No abstract

Traumatic brain injury (TBI) modulates several cell signaling pathways in the hippocampus critical for memory formation. Previous studies have found that the cAMP-protein kinase A signaling pathway is downregulated after TBI and that treatment with a phosphodiesterase (PDE) 4 inhibitor rolipram rescues the decrease in cAMP. In the present study, we examined the effect of rolipram on TBI-induced cognitive impairments. At 2 weeks after moderate fluid-percussion brain injury or sham surgery, adult male Sprague Dawley rats received vehicle or rolipram (0.03 mg/kg) 30 min before water maze acquisition or cue and contextual fear conditioning. TBI animals treated with rolipram showed a significant improvement in water maze acquisition and retention of both cue and contextual fear conditioning compared with vehicle-treated TBI animals. Cue and contextual fear conditioning significantly increased phosphorylated CREB levels in the hippocampus of sham animals, but not in TBI animals. This deficit in CREB activation during learning was rescued in TBI animals treated with rolipram. Hippocampal long-term potentiation was reduced in TBI animals, and this was also rescued with rolipram treatment. These results indicate that the PDE4 inhibitor rolipram rescues cognitive impairments after TBI, and this may be mediated through increased CREB activation during learning.

Learning and memory impairments are common in traumatic brain injury (TBI) survivors. However, there are no effective treatments to improve TBI-induced learning and memory impairments. TBI results in decreased cAMP signaling and reduced cAMP-response-element binding protein (CREB) activation, a critical pathway involved in learning and memory. TBI also acutely upregulates phosphodiesterase 4B2 (PDE4B2), which terminates cAMP signaling by hydrolyzing cAMP. We hypothesized that a subtype-selective PDE4B inhibitor could reverse the learning deficits induced by TBI. To test this hypothesis, adult male Sprague-Dawley rats received sham surgery or moderate parasagittal fluid-percussion brain injury. At 3 months postsurgery, animals were administered a selective PDE4B inhibitor or vehicle before cue and contextual fear conditioning, water maze training and a spatial working memory task. Treatment with the PDE4B inhibitor significantly reversed the TBI-induced deficits in cue and contextual fear conditioning and water maze retention. To further understand the underlying mechanisms of these memory impairments, we examined hippocampal long-term potentiation (LTP). TBI resulted in a significant reduction in basal synaptic transmission and impaired expression of LTP. Treatment with the PDE4B inhibitor significantly reduced the deficits in basal synaptic transmission and rescued LTP expression. The PDE4B inhibitor reduced tumor necrosis factor-␣ levels and increased phosphorylated CREB levels after TBI, suggesting that this drug inhibited molecular pathways in the brain known to be regulated by PDE4B. These results suggest that a subtype-selective PDE4B inhibitor is a potential therapeutic to reverse chronic learning and memory dysfunction and deficits in hippocampal synaptic plasticity following TBI.

The elderly have comparatively worse cognitive impairments from traumatic brain injury (TBI) relative to younger adults, but the molecular mechanisms that underlie this exacerbation of cognitive deficits are unknown. Experimental models of TBI have demonstrated that the cyclic AMP-protein kinase A (cAMP-PKA) signaling pathway is downregulated after brain trauma. Since the cAMPPKA signaling pathway is a key mediator of long-term memory formation, we investigated whether the TBI-induced decrease in cAMP levels is exacerbated in aged animals. Aged (19 months) and young adult (3 months) male Fischer 344 rats received sham surgery or mild (1.4–1.6 atmospheres, atm) or moderate (1.7–2.1 atm) parasagittal fluid-percussion brain injury. At various time points after surgery, the ipsilateral parietal cortex, hippocampus, and thalamus were assayed for cAMP levels. Mild TBI lowered cAMP levels in the hippocampus of aged, but not young adult animals. Moderate TBI lowered cAMP levels in the hippocampus and parietal cortex of both age groups. In the thalamus, cAMP levels were significantly lowered after moderate, but not mild TBI. To determine if the TBI-induced decreases in cAMP had physiological consequences in aged animals, hippocampal long-term potentiation (LTP) in the Schaffer collateral pathway of the CA1 region was assessed. LTP was significantly decreased in both young adult and aged animals after mild and moderate TBI as compared to sham surgery animals. Rolipram rescued the LTP deficits after mild TBI for young adult animals and caused a partial recovery for aged animals. However, rolipram did not rescue LTP deficits after moderate TBI in either young adult or aged animals. These results indicate that exacerbation of cognitive impairments in aged animals with TBI may be due to decreased cAMP levels and deficits in hippocampal LTP.

