Physiological measures of conduction velocity and refractory period for putative reward-relevant MFB axons arising in the rostral MFB

The Convergence Model of Brain Reward Circuitry: Implications for Relief of Treatment-Resistant Depression by Deep-Brain Stimulation of the Medial Forebrain Bundle

Deep-brain stimulation of the medial forebrain bundle (MFB) can provide effective, enduring relief of treatment-resistant depression. Panksepp provided an explanatory framework: the MFB constitutes the core of the neural circuitry subserving the anticipation and pursuit of rewards: the “SEEKING” system. On that view, the SEEKING system is hypoactive in depressed individuals; background electrical stimulation of the MFB alleviates symptoms by normalizing activity. Panksepp attributed intracranial self-stimulation to excitation of the SEEKING system in which the ascending projections of midbrain dopamine neurons are an essential component. In parallel with Panksepp’s qualitative work, intracranial self-stimulation has long been studied quantitatively by psychophysical means. That work argues that the predominant directly stimulated substrate for MFB self-stimulation are myelinated, non-dopaminergic fibers, more readily excited by brief electrical current pulses than the thin, unmyelinated axons of the midbrain dopamine neurons. The series-circuit hypothesis reconciles this view with the evidence implicating dopamine in MFB self-stimulation as follows: direct activation of myelinated MFB fibers is rewarding due to their trans-synaptic activation of midbrain dopamine neurons. A recent study in which rats worked for optogenetic stimulation of midbrain dopamine neurons challenges the series-circuit hypothesis and provides a new model of intracranial self-stimulation in which the myelinated non-dopaminergic neurons and the midbrain dopamine projections access the behavioral final common path for reward seeking via separate, converging routes. We explore the potential implications of this convergence model for the interpretation of the antidepressant effect of MFB stimulation. We also discuss the consistent finding that psychomotor stimulants, which boost dopaminergic neurotransmission, fail to provide a monotherapy for depression. We propose that non-dopaminergic MFB components may contribute to the therapeutic effect in parallel to, in synergy with, or even instead of, a dopaminergic component.

Dopamine neurons do not constitute an obligatory stage in the final common path for the evaluation and pursuit of brain stimulation reward

The neurobiological study of reward was launched by the discovery of intracranial self-stimulation (ICSS). Subsequent investigation of this phenomenon provided the initial link between reward-seeking behavior and dopaminergic neurotransmission. We re-evaluated this relationship by psychophysical, pharmacological, optogenetic, and computational means. In rats working for direct, optical activation of midbrain dopamine neurons, we varied the strength and opportunity cost of the stimulation and measured time allocation, the proportion of trial time devoted to reward pursuit. We found that the dependence of time allocation on the strength and cost of stimulation was similar formally to that observed when electrical stimulation of the medial forebrain bundle served as the reward. When the stimulation is strong and cheap, the rats devote almost all their time to reward pursuit; time allocation falls off as stimulation strength is decreased and/or its opportunity cost is increased. A 3D plot of time allocation versus stimulation strength and cost produces a surface resembling the corner of a plateau (the “reward mountain”). We show that dopamine-transporter blockade shifts the mountain along both the strength and cost axes in rats working for optical activation of midbrain dopamine neurons. In contrast, the same drug shifted the mountain uniquely along the opportunity-cost axis when rats worked for electrical MFB stimulation in a prior study. Dopamine neurons are an obligatory stage in the dominant model of ICSS, which positions them at a key nexus in the final common path for reward seeking. This model fails to provide a cogent account for the differential effect of dopamine transporter blockade on the reward mountain. Instead, we propose that midbrain dopamine neurons and neurons with non-dopaminergic, MFB axons constitute parallel limbs of brain-reward circuitry that ultimately converge on the final-common path for the evaluation and pursuit of rewards.

