Interactions Between LTP- and LTD-Inducing Stimulation in the Sensorimotor Cortex of the Awake Freely Moving Rat

The metaplastic effects of cordycepin in hippocampal CA1 area of rats

Metaplasticity is referred to adjustment in the requirements for induction of synaptic plasticity based on the prior history of activity. Synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), has been considered to be the neural processes underlying learning and memory. Previous observations that cordycepin (an adenosine derivative) improved learning and memory seemed to be contradictory to the findings that cordycepin inhibited LTP. Therefore, we speculated that the conflicting reports of cordycepin might be related to metaplasticity. In the current study, population spike (PS) in hippocampal CA1 area of rats was recorded by using electrophysiological method in vivo. The results showed that cordycepin reduced PS amplitude in hippocampal CA1 with a concentration-dependent relationship, and high frequency stimulation (HFS) failed to induce LTP when cordycepin was intrahippocampally administrated but improved LTP magnitude when cordycepin was pre-treated. Cordycepin increased LTD induced by activating N-Methyl-D-aspartate (NMDA) receptors and subsequently facilitated LTP induced by HFS. Furthermore, we found that 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), an adenosine A1 receptors antagonist, could block the roles of cordycepin on LTD and LTP. Collectively, cordycepin was able to modulate metaplasticity in hippocampal CA1 area of rats through adenosine A1 receptors. These findings would be helpful to reconcile the conflicting reports in the literatures and provided new insights into the mechanisms underlying cognitive function promotions with cordycepin treatment.

A unified computational model for cortical post-synaptic plasticity

Signalling pathways leading to post-synaptic plasticity have been examined in many types of experimental studies, but a unified picture on how multiple biochemical pathways collectively shape neocortical plasticity is missing. We built a biochemically detailed model of post-synaptic plasticity describing CaMKII, PKA, and PKC pathways and their contribution to synaptic potentiation or depression. We developed a statistical AMPA-receptor-tetramer model, which permits the estimation of the AMPA-receptor-mediated maximal synaptic conductance based on numbers of GluR1s and GluR2s predicted by the biochemical signalling model. We show that our model reproduces neuromodulator-gated spike-timing-dependent plasticity as observed in the visual cortex and can be fit to data from many cortical areas, uncovering the biochemical contributions of the pathways pinpointed by the underlying experimental studies. Our model explains the dependence of different forms of plasticity on the availability of different proteins and can be used for the study of mental disorder-associated impairments of cortical plasticity.

Manipulating fear associations via optogenetic modulation of amygdala inputs to prefrontal cortex

Fear-related disorders are thought to reflect strong and persistent fear memories. The basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) form strong reciprocal synaptic connections that play a key role in acquisition and extinction of fear memories. While synaptic contacts of BLA cells onto mPFC neurons are likely to play a crucial role in this process, the BLA connects with several additional nuclei within the fear circuit that could relay fear-associated information to the mPFC, and the contribution of direct monosynaptic BLA-mPFC inputs is not yet clear. Here we establish an optogenetic stimulation protocol that induces synaptic depression in BLA-mPFC synapses. In behaving mice, optogenetic high-frequency stimulation of BLA inputs to mPFC interfered with retention of cued associations, attenuated previously acquired cue-associated responses in mPFC neurons and facilitated extinction. Our findings demonstrate the contribution of BLA inputs to mPFC in forming and maintaining cued fear associations.

The persistence of long-term potentiation in the projection from ventral hippocampus to medial prefrontal cortex in awake rats

A potentially vital pathway in the processing of spatial memory is the pathway from ventral hippocampus to medial prefrontal cortex (vHPC-mPFC). To assess long-term potentiation (LTP) induction and maintenance across days in this pathway, the effects of several induction paradigms were compared in awake, freely moving rats. Two different high-frequency stimulation (HFS) protocols generated LTP lasting no longer than 1 week. However, after delivering HFS on three consecutive days, LTP lasted an average of 20 days, due mainly to the greater initial induction. Thus the pathway does not require extensive multi-day stimulation to induce LTP, as for other intra-neocortical pathways, but also it does not exhibit the extremely long-lasting and stable LTP previously observed in area CA1 and the dentate gyrus. By using bilaterally placed stimulating and recording electrodes, we found that HFS in one vHPC generated responses and LTP in the contralateral mPFC, even when the ipsilateral mPFC was inactivated by CNQX. We attribute this crossed response to a polysynaptic pathway from the vHPC to the contralateral mPFC. Finally, we found that repeated overnight exposure to an enriched environment also potentiated the vHPC-mPFC response, but this too was a transient effect lasting < 9 days, declining to baseline even before the enriched environment treatment was completed. Overall, these findings are consistent with the view that potentiation of vHPC-mPFC pathway may play a key role in promoting the hippocampus-mPFC interplay that, over days, leads to long-term storage in the frontal cortex of memories that are independent of the hippocampus.

