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

The aim of the present study was to investigate afferent projections from the medial thalamic nuclei (MT) to the frontal cortical areas using a single small iontophoretic injection of biotinylated dextran amine (BDA) and analysis of the anterogradely labeled fibers and varicosities. Projections from the mediodorsal (MD) nuclei were found primarily and extensively in the anterior cingulate cortex (ACC), whereas those from the centrolateral (CL) thalamic nucleus were found in the frontal motor cortex. The density of terminals in the ACC was high in layers II and III and sparse in layer I. The majority of projected fibers from the CL were found at a high density in layer V, with a moderate density in the superficial layers. The differential projection patterns were topographically organized in the medial prefrontal cortex and sensory motor cortex. These findings support the results of our previous electrophysiological studies suggesting that neurons in the medial thalamic nuclei relay nociceptive information to the limbic or sensory motor cortical areas. The present results agree with the current notion that the medial thalamo-frontal cortical network circuitry plays an important role in processing the emotional aspect of nociception.     

Attenuation of inhibitory synaptic transmission by glial dysfunction in rat thalamus

The thalamus serves as the obligatory gateway to the neocortex for sensory processing, and also serves as a pathway for corticocortical communication. In addition, the reciprocal synaptic connectivity between the thalamic reticular nucleus (TRN) and adjacent thalamic relay nuclei generates rhythmic activities similar to that observed during different arousal states and certain neurological conditions such as absence epilepsy. Epileptiform activity can arise from a variety of neural mechanisms, but in addition glia are thought to have an important role in such activities as well. Glia serve a central role in glutamine synthesis, a precursor for glutamate or GABA in nerve terminals. While alterations in glutamine shuttling from glia to neurons can influence GABA and glutamate neurotransmission; the consequences of such action on synaptic transmission and subsequent network activities within thalamic circuits is less understood. We investigated the consequences of altering glutamine transport on inhibitory transmission and intrathalamic activities using the in vitro thalamic slice preparation. Disruption of the glutamine shuttling by the neuronal glutamine transporter (system A transporter) antagonist, α-(methylamino)isobutyric acid (MeAIB), or the selective gliotoxic drug, fluorocitric acid (Fc) dramatically decreased intrathalamic rhythmic activities. At the single cell level, MeAIB and Fc significantly attenuated electrically evoked inhibitory postsynaptic currents (eIPSCs) in thalamic relay neurons; however, miniature IPSCs were unaffected. These data indicate that glutamate-glutamine shuttle is critical for sustaining thalamic synaptic transmission, and thereby alterations in this shuttle can influence intrathalamic rhythmic activities associated with absence epilepsy.     

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.     

Excitatory projection of the rat subicular complex to the cingulate cortex and synaptic integration with thalamic afferents

We studied the responses of rat cingulate cortex neurons to electrical stimulation of the subicular complex. Intracellular and 'quasi-intracellular' recordings from layer V posterior cingulate neurons showed that stimulation of the presubiculum or postsubiculum evoked EPSPs and action potentials. These were usually followed by shallow IPSPs averaging 122 ms in duration. Frequency potentiation of an EPSP was demonstrated in one case. Laminar analysis of field potentials provided evidence for a source of excitatory synaptic drive in layer II-III of the posterior cingulate cortex, where the subicular projections terminate, presumably on apical dendrites of layer V pyramids. Intracellular HRP injection of neurons showing EPSPs after subicular complex stimulation established that these responsive neurons were layer V pyramids. One cell with physiological properties characteristic of inhibitory interneurons was recorded in layer V. Stimulation of the thalamic nuclei lateralis and anterior ventralis also evoked EPSPs and action potentials in layer V cingulate neurons. In one cell it was possible to show that EPSPs evoked by presubicular stimulation and by nucleus anterior ventralis summed. These results indicate that subicular and thalamic afferents make excitatory synaptic contact onto dendrites of the same layer V cingulate neurons; that spatial summation can integrate the input from these two sources; and that inhibition from local interneurons limits the duration of this excitatory influence.     

