DRL performance in mice with deletion of tPA, uPA or PAI-1 genes

κ-Opioid Receptor Activation in Dopamine Neurons Disrupts Behavioral Inhibition

The dynorphin/κ-opioid receptor (KOR) system has been previously implicated in the regulation of cognition, but the neural circuitry and molecular mechanisms underlying KOR-mediated cognitive disruption are unknown. Here, we used an operational test of cognition involving timing and behavioral inhibition and found that systemic KOR activation impairs performance of male and female C57BL/6 mice in the differential reinforcement of low response rate (DRL) task. Systemic KOR antagonism also blocked stress-induced disruptions of DRL performance. KOR activation increased 'bursts' of incorrect responses in the DRL task and increased marble burying, suggesting that the observed disruptions in DRL performance may be attributed to KOR-induced increases in compulsive behavior. Local inactivation of KOR by injection of the long-acting antagonist nor-BNI in the ventral tegmental area (VTA), but not the infralimbic prefrontal cortex (PFC) or dorsal raphe nucleus (DRN), prevented disruption of DRL performance caused by systemic KOR activation. Cre-dependent genetic excision of KOR from dopaminergic, but not serotonergic neurons, also blocked KOR-mediated disruption of DRL performance. At the molecular level, we found that these disruptive effects did not require arrestin-dependent signaling, because neither global deletion of G-protein receptor kinase 3 (GRK3) nor cell-specific deletion of GRK3/arrestin-dependent p38α MAPK from dopamine neurons blocked KOR-mediated DRL disruptions. We then showed that nalfurafine, a clinically available G-biased KOR agonist, could also produce DRL disruptions. Together, these studies demonstrate that KOR activation in VTA dopamine neurons disrupts behavioral inhibition in a GRK3/arrestin-independent manner and suggests that KOR antagonists could be beneficial for decreasing stress-induced compulsive behaviors.

Impaired fibrinolysis and traumatic brain injury in mice

Traumatic brain injury (TBI) has been associated with intravascular coagulation, which may be a result of thromboplastin released following brain injury. Clots thus formed are lysed by plasmin, which is activated by tissue-type and urokinase-type plasminogen activators (uPA). To evaluate the association between traumatic intravascular coagulation and post-traumatic outcome, uPA knockout (uPA-/-) transgenic mice (n=12) or wild-type littermates (WT; n=12) were anesthetized and subjected to controlled cortical impact (CCI) brain injury. A second group of uPA-/- (n=12) and WT mice (n=12) were subjected to sham injury. Motor function was assessed over 2 weeks using the composite neuroscore test and cognition (learning) was assessed with the Morris Water Maze (MWM) at 2 weeks post-injury, whereupon the animals were sacrificed for cortical lesion volume analysis. Motor function was significantly worse in the brain-injured uPA-/- mice when compared to brain-injured WT mice at 48 h (p<0.05) and one week post-injury (p<0.05). These differences resolved by 2 weeks post-injury. There was no significant difference in post-injury cognitive function between uPA-/- mice and WT mice. However, at 2 weeks post-injury, the brain-injured uPA-/- had a significantly larger volume of cortical tissue loss than their WT counterparts (p<0.05). These results demonstrate that the absence of uPA in mice aggravates acute motor deficit and exacerbates cortical tissue loss following CCI brain injury, and suggests a neuroprotective role of the fibrinolytic process following TBI.

Extracellular proteases: biological and behavioral roles in the mammalian central nervous system

Extracellular proteases and their inhibitors have been implicated in both physiological and pathological states in the central nervous system (CNS). Given the presence of several classes of proteases, it is believed that each enzyme may undertake distinct biological roles. Some are indispensible for neuronal migration, neurite outgrowth and pathfinding, and synaptic plasticity. Others are required for neuronal death and tumor growth and invasion. Furthermore, studies from transgenic animals lacking or overexpressing one or more of the proteases have suggested that functional compensations and redundance among different members do exist. Normally, protease activity is tightly regulated by specific inhibitors to prevent disastrous proteolysis. Various insults can disrupt the fine control of proteolysis and caise pathological changes. Novel strategies have been attempted to maintain or restore protease-inhibitors homeostasis, thus minimizing damages to the CNS. They may provide us with effective therapeutic tools for fighting certain neurological disorders.

