Involvement of GABAergic modulation of the nucleus submedius (Sm) morphine-induced antinociception

Previous studies have shown that microinjection of morphine into the nucleus submedius (Sm) of the thalamus produces antinociception. The aim of the current study was to examine whether gamma-aminobutyric acid (GABA)ergic terminals in the Sm were involved in this antinociception. Under light anesthesia, the GABA(A) receptor antagonist bicuculline or agonist muscimol was microinjected into the Sm of the thalamus in Sm non-morphine-treated (control) or Sm morphine-treated (microinjection into the Sm in the thalamus) rats. Tail flick latencies (TFL) were measured in each of these groups of rats every 5 min. Bicuculline (100, 200, 500 ng in 0.5 microL) depressed the TF reflex in a dose-dependent fashion, and this effect was blocked by microinjection of the opioid receptor antagonist naloxone (0.5 microg) into the same Sm site. A small dose (100 ng) of bicuculline microinjected into Sm significantly enhanced the morphine-evoked inhibition of TF reflex. In contrast, administration of muscimol (250 ng) did not significantly influence the TF reflex in Sm non-morphine-treated rats, but it significantly attenuated the morphine-induced antinociception in the Sm morphine-treated rats. These results suggest that locally released GABA acting at GABA(A) receptors is involved in the modulation of Sm morphine-induced antinociception, and support the hypothesis that a disinhibitory effect elicited by morphine on GABAergic terminals in Sm may lead to activation of the Sm-ventrolateral orbital cortex (VLO)-perioqueductal gray (PAG) brainstem descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

Electrophysiological characterization of synaptic connections between layer VI cortical cells and neurons of the nucleus reticularis thalami in juvenile rats

Corticothalamic (CT) feedback projections to the thalamus outnumber sensory inputs from the periphery by orders of magnitude. However, their functional role remains elusive. CT projections may directly excite thalamic relay cells or indirectly inhibit them via excitation of the nucleus reticularis thalami (nRT), a nuclear formation composed entirely of gamma-aminobutyric acidergic neurons. The relative strengths of these two pathways will ultimately control the effects of CT projections on the output of thalamic relay cells. However, corticoreticular synapses have not yet been fully physiologically characterized. Here, local stimulation of layer VI cells by focal application of K+ or AMPA elicited excitatory postsynaptic potentials in nRT neurons with a mean peak amplitude of 2.4 +/- 0.1 mV (n = 75, mean +/- SEM), a mean rise time (10-90%) of 0.74 +/- 0.03 ms and a weighted decay time constant of 11 +/- 0.3 ms. A pharmacological profile of responses was drawn in both current-clamp and voltage-clamp modes, showing the presence of a small N-methyl-d-aspartate receptor-dependent component at depolarized potentials. In two pairs of synaptically coupled layer VI cell-nRT neuron, moderate rates of transmission failures were observed while the latencies were above 5 ms in both cases. Our results indicate that the corticoreticular pathway fulfills the criteria for 'modulatory' inputs and is temporally restricted. We suggest that it may be involved in coincidence detection of convergent corticoreticular signals.

Receptive field structure of burst and tonic firing in feline lateral geniculate nucleus

There are two recognised modes of firing activity in thalamic cells, burst and tonic. A low-threshold (LT) burst (referred to from now on as 'burst') comprises a small number of high-frequency action potentials riding the peak of a LT Ca(2+) spike which is preceded by a silent hyperpolarised state > 50 ms. This is traditionally viewed as a sleep-like phenomenon, with a shift to tonic mode at wake-up. However, bursts have also been seen in the wake state and may be a significant feature for full activation of recipient cortical cells. Here we show that for visual stimulation of anaesthetised cats, burst firing is restricted to a reduced area within the receptive field centre of lateral geniculate nucleus cells. Consistently, the receptive field size of all the recorded neurons decreased in size proportionally to the percentage of spikes in bursts versus tonic spikes, an effect that is further demonstrated with pharmacological manipulation. The role of this shrinkage may be distinct from that also seen in sleep-like states and we suggest that this is a mechanism that trades spatial resolution for security of information transfer.

