Involvement of nigrostriatal dopamine neurons in the contraversive rotational behavior evoked by electrical stimulation of the lateral hypothalamus

Electrical Stimulation Evokes Rotational Behavior In Tandem with Exocytotic-like Increases in Dopamine Measured by In Vivo Intracerebral Microdialysis

BACKGROUND

Electrical Stimulation is a traditional tool in neuroscience and is commonly used in vivo to evoke behavior and in vitro to study neural mechanisms. In vivo intracerebral microdialysis, also a traditional technique, is used to assay neurotransmitter release. However, the combination of these techniques is highly limited to studies using anesthetized animals; therefore, evoking and measuring exocytotic neurotransmitter release in awake models is lacking. Combining these techniques in an awake animal preparation is presented here with evidence to support the mechanistic action of electrical stimulation in vivo.

NEW METHODS

This report presents converging evidence to validate the combination of intracerebral electrical stimulation with microdialysis as a novel procedure to study exocytotic-like dopamine release in behaving animals.

RESULTS

It is shown that electrical stimulation of the medial forebrain bundle can be used to evoke frequency- and intensity-dependent exocytotic-like dopamine overflow and rotational behavior that are sensitive to Na+ channel blockade and Ca++ availability.

COMPARISON WITH EXISTING METHODS

Studies using modern techniques to evoke neurotransmitter release, combined with in vivo intracerebral microdialysis, and measured behavioral output are scarce. In contrast, commonly used pharmacological methods often are less precise and inefficient to evoke exocytotic dopamine release and behavior. Here we demonstrate, the combination of in vivo intracerebral microdialysis with electrical stimulation as a simple approach to simultaneously assess physiologically relevant neurotransmitter 'release' and behavior.

CONCLUSIONS

Research that aims to understand how dopamine neurotransmission is altered in behavioral disorders can utilize this innovative combination of electrical stimulation with in vivo intracerebral microdialysis.

Polymer-Encapsulated PC12 Cells: Long-Term Survival and Associated Reduction in Lesion-Induced Rotational Behavior

Intrastriatal implantation of a dopaminergic cell line surrounded by a permeable, thermoplastic membrane was investigated as a method of long-term dopamine (DA) delivery within the central nervous system (CNS). An increase in DA release from PC12 cell-loaded capsules maintained in vitro was associated with an increase in mitotic activity of the encapsulated cell line. A significant reduction in apomorphine-induced rotational behavior was observed after PC12 cell-containing capsules were implanted into unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, which was sustained for 24 wk. Four wk after implantation, micro-dialysis studies revealed the presence of DA near PC12 cell-containing capsules, which was comparable to extracellular striatal levels of unlesioned controls. Extracellular striatal DA was undetectable by microdialysis in lesioned animals near empty polymer capsules. Histological analysis after 24 wk in vivo demonstrated that encapsulated PC12 cells survived, continued to express tyrosine hydroxylase, and that encapsulation prevented tumorigenesis. The data suggested that the release of a diffusible substance, most likely DA, from an implant is sufficient to exert a long-term functional influence upon 6-OHDA unilaterally lesioned rats and that capsules containing DA-secreting cells may be an effective method of long-term DA delivery in the CNS.

Electrical stimulation of neural tissue to evoke behavioral responses

This review yields numerous conclusions. (1) Both unit recording and behavioral studies find that current activates neurons (i.e., cell bodies and axons) directly according to the square of the distance between the electrode and the neuron, and that the excitability of neurons can vary between 100 and 4000 microA/mm2 using a 0.2-ms cathodal pulse duration. (2) Currents as low as 10 microA, which is considered within the range of currents typically used during micro-stimulation, activate from a few tenths to several thousands of cell bodies in the cat motor cortex directly depending on their excitability; this indicates that even low currents activate more than a few neurons. (3) Electrode tip size has no effect on the current density--or effect current spread--at far field, but tip size limits the current-density generated at near field. (4) To minimize neuronal damage, the electrode should be discharged after each pulse and the pulse duration should not exceed the chronaxie of the stimulated tissue. (5) The amount of current needed to evoke behavioral responses depends not only on the excitability of the stimulated substrate but also on the type of behavior being studied.

