The afferent and efferent connections of the nucleus submedius in the rat

Projections from the five divisions of the orbital cortex to the thalamus in the rat

The orbital cortex (ORB) of the rat consists of five divisions: the medial (MO), ventral (VO), ventrolateral (VLO), lateral (LO), and dorsolateral (DLO) orbital cortices. No previous report has comprehensively examined and compared projections from each division of the ORB to the thalamus. Using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin, we describe the efferent projections from the five divisions of the ORB to the thalamus in the rat. We demonstrated that, with some overlap, each division of the ORB distributed in a distinct (and unique) manner to nuclei of the thalamus. Overall, ORB projected to a relatively restricted number of sites in the thalamus, and strikingly distributed entirely to structures of the medial/midline thalamus, while completely avoiding lateral regions or principal nuclei of the thalamus. The main termination sites in the thalamus were the paratenial nucleus (PT) and nucleus reuniens (RE) of the midline thalamus, the medial (MDm) and central (MDc) divisions of the mediodorsal nucleus, the intermediodorsal nucleus, the central lateral, paracentral, and central medial nuclei of the rostral intralaminar complex and the submedial nucleus (SM). With some exceptions, medial divisions of the ORB (MO, VO) mainly targeted "limbic-associated" nuclei such as PT, RE, and MDm, whereas lateral division (VLO, LO, DLO) primarily distributed to "sensorimotor-associated" nuclei including MDc, SM, and the rostral intralaminar complex. As discussed herein, the medial/midline thalamus may represent an important link (or bridge) between the orbital cortex and the hippocampus and between the ORB and medial prefrontal cortex. In summary, the present results demonstrate that each division of the orbital cortex projects in a distinct manner to nuclei of the thalamus which suggests unique functions for each division of the orbital cortex.

Organization of Afferents along the Anterior-posterior and Medial-lateral Axes of the Rat Orbitofrontal Cortex

The orbitofrontal cortex (OFC) has been anatomically divided into a number of subregions along its medial-lateral axis, which behavioral research suggests have distinct functions. Recently, evidence has emerged suggesting functional diversity is also present along the anterior-posterior axis of the rodent OFC. However, the patterns of anatomical connections that underlie these differences have not been well characterized. Here, we use the retrograde tracer cholera toxin subunit B (CTB) to simultaneously label the projections into the anterior lateral (ALO), posterior lateral (PLO), and posterior ventral (PVO) portions of the rat OFC. Our methodological approach allowed us to simultaneously compare the density and input patterns into these OFC subdivisions. We observed distinct and topographically organized projection patterns into ALO, PLO, and PVO from the mediodorsal and the submedius nuclei of the thalamus. We also observed different levels of connectivity strength into these OFC subdivisions from the amygdala, motor cortex, sensory cortices and medial prefrontal cortical structures, including medial OFC, infralimbic and prelimbic cortices. Interestingly, while labelling in some of these input regions revealed only a gradient in connectivity strength, other regions seem to project almost exclusively to specific OFC subdivisions. Moreover, differences in input patterns between ALO and PLO were as pronounced as those between PLO and PVO. Together, our results support the existence of distinct anatomical circuits within lateral OFC along its anterior-posterior axis.

Consolidating the Circuit Model for Addiction

Addiction is a disease characterized by compulsive drug seeking and consumption observed in 20-30% of users. An addicted individual will favor drug reward over natural rewards, despite major negative consequences. Mechanistic research on rodents modeling core components of the disease has identified altered synaptic transmission as the functional substrate of pathological behavior. While the initial version of a circuit model for addiction focused on early drug adaptive behaviors observed in all individuals, it fell short of accounting for the stochastic nature of the transition to compulsion. The model builds on the initial pharmacological effect common to all addictive drugs-an increase in dopamine levels in the mesolimbic system. Here, we consolidate this early model by integrating circuits underlying compulsion and negative reinforcement. We discuss the genetic and epigenetic correlates of individual vulnerability. Many recent data converge on a gain-of-function explanation for circuit remodeling, revealing blueprints for novel addiction therapies.

Widespread corticopetal projections from the oval paracentral nucleus of the intralaminar thalamic nuclei conveying orofacial proprioception in rats

The oval paracentral nucleus (OPC) was initially isolated from the paracentral nucleus (PC) within the intralaminar thalamic nuclei in rats. We have recently shown that the rat OPC receives proprioceptive inputs from jaw-closing muscle spindles (JCMSs). However, it remains unknown which cortical areas receive thalamic inputs from the OPC, and whether the cortical areas receiving the OPC inputs are distinct from those receiving inputs from the other intralaminar nuclei and sensory thalamic nuclei. To address this issue, we injected an anterograde tracer, biotinylated dextranamine (BDA), into the OPC, which was electrophysiologically identified by recording of proprioceptive inputs from the JCMSs. Many BDA-labeled axonal fibers and terminals from the OPC were ipsilaterally observed in the rostral and rostroventral regions of the primary somatosensory cortex (S1), the rostral region of the secondary somatosensory cortex (S2), and the most rostrocaudal levels of the granular insular cortex (GI). In contrast, a BDA injection into the caudal PC, which was located slightly rostral to the OPC, resulted in ipsilateral labeling of axonal fibers and terminals in the rostrolateral region of the medial agranular cortex and the rostromedial region of the lateral agranular cortex. Furthermore, injections of a retrograde tracer, Fluorogold, into these S1, S2, and GI regions, resulted in preferential labeling of neurons in the ipsilateral OPC among the intralaminar and sensory thalamic nuclei. These findings reveal that the rat OPC has widespread, but strong corticopetal projections, indicating that there exist divergent corticopetal pathways from the intralaminar thalamic nucleus, which process JCMS proprioceptive sensation.

Cortical Modulation of Nociception

Nociception is the neuronal process of encoding noxious stimuli and could be modulated at peripheral, spinal, brainstem, and cortical levels. At cortical levels, several areas including the anterior cingulate cortex (ACC), prefrontal cortex (PFC), ventrolateral orbital cortex (VLO), insular cortex (IC), motor cortex (MC), and somatosensory cortices are involved in nociception modulation through two main mechanisms: (i) a descending modulatory effect at spinal level by direct corticospinal projections or mostly by activation of brainstem structures (i.e. periaqueductal grey matter (PAG), locus coeruleus (LC), the nucleus of raphe (RM) and rostroventral medulla (RVM)); and by (ii) cortico-cortical or cortico-subcortical interactions. This review summarizes evidence related to the participation of the aforementioned cortical areas in nociception modulation and different neurotransmitters or neuromodulators that have been studied in each area. Besides, we point out the importance of considering intracortical neuronal populations and receptors expression, as well as, nociception-induced cortical changes, both functional and connectional, to better understand this modulatory effect. Finally, we discuss the possible mechanisms that could potentiate the use of cortical stimulation as a promising procedure in pain alleviation.

