Changes in magnetic resonance imaging and sex behavior after 6-OHDA injection in the medial preoptic area

Brain circuits activated by female sexual behavior evaluated by manganese enhanced magnetic resonance imaging

Magnetic resonance imaging (MRI) allows obtaining anatomical and functional information of the brain in the same subject at different times. Manganese-enhanced MRI (MEMRI) uses manganese ions to identify brain activity, although in high doses it might produce neurotoxic effects. Our aims were to identify a manganese dose that does not affect motivated behaviors such as sexual behavior, running wheel and the rotarod test. The second goal was to determine the optimal dose of chloride manganese (MnCl₂) that will allow us to evaluate activation of brain regions after females mated controlling (pacing) the sexual interaction. To achieve that, two experiments were performed. In experiment 1 we evaluated the effects of two doses of MnCl₂, 8 and 16 mg/kg. Subjects were injected with one of the doses of MnCl₂ 24 hours before the test on sessions 1, 5 and 10 and immediately thereafter scanned. Female sexual behavior, running wheel and the rotarod were evaluated once a week for 10 weeks. In experiment 2 we followed a similar procedure, but females paced the sexual interaction once a week for 10 weeks and were injected with one of the doses of MnCl₂ 24 hours before the test and immediately thereafter scanned on sessions 1, 5 and 10. The results of experiment 1 show that neither dose of MnCl₂ induces alterations on sexual behavior, running wheel and rotarod. Experiment 2 demonstrated that MEMRI allow us to detect activation of different brain regions after sexual behavior, including the olfactory bulb (OB), the bed nucleus of the stria terminalis (BNST), the amygdala (AMG), the medial preoptic area (MPOA), the ventromedial hypothalamus (VMH), the nucleus accumbens (NAcc), the striatum (STR) and the hippocampus (Hipp) allowing the identification of changes in brain circuits activated by sexual behavior. The socio sexual circuit showed a higher signal intensity on session 5 than the reward circuit and the control groups indicating that even with sexual experience the activation of the reward circuit requires the activation of the socio sexual circuit. Our study demonstrates that MEMRI can be used repeatedly in the same subject to evaluate the activation of brain circuits after motivated behaviors and how can this activation change with experience.

Altered dopaminergic regulation of the dorsal striatum is able to induce tic-like movements in juvenile rats

Motor tics are sudden, repetitive, involuntary movements representing the hallmark behaviors of the neurodevelopmental disease Tourette’s syndrome (TS). The primary cause of TS remains unclear. The initial observation that dopaminergic antagonists alleviate tics led to the development of a dopaminergic theory of TS etiology which is supported by post mortem and in vivo studies indicating that non-physiological activation of the striatum could generate tics. The striatum controls movement execution through the balanced activity of dopamine receptor D1 and D2-expressing medium spiny neurons of the direct and indirect pathway, respectively. Different neurotransmitters can activate or repress striatal activity and among them, dopamine plays a major role. In this study we introduced a chronic dopaminergic alteration in juvenile rats, in order to modify the delicate balance between direct and indirect pathway. This manipulation was done in the dorsal striatum, that had been associated with tic-like movements generation in animal models. The results were movements resembling tics, which were categorized and scored according to a newly developed rating scale and were reduced by clonidine and riluzole treatment. Finally, post mortem analyses revealed altered RNA expression of dopaminergic receptors D1 and D2, suggesting an imbalanced dopaminergic regulation of medium spiny neuron activity as being causally related to the observed phenotype.

Characterization of gray matter atrophy following 6-hydroxydopamine lesion of the nigrostriatal system

BACKGROUND

The unilaterally-lesioned 6-hydroxydopamine (6-OHDA) rat is one of the most commonly used experimental models of Parkinson's disease (PD). Here we investigated whether magnetic resonance imaging (MRI) that is widely used in human PD research, has the potential to non-invasively detect macroscopic structural brain changes in the 6-OHDA rat in ways translatable to humans.

METHODS

We measured the gray matter (GM) composition in the unilateral 6-OHDA rat in comparison to sham animals using whole-brain voxel-based morphometry (VBM) - an unbiased MR image analysis technique. The number of nigral dopamine (DA) neurons and the density of their cortical projections were examined post-mortem using immunohistochemistry.

RESULTS

VBM revealed widespread bilateral changes in gray matter volume (GMV) on a topographic scale in the brains of 6-OHDA rats, compared to sham-operated rats. The greatest changes were in the lesioned hemisphere, which displayed reductions of GMV in motor, cingulate and somatosensory cortex. Histopathological results revealed dopaminergic cell loss in the substantia nigra (SN) and a denervation in the striatum, as well as in the frontal, somatosensory and cingulate cortices.

