Alcohol potentiates multiple GABAergic inputs to dorsal striatum fast-spiking interneurons

Parvalbumin-expressing dorsal striatal fast-spiking interneurons, comprising ∼1% of the total dorsal striatal neuronal population, are necessary for the expression of compulsive-like ethanol consumption mice. Fast-spiking interneurons are driven to fire by glutamatergic inputs derived primarily from the cortex. However, these neurons also receive substantial GABAergic input from two sources: the globus pallidus and the reticular nucleus of the thalamus. How ethanol modulates inhibitory input onto fast-spiking neurons is unclear and, more broadly, alcohol effects on GABAergic synaptic transmission onto GABAergic interneurons are understudied. Examining this, we found that acute bath application of ethanol (50 mM) potentiated GABAergic transmission from both the globus pallidus and the reticular nucleus of the thalamus onto fast-spiking interneurons in mouse of both sexes. This ethanol-induced potentiation required postsynaptic calcium and was not accompanied by a sustained change in presynaptic GABA release probability. Examining whether this ethanol effect persisted following chronic intermittent ethanol exposure, we found attenuated acute-ethanol potentiation of GABAergic transmission from both the globus pallidus and the reticular nucleus of the thalamus onto striatal fast-spiking interneurons. These data underscore the impact of ethanol on GABAergic signaling in the dorsal striatum and support the notion that ethanol may disinhibit the dorsolateral striatum.

The antipsychotic drug brexpiprazole reverses phencyclidine-induced disruptions of thalamocortical networks

Brexpiprazole (BREX), a recently approved antipsychotic drug in the US and Canada, improves cognitive dysfunction in animal models, by still largely unknown mechanisms. BREX is a partial agonist at 5-HT_1A and D₂ receptors and antagonist at α_1B- and α_2C-adrenergic and 5-HT_2A receptors all with a similar potency. The NMDA receptor antagonist phencyclidine (PCP), used as pharmacological model of schizophrenia, activates thalamocortical networks and decreases low frequency oscillations (LFO; <4 Hz). These effects are reversed by antipsychotics. Here we assessed the ability of BREX to reverse PCP-induced hyperactivity of thalamocortical circuits, and the involvement of 5-HT_1A receptors in its therapeutic action. BREX reversed PCP-induced neuronal activation at a lower dose in centromedial/mediodorsal thalamic nuclei (CM/MD; 0.5mg/kg) than in pyramidal medial prefrontal cortex neurons (mPFC, 2mg/kg), perhaps due to antagonism at α_1B-adrenoceptors, abundantly expressed in the thalamus. Conversely, a cumulative 0.5 mg/kg dose reversed a PCP-induced LFO decrease in mPFC but not in CM/MD. BREX reduced LFO in both areas, yet with a different dose-response, and moderately excited mPFC neurons. The latter effect was reversed by the 5-HT_1A receptor antagonist WAY-100635. Thus, BREX partly antagonizes PCP-induced thalamocortical hyperactivity, differentially in mPFC versus CM/MD. This regional selectivity may be related to the differential expression of α_1B-, α_2C-adrenergic and 5-HT_2A receptors in both regions and/or different neuronal types. Furthermore, the pro-cognitive properties of BREX may be related to the 5-HT_1A receptor-mediated increase in mPFC pyramidal neuron activity. Overall, the present data provide new insight on the brain elements involved in BREX's therapeutic actions.

Enhanced functional connectivity and volume between cognitive and reward centers of naïve rodent brain produced by pro-dopaminergic agent KB220Z

Dopaminergic reward dysfunction in addictive behaviors is well supported in the literature. There is evidence that alterations in synchronous neural activity between brain regions subserving reward and various cognitive functions may significantly contribute to substance-related disorders. This study presents the first evidence showing that a pro-dopaminergic nutraceutical (KB220Z) significantly enhances, above placebo, functional connectivity between reward and cognitive brain areas in the rat. These include the nucleus accumbens, anterior cingulate gyrus, anterior thalamic nuclei, hippocampus, prelimbic and infralimbic loci. Significant functional connectivity, increased brain connectivity volume recruitment (potentially neuroplasticity), and dopaminergic functionality were found across the brain reward circuitry. Increases in functional connectivity were specific to these regions and were not broadly distributed across the brain. While these initial findings have been observed in drug naïve rodents, this robust, yet selective response implies clinical relevance for addicted individuals at risk for relapse, who show reductions in functional connectivity after protracted withdrawal. Future studies will evaluate KB220Z in animal models of addiction.

