Cocaine Withdrawal Impairs mGluR5-Dependent Long-Term Depression in Nucleus Accumbens Shell Neurons of Both Direct and Indirect Pathways.

We previously reported that animals withdrawn from repeated cocaine exposure exhibited a selective deficit in the ability to elicit metabotropic glutamate receptor 5 (mGluR5)-dependent long-term depression (LTD) in the nucleus accumbens (NAc) shell. To determine whether such impairment occurs in the NAc in a cell-type-specific manner, we used bacterial artificial chromosome (BAC) transgenic mice expressing enhanced green fluorescent protein (eGFP) under the control of gene regulatory elements for the dopamine D1 receptor (Drd1) or dopamine D2 receptor (Drd2) to identify distinct subpopulations of medium spiny neurons (MSNs). We found that bath application of group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) reliably induced LTD in both NAc shell and core MSNs of wild-type, hemizygous Drd1-eGFP, and Drd2-eGFP mice. Confirming our previous results, cocaine withdrawal selectively impaired DHPG-LTD in NAc shell Drd1-expressing direct and Drd2-expressing indirect pathway MSNs. We also found that the expression of DHPG-LTD in NAc MSNs was not affected by the Ca(2+)-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 1-naphthyl acetyl spermine. Furthermore, systemic administration of mGluR5-negative allosteric modulator fenobam before the daily injection of cocaine preserved mGluR5 function and significantly reduced the expression of cocaine-induced behavioral sensitization. These results reveal that withdrawal from repeated cocaine exposure may result in the impairment of NAc mGluR5-LTD in a subregion- but not cell-type-specific manner and suggests that pharmacological antagonism of mGluR5 may represent a potential strategy for reducing cocaine-induced addictive behaviors.

Long-term potentiation at excitatory synaptic inputs to the intercalated cell masses of the amygdala.

The intercalated cell masses (ITCs) of the amygdala are clusters of GABAergic interneurons that surround the basolateral complex of the amygdala. ITCs have been increasingly implicated in the acquisition and extinction of conditioned fear responses, but the underlying cellular mechanisms remain unexplored. Here, we report that repetitive stimulation of lateral amygdala (LA) afferents with a modified theta burst stimulation (TBS) protocol and induces long-term potentiation (LTP) of excitatory synapses onto medial paracapsular ITC (Imp) neurons. This TBS-induced LTP is; (1) induced and expressed post-synaptically, (2) involves a rise in post-synaptic Ca2+ and the activation of NR2B-containing N-methyl-D-aspartate receptors (NMDARs), (3) dependent on calcium/calmodulin-dependent protein kinase II and cAMP-dependent protein kinase activation, and (4) associated with increased exocytotic delivery of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) to the post-synaptic membrane. Remarkably, auditory fear conditioning led to a persistent increase in AMPAR/NMDAR ratio of glutamatergic synaptic currents and occluded TBS-induced LTP at LA-Imp synapses. Furthermore, extinction training rescued the effect of fear conditioning on AMPAR/NMDAR ratio and LTP induction. These results show that a prominent form of LTP can be elicited at LA-Imp synapses and suggest that this synaptic plasticity may contribute to the expression of fear conditioning.

Impairment of long-term depression in the anterior cingulate cortex of mice with bone cancer pain.

The anterior cingulate cortex (ACC) has been shown to play an important role in pain-related perception and chronic pain. However, little is known about the molecular mechanisms involved. To address this issue, we analyzed excitatory synaptic transmission and long-term synaptic plasticity in layer II/III pyramidal neurons within the rostral ACC (rACC) from mice with bone cancer pain induced by intra-tibia implantation of osteolytic fibrosarcoma cells. Ex vivo whole-cell patch-clamp recordings from rACC neurons showed no significant alterations in presynaptic glutamate release probability and postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic responses in mice with bone cancer pain. However, mechanical allodynia occurred in conjunction with decreased N-methyl-d-aspartate (NMDA)/AMPA ratio of synaptic currents elicited in bilateral rACC neurons. In addition, the induction of NMDA receptor-dependent long-term depression (LTD) at rACC synapses was impaired in rACC neurons of tumor-bearing mice. Western blot analysis revealed a significant decrease in the levels of NR1, NR2A, and NR2B subunits of NMDA receptors in the rACC under bone cancer pain condition. No significant changes in overall mRNA levels for any of the NMDA receptor subunits or calpain activity were observed in the rACC of tumor-bearing mice. These results indicate that tumor-induced injury or remodeling of primary afferent sensory nerve fibers that innervate the tumor-bearing bone may cause a persistent decrease in NMDA receptor expression in rACC neurons, resulting in a loss of LTD induction, thereby leading to long-term alterations of rACC activity and creating exaggerated pain behaviors.

