Lesions of the entopeduncular nucleus in rats prevent apomorphine-induced deficient sensorimotor gating

What rodent models of deep brain stimulation can teach us about the neural circuit regulation of prepulse inhibition in neuropsychiatric disorders

Deep brain stimulation (DBS) is routinely used for treatment of movement disorders and it is also under investigation for neuropsychiatric disorders with deficient sensorimotor gating, such as schizophrenia, Tourette's syndrome and obsessive compulsive disorder. Electrical stimulation induces excitation and inhibition both at the stimulation site and at projection sites, thus modulating synchrony and oscillatory behavior of neuronal networks. We first provide background information on DBS in neuropsychiatric disorders accompanied by deficient sensorimotor gating. We then introduce prepulse inhibition (PPI) as a measure for sensorimotor gating in these disorders. Thereafter, we report on the use of DBS in rat models with deficient PPI induced by pharmacologic, genetic and neurodevelopmental manipulation. These models offer the opportunity to define the neuronal circuit regulation that is of relevance to PPI and its deficits in neuropsychiatric disorders with disturbed sensorimotor gating. Finally, we report on the use of the PPI paradigm in human patients operated for DBS on/off stimulation, which may further elucidate the neuronal network involved in regulation of PPI.

Capsular stroke modeling based on somatotopic mapping of motor fibers

Recently, several capsular stroke models have been reported with different targets of destruction. This study was performed to establish an accurate internal capsule (IC) target for capsular stroke modeling in rats. We injected adeno-associated virus serotype 5 (AAV)-CaMKII-EYFP into forelimb motor cortex and AAV-CaMKII-mCherry into hindlimb motor cortex (n = 9) to anterogradely trace the pyramidal fibers and map their somatotopic distribution in the IC. On the basis of the neural tracing results, we created photothrombotic infarct lesions in rat forelimb and hindlimb motor fiber (FMF and HMF) areas of the IC (n = 29) and assessed motor behavior using a forelimb-use asymmetry test, a foot-fault test, and a single-pellet reaching test. We found that the FMFs and HMFs were primarily distributed in the inferior portion of the posterior limb of the IC, with the FMFs located largely ventral to the HMFs but with an area of partial overlap. Photothrombotic lesions in the FMF area resulted in persistent motor deficits. In contrast, lesions in the HMF area did not result in persistent motor deficits. These results indicate that identification of the somatotopic distribution of pyramidal fibers is critical for accurate targeting in animal capsular stroke models: only infarcts in the FMF area resulted in long-lasting motor deficits.

Ongoing behavioral state information signaled in the lateral habenula guides choice flexibility in freely moving rats

The lateral habenula (LHb) plays a role in a wide variety of behaviors ranging from maternal care, to sleep, to various forms of cognition. One prominent theory with ample supporting evidence is that the LHb serves to relay basal ganglia and limbic signals about negative outcomes to midbrain monoaminergic systems. This makes it likely that the LHb is critically involved in behavioral flexibility as all of these systems have been shown to contribute when flexible behavior is required. Behavioral flexibility is commonly examined across species and is impaired in various neuropsychiatric conditions including autism, depression, addiction, and schizophrenia; conditions in which the LHb is thought to play a role. Therefore, a thorough examination of the role of the LHb in behavioral flexibility serves multiple functions including understanding possible connections with neuropsychiatric illnesses and additional insight into its role in cognition in general. Here, we assess the LHb’s role in behavioral flexibility through comparisons of the roles its afferent and efferent pathways are known to play. Additionally, we provide new evidence supporting the LHb contributions to behavioral flexibility through organization of specific goal directed actions under cognitively demanding conditions. Specifically, in the first experiment, a majority of neurons recorded from the LHb were found to correlate with velocity on a spatial navigation task and did not change significantly when reward outcomes were manipulated. Additionally, measurements of local field potential (LFP) in the theta band revealed significant changes in power relative to velocity and reward location. In a second set of experiments, inactivation of the LHb with the gamma-aminobutyric acid (GABA) agonists baclofen and muscimol led to an impairment in a spatial/response based repeated probabilistic reversal learning task. Control experiments revealed that this impairment was likely due to the demands of repeated switching behaviors as rats were unimpaired on initial discrimination acquisition or retention of probabilistic learning. Taken together, these novel findings compliment other work discussed supporting a role for the LHb in action selection when cognitive or emotional demands are increased. Finally, we discuss future mechanisms by which a superior understanding of the LHb can be obtained through additional examination of behavioral flexibility tasks.