Bidirectional regulation of synaptic plasticity in the basolateral amygdala induced by the D1-like family of dopamine receptors and group II metabotropic glutamate receptors

Competing mechanisms of long-term potentiation (LTP) and long-term depression (LTD) in principal neurons of the basolateral amygdala (BLA) are thought to underlie the acquisition and consolidation of fear memories, and their subsequent extinction. However, no study to date has examined the locus of action and/or the cellular mechanism(s) by which these processes interact. Here, we report that synaptic plasticity in the cortical pathway onto BLA principal neurons is frequency-dependent and shows a transition from LTD to LTP at stimulation frequencies of ∼10 Hz. At the crossover point from LTD to LTP induction we show that concurrent activation of D1 and group II metabotropic glutamate (mGluR2/3) receptors act to nullify any net change in synaptic strength. Significantly, blockade of either D1 or mGluR2/3 receptors unmasked 10 Hz stimulation-induced LTD and LTP, respectively. Significantly, prior activation of presynaptic D1 receptors caused a time-dependent attenuation of mGluR2/3-induced depotentiation of previously induced LTP. Furthermore, studies with cell type-specific postsynaptic transgene expression of designer receptors activated by designer drugs (DREADDs) suggest that the interaction results via bidirectional modulation of adenylate cyclase activity in presynaptic glutamatergic terminals. The results of our study raise the possibility that the temporal sequence of activation of either presynaptic D1 receptors or mGluR2/3 receptors may critically regulate the direction of synaptic plasticity in afferent pathways onto BLA principal neurons. Hence, the interaction of these two neurotransmitter systems may represent an important mechanism for bidirectional metaplasticity in BLA circuits and thus modulate the acquisition and extinction of fear memory.

Enhanced detection threshold for in vivo cortical stimulation produced by Hebbian conditioning

Normal brain function requires constant adaptation, as an organism learns to associate important sensory stimuli with the appropriate motor actions. Neurological disorders may disrupt these learned associations and require the nervous system to reorganize itself. As a consequence, neural plasticity is a crucial component of normal brain function and a critical mechanism for recovery from injury. Associative, or Hebbian, pairing of pre- and post-synaptic activity has been shown to alter stimulus-evoked responses in vivo; however, to date, such protocols have not been shown to affect the animal's subsequent behavior. We paired stimulus trains separated by a brief time delay to two electrodes in rat sensorimotor cortex, which changed the statistical pattern of spikes during subsequent behavior. These changes were consistent with strengthened functional connections from the leading electrode to the lagging electrode. We then trained rats to respond to a microstimulation cue, and repeated the paradigm using the cue electrode as the leading electrode. This pairing lowered the rat's ICMS-detection threshold, with the same dependence on intra-electrode time lag that we found for the functional connectivity changes. The timecourse of the behavioral effects was very similar to that of the connectivity changes. We propose that the behavioral changes were a consequence of strengthened functional connections from the cue electrode to other regions of sensorimotor cortex. Such paradigms might be used to augment recovery from a stroke, or to promote adaptation in a bidirectional brain-machine interface.

Toward rational design of electrical stimulation strategies for epilepsy control

Electrical stimulation is emerging as a viable alternative for patients with epilepsy whose seizures are not alleviated by drugs or surgery. Its attractions are temporal and spatial specificity of action, flexibility of waveform parameters and timing, and the perception that its effects are reversible unlike resective surgery. However, despite significant advances in our understanding of mechanisms of neural electrical stimulation, clinical electrotherapy for seizures relies heavily on empirical tuning of parameters and protocols. We highlight concurrent treatment goals with potentially conflicting design constraints that must be resolved when formulating rational strategies for epilepsy electrotherapy, namely, seizure reduction versus cognitive impairment, stimulation efficacy versus tissue safety, and mechanistic insight versus clinical pragmatism. First, treatment markers, objectives, and metrics relevant to electrical stimulation for epilepsy are discussed from a clinical perspective. Then the experimental perspective is presented, with the biophysical mechanisms and modalities of open-loop electrical stimulation, and the potential benefits of closed-loop control for epilepsy.