A thalamic bridge from sensory perception to cognition

The ability to adapt to dynamic environments requires tracking multiple signals with variable sensory salience and fluctuating behavioral relevance. This complex process requires integrative crosstalk between sensory and cognitive brain circuits. Functional interactions between cortical and thalamic regions are now considered essential for both sensory perception and cognition but a clear account of the functional link between sensory and cognitive circuits is currently lacking. This review aims to document how thalamic nuclei may effectively act as a bridge allowing to fuse perceptual and cognitive events into meaningful experiences. After highlighting key aspects of thalamocortical circuits such as the classic first-order/higher-order dichotomy, we consider the role of the thalamic reticular nucleus from directed attention to cognition. We next summarize research relying on Pavlovian learning paradigms, showing that both first-order and higher-order thalamic nuclei contribute to associative learning. Finally, we propose that modulator inputs reaching all thalamic nuclei may be critical for integrative purposes when environmental signals are computed. Altogether, the thalamus appears as the bridge linking perception, cognition and possibly affect.

Descending Modulation of Laryngeal Vagal Sensory Processing in the Brainstem Orchestrated by the Submedius Thalamic Nucleus

The nodose and jugular vagal ganglia supply sensory innervation to the airways and lungs. Jugular vagal airway sensory neurons wire into a brainstem circuit with ascending projections into the submedius thalamic nucleus (SubM) and ventrolateral orbital cortex (VLO), regions known to regulate the endogenous analgesia system. Here we investigate whether the SubM-VLO circuit exerts descending regulation over airway vagal reflexes in male and female rats using a range of neuroanatomical tracing, reflex physiology, and chemogenetic techniques. Anterograde and retrograde neuroanatomical tracing confirmed the connectivity of the SubM and VLO. Laryngeal stimulation in anesthetized rats reduced respiration, a reflex that was potently inhibited by activation of SubM. Conversely, inhibition of SubM potentiated laryngeal reflex responses, while prior lesions of VLO abolished the effects of SubM stimulation. In conscious rats, selective chemogenetic activation of SubM neurons specifically projecting to VLO significantly inhibited respiratory responses evoked by inhalation of the nociceptor stimulant capsaicin. Jugular vagal inputs to SubM via the medullary paratrigeminal nucleus were confirmed using anterograde transsynaptic conditional herpes viral tracing. Respiratory responses evoked by microinjections of capsaicin into the paratrigeminal nucleus were significantly attenuated by SubM stimulation, whereas those evoked via the nucleus of the solitary tract were unaltered. These data suggest that jugular vagal sensory pathways input to a nociceptive thalamocortical circuit capable of regulating jugular sensory processing in the medulla. This circuit organization suggests an intersection between vagal sensory pathways and the endogenous analgesia system, potentially important for understanding vagal sensory processing in health and mechanisms of hypersensitivity in disease.SIGNIFICANCE STATEMENT Jugular vagal sensory pathways are increasingly recognized for their important role in defensive respiratory responses evoked from the airways. Jugular ganglia neurons wire into a central circuit that is notable for overlapping with somatosensory processing networks in the brain rather than the viscerosensory circuits in receipt of inputs from the nodose vagal ganglia. Here we demonstrate a novel and functionally relevant example of intersection between vagal and somatosensory processing in the brain. The findings of the study offer new insights into interactions between vagal and spinal sensory processing, including the medullary targets of the endogenous analgesia system, and offer new insights into the central processes involved in airway defense in health and disease.

Activation of ventrolateral orbital cortex improves mouse neuropathic pain-induced anxiodepression

Depression and anxiety are frequently observed in patients suffering from neuropathic pain. The underlying mechanisms remained unclear. The ventrolateral orbital cortex (VLO) has attracted considerable interest in its role in antidepressive effect in rodents. In the present study, we further investigated the role of the VLO in the anxiodepressive consequences of neuropathic pain in a chronic constriction injury of infraorbital nerve-induced trigeminal neuralgia (TN) mouse model. Elevated plus maze, open field, forced swimming, tail suspension, and sucrose preference tests were used to evaluate anxiodepressive-like behaviors. The results show that chemogenetic activation of bilateral VLO neurons, especially CaMK2A+ pyramidal neurons, blocked the TN-induced anxiodepressive-like behaviors. Chemogenetic and optogenetic activation of VGLUT2+ or inhibition of VGAT+ VLO neurons was sufficient to produce an antianxiodepressive effect in TN mice. Pharmacological activation of D1-like receptors (D1Rs) but not D2Rs in the VLO significantly alleviated TN-induced depressive-like behaviors. Electrophysiological recordings revealed a decreased excitability of VLO excitatory neurons following neuropathic pain. Furthermore, activation of submedius thalamic nucleus-VLO (Sm-VLO) projection mimicked the antianxiodepressive effect of VLO excitation. Conversely, activation of VLO-periaqueductal gray matter (PAG) projection had no effect on TN-induced anxiodepressive behaviors. This study provides a potentially novel mechanism-based therapeutic strategy for the anxiodepressive consequences of neuropathic pain.

Thalamic Input to Orbitofrontal Cortex Drives Brain-wide, Frequency-Dependent Inhibition Mediated by GABA and Zona Incerta

Anatomical and behavioral data suggest that the ventrolateral orbitofrontal cortex (VLO), which exhibits extensive connectivity and supports diverse sensory and cognitive processes, may exert global influence over brain activity. However, this hypothesis has never been tested directly. We applied optogenetic fMRI to drive various elements of VLO circuitry while visualizing the whole-brain response. Surprisingly, driving excitatory thalamocortical projections to VLO at low frequencies (5-10 Hz) evoked widespread, bilateral decreases in brain activity spanning multiple cortical and subcortical structures. This pattern was unique to thalamocortical projections, with direct stimulations of neither VLO nor thalamus eliciting such a response. High-frequency stimulations (25-40 Hz) of thalamocortical projections evoked dramatically different-though still far-reaching-responses, in the form of widespread ipsilateral activation. Importantly, decreases in brain activity evoked by low-frequency thalamocortical input were mediated by GABA and activity in zona incerta. These findings identify specific circuit mechanisms underlying VLO control of brain-wide neural activities.