Developmental GnRH signaling is not required for sexual differentiation of kisspeptin neurons but is needed for maximal Kiss1 gene expression in adult females

Kisspeptin, encoded by Kiss1, stimulates reproduction. In rodents, one Kiss1 population resides in the hypothalamic anterior ventral periventricular nucleus and neighboring rostral periventricular nucleus (AVPV/PeN). AVPV/PeN Kiss1 neurons are sexually dimorphic (greater in females), yet the mechanisms regulating their development and sexual differentiation remain poorly understood. Neonatal estradiol (E₂) normally defeminizes AVPV/PeN kisspeptin neurons, but emerging evidence suggests that developmental E₂ may also influence feminization of kisspeptin, although exactly when in development this process occurs is unknown. In addition, the obligatory role of GnRH signaling in governing sexual differentiation of Kiss1 or other sexually dimorphic traits remains untested. Here, we assessed whether AVPV/PeN Kiss1 expression is permanently impaired in adult hpg (no GnRH or E₂) or C57BL6 mice under different E₂ removal or replacement paradigms. We determined that 1) despite lacking GnRH signaling in development, marked sexual differentiation of Kiss1 still occurs in hpg mice; 2) adult hpg females, who lack lifetime GnRH and E₂ exposure, have reduced AVPV/PeN Kiss1 expression compared to wild-type females, even after chronic adulthood E₂ treatment; 3) E₂ exposure to hpg females during the pubertal period does not rescue their submaximal adult Kiss1 levels; and 4) in C57BL6 females, removal of ovarian E2 before the pubertal or juvenile periods does not impair feminization and maximal adult AVPV/PeN Kiss1 expression nor the ability to generate LH surges, indicating that puberty is not a critical period for Kiss1 development. Thus, sexual differentiation still occurs without GnRH, but GnRH or downstream E₂ signaling is needed sometime before juvenile development for complete feminization and maximal Kiss1 expression in adult females.

Role of the bed nucleus of the stria terminalis in the cardiovascular responses to acute restraint stress in rats

The aim of this work was to test the hypothesis that the bed nucleus of the stria terminalis (BST) and noradrenergic neurotransmission therein mediate cardiovascular responses to acute restraint stress in rats. Bilateral microinjection of the non-specific synaptic blocker CoCl(2) (0.1 nmol/100 nl) into the BST enhanced the heart rate (HR) increase associated with acute restraint without affecting the blood pressure increase, indicating that synapses within the BST influence restraint-evoked HR changes. BST pretreatment with the selective alpha(1)-adrenoceptor antagonist WB4101 (15 nmol/100 nl) caused similar effects to cobalt, indicating that local noradrenergic neurotransmission mediates the BST inhibitory influence on restraint-related HR responses. BST treatment with equimolar doses of the alpha(2)-adrenoceptor antagonist RX821002 or the beta-adrenoceptor antagonist propranolol did not affect restraint-related cardiovascular responses, reinforcing the inference that alpha(1)-adrenoceptors mediate the BST-related inhibitory influence on HR responses. Microinjection of WB4101 into the BST of rats pretreated intravenously with the anticholinergic drug homatropine methyl bromide (0.2 mg/kg) did not affect restraint-related cardiovascular responses, indicating that the inhibitory influence of the BST on the restraint-evoked HR increase could be related to an increase in parasympathetic activity. Thus, our results suggest an inhibitory influence of the BST on the HR increase evoked by restraint stress, and that this is mediated by local alpha(1)-adrenoceptors. The results also indicate that such an inhibitory influence is a result of parasympathetic activation.

Effects of mental illness and aging in two thalamic nuclei

We previously reported a schizophrenia associated reduction of neuronal and oligodendrocyte number in the anterior principal thalamic nucleus (APN) in a cohort of severely impaired elderly subjects with schizophrenia (SZ) relative to age matched nonpsychiatric controls (NCs). The present study was undertaken to determine 1) if those findings could be replicated in an independent sample of less chronically impaired subjects with SZ and NCs stratified across a broader age range; 2) if the findings are specific to SZ or are also seen in unipolar major depressive (MDD) or bipolar disorder (BPD); and 3) if the findings are specific to the APN or also seen in another thalamic nucleus. Computer assisted stereological methods were employed to determine the number of neurons and oligodendrocytes in the APN and centromedian nucleus (CMN) of the Nissl-stained thalamic sections maintained by the Stanley Foundation Brain Bank. This collection includes specimens from NCs and age matched subjects with diagnoses of SZ, MDD, or BPD who died between the ages of 25 and 68. Data were analyzed by mixed-effects linear regressions adjusting for demographic variables and known history of exposure to psychotropic medications. Oligodendrocyte number was decreased in both nuclei relative to NCs in subjects with SZ and in that subset of subjects with BPD who had experienced psychotic episodes. Compared to NCs both of these patient groups also exhibited an attenuation of an age-related increase in the number of oligodendrocytes. Contrary to our previous report, we did not detect a SZ-associated deficit in neuronal number in the APN. A history of exposure to neuroleptics, however, was associated with a decrease in neuronal number in both nuclei, but this decrease did not vary in relation to cumulative lifetime neuroleptic exposure in fluphenazine equivalents. Among subjects with psychiatric diagnoses, exposure to lithium was associated with an increase in the number of oligodendrocytes. No effects were detected for exposure to anticonvulsants or for abuse of alcohol or other substances.

