Fiber pathways associated with cerebellar self-stimulation in the rat: a retrograde and anterograde tracing study

From back to front: A functional model for the cerebellar modulation in the establishment of conditioned preferences for cocaine-related cues

It is now increasingly clear that the cerebellum may modulate brain functions altered in drug addiction. We previously demonstrated that cocaine-induced conditioned preference increased activity at the dorsal posterior cerebellar vermis. Unexpectedly, a neurotoxic lesion at this region increased the probability of cocaine-induced conditioned preference acquisition. The present research aimed at providing an explanatory model for such as facilitative effect of the cerebellar lesion. First, we addressed a tracing study in which we found a direct projection from the lateral (dentate) nucleus to the ventral tegmental area (VTA) that also receives Purkinje axons from lobule VIII in the vermis. This pathway might control the activity and plasticity of the cortico-striatal circuitry. Then we evaluated cFos expression in different regions of the medial prefrontal cortex and striatum after a lesion in lobule VIII before conditioning. Additionally, perineuronal net (PNN) expression was assessed to explore whether the cerebellar lesion might affect synaptic stabilization mechanisms in the medial prefrontal cortex (mPFC). Damage in this region of the vermis induced general disinhibition of the mPFC and striatal subdivisions that receive dopaminergic projections, mainly from the VTA. Moreover, cerebellar impairment induced an upregulation of PNN expression in the mPFC. The major finding of this research was to provide an explanatory model for the function of the posterior cerebellar vermis on drug-related memory. In this model, damage of the posterior vermis would release striatum-cortical networks from the inhibitory tonic control exerted by the cerebellar cortex over VTA, thereby promoting drug effects.

Cerebellar modulation of the reward circuitry and social behavior

The cerebellum has been implicated in a number of nonmotor mental disorders such as autism spectrum disorder, schizophrenia, and addiction. However, its contribution to these disorders is not well understood. In mice, we found that the cerebellum sends direct excitatory projections to the ventral tegmental area (VTA), one of the brain regions that processes and encodes reward. Optogenetic activation of the cerebello-VTA projections was rewarding and, in a three-chamber social task, these projections were more active when the animal explored the social chamber. Intriguingly, activity in the cerebello-VTA pathway was required for the mice to show social preference in this task. Our data delineate a major, previously unappreciated role for the cerebellum in controlling the reward circuitry and social behavior.

Drive and Reinforcement Circuitry in the Brain: Origins, Neurotransmitters, and Projection Fields

Brain stimulation has identified two central subsets of stimulation sites with motivational relevance. First, there is a large and disperse set of sites where stimulation is reinforcing, increasing the frequency of the responses it follows, and second, a much more restricted set of sites where-along with reinforcement-stimulation also has drive-like effects, instigating feeding, copulation, predation, and other motivated acts in otherwise sated or peaceful animals. From this work a dispersed but synaptically interconnected network of reinforcement circuitry is emerging: it includes afferents to the ventral tegmental area and substantia nigra; the dopamine systems themselves; glutamatergic afferents to the striatum; and one of two dopamine-receptor-expressing efferent pathways of the striatum. Stimulation of a limited subset of these sites, including descending inhibitory medial forebrain bundle fibers, induces both feeding and reinforcement, and suggests the possibility of a subset of fibers where stimulation has both drive-like and reinforcing effects. This review stresses the common findings of sites and connectivity between electrical and optogenetic studies of core drive and reinforcement sites. By doing so, it suggests the biological importance of optogenetic follow-up of less-publicized electrical stimulation findings. Such studies promise not only information about origins, neurotransmitters, and connectivity of related networks, by covering more sensory and at least one putative motor component they also promote a much deeper understanding of the breadth of motivational function.

Dual roles of dopamine in food and drug seeking: the drive-reward paradox

The question of whether (or to what degree) obesity reflects addiction to high-energy foods often narrows to the question of whether the overeating of these foods causes the same long-term neuroadaptations as are identified with the late stages of addiction. Of equal or perhaps greater interest is the question of whether common brain mechanisms mediate the acquisition and development of eating and drug-taking habits. The earliest evidence on this question is rooted in early studies of brain stimulation reward. Lateral hypothalamic electrical stimulation can be reinforcing in some conditions and can motivate feeding in others. That stimulation of the same brain region should be both reinforcing and drive inducing is paradoxical; why should an animal work to induce a drive-like state such as hunger? This is known as the drive-reward paradox. Insights into the substrates of the drive-reward paradox suggest an answer to the controversial question of whether the dopamine system--a system downstream from the stimulated fibers of the lateral hypothalamus--is more critically involved in wanting or in liking of various rewards including food and addictive drugs. That the same brain circuitry is implicated in the motivation for and the reinforcement by both food and addictive drugs extends the argument for a common mechanism underlying compulsive overeating and compulsive drug taking.