Brain stimulation reward is altered by affecting dopamine-glutamate interactions in the central extended amygdala

This work compares the effects on brain stimulation reward (BSR) when combining D2 dopamine receptor and AMPA glutamate receptor manipulations in the sublenticular central extended amygdala (SLEAc) and the nucleus accumbens shell (NAc shell). Thirty-seven male Long Evans rats received medial forebrain bundle (MFB) stimulation electrodes and bilateral injection guide cannulae aimed at either the SLEAc or the NAc shell. The rate-frequency paradigm was used to assess drug-induced changes in stimulation reward effectiveness and in response rate following 0.5 μl infusions of 0.50 μg of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) (AMPA receptor antagonist), 10.0 μg of quinpirole (D2 receptor agonist), 0.25 μg of AMPA (AMPA receptor agonist), 3.0 μg of eticlopride (D2 receptor antagonist), 0.50 μg of NBQX with 10.0 μg of quinpirole, and 0.25 μg of AMPA with 3.0 μg of eticlopride. The drugs were injected both ipsi- and contralateral to the stimulation site. AMPA blockade and D2 stimulation synergized to reduce BSR's reward efficacy when directed at the SLEAc contralateral to the stimulation site whereas changes in reward efficacy were primarily D2-dependent following injections into the ipsilateral SLEAc. When injected into the NAc shell the drugs had only one significant effect on the frequency required to maintain half-maximal responding: injections of NBQX with quinpirole ipsilateral to the stimulation site increased required frequency significantly more than did injections of saline. Contrary to expectations, stimulating AMPA receptors with and without co-blockade of D2 receptors also decreased the stimulation's reward efficacy, although these effects may reflect general behavioral disruption more than effects on reward per se. These results indicate a role for the SLEAc in BSR and also suggest that SLEAc neurons ipsi- and contralateral to the stimulated MFB play their roles in BSR through different mechanisms.

Scarce means with alternative uses: robbins' definition of economics and its extension to the behavioral and neurobiological study of animal decision making

Almost 80 years ago, Lionel Robbins proposed a highly influential definition of the subject matter of economics: the allocation of scarce means that have alternative ends. Robbins confined his definition to human behavior, and he strove to separate economics from the natural sciences in general and from psychology in particular. Nonetheless, I extend his definition to the behavior of non-human animals, rooting my account in psychological processes and their neural underpinnings. Some historical developments are reviewed that render such a view more plausible today than would have been the case in Robbins’ time. To illustrate a neuroeconomic perspective on decision making in non-human animals, I discuss research on the rewarding effect of electrical brain stimulation. Central to this discussion is an empirically based, functional/computational model of how the subjective intensity of the electrical reward is computed and combined with subjective costs so as to determine the allocation of time to the pursuit of reward. Some successes achieved by applying the model are discussed, along with limitations, and evidence is presented regarding the roles played by several different neural populations in processes posited by the model. I present a rationale for marshaling convergent experimental methods to ground psychological and computational processes in the activity of identified neural populations, and I discuss the strengths, weaknesses, and complementarity of the individual approaches. I then sketch some recent developments that hold great promise for advancing our understanding of structure–function relationships in neuroscience in general and in the neuroeconomic study of decision making in particular.

Ventral pallidum roles in reward and motivation

In recent years the ventral pallidum has become a focus of great research interest as a mechanism of reward and incentive motivation. As a major output for limbic signals, the ventral pallidum was once associated primarily with motor functions rather than regarded as a reward structure in its own right. However, ample evidence now suggests that ventral pallidum function is a major mechanism of reward in the brain. We review data indicating that (1) an intact ventral pallidum is necessary for normal reward and motivation, (2) stimulated activation of ventral pallidum is sufficient to cause reward and motivation enhancements, and (3) activation patterns in ventral pallidum neurons specifically encode reward and motivation signals via phasic bursts of excitation to incentive and hedonic stimuli. We conclude that the ventral pallidum may serve as an important 'limbic final common pathway' for mesocorticolimbic processing of many rewards.

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.

Critical assessment of how to study addiction and its treatment: human and non-human animal models

Laboratory models, both animal and human, have made enormous contributions to our understanding of addiction. For addictive disorders, animal models have the great advantage of possessing both face validity and a significant degree of predictive validity, already demonstrated. Another important advantage to this field is the ability of reciprocal interplay between preclinical and clinical experiments. These models have made important contributions to the development of medications to treat addictive disorders and will likely result in even more advances in the future. Human laboratory models have gone beyond data obtained from patient histories and enabled investigators to make direct observations of human drug self-administration and test the effects of putative medications on this behavior. This review examines in detail some animal and human models that have led not only to important theories of addiction mechanisms but also to medications shown to be effective in the clinic.