Impaired thalamocortical connectivity in humans during general-anesthetic-induced unconsciousness

Whereas converging lines of evidence suggest that anesthetic-induced unconsciousness may result from disruption of functional interactions within neural networks involving the thalamus and cerebral cortex, the effects anesthetics have on human thalamocortical connectivity remain unexamined with current neuroimaging techniques. To address this issue we retrospectively analyzed positron emission tomography data from 11 volunteers scanned for regional cerebral glucose utilization (rCMRglu) when awake and again during isoflurane- (n = 6) or halothane- (n = 5) induced unconsciousness using statistical parametric mapping (SPM99) and structural equation modeling. A main effect analysis, contrasting awake and unconscious metabolic activity, localized a discrete region of the left va/vl thalamus whose relative rCMRglu activity was significantly suppressed (P < 0.05, corrected) during the unconscious state. To identify brain regions whose functional connectivity with this region of the thalamus was impaired during the unconscious state, a psychophysiological interaction analysis was performed. This analysis revealed effects predominantly in topographically related areas of the primary motor and supplementary motor association cortices. Structural equation modeling of a neuroanatomical network encompassing these empirically identified regions revealed significant state-related changes in effective connectivity (chi(2)diff (6)-15.88; P < 0.05) which primarily involved impairment of thalamocortical and corticocortical projections during the unconscious state. These findings support the hypothesis that a mechanistic component underlying general-anesthetic-induced unconsciousness involves disruption of functional interactions within thalamocortical neural networks.

Dynamic GABA(A) receptor subtype-specific modulation of the synchrony and duration of thalamic oscillations

Networks of interconnected inhibitory neurons, such as the thalamic reticular nucleus (TRN), often regulate neural oscillations. Thalamic circuits generate sleep spindles and may contribute to some forms of generalized absence epilepsy, yet the exact role of inhibitory connections within the TRN remains controversial. Here, by using mutant mice in which the thalamic effects of the anti-absence drug clonazepam (CZP) are restricted to either relay or reticular nuclei, we show that the enhancement of intra-TRN inhibition is both necessary and sufficient for CZP to suppress evoked oscillations in thalamic slices. Extracellular and intracellular recordings show that CZP specifically suppresses spikes that occur during bursts of synchronous firing, and this suppression grows over the course of an oscillation, ultimately shortening that oscillation. These results not only identify a particular anatomical and molecular target for anti-absence drug design, but also elucidate a specific dynamic mechanism by which inhibitory networks control neural oscillations.

Targeting thalamic nuclei is not sufficient for the full anti-absence action of ethosuximide in a rat model of absence epilepsy

Absence epilepsy is characterised by recurrent periods of physical and mental inactivity coupled to bilateral, synchronous spike and wave discharges (SWDs) on the electroencephalogram. The mechanism of action of ethosuximide (ETX), a drug specific for absence seizures, is believed to involve a reduction in the low threshold T-type Ca(2+) current in thalamocortical and nucleus reticularis thalami (NRT) neurones, although other electrophysiological data have questioned this. Here, we employed a genetic rat model of absence seizures to investigate the effects of directly administering ETX to the thalamus.SWDs were immediately and substantially reduced (approximately 90%) by systemic administration of ETX (177-709 micromol/kg), or by bilateral microinfusion into the thalamus of the GABA(B) antagonist, CGP 36742 (5-27 nmol per side). However, infusion of ETX (1-200 nmol per side) into the ventrobasal complex or the NRT resulted in a reduction of SWDs that was delayed (30-60 min) and less marked (approximately 50%). Administration of ETX (0.2 mM to 1M) to a greater volume of thalamus by reverse microdialysis also produced significant but delayed reduction of SWDs at concentrations >1mM. Only at 5mM were seizures significantly reduced (approximately 70%) within 30 min of administration. These results suggest that targeting of the thalamus alone may be insufficient for an immediate and full anti-absence action for ETX. Concomitant or exclusive actions in the cortex remain a possibility.