A thalamocortical circuit for updating action-outcome associations

The ability to flexibly use knowledge is one cardinal feature of goal-directed behaviors. We recently showed that thalamocortical and corticothalamic pathways connecting the medial prefrontal cortex and the mediodorsal thalamus (MD) contribute to adaptive decision-making (Alcaraz et al., 2018). In this study, we examined the impact of disconnecting the MD from its other main cortical target, the orbitofrontal cortex (OFC) in a task assessing outcome devaluation after initial instrumental training and after reversal of action-outcome contingencies. Crossed MD and OFC lesions did not impair instrumental performance. Using the same approach, we found however that disconnecting the OFC from its other main thalamic afferent, the submedius nucleus, produced a specific impairment in adaptive responding following action-outcome reversal. Altogether, this suggests that multiple thalamocortical circuits may act synergistically to achieve behaviorally relevant functions.

The Cognitive Thalamus as a Gateway to Mental Representations

Historically, the thalamus has been viewed as little more than a relay, simply transferring information to key players of the cast, the cortex and hippocampus, without providing any unique functional contribution. In recent years, evidence from multiple laboratories researching different thalamic nuclei has contradicted this idea of the thalamus as a passive structure. Dated models of thalamic functions are being pushed aside, revealing a greater and far more complex contribution of the thalamus for cognition. In this Viewpoints article, we show how recent data support novel views of thalamic functions that emphasize integrative roles in cognition, ranging from learning and memory to flexible adaption. We propose that these apparently separate cognitive functions may indeed be supported by a more general role in shaping mental representations. Several features of thalamocortical circuits are consistent with this suggested role, and we highlight how divergent and convergent thalamocortical and corticothalamic pathways may complement each other to support these functions. Furthermore, the role of the thalamus for subcortical integration is highlighted as a key mechanism for maintaining and updating representations. Finally, we discuss future areas of research and stress the importance of incorporating new experimental findings into existing knowledge to continue developing thalamic models. The presence of thalamic pathology in a number of neurological conditions reinforces the need to better understand the role of this region in cognition.

The orbitofrontal cortex projects to the parvafox nucleus of the ventrolateral hypothalamus and to its targets in the ventromedial periaqueductal grey matter

Although connections between the orbitofrontal cortex (OFC)-the seat of high cognitive functions-the lateral hypothalamus and the periaqueductal grey (PAG) have been recognized in the past, the precise targets of the descending fibres have not been identified. In the present study, viral tracer-transport experiments revealed neurons of the lateral (LO) and the ventrolateral (VLO) OFC (homologous to part of Area 13 in primates) to project to a circumscribed region in the ventrolateral hypothalamus, namely, the horizontally oriented, cylindrical parvalbumin- and Foxb1-expressing (parvafox) nucleus. The fine collaterals stem from coarse axons in the internal capsule and form excitatory synapses specifically with neurons of the parvafox nucleus, avoiding the rest of the hypothalamus. In its further caudal course, this contingent of LO/VLO-axons projects collaterals to the Su3- and the PV2 nuclei, which lie ventral to the aqueduct in the (PAG), where the terminals fields overlap those deriving from the parvafox nucleus itself. The targeting of the parvafox nucleus by the LO/VLO-projections, and the overlapping of their terminal fields within the PAG, suggest that the two cerebral sites interact closely. An involvement of this LO/VLO-driven circuit in the somatic manifestation of behavioural events is conceivable.

Thalamo-insular pathway conveying orofacial muscle proprioception in the rat

Little is known about how proprioceptive signals arising from muscles reach to higher brain regions such as the cerebral cortex. We have recently shown that a particular thalamic region, the caudo-ventromedial edge (VPMcvm) of ventral posteromedial thalamic nucleus (VPM), receives the proprioceptive signals from jaw-closing muscle spindles (JCMSs) in rats. In this study, we further addressed how the orofacial thalamic inputs from the JCMSs were transmitted from the thalamus (VPMcvm) to the cerebral cortex in rats. Injections of a retrograde and anterograde neuronal tracer, wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), into the VPMcvm demonstrated that the thalamic pathway terminated mainly in a rostrocaudally narrow area in the dorsal part of granular insular cortex rostroventrally adjacent to the rostralmost part of the secondary somatosensory cortex (dGIrvs2). We also electrophysiologically confirmed that the dGIrvs2 received the proprioceptive inputs from JCMSs. To support the anatomical evidence of the VPMcvm-dGIrvs2 pathway, injections of a retrograde neuronal tracer Fluorogold into the dGIrvs2 demonstrated that the thalamic neurons projecting to the dGIrvs2 were confined in the VPMcvm and the parvicellular part of ventral posterior nucleus. In contrast, WGA-HRP injections into the lingual nerve area of core VPM demonstrated that axon terminals were mainly labeled in the core regions of the primary and secondary somatosensory cortices, which were far from the dGIrvs2. These results suggest that the dGIrvs2 is a specialized cortical region receiving the orofacial proprioceptive inputs. Functional contribution of the revealed JCMSs-VPMcvm-dGIrvs2 pathway to Tourette syndrome is also discussed.

A neuroanatomical framework for the central modulation of respiratory sensory processing and cough by the periaqueductal grey

Sensory information arising from the airways is processed in a distributed brain network that encodes for the discriminative and affective components of the resultant sensations. These higher brain networks in turn regulate descending motor control circuits that can both promote or suppress behavioural responses. Here we explore the existence of possible descending neural control pathways that regulate airway afferent processing in the brainstem, analogous to the endogenous descending analgesia system described for noxious somatosensation processing and placebo analgesia. A key component of this circuitry is the midbrain periaqueductal grey, a region of the brainstem recently highlighted for its altered activity in patients with chronic cough. Understanding the nature and plasticity of descending neural control may help identify novel central therapeutic targets to alleviate the neuronal hypersensitivity underpinning many symptoms of respiratory disease.

Decreased spontaneous activity and altered evoked nociceptive response of rat thalamic submedius neurons to lumbar vertebra thrust

The thalamus is a central structure important to modulating and processing all mechanoreceptor input destined for the cortex. A large number of diverse mechanoreceptor endings are stimulated when a high velocity low amplitude thrust is delivered to the lumbar spine during spinal manipulation. The objective of this study was to determine if a lumbar thrust alters spontaneous and/or evoked nociceptive activity in medial thalamic submedius (Sm) neurons. Extracellular recordings were obtained from 94 thalamic Sm neurons in 54 urethane-anesthetized adult Wistar rats. Spontaneous activity was recorded 5 min before and after an L₅ control (no thrust) and thrust (85% rat body weight; 100 ms) procedure. In a subset of responsive nociceptive-specific neurons, mean changes in noxious-evoked response (10-s pinch with clip; 795 g) at three sites (tail, contra- and ipsilateral hindpaw) were determined following an L₅ thrust. Mean changes in Sm spontaneous activity (60 s bins) and evoked noxious response were compared using a mixed model repeated measures ANOVA with Bonferroni post hoc t tests and paired t tests, respectively. Compared to control, spontaneous Sm activity decreased 180-240 s following the lumbar thrust (p < 0.005). Inhibitory evoked responses were attenuated in the contralateral hindpaw following an L₅ thrust compared to control (p < 0.05). No other changes in spontaneous or noxious-evoked Sm activity were found. A delayed, but prolonged suppression of spontaneous Sm activity along with changes in noxious-evoked inhibitory responses in the contralateral hindpaw following lumbar vertebra thrust suggest that thalamic submedius neurons may play a role in central pain modulation related to manual therapy intervention.