The central extended amygdala network as a proposed circuit underlying reward valuation

The phenomenon of medial forebrain bundle self-stimulation offers a powerful model of reward-based behavior. In particular, it appears to activate a neural system whose natural function is to compute the survival value or utility of present stimuli and to help orchestrate responses toward those inputs. Although the anatomical identity of this system is as yet unknown, recent descriptions of anatomical macrosystems within the basal forebrain lead to the proposal that it may be largely contained within the central extended amygdala network. This paper reviews decades' worth of behavioral and neurophysiological investigations of brain stimulation reward that support or are at least consistent with this idea. The proposed network circuitry underlying self-stimulation is also placed into the larger context of basal forebrain function, specifically, the role of the ventral striatopallidum in linking motivation to behavior, the role of the amygdala in detecting motivationally significant inputs, and the role of the magnocellular complex in communicating reward information to cortical and hippocampal targets.

Time-dependent recovery from the effects of 6-hydroxydopamine lesions of the rat nucleus accumbens on cocaine self-administration and the levels of dopamine in microdialysates

RATIONALE

Neurotoxin induced lesions of dopamine-releasing neurons that innervate the nucleus accumbens (NAcc) alter cocaine self-administration. In addition, elevated extracellular levels of NAcc dopamine (DA) are thought to be central to the biological mechanisms that underlie this behavior.

OBJECTIVES

This study assessed the long-term effects of 6-hydroxydopamine (6-OHDA) induced lesions of the NAcc on cocaine self-administration and the dialysate levels of dopamine ([DA](d)) in this structure to determine if recovery of drug intake was correlated with the DA response.

METHODS

Rats implanted with jugular catheters and bilateral cannulas were trained to self-administer cocaine and subsequently received bilateral intracranial micro-injections of 6-OHDA or vehicle into the NAcc. The levels of DA and cocaine were determined in microdialysates of the NAcc collected during experimental sessions 6-7, 14-16, 29-30, and 44-46 days post-treatment.

RESULTS

The 6-OHDA induced lesions significantly reduced cocaine self-administration for 3 weeks while vehicle treatment had a moderate effect for the first several days. Cocaine-induced increases in NAcc [DA](d) did not return to sham/vehicle treated control levels for 6 weeks in the lesioned group and DA content in the NAcc was 46% of control at 44 days post-lesion.

CONCLUSIONS

Although dopaminergic lesions of the NAcc produced profound effects on cocaine self-administration, responding recovered to control levels before cocaine-induced increases in NAcc [DA](d) while content of DA in the NAcc did not recover. These data suggest that the plasticity of neuronal systems in the NAcc related to cocaine self-administration and their response following 6-OHDA lesions is more complex than restoration of DAergic tone.

Mapping the Neural Substrate Underlying Brain Stimulation Reward with the Behavioral Adaptation of Double-Pulse Methods

Behavioral adaptations of double-pulse methods--primarily collision and refractory period tests--have been employed to unveil the electrophysiological and anatomical characteristics of neural networks of known function. These paradigms are based on trade-off functions: a determination of different combinations of stimuli that yield the same behavioral output. A detailed explanation of the logic and methodology underlying these techniques is elaborated in this paper. The implementation of such approaches to the study of brain stimulation reward (BSR) has provided a means of discriminating between the neurons underlying this behavior from other cells activated by the stimulating electrode, endowing them with a particularly powerful scientific scope. An increasingly detailed portrait of the BSR substrate, both within and outside the medial forebrain bundle, has been emerging as a result of these investigations and is reviewed in this paper. Finally, the challenges associated with these paradigms are discussed and potential solutions as well as future experimental ventures proposed. Attention is drawn to the major contribution of these methods to our understanding of the neural pathways and characteristics underlying BSR.

The substrate for brain-stimulation reward in the lateral preoptic area: III. Connections to the lateral hypothalamic area

Double-pulse tests were used to estimate the refractory periods and anatomical linkage of the reward-relevant fibers that course between the lateral preoptic and lateral hypothalamic areas. In the 1st study, pairs of conditioning and test pulses were delivered to each site, and the interval between pulses varied; recovery from refractoriness was similar at both sites, with the curves generally rising from 0.6 to 2.0 ms. In the 2nd study, the pairs of pulses were delivered to both sites. Six of 7 rats showed evidence of axonal collision, with estimates of conduction velocity that ranged from 0.48 to 8.95 m/s across rats. These results suggest that a wide spectrum of fiber types characterizes the reward-relevant axons that course uninterruptedly between these 2 regions.