Effects of ventrobasal lesion and cortical cooling on fast oscillations (>200 Hz) in rat somatosensory cortex

High-frequency oscillatory activity (>200 Hz) termed "fast oscillations" (FO) have been recorded in the rodent somatosensory cortex and may reflect very rapid integration of vibrissal information in sensory cortex. Yet, while electrophysiological correlates suggest that FO is generated within intracortical networks, contributions of subcortical structures along the trigeminal pathway remain uncertain. Using surface and laminar electrode arrays, in vivo recordings of vibrissal and electrically evoked FO were made within somatosensory cortex of anesthetized rodents before and after ablation of the ventrobasal thalamus (VB) or during reversible cortical cooling. In VB-lesioned animals, vibrissal stimulation failed to evoke FO, while epicortical stimulation in lesioned animals remained effective in generating FO. In nonlesioned animals, cortical cooling eliminated vibrissal-evoked FO despite the persistence of thalamocortical input. Vibrissal-evoked FO returned with the return to physiological temperatures. Results from this study indicate that somatosensory cortex alone is able to initiate and sustain FO. Moreover, these data suggest that cortical network interactions are solely responsible for the generation of FO, while synchronized thalamocortical input serves as the afferent trigger.

[Effect of atropine on the inhibition of melatonin to the unit discharges evoked in the posterior group of thalamic nuclei in cats]

AIM

To study the effect of atropine, muscarinic cholinergic antagonist, on the central analgesic action of melatonin (MT) and to explore the mechanism of MT analgesia.

METHODS

As an indicator of visceral pain, the unit discharges of the neurons in the posterior group of thalamic nuclei (PO) were caused by stimulating the great splanchnic nerve (GSN) of the cat. The cranial stereotaxic and extracellular glass microelectrode record technique were used. The drugs were given through the intra-cranial-ventricle (icv).

RESULTS

0.1% MT (10 micrograms.kg-1, icv) was shown to inhibit the unit discharge of the neurons in PO of the cat, whether the long latency or the short latency, which was evoked by stimulating GSN. The inhibition of 0.1% MT (10 micrograms.kg-1, icv) on the short latency discharge of neurons in PO was antagonized by 0.1% atropine (20 micrograms, icv). However, 0.1% atropine (20 micrograms, icv) did not show antagonistic effect on the inhibition of 0.1% morphine (5 micrograms, icv) at the same latency.

CONCLUSION

MT exhibited central analgesic action with mechanism different from morphine. It was suggested that the cholinergic system may be involved in analgesic process of MT.

Central nuclei mediating estrogen-induced changes in autonomic tone and baroreceptor reflex in male rats

The current investigation examines the significance of estrogen in central cardiovascular regulatory nuclei in modulating autonomic tone and baroreceptor reflex function. Experiments were done in anaesthetized male Sprague-Dawley rats. Changes in autonomic tone were assessed by monitoring vagal and renal efferent nerve activities before and following bilateral injection of estrogen into select central autonomic nuclei. In the first study, selective blockade of neurotransmission through the central nucleus of the amygdala (CNA), lateral hypothalamic area (LHA) and ventral posteromedial thalamic nucleus (VPM) using the local anaesthetic lidocaine was done to determine which nuclei were involved in mediating the autonomic changes observed following bilateral injections of estrogen into the insular cortex (IC). In the second study, the role of the parabrachial nucleus (PBN) in mediating the autonomic changes observed following bilateral estrogen injections into the CNA, LHA, VPM and IC was determined by blocking neurotransmission through the PBN using lidocaine.Injections of estrogen into the IC produced a significant increase in renal sympathetic nerve activity (RSNA; from 10+/-2 to 24+/-4 microV/sec; p<0.05). This estrogen-induced increase in RSNA was significantly attenuated when lidocaine was pre-injected into the LHA, CNA or PBN (55+/-6, 33+/-4 and 91+/-7% decrease respectively; p<0.05) but not when injected into the VPM (16+/-6% decrease; p>0.05). Injection of estrogen into the CNA resulted in a significant decrease in RSNA (48+/-5%; p<0.05) whereas estrogen injection into the LHA resulted in a significant increase (28+/-4%; p<0.05) in RSNA. Pre-injection of lidocaine into the PBN resulted in complete blockade of the autonomic changes observed following estrogen injection into the CNA but did not affect the changes observed following estrogen injection into the LHA. These results suggest that estrogen acting in forebrain and midbrain cardiovascular nuclei activated efferent pathways which synapse in the LHA, CNA and/or PBN prior to projecting to autonomic preganglionic nuclei to affect autonomic tone. These nuclei may therefore provide an added level of processing and/or integration of the autonomic response(s) following activation by local or systemic estrogen.