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 novel rat medial prefrontal cortical slice preparation to investigate synaptic transmission from amygdala to layer V prelimbic pyramidal neurons

Electrophysiological recordings from identified synapses in CNS slice preparations in vitro provide important information regarding the connectivity of neuronal circuits and the underlying cellular mechanisms responsible for neuronal excitability and synaptic transmission. We present an anatomical, electrophysiological, and pharmacological characterization of a novel brain slice preparation (BLA-mPFC) to investigate basolateral amygdala synaptic input to rat layer V medial prefrontal cortex pyramidal neurons. A fluorescent tracer (DiI) unilaterally infused in vivo into the basolateral amygdala was used to detect amygdala efferent fibers innervating layer V of the prelimbic and infralimbic cortices within prefrontal cortex slices. In vitro, evoked synaptic responses elicited by stimulating identified basolateral amygdala pathway terminals within the acute BLA-mPFC slice preparation yielded monosynaptic excitatory postsynaptic responses in layer V pyramidal neurons from the prelimbic cortex as determined by extracellular and intracellular recordings. The BLA-mPFC preparation provides essential knowledge of amygdaloid input to the medial prefrontal cortex where information from various brain areas is integrated and returned to subcortical structures, such as the amygdala itself. In addition to investigating normal synaptic function, this preparation provides opportunities to investigate this synapse in animals which have received drugs chronically or have been manipulated genetically to model specific mental diseases known to involve prefrontal cortex and/or amygdala pathology (e.g., schizophrenia, addiction, anxiety, and depression).     

An anatomic basis for the communication of hypothalamic, cortical and mesolimbic circuitry in the regulation of energy balance

Central neural control of complex feeding behaviour is likely to be influenced by a number of factors including homeostatic responses to peripheral nutrient status, cortical integration of feeding-related cues and the underlying reward value of food. We have used retrogradely transported neurotropic viruses, as tools to map chains of synaptically-connected neurons, in conjunction with neurochemical markers of feeding-related peptides to expand the blueprint of the circuitries that underlie these different components of feeding behaviour. We have identified projections to insular and anterior cingulate cortex, extending from the arcuate nucleus through synaptic relays in the lateral hypothalamic area and midline thalamic nuclei. Cortically projecting neurons from the hypothalamic arcuate nucleus were found predominantly in its lateral aspects and contained anorexigenic peptides with no representation amongst more medially-positioned neurons containing orexigenic peptides. Largely overlapping pathways were shown to project multisynaptically to the shell of the nucleus accumbens but those with origins in the arcuate nucleus had either orexigenic or anorexigenic phenotypes. Similar to the cortical projections, those relaying to the nucleus accumbens in the lateral hypothalamus contained the orexigenic peptides orexin-A and melanin-concentrating hormone in ∼30% of cases. Common to the neural pathways directed to all three virally-injected areas were nodes of synaptic relays in the lateral hypothalamus and midline thalamic nuclei. These regions are well positioned to integrate sensory information about energy homeostasis and the reward value of food in the passage of this information to the 'ingestive cortex'.     

Spatiotemporal organization and thalamic modulation of seizures in the mouse medial thalamic-anterior cingulate slice

Purpose: Seizure‐like activities generated in anterior cingulate cortex (ACC) are usually classified as simple partial and are associated with changes in autonomic function, motivation, and thought. Previous studies have shown that thalamic inputs can modulate ACC seizure, but the exact mechanisms have not been studied thoroughly. Therefore, we investigated the role of thalamic inputs in modulating ACC seizure‐like activities. In addition, seizure onset and propagation are difficult to determine in vivo in ACC. We studied the spatiotemporal changes in epileptiform activity in this cortex in a thalamic–ACC slice to clearly determine seizure onset. Methods: We used multielectrode array (MEA) recording and calcium imaging to investigate the modulatory effect of thalamic inputs in a thalamic–ACC slice preparation. Key Findings: Seizure‐like activities induced with 4‐aminopyridine (4‐AP; 250 μm ) and bicuculline (5–50 μm ) in ACC were attenuated by glutamate receptor antagonists, and the degree of disinhibition varied with the dose of bicuculline. Seizure‐like activities were decreased with 1 Hz thalamic stimulation, whereas corpus callosum stimulation could increase ictal discharges. Amplitude and duration of cingulate seizure‐like activities were augmented after removing thalamic inputs, and this effect was not observed with those induced with elevated bicuculline (50 μm ). Seizure‐like activities were initiated in layers II/III and, after thalamic lesions, they occurred mainly in layers V/VI. Two‐dimensional current‐source density analyses revealed sink signals more frequently in layers V/VI after thalamic lesions, indicating that these layers produce larger excitatory synchronization. Calcium transients were synchronized after thalamic lesions suggesting that ACC seizure‐like activities are subjected to desynchronizing modulation by thalamic inputs. Therefore, ACC seizure‐like activities are subject to desynchronizing modulation from medial thalamic inputs to deep layer pyramidal neurons. Significance: Cingulate seizure‐like activities were modulated significantly by thalamic inputs. Repeated stimulation of the thalamus efficiently inhibited epileptiform activity, demonstrating that the desynchronization was pathway‐specific. The clinical implications of deep thalamic stimulation in the modulation of cingulate epileptic activity require further investigation.     