Dynamic changes in cytochrome oxidase activity in the amygdala following lesions of rewarding sites in the lateral hypothalamus

The aim of this study was to evaluate neural changes in oxidative metabolism in amygdaloid sub-nuclei following unilateral electrolytic lesions of lateral hypothalamic sites that supported brain stimulation reward. A histochemical analysis of cytochrome oxidase activity, comparing lesioned to non-lesioned sides in the amygdala, revealed a significant reduction of oxidative metabolism in the cortical nucleus and, to a lesser degree, in the adjacent piriform cortex; this effect was observed 2-4 weeks after the lesion, with complete recovery by the eighth week in the case of the cortical nucleus only. No particular pattern in cytochrome oxidase activity was detected in other amygdaloid sub-nuclei that were examined, including the basolateral and medial nucleus. Within both structures, the most pronounced decreases in metabolic activity were observed at roughly the same level, corresponding to the posterolateral and posteromedial levels of the cortical nucleus and just anterior to the amygdalopiriform transition. These results suggest that within the amygdaloid complex, the cortical sub-nuclei and possibly the neighbouring piriform cortex contribute more to modulating lateral hypothalamic self-stimulation than components of the central extended amygdala.

The substrate for brain-stimulation reward in the lateral preoptic area. II. Connections to the ventral tegmental area

This experiment investigated the existence of a direct anatomical connection between lateral preoptic and ventral tegmental areas that mediate brain stimulation reward using the behavioral adaptation of the collision test. This test is a double-pulse, two-electrode technique based on the axonal conduction failure that occurs when two separate sites in the same axon bundle are concurrently stimulated. This anatomical arrangement is inferred from the shape of the function relating the effectiveness of double-pulse stimulation to the interval between pulses. In this study, nine rats with a total of 44 pairs of sites were examined. In two pairs only was there a profile suggestive of an axonal collision effect, while the double-pulse effectiveness curve consistent with the properties of transynaptic collision was apparent for a single pair of sites; the remaining 93% were associated with relatively flat effectiveness curves. While electrode misalignment could be responsible for these results, there was adequate sampling to suggest that the preponderance of first stage signals that give rise to the rewarding effects mediated by the lateral preoptic and ventral tegmental areas do not travel along the same fiber bundle. However, stimulation applied to both sites concurrently produces a summation that is roughly 40% greater than stimulation at either site alone, suggesting reasonable integration of the reward signals generated by lateral preoptic and ventral tegmental area stimulation.

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.

Regional differences in desensitization of c-Fos expression following repeated self-stimulation of the medial forebrain bundle in the rat

The acute self-stimulation of the medial forebrain bundle was reported to induce the expression of c-Fos, the protein product of c-fos, an immediate early gene, in the central nervous system. In the present study, we examined regional changes in c-Fos expression in several reward-related areas of rat brain in response to short- and long-term exposure to self-stimulation of the medial forebrain bundle. Short-term one-hour stimulation of the medial forebrain bundle for one day after training, which evoked steady self-stimulation behavior, significantly increased the number of c-Fos-positive neurons bilaterally in all of 15 brain structures assayed, as compared to the non-stimulation control. Among them, structures showing a larger number of the stained neurons on the stimulated side were the anterior olfactory nucleus, amygdala, medial caudate-putamen complex, lateral septum, bed nucleus of the stria terminals, ventral pallidum, substantia innominata, lateral preoptic area, medial preoptic area, lateral hypothalamus rostral to the stimulating electrodes, and substantia nigra. Long-term stimulation of the medial forebrain bundle once daily for five successive days, which maintained consistently stable self-stimulation behavior, also increased the number of c-Fos-positive neurons in the aforementioned structures, as compared to the control. However, the long-term rewarding stimulation diminished the increased number of labeled neurons, as compared to the short-term rewarding stimulation. Seven areas, medial caudate-putamen complex, ventral pallidum, substantia innominata, lateral preoptic area, medial preoptic area, rostral lateral hypothalamus and substantia nigra, showed asymmetrical, ipsilateral predominance after the short- and long-term stimulation. However, the stained neuron count in those areas after the long-term stimulation was reduced to less than 50% of that found after the short-term stimulation with the exception of lateral preoptic area and rostral lateral hypothalamus. The results suggest that the development of desensitization of c-Fos response may differ among the reward-relevant brain regions as a consequence of repeated self-stimulation. They also indicate that a larger portion of neurons in the lateral preoptic area and rostral lateral hypothalamus may be implicated in both short- and long-term self-stimulations of the medial forebrain bundle.

