Shanelle W. Ko

Roles of NMDA NR2B Subtype Receptor in Prefrontal Long-Term Potentiation and Contextual Fear Memory

Cortical plasticity is thought to be important for the establishment, consolidation, and retrieval of permanent memory. Hippocampal long-term potentiation (LTP), a cellular mechanism of learning and memory, requires the activation of glutamate N-methyl-D-aspartate (NMDA) receptors. In particular, it has been suggested that NR2A-containing NMDA receptors are involved in LTP induction, whereas NR2B-containing receptors are involved in LTD induction in the hippocampus. However, LTP in the prefrontal cortex is less well characterized than in the hippocampus. Here we report that the activation of the NR2B and NR2A subunits of the NMDA receptor is critical for the induction of cingulate LTP, regardless of the induction protocol. Furthermore, pharmacological or genetic blockade of the NR2B subunit in the cingulate cortex impaired the formation of early contextual fear memory. Our results demonstrate that the NR2B subunit of the NMDA receptor in the prefrontal cortex is critically involved in both LTP and contextual memory.

Deficits in Trace Fear Memory and Long-Term Potentiation in a Mouse Model for Fragile X Syndrome

Trace fear memory requires the activity of the anterior cingulate cortex (ACC) and is sensitive to attention-distracting stimuli. Fragile X syndrome is the most common form of mental retardation with many patients exhibiting attention deficits. Previous studies in fragile X mental retardation 1 (FMR1) knock-out (KO) mice, a mouse model for fragile X, focused mainly on hippocampal-dependent plasticity and spatial memory. We demonstrate that FMR1 knock-out mice show a defect in trace fear memory without changes in locomotion, anxiety, and pain sensitivity. Whole-cell path-clamp recordings in the ACC show that long-term potentiation (LTP) was completely abolished. A similar decrease in LTP was found in the lateral amygdala, another structure implicated in fear memory. No significant changes were found in basal synaptic transmission. This suggests that synaptic plasticity in the ACC and amygdala of FMR1 KO mice plays an important role in the expression of behavioral phenotypes similar to the symptoms of fragile X syndrome.

Upregulation of Forebrain NMDA NR2B Receptors Contributes to Behavioral Sensitization after Inflammation

Transgenic overexpression of NMDA NR2B receptors in forebrain regions increased behavioral responses to persistent inflammatory pain. However, it is not known whether inflammation leads to the upregulation of NR2B receptors in these regions. Here, we show that peripheral inflammation increased the expression of NMDA NR2B receptors and NR2B receptor-mediated synaptic currents in the anterior cingulate cortex (ACC). In freely moving mice, the increase in NR2B receptors after inflammation contributed to enhanced NMDA receptor-mediated responses in the ACC. Inhibition of NR2B receptors in the ACC selectively reduced behavioral sensitization related to inflammation. Our results demonstrate that the upregulation of NR2B receptors in the ACC contributes to behavioral sensitization caused by inflammation.

Enhanced Presynaptic Neurotransmitter Release in the Anterior Cingulate Cortex of Mice with Chronic Pain

The anterior cingulate cortex (ACC) is a forebrain structure known for its roles in learning and memory. Recent studies show that painful stimuli activate the prefrontal cortex and that brain chemistry is altered in this area in patients with chronic pain. Components of the CNS that are involved in pain transmission and modulation, from the spinal cord to the ACC, are very plastic and undergo rapid and long-term changes after injury. Patients suffering from chronic pain often complain of memory and concentration difficulties, but little is known about the neural circuitry underlying these deficits. To address this question, we analyzed synaptic transmission in the ACC from mice with chronic pain induced by hindpaw injection of complete Freunds adjuvant (CFA). In vitro whole-cell patch-clamp recordings revealed a significant enhancement in neurotransmitter release probability in ACC synapses from mice with chronic pain. Trace fear memory, which requires sustained attention and the activity of the ACC, was impaired in CFA-injected mice. Using knock-out mice, we found that calmodulin-stimulated adenylyl cyclases, AC1 and/or AC8, were crucial in mediating the long-lasting enhanced presynaptic transmitter release in the ACC of mice with chronic pain. Our findings provide strong evidence that presynaptic alterations caused by peripheral inflammation contribute to memory impairments after injury.

Hot receptors in the brain

Two major approaches have been employed for the development of novel drugs to treat chronic pain. The most traditional approach identifies molecules involved in pain as potential therapeutic targets and has focused mainly on the periphery and spinal cord. A more recent approach identifies molecules that are involved in long-term plasticity. Drugs developed through the latter approach are predicted to treat chronic, but not physiological or acute, pain. The TRPV1 (transient receptor potential vanilloid-1) receptor is involved in nociceptive processing, and is a candidate therapeutic target for pain. While most research on TRPV1 receptors has been conducted at the level of the spinal cord and peripheral structures, considerably less research has focused on supraspinal structures. This short paper summarizes progress made on TRPV1 receptors, and reviews research on the expression and function of TRPV1 receptors in supraspinal structures. We suggest that the TRPV1 receptor may be involved in pain processing in higher brain structures, such as the anterior cingulate cortex. In addition, some regions of the brain utilize the TRPV1 receptor for functions apparently unrelated to pain.