Long-term potentiation and depression of synaptic transmission in rat posterior cingulate cortex

We used stimulation of corpus callosum (CAL) and the subiculo-cingulate tract (SCT), in an in vitro brain slice preparation, to study activity-dependent changes in synaptic efficacy in posterior cingulate cortex (PCC). SCT stimulation monosynaptically excites the apical dendrites of deep laminae (V–VI) pyramidal neurons, while CAL afferents drive these same cells via synapses on their basal dendrites. In contrast, most superficial laminae (II/III–IV) pyramids appear to be driven polysynaptically via ascending axonal collaterals of deep pyramids. In slices retaining these connectivities, we contrasted characteristics of synaptic plasticity in superficial vs deep laminae field and intracellular potentials evoked by conditioning stimuli given at frequencies of 100, 20, 8, 5 and 1 Hz. Tetanic stimulation (100 Hz) of SCT or CAL yielded homosynaptic long-term potentiation (LTP) of each pathway, while stimulus trains of 8–20 Hz did not. 1–5 Hz stimulation of SCT and CAL elicited homosynaptic long-term depression (LTD) of synaptic strength in each pathway. Associative LTD was induced by interleaving 5 Hz pulses to the SCT pathway with 100 Hz θ-burst stimulation of CAL, but was not induced when these stimulus loci were switched. Heterosynaptic non-associative LTD was also observed in the alternate pathway following tetanization of either SCT or CAL. In all cases, LTP and LTD were observed only in deep laminae recordings. In contrast, superficial records showed only paired-pulse facilitation and short-term post-tetanic potentiation. In in vivo experiments in anaesthetized rats, PCC responses to SCT stimulation were contrasted with responses to stimulation of anteroventral and anterodorsal thalamic nuclei (AV/AD). SCT-elicited field potentials closely resembled those evoked in the slice, with maximal amplitude tuned to the 4–8 Hz frequency band. AV/VD stimulation elicited field potentials which were not frequency tuned. Overall, these data suggest that the acute circuit properties of PCC superficial laminae, modulated by thalamic input and synaptic plasticity in deep laminae, can transform hippocampal synaptic inflow before relaying it to extracingulate targets.     

Long-lasting inhibitory synaptic depression is age- and calcium-dependent.

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.     

Neurotoxicity and Reactive Astrogliosis in the Anterior Cingulate Cortex in Acute Ciguatera Poisoning

Ciguatoxins (CTXs) cause long-term disturbance of cerebral functions. The primary mechanism of neurotoxicity is related to their interaction with voltage-gated sodium channels. However, until now, the neurological targets for CTXs in the brain of intact animals have not been described. In our study, 1 day following oral exposure to 0.26 ng/g of Pacific ciguatoxin 1 (P-CTX-1), we performed in vivo electrophysiological recordings in the rat anterior cingulate cortex (ACC) and identified the increase in spontaneous firings and enhanced responses to visceral noxious stimulation. Local field recordings characterized the P-CTX-1-induced synaptic potentiation and blockage of the induction of electrical stimulation-induced long-term potentiation in the medial thalamus (MT)-ACC pathway. Furthermore, intracerebroventricular administration of P-CTX-1 at doses of 1.0, 5.0, and 10 nM produced a dose-dependent increase in ACC neuronal firings and MT-ACC synaptic transmission. Further studies showed upregulated Na⁺ channel expression in astrocytes under pathological conditions. We hypothesized that the astrocytes might have been activated in the ciguatera poisoning in vivo. Increases in glial fibrillary acid protein expression were detected in reactive astrocytes in the rat ACC. The activation of astroglia was further indicated by activation of the gap junction protein connexin 43 and upregulation of excitatory amino acid transporter 2 expression suggesting that glutamate was normally rapidly cleared from the synaptic cleft during acute ciguatera poisoning. However, neurotoxicity and reactive astrogliosis were not detected in the ACC after 7 days of P-CTX-1 exposure. The present results are the first characterization of P-CTX-1-invoked brain cortex neuronal excitotoxicity in vivo and supported the theme that neuron and astroglia signals might play roles in acute ciguatera poisoning.     

