Loayza, Joanna
(2023)
Further studies in rat models of neural disinhibition: behavioural, in vivo electrophysiological, and translational imaging studies.
PhD thesis, University of Nottingham.
Abstract
Neural disinhibition, i.e., reduced inhibitory GABA transmission, has been implicated in various neuropsychiatric disorders. In particular, GABAergic abnormalities in the hippocampus and the striatum have been associated with schizophrenia and Tourette Syndrome, respectively. Regional neural disinhibition may affect behaviour by disrupting regional neural function and by causing changes in projection sites of the disinhibited region. In schizophrenia, one of such projection sites is the septum and in Tourette Syndrome the motor cortex. To further examine the impact of regional disinhibition (ventral hippocampus or anterior dorsal striatum), we combined intra-cerebral microinfusions of a GABA-A receptor antagonist (picrotoxin) with translational neural imaging (MRS, rsFC MRI), electrophysiological measurements and behavioural methods (locomotor activity and prepulse inhibition) in Lister hooded rats.
First, we used 1H-Magnetic Resonance Spectroscopy (MRS) at 7T, to measure neuro-metabolites in the septum following ventral hippocampal disinhibition (Chapter 2). This study was based on our previous Singlephoton emission computed tomography (SPECT) imaging study that revealed marked metabolic activation in several extra-hippocampal sites, including the septum, after ventral hippocampus disinhibition (Williams et al., in preparation). This experiment demonstrated no clear effects of ventral hippocampal disinhibition on any of the neuro-metabolites measured in the septum, including glutamate, glutamine and GABA, indicating that marked acute metabolic activation in the lateral septum (as detected by SPECT) is potentially not accompanied by neuro-metabolites changes measurable by MRS (possibly reflecting homeostatic metabolic mechanisms).
Second, we used electrophysiological measurements in anaesthetised rats (Chapter 3) to examine the effects of striatal disinhibition, induced by picrotoxin infusions (300 ng picrotoxin in 0.5 µl saline) into the anterior dorsal striatum, on neural activity in the vicinity of the infusion site. Our findings revealed large local field potential (LFP) spike-wave discharges, consisting of a single negative spike followed by a positive wave. Furthermore, picrotoxin in the dorsal striatum enhanced multi-unit burst firing, reflected by significant increases in mean spike frequency in burst per block, mean peak spike frequency in burst and percentage spikes in burst and by increased frequency of bursts, reflected by significant decreases of mean interburst interval per block. This is a new finding in the striatum and is consistent with disinhibition-induced enhancement of burst firing that we previously reported in the prefrontal cortex and hippocampus (Pezze et al., 2014; McGarrity et al., 2017), suggesting that in all these regions, GABA-A receptor-mediated inhibition is particularly important to control neural burst firing. No tic-like movements were visible in the anesthetised rats.
Third, we characterised tic-like forelimb movements caused by infusing picrotoxin into the right anterior dorsal striatum (300 ng and 200 ng picrotoxin in 0.5 µl saline) of rats (Chapter 4). Such infusions reliably induced tic-like movements in the left forelimb within the same rat and across rats. Most rats expressed highest frequency of tic-like movements in the first 5 to 35 min post-infusion. Tic-like movements were characterised. The most common tic-like movement involved the rat to lift its left forelimb, thereby rotating its head and torso to the right around the body’s long axis, before putting the left forelimb back down again, and thereby moving its head and torso into its starting position. There were also some more pronounced forelimb movements that lasted for several seconds and led to a whole rotation of the body around its long axis. The movements observed in this study were compared to tics in Tourette syndrome.
Fourth, we investigated the impact of anterior dorsal striatal disinhibition (300 ng picrotoxin in 0.5 µl saline) on prepulse inhibition (PPI) of the acoustic startle response and locomotor activity (Chapter 5). Picrotoxin infusion caused tic-like movements, had no effect on prepulse inhibition, tended to reduce startle and significantly increased locomotor activity and fine motor count. No PPI deficit following striatal disinhibition indicates that GABAergic inhibition in the dorsal striatum is not critical for prepulse inhibition. Our finding that marked tic-like movements were produced alongside intact PPI does not support the possibility that PPI disruption is necessary for tic-like movements (as suggested by Swerdlow and colleagues). The timeline of the fine motor count was similar to the timeline of the tic-like movements, and therefore, fine motor counts may thus be used as an automated measure of tic-like movements. The locomotor hyperactivity alongside tic-like movements indicates that dorsal striatal activity is not only involved in generating movements of individual body parts, but also in modulating locomotor activity and suggests that striatal disinhibition that contributes to tic-like movements may also contribute to hyperactivity, which is often comorbid with Tourette Syndrome (Robertson, 2015).
Lastly, we aimed to provide proof of principle that standard resting state functional connectivity Magnetic Resonance Imaging (rsFC MRI) measurements are possible in rats with pre-implanted guide cannulae and microinfusions in the anterior dorsal striatum (Chapter 6). Although we obtained sufficient quality rsFC MRI data to reveal the resting-state default mode network of an unoperated rat using the anterior cingulate as a seed, we were unable to obtain any resting-state default mode network of an operated rat, due to the significant signal loss from the guide cannula. We also tried to obtain spectra from the motor cortex following striatal disinhibition (using MRS), however, were unable to get good Signal to Noise ratio values for the water signal of the voxel placed in the motor cortex (SNR H2O). Although some neuro-metabolites, including glutamate, were measurable in the motor cortex of rats with a dorsal striatal guide cannula, others, including GABA, were not. When comparing the SNR H2O values between our studies, it appears that SNR H2O values decrease with increased distance between regions (disinhibited region and ROI for spectrum).
Concluding, disinhibiting the ventral hippocampus, has no clear effects on any neuro-metabolites in the septum, as measured by MRS. Dorsal striatal disinhibition causes large spike wave discharges and enhances burst firing in the striatum under anaesthesia. In behaving rats, striatal disinhibition causes tic-like movements, increases locomotor activity and fine movement counts, and has no effect on prepulse inhibition. Obtaining sufficient quality rsFC MRI data to reveal the resting-state default mode network is not possible following implantation of a guide cannula, as the cannula leads to too much signal loss. The signal for MRS is too poor with striatal cannula implants and a voxel placed in the motor cortex.
Actions (Archive Staff Only)
 |
Edit View |
Tools
Tools