Prenatal cocaine exposure enhances long-term potentiation induction in rat medial prefrontal cortex.

Prenatal exposure to cocaine has been reported to produce long-lasting cognitive deficits, but the underlying mechanisms remain largely unknown. Here, we report that the induction of long-term potentiation (LTP) at excitatory synapses onto layer V pyramidal neurons in the medial prefrontal cortex (mPFC) is facilitated in rats exposed to cocaine in utero (3 mg/kg, intravenous twice daily during embryonic days 10-20). This facilitated LTP is caused by a reduction of A-type γ-aminobutyric acid (GABA(A)) receptor-mediated inhibition of mPFC pyramidal neurons. Biochemical experiments revealed a significant decrease in the surface expression of GABA(A) receptor α1 subunits and total protein levels of γ2 and δ subunits in mPFC slices from rats exposed to cocaine in utero. Prenatal cocaine exposure also leads to enhanced mPFC pyramidal neuronal excitability. However, the development of behavioural sensitization to repeated cocaine administration was impaired in rats that were exposed to cocaine in utero. These results suggest that prenatal cocaine exposure causes a long-lasting reduction of GABAergic inhibition in mPFC layer V pyramidal neurons, leading to an increased susceptibility of excitatory synapses to LTP induction during the postnatal period.

Estrogen modulates sexually dimorphic contextual fear extinction in rats through estrogen receptor beta.

Females and males are different in brain and behavior. These sex differences occur early during development due to a combination of genetic and hormonal factors and continue throughout the lifespan. Previous studies revealed that male rats exhibited significantly higher levels of contextual fear memory than female rats. However, it remains unknown whether a sex difference exists in the contextual fear extinction. To address this issue, male, normally cycling female, and ovariectomized (OVX) female Sprague-Dawley rats were subjected to contextual fear conditioning and extinction trials. Here we report that although male rats exhibited higher levels of freezing than cycling female rats after contextual fear conditioning, female rats subjected to conditioning in the proestrus and estrus stage exhibited an enhancement of fear extinction than male rats. An estrogen receptor (ER) beta agonist diarylpropionitrile but not an ERalpha agonist propyl-pyrazole-triol administration also enhanced extinction of contextual fear in OVX female rats, suggesting that estrogen-mediated facilitation of extinction involves the activation of ERbeta. Intrahippocampal injection of estradiol or diarylpropionitrile before extinction training in OVX female rats remarkably reduced the levels of freezing response during extinction trials. In addition, the locomotion or anxiety state of female rats does not vary across the ovarian cycle. These results reveal a crucial role for estrogen in mediating sexually dimorphic contextual fear extinction, and that estrogen-mediated enhancement of fear extinction involves the activation of ERbeta.

Repeated cocaine administration decreases 5-HT(2A) receptor-mediated serotonergic enhancement of synaptic activity in rat medial prefrontal cortex.

Neural adaptations in the medial prefrontal cortex (mPFC) are thought to be crucial in the development and maintenance of addictive behaviors. The mPFC receives a dense serotonergic (5-hydroxytryptamine, 5-HT) innervation from raphe nuclei and 5-HT exerts complex actions on mPFC pyramidal neurons. The present study, using a rat model of behavioral sensitization to cocaine, was designed to determine whether repeated cocaine exposure in vivo is capable of altering 5-HT-induced regulation of glutamatergic transmission in the mPFC. In layer V pyramidal neurons of the mPFC, application of 5-HT, through activation of 5-HT(2A) receptors, induced a massive enhancement of spontaneous excitatory postsynaptic currents (sEPSCs). Repeated cocaine administration for 5 days resulted in an attenuation in the ability of 5-HT to enhance sEPSCs. This effect was prevented when cocaine was co-administered with the selective 5-HT(2A) receptor antagonist ketanserin and was mimicked by repeated 5-HT(2A) receptor agonist (-)4-iodo-2,5-dimethoxyphenylisopropylamine administration. Repeated cocaine administration is not associated with any changes in the levels of 5-HT(2A) receptors or regulator of GTP-binding protein signaling 4. These results suggest that cocaine-induced inhibition of 5-HT(2A) receptor-mediated enhancement of glutamatergic transmission in the mPFC may be caused, at least in part, by the impairment of coupling of 5-HT(2A) receptors with GTP-binding proteins during cocaine withdrawal. These alterations in 5-HT(2A) receptor responsiveness in the mPFC may be relevant to the development of behavioral sensitization and withdrawal effects following repeated cocaine administration.