Tissue plasminogen activator promotes the effects of corticotropin-releasing factor on the amygdala and anxiety-like behavior

Stress-induced plasticity in the brain requires a precisely orchestrated sequence of cellular events involving novel as well as well known mediators. We have previously demonstrated that tissue plasminogen activator (tPA) in the amygdala promotes stress-induced synaptic plasticity and anxiety-like behavior. Here, we show that tPA activity in the amygdala is up-regulated by a major stress neuromodulator, corticotropin-releasing factor (CRF), acting on CRF type-1 receptors. Compared with WT, tPA-deficient mice responded to CRF treatment with attenuated expression of c-fos (an indicator of neuronal activation) in the central and medial amygdala but had normal c-fos responses in paraventricular nuclei. They exhibited reduced anxiety-like behavior to CRF but had a sustained corticosterone response after CRF administration. This effect of tPA deficiency was not mediated by plasminogen, because plasminogen-deficient mice demonstrated normal behavioral and hormonal changes to CRF. These studies establish tPA as an important mediator of cellular, behavioral, and hormonal responses to CRF.

Evidence for disrupted NMDA receptor function in tissue plasminogen activator knockout mice

Tissue plasminogen activator (tPA), a serine protease immediate-early gene product expressed in brain areas important in learning and memory, has been shown to cleave the NR1 subunit of the NMDA receptor leading to a potentiated Ca(2+) influx. Mice lacking tPA (tPA-/- mice) have disrupted late phase-LTP in the hippocampus, possibly as a consequence of reduced Ca(2+) flux through NMDA receptors. In the present experiments, we investigated whether the NMDA antagonist dizocilpine might alter performance in tPA-/- mice in behavioural tasks shown to be sensitive to hippocampal lesions. tPA-/- mice and wild-type controls (WT) showed similar rates of acquisition and performance of a spatial working memory task (eight-arm radial maze). Dizocilpine (0.03-0.3 mg/kg, i.p.), given acutely, disrupted performance by increasing the number of errors equally across both genotypes. At asymptotic performance of a differential reinforcement of low response rate operant task (DRL), acute dizocilpine (0.03-0.3 mg/kg) impaired performance, but no differences between genotypes were observed. However, dizocilpine (0.1 mg/kg), given repeatedly during acquisition of a signalled-DRL15" task, retarded acquisition in tPA-/- but not WT mice. This treatment regime had no effect on locomotor activity in either genotype. tPA-/- mice showed no spatial learning deficits, but were more sensitive to dizocilpine during acquisition (though not expression) of a DRL task. This supports a role for tPA in modification of the NMDA receptor, although absence of tPA does not have consequences for all forms of NMDA-dependent mediated learning.

Impulsive-like behavior in differential-reinforcement-of-low-rate 36 s responding in mice depends on training history

Prior behavioral history in operant conditioning paradigms may induce impulsive-like responding as shown in rats. Little is known to what extent behavioral history influences subsequent behavior in mice, therefore the present study investigated the effects of lever-pressing under a fixed-ratio 5 schedule of reinforcement on subsequent differential-reinforcement-of-low-rate (DRL) 36 s performance in wild type mice compared to the behavior of 5-HT1B receptor knockout mice. Acquisition of both autoshaping and fixed-ratio 5 training was faster in 5-HT1B receptor knockout compared to wild type mice. Nevertheless, in the DRL 36 s procedure no differences were observed between genotypes. Both wild type and 5-HT1B receptor knockout mice displayed premature or impulsive-like responding in the DRL 36 s procedure, for example a peak location of responses around 20 s and high rates of responding. Taken together, the present data suggest that impulsive-like responding in the DRL 36 s procedure in mice depends on prior behavioral history.

Operant learning and differential-reinforcement-of-low-rate 36-s responding in 5-HT1A and 5-HT1B receptor knockout mice

Previous studies with mice lacking 5-HT(1A) (1AKO) and 5-HT(1B) (1BKO) receptors in hippocampus-dependent learning and memory paradigms, suggest that these receptors play an important role in learning and memory, although their precise role is unclear. In the present study, 1AKO and 1BKO mice were studied in operant behavioural paradigms of decision making and response inhibition, to further study the putative involvement of these receptors in prefrontal cortex-dependent learning and memory. Moreover, because 1AKO mice have been shown to exhibit an antidepressant-like phenotype and 1BKO mice to be more impulsive in ethological studies, mice were trained in a differential-reinforcement-of-low-rates (DRL) procedure. Overall, results indicate that 1AKO and 1BKO mice display subtle differences in operant paradigms of decision making and response inhibition compared to wild type (WT) mice. In addition, when responding under a DRL 36-s schedule had stabilised, 1BKO mice showed a phenotype indicative of increased impulsivity, whereas 1AKO mice did not differ from WT mice. In conclusion, 5-HT(1B) receptors appear to play an important role in impulsivity and a minor role in prefrontal cortex-dependent learning and memory as shown by the results obtained in serial reversal learning and extinction. In contrast, 5-HT(1A) receptors appear to be involved in facilitation of autoshaping, but their role in impulsivity and prefrontal cortex-dependent learning and memory appears to be limited.