Possible relationship between the stress-induced synaptic response and metaplasticity in the hippocampal CA1 field of freely moving rats

Hippocampal long-term potentiation (LTP) is suppressed not only by stress paradigms but also by low frequency stimulation (LFS) prior to LTP-inducing high frequency stimulation (HFS; tetanus), termed metaplasticity. These synaptic responses are dependent on N-methyl-D-aspartate receptors, leading to speculations about the possible relationship between metaplasticity and stress-induced LTP impairment. However, the functional significance of metaplasticity has been unclear. The present study elucidated the electrophysiological and neurochemical profiles of metaplasticity in the hippocampal CA1 field, with a focus on the synaptic response induced by the emotional stress, contextual fear conditioning (CFC). The population spike amplitude in the CA1 field was decreased during exposure to CFC, and LTP induction was suppressed after CFC in conscious rats. The synaptic response induced by CFC was mimicked by LFS, i.e., LFS impaired the synaptic transmission and subsequent LTP. Plasma corticosterone levels were increased by both CFC and LFS. Extracellular levels of gamma-aminobutyric acid (GABA), but not glutamate, in the hippocampus increased during exposure to CFC or LFS. Furthermore, electrical stimulation of the medial prefrontal cortex (mPFC), which caused decreases in freezing behavior during exposure to CFC, counteracted the LTP impairment induced by LFS. These findings suggest that metaplasticity in the rat hippocampal CA1 field is related to the neural basis of stress experience-dependent fear memory, and that hippocampal synaptic response associated stress-related processes is under mPFC regulation.

Metaplasticity: new insights through electrophysiological investigations

The term synaptic plasticity describes the ability of excitatory synapses to undergo activity-driven long-lasting changes in the efficacy of basal synaptic transmission. This change may be expressed as a long-term potentiation (LTP) or as a long-term depression (LTD). Metaplasticity is a higher-order form of synaptic plasticity that regulates the expression of both LTP and LTD through processes that are initiated by cellular activity that precedes a later bout of plasticity-inducing synaptic activity. Activation by prior synaptic activity and later expression as a facilitation or inhibition of activity-dependent synaptic plasticity are fundamental properties of metaplasticity. The intracellular mechanisms which support metaplasticity appear to be closely linked to those of synaptic plasticity, hence there are significant technical challenges to overcome in order to elucidate those mechanisms specific to metaplasticity. This review will examine the progress in the characterization of metaplasticity over the last decade or so with a focus on findings gained using electrophysiological techniques. It will look at the techniques applied, the brain regions investigated and the knowledge gained from the application of a wide range of protocols designed to examine the influence of varied forms of prior synaptic activity on later, activity-induced, synaptic plasticity.

Sensory deprivation unmasks a PKA-dependent synaptic plasticity mechanism that operates in parallel with CaMKII

Calcium/calmodulin kinase II (CaMKII) is required for LTP and experience-dependent potentiation in the barrel cortex. Here, we find that whisker deprivation increases LTP in the layer IV to II/III pathway and that PKA antagonists block the additional LTP. No LTP was seen in undeprived CaMKII-T286A mice, but whisker deprivation again unmasked PKA-sensitive LTP. Infusion of a PKA agonist potentiated EPSPs in deprived wild-types and deprived CaMKII-T286A point mutants but not in undeprived animals of either genotype. The PKA-dependent potentiation mechanism was not present in GluR1 knockouts. Infusion of a PKA antagonist caused depression of EPSPs in undeprived but not deprived cortex. LTD was occluded by whisker deprivation and blocked by PKA manipulation, but not blocked by cannabinoid antagonists. NMDA receptor currents were unaffected by sensory deprivation. These results suggest that sensory deprivation causes synaptic depression by reversing a PKA-dependent process that may act via GluR1.