Microinjection of morphine into thalamic nucleus submedius depresses bee venom-induced inflammatory pain in the rat

Previous studies have provided evidence of the existence of a pain modulatory feedback pathway consisting of thalamic nucleus submedius (Sm)-ventrolateral orbital cortex-periaqueductal grey pathway, which is activated during acute pain and leads to depression of transmission of nociceptive information in the spinal dorsal horn. The aim of this study was to test the hypothesis that morphine microinjection into the Sm decreased spontaneous pain and bilateral thermal hyperalgesia, as well as ipsilateral mechanical allodynia, induced by subcutaneous injections of bee venom into the rat hind paw. Morphine (1.0, 2.5 or 5.0 microg in 0.5 microL) injected into the Sm, contralateral to the bee venom-injected paw, depressed spontaneous nociceptive behaviour in a dose-dependent manner. Furthermore, morphine significantly decreased bilateral thermal hyperalgesia and ipsilateral mechanical allodynia 2 h after bee venom injection. These morphine-induced effects were antagonized by 1.0 microg naloxone (an opioid antagonist) microinjected into the Sm 5 min before morphine administration. The results provided further support for the important role of the Sm and Sm-opioid receptors in inhibiting nociceptive behaviour and indicated for the first time that Sm opioid receptors were also effective in inhibiting the hypersensitivity provoked by bee venom-induced inflammation.

Cellular dynamics of cholinergically induced alpha (8-13 Hz) rhythms in sensory thalamic nuclei in vitro

Although EEG alpha (8-13 Hz) rhythms are traditionally thought to reflect an "idling" brain state, they are also linked to several important aspects of cognition, perception, and memory. Here we show that reactivating cholinergic input, a key component in normal cognition and memory operations, in slices of the cat primary visual and somatosensory thalamus, produces robust alpha rhythms. These rhythms rely on activation of muscarinic receptors and are primarily coordinated by activity in the recently discovered, gap junction-coupled subnetwork of high-threshold (HT) bursting thalamocortical neurons. By performing extracellular field recordings in combination with intracellular recordings of these cells, we show that (1) the coupling of HT bursting cells is sparse, with individual neurons typically receiving discernable network input from one or very few additional cells, (2) the phase of oscillatory activity at which these cells prefer to fire is readily modifiable and determined by a combination of network input, intrinsic properties and membrane polarization, and (3) single HT bursting neurons can potently influence the local network state. These results substantially extend the known effects of cholinergic activation on the thalamus and, in combination with previous studies, show that sensory thalamic nuclei possess powerful and dynamically reconfigurable mechanisms for generating synchronized alpha activity that can be engaged by both descending and ascending arousal systems.

Effects of non-competitive AMPA receptor antagonists injected into some brain areas of WAG/Rij rats, an animal model of generalized absence epilepsy

CFM-2 [1-(4-aminophenyl)-3,5-dihydro-7,8-dimethoxy-4H-2,3-benzodiazepin-4-one] and THIQ-10c [N-acetyl-1-(4-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline], are two non-competitive 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid (AMPA) receptor antagonists, which demonstrated to antagonize generalized tonic-clonic seizures in different animal models. We have evaluated the effects of such compounds in a genetic animal model of absence epilepsy, the WAG/Rij rat. Animals were focally microinjected into specific brain areas of the cortico-thalamic circuit in order to evaluate the effects of these compounds on the number and duration of epileptic spike-wave discharges (SWDs) and better characterize the role of AMPA neurotransmission in this animal model. The focal microinjection of the two AMPA antagonists into some thalamic nuclei (ventralis posteromedialis (VPM), reticularis (NRT), ventralis posterolateralis (VPL) and the primary somatosensory forelimb region (S1FL)) was, generally, not able to significantly modify the occurrence of SWDs. Whereas, both compounds were able to reduce the number and duration of SWDs dose-dependently when microinjected into the peri-oral region of the primary somatosensory cortex (S1po). These findings suggest that AMPA receptor antagonists might play a role in absence epilepsies and that it might depend on the involvement of specific neuronal areas.