Effects of NMDA lesions of the medial basal forebrain on LH and VTA self-stimulation

Rewarding stimulation of the medial forebrain bundle (MFB) increases Fos-like immunoreactivity in many brain areas, including an ipsilateral, basal forebrain region extending from the medial preoptic area (MPO) to the lateral preoptic area, and substantia innominata. Excitotoxic lesions of the lateral portion of this region have been found to produce large sustained or transient increases in the number of pulses required to maintain half-maximal lever-pressing (required number of pulses) for MFB stimulation. In the present study, changes in self-stimulation of the lateral hypothalamus and ventral tegmental area were assessed following excitotoxic lesions of more medial structures, including the MPO and bed nucleus of the stria terminalis. Increases in the required number of pulses (up to 0.16 log10 units) were seen in only 2 of 10 subjects. In two other rats, the reward effectiveness of the stimulation was moderately increased after the lesion as manifested in decreases of up to 0.14 log10 units in the required number. No appreciable change from baseline was seen in the remaining six subjects. The simplest interpretation of these results is that neurons with cell bodies in the medial portion of the basal forebrain may make a smaller contribution to the rewarding effect of MFB stimulation than neurons in the lateral portion.

Medial forebrain bundle lesions fail to structurally and functionally disconnect the ventral tegmental area from many ipsilateral forebrain nuclei: implications for the neural substrate of brain stimulation reward

Lesions in the medial forebrain bundle rostral to a stimulating electrode have variable effects on the rewarding efficacy of self-stimulation. We attempted to account for this variability by measuring the anatomical and functional effects of electrolytic lesions at the level of the lateral hypothalamus (LH) and by correlating these effects to postlesion changes in threshold pulse frequency (pps) for self-stimulation in the ventral tegmental area (VTA). We implanted True Blue in the VTA and compared cell labeling patterns in forebrain regions of intact and lesioned animals. We also compared stimulation-induced regional [14C]deoxyglucose (DG) accumulation patterns in the forebrains of intact and lesioned animals. As expected, postlesion threshold shifts varied: threshold pps remained the same or decreased in eight animals, increased by small but significant amounts in three rats, and increased substantially in six subjects. Unexpectedly, LH lesions did not anatomically or functionally disconnect all forebrain nuclei from the VTA. Most septal and preoptic regions contained equivalent levels of True Blue label in intact and lesioned animals. In both intact and lesioned groups, VTA stimulation increased metabolic activity in the fundus of the striatum (FS), the nucleus of the diagonal band, and the medial preoptic area. On the other hand, True Blue labeling demonstrated anatomical disconnection of the accumbens, FS, substantia innominata/magnocellular preoptic nucleus (SI/MA), and bed nucleus of the stria terminalis. [14C]DG autoradiography indicated functional disconnection of the lateral preoptic area and SI/MA. Correlations between patterns of True Blue labeling or [14C]deoxyglucose accumulation and postlesion shifts in threshold pulse frequency were weak and generally negative. These direct measures of connectivity concord with the behavioral measures in suggesting a diffuse net-like connection between forebrain nuclei and the VTA.

Rewarding brain stimulation induces only sparse Fos-like immunoreactivity in dopaminergic neurons