CONCLUSION

Unilateral nigrostriatal 6-OHDA lesioning leads to widespread GMV changes, which extend beyond the nigrostriatal system and resemble advanced Parkinsonism. This study highlights the potential of structural MRI, and VBM in particular, for the system-level phenotyping of rodent models of Parkinsonism and provides a methodological framework for future studies in novel rodent models as they become available to the research community.

Magnetic resonance imaging (MRI) to study striatal iron accumulation in a rat model of Parkinson's disease

Abnormal accumulation of iron is observed in neurodegenerative disorders. In Parkinson's disease, an excess of iron has been demonstrated in different structures of the basal ganglia and is suggested to be involved in the pathogenesis of the disease. Using the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease, the edematous effect of 6-OHDA and its relation with striatal iron accumulation was examined utilizing in vivo magnetic resonance imaging (MRI). The results revealed that in comparison with control animals, injection of 6-OHDA into the rat striatum provoked an edematous process, visible in T2-weighted images that was accompanied by an accumulation of iron clearly detectable in T2*-weighted images. Furthermore, Prussian blue staining to detect iron in sectioned brains confirmed the existence of accumulated iron in the areas of T2* hypointensities. The presence of ED1-positive microglia in the lesioned striatum overlapped with this accumulation of iron, indicating areas of toxicity and loss of dopamine nerve fibers. Correlation analyses demonstrated a direct relation between the hyperintensities caused by the edema and the hypointensities caused by the accumulation of iron.

Dopaminergic projections to the medial preoptic area of postpartum rats

Dopamine receptor activity in the rodent medial preoptic area (mPOA) is crucial for the display of maternal behaviors, as well as numerous other physiological and behavioral functions. However, the origin of dopaminergic input to the mPOA has not been identified through neuroanatomical tracing. To accomplish this, the retrograde tracer Fluorogold was iontophoretically applied to the mPOA of postpartum laboratory rats, and dual-label immunocytochemistry for Fluorogold and tyrosine hydroxylase later performed to identify dopaminergic cells of the forebrain and midbrain projecting to the mPOA. Results indicate that the number of dopaminergic cells projecting to the mPOA is moderate ( approximately 90 cells to one hemisphere), and that these cells have an unexpectedly wide distribution. Even so, more than half of the dual-labeled cells were found in either what has been considered extensions of the A10 dopamine group (particularly the ventrocaudal posterior hypothalamus and adjacent medial supramammillary nucleus), or in the A10 group of the ventral tegmental area. The rostral hypothalamus and surrounding region also contained numerous dual-labeled cells, with the greatest number found within the mPOA itself (including in the anteroventral preoptic area and preoptic periventricular nucleus). Notably, dual-labeled cells were rare in the zona incerta (A13), a site previously suggested to provide dopaminergic input to the mPOA. This study is the first to use anatomical tracing to detail the dopaminergic projections to the mPOA in the laboratory rat, and indicates that much of this projection originates more caudally than previously suggested.

Chemosensory cues are essential for mating-induced dopamine release in MPOA of male Syrian hamsters

The medial preoptic area (MPOA) is crucial for male sex behavior. Dopamine (DA) is released in MPOA during copulation, and contributes to the reinforcing effects of mating. The aim of the present study was to identify sensory stimuli responsible for mating-induced DA release. Specifically, we determined if chemosensory cues are essential for mating-induced MPOA DA release using in vivo microdialysis in male Syrian hamsters. Hamsters were used because chemosensory cues from the olfactory mucosa and vomeronasal organ are essential for sexual behavior in this species. Sexually experienced adult male hamsters were implanted with a microdialysis guide cannula over MPOA. At the same time, males received sham olfactory bulbectomy (Sham Bx, n = 11), bilateral bulbectomy (Bibx, n = 6), or unilateral bulbectomy (Ubx) ipsilateral (Ipsi Ubx, n = 9) or contralateral (Contra Ubx, n = 8) to the microdialysis probe. This model takes advantage of the predominantly ipsilateral projections of the olfactory bulbs. Microdialysis samples were collected from the MPOA during baseline, exposure to a receptive female, and after removal of female. Extracellular DA was measured using high-performance liquid chromatography with electrochemical detection. During mating, DA increased in MPOA of Sham Bx males (to 146.7 +/- 17.5% of baseline). Bibx males did not mate, and MPOA DA did not increase (96.1 +/- 15.8% of baseline). Although both groups of Ubx males mated to ejaculation, MPOA DA increased significantly only in Contra Ubx males (to 161.8 +/- 35.3% of baseline), and not in males with Ipsi Ubx (107.6 +/- 11.5% of baseline). The results demonstrate that chemosensory cues are essential for MPOA DA release during mating in male Syrian hamsters.