A role of p38 mitogen-activated protein kinase in adenosine A1 receptor-mediated synaptic depotentiation in area CA1 of the rat hippocampus.

BACKGROUND: Although long-term potentiation (LTP) of synaptic strength is very persistent, current studies have provided evidence that various manipulations or pharmacological treatment when applied shortly after LTP induction can reverse it. This kind of reversal of synaptic strength is termed as depotentiation and may have a function to increase the flexibility and storage capacity of neuronal networks. Our previous studies have demonstrated that an increase in extracellular levels of adenosine and subsequent activation of adenosine A1 receptors are important for the induction of depotentiation; however, the signaling downstream of adenosine A1 receptors to mediate depotentiation induction remains elusive. RESULTS: We confirm that depotentiation induced by low-frequency stimulation (LFS) (2 Hz, 10 min, 1200 pulses) was dependent on adenosine A1 receptor activation, because it was mimicked by bath-applied adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) and was inhibited by the selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Pretreatment of the hippocampal slices with the selective p38 mitogen-activated protein kinase (MAPK) inhibitors, 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl]-5-(4-pyrudyl)-1H-imidazole (SB203580) or trans-1-(4-hydroxycyclohexyl)-4-(fluorophenyl)-5-(2-methoxypyrimidin-4-yl)imidazole (SB239063), prevented the induction of depotentiation by LFS and CPA. In agreement with electrophysiological observation, both LFS- and CPA-induced depotentiation are associated with an increase in p38 MAPK activation, which are blocked by DPCPX or SB203580 application. CONCLUSION: These results suggest that activation of adenosine A1 receptor and in turn triggering p38 MAPK signaling may contribute to the LFS-induced depotentiation at hippocampal CA1 synapses.

Effects of neonatal corticosteroid treatment on hippocampal synaptic function.

There is growing concern about long-term neurodevelopmental outcomes after neonatal corticosteroid treatment for chronic lung disease (CLD). Here, we use a protocol with tapering doses of dexamethasone (DEX) or hydrocortisone (HC) proportional to those used in preterm infants to examine the long-term consequences of these treatments on hippocampal synaptic plasticity and associative memory in later life. We found that neonatal DEX, but not HC, treatment impairs long-term potentiation (LTP) but enhances long-term depression (LTD) induction in adolescent rats. The effects of neonatal DEX treatment on LTP and LTD were prevented when the animals were given glucocorticoid receptor antagonist, RU38486, before DEX administration. We also found that neonatal DEX, but not HC, treatment induces a profound increase in the autophosphorylation of a isoform of Ca2+/calmodulin-dependent protein kinase II at threonine-286 and a decrease in the protein phosphatase 1 expression. In addition, only neonatal DEX treatment disrupts memory retention in rats subjected to passive avoidance learning tasks. These results demonstrate that only neonatal DEX treatment alters the hippocampal synaptic plasticity and associative memory formation in later life and thus suggest that HC may be a safer alternative to DEX for the treatment of CLD in the neonatal period.

The synthetic cannabinoids attenuate allodynia and hyperalgesia in a rat model of trigeminal neuropathic pain.

Trigeminal neuralgia is a disorder of paroxysmal and severely disabling facial pain and continues to be a real therapeutic challenge. At present there are few effective drugs. Here we have evaluated the effects of the synthetic cannabinoid WIN 55,212-2 on mechanical allodynia and thermal hyperalgesia in a rat model of trigeminal neuropathic pain produced by a chronic constriction injury (CCI) of the infraorbital branch of the trigeminal nerve (ION). Relative to sham operation controls, rats with the CCI-ION consistently displayed hyperresponsiveness to von Frey filament and heat stimulation of the vibrissal pad. Both mechanical allodynia and thermal hyperalgesia are seen both ipsilateral and contralateral to the side of nerve injury, but is significantly more severe ipsilaterally. Administration of WIN 55,212-2 (0.3-5 mg/kg i.p.) dose-dependently increased the mechanical and heat withdrawal thresholds. WIN 55,212-2 (0.3-3 mg/kg i.p.) produced no significant motor deficits in animals using the rotarod test. The effect of WIN 55,212-2 was mimicked by cannabinoid CB1 receptor agonist HU 210 and was antagonized by CB1 receptor antagonist AM 251, but not by CB2 receptor antagonist AM 630 or vanilloid receptor 1 antagonist capsazepine, suggesting the involvement of CB1 receptors. CCI-ION also induced a time-dependent upregulation of CB1 receptors primarily within the ipsilateral superficial laminae of the trigeminal caudal nucleus revealed by both Western blot and immunohistochemistry. Taken together, these results suggest that cannabinoids may be a useful therapeutic approach for the clinical management of trigeminal neuropathic pain disorders.