Effects of the essential oil from citrus lemon in male and female rats exposed to a persistent painful stimulation

The ability of olfaction to modulate behavior in mammalian species has repeatedly been demonstrated. Here we tested the properties of the volatile components of lemon essential oil. Male and female rats were allowed to inhale the aroma while experiencing a persistent nociceptive input (50 microl formalin, 5%); in the same animals the c-Fos immunohistochemistry was used to test the degree of neuronal activation of areas belonging to the limbic system. In formalin-treated animals, lemon essential oil decreased licking the injected paw, in both sexes; flinching and flexing were decreased in males and increased in females in the interphase (5-20 min) of the formalin test. Essential oil increased the c-Fos expression in the arcuate n. of the hypothalamus. Essential oil and formalin increased c-Fos in the paraventricular n. of the hypothalamus and in the dentate gyrus of the hippocampus. In the paraventricular n. of the thalamus formalin induced higher c-Fos than control in both sexes; when formalin treatment was carried out in presence of essential oil, c-Fos further increased in males, but remained at control levels in females. The present results clearly indicate the ability of lemon essential oil to modulate the behavioral and neuronal responses related to nociception and pain.

Selective action of orexin (hypocretin) on nonspecific thalamocortical projection neurons

As is evident from the pathological consequences of its absence in narcolepsy, orexin (hypocretin) appears to be critical for the maintenance of wakefulness. Via diffuse projections through the brain, orexin-containing neurons in the hypothalamus may act on a number of wake-promoting systems. Among these are the intralaminar and midline thalamic nuclei, which project in turn in a widespread manner to the cerebral cortex within the nonspecific thalamocortical projection system. Testing the effect of orexin in rat brain slices, in two nuclei of this system, centromedial (CM) nuclei and rhomboid nuclei, we found that it depolarized and excited all neurons tested through a direct postsynaptic action. An additional analysis of this effect in CM neurons indicates that it results from the decrease of a potassium conductance. By a detailed comparison of the effects of orexin A and B, we established that orexin B was more potent than orexin A, indicating the probable mediation by orexin type 2 receptors. In contrast to its effect on the nonspecific thalamocortical projection neurons, orexin had no effect on the specific sensory relay neurons of the somatic, ventral posterolateral, and visual dorsal lateral geniculate nuclei. Orexin differs in this regard from norepinephrine and acetylcholine, to which neurons in the specific and nonspecific systems are sensitive. Orexin may thus act in the thalamus to promote wakefulness by exciting neurons of the nonspecific thalamocortical projection system, which, through widespread projections to the cerebral cortex, stimulate and maintain cortical activation.

