decrease in ICSS with stimulation of the posterior hypothalamic electrode ipsilateral to the lesion. These results suggest that ICSS in the anterior part of the medial forebrain bundle is sustained by long fibers originating in the middle part of the lateral hypothalamus, while ICSS in the posterior part of the lateral hypothalamus may not depend on the neurons located in the lesioned area.     

Two substrates for medial forebrain bundle self-stimulation: myelinated axons and dopamine axons

The directly activated substrates for medial forebrain bundle (MFB) self-stimulation are primarily low threshold, myelinated axons with absolute refractory periods of 0.4 to 1.2 msec, conduction velocities of 1 to 8 m/sec and current-distance constants of 1000 to 3000 microA/mm2. When small electrode tips or high currents are used, however, a second population of long refractory period (1.2 to 5 msec) axons is added. The excitability properties of this second population are almost identical with those of dopamine (DA) axons. Furthermore, the long-refractory period effects of MFB self-stimulation are reduced, but not completely blocked, by peripheral injections of alpha-flupenthixol, suggesting that dopamine axons make small contributions to MFB self-stimulation when small tips are used. Collision data, strength-duration data and refractory period data in various self-stimulation experiments are compared. Asymmetric collision effects, recently observed in cortical and striatal sites mediating electrically evoked turning, may help determine where synapses are located in circuits mediating electrically evoked behaviors. A neural model of symmetric, asymmetric and mixed collision is proposed.     

Serotonergic mediation of reward within the medial raphe nucleus: some persistent problems in interpretation.

Adult male Sprague Dawley rats were implanted with unipolar stimulating electrodes aimed at the medial forebrain bundle (MFB) or the medial raphe nucleus (MR). All MFB implanted subjects self-stimulated at high stable rates for at least three weeks. Only a minority (1/3) of MR rats self-stimulated at all. Rates for the MR group were considerably more variable, and could not be maintained for more than two weeks. Treatment with methysergide increased MFB self-stimulation but decreased MR self-stimulation. While this result suggests serotonergic mediation of self-stimulation this conclusion must be interpreted cautiously since parachlorophenylalanine (PCPA) reinstated self-stimulation in raphe animals which had spontaneously ceased responding.     

Neuropharmacological and electrophysiological evidence implicating the mesolimbic dopamine system in feeding responses elicited by electrical stimulation of the medial forebrain bundle

The contribution of the mesolimbic dopamine pathway to feeding behavior was investigated in rats in which feeding responses were elicited by electrical stimulation of the medial forebrain bundle at the level of the lateral hypothalamus. Injections of spiroperidol, a dopamine antagonist, into the nucleus accumbens ipsilateral to the stimulating electrode significantly attenuated the elicited feeding responses whereas injecting spiroperidol into the contralateral nucleus accumbens had no effect. The spontaneous discharge rates of neurons of the ventral tegmental area, identified by their electrophysiological characteristics as dopaminergic, were both increased and decreased in response to single pulse stimulation of sites in the medial forebrain bundle from which feeding responses had been elicited. These observations suggest that mesolimbic dopaminergic neurons may have a role in feeding behavior and indicate the need for chronic electrophysiological recording experiments to see whether or not the activity of these neurons is correlated with the initiation of elicited and spontaneous feeding responses.     

Region-specific reductions of intracranial self-stimulation after uncontrollable stress: Possible effects on reward processes

Rates of responding for intracranial self-stimulation from the medial forebrain bundle, nucleus accumbens and substantia nigra were evaluated in mice that had been exposed to either escapable shock, yoked inescapable shock or no shock treatment. Whereas performance was unaffected by escapable shock, marked reductions of responding from the medial forebrain bundle and nucleus accumbens were evident following the uncontrollable shock treatment. Responding from the substantia nigra was unaffected by the stress treatment. Uncontrollable shock is thought to reduce the rewarding value of responding for electrical brain stimulation from those brain regions in which stressors are known to influence dopamine activity.     

Lidocaine inactivation of the ventral pallidum affects responding for brain stimulation reward more than it affects the stimulation's reward value

The ventral pallidum (VP) supports self-stimulation and has anatomical connections that suggest it could be linked to medial forebrain bundle (MFB) self-stimulation. Dorsal VP appears to be more related to dorsal striatopallidum and thus to cognitive control of movement, while ventral VP appears to be more related to linking motivation to action. In this study we challenged MFB self-stimulation by temporarily inactivating dorsal and ventral VP. We assessed changes in performance capacity and stimulation reward value using the rate-frequency curve shift paradigm. VP inactivation, especially in the ventral aspect of the VP ipsilateral to the stimulation site, was more likely to substantially impair maximum response rate than to affect the frequency required to maintain half-maximal responding. These effects were transient, typically disappearing within 20 min following inactivation. Contralateral inactivation was relatively ineffective and bilateral inactivation was surprisingly less effective than ipsilateral inactivation alone, although bilaterally symmetric injection sites were largely confined to the dorsal VP. The fact that inactivation-induced changes in maximum response rate were more prominent than changes in the frequency required to maintain half-maximal responding suggests a role for the ventral VP in linking reward to responding rather than detecting or computing reward value.