Lesion of the ventral tegmental area amplifies stimulation-induced Fos expression in the rat brain

The reinstatement of the expression phase of morphine-induced conditioned place preference in male Wistar rats under ventral tegmental area stimulation and brief inactivation

Background and purpose

Previous research has found that the electrical stimulation of the ventral tegmental area (VTA) is involved in drug-dependent behaviors and plays a role in reward-seeking. However, the mechanisms remain unknown, especially the effect of electrical stimulation on this area. Therefore, this study aimed to investigate how the electrical stimulation and the temporary inactivation of VTA affect the morphine- dependent behavior in male rats.

Experimental approach

The adult Wistar male rats were anesthetized with ketamine and xylazine. The stimulation electrode (unilaterally) and the microinjection cannula (bilaterally) were implanted into the VTA, stereotaxically. Then, the rats underwent three-day of repeated conditioning with subcutaneous morphine (0.5 or 5 mg/kg) injections, in the conditioned place preference apparatus, followed by four-day forced abstinence, which altered their conditioning response to a morphine (0.5 mg/kg) priming dose on the ninth day. On that day, rats were given high- or low-intensity electrical stimulation or reversible inactivation with lidocaine (0.5 pL/site) in the VTA.

Findings/Results

Results showed that the electrical stimulation of the VTA with the high intensity (150 μA/rat), had a minimal effect on the expression of morphine-induced place conditioning in rats treated with a high dose (5 mg/kg) of morphine. However, the reversible inactivation of the VTA with lidocaine greatly increased place preference in rats treated with a low dose (0.5 mg/kg) of morphine. Additionally, the reinstatement of 0.5 mg/kg morphine-treated rats was observed after lidocaine infusion into the VTA.

Conclusion and implications

These results suggest that VTA electrical stimulation suppresses neuronal activation, but the priming dose causes reinstatement. The VTA may be a potential target for deep brain stimulation-based treatment of intractable disorders induced by substance abuse.

Behavioral Reaction and c-fos Expression after Opioids Injection into the Pedunculopontine Tegmental Nucleus and Electrical Stimulation of the Ventral Tegmental Area

The pedunculopontine tegmental nucleus (PPN) regulates the activity of dopaminergic cells in the ventral tegmental area (VTA). In this study, the role of opioid receptors (OR) in the PPN on motivated behaviors was investigated by using a model of feeding induced by electrical VTA-stimulation (Es-VTA) in rats (male Wistar; n = 91). We found that the OR excitation by morphine and their blocking by naloxone within the PPN caused a change in the analyzed motivational behavior and neuronal activation. The opioid injections into the PPN resulted in a marked, dose-dependent increase/decrease in latency to feeding response (FR), which corresponded with increased neuronal activity (c-Fos protein), in most of the analyzed brain structures. Morphine dosed at 1.25/1.5 µg into the PPN significantly reduced behavior induced by Es-VTA, whereas morphine dosed at 0.25/0.5 µg into the PPN did not affect this behavior. The opposite effect was observed after the naloxone injection into the PPN, where its lowest doses of 2.5/5.0 μg shortened the FR latency. However, its highest dose of 25.0 μg into the PPN nucleus did not cause FR latency changes. In conclusion, the level of OR arousal in the PPN can modulate the activity of the reward system.

Stimulation and transient inactivation of ventral tegmental area modify reinstatement of acquisition phase of morphine-induced conditioned place preference in male rats

For processing the development of psychological dependency, opioid reinforcement, and opiate-related associative reward, learning, and memory in the brain, the ventral tegmental area (VTA) is considered the key zone. As the responsible region for the morphine role in conditioned place preference (CPP), this area has an important role. So, the present research was conducted to investigate the effects of different intensities of electrical stimulation on VTA utilizing CPP, with two morphine doses. Reversible inactivation of VTA was performed via bilateral microinjection of Lidocaine into this area with two implanted separate cannulas. Our findings indicated that 5 mg/kg morphine-induced CPP was suppressed by 150 μA VTA electrical stimulation. The results also showed that bilateral Intra-VTA administration of Lidocaine significantly decreased the 5 mg/kg morphine-induced CPP acquisition phase in comparison with their respective sham group, which reversed in the reinstatement test. It should be concluded that these findings are important for the detection of mesolimbic nervous system ties and could help to find new ways to attenuate the rewarding action of morphine.

Ventral tegmental area inactivation alters hormonal, metabolic, and locomotor responses to inescapable stress

Context: The involvement of unilateral and bilateral inhibition of the ventral tegmental area (VTA) in response to stress is not well understood. Objective: In the present study, the effect of unilateral and bilateral inhibition of the VTA on hormonal, metabolic, and locomotor responses to stress was assessed. Material and methods: Male rats seven days after cannulation into the VTA received electro foot-shock stress for seven consecutive days. Twenty minutes before induction of stress, 2% lidocaine hydrochloride or sterile saline (control) was injected either uni- or bi-laterally into the VTA. Results: Results showed that stress significantly increased serum corticosterone level, adrenal gland weight and anorexia, reduced weight gain, food-intake, and locomotor activity. However, bilateral inactivation of VTA prevented stress-induced these parameters changes. Conclusion: The present study demonstrated that the bilateral VTA blockade effectively relieves the symptoms of stress, while the unilateral VTA blockade does not significantly improve the changes caused by stress.