Inhibitory interactions between ferret thalamic reticular neurons

The thalamic reticular nucleus (nRt) provides an important inhibitory input to thalamic relay nuclei and is central in the generation of both normal and abnormal thalamocortical activities. Although local inhibitory interactions between these neurons may play an important role in controlling thalamocortical activities, the physiological features of this interaction have not been fully investigated. Here we sought to establish the nature of inhibitory interaction between nRt neurons with intracellular and extracellular recordings in slices of ferret nRt maintained in vitro. In many nRt neurons, intracellular recordings revealed spontaneous inhibitory postsynaptic potentials (IPSPs). In addition, the local excitation of nRt cells with glutamate led to the generation of IPSPs in the intracellularly recorded nRt neuron. These evoked IPSPs exhibited an average reversal potential of -72 mV and could be blocked by picrotoxin, a GABA(A)-receptor antagonist. These results indicate that nRt neurons interact locally through the activation of GABA(A) receptor-mediated inhibitory postsynaptic potentials. This lateral inhibition may play an important role in controlling the responsiveness of these cells to cortical and thalamic excitatory inputs in both normal and abnormal thalamocortical function.

Enhancement of spike and wave discharges by microinjection of bicuculline into the reticular nucleus of rats with absence epilepsy

The aim of this study was to demonstrate the effect of administration of gamma-aminobutyric acid (GABA)(A) receptor antagonist, bicuculline, into the reticular nucleus of the thalamus (nRt) on spike and wave discharges (SWD) and cardiovascular regulation in conscious rats with genetic absence epilepsy. Rats were instrumented with guide cannulas for drug injection and extradural electrodes for electroencephalogram recording. After a 1 week recovery period, iliac arterial catheters were inserted for direct measurement of blood pressure and heart rate. Administration of bicuculline into the nRt produced increases in spontaneous SWD and failed to alter blood pressure and heart rate. These data suggest that GABA(A) receptors located within the nRt are involved in the incidence of SWD, whereas they do not seem to be involved in cardiovascular regulation of rats with genetic absence epilepsy.

Neuromodulatory role of serotonin in the ferret thalamus

Serotonergic fibers broadly innervate the thalamus and may influence the sleep wake cycle, attention, and other processes through modulation of neurons in this structure. However, the actions of serotonin in the dorsal thalamus have been investigated in detail only in the dorsal lateral geniculate nucleus. In the present study, we examined the action of serotonin in several different regions of the ferret dorsal thalamus, including the associative nuclei, using the in vitro slice preparation and intracellular recording techniques. In nearly all nuclei examined, the predominant action of serotonin was one of hyperpolarization and inhibition of the tonic firing mode. The magnitude of the hyperpolarizing response decreased with age and varied greatly across and somewhat within nuclei maintaining the following relationship (in descending order of magnitude): lateral posterior, lateral dorsal, pulvinar, mediodorsal, center median, anteroventral, central lateral, ventral basal, and medial geniculate. This hyperpolarization is elicited through two mechanisms: one direct and the other via local interneurons. The direct action occurs through an increase in potassium conductance mediated through the 5-HT(1A) receptor. This conclusion is supported by the findings that it persists in the presence of tetrodotoxin and block of GABAergic synaptic transmission, the reversal potential shifts in a Nernstian fashion with changes in extracellular potassium concentration, and the response is antagonized by the 5-HT(1A) antagonist WAY100635 and mimicked by the application of the 5-HT(1A)-selective agonist 8-OH DPAT. The second mechanism by which 5-HT evoked a hyperpolarization was through the activation of local interneurons. In slices in which GABA receptors were not blocked, 5-HT application increased the frequency and amplitude of spontaneous inhibitory postsynaptic potentials (IPSPs) occurring in thalamocortical neurons. Application of 5-HT to physiologically or morphologically identified interneurons evoked a prolonged suprathreshold depolarization. Our results suggest that serotonergic inputs act differentially across the thalamus in a complex manner involving direct and indirect mechanisms. It appears that 5-HT has a greater direct postsynaptic inhibitory influence in the posterior, medial, and intralaminar nuclei than in the primary sensory nuclei.

