The Connections of the Medial Part of the Subthalamic Nucleus in the Rat: Evidence for a Parallel Organization

Targeting the subthalamic nucleus in a preclinical model of alcohol use disorder

BACKGROUND

The subthalamic nucleus (STN) has only recently been considered to have a role in reward processing. In rats, inactivation of the STN by lesion or high-frequency stimulation (HFS) decreases motivation for cocaine but increases motivation for sucrose. For ethanol, the effect of STN lesion depends on the individual's baseline intake; decreasing motivation for ethanol in rats with lower ethanol intake, while increasing motivation for ethanol in rats with higher-but still limited-ethanol intake. However, the involvement of the STN in behaviour more closely resembling some aspects of alcohol use disorder has not been assessed. This study aimed to determine the effect of STN lesions on the escalation of ethanol intake, subsequent increases in the motivation to "work" for ethanol and the choice of ethanol over a non-drug alternative.

RESULTS

We found that STN lesion prevented increases in ethanol intake observed during intermittent ethanol access and after a long period of ethanol privation. STN lesion also decreased the motivation to work for ethanol after escalated intake. Surprisingly, STN lesion increased the choice of alcohol over saccharin. This was associated with a blunting of the hedonic responses to the taste of the reinforcement alternatives.

CONCLUSION

These results evidence the involvement of the STN in different ethanol-motivated behaviours and therefore position the STN as an interesting target for the treatment of alcohol use disorders.

The subthalamic nucleus keeps you high on emotion: behavioral consequences of its inactivation

The subthalamic nucleus (STN) belongs to the basal ganglia and is the current target for the surgical treatment of neurological and psychiatric disorders such as Parkinson's Disease (PD) and obsessive compulsive disorders (OCD), but also a proposed site for the treatment of addiction. It is therefore very important to understand its functions in order to anticipate and prevent possible side-effects in the patients. Although the involvement of the STN is well documented in motor, cognitive and motivational processes, less is known regarding emotional processes. Here we have investigated the direct consequences of STN inactivation by excitotoxic lesions on emotional processing and reinforcement in the rat. We have used various behavioral procedures to assess affect for neutral, positive and negative reinforcers in STN lesioned rats. STN lesions reduced affective responses for positive (sweet solutions) and negative (electric foot shock, Lithium Chloride-induced sickness) reinforcers while they had no effect on responses for a more neutral reinforcer (novelty induced place preference (NIPP)). Furthermore, when given the choice between saccharine, a sweet but non caloric solution, and glucose, a more bland but caloric solution, in contrast to sham animals that preferred saccharine, STN lesioned animals preferred glucose over saccharine. Taken altogether these results reveal that STN plays a critical role in emotional processing. These results, in line with some clinical observations in PD patients subjected to STN surgery, suggest possible emotional side-effects of treatments targeting the STN. They also suggest that the increased motivation for sucrose previously reported cannot be due to increased pleasure, but could be responsible for the decreased motivation for cocaine reported after STN inactivation.

Deep brain stimulation mechanisms: beyond the concept of local functional inhibition

Deep brain electrical stimulation has become a recognized therapy in the treatment of a variety of motor disorders and has potentially promising applications in a wide range of neurological diseases including neuropsychiatry. Behavioural observation that electrical high-frequency stimulation of a given brain area induces an effect similar to a lesion suggested a mechanism of functional inhibition. In vitro and in vivo experiments as well as per operative recordings in patients have revealed a variety of effects involving local changes of neuronal excitability as well as widespread effects throughout the connected network resulting from activation of axons, including antidromic activation. Here we review current data regarding the local and network activity changes induced by high-frequency stimulation of the subthalamic nucleus and discuss this in the context of motor restoration in Parkinson's disease. Stressing the important functional consequences of axonal activation in deep brain stimulation mechanisms, we highlight the importance of developing anatomical knowledge concerning the fibre connections of the putative therapeutic targets.