Relapse to Cocaine-Seeking After Hippocampal Theta Burst Stimulation

Treatment efforts for cocaine addiction are hampered by high relapse rates. To map brain areas underlying relapse, we used electrical brain stimulation and intracranial injection of pharmacological compounds after extinction of cocaine self-administration behavior in rats. Electrical stimulation of the hippocampus containing glutamatergic fibers, but not the medial forebrain bundle containing dopaminergic fibers, elicited cocaine-seeking behavior dependent on glutamate in the ventral tegmental area. This suggests a role for glutamatergic neurotransmission in relapse to cocaine abuse. The medial forebrain bundle electrodes supported intense electrical self-stimulation. These findings suggest a dissociation of neural systems subserving positive reinforcement (self-stimulation) and incentive motivation (relapse).

Lesions and inactivation implicate dorsolateral hindbrain in MFB self-stimulation

Two experiments explored the role of the motor nucleus of the trigeminal nerve (Mo5) and surrounding area in the rewarding effects of medial forebrain bundle (MFB) stimulation. In the first, eight rats received serial bilateral lesions of the target region. The reward value of MFB stimulation was assessed at 200, 400, and 800 microA using the rate-frequency curve shift paradigm. In five rats, no lesions affecting the motor nucleus or its surrounding area affected the frequency required to maintain half-maximal response rate at any current. One rat with a relatively ventrally placed lesion showed substantial enhancement of stimulation reward value at two currents, while two rats with lesions affecting the area around the descending fibers of the superior cerebellar peduncle (scp) showed substantial increases in required frequency. In the second experiment, six rats received uni- and bilateral injections of lidocaine to temporarily inactivate the target area. Two rats with injections centered near the descending fibers of the scp showed substantial increases in required frequency, as great as 0.30 log(10) units. Two rats with injections slightly rostral to these showed little change in required frequency. Two rats with injections in the ventral cerebellum, just lateral to the fastigial nucleus, showed increases in required frequency, particularly following injections contralateral to the MFB stimulation site. These data are interpreted to imply a role for the area around the lateral pole of the scp, perhaps including axons arising from the cerebellum, in MFB stimulation reward.

Pontine and mesencephalic substrates of self-stimulation

Single or twin, moveable monopolar stimulating electrodes were implanted in male adult rats in order to map the medial pons and mesencephalon for self-stimulation behaviour. The electrodes were implanted 6 mm below the surface of the skull and subsequently moved down by steps of 0.13 or 0.16 mm. Each bar press in a Skinner box delivered a train (0.4 s in duration) of cathodal rectangular pulses of fixed intensity (200 microA) and width (0.1 ms). Self-stimulation was recorded from zero to the maximum performance by varying the number of pulses per stimulating train. The rewarding efficacy of the stimulation at each electrode location was inferred from determination of the pulse period corresponding to the threshold and half-maximal performance. Out of 361 mesencephalic and pontine sites sampled, 289 supported self-stimulation. Within the metencephalon, the study revealed a continuous band of positive sites, extending over a dorso-ventral distance of 4 mm, between the floor of the aqueduct and the pontine nuclei. Hence, all electrode locations in the central grey, dorsal raphe and median raphe supported self-stimulation. Within the mesencephalon, the positive band was restricted between the floor of the central grey and the middle part of the interpeduncular nucleus. At the rostral mesencephalon, it shifted laterally towards the substantia nigra. The overlap between the self-stimulation sites and some of the best known ascending and descending pathways is discussed.

Post-stimulation excitability of mediodorsal thalamic self-stimulation

The post-stimulation excitability of the substrate for brain stimulation reward in the mediodorsal thalamus was assessed using equal- and unequal-pulse procedures. In 3 rats, refractory periods were found to begin no earlier than 1 ms and to end as late as 10 ms. Using test (T) pulses 1.5 times the amplitude of condition (C) pulses, the contribution of absolute and relative refractory periods was determined in one subject. No change in the slope of the recovery function was obtained in this condition, suggesting that several populations of neurons with different absolute refractory periods compose the behaviorally relevant substrate. A large supernormal contribution, evaluated by increasing the C amplitude to 1.5T, occurred between 3 and 10 ms with a peak at 7.5 ms. These results suggest that mediodorsal thalamic self-stimulation is mediated by a wide range of small, probably unmyelinated fibers.