Performance of different synchronization measures in real data: a case study on electroencephalographic signals

We study the synchronization between left and right hemisphere rat electroencephalographic (EEG) channels by using various synchronization measures, namely nonlinear interdependences, phase synchronizations, mutual information, cross correlation, and the coherence function. In passing we show a close relation between two recently proposed phase synchronization measures and we extend the definition of one of them. In three typical examples we observe that except mutual information, all these measures give a useful quantification that is hard to be guessed beforehand from the raw data. Despite their differences, results are qualitatively the same. Therefore, we claim that the applied measures are valuable for the study of synchronization in real data. Moreover, in the particular case of EEG signals their use as complementary variables could be of clinical relevance.

Visual pathways involved in fear conditioning measured with fear-potentiated startle: behavioral and anatomic studies

Visual pathways to the amygdala, a brain structure critical for classical fear conditioning, were investigated. Conditioned fear was measured in rats as increased acoustic startle amplitude in the presence versus absence of a light or an odor paired previously with foot shock (fear-potentiated startle). Post-training lesions of both the lateral geniculate body (LG) and lateral posterior nucleus (LP) of the thalamus together, but not lesions of LG or LP alone, completely blocked the expression of fear-potentiated startle to a visual conditioned stimulus (CS) but not to an olfactory CS. These lesions also did not block contextual fear conditioning using startle or freezing as measures. Local infusion of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f] quinoxaline-7-sulfonamide disodium, an AMPA antagonist, into the visual thalamus immediately before testing also blocked fear-potentiated startle to a visual CS, suggesting that the lesion effects were not attributable to damage of fibers of passage. Iontophoretic injections into the LP of the anterograde tracer biotinylated dextran amine resulted in heavy anterograde labeling in two amygdala-fugal cortical areas: area TE2 and dorsal perirhinal cortex (PR), and moderate labeling in the lateral amygdaloid nucleus (L). These results suggest that, during classical fear conditioning, a visual stimulus can be transmitted to the amygdala via either lemniscal (i.e., LG --> V1, V2 --> TE2/PR) or non-lemniscal (i.e., LP --> V2, TE2/PR) thalamo-cortico-amygdala pathways, or direct thalamo-amygdala (i.e., LP --> L) projections.

The neural mechanisms of oral and facial pain

Pain is a complex and variable phenomenon that can be influenced by many factors. The neural pathways serving pain are not passive conduits, but are part of a dynamic system which can result in different levels of pain resulting from similar injuries under different circumstances. The passage of signals in these pathways may be inhibited or enhanced at almost any level, from the peripheral sensory receptors to the higher centres of the brain. This review will describe recent developments in our understanding of these mechanisms and how this knowledge may be used in controlling pain.

The midline thalamus: alterations and a potential role in limbic epilepsy

PURPOSE

In limbic or mesial temporal lobe epilepsy, much attention has been given to specific regions or cell populations (e.g., the hippocampus or dentate granule cells). Epileptic seizures may involve broader changes in neural circuits, and evidence suggests that subcortical regions may play a role. In this study we examined the midline thalamic regions for involvement in limbic seizures, changes in anatomy and physiology, and the potential role for this region in limbic seizures and epilepsy.

METHODS

Using two rat models for limbic epilepsy (hippocampal kindled and chronic spontaneous limbic epilepsy) we examined the midline thalamus for evidence of involvement in seizure activity, alterations in structure, changes in the basic in vitro physiology of the thalamic neurons. We also explored how this region may influence limbic seizures.

RESULTS

The midline thalamus was consistently involved with seizure activity from the onset, and there was significant neuronal loss in the medial dorsal and reuniens/rhomboid nuclei. In addition, thalamic neurons had changes in synaptically mediated and voltage-gated responses. Infusion of lidocaine into the midline thalamus significantly shortened afterdischarge duration.

CONCLUSIONS

These observations suggest that this thalamic region is part of the neural circuitry of limbic epilepsy and may play a significant role in seizure modulation. Local neuronal changes can enhance the excitability of the thalamolimbic circuits.