Characterization of long-term potentiation of primary afferent transmission at trigeminal synapses of juvenile rats: essential role of subtype 5 metabotropic glutamate receptors.

Recent work has demonstrated that a brief high-frequency conditioning stimulation to the primary afferent nerve fibers can induce a long-term potentiation (LTP) of synaptic transmission in neurons in the superficial layer of the trigeminal caudal nucleus; however, the cellular and molecular mechanisms underlying this synaptic potentiation remain unclear. Using both extracellular field potential and whole-cell patch-clamp recordings in brainstem parasagital slices of juvenile rat with the mandibular nerve attached, we show here that the induction of trigeminal primary afferent LTP: (1) does not require the activation of ionotropic glutamate receptors; (2) is dependent on extracellular Ca(2+) and the release of Ca(2+) from intracellular stores; (3) is specifically prevented by the metabotropic glutamate receptor subtype 5 (mGluR5) antagonist 2-methyl-6-(phenylethynyl)pyridine but not the mGluR1 antagonist LY367385, group II mGluR antagonist LY341495 or group III mGluR antagonist MAP4; (4) is mimicked by the bath-applied group I mGluR agonist (S)-3,5-dihydroxyphenylglycine and mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine; (5) requires the activation of phospholipase C (PLC) and protein kinase C (PKC); and (6) is concomitantly with a decrease in paired-pulse depression. These results demonstrate that the activation of mGluR5 and in turn triggering a PLC/PKC-dependent signaling cascade may contribute to the induction of LTP of primary afferent synaptic transmission in the superficial layer of trigeminal caudal nucleus of juvenile rats. This may be relevant to the processing of nociceptive information.

Therapeutic potential of cannabinoids in trigeminal neuralgia.

Trigeminal neuralgia is a disorder of paroxysmal and severely disabling facial pain and continues to be a real therapeutic challenge to the clinicians. While the exact cause and pathology of this disorder is uncertain, it is thought that trigeminal neuralgia caused by irritation of the trigeminal nerve. This irritation results from damage due to the change in the blood vessels, the presence of a tumor or other lesions that cause the compression of the trigeminal root. The pain of trigeminal neuralgia is characterized by unilateral pain attacks that start abruptly and last for varying periods of time from minutes to hours. The quality of pain is usually sharp, stabbing, lancinating, and burning. The attacks are initiated by mild stimuli such as light touch of the skin, eating, chewing, washing the face, brushing the teeth, and exposure to wind. Although antiepileptic drug therapy may be beneficial in the treatment of trigeminal neuralgia, up to one-half of the patients become refractory or intolerant to these medications. At present there are few other effective drugs. In cases of lacking effect after pharmacotherapy, surgical options may be considered. Currently there is growing amount of evidence to suggest that the psychoactive ingredient in cannabis and individual cannabinoids may be effective in alleviating neuropathic pain and hyperalgesia. Evidence suggests that cannabinoids may prove useful in pain modulation by inhibiting neuronal transmission in pain pathways. Considering the pronounced antinociceptive effects produced by cannabinoids, they may be a promising therapeutic approach for the clinical management of trigeminal neuralgia.

Valacyclovir treatment ameliorates the persistently increased pentylenetetrazol-induced seizure susceptibility in mice with herpes simplex virus type 1 infection.