Sustained relief of dystonia following cessation of deep brain stimulation

We describe the unusual clinical course of a patient with cranial dystonia (i.e., Meige syndrome) and additional upper limb involvement, who developed sustained relief of motor symptoms following cessation of a prolonged course of bilateral pallidal deep brain stimulation (DBS). Early response to therapy proved titratable and reversible; however, the patient gained independence from DBS in the fifth postoperative year and has since been more than a year without treatment or exacerbation of motor symptoms. Among the potential explanations for these neurological benefits lies the intriguing possibility that DBS therapy may have the capacity to induce plastic change that lessens or obviates the need for further treatment in susceptible patients.

Background firing rates of orbitofrontal neurons reflect specific characteristics of operant sessions and modulate phasic responses to reward-associated cues and behavior

The orbitofrontal cortex plays an important role in the ability of animals to adjust their behavior in response to behavioral outcomes. Multiple studies have demonstrated that responses of orbitofrontal neurons during operant sessions reflect the outcome of particular behaviors. These studies have focused on rapid neural responses to short-duration events such as instrumental behavior and reward-associated discrete cues. We hypothesize that longer-lasting changes in firing are also important for information processing in the orbitofrontal cortex. In the present study, we recorded the activity of 115 single orbitofrontal neurons during a multiphase operant task in which the relationship between a lever-press response and a sucrose reward was varied between the different phases. Approximately one-half of the orbitofrontal neurons exhibited a change in background firing during the operant phases. These changes were observable across multiple behavioral and stimulus events and thus reflected a general shift in background firing. The majority of changes were selective for one or the other of the operant phases. Selective changes contributed to unique patterns of phasic firing time locked to cues and operant behavior in the two operant phases. These findings are consistent with the interpretation that changes in background firing of orbitofrontal neurons reflect operant session characteristics associated with behavioral outcome, and indicate further that changes in background firing contribute to the outcome selectivity of phasic firing patterns. More generally, we propose that the background firing rates of orbitofrontal neurons reflect contextual information, and facilitate context-appropriate event-related information processing and behavioral responses.

Sensitization of rat dorsal horn neurons by NGF-induced subthreshold potentials and low-frequency activation. A study employing intracellular recordings in vivo

Intramuscular injection of NGF in human subjects has been reported not to elicit pain, whereas 5% NaCl does. On the other hand, NGF injections induce a long-lasting hyperalgesia. In the present study, the possible neuronal basis of these effects was studied at the spinal level. In anesthetized rats, neurons in the segment L4 were recorded intracellularly before (n=65), during (n=15), and after injections of NGF (n=50) as well as during and after 5% NaCl (during: n=12, after: n=39) into the gastrocnemius-soleus (GS) muscle. The neuronal responses to electrical and mechanical stimuli were tested and possible changes caused by the stimulants recorded. Of those neurons that responded to the NGF injections (7 out of 15), the majority exhibited subthreshold excitatory postsynaptic potentials (EPSPs). Only 3 out of 15 neurons reacted with action potentials (APs) at a low frequency. Already 5 to 30 min after NGF injection, some of these neurons showed signs of a sensitization. In comparison to NGF, hypertonic saline i.m. elicited APs at a higher frequency in a larger number of neurons (9 out of 12). One day after NGF i.m., the proportion of dorsal horn neurons responding with APs to electrical stimulation of the GS nerves had increased significantly from 4.6% to 28.0%. Despite the stronger excitatory effect of 5% NaCl, the sensitization of the dorsal horn neurons after hypertonic saline was less than that after NGF (15.3%). Behavioral experiments showed that NGF injections induced stronger mechanical allodynia and hyperalgesia than hypertonic saline i.m. The data demonstrate that low-frequency activation or even subthreshold potentials in dorsal horn neurons evoked by unmyelinated muscle afferents are an effective means of sensitizing these neurons.

Effects of GABAergic inhibition on neocortical long-term potentiation in the chronically prepared rat