Application of radiosurgical techniques to produce a primate model of brain lesions

Behavioral analysis of subjects with discrete brain lesions provides important information about the mechanisms of various brain functions. However, it is generally difficult to experimentally produce discrete lesions in deep brain structures. Here we show that a radiosurgical technique, which is used as an alternative treatment for brain tumors and vascular malformations, is applicable to create non-invasive lesions in experimental animals for the research in systems neuroscience. We delivered highly focused radiation (130-150 Gy at ISO center) to the frontal eye field (FEF) of macaque monkeys using a clinical linear accelerator (LINAC). The effects of irradiation were assessed by analyzing oculomotor performance along with magnetic resonance (MR) images before and up to 8 months following irradiation. In parallel with tissue edema indicated by MR images, deficits in saccadic and smooth pursuit eye movements were observed during several days following irradiation. Although initial signs of oculomotor deficits disappeared within a month, damage to the tissue and impaired eye movements gradually developed during the course of the subsequent 6 months. Postmortem histological examinations showed necrosis and hemorrhages within a large area of the white matter and, to a lesser extent, in the adjacent gray matter, which was centered at the irradiated target. These results indicated that the LINAC system was useful for making brain lesions in experimental animals, while the suitable radiation parameters to generate more focused lesions need to be further explored. We propose the use of a radiosurgical technique for establishing animal models of brain lesions, and discuss the possible uses of this technique for functional neurosurgical treatments in humans.

Voltage-sensitive dye imaging of primary motor cortex activity produced by ventral tegmental area stimulation

The primary motor cortex (M1) receives dopaminergic projections from the ventral tegmental area (VTA) through the mesocortical dopamine pathway. However, few studies have focused on changes in M1 neuronal activity caused by VTA activation. To address this issue, we used voltage-sensitive dye imaging (VSD) to reveal the spatiotemporal dynamics of M1 activity induced by single-pulse stimulation of VTA in anesthetized rats. VSD imaging showed that brief electrical stimulation of unilateral VTA elicited a short-latency excitatory-inhibitory sequence of neuronal activity not only in the ipsilateral but also in the contralateral M1. The contralateral M1 response was not affected by pharmacological blockade of ipsilateral M1 activity, but it was completely abolished by corpus callosum transection. Although the VTA-evoked neuronal activity extended throughout the entire M1, we found the most prominent activity in the forelimb area of M1. The 6-OHDA-lesioned VTA failed to evoke M1 activity. Furthermore, both excitatory and inhibitory intact VTA-induced activity was entirely extinguished by blocking glutamate receptors in the target M1. When intracortical microstimulation of M1 was paired with VTA stimulation, the evoked forelimb muscle activity was facilitated or inhibited, depending on the interval between the two stimuli. These findings suggest that VTA neurons directly modulate the excitability of M1 neurons via fast glutamate signaling and, consequently, may control the last cortical stage of motor command processing.

Brain hemispheric differences in the neurochemical effects of lead, prenatal stress, and the combination and their amelioration by behavioral experience

Brain lateralization, critical to mediation of cognitive functions and to "multitasking," is disrupted in conditions such as attention deficit disorder and schizophrenia. Both low-level lead (Pb) exposure and prenatal stress (PS) have been associated with mesocorticolimbic system-mediated executive-function cognitive and attention deficits. Mesocorticolimbic systems demonstrate significant laterality. Thus, altered brain lateralization could play a role in this behavioral toxicity. This study examined laterality of mesocorticolimbic monoamines (frontal cortex, nucleus accumbens, striatum, midbrain) and amino acids (frontal cortex) in male and female rats subjected to lifetime Pb exposure (0 or 50 ppm in drinking water), PS (restraint stress on gestational days 16-17), or the combination with and without repeated learning behavioral experience. Control males exhibited prominent laterality, particularly in midbrain and also in frontal cortex and striatum; females exhibited less laterality, and this was primarily striatal. Lateralized Pb ± PS induced neurotransmitter changes were assessed only in males because of limited sample sizes of Pb + PS females. In males, Pb ± PS changes occurred in left hemisphere of frontal cortex and right hemisphere of midbrain. Behavioral experience modified the laterality of Pb ± PS-induced neurotransmitter changes in a region-dependent manner. Notably, behavioral experience eliminated Pb ± PS neurotransmitter changes in males. These findings underscore the critical need to evaluate both sexes and brain hemispheres for the mechanistic understanding of sex-dependent differences in neuro- and behavioral toxicity. Furthermore, assessment of central nervous system mechanisms in the absence of behavioral experience, shown here for males, may constitute less relevant models of human health effects.

Blood natural killer cell cytotoxicity enhancement correlates with an increased activity in brain motor structures following chronic stimulation of the bed nucleus of the stria terminalis in rats

The present study indicates that a chronic 14 day electrical stimulation of the bed nucleus of the stria terminalis (BST) increased blood but not spleen natural killer cell (NK) cytotoxicity and a large granular lymphocyte (LGL) number. These immune changes positively correlated with the increased activity in brain cortical and subcortical motor structures that influence the BST stimulation-induced behavioral response. No significant changes in blood and spleen leukocyte population numbers and plasma corticosterone concentration after the stimulation were found. The obtained results suggest that this immunoenhancing effect on blood NK cell function and number is dependent on the behavioral outcome of the BST stimulation rather than endocrine response.