GABAA receptor-mediated tonic inhibition in thalamic neurons

Tonic GABAA receptor-mediated inhibition is typically generated by delta subunit-containing extrasynaptic receptors. Because the delta subunit is highly expressed in the thalamus, we tested whether thalamocortical (TC) neurons of the dorsal lateral geniculate nucleus (dLGN) and ventrobasal complex exhibit tonic inhibition. Focal application of gabazine (GBZ) (50 microM) revealed the presence of a 20 pA tonic current in 75 and 63% of TC neurons from both nuclei, respectively. No tonic current was observed in GABAergic neurons of the nucleus reticularis thalami (NRT). Bath application of 1 microM GABA increased tonic current amplitude to approximately 70 pA in 100% of TC neurons, but it was still not observed in NRT neurons. In dLGN TC neurons, the tonic current was sensitive to low concentrations of the delta subunit-specific receptor agonists allotetrahydrodeoxycorticosterone (100 nM) and 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol (THIP) (100 nM) but insensitive to the benzodiazepine flurazepam (5 microM). Bath application of low concentrations of GBZ (25-200 nM) preferentially blocked the tonic current, whereas phasic synaptic inhibition was primarily maintained. Under intracellular current-clamp conditions, the preferential block of the tonic current with GBZ led to a small depolarization and increase in input resistance. Using extracellular single-unit recordings, block of the tonic current caused the cessation of low-threshold burst firing and promoted tonic firing. Enhancement of the tonic current by THIP hyperpolarized TC neurons and promoted burst firing. Thus, tonic current in TC neurons generates an inhibitory tone. Its modulation contributes to the shift between different firing modes, promotes the transition between different behavioral states, and predisposes to absence seizures.

Selective changes in thalamic and cortical GABAA receptor subunits in a model of acquired absence epilepsy in the rat

Neonatal treatment of Long-Evans Hooded rats with the cholesterol synthesis inhibitor (CSI) AY9944 has been shown to increase occurrence of spike-waves in EEG recordings and decrease benzodiazepines sensitivity of GABA(A) receptor-mediated responses in neurons from the thalamic reticular nuclei (nRt, Wu et al., 2004). The present experiments were designed to investigate the changes in the gamma2 and alpha1 subunits of the GABA(A) receptor in CSI model rats as possible mechanisms of these changes. Western blot, immunohistochemistry and real-time PCR techniques were performed to measure the levels of GABA(A) receptor gamma2 and alpha1 subunit transcripts and protein in the nRt and ventrobasal (VB) relay nuclei of thalamus and in somatosensory cortex. In CSI model animals, Western blot results showed that gamma2 subunit expression significantly decreased in thalamus (control, n=6: 0.17+/-0.02 relative to actin vs. CSI model, n=6: 0.11+/-0.01, P<0.05) but neither in cortex nor in hippocampal tissues. Conversely, alpha1 subunit expression decreased in CSI model somatosensory cortex, but not in nRt and VB. The present results demonstrate that neonatal block of cholesterol synthesis produces region- and subunit-specific decreases in GABA(A) receptor subunits in thalamus and cortex. Selective reductions in GABA(A) receptor subunits in thalamus may play a role in pathophysiology of absence epilepsy.

Effects in vitro and in vivo of a gap junction blocker on epileptiform activities in a genetic model of absence epilepsy

We investigated the effects of carbenoxolone (CBX), a gap junctions (GJ) blocker, on epileptiform activities in vivo and in vitro. In a first series of experiments, i.p. CBX decreased the cumulative duration of cortical spike-wave discharges (SWD) in adult Genetic Absence Epilepsy Rats from Strasbourg (GAERS) without reduction in the SW amplitude or frequency. Since SWD are generated in thalamocortical networks, we studied the effect of CBX on thalamic and cortical activities elicited by 4-aminopyridine (4AP) in thalamocortical slices from GAERS or non-epileptic rats (NER). Spontaneous ictal-like activities (ILA) were recorded simultaneously in thalamus and somatosensory cortex. However, experiments where these structures were surgically separated showed that ILA were generated in the cortex and recorded by volume conduction in the thalamus. GABA-dependent negative field potentials were also recorded in the cortex, either isolated or initiating ILA. After bath-applying CBX (100 microM), the frequency and cumulative duration of ILA decreased but less rapidly in GAERS than in NER slices and they disappeared at a time point when GABA-dependent negative potentials remained. These data suggest that GJ do not mediate the 4AP induced interneuronal synchronisation but may be implicated in the spreading of the synchronised activities from interneuronal networks to principal neurones. Our results show that CBX exerts an antiepileptic action in vivo, and that GJ blockers limits spread of synchronised activities in vitro. They may represent an appropriate target for development of new antiepileptic drugs.

