Differential projections from the mediodorsal and centrolateral thalamic nuclei to the frontal cortex in rats

Synaptic organization and input-specific short-term plasticity in anterior cingulate cortical neurons with intact thalamic inputs

The absence of a slice preparation with intact thalamocortical pathways has held back elucidation of the cellular and synaptic mechanisms by which thalamic signals are differentially transmitted to and processed in the anterior cingulate cortex (ACC). In this report we introduce an innovative mouse brain slice preparation in which it is possible to explore the electrophysiological properties of ACC neurons with intact long-distance inputs from medial thalamic (MT) nuclei by intracellular recordings; this MT-ACC neuronal pathway plays an integral role in information transmission. Biocytin-labeled fibers in a functional slice could be traced anterogradely or retrogradely from the MT via the reticular thalamic nuclei, striatum and corpus callosum to the cingulate cortical areas. Eighty-seven cells downstream of the thalamic projections in 49 slices were recorded intracellularly. Intracellular recordings in the ACC showed that thalamocingulate transmission involves both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors. Thalamus-evoked responses recorded extracellularly in the ACC were activated and progressed along a deep-superficial-deep trajectory loop across the ACC layers. We observed enhanced paired-pulse facilitation and tetanic potentiation of thalamocingulate synapses, suggestive of input-specific ACC plasticity and selective processing of information relayed by thalamocingulate pathways. Furthermore, we observed differential responses of ACC neurons to thalamic burst stimulation, which underscores the importance of MT afferents in relaying sensory information to the ACC. This new slice preparation enables the contribution of MT-evoked ACC synaptic transmission to short-term plasticity in the neuronal circuitry underlying sensory information processing to be examined in detail.

Neuronal mechanism for neuropathic pain

Among different forms of persistent pain, neuropathic pain presents as a most difficult task for basic researchers and clinicians. Despite recent rapid development of neuroscience and modern techniques related to drug discovery, effective drugs based on clear basic mechanisms are still lacking. Here, I will review the basic neuronal mechanisms that maybe involved in neuropathic pain. I will present the problem of neuropathic pain as a rather difficult task for neuroscientists, and we may have to wait for a long time before we fully understand how brain encode, store, and retrieve painful information after the injury. I propose that neuropathic pain as a major brain disease, rather being a clinic problem due to peripheral injury.

Analysis of 5-HT6 and 5-HT7 receptor gene expression in rats showing differences in novelty-seeking behavior

Sensation-seeking is a human personality trait associated with a greater propensity to use psychoactive substances. A rat model showing face validity of this human trait has been developed. The model is based on the variety of behavioral responses that rats exhibit in a novel and inescapable environment, with some animals (high-responders, HR) being highly active, and others (low-responders, LR) showing less exploration. More active rats (HR) also show increased drug-taking and decreased anxiety-like behavior. There is evidence that response to novelty may rely on differential 5-HT-mediated neurotransmission. This research focuses on the recently discovered 5-HT6 and 5-HT7 receptors which share affinity for neuroleptic drugs and hallucinogens. To date, emerging evidence suggests that 5-HT6 and 5-HT7 may be involved in cognition and mood regulation, respectively. To further our knowledge of their behavioral attributes, we compared patterns of gene expression for these receptors in the brains of HR and LR rats. As a control, gene expression for the 5-HT3 receptor was investigated because its contribution to anxiety and addiction is only weakly demonstrated. Transcript levels for 5-HT6 in the olfactory tubercle inversely correlated with the level of locomotion in a novel environment. Phenotype differences in mRNA signal for 5-HT6 showed a complex pattern in the dentate gyrus. LR rats were statistically higher in the most anterior region of the dentate gyrus, while HR rats were higher in median areas of the dentate gyrus. Levels of 5-HT7 transcript in HR rats were significantly lower than LR rats in pivotal areas for information trafficking, such as thalamo-cortical projection areas and dorsal hippocampus. By contrast, phenotype differences in 5-HT3 expression were not found in areas of the limbic cortex and mesolimbic system. Taken together, these results provide new insight into the potential contribution of 5-HT to novelty-seeking behavior and associated behaviors such as substance abuse.