Differential regulation of synaptic plasticity of the hippocampal and the hypothalamic inputs to the anterior thalamus

The hippocampus projects to the anterior thalamic nuclei both directly and indirectly via the mammillary bodies, but little is known about the electrophysiological properties of these convergent pathways. Here we demonstrate, for the first time, the presence of long‐term plasticity in anterior thalamic nuclei synapses in response to high‐ and low‐frequency stimulation (LFS) in urethane‐anesthetized rats. We compared the synaptic changes evoked via the direct vs. the indirect hippocampal pathways to the anterior thalamus, and found that long‐term potentiation (LTP) of the thalamic field response is induced predominantly through the direct hippocampal projections. Furthermore, we have estimated that that long‐term depression (LTD) can be induced only after stimulation of the indirect connections carried by the mammillothalamic tract. Interestingly, basal synaptic transmission mediated by the mammillothalamic tract undergoes use‐dependent, BDNF‐mediated potentiation, revealing a distinct form of plasticity specific to the diencephalic region. Our data indicate that the thalamus does not passively relay incoming information, but rather acts as a synaptic network, where the ability to integrate hippocampal and mammillary body inputs is dynamically modified as a result of previous activity in the circuit. The complementary properties of these two parallel pathways upon anterior thalamic activity reveal that they do not have duplicate functions. © 2009 Wiley‐Liss, Inc.     

Synaptic reorganization in the substantia gelatinosa after peripheral nerve neuroma formation: aberrant innervation of lamina II neurons by Abeta afferents.

Intracellular recording and extracellular field potential (FP) recordings were obtained from spinal cord dorsal horn neurons (laminae I-IV) in a rat transverse slice preparation with attached dorsal roots. To study changes in synaptic inputs after neuroma formation, the sciatic nerve was sectioned and ligated 3 weeks before in vitro electrophysiological analysis. Horseradish peroxidase labeling of dorsal root axons indicated that Abeta fibers sprouted into laminae I-II from deeper laminae after sciatic nerve section. FP recordings from dorsal horns of normal spinal cord slices revealed long-latency synaptic responses in lamina II and short-latency responses in lamina III. The latencies of synaptic FPs recorded in lamina II of the dorsal horn after sciatic nerve section were reduced. The majority of monosynaptic EPSPs recorded with intracellular microelectrodes from lamina II neurons in control slices were elicited by high-threshold nerve stimulation, whereas the majority of monosynaptic EPSPs recorded in lamina III were elicited by low-threshold nerve stimulation. After sciatic nerve section, 31 of 57 (54%) EPSPs recorded in lamina II were elicited by low-threshold stimulation. The majority of low-threshold EPSPs in lamina II neurons after axotomy displayed properties similar to low-threshold EPSPs in lamina III of control slices. These results indicate that reoccupation of lamina II synapses by sprouting Abeta fibers normally terminating in lamina III occurs after sciatic nerve neuroma formation. Furthermore, these observations indicate that the lamina II neurons receive inappropriate sensory information from low-threshold mechanoreceptor after sciatic nerve neuroma formation.     

Nociceptive spinothalamic tract and postsynaptic dorsal column neurons are modulated by paraventricular hypothalamic activation