Neuronal activity of the prefrontal cortex is reduced in rats selectively bred for deficient sensorimotor gating

Rats selectively bred for deficient prepulse inhibition (PPI), an operant measure of sensorimotor gating in which a weak prepulse stimulus attenuates the response to a subsequent startling stimulus, may be used to study certain pathophysiological mechanisms and therapeutic strategies for neuropsychiatric disorders with abnormalities in information processing, such as schizophrenia and Tourette's syndrome (TS). Little is known about neuronal activity in the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAC), which are involved in the modulation of PPI. Here, we examined neuronal activity in these structures, and also in the entopeduncular nucleus (EPN), since lesions of this region alleviate the PPI deficit. Male rats with breeding-induced high and low expression of PPI (n=7, each) were anesthetized with urethane (1.4 mg/kg). Single-unit activity and local field potentials were recorded in the mPFC, the NAC and in the EPN. In the mPFC discharge rate, measures of irregularity and burst activity were significantly reduced in PPI low compared to PPI high rats (P<0.05), while analysis in the NAC showed approximately inverse behavior. In the EPN no difference between groups was found. Additionally, the oscillatory theta band activity (4-8 Hz) was enhanced and the beta band (13-30 Hz) and gamma band (30-100 Hz) activity was reduced in the NAC in PPI low rats. Reduced neuronal activity in the mPFC and enhanced activity in the NAC of PPI low rats, together with altered oscillatory behavior are clearly associated with reduced PPI. PPI low rats may thus be used to study the pathophysiology and therapeutic strategies for neuropsychiatric disorders accompanied by deficient sensorimotor gating.

Effect of deep brain stimulation in rats selectively bred for reduced prepulse inhibition

BACKGROUND

Sensorimotor gating, measured as prepulse inhibition (PPI) of the acoustic startle reaction (ASR), is disturbed in certain neuropsychiatric disorders, such as schizophrenia, obsessive compulsive disorder, and Tourette's syndrome (TS). Deep brain stimulation (DBS) of the centromedian-parafascicular complex (CM-Pf), globus pallidus internus (in rats the entopeduncular nucleus - EPN), and the ventral striatum (in rats the nucleus accumbens - NAC) has been used for treatment in TS.

OBJECTIVE

We tested whether DBS of these regions would alleviate breeding-induced low PPI in rats.

METHODS

Rats with breeding-induced low and high PPI were bilaterally implanted with electrodes in the CM-Pf, the EPN, or the NAC. After two weeks, they were stimulated or sham stimulated for epochs of 6 days (in the EPN with a current of 20% below the individual threshold for stimulation-induced side effects, in the NAC or CM-Pf with 100 μA and 150 μA). On the 6th day the rats were tested for PPI of ASR.

RESULTS

Stimulation in the CM-Pf with 150 μA significantly alleviated PPI, while NAC stimulation was less effective. In PPI low rats electrode implantation in the EPN already improved PPI, while subsequent stimulation had no additional effect. Startle reaction of PPI low rats was not affected by stimulation of either region.

CONCLUSION

The CM-Pf and the EPN are important for the modulation of sensorimotor gating in rats with breeding-induced low PPI. These rats may therefore be useful to further investigate the pathophysiological mechanisms of deficient sensorimotor gating and also mechanisms of action of DBS in these circumstances.