Parallel inputs from the mediodorsal thalamus to the prefrontal cortex in the rat

There is a growing interest in determining the functional contribution of thalamic inputs to cortical functions. In the context of adaptive behaviours, identifying the precise role of the mediodorsal thalamus (MD) in particular remains difficult despite the large amount of experimental data available. A better understanding of the thalamocortical connectivity of this region may help to capture its functional role. To address this issue, this study focused exclusively on the specific connections from the MD to the prefrontal cortex (PFC) by means of direct comparisons of labelling produced by single and dual injections of retrograde tracers in the different subdivisions of the PFC in the rat. We show that at least three parallel and essentially separate thalamocortical pathways originate from the MD, as follows: projections to the dorsal (1) and the ventral (2) subdivisions of the mPFC follow a mediolateral topography at the thalamic level (i.e. medial thalamic neurons target the mPFC ventrally whereas lateral thalamic neurons project dorsally), whereas a considerable innervation to the OFC (3) includes thalamic cells projecting to both the lateral and the ventral OFC subdivisions. These observations provide new insight on the functions of the MD and suggest a specific focus on each of these pathways for future functional studies.

Activation of α1 adrenoceptors in ventrolateral orbital cortex attenuates allodynia induced by spared nerve injury in rats

Recent studies have demonstrated that noradrenaline acting in the ventrolateral orbital cortex (VLO) can potentially reduce allodynia induced by spared nerve injury (SNI), and this effect is mediated by α2 adrenoceptor. The present study examined the effect of the α1 adrenoceptors in the VLO on allodynia induced by SNI in the rats. The mechanical paw withdrawal threshold (PWT) was measured using von-Frey filaments. Microinjection of selective α1 adrenoceptor agonist methoxamine (20, 50, 100 μg in 0.5 μl) into the VLO, contralateral to the site of nerve injury, increased PWT in a dose-dependent manner. This effect was antagonized by pre-microinjection of the selective α1 adrenoceptor antagonist benoxathian into the same VLO site, and blocked by electrolytic lesion of the ventrolateral periaqueductal gray (PAG). Furthermore, pre-administration of non-selective glutamate receptor antagonist kynurenic acid, phospholipase C (PLC) inhibitor U73122, and protein kinase C (PKC) inhibitor chelerythrine to the VLO also blocked methoxamine-induced inhibition of allodynia. These results suggest that activation of α1 adrenoceptors in the VLO can potentially reduce allodynia induced by SNI. This effect may be direct excitation of the VLO neurons, via PLC-PKC signaling pathway, projecting to the PAG or facilitating glutamate release and then indirectly exciting the VLO output neurons projecting to the PAG, leading to activation of the PAG-brainstem descending inhibitory system which depresses the nociceptive transmission at the spinal cord level.

Flexible Use of Predictive Cues beyond the Orbitofrontal Cortex: Role of the Submedius Thalamic Nucleus

The orbitofrontal cortex (OFC) is known to play a crucial role in learning the consequences of specific events. However, the contribution of OFC thalamic inputs to these processes is largely unknown. Using a tract-tracing approach, we first demonstrated that the submedius nucleus (Sub) shares extensive reciprocal connections with the OFC. We then compared the effects of excitotoxic lesions of the Sub or the OFC on the ability of rats to use outcome identity to direct responding. We found that neither OFC nor Sub lesions interfered with the basic differential outcomes effect. However, more specific tests revealed that OFC rats, but not Sub rats, were disproportionally relying on the outcome, rather than on the discriminative stimulus, to guide behavior, which is consistent with the view that the OFC integrates information about predictive cues. In subsequent experiments using a Pavlovian contingency degradation procedure, we found that both OFC and Sub lesions produced a severe deficit in the ability to update Pavlovian associations. Altogether, the submedius therefore appears as a functionally relevant thalamic component in a circuit dedicated to the integration of predictive cues to guide behavior, previously conceived as essentially dependent on orbitofrontal functions. Significance statement: In the present study, we identify a largely unknown thalamic region, the submedius nucleus, as a new functionally relevant component in a circuit supporting the flexible use of predictive cues. Such abilities were previously conceived as largely dependent on the orbitofrontal cortex. Interestingly, this echoes recent findings in the field showing, in research involving an instrumental setup, an additional involvement of another thalamic nuclei, the parafascicular nucleus, when correct responding requires an element of flexibility (Bradfield et al., 2013a). Therefore, the present contribution supports the emerging view that limbic thalamic nuclei may contribute critically to adaptive responding when an element of flexibility is required after the establishment of initial learning.

IL-10 and IL-1β mediate neuropathic-pain like behavior in the ventrolateral orbital cortex

Previous evidence has shown that the glial cells can be activated by peripheral nerve injury and release both pro-inflammatory and anti-inflammatory cytokines, which play crucial roles in the establishment and maintenance of neuropathic pain. The present study examined the roles of anti-inflammatory cytokine IL-10 and pro-inflammatory IL-1β on allodynia induced by spared nerve injury (SNI) in the ventrolateral orbital cortex (VLO) in the rat. The mechanical paw withdrawal threshold (PWT) was measured using von-Frey filaments. Microinjection of IL-10 (0.1, 0.5, 1 μg/0.5 μl) into the VLO, contralateral to the site of nerve injury attenuated allodynia; PWT increased in a dose-dependent manner. Similar to IL-10, administration of rabbit anti-rat IL-1β antibody (0.1, 1.0 and 10 ng/0.5 μl) into the same VLO site also alleviated allodynia with a dose-dependent fashion. Moreover, western blotting results showed expression levels of IL-10 and IL-1β significantly up-regulated in the contralateral VLO of SNI rats as compared with that of sham-operated rats. These results suggest that anti-inflammatory cytokine IL-10 and pro-inflammatory cytokine IL-1β mediate neuropathic-pain like behavior at the cerebral cortex level; IL-10 released from activated glial cells in the VLO can potentially attenuate allodynia while IL-1β released from activated glial cells in the VLO can potentially maintain or facilitate allodynia. These results provide new insights and site for therapy at the cerebral cortex level in neuropathic pain condition.