Restricted Expression of LGR8 in Intralaminar Thalamic Nuclei of Rat Brain Suggests a Role in Sensorimotor Systems

LGR8 was recently identified as a cognate receptor for insulin-like peptide-3 (INSL3), and INSL3-LGR8 signaling is best known for its role in testis descent during development. LGR8 mRNA has been detected in various human tissues including brain, but the regional and cellular distribution of LGR8 expression in the mammalian central nervous system is unknown. Therefore, in this study we investigated the presence and localization of LGR8 mRNA in rat brain using reverse transcription-polymerase chain reaction and in situ hybridization histochemistry. Results revealed a distinct distribution of LGR8 in forebrain, with transcripts principally restricted to the posterior thalamus and highest densities detected in the parafascicular nucleus of both adult and developing rats. Unexpectedly, INSL3 mRNA was not detected in brain by similar methods, but preliminary electrophysiologic studies of parafascicular neurons revealed that INSL3 altered their activity. These findings suggest that LGR8 signaling may be involved in sensorimotor control in the rat and perhaps other species, particularly via actions on parafascicular neurons that project to basal ganglia and are depleted in Parkinson's disease.

Antiabsence effects of carbenoxolone in two genetic animal models of absence epilepsy (WAG/Rij rats and mice)

Carbenoxolone (CBX), the succinyl ester of glycyrrhetinic acid, is an inhibitor of gap junctional intercellular communication. We have tested its possible effects upon two genetic animal models of epilepsy (WAG/Rij rats and lethargic (lh/lh) mice). Systemic administration of CBX was unable to significantly affect the occurrence of absence seizures in WAG/Rij rats. In particular, intravenous (5-40 mg/kg) or intraperitoneal (i.p.; 10-80 mg/kg) administration of CBX was unable to significantly modify the number and duration of spike-wave discharges (SWDs) in WAG/Rij rats, whereas the bilateral microinjection (0.05, 0.1, 0.5 and 1 microg/0.5 microl) of CBX into nucleus reticularis thalami (NRT) and nucleus ventralis posterolateralis (VPL) thalami produced a decrease in the duration and the number of SWDs. Bilateral microinjection of CBX into nucleus ventroposteromedial (VPM) thalami did not produce any significant decrease in the number and duration of SWDs. On the contrary, i.p. (5-40 mg/kg) or intracerebroventricular (0.5, 1, 2 and 4 microg/2 microl) administration of CBX in lh/lh mice induced a marked decrease in the number and duration of SWDs in a dose-dependent manner. At the doses used no movement disorders, or other behavioural changes, were recorded in both WAG/Rij rats and lh/lh mice. No effects were observed in both animal models following systemic or focal administration of glycyrrhizin into the same brain areas where CBX was shown to be effective.

GABAergic modulation mediates antinociception produced by serotonin applied into thalamic nucleus submedius of the rat

Our previous studies have indicated that the thalamic nucleus submedius (Sm) is involved in modulation of nociception as part of an ascending component of an endogenous analgesic system consisting of spinal cord-Sm-ventrolateral orbital cortex (VLO)-periaqueductal gray (PAG)-spinal cord loop and that microinjection of 5-hydroxytryptamine (5-HT) into Sm produces antinociception. The aim of the present study was to examine whether the gamma-aminobutyric acid (GABA)ergic modulation is involved in the Sm 5-HT-evoked antinociception. Experiments were carried out on lightly anesthetized rats with an implanted cannula targeting the Sm nucleus. The microinjection of GABA(A) receptor antagonist bicuculline dose-dependently depressed the tail flick (TF) reflex. A smaller dose (100 ng) of bicuculline enhanced the inhibition of TF reflex produced by 5-HT application into Sm, whereas application of GABA (2.5 microg) did not influence the TF reflex but significantly attenuated the 5-HT-evoked inhibition. These results indicate that GABA(A) receptor may be involved in mediating the 5-HT-induced antinociception in Sm possibly through a disinhibition mechanism.

Colonic nociception via nucleus submedius is modulated by pontine centres in the rat

The rat thalamic nucleus submedius responds to noxious pressure stimuli in the colon. Some neurons in and near Barrington's nucleus, a pontine center related to bladder function, also respond to colon distension. We hypothesized that colonic nociception may be relayed via Barrington's nucleus to the nucleus submedius. Experiments were carried out in 22 urethane-anesthetized male rats. Noxious stimuli were applied to the toes using standardized clips and to the colon by inflation of the balloon to 80 mmHg for 30 s using a barostat. The brain was exposed to allow recording from the nucleus submedius with a monopolar tungsten electrode and the activity of rectus muscle was assessed via silver wire electrodes. A glass pipette was inserted into Barrington's nucleus for injection of 5 mM CoCl2, a temporary neural blocker. The site of CoCl2 injection was confirmed by the presence of FluoroGold which was incorporated into the CoCl2 solution. We recorded 51 units in submedius that were excited by noxious toe pinch, 4 were inhibited. Colon distension to 80 mmHg produced visceromotor responses, excited 23 units in submedius and inhibited 13 units. Injection of CoCl2 into the region of Barrington's nucleus blocked the response to colon distension in 10 of 12 Sm units tested, but had no influence on the accompanying visceromotor response. These data point to a previously unrecognized relationship between Barrington's nucleus and submedius that may subserve colon nociception.