Previously, we demonstrated that stimulation of the paraventricular hypothalamic nucleus diminishes the nociceptive dorsal horn neuronal responses, and this decrease was mediated by oxytocin in the rat. In addition, we have proposed that oxytocin indirectly inhibits sensory transmission in dorsal horn neurons by exciting spinal inhibitory GABAergic interneurons. The main purpose of the present study was to identify which of the neurons projecting to supraspinal structures to transmit somatic information are modulated by the hypothalamic-spinal descending activation. In anaesthetized rats, single-unit extracellular and juxtacellular recordings were made from dorsal horn lumbar segments, which receive afferent input from the toe and hind-paw regions. The projecting spinothalamic tract and postsynaptic dorsal column system were identified antidromically. Additionally, in order to label the projecting dorsal horn neurons, we injected fluorescent retrograde neuronal tracers into the ipsilateral gracilis nucleus and contralateral ventroposterolateral thalamic nucleus. Hence, juxtacellular recordings were made to iontophoretically label the recorded neurons with a fluorescent dye and identify the recorded projecting cells. We found that only nociceptive evoked responses in spinothalamic tract and postsynaptic dorsal column neurons were significantly inhibited (48.1 ± 4.6 and 47.7 ± 8.2%, respectively) and non-nociceptive responses were not affected by paraventricular hypothalamic nucleus stimulation. We conclude that the hypothalamic-spinal system selectively affects the transmission of nociceptive information of projecting spinal cord cells.     

Inhibitory short‐term plasticity modulates neuronal activity in the rat entopeduncular nucleus in vitro

The entopeduncular nucleus (EP) is one of the basal ganglia output nuclei integrating synaptic information from several pathways within the basal ganglia. The firing of EP neurons is modulated by two streams of inhibitory synaptic transmission, the direct pathway from the striatum and the indirect pathway from the globus pallidus. These two inhibitory pathways continuously modulate the firing of EP neurons. However, the link between these synaptic inputs to neuronal firing in the EP is unclear. To investigate this input–output transformation we performed whole‐cell and perforated‐patch recordings from single neurons in the entopeduncular nucleus in rat brain slices during repetitive stimulation of the striatum and the globus pallidus at frequencies within the in vivo activity range of these neurons. These recordings, supplemented by compartmental modelling, showed that GABAergic synapses from the striatum, converging on EP dendrites, display short‐term facilitation and that somatic or proximal GABAergic synapses from the globus pallidus show short‐term depression. Activation of striatal synapses during low presynaptic activity decreased postsynaptic firing rate by continuously increasing the inter‐spike interval. Conversely, activation of pallidal synapses significantly affected postsynaptic firing during high presynaptic activity. Our data thus suggest that low‐frequency striatal output may be encoded as progressive phase shifts in downstream nuclei of the basal ganglia while high‐frequency pallidal output may continuously modulate EP firing.     

Plasticity changes of neuronal activities in central lateral nucleus by stimulation of the anterior cingulate cortex in rat

The medial thalamus (MT) and anterior cingulate cortex (ACC) are essential components in mediating the affective emotional-aspect of pain. Whether ACC modulates the neuron activity in MT has not been elucidated and clarifying this point will further reveal the neurobiological mechanism underlying pain related emotions. In the present study, we used in vivo single unit recording and retrograde tracing technique to demonstrate that the majority of examined neurons in the central lateral nucleus (CL), an important nucleus of MT, responded to noxious stimulation. Tetanic stimulation in the ACC increased spike activities of nociceptive-responding neurons in the CL; retrograde tracing by fluorogold in the CL showed the positive neurons are distributed bilaterally in the ACC. Taken together, we demonstrated descending modulation to nociceptive responses of CL neurons by direct projections from the ACC, which may underlie the neuronal mechanism of negative pain emotions.     

Dorsal spinocerebellar tract neuronal activity in the intact chronic cat

The ability to electrophysiologically identify the axonal projections of lumbar neurons recorded in chronic unanesthetized intact awake animals is a formidable but essential requirement toward understanding ascending sensory transmission under naturally occurring conditions. Chronic immobilization procedures previously introduced by Morales et al. (1981) for intracellular studies of motoneurons are modified and then integrated with procedures for antidromic cellular identification and extracellular recording of upper (or lower) dorsal lumbar spinocerebellar tract (DSCT) neuronal activity, in conjunction with behavioral state recording and drug microiontophoresis. These implant procedures provide up to 6 months of stable recording conditions and, when combined with other techniques, allow individual DSCT neurons to be monitored over multiple cycles of sleep and wakefulness, following the induction into and recovery from barbiturate anesthesia and/or during the juxtacellular microiontophoretic ejection of inhibitory or excitatory amino acid neurotransmitters. The combination of such techniques allows a comprehensive examination of synaptic transmission through the DSCT and other lumbar sensory pathways in the intact normally respiring cat and its modulation during the general anesthetic state. These techniques permit investigations of the supraspinal controls impinging on lumbar sensory tract neurons during wakefulness and other behavioral states such as active sleep.