Dopamine lesion-induced changes in subthalamic nucleus activity are not associated with alterations in firing rate or pattern in layer V neurons of the anterior cingulate cortex in anesthetized rats

Dysfunctional activity in the subthalamic nucleus (STN) is thought to underlie movement deficits of patients with Parkinson's disease. Alterations in STN firing patterns are also evident in the anesthetized rat model of Parkinson's disease, where studies show that loss of striatal dopamine and concomitant changes in the indirect pathway are associated with bursty and oscillatory firing patterns in STN output. However, the extent to which alterations in cortical activity contribute to changes in STN activity is unclear. As pyramidal neurons in the cingulate cortex project directly to the STN, cingulate output was assessed after dopamine lesion by simultaneously recording single-unit and local field potential (LFP) activities in STN and anterior cingulate cortex in control, dopamine-lesioned and non-lesioned hemispheres of urethane-anesthetized rats. Correlated oscillations were observed in cross-correlograms of spike trains from STN and cingulate layer V neurons with broad waveforms indicative of pyramidal neurons. One-2 weeks after dopamine cell lesion, firing rate, incidence of bursty and 0.3-2.5 Hz oscillatory activity of neurons and LFP power in the STN all increased significantly. In contrast, firing rate, incidence of bursty and 0.3-2.5 Hz oscillatory activity of cingulate layer V putative pyramidal neurons and power in cingulate LFPs did not differ significantly between dopamine-lesioned, non-lesioned or control hemispheres, despite significant loss of dopamine in the lesioned cingulate cortex. Data show that alterations in STN activity in the dopamine-lesioned hemisphere are not associated with alterations in neuronal activity in layer V of the anterior cingulate cortex in anesthetized rats.

Subthalamic nucleus lesions increase impulsive action and decrease impulsive choice - mediation by enhanced incentive motivation?

The subthalamic nucleus (STN) is traditionally thought of as part of a system involved in motor control but recent evidence suggests that it may also play a role in other psychological processes. Here we examined the effects of STN lesions on two measures of impulsivity and found that STN lesions increased 'impulsive action' (produced behavioral disinhibition), as measured by performance on a differential reinforcement of low rates of responding task, but decreased 'impulsive choice' (impulsive decision making), as measured by a delay discounting task. In addition, amphetamine and food restriction increased 'impulsive action' and decreased 'impulsive choice' to a greater extent in STN-lesioned animals than in sham controls. We speculate that these apparently discrepant effects may be because STN lesions enhance the incentive salience assigned to rewards. These findings suggest that the STN may serve as a novel target for the treatment of psychological disorders characterized by deficits in behavioral control, such as drug addiction and attention deficit hyperactivity disorder.

Transcranial magnetic stimulation of the human motor cortex influences the neuronal activity of subthalamic nucleus

The critical role of the subthalamic nucleus (STN) in the control of movement and parkinsonian symptoms is well established. Research in animals suggests that the cerebral cortex plays an important role in regulating the activity of the STN but this control is not known in humans. The most extensive cortical innervation of the STN originates from motor areas. Here, we used transcranial magnetic stimulation (TMS) during intraoperative single-unit recordings from STN, in six patients with Parkinson's disease (PD) undergoing implantation of deep brain stimulators, to determine whether TMS of the motor cortex (MC) modulates the activity of STN and to investigate in vivo the functional organization of the corticosubthalamic circuit in the human brain. Single-pulse TMS of the MC induced an excitation in 74.9% of neurons investigated. This activation was followed by a long-lasting inhibition of the STN neuronal activity that did not correlate with PD severity. Responsive neurons to TMS of the hand area of motor cortex were located mainly in the lateral and dorsal region of the subthalamus while unresponsive cells had a prevalently medial distribution. This is the first report of TMS-induced modulation of STN neuronal activity in humans. These findings open up new avenues for in vivo studies of corticosubthalamic interactions in human brain and PD.