Mild elevations in core temperature can occur in individuals involved in strenuous activities that are risky for potentially sustaining a mild traumatic brain injury (mTBI) or concussion. Recently, we have discovered that mild elevations in brain temperature can significantly aggravate the histopathological consequences of mTBI. However, whether this exacerbation of brain pathology translates into behavioral deficits is unknown. Therefore, we investigated the behavioral consequences of elevating brain temperature to mildly hyperthermic levels prior to mTBI. Adult male Sprague Dawley rats underwent mild fluid-percussion brain injury or sham surgery while normothermic (37 °C) or hyperthermic (39 °C) and were allowed to recover for 7 days. Animals were then assessed for cognition using the water maze and cue and contextual fear conditioning. We found that mTBI alone at normothermia had no effect on long-term cognitive measures whereas mTBI animals that were hyperthermic for 15 min prior to and for 4 h after brain injury were significantly impaired on long-term retention for both the water maze and fear conditioning. In contrast, hyperthermic mTBI animals cooled within 15 min to normothermia demonstrated no significant long-term cognitive deficits. Mild TBI irrespective of temperature manipulations resulted in significant short-term working memory deficits. Cortical atrophy and contusions were detected in all mTBI treatment groups and contusion volume was significantly less in hyperthermic mTBI animals that were cooled as compared to hyperthermic mTBI animals that remained hyperthermic. These results indicate that brain temperature is an important variable for mTBI outcome and that mildly elevated temperatures at the time of injury result in persistent cognitive deficits. Importantly, cooling to normothermia after mTBI prevents the development of long-term cognitive deficits caused by hyperthermia. Reducing temperature to normothermic levels soon after mTBI represents a rational approach to potentially mitigate the long-term consequences of mTBI.

Innovative molecular and genetic techniques have recently led to the identification of genetically defined populations of ipsilaterally projecting excitatory interneurons with probable functions in the rhythm-generating kernel of the central pattern generators (CPGs). The role of interneuronal populations in specific motor function is determined by their synaptic inputs, intrinsic properties, and target neurons. In this review we examine whether Hb9-expressing interneurons (Hb9 INs) fulfill a set of criteria that are the hallmarks of rhythm generators in the locomotor circuitry. Induced locomotor-like activity in this distinct population of ventral interneurons is in phase with bursts of motor activity, raising the possibility that they are part of the locomotor generator. To increase our understanding of the integrative function of Hb9 INs in the locomotor CPG, we investigated the cellular mechanisms underlying their rhythmic activity and examined the properties of synaptic inputs from low-threshold afferents and possible synaptic contacts with segmental motoneurons. Our findings suggest that the rhythmogenic Hb9 INs are integral components of the sensorimotor circuitry that regulate locomotor-like activity in the spinal cord.

Commissural inhibitory interneurons (INs) are integral components of the locomotor circuitry that coordinate left-right motor activity during movements. We have shown that GABA-mediated synaptic transmission plays a key role in generating alternating locomotor-like activity in the mouse spinal cord (Hinckley et al., 2005a). The primary objective of our study was to determine whether properties of lamina VIII (LVIII) GABAergic INs in the spinal cord of GAD67::GFP transgenic mice fit the classification of rhythm-coordinating neurons in the locomotor circuitry. The relatively large GFP+ INs had comparable morphological and electrophysiological properties, suggesting that they comprised a homogenous neuronal population. They displayed multipolar and complex dendritic arbors in ipsilateral LVII-LVIII and their axonal projections crossed the ventral commissure and branched into contralateral ventral, medial and dorsal laminae. Putative synaptic contacts evident as bouton-like varicosities were detected in close apposition to lateral motoneurons, Renshaw cells, other GFP+ INs and unidentified neurons. Exposure to a rhythmogenic cocktail triggered locomotor-like rhythmic firing in the majority of LVIII GFP+ INs. Their induced oscillatory activity was out of phase with bursts of contralateral motoneurons and in phase with bouts of ipsilateral motor activity. Membrane voltage oscillations were elicited by rhythmic increases in excitatory synaptic drive and might have been augmented by three types of voltage-activated cationic currents known to increase neuronal excitability. Based on their axonal projections and activity pattern we propose that this population of GABAergic INs forms a class of local commissural inhibitory interneurons that are integral component of the locomotor circuitry.

Traumatic brain injury (TBI) results in significant impairments in hippocampal synaptic plasticity. A molecule critically involved in hippocampal synaptic plasticity, 3′,5′-cyclic adenosine monophosphate, is downregulated in the hippocampus after TBI, but the mechanism that underlies this decrease is unknown. To address this question, we determined whether phosphodiesterase (PDE) expression in the hippocampus is altered by TBI. Young adult male Sprague Dawley rats received sham surgery or moderate parasagittal fluid-percussion brain injury. Animals were analyzed by western blotting for changes in PDE expression levels in the hippocampus. We found that PDE1A levels were significantly increased at 30 min, 1 h and 6 h after TBI. PDE4B2 and 4D2 were also significantly increased at 1, 6, and 24 h after TBI. Additionally, phosphorylation of PDE4A was significantly increased at 6 and 24 h after TBI. No significant changes were observed in levels of PDE1B, 1C, 3A, 8A, or 8B between 30 min to 7 days after TBI. To determine the spatial profile of these increases, we used immunohistochemistry and flow cytometry at 24 h after TBI. PDE1A and phospho-PDE4A localized to neuronal cell bodies. PDE4B2 was expressed in neuronal dendrites, microglia and infiltrating CD11b+ immune cells. PDE4D was predominantly found in microglia and infiltrating CD11b+ immune cells. To determine if inhibition of PDE4 would improve hippocampal synaptic plasticity deficits after TBI, we treated hippocampal slices with rolipram, a pan-PDE4 inhibitor. Rolipram partially rescued the depression in basal synaptic transmission and converted a decaying form of long-term potentiation (LTP) into long-lasting LTP. Overall, these results identify several possible PDE targets for reducing hippocampal synaptic plasticity deficits and improving cognitive function acutely after TBI.