Herpes simplex virus type 1 (HSV-1) is an important pathogen related to epilepsy. We have shown previously that corneal inoculation of mice with HSV-1 causes acute spontaneous behavioral and electrophysiological seizures and increases hippocampal excitability and kainite-induced seizure susceptibility. In this study, we aimed to determine whether early-life HSV-1 infection in mice might cause short- and long-term enhanced susceptibility to pentylenetetrazol (PTZ)-induced seizures and to evaluate whether early antiviral drug therapy was effectively ameliorating this deficit. Seizure threshold was calculated by the latency of onset of the myoclonic jerk, generalized clonus, and maximal tonic-clonic convulsion. We demonstrate that the localization of viral antigens was predominantly within the bilateral temporal areas (amygdala, piriform, and entorhinal cortex) of HSV-1-infected mice. We also present evidence that mice of all HSV-1-infected groups had a shorter latency and higher severity to PTZ-induced seizures than in age-matched, mock-infected controls. Treatment of HSV-1-infected mice with valacyclovir, a potent inhibitor of HSV-1 replication, produced a dose-dependent decrease in the signs of neurological deficits, pathological damages, and PTZ-induced seizure severity. Our results are consistent with the hypothesis that early-life HSV-1 infection leads to persistent enhancement of neuronal excitability in limbic circuits, which could result in an overall increased propensity to induce seizures later in life. Additionally, prompt optimal antiviral therapy effectively decreases seizure susceptibility in HSV-1-infected mice by limiting the level of viral replication and inflammatory response induced by virus. The present study provides not only experimental evidence, but also a new therapeutic strategy in HSV-1-associated human epilepsy.

Seizure, neuron loss, and mossy fiber sprouting in herpes simplex virus type 1-infected organotypic hippocampal cultures.

PURPOSE: Epileptic seizures are frequently seen after viral encephalitis. Herpes simplex virus type 1 (HSV-1) encephalitis is the most common cause of acquired epilepsy in humans. However, little information is available about the neuropathogenesis of HSV-1-associated seizures. We have developed an in vitro HSV-1-infected organotypic hippocampal slice culture to elucidate the underlying mechanisms of HSV-1-associated acute seizure activity. METHODS: Hippocampal slice cultures were prepared from postnatal day 10 to 12 rat pups. Wild-type HSV-1 strain RE (1 x 10(5) PFU) was applied to cultures at 14 days in vitro. The excitability of CA3 pyramidal cells and hippocampal network properties were measured with electrophysiological recordings. Hematoxylin-eosin (H&E) and Timm stains were used. RESULTS: HSV-1 infection induces epileptiform activity, neuron loss, and subsequently a dramatic increase of mossy fiber sprouting in the supragranular area. With intracellular recordings, surviving CA3 pyramidal cells exhibited a more depolarizing resting membrane potential concomitant with an increase in membrane input resistance and had a lower threshold to generate synchronized bursts and a decrease in the amplitude of afterhyperpolarization than did controls. When the antiherpes agent acyclovir was applied with a delay of 1 or 24 h after HSV-1 infection, a dramatic inhibition of HSV-1 replication and protection of the neuron loss were observed. CONCLUSIONS: These results suggest that a direct change in the excitability of the hippocampal CA3 neuronal network and HSV-1-induced neuron loss resulting in subsequent mossy fiber reorganization may play an important role in the generation of epileptiform activity.

Cannabinoid-induced presynaptic inhibition at the primary afferent trigeminal synapse of juvenile rat brainstem slices.

Systemic or intraventricular administration of cannabinoids causes analgesic effects, but relatively little is known for their cellular mechanism. Using brainstem slices with the mandibular nerve attached, we examined the effect of cannabinoids on glutamatergic transmission in superficial trigeminal caudal nucleus of juvenile rats. The exogenous cannabinoid receptor agonist WIN 55,212-2 (WIN), as well as the endogenous agonist anandamide, hyperpolarized trigeminal caudal neurones and depressed the amplitude of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) monosynaptically evoked by stimulating mandibular nerves in a concentration-dependent manner. The inhibitory action of WIN was blocked or fully reversed by the CB1 receptor antagonist SR 141716A. WIN had no effect on the amplitude of miniature excitatory postsynaptic currents (mEPSCs) recorded in the presence of tetrodotoxin or cadmium. The inhibitory effect of WIN on EPSCs was greater for those with longer synaptic latency, suggesting that cannabinoids have a stronger effect on C-fibre EPSPs than on Adelta-fibre EPSPs. Ba2+ (100 microm) blocked the hyperpolarizing effect of cannabinoids, but did not affect their inhibitory effect on EPSPs. The N-type Ca2+ channel blocker omega-conotoxin GVIA (omega-CgTX) occluded the WIN-mediated presynaptic inhibition, whereas the P/Q-type Ca2+ channel blocker omega-agatoxin TK (omega-Aga) had no effect. These results suggest that cannabinoids preferentially activate CB1 receptors at the nerve terminal of small-diameter primary afferent fibres. Upon activation, CB1 receptors may selectively inhibit presynaptic N-type Ca2+ channels and exocytotic release machinery, thereby attenuating the transmitter release at the trigeminal nociceptive synapses.