Effect of repeated nicotine exposure on high-affinity nicotinic acetylcholine receptor density in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHRs) are often used as a model of attention deficit hyperactivity disorder (ADHD) and to investigate the effects of hypertension on cognitive function. Further, they appear to have reduced numbers of central nicotinic acetylcholine receptors (nAChRs) and, therefore, may be useful to model certain aspects of Alzheimer's disease (AD) and other forms of dementia given that a decrease in nAChRs is thought to contribute to cognitive decline in these disorders. In the present study, based on reports that chronic nicotine exposure increases nAChRs in several mammalian models, we tested the hypothesis that repeated exposures to a relatively low dose of the alkaloid would ameliorate the receptor deficits in SHR. Thus, young-adult SHRs and age-matched Wistar-Kyoto (WKY) control rats were treated with either saline or nicotine twice a day for 14 days (total daily dose = 0.7 mg/kg nicotine base) and then sacrificed. Quantitative receptor autoradiography with [125I]-IPH, an epibatidine analog, revealed: (1) that high-affinity nAChRs were higher in saline-treated WKY (control) rats compared to saline-treated SHRs in 18 of the 19 brain region measured, although statistically different only in the mediodorsal thalamic nuclei, (2) that nicotine significantly increased nAChR binding in WKY rats in six brain areas including cortical regions and the anterior thalamic nucleus, (3) that there were no cases where nicotine significantly increased nAChR binding in SHRs. These results indicate that subjects deficient in nAChRs may be less sensitive to nAChR upregulation with nicotine than normal subjects and require higher doses or longer periods of exposure.

Stimulant drug action in attention deficit hyperactivity disorder (ADHD): inference of neurophysiological mechanisms via quantitative modelling

OBJECTIVE

To infer the neural mechanisms underlying tonic transitions in the electroencephalogram (EEG) in 11 adolescents diagnosed with attention deficit hyperactivity disorder (ADHD) before and after treatment with stimulant medication.

METHODS

A biophysical model was used to analyse electroencephalographic (EEG) measures of tonic brain activity at multiple scalp sites before and after treatment with medication.

RESULTS

It was observed that stimulants had the affect of significantly reducing the parameter controlling activation in the intrathalamic pathway involving the thalamic reticular nucleus (TRN) and the parameter controlling excitatory cortical activity. The effect of stimulant medication was also found to be preferentially localized within subcortical nuclei projecting towards frontal and central scalp sites.

CONCLUSIONS

It is suggested that the action of stimulant medication occurs via suppression of the locus coeruleus, which in turn reduces stimulation of the TRN, and improves cortical arousal. The effects localized to frontal and central sites are consistent with the occurrence of frontal delta-theta EEG abnormalities in ADHD, and existing theories of hypoarousal.

SIGNIFICANCE

To our knowledge, this is the first study where a detailed biophysical model of the brain has been used to estimate changes in neurophysiological parameters underlying the effects of stimulant medication in ADHD.

Effect of Amiloride on Gustatory Responses in the Ventroposteromedial Nucleus of the Thalamus in Rats

The existence of gustatory neuron types has been demonstrated in the chorda tympani nerve and the nucleus of the solitary tract (NTS) of rats and hamsters through the oral application of amiloride, a sodium channel blocker. At these lower-order levels, amiloride was shown to reduce the response to sodium and lithium salts in sodium- and sugar-oriented cells, while leaving those of acid- and quinine-oriented neurons unmodified. We extended this investigation to higher-order levels by determining whether amiloride suppressed the responses of cells at the 4th-order gustatory relay in the thalamus, which neurons were affected, the degree of suppression, and whether the subsequent neural code for sodium was altered. We stimulated the whole oral cavity of anesthetized rats with a variety of tastants while recording the responses of 42 single thalamic neurons before and after the application of amiloride. The results revealed a similar pattern to that reported in the NTS. Amiloride inhibited only sodium- and sugar-oriented neurons, and specifically their responses to sodium- or lithium-containing stimuli. Moreover, there was a significant relationship between the degree of sodium specificity of a neuron and its sensitivity to inhibition by amiloride. These results demonstrate a relationship between a cell's response profile and its susceptibility to amiloride, and so offer evidence that gustatory neuron types exist through the level of the thalamus in rats. Thus membership in a neuronal group retains functional significance based on a receptor event 4 synapses away.