Long-term potentiation (LTP) is a form of activity-dependent synaptic plasticity that is a candidate cellular mechanism for some forms of learning and memory. Although GABAergic synaptic inhibition plays a critical role in regulating of synaptic plasticity, there is still little known about the GABAergic modulation on LTP induction in chronic preparation. In the present study we examined the effect of GABA(A) agonist, diazepam (DZM), and antagonist, picrotoxin (PTX) on the induction of LTP in the somatosensory cortex of freely moving rats for a long-term period. In adult rats a bipolar stimulating and recording electrode were implanted into corpus callusom and somatosensory cortex, respectively. Two weeks after the surgery, evoked field potential responses were recorded before, during (12 days), and after (1 month) induction period of LTP by high-frequency stimulation. The LTP characteristics were compared between control, DZM and PTX groups during the time course of recording in each rat. Administration of DZM prior to train, blocked the induction of neocortical LTP, while the PTX increased the development of LTP making the highest differential levels of LTP about 12 days after the initiation of LTP induction. Our findings suggest that the augmentation of LTP by PTX can be explained by an interaction between excitatory and inhibitory pathways. Suppression of neocortical inhibitory inputs by PTX causes enhancement in LTP induction. These results suggest that GABAergic system has an important role in synaptic plasticity and long-term modification of somatosensory cortex in freely moving rat.

Long-term depression in the sensorimotor cortex induced by repeated delivery of 10 Hz trains in vivo

Memory consolidation in the neocortex is thought to be mediated in part by bi-directional modifications of synaptic strength. The sensorimotor cortex shows marked spontaneous activity near 10 Hz during both waking and sleep in the form of electroencephalographic spindle waves, and is also sensitive to electrical activation of inputs at 10 Hz. Induction of long-term synaptic depression in corpus callosum inputs to layer V of the sensorimotor cortex of the awake, adult rat requires repeated low-frequency stimulation over many days. To determine if 10 Hz stimulation may facilitate the induction of long-term depression, we compared the amounts of long-term depression induced by conventional 1 Hz trains, repeated delivery of 450 pairs of stimulation pulses using a 100 ms interpulse interval, and 45 short, 2 s, 10 Hz trains. Each pattern was delivered daily for 10 days and was matched for total duration and number of pulses. Changes in synaptic responses were assessed by monitoring field potentials evoked by stimulation of the corpus callosum. A facilitation of synaptic responses in layer V was observed during delivery of both paired-pulse trains and 10 Hz trains. There was no significant difference in long-term depression induced by 1 Hz stimulation and repeated paired-pulse stimulation, but 10 Hz trains induced significantly greater long-term depression than 1 Hz trains in both the early monosynaptic and late polysynaptic field potential components. The effectiveness of short 10 Hz trains for the induction of long-term depression suggests that synchronous population activity at frequencies near 10 Hz such as spindle waves may contribute to endogenous synaptic depression in sensorimotor cortex.

Nitric Oxide and Synaptic Dynamics in the Adult Brain: Physiopathological Aspects

The adult brain retains the capacity to rewire mature neural circuits in response to environmental changes, brain damage or sensory and motor experiences. Two plastic processes, synaptic remodeling and neurogenesis, have been the subject of numerous studies due to their involvement in the maturation of the nervous system, their prevalence and re-activation in adulthood, and therapeutic relevance. However, most of the research looking for the mechanistic and molecular events underlying synaptogenic phenomena has been focused on the extensive synaptic reorganization occurring in the developing brain. In this stage, a vast number of synapses are initially established, which subsequently undergo a process of activity-dependent refinement guided by target-derived signals that act as synaptotoxins or synaptotrophins, promoting either loss or consolidation of pre-existing synaptic contacts, respectively. Nitric oxide (NO), an autocrine and/or paracrine-acting gaseous molecule synthesized in an activity-dependent manner, has ambivalent actions. It can act by mediating synapse formation, segregation of afferent inputs, or growth cone collapse and retraction in immature neural systems. Nevertheless, little information exists about the role of this ambiguous molecule in synaptic plasticity processes occurring in the adult brain. Suitable conditions for elucidating the role of NO in adult synaptic rearrangement include physiopathological conditions, such as peripheral nerve injury. We have recently developed a crush lesion model of the XIIth nerve that induces a pronounced stripping of excitatory synaptic boutons from the cell bodies of hypoglossal motoneurons. The decline in synaptic coverage was concomitant with de novo expression of the neuronal isoform of NO synthase in motoneurons. We have demonstrated a synaptotoxic action of NO mediating synaptic withdrawal and preventing synapse formation by cyclic GMP (cGMP)-dependent and, probably, S-nitrosylation-mediated mechanisms, respectively. This action possibly involves the participation of other signaling molecules working together with NO. Brain-derived neurotrophic factor (BDNF), a target-derived synaptotrophin synthesized and released postsynaptically in an activity-dependent form, is a potential candidate for effecting such a concerted action. Several items of evidence support an interrelationship between NO and BDNF in the regulation of synaptic remodeling processes in adulthood: i) BDNF and its receptor TrkB are expressed by motoneurons and upregulated by axonal injury; ii) they promote axon arborization and synaptic formation, and modulate the structural dynamics of excitatory synapses; iii) NO and BDNF each control the production and activity of the other at the level of individual synapses; iv) the NO/cGMP pathway inhibits BDNF secretion; and finally, v) BDNF protects F-actin from depolymerization by NO, thus preventing the collapsing and retracting effects of NO on growth cones. Therefore, we propose a mechanism of action in which the NO/BDNF ratio regulates synapse dynamics after peripheral nerve lesion. This hypothesis also raises the possibility that variations in this NO/BDNF balance constitute a common hallmark leading to synapse loss in the progression of diverse neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's and Parkinson's diseases.