Molecular, chemical, and anatomical characterization of globus pallidus dopamine D2 receptor mRNA-containing neurons

Essential for normal movement, the globus pallidus (GP) is a prominent nucleus whose neurons project to all other basal ganglia nuclei. The GP is composed of at least two distinct neuron populations. GP neurons of the rodent contain either the calcium-binding protein parvalbumin (PV) or preproenkephalin (PPE) mRNA, differentially innervate several basal ganglia structures, and have distinct immediate early gene responses to dopamine agonists or antagonists. Recent research has revealed that dopamine directly influences GP neurons, with D2 receptors contributing to both pre- and postsynaptic effects of dopaminergic agents. The existence of D2 mRNA-expressing (D2+) GP neurons has been established, but little is known concerning their numbers, regional distribution, or relationship to pallidal subpopulations identified on the basis of PV immunocytochemistry, PPE mRNA, or axonal targets. Detection of pallidal D2 mRNA with a 35S-cRNA probe revealed that D2+ neurons are found throughout the GP, comprising approximately one-half of pallidal neurons, but they are most dense within a dorsoventral band in lateral GP. While a substantial proportion (42-51%) of all chemically and anatomically labeled pallidal neuron subpopulations expressed D2 transcript, the D2+ neurons exhibited both population-based and regional heterogeneities. Overall, the pallidostriatal cells had a greater density of D2 mRNA than did pallidosubthalamic cells. Also, compared to other pallidal regions, the ventromedial GP contained fewer D2+ cells, and the PPE mRNA-expressing cells in this region had lower densities of D2 mRNA per neuron. These results reveal heterogeneous chemical and anatomical properties of the extensive population of D2+ GP neurons, a potential cellular substrate for dopamine's effects in pallidum.

Basal ganglia and processing of cortical information: functional interactions between trans-striatal and trans-subthalamic circuits in the substantia nigra pars reticulata

The substantia nigra pars reticulata (SNR), a major output station of basal ganglia, receives information from the cerebral cortex through three main pathways, i.e. a direct inhibitory trans-striatal pathway, an indirect excitatory trans-striatal pathway that involves the pallidum and the subthalamus and a direct excitatory trans-subthalamic pathway. In order to determine how cortical information flow originating from functionally distinct cortical areas and processed through the trans-striatal and trans-subthalamic pathways is integrated within the SNR, the responses induced by electrical stimulation of prefrontal, motor and auditory cortex in SNR cells were analyzed in anesthetized rats. Further confirming that direct striato-nigral pathways related to these functionally distinct cortical areas are organized in parallel channels, stimulation of the prefrontal, motor and auditory cortex induced an inhibitory response on distinct subpopulations of SNR cells. Within a given channel, the direct trans-striatal and the trans-subthalamic pathways converge on a large number of nigral cells. In addition, the present study reveals that nigral cells receiving an inhibitory input from a given cortical area through the direct trans-striatal pathway can also receive an excitatory input from a functionally distinct cortical area through the trans-subthalamic pathways. Such a convergence mainly occurred between the direct striato-nigral pathway issued from the auditory cortex and the trans-subthalamic pathways issued from the motor cortex. These data reveal the existence of a converging influence of trans-subthalamic and direct striato-nigral pathways not only within but also across channels. Within a given cortico-basal ganglia channel, the trans-subthalamic pathways likely contribute to the temporal shaping of the striato-nigral inhibition and thus of the disinhibition of the related nigral target nuclei in the thalamus and mesencephalon. Across channels, the specific interactions between trans-subthalamic and direct striato-nigral pathways could contribute to prevent inhibition of subpopulations of nigral cells implicated in competing functions.