Normalization of extracellular dopamine in striatum following recovery from a partial unilateral 6-OHDA lesion of the substantia nigra: a microdialysis study in freely moving rats

It has been hypothesized that striatal dopamine (DA) terminals undergo compensatory changes in response to partial damage of the mesostriatal DA system, which results in higher concentrations of DA in the extracellular space than would be predicted by DA concentrations in post-mortem tissue. But, this hypothesis has never been tested directly in vivo, and therefore, the present study was designed signed to do so. Microdialysis was used in freely moving rats to estimate the concentration of DA, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) in striatal extracellular fluid; simultaneously from the hemisphere with unilateral 6-hydroxydopamine (6-OHDA) lesion of the substantia nigra and from the intact hemisphere. It was found that following recovery from a 6-OHDA lesion, and during the resting state, the extracellular concentrations of DA were normal on the lesion side, even after that side was depleted of up to 99.0% of the DA measured in post-mortem tissue. Furthermore, the extracellular concentrations of DA were elevated in the intact hemisphere of animals with a greater than 95% DA depletion. In rats with a less than 95% DA depletion amphetamine (1.5 mg/kg) caused a large increase in the extracellular concentration of DA in both the lesion and intact hemispheres (intact greater than lesion), but in rats with a greater than 95% tissue DA depletion amphetamine only enhanced extracellular DA on the intact side; on the lesion side amphetamine produced a progressive decrease in extracellular DA to nondetectable levels. Animals rotated towards the lesion side. Unlike DA, the extracellular concentrations of DOPAC and HVA were greatly reduced on the lesion side, and the extent of the depletion was highly correlated with lesion size. It is concluded that following partial unilateral damage to mesostriatal DA projections there are massive changes in the remaining DA terminals that are sufficient to normalize the extracellular (and presumably synaptic) concentrations of DA. The normalization of extracellular DA concentrations seen after extensive (but incomplete) damage to the mesostriatal system must play a major role in the sparing and recovery of behavioral function that is so characteristics of this system. After extensive damage the capacity of the remaining DA neurons to respond to increased demand is limited, however, and this may explain why behavioral deficits can be reinstated by stimuli that challenge the system.

Hypothalamic substrates for brain stimulation-induced grooming, digging and circling in the rat

Despite a great number of studies concerned with the induction of specific behavioural responses from the rat hypothalamus by electrical brain stimulation, hypothalamic response areas and underlying neural substrates have never been determined accurately. In this study the boundaries of the hypothalamic response areas for grooming, digging and circling were delimited using moveable electrodes, an enriched environment containing a variety of goal objects, and an appropriate statistical technique. A total of 641 hypothalamic sites in 71 male CPB/WU Wistar rats were electrically stimulated. Results are plotted on a detailed stereotaxic brain atlas of the rat hypothalamus. Positive sites for any behavioural response cluster into restricted hypothalamic areas. Discriminant analysis of both positive and negative electrode localizations yields areas with high, intermediate or low probabilities of inducing the behavioural response concerned. Each response has its own response area where probabilities are high, although there may be overlap. Even within response areas a distinction can be made between areas in which the response can be induced at relatively high or low threshold current intensities. Lowest threshold sites within electrode tracks are often clustered. In search of neuroanatomical correlates, grooming is related to the distribution of ACTH-immunoreactive neural elements, digging is related to the distribution of efferent fibres from the bed nucleus of the stria terminalis, and circling is related to the distribution of dopaminergic fibres of the nigrostriatal pathway. The results clearly point to the stimulation site being the most important determinant of the evoked behavioural response. Evidently behavioural specificity does exist within the hypothalamus.

Sensitization to stress: the enduring effects of prior stress on amphetamine-induced rotational behavior

In rats with a unilateral 6-OHDA lesion of the substantia nigra exposure to footshock or immobilization stress produced a long-lasting enhancement in the rotational behavior evoked by a subsequent injection of amphetamine. However, the effect was dependent on the environmental context in which stress was applied. It is suggested that stress may induce enduring changes in brain and behavior similar to those produced by psychomotor stimulant drugs.