Modulation of the behavioral and neurochemical effects of psychostimulants by kappa-opioid receptor systems

The repeated, intermittent use of cocaine and other drugs of abuse produces profound and often long-lasting alterations in behavior and brain chemistry. It has been suggested that these consequences of drug use play a critical role in drug craving and relapse to addiction. This article reviews the effects of psychostimulant administration on dopaminergic and excitatory amino acid neurotransmission in brain regions comprising the brain's motive circuit and provides evidence that the activation of endogenous kappa-opioid receptor systems in these regions opposes the behavioral and neurochemical consequences of repeated drug use. The role of this opioid system in mediating alterations in mood and affect that occur during abstinence from repeated psychostimulant use are also discussed.

Antinociceptive effect of a group II metabotropic glutamate receptor antagonist in the thalamus of monoarthritic rats

Adult rats were rendered monoarthritic (MA) by injection of 50 microl of complete Freund's adjuvant (CFA) into the tibiotarsal joint. The ankle-bend (AB) test of nociception was performed in those animals before and during 60 min after the stereotaxic injection of 2 microl of either saline (controls) or (2S)-alpha-ethylglutamic acid (EGLU, 80 nmol in 2 microl), a group II metabotropic glutamate receptors (mGluR) antagonist, in the reticular thalamic nucleus (Rt) contralateral to the arthritic joint. AB scores reached near maximum values before the stereotaxic injections (18.7+/-0.8), and remained constant throughout the entire experimental period in the control group, denoting marked allodynia. In the EGLU-treated group, AB scores gradually decreased after EGLU injection, with minimum values at 10 min (7.7+/-1.6), recovering to scores near maximum at 60 min (19.7+/-0.3). The data point to an activation of group II mGluR by noxious inputs in the Rt of MA rats, suggesting their participation in inhibiting local gamma-aminobutyric acid (GABA)ergic inhibitory neurones.

Effects of bilateral microinjections of ibotenic acid in the thalamic reticular nucleus on delta oscillations and sleep in freely-moving rats

The thalamic reticular nucleus (NRT) consists of a large pool of GABAergic neurons located on each side on the anterior, lateral, and ventral surfaces of the dorsal thalamus. The NRT is divided up into sectors. The aim of this study was to investigate the effects of bilateral lesions of the NRT on sleep and sleep oscillations. Only the results concerning delta oscillations will be reported here. As a first step we produced stereotaxically placed electrolytic lesions. The rats presented continuous circling behavior with electroencephalographic (EEG) theta and delta activity and subsequent sudden death. To avoid disruption of the bundles of fibers that pass through the NRT to and from the cerebral cortex, we used the excitotoxic ibotenic acid. Given its high toxicity, we concentrated on the rostral pole of the NRT, which is believed to have powerful effects on the synchronization of oscillatory activity during sleep. Immediately after surgery, the rats fell into a deep sleep during which there was an increase in EEG slow-wave activity and no spindles. On postoperative day 2, corresponding to the destruction period, the sleep/wake cycle partially recovered, but NREM sleep was quantitatively diminished and showed abnormalities (increased latency to sleep onset, sleep fragmentation, gradual elimination of the delta rhythm). It is concluded that the rostral pole of the NRT contributes to normal and pathological EEG synchronization and the organization of sleep in rats.

Neurochemical changes after morphine, dizocilpine or riluzole in the ventral posterolateral thalamic nuclei of rats with hyperalgesia