Hippocampo-prefrontal cortex pathway: anatomical and electrophysiological characteristics

The hippocampus, the prefrontal cortex, and interconnected neural circuits are implicated in several aspects of cognitive and memory processes. The present review is dedicated to the description of the anatomo-functional characteristics of the hippocampo-prefrontal pathway and related neuronal circuits in the rat. This pathway, which originates from the hippocampal CA1/subiculum fields, innervates the prelimbic/medial orbital areas of the prefrontal cortex (PL/MO). Its synaptic influence on cortical pyramidal neurons consists in an early monosynaptic excitation followed by an inhibition and, in some cases, a late excitation. These later effects are likely due to the subsequent activation of the local cortical network. PL/MO areas and the CA1/subiculum both send projections to the nucleus accumbens, a region of the ventral striatum which is particularly implicated in goal-directed behavior. Therefore, emphasis is placed on respective projections from PL/MO areas and from the CA1/subiculum on the "core" and the "shell" regions of the nucleus accumbens, as well as on their interconnected circuits. Signals which are directed to the prefrontal cortex through these circuits might modulate hippocampo-prefrontal inputs. Finally, the direct and/or indirect relationships of the hippocampus, prefrontal cortex, and nucleus accumbens with the ventral tegmental area/substantia nigra pars compacta complex (VTA/SNC) (where dopamine neurons are located) will also be described, because these neurons are known to modulate synaptic transmission and plasticity in their target structures and to play a fundamental role in motivational processes.

The connections of the dopaminergic system with the striatum in rats and primates: an analysis with respect to the functional and compartmental organization of the striatum

This Commentary compares the connections of the dopaminergic system with the striatum in rats and primates with respect to two levels of striatal organization: a tripartite functional (motor, associative and limbic) subdivision and a compartmental (patch/striosome-matrix) subdivision. The topography of other basal ganglia projections to the dopaminergic system with respect to their tripartite functional subdivision is also reviewed. This examination indicates that, in rats and primates, the following observations can be made. (1) The limbic striatum reciprocates its dopaminergic input and in addition innervates most of the dopaminergic neurons projecting to the associative and motor striatum, whereas the motor and associative striatum reciprocate only part of their dopaminergic input. Therefore, the connections of the three striatal subregions with the dopaminergic system are asymmetrical, but the direction of asymmetry differs between the limbic versus the motor and associative striatum. (2) The limbic striatum provides the main striatal input to dopamine cell bodies and proximal dendrites, with some contribution from a subset of neurons in the associative and motor striatum (patch neurons in rats; an unspecified group of neurons in primates), while striatal input to the ventrally extending dopamine dendrites arises mainly from a subset of neurons in the associative and motor striatum (matrix neurons in rats; an unspecified group of neurons in primates). (3) Projections from functionally corresponding subdivisions of the striatum, pallidum and subthalamic nucleus to the dopaminergic system overlap, but the specific targets (dopamine cells, dopamine dendrites, GABA cells) of these projections differ. Major differences include the following. (1) In rats, neurons projecting to the motor and associative striatum reside in distinct regions, while in primates they are arranged in interdigitating clusters. (2) In rats, the terminal fields of projections arising from the motor and associative striatum are largely segregated, while in primates they are not. (3) In rats, patch- and matrix-projecting dopamine cells are organized in spatially, morphologically, histochemically and hodologically distinct ventral and dorsal tiers, while in primates there is no (bi)division of the dopaminergic system that results in two areas which have all the characteristics of the two tiers in rats. Based on the anatomical data and known dopamine cell physiology, we forward an hypothesis regarding the influence of the basal ganglia on dopamine cell activity which captures at least part of the complex interplay taking place within the substantia nigra between projections arising from the different basal ganglia nuclei. Finally, we incorporate the striatal connections with the dopaminergic system into an open-interconnected scheme of basal ganglia-thalamocortical circuitry.