Guided delivery of adeno-associated viral vectors into the primate brain

In this review, we discuss recent developments in the delivery of adeno-associated virus-based vectors (AAV), particularly with respect to the role of axonal transport in vector distribution in the brain. The use of MRI-guidance and new stereotactic aiming devices have now established a strong foundation for neurological gene therapy to become an accepted procedure in interventional neurology.

Transneuronal tracing of airways-related sensory circuitry using herpes simplex virus 1, strain H129

Sensory input from the airways to suprapontine brain regions contributes to respiratory sensations and the regulation of respiratory function. However, relatively little is known about the central organization of this higher brain circuitry. We exploited the properties of the H129 strain of herpes simplex virus 1 (HSV-1) to perform anterograde transneuronal tracing of the central projections of airway afferent nerve pathways. The extrathoracic trachea in Sprague-Dawley rats was inoculated with HSV-1 H129, and tissues along the neuraxis were processed for HSV-1 immunoreactivity. H129 infection appeared in the vagal sensory ganglia within 24 h and the number of infected cells peaked at 72 h. Brainstem nuclei, including the nucleus of the solitary tract and trigeminal sensory nuclei were infected within 48 h, and within 96 h infected cells were evident within the pons (lateral and medial parabrachial nuclei), thalamus (ventral posteromedial, ventral posterolateral, submedius, and reticular nuclei), hypothalamus (paraventricular and lateral nuclei), subthalamus (zona incerta), and amygdala (central and anterior amygdala area). At later times H129 was detected in cortical forebrain regions including the insular, orbital, cingulate, and somatosensory cortices. Vagotomy significantly reduced the number of infected cells within vagal sensory nuclei in the brainstem, confirming the main pathway of viral transport is through the vagus nerves. Sympathetic postganglionic neurons in the stellate and superior cervical ganglia were infected by 72 h, however, there was no evidence for retrograde transynaptic movement of the virus in sympathetic pathways in the central nervous system (CNS). These data demonstrate the organization of key structures within the CNS that receive afferent projections from the extrathoracic airways that likely play a role in the perception of airway sensations.

The mouse olfactory peduncle

The olfactory peduncle, the region connecting the olfactory bulb with the basal forebrain, contains several neural areas that have received relatively little attention. The present work includes studies that provide an overview of the region in the mouse. An analysis of cell soma size in pars principalis (pP) of the anterior olfactory nucleus (AON) revealed considerable differences in tissue organization between mice and rats. An unbiased stereological study of neuron number in the cell-dense regions of pars externa (pE) and pP of the AON of 3-, 12-, and 24-month-old mice indicated that pE has about 16,500 cells in 0.043 mm(3) and pP about 58,300 cells in 0.307 mm(3) . Quantitative Golgi studies of pyramidal neurons in pP suggested that mouse neurons are similar to although smaller than those of the rat. An immunohistochemical analysis demonstrated that all peduncular regions (pE, pP, the dorsal peduncular cortex, ventral tenia tecta, and anterior olfactory tubercle and piriform cortex) have cells that express either calbindin, calretinin, parvalbumin, somatostatin, vasoactive intestinal polypeptide, neuropeptide Y, or cholecystokinin (antigens commonly co-expressed by subspecies of γ-aminobutyric acid [GABA]ergic neurons), although the relative numbers of each cell type differ between zones. Finally, an electron microscopic comparison of the organization of myelinated fibers in lateral olfactory tract in the anterior and posterior peduncle indicated that the region is less orderly in mice than in rats. The results provide a caveat for investigators who generalize data between species, as both similarities and differences between the laboratory mouse and rat were observed.

Ascending multisynaptic pathways from the trigeminal ganglion to the anterior cingulate cortex

By means of retrograde transneuronal transport of rabies virus, ascending multisynaptic pathways from the trigeminal ganglion (TG) to the anterior cingulate cortex (ACC) were identified in the rat. After rabies injection into an electrophysiologically defined trigeminal projection region of the ACC, transsynaptic labeling of second-order neurons via the medial thalamus (including the parafascicular nucleus) was located in the spinal trigeminal nucleus pars caudalis. Third-order neuron labeling occurred in the TG. Most of these TG neurons were medium- or large-sized cells giving rise to myelinated Aδ or Aβ afferent fibers, respectively. By contrast, TG neurons labeled transsynaptically from the orofacial region of the primary somatosensory cortex contained many small cells associated with unmyelinated C afferent fibers. Furthermore, the TG neurons retrogradely labeled with fluorogold injected into the mental nerve were smaller in their sizes compared to those labeled with rabies. Our extracellular unit recordings revealed that a majority of ACC neurons responded to trigeminal nerve stimulation with latencies of shorter than 20ms. Thus, somatosensory information conveyed to the ACC by multisynaptic ascending pathways derived predominantly from myelinated primary afferents (i.e., the medial nociceptive system) and may be used to subserve affective-motivational aspects of pain. Lack of overlap with the lateral nociceptive system is notable and suggests that the medial and lateral nociceptive systems perform separate and non-overlapping functions.

Cerebral cortex modulation of pain

Pain is a complex experience encompassing sensory-discriminative, affective-motivational and cognitiv e-emotional components mediated by different mechanisms. Contrary to the traditional view that the cerebral cortex is not involved in pain perception, an extensive cortical network associated with pain processing has been revealed using multiple methods over the past decades. This network consistently includes, at least, the anterior cingulate cortex, the agranular insular cortex, the primary (SI) and secondary somatosensory (SII) cortices, the ventrolateral orbital cortex and the motor cortex. These cortical structures constitute the medial and lateral pain systems, the nucleus submedius-ventrolateral orbital cortex-periaqueductal gray system and motor cortex system, respectively. Multiple neurotransmitters, including opioid, glutamate, GABA and dopamine, are involved in the modulation of pain by these cortical structures. In addition, glial cells may also be involved in cortical modulation of pain and serve as one target for pain management research. This review discusses recent studies of pain modulation by these cerebral cortical structures in animals and human.