6-Hydroxydopamine-induced lesions in a rat model of hemi-Parkinson's disease monitored by magnetic resonance imaging

Injection with 6-hydroxydopamine (6-OHDA) into the nigrostriatal pathway results in loss of nigrostriatal dopaminergic neurons, which has been used widely as an animal model of Parkinson's disease. In the present study, location and extent of lesions 1 day after 6-OHDA injections (2, 4, 8, or 16 microg as a free base) in the substantia nigra (SN) were evaluated in rats by T(2)-weighted magnetic resonance imaging (MRI). The changes in MRI were also compared to immunohistochemical and behavioral changes. Hyperintense area in MRI was found at the region corresponding to 6-OHDA injection in a dose-dependent manner and was accompanied by a loss of tyrosine hydroxylase (TH)-positive cells. The shape of hyperintense area in the SN appeared to be composed of two components (i.e., circular and longitudinal regions). Administration of a larger dose of 6-OHDA (8-16 microg) was accompanied by an increase in hyperintense area and loss of TH-positive cells beyond the SN. The hyperintense area was observed on the first and third days after 6-OHDA injection, but the size and intensity declined to near normal levels on the ninth day. Rotational behavior induced by methamphetamine reached maximal levels at 4 microg 6-OHDA, and the behavior was maintained with doses up to 16 microg of 6-OHDA. Intrastriatal injection with 6-OHDA was less effective. These results suggest that MRI provides highly valuable information for verifying the size and location of intended lesions as well as for determining the optimal dose of neurotoxins in individual animals.

Role of noradrenergic fibers of the preoptic area in regulating sleep

The preoptic area (POA) has noradrenergic (NE) terminals, and this area controls sleep apart from body temperature and reproduction. The destruction of catecholaminergic (CA) terminals in the POA produced a decrease in sleep in rats. This effect was shown to be due to the destruction of NE and not dopaminergic terminals. The rats, which were hyperthermic after the destruction of CA fibers in the POA, preferred a lower ambient temperature. Though they were unable to have normal amount of sleep after lesion, it did not affect their behavioral thermoregulation. Acute total sleep deprivation for 48 h led to a significant decrease in noradrenaline, increase in the level of metabolites of monoamines, and an enhancement in the number of dendritic spines at the medial preoptic area (mPOA). Enhanced sleep pressure during sleep deprivation could have led to a higher release of noradrenaline, and an increase in dendritic spines in the mPOA. Arousal was produced by application of noradrenaline at the mPOA, whereas the alpha antagonists produced sleep in free-moving rats. This was in contrast to the increased wakefulness produced by the destruction of NE terminals. As wakefulness and sleep, respectively, were induced on local application of alpha-2 antagonist and agonists, it was suspected that the noradrenaline and alpha antagonists might have acted on the alpha-2 receptors, which are predominantly present on the pre-synaptic terminals. Sleep produced by noradrenaline, which was locally applied at the mPOA, after destroying the NE terminals, further confirmed this possibility. Hypothermia and sexual arousal produced by application of alpha- and beta-adrenergic agonists at the mPOA would have contributed towards the wakefulness induced by these drugs in normal rats. Thus, the available evidence shows that the NE fibers in the POA are involved in the induction of sleep.

Stereotaxic assembly and procedures for simultaneous electrophysiological and MRI study of conscious rat

A stereotaxic restraining assembly was designed and developed for simultaneous electrophysiological recordings and functional MRI (fMRI) data acquisition from a conscious rat. The design of the nonmagnetic stereotaxic apparatus facilitated the restraining of head and body of the unanesthetized conscious animal during MRI experiments. The apparatus was made of Teflon and Perspex materials with an appropriate size and shape for a 4.7 T / 40 cm animal MRI scanner. Electrodes made from nonmagnetic silver wire were implanted on the skull for recording the electroencephalogram (EEG), the electro-oculogram (EOG), and the electromyogram (EMG), while polycarbonate screws were used for anchoring the electrode assembly. There were no major distortions or artifacts observed in the electrophysiological tracings and MR images. Electrophysiological recordings during fMRI acquisitions are useful to study different neurophysiological mechanisms of sleep and pathophysiology of seizure activity. Integration of electrophysiological recordings (with their good temporal resolution) and MRI (with its superior spatial resolution) is helpful in characterizing the functional state of different brain regions.

Changes in sleep-wakefulness after 6-hydroxydopamine lesion of the preoptic area

This study was undertaken to assess the role of catecholamine fibers, terminating in the preoptic area, in regulating sleep-wakefulness in rats. Sleep-wakefulness was assessed on the basis of 24h electroencephalogram, electromyogram and electro-oculogram recordings before and after destruction of catecholaminergic terminals at the medial preoptic area by bilateral intracerebral injection of 6-hydroxydopamine (8 microg in 0.2 microl). There was a mild reduction in sleep and increase in wakefulness after the lesion. The increase in active wakefulness observed after eight days of lesion persisted even on the 12th day. In spite of the reduction in sleep, the day-night sleep ratio was not affected by 6-hydroxydopamine lesion of the preoptic area. The results indicate that the noradrenergic fibers at the preoptic area have a hypnogenic role.