Effects of neonatal C-fiber depletion on neocortical long-term potentiation and depression

Capsaicin (Cap)-induced depletion of C-fiber afferents results in plasticity of somatosensory system which is manifested as a functional alteration at different levels of the somatosensory pathway. In the present study we examined the effect of Cap-induced neonatal depletion of C-fibers on the induction of long-term potentiation (LTP) and long-term depression (LTD) in the neocortex of freely moving rats. A stimulating electrode was implanted into corpus callosum and a recording electrode was implanted in the somatosensory cortex of control (Con: normal, without electrical stimulation), trained (normal, with electrical stimulation) and Cap-treated (C-fiber depleted, with electrical stimulation) adult rats. Two weeks after the surgery, evoked field potential responses were recorded before, during (12 days) and after (1 month) the induction period of LTP and LTD. The LTP and LTD response characteristics during the time course of recording were compared between different groups. In the train group, LTP and LTD appeared after 3 days of stimulation. LTP magnitude peaked after about 6 days while LTD magnitude peaked in about 12 days. C-fiber depletion postponed the development of LTP and LTD making the highest differential levels of LTP about 6 days after the initiation of LTP induction. The impact of C-fiber depletion on slowing the time course of LTD induction was more prolonged and lasted until day 12 of the initiation of LTD induction. These results suggest that intact C-fibers are necessary for normal plasticity and long-term synaptic modification of the somatosensory system.

Synaptic Plasticity in Local Cortical Network In Vivo and Its Modulation by the Level of Neuronal Activity

Neocortical neurons maintain high firing rates across all behavioral states of vigilance but the discharge patterns vary during different types of brain oscillations, which are assumed to play an important role in information processing and memory consolidation. In the present study, we report that trains of stimuli applied to local neocortical networks of cats, at frequencies that mimic endogenous brain rhythms, produced depression or potentiation of postsynaptic potentials, which lasted for several minutes. This form of synaptic plasticity was not mediated through NMDA receptors since it persisted after blockade of these receptors, but was strongly modulated by the level of background neuronal activity. Using different preparations in vivo, we found that increased background neuronal activity decreased the probability of plastic changes but enhanced the probability of potentiation over depression. Conversely, when the level of background neuronal activity was low, plasticity was observed in all neurons, but mainly depression was induced. Our results demonstrate that high levels of neuronal activity in the cortical network promote potentiation and insure the stability of synaptic connections.

Corticotrophin-releasing hormone decreases synaptic transmission in rat sensorimotor cortex in vivo

Corticotrophin-releasing hormone is a key regulator of the mammalian stress response. Although its actions on behavior are well documented, the actions of corticotrophin-releasing hormone in cortical neuronal systems are poorly understood. In the present experiments, adult male Sprague-Dawley rats were anesthetized and field excitatory post-synaptic potential recordings were made from sensorimotor cortex layer II/III and layer V cells. Infusions of corticotrophin-releasing hormone (100 ng/nl) directly into the sensorimotor cortex produced a significant depression of the initial excitatory component of evoked responses that could be prevented by prior administration of a corticotrophin-releasing hormone antagonist. Although requiring the activation of corticotrophin-releasing hormone receptors, the depression was also dependent upon N-methyl-D-aspartate receptor activity and could be blocked by the competitive N-methyl-D-aspartate antagonist -3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonate. These findings demonstrate that corticotrophin-releasing hormone has a novel depressant-like action in sensorimotor cortex in vivo that may play a role in modulating motor activity during periods of stress.