Tracing thalamo-cortical connections in tenrecA further attempt to characterize poorly differentiated neocortical regions, particularly the motor cortex

The hedgehog tenrec (Afrosoricidae) has a very poorly differentiated neocortex. Previously its primary sensory regions have been characterized with hodological and electrophysiological techniques. Unlike the marsupial opossum the tenrec may also have a separate motor area as far as there are cortico-spinal cells located rostral to the primary somatosensory cortex. However, not knowing its thalamic input it may be premature to correlate this area with the true (mirror-image-like) primary motor cortex in higher mammals. For this reason the tenrec's thalamo-cortical connections were studied following tracer injections into various neocortical regions. The main sensory areas were confirmed by their afferents from the principal thalamic nuclei. The dorsal lateral geniculate nucleus, in addition, was connected with the retrosplenial area and a rostromedial visual region. Unlike the somatosensory cortex the presumed motor area did not receive afferents from the ventrobasal thalamus but fibers from the cerebello-thalamic target regions. These projections, however, were not restricted to the motor area, but involved the entire somatosensorimotor field as well as adjacent regions. The projections appeared similar to those arising in the rat thalamic ventromedial nucleus known to have a supporting function rather than a specific motor task. The question was raised whether the input from the basal ganglia might play a crucial role in the evolution of the mammalian motor cortex? Certainly, in the tenrec, the poor differentiation of the motor cortex coincides with the virtual absence of an entopeduncular projection to the ventrolateral thalamus.

Synaptic connections between GABAergic elements and serotonergic terminals or projecting neurons in the ventrolateral orbital cortex

The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system, consisting of the spinal cord-thalamic nucleus submedius-VLO periaqueductal gray (PAG)-spinal cord loop. The present study examined morphological connections of GABAergic (gamma-aminobutyric acidergic) neurons and serotonergic projection terminals from the dorsal raphe nucleus (DR), as well as the relationship between GABAergic terminals and VLO neurons projecting to the PAG, by using anterograde and retrograde tracing combined with immunofluorescence, immunohistochemistry, and electron microscopy methods. Results indicate that the majority (93%) of GABAergic neurons in the VLO also express the 5-HT(1A) (5-hydroxytryptamine 1A) receptor, and serotonergic terminals originating from the DR nucleus made symmetrical synapses with GABAergic neuronal cell bodies and dendrites within the VLO. GABAergic terminals also made symmetrical synapses with neurons expressing GABA(A) receptors and projecting to the PAG. These results suggest that a local neuronal circuit, consisting of 5-HTergic terminals, GABAergic interneurons, and projection neurons, exists in the VLO, and provides morphological evidence for the hypothesis that GABAergic modulation is involved in 5-HT(1A) receptor activation-evoked antinociception.

GABAergic modulation is involved in the ventrolateral orbital cortex 5-HT 1A receptor activation-induced antinociception in the rat

The ventrolateral orbital cortex (VLO) is a component of an endogenous analgesic system consisting of an ascending pathway from the spinal cord to VLO via the thalamic nucleus submedius (Sm) and a descending pathway relaying in the periaqueductal gray matter (PAG). This study examines whether the activation of 5-HT 1A receptors in VLO produces antinociception and whether GABAergic modulation is involved in the VLO 5-HT 1A receptor activation-evoked antinociception. The radiant heat-evoked tail flick (TF) reflex was used as an index of nociceptive response in lightly anesthetized rats. Microinjection of the 5-HT 1A receptor agonist 8-OH-DPAT (1.0, 2.0, 5.0 microg) into VLO produced dose-dependent antinociception, which was reversed by the 5-HT 1A receptor antagonist (NAN-190, 20 mug). We also found that VLO application of the GABA A receptor antagonist bicuculline or picrotoxin (100 ng) enhanced the 8-OH-DPAT-induced inhibition of the TF reflex, whereas the GABA A receptor agonist muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the 8-OH-DPAT-induced inhibition. These results suggest that 5-HT 1A receptors are involved in VLO-induced antinociception and that GABAergic disinhibitory mechanisms participate in the 5-HT 1A receptor-mediated effect. These findings provide support for the hypothesis that 5-HT 1A receptor activation may inhibit the inhibitory action of the GABAergic interneurons on the output neurons projecting to PAG leading to activation of the brainstem descending inhibitory system and depression of nociceptive inputs at the spinal cord level.

The role of 5-HT receptor subtypes in the ventrolateral orbital cortex of 5-HT-induced antinociception in the rat

The present study examined the involvement of 5-HT in the ventrolateral orbital cortex (VLO) on descending antinociception and determined which subtypes of 5-HT receptors mediated this effect. This study focused on the effects of 5-HT microinjection in the VLO of lightly anesthetized male rats on the radiant heat-evoked tail flick (TF) reflex, as well as the influence of 5-HT(1A), 5-HT(2), 5-HT(3), and 5-HT(4) receptor subtype antagonists on the effect of 5-HT. Results showed that 5-HT microinjection (2, 5, 10 microg, in 0.5 microl) into the VLO depressed the TF reflex in a dose-dependent manner. Pretreatment with 5-HT receptor antagonists (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide (NAN-190), cyproheptadine hydrochloride (CPT) and 1-methyl-N-(8-methyl-8-azabicyclo[3.2.3]-oct-3-yl)-1H-indazole-3-carboxamide maleate salt (LY-278,584)), specific for 5-HT(1A), 5-HT(2) and 5-HT(3) receptors, respectively, partially reversed the 5-HT-evoked inhibition. In contrast, the 5-HT(4) receptor antagonist, 1-[2-[(methylsulfonyl)-amino]ethyl]-4-piperidinyl]methyl1-methyl-1H-indole-3-carboxylate (GR 113808), had no effect on the inhibition of 5-HT. Microinjections of NAN-190, CPT and LY-278,584 alone into the VLO had no effect on the TF reflex. These results suggest that 5-HT(1A), 5-HT(2) and 5-HT(3), but not 5-HT(4) receptors, are involved in mediating 5-HT-induced antinociception in the VLO. According to different properties and distribution patterns of the 5-HT receptor subtypes on neurons, the possible mechanism of 5-HT activation of the VLO-periaqueductal gray (PAG) descending antinociceptive pathway is discussed.