Herpes simplex virus type 1 inoculation enhances hippocampal excitability and seizure susceptibility in mice.

Herpes simplex virus type 1 (HSV-1) is the major pathogen related to epilepsy. However, little is known about the pathogenesis of HSV-1-associated epilepsy. Here, we report that corneal inoculation of mice with HSV-1 induces acute spontaneous behavioural and electrophysiological seizures and chronically increases hippocampal excitability and seizure susceptibility. In slices from infected mice, the surviving hippocampal CA3 pyramidal neurons exhibited a more depolarizing resting membrane potential concomitant with an increase in membrane input resistance. They also had a lower threshold for generating synchronized bursts and a decrease in the amplitude of afterhyperpolarization (AHP) than did controls. These results suggest that a direct change in the excitability of the hippocampal CA3 neuronal network could play an important role in facilitating the development of acute seizures and subsequent epilepsy.

Role for cAMP and protein phosphatase in the presynaptic expression of mouse hippocampal mossy fibre depotentiation.

Long-term potentiation (LTP) at the hippocampal mossy fibre-CA3 synapses can be reversed (depotentiated) by long trains of low-frequency stimulation (LFS). In the present study, we showed that this depotentiation is triggered by a presynaptic group II metabotropic glutamate receptor (mGluR), which reduces cytosolic cAMP level, leading to a reversal of cellular processes responsible for mossy fibre LTP expression. Furthermore, we found that both the presynaptic activity-induced elevation of Ca(2+) and the activation of protein phosphatase (PP) activity are required for the induction of depotentiation. Thus, we conclude that mossy fibre depotentiation is expressed presynaptically through the activation of both presynaptic mGluR- and PP-coupled signalling cascades, and that the bidirectional long-term plasticity at the mossy fibre-CA3 synapses is likely to be regulated by presynaptic Ca(2+)-dependent processes.

Alterations in the balance of protein kinase and phosphatase activities and age-related impairments of synaptic transmission and long-term potentiation.

Aging is associated with an impaired ability to maintain long-term potentiation (LTP), but the underlying cause of the impairment remains unclear. To gain a better understanding of the cellular and molecular mechanisms responsible for this impairment, the synaptic transmission and plasticity were studied in the CA1 region of hippocampal slices from adult (6-8 months) and poor-memory (PM)-aged (23-24 months) rats. The one-way inhibitory avoidance learning task was used as the behavioral paradigm to screen PM-aged rats. With intracellular recordings, CA1 neurons of PM-aged rats exhibited a more hyperpolarized resting membrane potential, reduced input resistance, and increased amplitude of afterhyperpolarization and spike threshold, compared with those in adult rats. Although a reduction in the size of excitatory synaptic response was observed in PM-aged rats, no obvious differences were found between adult and PM-aged rats in the pharmacological properties of excitatory synaptic response, paired-pulse facilitation, or frequency-dependent facilitation, which was tested with trains of 10 pulses at 1, 5, and 10 Hz. Slices from the PM-aged rats displayed significantly reduced early-phase long-term potentiation (E-LTP) and late-phase LTP (L-LTP), and the entire frequency-response curve of LTP and LTD is modified to favor LTD induction. The susceptibility of time-dependent reversal of LTP by low-frequency afferent stimulation was also facilitated in PM-aged rats. Bath application of the protein phosphatase inhibitor, calyculin A, enhanced synaptic response in slices from PM-aged, but not adult, rats. In contrast, application of the cAMP-dependent protein kinase inhibitors, Rp-8-CPT-cAMPS and KT5720, induced a decrease in synaptic transmission only in slices from the adult rats. Furthermore, the selective beta-adrenergic receptor agonist, isoproterenol, and pertussis toxin-sensitive G-protein inhibitor, N-ethylmaleimide, effectively restored the deficit in E-LTP and L-LTP of PM-aged rats. These results demonstrate that age-related impairments of synaptic transmission and LTP may result from alterations in the balance of protein kinase/phosphatase activities.