Prefrontal cortex-basal ganglia circuits in the rat: involvement of ventral pallidum and subthalamic nucleus

The core of the nucleus accumbens (NAcc core) is the principal input structure to the basal ganglia circuitry for the prelimbic and medial orbital areas (PL/MO) of the prefrontal cortex. As is now well recognized in the rat, the main basal ganglia output of this prefrontal channel is the dorsomedial part of the substantia nigra pars reticulata (SNR) and not the ventral pallidum as previously suggested. There is evidence suggesting that the ventral pallidum is rather involved with the subthalamic nucleus (STN) in an indirect NAcc-SNR pathway. Indeed, we have recently shown that the NAcc core sends an inhibitory input to the lateral ventral pallidum (VPl), which projects to the medial STN. In the present study, we injected biocytin into the medial STN, at a site where neurons presented an inhibitory response to VPl stimulation. This produced anterogradely labelled fibres in the medial SNR and in the VPl. Furthermore, the stimulation of the VPl induced an inhibition in a majority of the STN cells identified, by the antidromic activation method, as projecting to SNR (76.6%) and/or back to the VPl (72.7%). In conclusion, these data further demonstrate the existence of an indirect striato-nigral pathway in the PL/MO channel and indicate that VPl is involved in an inhibitory feedback circuit, which modulates the discharge of medial STN. These results indicate that the medial STN is implicated in the limbic/cognitive functions of the basal ganglia.

Electrophysiological and Fos immunohistochemical evidence for the excitatory nature of the parafascicular projection to the globus pallidus

Extracellular recordings and immunohistological detection of c-Fos-like immunoreactive proteins were used to determine the synaptic effect of the parafascicular projection to the globus pallidus. Electrical stimulation of the parafascicular neurons induced a single-spike excitatory response with a stable latency of 2.3 ms, suggesting a monosynaptically driven effect. Pharmacological stimulation of the parafascicular nucleus with carbachol increased tonically the pallidal discharge rate by 142%. The discharge rate of the pallidal neurons was described by 37% in parafascicular-lesioned rats. These results demonstrate the excitatory nature and the tonic action of the parafasciculopallidal projection. Carbachol activation of parafascicular neurons also induced the synthesis of c-Fos-like immunoreactive proteins in the pallidal neurons. Control experiments in subthalamic-lesioned rats showed that the parafascicular excitation of the pallidal neurons remained, but both electrophysiological and expression of c-Fos-like immunoreactive proteins were attenuated. This suggests that the direct parafascicular excitation of the pallidal neurons is indirectly reinforced by the previously described parafascicular excitatory input to the subthalamic nucleus. Conversely, the effect of this last input to the subthalamic nucleus is dramatically enhanced in rats with pallidal lesion. Our results demonstrate the complex role of the parafascicular nucleus in activating both the globus pallidus and the subthalamic nucleus, two closely related structures. These results illustrate the integrative capacities of the globus pallidus, whose activity is modulated by multiple afferents.

Differential synaptic innervation of neurons in the internal and external segments of the globus pallidus by the GABA- and glutamate-containing terminals in the squirrel monkey

The present study aimed at comparing the pattern of synaptic innervation of neurons in the external (GPe) and internal (GPi) pallidum by gamma-aminobutyric acid (GABA)- and glutamate-immunoreactive terminals in the squirrel monkey. Four major populations of terminals were encountered in GPe and GPi. Our findings combined with those obtained in previous tract-tracing studies reveal that the synaptic innervation of perikarya in GPe is strikingly different from that in GPi. Although the GABA-positive type I boutons (from the striatum) represent 85% of the terminals in contact with somata in GPe, only 32% of the axosomatic synapses involve this type of terminal in GPi. However, the type II terminals (from GPe), which display a moderate level of GABA and glutamate immunoreactivities, account for 48% of the boutons in contact with perikarya in GPi but only 10% in GPe. In both pallidal segments, less than 10% of the axosomatic synapses involve the glutamate-immunoreactive type III terminals (from the subthalamic nucleus). Finally, the type IIa boutons (unknown source), which show levels of immunoreactivities similar to the type II terminals, account for 12% of the boutons in contact with perikarya in GPi but only 4% in GPe. In contrast to perikarya, the innervation of dendritic shafts is similar in both GPe and GPi; more than 80% of the axodendritic synapses involve the type I terminals, 10-15% involve the type III terminals, less than 5% are formed by the type II boutons, and less than 1% involve the type IIa terminals. Three other categories of boutons (types IV, V, VI) account for less than 1% of the total population of terminals in GPe and GPi. In conclusion, our findings demonstrate a differential synaptic innervation of neuronal perikarya in GPe and GPi in primates. These data suggest that the two pallidal segments are separate functional entities of which the neuronal activity is largely controlled by extrinsic inputs that are differentially distributed at the level of single cells.