Increased Anxiety-Like Behavior and Enhanced Synaptic Efficacy in the Amygdala of GluR5 Knockout Mice

GABAergic transmission in the amygdala modulates the expression of anxiety. Understanding the interplay between GABAergic transmission and excitatory circuits in the amygdala is, therefore, critical for understanding the neurobiological basis of anxiety. Here, we used a multi-disciplinary approach to demonstrate that GluR5-containing kainate receptors regulate local inhibitory circuits, modulate the excitatory transmission from the basolateral amygdala to the central amygdala, and control behavioral anxiety. Genetic deletion of GluR5 or local injection of a GluR5 antagonist into the basolateral amygdala increases anxiety-like behavior. Activation of GluR5 selectively depolarized inhibitory neurons, thereby increasing GABA release and contributing to tonic GABA current in the basolateral amygdala. The enhanced GABAergic transmission leads to reduced excitatory inputs in the central amygdala. Our results suggest that GluR5 is a key regulator of inhibitory circuits in the amygdala and highlight the potential use of GluR5-specific drugs in the treatment of pathological anxiety.

Genetic reduction of chronic muscle pain in mice lacking calcium/calmodulin-stimulated adenylyl cyclases

BackgroundThe Ca2+/calmodulin-stimulated adenylyl cyclase (AC) isoforms AC1 and AC8, couple NMDA receptor activation to cAMP signaling pathways in neurons and are important for development, learning and memory, drug addiction and persistent pain. AC1 and AC8 in the anterior cingulate cortex (ACC) and the spinal cord were previously shown to be important in subcutaneous inflammatory pain. Muscle pain is different from cutaneous pain in its characteristics as well as conducting fibers. Therefore, we conducted the present work to test the role of AC1 and AC8 in both acute persistent and chronic muscle pain.ResultsUsing an acute persistent inflammatory muscle pain model, we found that the behavioral nociceptive responses of both the late phase of acute muscle pain and the chronic muscle inflammatory pain were significantly reduced in AC1 knockout (KO) and AC1&8 double knockout (DKO) mice. Activation of other adenylyl cyclases in these KO mice by microinjection of forskolin into the ACC or spinal cord, but not into the peripheral tissue, rescued the behavioral nociceptive responses. Additionally, intra-peritoneal injection of an AC1 inhibitor significantly reduced behavioral responses in both acute persistent and chronic muscle pain.ConclusionThe results of the present study demonstrate that neuronal Ca2+/calmodulin-stimulated adenylyl cyclases in the ACC and spinal cord are important for both late acute persistent and chronic inflammatory muscle pain.

Kainate receptors and pain: from dorsal root ganglion to the anterior cingulate cortex.

Ionotropic glutamate receptors contain three subtypes: NMDA, AMPA and kainate receptors. The former two receptor subtypes have well defined roles in nociception, while the role of kainate receptors in pain is not as well characterized. Kainate receptors are expressed in nociceptive pathways, including the dorsal root ganglion, spinal cord, thalamus and cortex. Electrophysiological studies show that functional kainate receptors are located postsynaptically, where they mediate a portion of excitatory synaptic transmission, or are located presynaptically, where they modulate excitatory or inhibitory neurotransmission. Recent genetic and pharmacological studies suggest that kainate receptors can regulate nociceptive responses. These results highlight kainate receptors as a target for the development of new treatments for chronic pain.

Contribution of CaMKIV to injury and fear- induced ultrasonic vocalizations in adult mice

Calcium-calmodulin dependent protein kinase IV (CaMKIV) is a protein kinase that activates the transcription factor CREB. Our previous work demonstrated that mice lacking CaMKIV had a defect in fear memory while behavioral responses to noxious stimuli were unchanged. Here, we measured ultrasonic vocalizations (USVs) before and after fear conditioning and in response to a noxious injection of capsaicin to measure behavioral responses to emotional stimuli. Consistent with previous findings, behavioral nociceptive responses to capsaicin were undistinguishable between wild-type and CaMKIV-/- mice. Wild-type animals showed a selective increase in 50 kHz USVs in response to capsaicin while such an increase was absent in CaMKIV-/- mice. The foot shock given during fear conditioning caused an increase in 30 kHz USVs in both wild-type and CaMKIV-/- mice. When returned to the context one hour later, USVs from the wild-type were significantly decreased. Additionally, the onset of a tone, which had previously been paired with the foot shock, caused a significant decrease in USVs during auditory conditioning. CaMKIV-/- mice showed significantly less reduction in USVs when placed in the same context three days after receiving the shock, consistent with the decrease in freezing reported previously. Our results provide a new approach for investigating the molecular mechanism for emotional vocalization in mice and suggest that CaMKIV dependent signaling pathways play an important role in the emotional response to pain and fear.

Genetic alteration of anxiety and stress-like behavior in mice lacking CaMKIV

Calcium-calmodulin-dependent protein kinase IV (CaMKIV) phosphorylates the major transcription factor cyclic AMP-response element binding protein (CREB), which plays a role in emotional behavior. Here, CaMKIV knockout mice (CaMKIV-/-) were tested in a battery of stress and anxiety-related behavioral tests, to determine if CaMKIV plays a role in emotional behavior. CaMKIV-/-exhibited a decrease in anxiety-like behavior in both the elevated plus maze and dark-light emergence tests when compared to wild-type mice. Both the acoustic startle response and prepulse inhibition of startle were decreased with the deletion of CaMKIV. In addition, CaMKIV-/- mice displayed a lack of stress-induced analgesia following restraint or cold swim stress. Our results demonstrate a key role for CaMKIV in anxiety and stress-related behavior.