Genetic and pharmacological studies of GluR5 modulation of inhibitory synaptic transmission in the anterior cingulate cortex of adult mice

In the anterior cingulate cortex (ACC), GluR5-containing kainate receptor mediated the small portion of excitatory postsynaptic current. However, little is known about its role in modulation of neurotransmitter release in this brain region. In the present study, we address this question by using selective GluR5 agonist and antagonist, as well as GluR5(-/-) mice. Our results showed that activation of GluR5 induced action potential-dependent GABA release, which is also required for the activation of voltage-dependent calcium channel and Ca(2+) influx. The effect of GluR5 activation is selective to the GABAergic, but not glutamatergic synaptic transmission. Endogenous activation of GluR5 also enhanced GABA release to ACC pyramidal neurons and the corresponding postsynaptic tonic GABA current. Our results suggest the somatodendritic, but not presynaptic GluR5, in modulation of GABA release. The endogenous GluR5 activation and the subsequent tonic GABA current may play an inhibitory role in ACC-related brain functions.

Long-term depression requires postsynaptic AMPA GluR2 receptor in adult mouse cingulate cortex

Synaptic long-term depression (LTD) is thought to be important for various brain functions such as learning, memory, and development. Although anterior cingulated cortex (ACC) has been demonstrated to contribute to learning and memory, no studies has been reported about the synaptic mechanisms for cingulate LTD. Here, we used integrative genetic, pharmacological and electrophysiological approaches to demonstrate that AMPA GluR2, but not GluR3, subunit is critical for cingulate LTD. We found that LTD was absent in adult cingulate slices of GluR2 knockout mice. Furthermore, postsynaptic injections of peptides that inhibit AMPA GluR2-PDZ interactions blocked the induction of LTD. To determine if the requirement for AMPA receptor-PDZ interaction is time-dependent, we injected the same inhibiting peptide into the postsynaptic cells 5 min after the induction of LTD. We found that LTD was not affected by the peptide, providing the first evidence that postsynaptic AMPA GluR2-mediated depression occurs rapidly (within t = 5 min). Genetic deletion of GluR3 did not affect cingulate LTD. Our results provide the first study of cingulate LTD mechanism using whole-cell patch-clamp recording in adult cortical slices and demonstrate that postsynaptic AMPA GluR2 subunit is crucial for synaptic depression in the ACC of adult mice.

Corticofugal influences on thalamic neurons during nociceptive transmission in awake rats

Pain is a multidimensional phenomenon and processed in a neural network. The supraspinal, brain mechanisms are increasingly recognized in playing a major role in the representation and modulation of pain. The aim of the current study is to investigate the functional interactions between cortex and thalamus during nociceptive processing, by observing the pain-related information flow and neuronal correlations within thalamo-cortical pathways. Pain-evoked, single-neuron activity was recorded in awake Sprague-Dawley rats with a Magnet system. Eight-wire microarrays were implanted into four different brain regions, i.e., the primary somatosensory (SI) and anterior cingulate cortex (ACC), as well as ventral posterior (VP) and medial dorsal thalamus (MD). Noxious radiant heat was delivered to the rat hind paws on the side contralateral to the recording regions. A large number of responsive neurons were recorded in the four brain areas. Directed coherence analysis revealed that the amount of information flow was significantly increased from SI cortex to VP thalamus following noxious stimuli, suggesting that SI cortex has descending influence on thalamic neurons during pain processing. Moreover, more correlated neuronal activities indicated by crosscorrelation histograms were found between cortical and thalamic neurons, with cortical neurons firing ahead of thalamic units. On basis of the above findings, we propose that nociceptive responses are modulated by corticothalamic feedback during nociceptive transmission, which may be tight in the lateral pathway, while loose in the medial pathway.

Does the medial thalamus play a role in the negative affective component of visceral pain in rats?

Pain consists of sensory and negative affective components. Using a conditioned place aversion (CPA) paradigm, we investigated whether the medial thalamus (MT) played a role in the affective component of visceral pain induced by intraperitoneal injection of acetic acid into male Long-Evan rats. Acetic acid produced writhing response as well as CPA. The bilateral MT-lesions resulted in slight reduction of writhing response, but CPA was not affected. The results suggest that while MT may play a role in visceral nociception, it does not participate in the negative affective component of visceral pain.

Short-term synaptic plasticity in layer II/III of the rat anterior cingulate cortex