Synaptic innervation of neurones in the internal pallidal segment by the subthalamic nucleus and the external pallidum in monkeys

In order to better understand the way by which the subthalamic nucleus interacts with the globus pallidus to control the output of the basal ganglia, we carried out a series of experiments to investigate the pattern of synaptic innervation of the pallidal neurones by the subthalamic terminals in the squirrel monkey. To address this problem we used the anterograde transport of biocytin. Following injections of biocytin in the subthalamic nucleus, rich plexuses of labelled fibres and varicosities formed bands that lay along the medullary lamina in both segments of the ipsilateral pallidum. At the electron microscopic level, two populations of biocytin-containing terminals were identified in the internal pallidum (GPi). A first group of small to medium-sized terminals (type 1; mean cross-sectional area +/- S.D. = 0.41 +/- 0.04 microns 2) contained round vesicles and formed asymmetric synapses with dendritic shafts (95%) of mixed sizes (maximum diameter ranging from 0.3 to 4.0 microns) and spine-like structures (5%). The second group of terminals (type 2) contained pleiomorphic vesicles, had a larger cross-sectional area (mean +/- S.D. = 0.9 +/- 0.4 micron 2) and formed symmetric synapses predominantly with perikarya (41%) and large dendrites (57%). In some cases, the two types of terminals converged at the level of single GPi neurones. Postembedding immunogold method revealed that the type 2 terminals displayed gamma-aminobutyric acid (GABA) immunoreactivity, whereas the type 1 terminals did not. In the external pallidum (GPe), injections in the subthalamic nucleus labelled both type 1 or type 2 terminals. However, the labelled type 2 boutons were much less abundant in GPe than in GPi. The presence of biocytin-labelled perikarya in GPe and the fact that the type 2 terminals displayed GABA immunoreactivity led us to suspect that these terminals were derived from axons of GPe neurones. In agreement with this hypothesis, injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in GPe labelled terminals in GPi that displayed the morphological features and a pattern of synaptic organization similar to the type 2 terminals. In conclusion, the results of our study demonstrate that the subthalamopallidal terminals form asymmetric synapses that are distributed along the dendritic tree of GPe and GPi neurones. In contrast, the GPe projection to GPi gives rise to large GABA-containing terminals that form symmetric synapses predominantly with the proximal region of pallidal neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Descending efferent connections of the sub-pallidal areas in the cat: projections to the subthalamic nucleus, the hypothalamus, and the midbrain

The efferent connections of the sub-pallidal regions to the mediodorsal thalamic nucleus, the subthalamic nucleus, the lateral hypothalamic area, and the midbrain were investigated in the cat, using Phaseolus vulgaris--leucoagglutinin (PHA-L) as an anterograde label. The results indicate that the sub-pallidal regions of the cat project to the (dorso)medial tip of the subthalamic nucleus and the adjoining lateral hypothalamic area as well as to the ventral tegmental area and the greater extent of the dorsolateral tier of the substantia nigra pars compacta. Extensive projections were also found to the peripeduncular nucleus. The central gray as well as the mesencephalic locomotor region receive some input from the basal forebrain too. In contrast only very limited projections were found to the mediodorsal thalamic nucleus. The results are discussed in view of the possible role of these output regions in oro-facial dyskinesia.