Localization of neuropeptide Y receptor Y5 mRNA in the guinea pig brain by in situ hybridization

Neuropeptide Y (NPY) has prominent stimulatory effects on food intake in virtually all animals that have been studied. In mammals, the effect is primarily mediated by receptors Y1 and Y5, which seem to contribute to different aspects of feeding behavior in guinea pigs and rats/mice. Interestingly, differences in receptor distribution among mammalian species have been reported. To get a broader perspective on the role of Y5, we describe here studies of guinea pig (Cavia porcellus), a species which due to its phylogenetic position in the mammalian radiation is an interesting complement to previous studies in rat and mouse. Guinea pig brain sections were hybridized with two 35S-labeled oligonucleotides complementary to Y5 mRNA. The highest expression levels of Y5 mRNA were observed in the hippocampus and several hypothalamic and brain stem nuclei implicated in the regulation of feeding, such as the paraventricular, arcuate and ventromedial hypothalamic nuclei. This contrasts with autoradiography studies that detected low Y5-like binding in these areas, a discrepancy observed also in rat and human. Y5 mRNA expression was also seen in the striatum, in great contrast to mouse and rat. Taken together, these data show that Y5 mRNA distribution displays some interesting species differences, but that its expression in feeding centers seems to be essentially conserved among mammals, adding further support for an important role in food intake.

Modifications of local cerebral glucose utilization in thalamic structures following injection of a dopaminergic agonist in the nucleus accumbens—involvement in antiepileptic effects?

Dopaminergic transmission in the nucleus accumbens (NAcc) is implicated in different aspects of reward and motivational mechanisms. More recently, it has been suggested that this nucleus could also be involved in the modulation of generalized epileptic seizures. In particular, microinjection of dopaminergic agonists in the NAcc suppresses the occurrence of epileptic seizures in a model of absence seizures, the GAERS (generalized absence epileptic rats from Strasbourg). The aim of this study was to identify the structures involved in this effect. Local cerebral metabolic rates for glucose utilization (LCMRglc) were measured in different parts of the basal ganglia and output structures after apomorphine injection in the NAcc in GAERS and in the inbred non-epileptic rats (NE), concomitantly with seizure suppression. Apomorphine injection in the NAcc induced a significant increase of glucose intake in the anteromedial, mediodorsal and ventrolateral nuclei of the thalamus in NE rats, while no significant changes were observed in the basal ganglia structures (globus pallidus, subthalamic nucleus, substantia nigra). Furthermore, microinjections of muscimol (100 and 200 pmol/side) in the mediodorsal nucleus of the thalamus in GAERS rats suppressed seizures. These results suggest that the mediodorsal nucleus of the thalamus could be involved in absence seizures modulation. Along with data from the literature, our data suggest that this nucleus could participate in the control of the basal ganglia over generalized epileptic seizures.

Chemical stimulation of the thalamic reticular nucleus inhibits the neuronal activity of the posterior insular cortex in rats

Extracellular neuronal responses were recorded from the posterior insular cortex following electrical and chemical stimulation of the thalamic reticular nucleus (Rt) regions. In the present study, most neurons (29/32) were first characterized for their responses to electrical stimulation of the superior laryngeal (SL) nerve or glossopharyngeal (IXth) nerve. In the first experiment, 15 neurons in the posterior insular cortex were examined for their responses to electrical stimulation of the Rt regions. It was found that effective stimulation sites to evoke action potentials in the posterior insular cortex were the ventromedial portion of the Rt and its adjacent regions. In the second experiment, 17 neurons in the posterior insular cortex were examined for their responses by pressure injection of glutamate (Glu) into the Rt regions. Of the 17 neurons, 13 were inhibited in the spontaneous discharge rate following injection of Glu into the Rt, and the remaining four were unaffected. Histologically, it was demonstrated that Glu injection sites for the case of inhibition were located near or within the Rt. On the other hand, the injection sites for all four non-responsive neurons were located outside of the Rt. These data suggest that excitation of the Rt (GABAergic neurons) causes depression of the neuronal activity in the thalamic relay nucleus and then this may in turn induce depressed neuronal activity in the posterior insular cortex. The results here indicate that neuronal activity in the posterior insular cortex is controlled by the Rt, which has been reported in other sensory systems.