Recent in vivo electrophysiological studies in our laboratory demonstrated medial thalamus (MT) induced short-term facilitation in the middle layers of the anterior cingulate cortex (ACC). The aim of the present study was to investigate different forms of short-term plasticity (STP) in layer II/III of the ACC in an in vitro slice preparation. Extracellular field potentials in layer II/III consisting of an early component (fAP) and a late component (fPSP) were activated by electrical stimulation of the deep layers. The fPSP and intracellularly recorded excitatory post-synaptic potential (EPSP) could be facilitated by paired-pulse stimulation at a low frequency (0.033Hz, pulse interval 20-400ms). An initial facilitation and subsequent depression were obtained when high frequency (12.5, 25 and 50Hz) tetanus stimulations were applied to the ACC slice. A post-tetanic augmentation 30s in duration was also observed. The effects of tetanic stimulation were altered in the presence of an increased or a decreased calcium concentration. Application of omega-conotoxin GVIA (CTX) in normal calcium concentration conditions decreased overall responses during tetanic stimulation similar to reducing calcium exposure. However CTX application did not increase paired-pulse facilitation (PPF) as is seen under low calcium conditions. These results indicate that calcium is involved in the formation of certain features of STP in layer II/III of the ACC and that N-type calcium channels contribute to some, but not all, components of these plastic changes. Two-site electrical stimulation testing showed that two separate presynaptic inputs can produce short-term facilitation. Our findings implicate a post-synaptic mechanism in STP in layer II/III of the ACC.

Intracortical circuits in rat anterior cingulate cortex are activated by nociceptive inputs mediated by medial thalamus

We investigated the afferents and intracortical synaptic organization of the anterior cingulate cortex (ACC) during noxious electrical stimulation. Extracellular field potentials were recorded simultaneously from 16 electrodes spanning all layers of the ACC in male Sprague-Dawley rats anesthetized by halothane inhalation. Laminar-specific transmembrane currents were calculated with the current source density analysis method. Two major groups of evoked sink currents were identified: an early group (latency = 54.04 +/- 2.12 ms; 0.63 +/- 0.07 mV/mm(2)) in layers V-VI and a more intense late group (latency = 80.07 +/- 4.85 ms; 2.16 +/- 0.22 mV/mm(2)) in layer II/III and layer V. Multiunit activities were evoked mainly in layer V and deep layer II/III with latencies similar to that of the early and late sink groups. The evoked EPSP latencies of pyramidal neurons in layers II/III and V related closely with the sink currents. The sink currents were inhibited by intracortical injection of CNQX (1 mM, 1 microl), a glutaminergic receptor antagonist, and enhanced by intraperitoneal (5 mg/kg) and intracortical (10 microg/microl, 1 microl) injection of morphine, a mu-opioid receptor agonist. Paired-pulse depression was observed with interpulse intervals of 50 to 1,000 ms. High-frequency stimulation (100 Hz, 11 pulses) enhanced evoked responses in the ACC and evoked medial thalamic (MT) unit activities. MT lesions blocked evoked responses in the ACC. Our results demonstrated that two distinct synaptic circuits in the ACC were activated by noxious stimuli and that the MT is the major thalamic relay that transmits nociceptive information to the ACC.

Short-term facilitation in the anterior cingulate cortex following stimulation of the medial thalamus in the rat

The present study examined the distribution and localization of synaptic activities (field potentials, multiunit activities and sink source currents) evoked in the anterior cingulate cortex (ACC) by electrical paired pulse stimulation of the ipsilateral medial thalamus (MT). Male Sprague-Dawley rats were anesthetized with halothane (1.0-1.5%), and electrical paired pulses stimuli (100-300 microA, inter-pulse interval, 100 ms) were delivered to the MT. Tungsten microelectrodes and a multichannel Michigan probe were used to record the evoked field potentials and multiunit activities in the ACC. Paired pulse stimulation facilitated field potentials and multiunit activities elicited from several MT nuclei. The second component of the negative field potential (com2) was augmented to about 2.5 times that of the first component (com1), and the integrated multiunit activities were facilitated by about 1.6-fold. Paired stimulation produced an expansion of the maximal negative potential from layer II/III into the deeper layers of the cingulate cortex area 1 (Cg1). Furthermore, the potentiated activity spread into adjacent secondary motor cortex (M2) and prelimbic cortex (PrL). Meanwhile, the area covered by the maximal integrated multiunit activities expanded from layer V (com1) to layers II-V (com2) in M2, Cg1 and PrL. The current source density (CSD) analysis revealed that the short latency sinks were located in layer II/III and layer V/VI. The sink currents were potentiated and expanded to more superficial and to deeper layers when a second pulse was delivered with a 100-ms time delay. Sink currents and the paired pulse facilitation (PPF) were reduced by morphine treatment (5 mg/kg, i.v.), and this effect could be blocked by naloxone. Electrical stimulation at 10 Hz in the MT induced more pronounced c-fos immunolabeling of neurons in the medial prefrontal cortex than did 1-Hz stimulation. The short-term facilitation occurred in the middle layers and expanded to the deeper layers of the ACC. These changes may mediate the effective signal transference in the specific frequency associated with painful responses.