A number of studies suggest the involvement of glutamate in central hyperalgesia through NMDA receptors in animal models of inflammation. Most studies analyze glutamate effects at the spinal cord level. In this work, the effects of morphine, dizocilpine and riluzole on the hyperalgesia induced by carrageenan administration in the rat paw model were investigated. The effects of morphine and riluzole on the release of glutamate and aspartate and on the concentrations of citrulline and arginine in dialysates of the ventral posterolateral nucleus of the thalamus were also examined. All three drugs decreased hyperalgesia when administered prior to carrageenan injection. Morphine decreased the glutamate concentration in dialysates of the ventral posterolateral nucleus but did not affect the concentrations of the other amino acids. The effect of morphine was observed in the absence of painful stimulation and when pressure applied to the rat paw induced a nociceptive reaction. Riluzole decreased the concentrations of glutamate and aspartate and those of citrulline and arginine in the presence or absence of painful stimulation. These experiments suggest that morphine and riluzole attenuate the hyperalgesia induced by injection of carrageenan in the rat hind paw, at least partly, by decreasing glutamate release in the ventral posterolateral thalamic nucleus.

Antinociceptive effects of morphine injected into the nucleus parafascicularis thalami of the rat

The antinociceptive action of morphine microinjected into the nucleus parafascicularis thalami (nPf) on pain behaviors organized at different levels of the neuraxis was examined in the rat. Behaviors organized at spinal (spinal motor reflexes, SMRs), medullary (vocalizations during shock, VDSs), and forebrain (vocalization afterdischarges, VADs) levels were elicited by noxious tailshock. Morphine administered into nPf generated dose-dependent increases in thresholds of VDS and VAD, but failed to elevate SMR thresholds. Increases in vocalization thresholds were reversed in a dose-dependent manner by the microinjection of the mu-opiate receptor antagonist, methylnaloxonium, into nPf. Results are discussed in terms of the relative influence of nPf-administered morphine on nociceptive processing at spinal versus supraspinal levels of the neuraxis.

Nigrostriatal lesions alter oral dyskinesia and c-Fos expression induced by the serotonin agonist 1-(m-chlorophenyl)piperazine in adult rats

The loss of dopaminergic innervation of the basal ganglia, a group of subcortical regions involved in motor control, is the hallmark of Parkinson's disease. The resulting molecular and cellular alterations mediate behavioral deficits and may modify neuronal responses to other neurotransmitters. In the present study, we sought to determine the effects of chronic dopamine (DA) depletion on responses mediated by stimulation of serotonergic 2C (5-HT(2C)) receptors, a serotonergic receptor subtype present in discrete regions of the basal ganglia. Specifically, the effects of unilateral lesions of nigrostriatal DA neurons on oral dyskinesia and Fos protein expression induced by the non-selective 5-HT(2C) agonist 1-(m-chlorophenyl)piperazine (m-CPP) were examined. Confirming previous findings, both peripheral and local injections of m-CPP into the subthalamic nucleus elicited oral dyskinesia. Nigrostriatal lesions markedly enhanced oral bouts induced by peripheral but not intrasubthalamic administration of m-CPP. In intact rats, Fos expression was increased by m-CPP (1 mg/kg, i.p.) in the striatum and the subthalamic nucleus. After nigrostriatal lesions, m-CPP-induced Fos expression remained unchanged in the subthalamic nucleus but was reduced in the medial quadrants of the striatum and was markedly enhanced in the entopeduncular nucleus. These data demonstrate regionally specific alterations in behavioral and cellular responses to a serotonergic agonist in an animal model of Parkinson's disease.

Amygdaloid-thalamic interactions mediate the antinociceptive action of morphine microinjected into the periaqueductal gray

The bilateral administration of the serotonin receptor antagonist methysergide (2.5 microg, 5 microg, and 10 microg) into either the central nucleus of the amygdala (ACe) or nucleus parafascicularis thalami (nPf) produced dose-dependent inhibition of the antinociceptive action of ventrolateral periaqueductal gray (vPAG)-administered morphine. Unilateral administration of these doses of methysergide into either the ACe or nPf had no effect on morphine-induced antinociception. However, the combined unilateral administration of these doses of methysergide into the ACe and nPf produced dose-dependent inhibition of morphine antinociception that was identical to that observed after its bilateral administration into either site. This latter finding is interpreted as evidence that a functional interaction between the ACe and nPf supports the antinociceptive action of morphine administered into the vPAG.