Roles of different subtypes of opioid receptors in mediating the ventrolateral orbital cortex opioid-induced inhibition of mirror-neuropathic pain in the rat

Previous studies have demonstrated that opioid receptors in the prefrontal ventrolateral orbital cortex (VLO) are involved in anti-nociception. The aim of this current study was to examine whether opioid receptors in the VLO have effects on the hypersensitivity induced by contralateral L5 and L6 spinal nerve ligation (SNL), termed as mirror neuropathic pain (MNP) in the male rat. Morphine (1.0, 2.5, 5.0 microg) microinjected into the VLO contralateral to the SNL depressed the mechanical paw withdrawal assessed by von Frey filaments and the cold plate (4 degrees C)-induced paw lifting in a dose-dependent manner on the side without SNL. These effects were antagonized by microinjection of the non-selective opioid receptor antagonist naloxone (1.0 mug) into the same VLO site. Microinjection of endomorphin-1 (5.0 microg), a highly selective mu-opioid receptor agonist, and [d-Ala(2), d-Leu(5)]-enkephalin (DADLE, 10 microg), a delta-/mu-receptor agonist, also depressed the MNP. The effects of both drugs were blocked by selective mu-receptor antagonist beta-funaltrexamine (beta-FNA, 3.75 microg), but the effect of the DADLE was not influenced by the selective delta-receptor antagonist naltrindole (5.0 microg). Microinjection of the kappa-opioid receptor agonist spiradoline mesylate salt (U-62066) (100 microg) had no effect on the MNP. These results suggest that the VLO is involved in opioid-induced inhibition of the MNP and the effect is mediated by mu- (but not delta- and kappa-) opioid receptors.

Reversible attenuation of neuropathic-like manifestations in rats by lesions or local blocks of the intralaminar or the medial thalamic nuclei

BACKGROUND AND AIM

Thalamic somatosensory nuclei have been classified into medial and lateral systems based on their role in nociception. An imbalance between these two systems may result in abnormal somatic sensations and spontaneous pain. This study aims to investigate the effects of transient or permanent block of the medial and intralaminar nuclear groups on the neuropathic-like behavior in a rat model for mononeuropathy.

METHODS

Neuropathy was induced on one hind paw in different groups of rats following the spared nerve injury model. When the resulting hyperalgesia and allodynia (tactile and cold) reached a maximum plateau, the rats received either chemical or electrolytic lesion or lidocaine (2%) microperfusion, placed in the various thalamic nuclear groups.

RESULTS

All procedures produced transient but significant decrease of neuropathic manifestations. The magnitude and duration of decrease depended on the type and the site of the block. These effects can be ranked in increasing order as follows, electrolytic<chemical<lidocaine micro-perfusion according to the procedure, and as rostro-medial<ventro-median<parafascicular nuclei, according to the site of the block. Thermal hyperalgesia was the most affected while cold allodynia showed the least attenuation. Neuropathic manifestations returned to their pre-lesion levels after 2-3 weeks, along with frequently observed delayed hyper-responsiveness to the hotplate test.

CONCLUSION

The observed results demonstrate the involvement of the medial and intralaminar thalamic nuclei in the processing of neuropathic-like manifestations, and the reversibility of the effects suggests the flexibility of the neural network involved in supraspinal processing of nociceptive information.

μ- but not δ- and κ-opioid receptors in the ventrolateral orbital cortex mediate opioid-induced antiallodynia in a rat neuropathic pain model

Previous studies have indicated that the ventrolateral orbital cortex (VLO) is involved in opioid-mediated antinociception in the tail flick test and formalin test. The aim of the current study was to examine the effect of opioids microinjected into the VLO on allodynia in the rat L5/L6 spinal nerve ligation (SNL) model of neuropathic pain and determine the roles of different subtypes of opioid receptors in this effect. The allodynia was assessed by both mechanical (von Frey filaments) and cold plate (4 degrees C) stimuli. Morphine (1.0, 2.5, and 5.0 microg) microinjected into the VLO contralateral to the nerve ligation dose-dependently depressed the mechanical and cold allodynia and these effects were reversed by nonselective opioid receptor antagonist naloxone (1.0 microg) administrated into the same site. Microinjection of endomorphin-1 (5.0 microg), a highly selective mu-opioid receptor agonist, and [D-Ala2, D-Leu5]-enkephalin (DADLE, 10 microg), a delta-/mu-opioid receptor agonist, also depressed the allodynia, and the effects of both drugs were blocked by selective mu-receptor antagonist beta-funaltrexamine (beta-FNA, 3.75 microg), but the effects of DADLE were not influenced by the selective delta-receptor antagonist naltrindole (5.0 microg). Microinjection of U-62066 (100 microg), a kappa-opioid receptor agonist, into the VLO had no effect on the allodynia. These results suggest that the VLO is involved in opioid-induced antiallodynia and mu- but not delta- and kappa-opioid receptor mediates these effects in the rat with neuropathic pain.

Involvement of GABAergic modulation of antinociception induced by morphine microinjected into the ventrolateral orbital cortex

Previous studies have shown that microinjection of morphine into the prefrontal ventrolateral orbital cortex (VLO) produces antinociception. The current study examined whether gamma-aminobutyric acid (GABA) containing neurons in the VLO were involved in this antinociception. Under light anesthesia, the GABA(A) receptor antagonist bicuculline and picrotoxin or agonist muscimol and THIP was microinjected into the VLO in non-morphine-treated (control) and morphine-treated (microinjection into the VLO) rats. Noxious heat-evoked tail flick (TF) latencies (TFLs) were measured in all of these groups of rats every 5 min. Bicuculline or picrotoxin (100, 200, 500 ng in 0.5 microl) depressed the TF reflex in a dose-related fashion. A smaller dose (100 ng) of bicuculline or picrotoxin microinjected into VLO significantly enhanced the VLO morphine-evoked inhibition of the TF reflex. In contrast, administration of muscimol (250 ng) or THIP (1.0 microg) significantly attenuated the morphine-induced antinociception in the VLO morphine-treated rats. These results suggest that the GABA(A) receptor is involved in the modulation of VLO morphine-induced antinociception, and provide a behavioral support for the hypothesis that morphine may directly inhibit the GABAergic inhibitory interneurons leading to indirect activation of the descending antinociceptive pathway through a disinhibitory effect on the VLO output neurons and depression of the nociceptive inputs at the spinal cord level.