Molecular mechanisms of pain in the anterior cingulate cortex

It is well known that peripheral sensory stimuli, including pain, trigger a series of neuronal activities along the somatosensory pathways as well as the neuronal network in the high brain structures. These neuronal activities not only produce appropriate physiological responses but also induce long-term plastic changes in some of the central synapses. It is believed that long-term synaptic changes help the brain to process and store new information. Such learning is critical for animals and humans to gain new knowledge of changing environment, generate appropriate emotional responses, and avoid dangerous stimuli in the future. In the case of permanent injury, however, the brain fails to distinguish the difference between "useful" and painful stimuli. Long-term synaptic changes work against the system and at least in part contribute to chronic pain. In this short article, the possible molecular mechanisms for long-term plasticity within the anterior cingulate cortex (ACC) will be discussed and reviewed, and it is hypothesized that potentiation of excitatory responses within the ACC contributes to chronic pain and pain-related mental disorders.

A parametric assessment of GABA antagonist effects on paired-pulse facilitation in the rat anterior cingulate cortex

Paired-pulse facilitation (PPF) is a form of short-term plasticity that can be used qualitatively to characterize the synaptic effects of neuroactive compounds. As we have shown previously, CNQX has a marked effect on PPF which can be measured quantitatively. The aim of the present study was to examine quantitatively possible differences in the effects of the post- and pre-synaptic GABA antagonists on PPF in vitro. Experiments were performed on slices taken from the coronal anterior cingulate cortex (ACC) of Sprague-Dawley rats. The stimuli consisted of a pair of biphasic pulses with an inter-pulse interval of 40ms. Evoked extracellular field potentials in layers 2/3 of the ACC were recorded. Quantitative assessment of PPF was achieved by calculating two parameters, the PPFmax (theoretical maximal PPF) and the Stmax (stimulus intensity that produces the PPFmax). Picrotoxin treatment produced increases in both the PPFmax and Stmax, by increasing the stimulus producing the half-maximal effect. In contrast, CGP-55845 treatment produced an increase in only the PPFmax, which was due to an alteration in the asymptotic values of the response amplitudes. Our findings show that the effect of different GABA receptor antagonists on short-term synaptic facilitation in the ACC may be assessed and specified quantitatively.

Anterior cingulate cortical neuronal activity during perception of noxious thermal stimuli in monkeys

It has been reported that the anterior cingulate cortex (ACC) has a variety of functions relating to pain as well as pain perception. However, the underlying mechanisms for those functions remain unclear. To elucidate the functional role of the ACC in pain perception and pain-related functions such as attention to pain and escape from pain, single neuronal activity was recorded from the ACC, and the behavioral correlates of this neuronal activity was studied. A total of 667 neurons were recorded from the ACC in awake behaving monkeys. Twenty-one had modulated activity during a heat-detection task. Eighteen of these increased their firing frequency following an increase in stimulus temperature, whereas three of them had decreased firing during heating of the face. Seventy-five percent of heat-evoked responses of heat-responsive ACC neurons were significantly depressed when monkeys detected the change in magnitude of illumination of a light presented on the front panel. The neuronal activity was significantly higher when monkeys escaped from a noxious heat stimulus than when the monkeys detected a small change in temperature (T2) above a larger initial shift (T1). No relationship between firing frequency and detection latency of the T2 stimulation was observed. These findings suggest that ACC nociceptive neurons are involved in attention to pain and escape from pain but not in the sensory discriminative aspect of pain.

Degenerative neuronal changes in the rat thalamus induced by status epilepticus at different developmental stages

SE was induced in Wistar rats at post-natal (P) days 12, 15, 18, 21, and 25 to determine distribution and severity of thalamic damage in relation to time after SE. Six different intervals from 4 h up to 1 week were studied using Fluoro-Jade B (FJB) staining. Severity of damage was semi-quantified for every age-and-interval group. Distribution of neuronal damage within various thalamic nuclei was mapped by a computer-aided digitizing system. A consistent neuronal damage occurred in functionally heterogenous thalamic nuclei. Damage was found in all age groups although its extension and time course as well as the number of involved thalamic nuclei varied. Number of injured thalamic nuclei rapidly increased with age on SE-onset. In P12 group, degenerating neurons were consistently seen in the mediodorsal and lateral dorsal thalamic nuclei. Since P15, neurodegeneration was observed additionally in midline, ventral and caudal thalamic nuclei (visual and auditory thalamic nuclei), in the lateral posterior and in the reticular nucleus. In P21 and P25 animals, the majority of thalamic nuclei exhibited marked neuronal damage. Nuclei with a small number (anterior and intralaminar) or no FJB-positive neurons (the ventral nucleus of the lateral geniculate body) were exceptional. The pattern of thalamic damage is age-specific; its extent and severity increases with age.