In this study, c-fos immunohistochemistry was used to identify the brain regions activated by rewarding brain stimulation in rats. Rats had monopolar electrodes implanted in the medial forebrain bundle and were allocated to either a self-stimulation (n = 4), yoked stimulation (n = 4) or no stimulation (n = 6) group. In a single 1 h test session, each rat in the self-stimulation group made 1000 nose poke responses with each response followed by a 0.5 s train of brain stimulation. Rats in the yoked-stimulation group were paired with a partner in the self-stimulation group and received brain stimulation whenever their partner did. However, their nose poke responses did not trigger stimulation. This yoked procedure was thus used to identify any Fos-like immunoreactivity due to operant responding. Rats in the no stimulation group were placed in the same apparatus as the other rats but received no brain stimulation and were thus used to assess baseline Fos-like immunoreactivity. Results showed that stimulation increased Fos-like immunoreactivity in many areas of the brain in both the self-stimulation and yoked groups. The areas with the highest Fos-like immunoreactivity were ipsilateral to the electrode site and included the medial prefrontal cortex, lateral septum, nucleus accumbens (shell), the medial and lateral preoptic areas, bed nucleus of the stria terminalis, central amygdala, lateral habenula, dorsomedial hypothalamus, lateral hypothalamus and the anterior ventral tegmental area. Bilateral Fos-like immunoreactivity was evident in the nucleus accumbens core, paraventricular nucleus of the hypothalamus, the retrorubral fields and the locus coeruleus. A double-labelling procedure identifying both Fos and tyrosine hydroxylase was used to show that very few (< 5%) of the A10 dopamine cell bodies in the ventral tegmental area expressed Fos following brain stimulation. In contrast, most of the noradrenergic neurons of the locus coeruleus (A6), rubrospinal tract (A5) and pontine tegmental area (A7) were Fos positive. Overall, the results show that rewarding, brain stimulation induces Fos-like immunoreactivity in many forebrain regions but only sparsely in mesolimbic and mesocortical dopamine neurons. The similar patterns of Fos-like immunoreactivity seen in the self-stimulation and yoked-stimulation groups suggests that the operant responding for brain stimulation causes minimal Fos expression in itself.

Fos-like immunoreactivity in the caudal diencephalon and brainstem following lateral hypothalamic self-stimulation

Fos immunohistochemistry was used to stain neurons in the caudal diencephalon, midbrain and hindbrain driven by rewarding stimulation of the lateral hypothalamus (LH). Increases in Fos-like immunoreactivity were most pronounced ipsilateral to the site of stimulation and tended to be confined within discrete structures such as the posterior LH, arcuate nucleus, ventral tegmental area (VTA), central gray, dorsal raphé, pedunculopontine area (PPTg), parabrachial nucleus, and locus coeruleus. At least two of these structures, the VTA and PPTg, have been implicated in medial forebrain bundle self-stimulation.

Fos-like immunoreactivity in forebrain regions following self-stimulation of the lateral hypothalamus and the ventral tegmental area

According to the descending-path hypothesis, the direct excitation of descending fibers linking the lateral hypothalamus (LH) and ventral tegmental area (VTA) contributes to the rewarding effect produced by electrical stimulation of the medial forebrain bundle (MFB). To visualize forebrain neurons activated by stimulation of both the LH and VTA, Fos-like immunoreactivity (FLIR) in forebrain regions was assessed following self-stimulation of these two sites in male rats. Among the regions where FLIR was greater in the stimulated hemisphere following either LH or VTA stimulation were the anterior LH, the substantia innominata, and the bed nucleus of the stria terminalis, and olfactory tubercle. These findings are analyzed with reference to the effects of forebrain lesions on self-stimulation of the MFB. Advantages and limitations of using FLIR to identify neurons activated by rewarding stimulation are discussed.

Neural basis of utility estimation

The allocation of behavior among competing activities and goal objects depends on the payoffs they provide. Payoff is evaluated among multiple dimensions, including intensity, rate, delay, and kind. Recent findings suggest that by triggering a stream of action potentials in myelinated, medial forebrain bundle axons, rewarding electrical brain stimulation delivers a meaningful intensity signal to the process that computes payoff.

Increased ipsilateral expression of Fos following lateral hypothalamic self-stimulation

Immunohistochemical labeling of Fos protein was used to visualize neurons activated by rewarding stimulation of the lateral hypothalamic level of the medial forebrain bundle (MFB). Following training and stabilization of performance, seven rats were allowed to self-stimulate for 1 h prior to anesthesia and perfusion. Brains were then processed for immunohistochemistry. Two control subjects were trained and tested in an identical manner except that the stimulator was disconnected during the final 1 h test. Among the structures showing a greater density of labeled neurons on the stimulated side of the brains of the experimental subjects were the septum, lateral preoptic area (LPO), medial preoptic area, bed nucleus of the stria terminalis, substantia innominata (SI), and the lateral hypothalamus (LH). Several of these structures, the LPO, SI, and LH, have been implicated in MFB self-stimulation by the results of psychophysical, electrophysiological, and lesion studies.