The roles of different subtypes of opioid receptors in mediating the nucleus submedius opioid-evoked antiallodynia in a neuropathic pain model of rats

Previous studies have indicated that thalamic nucleus submedius is involved in opioid-mediated antinociception in tail flick test and formalin test. The current study examined the effects of opioids microinjected into the thalamic nucleus submedius on the allodynia developed in neuropathic pain model rats, and determined the roles of different subtypes of opioid receptors in the thalamic nucleus submedius opioid-evoked antiallodynia. The allodynic behaviors induced by L5/L6 spinal nerve ligation were assessed by mechanical (von Frey filaments) and cold (4 degrees C plate) stimuli. Morphine (1.0, 2.5, and 5.0 microg) microinjected into the thalamic nucleus submedius contralateral to the nerve injury paw produced a dose-dependent inhibition of the mechanical and cold allodynia, and these effects were reversed by microinjection of the non-selective opioid receptor antagonist naloxone (1.0 microg) into the same site. Microinjection of endomorphin-1 (5.0 microg), a highly selective mu-opioid receptor agonist, and [D-Ala2, D-Leu5]-enkephalin (10 microg), a delta-/mu-opioid receptor agonist, also inhibited the allodynic behaviors, and these effects were blocked by selective mu-opioid receptor antagonist beta-funaltrexamine hydrochloride (3.75 microg). However, the [D-Ala2, D-Leu5]-enkephalin-evoked antiallodynic effects were not influenced by the selective delta-opioid receptor antagonist naltrindole (5.0 microg). Microinjection of the selective kappa-receptor agonist spiradoline mesylate salt (100 microg) into the thalamic nucleus submedius failed to alter the allodynia induced by spinal nerve ligation. These results suggest that the thalamic nucleus submedius is involved in opioid-evoked antiallodynia which is mediated by mu- but not delta- and kappa-opioid receptor in the neuropathic pain model rats.

A field guide to the anterior olfactory nucleus (cortex)

While portions of the mammalian olfactory system have been studied extensively, the anterior olfactory nucleus (AON) has been relatively ignored. Furthermore, the existing research is dispersed and obscured by many different nomenclatures and approaches. The present review collects and assembles the relatively sparse literature regarding the portion of the brain situated between the olfactory bulb and primary olfactory (piriform) cortex. Included is an overview of the area's organization, the functional, morphological and neurochemical characteristics of its cells and a comprehensive appraisal of its efferent and afferent fiber systems. Available evidence suggests the existence of subdivisions within the AON and demonstrates that the structure influences ongoing activity in many other olfactory areas. We conclude with a discussion of the AON's mysterious but complex role in olfactory information processing.

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.

Thalamic nucleus submedius receives GABAergic projection from thalamic reticular nucleus in the rat

GABAergic projection from thalamic reticular nucleus to thalamic nucleus submedius in the medial thalamus of the rat was studied by using immunohistochemistry for GABA, retrograde labeling with Fluoro-Gold combined with immunohistochemistry for GABA, and anterograde labeling with biotinylated dextranamine. Immunohistochemistry displayed that only GABA immunoreactive terminals were observed in the thalamic nucleus submedius, while GABA immunoreactive neuronal cell bodies were located in the thalamic reticular nucleus and lateral geniculate nucleus. Injection of Fluoro-Gold into the thalamic nucleus submedius resulted in massive retrogradely labeled neuronal cell bodies in the rostroventral portion of the ipsilateral thalamic reticular nucleus and a few in the contralateral thalamic reticular nucleus, and most of these cell bodies showed GABA immunopositive staining. Many biotinylated dextranamine anterogradely labeled fibers and terminals in the thalamic nucleus submedius were observed after injection of biotinylated dextranamine into the thalamic reticular nucleus. The present results provide a morphological evidence for a hypothesis that a disinhibitory effect on output neurons elicited by opioid or 5-hydroxytryptamine inhibiting a GABAergic terminal in the thalamic nucleus submedius may lead to activation of the descending inhibitory system and depression of the nociceptive inputs at the spinal cord level.

Afferent projections to nucleus reuniens of the thalamus

The nucleus reuniens (RE) is the largest of the midline nuclei of the thalamus and the major source of thalamic afferents to the hippocampus and parahippocampal structures. Nucleus reuniens has recently been shown to exert powerful excitatory actions on CA1 of the hippocampus. Few reports on any species have examined afferent projections to nucleus reuniens. By using the retrograde anatomical tracer Fluorogold, we examined patterns of afferent projections to RE in the rat. We showed that RE receives a diverse and widely distributed set of afferents projections. The main sources of input to nucleus reuniens were from the orbitomedial, insular, ectorhinal, perirhinal, and retrosplenial cortices; CA1/subiculum of hippocampus; claustrum, tania tecta, lateral septum, substantia innominata, and medial and lateral preoptic nuclei of the basal forebrain; medial nucleus of amygdala; paraventricular and lateral geniculate nuclei of the thalamus; zona incerta; anterior, ventromedial, lateral, posterior, supramammillary, and dorsal premammillary nuclei of the hypothalamus; and ventral tegmental area, periaqueductal gray, medial and posterior pretectal nuclei, superior colliculus, precommissural/commissural nuclei, nucleus of the posterior commissure, parabrachial nucleus, laterodorsal and pedunculopontine tegmental nuclei, nucleus incertus, and dorsal and median raphe nuclei of the brainstem. The present findings of widespread projections to RE, mainly from limbic/limbic-associated structures, suggest that nucleus reuniens represents a critical relay in the transfer of limbic information (emotional/cognitive) from RE to its major targets, namely, to the hippocampus and orbitomedial prefrontal cortex. RE appears to be a major link in the two-way exchange of information between the hippocampus and the medial prefrontal cortex.

Mu but not delta and kappa opioid receptor involvement in ventrolateral orbital cortex opioid-evoked antinociception in formalin test rats

The present study was designed to investigate the roles of different subtypes of opioid receptors in ventrolateral orbital cortex (VLO) opioid-evoked antinociception in formalin test by using an automatic detection system for recording the nociceptive behavior (agitation) and a manual method for detecting the duration of licking the injected paw in the conscious rat. Formalin (5%, 50 microl) s.c. injected into the hindpaw produced a biphasic agitation response or lengthening duration of licking. Morphine (5 microg) microinjected unilaterally into VLO significantly inhibited the agitation response and the licking time, and these effects were blocked by pre-administration of the non-selective opioid receptor antagonist naloxone (1.0 microg) into the same site. Microinjection of endomorphin-1 (5 microg), a selective micro-receptor agonist, and [D-Ala2, D-Leu5]-enkephalin (DADLE, 10 microg), a delta-/micro-receptor agonist also inhibited the nociceptive behaviors, and both the effects were blocked by selective mu-receptor antagonist beta-funaltrexamine hydrochloride (beta-FNA; 3.75 microg), but the DADLE-evoked inhibition was not influenced by the selective delta-receptor antagonist naltrindole (5 microg). Microinjection of selective kappa-receptor agonist (+/-)-trans-U-50488 methanesulfonate salt (1.5 microg) failed to alter the nociceptive behaviors induced by formalin injection. The beta-FNA and naloxone applied into VLO and morphine into the adjacent regions ventral and dorsal to VLO had no effect on the formalin-evoked nociceptive behaviors. These results suggest that mu- but not delta- or kappa-opioid receptor is involved in the VLO opioid-evoked antinociception in formalin test rat.

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.