Activation of dopamine D4 receptors modulates [3H]GABA release in slices of the rat thalamic reticular nucleus

The thalamic reticular nucleus (nRt) is innervated by dopaminergic projections from the sustantia nigra compacta (SNc) and is rich in dopamine D4 receptors, however, the functional effects of dopamine on this structure are unknown. We examined whether the D1 receptor agonist SKF 38393, or the D2 class receptor agonist quinpirole, modify depolarization evoked Ca(2+)-dependent [3H]GABA release. SKF 38393 was without effects, whereas quinpirole inhibited [3H]GABA release with an IC50 of 81 +/- 33 nM. Dose-dependence determinations of agonists (quinpirole and PD 168, 077) and antagonists (L-745,870, U-101958, clozapine and raclopride) with different affinities for different D2 class subtype receptors showed that a D4 receptor mediates quinpirole inhibition. We used methylphenidate, an agent that acts by increasing interstitial dopamine, to determine whether endogenous dopamine modulates [3H]GABA release. Methylphenidate inhibited [3H]GABA release showing that the nRt contains sufficient endogenous dopamine to activate D4 receptors. This inhibition was completely reversed by selectively blocking D4 receptors with L-745,870 or U-101958 indicating that the catecholamine receptors that modulate GABA release are D4 receptors. Given the importance of the nRt in the control of attention, sensory processing and the generation of rhythmic activity during slow wave sleep, it is possible that abnormal nRt function may generate some of the manifestations of the disorders of dopaminergic transmission.

Group II and III metabotropic glutamate receptors and the control of the nucleus reticularis thalami input to rat thalamocortical neurones in vitro

Intracellular recordings were made from neurones in the thalamic reticular nucleus (TRN) and ventro-basal (VB) thalamus in slices of rat midbrain in vitro. Electrical stimulation of the medial lemniscus or TRN resulted in the generation of complex synaptic potentials containing disynaptic inhibitory post-synaptic potentials (IPSPs) in VB thalamocortical neurones. Analysis of the excitatory synaptic responses in TRN neurones indicates they can produce burst output response irrespective of the level of sub-threshold membrane potential. This suggests that network-evoked IPSPs in VB thalamocortical neurones occur following a burst of TRN action potentials. Using ionotropic glutamate receptor antagonists, the activation of these disynaptic events was blocked, and the monosynaptic IPSPs that resulted from the direct activation of the TRN could be isolated. The selective Group II agonists LY354740 (1-10 microM) and N-acetyl-aspartyl-glutamate (NAAG; 100-500 microM) both caused a reversible depression of these monosynaptic TRN IPSPs without any effect on membrane potential or input resistance. Likewise, the specific Group III agonist L-2-amino-4-phosphonobutanoate (10-500 microM), but not (RS)-4-phosphonophenylglycine (1 and 30 microM) also caused a reversible depression of these IPSPs, again without any effect on membrane potential or input resistance.Thus, the IPSPs recorded in VB thalamocortical neurones, evoked by TRN activation, can be depressed by the activation of either Group II or III metabotropic glutamate receptors. This is consistent with the location of these receptor types on the presynaptic terminals of TRN axons in the VB thalamus. This raises the possibility that, during periods of intense excitatory activity, glutamate release could influence the release of GABA from TRN axon terminals in the thalamus. In addition, as NAAG is located in the axons and terminals arising from the TRN, there is the possibility that this dipeptide is also released by these terminals to control the release of GABA during periods of high activity in the TRN.

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.

Group II metabotropic glutamate receptors are a common target of N-anisoyl-GABA and 1S,3R-ACPD in enhancing ACh release in the prefrontal cortex of freely moving SHRSP

Aniracetam is a therapeutically useful cognition enhancer for treating various neuropsychiatric symptoms occurring after cerebral infarction. We recently reported that local perfusion of its major metabolites N-anisoyl-GABA and p-anisic acid, but not aniracetam itself, enhanced acetylcholine (ACh) release with a delayed onset in cerebral regions of stroke-prone spontaneously hypertensive rats (SHRSP). In this study, we examined the possible involvement of metabotropic and ionotropic glutamate (mGlu and AMPA) receptors in the N-anisoyl-GABA-induced ACh release using brain in vivo microdialysis. Basal ACh release in SHRSP was commonly lower in the nucleus reticularis thalami, dorsal hippocampus and prefrontal cortex than that in age-matched Wistar Kyoto rats. The delayed ACh release in the prefrontal cortex of SHRSP was completely blocked by MCPG, a group I and II mGlu receptor antagonist, and MCCG, a group II-selective mGlu receptor antagonist. In contrast, it was largely unaffected by AIDA, a group I-selective mGlu receptor antagonist, or by YM90K, an AMPA receptor antagonist. 1S,3R-ACPD, a preferential group II mGlu receptor agonist, enhanced ACh release with a similar latency and the effect was antagonized by MCCG, whereas AMPA induced a prompt ACh release. These results indicate that N-anisoyl-GABA and 1S,3R-ACPD share a common mechanism mediated by group II mGlu receptors in enhancing ACh release. The findings suggest a possible mechanism for aniracetam's clinical efficacy in stroke patients with cholinergic deficits.