Polysynaptic Connections Research Articles

Structural Pathways Supporting Swift Acquisition of New Visuomotor Skills.

Human skill learning requires fine-scale coordination of distributed networks of brain regions linked by white matter tracts to allow for effective information transmission. Yet how individual differences in these anatomical pathways may impact individual differences in learning remains far from understood. Here, we test the hypothesis that individual differences in structural organization of networks supporting task performance predict individual differences in the rate at which humans learn a visuomotor skill. Over the course of 6 weeks, 20 healthy adult subjects practiced a discrete sequence production task, learning a sequence of finger movements based on discrete visual cues. We collected structural imaging data, and using deterministic tractography generated structural networks for each participant to identify streamlines connecting cortical and subcortical brain regions. We observed that increased white matter connectivity linking early visual regions was associated with a faster learning rate. Moreover, the strength of multiedge paths between motor and visual modules was also correlated with learning rate, supporting the potential role of extended sets of polysynaptic connections in successful skill acquisition. Our results demonstrate that estimates of anatomical connectivity from white matter microstructure can be used to predict future individual differences in the capacity to learn a new motor–visual skill, and that these predictions are supported both by direct connectivity in visual cortex and indirect connectivity between visual cortex and motor cortex.

Recruitment of Polysynaptic Connections Underlies Functional Recovery of a Neural Circuit after Lesion.

The recruitment of additional neurons to neural circuits often occurs in accordance with changing functional demands. Here we found that synaptic recruitment plays a key role in functional recovery after neural injury. Disconnection of a brain commissure in the nudibranch mollusc, Tritonia diomedea, impairs swimming behavior by eliminating particular synapses in the central pattern generator (CPG) underlying the rhythmic swim motor pattern. However, the CPG functionally recovers within a day after the lesion. The strength of a spared inhibitory synapse within the CPG from Cerebral Neuron 2 (C2) to Ventral Swim Interneuron B (VSI) determines the level of impairment caused by the lesion, which varies among individuals. In addition to this direct synaptic connection, there are polysynaptic connections from C2 and Dorsal Swim Interneurons to VSI that provide indirect excitatory drive but play only minor roles under normal conditions. After disconnecting the pedal commissure (Pedal Nerve 6), the recruitment of polysynaptic excitation became a major source of the excitatory drive to VSI. Moreover, the amount of polysynaptic recruitment, which changed over time, differed among individuals and correlated with the degree of recovery of the swim motor pattern. Thus, functional recovery was mediated by an increase in the magnitude of polysynaptic excitatory drive, compensating for the loss of direct excitation. Since the degree of susceptibility to injury corresponds to existing individual variation in the C2 to VSI synapse, the recovery relied upon the extent to which the network reorganized to incorporate additional synapses.

Unravelling Spinal Circuits of Pain and Mechanical Allodynia

How do spinal circuits mediating tactile sensation and pain get entangled to evoke allodynia, i.e., pain sensation, in response to a normally innocuous stimulus? Recent breakthroughs are now closing this long-standing, critical gap. VGLUT3-expressing neurons and their polysynaptic connectivity to calretinin-expressing neurons are now identified as key determinants of the spinal circuitry underlying mechanical allodynia.

Discrete BDNF Neurons in the Paraventricular Hypothalamus Control Feeding and Energy Expenditure

Brain-derived neurotrophic factor (BDNF) is a key regulator of energy balance; however, its underlying mechanism remains unknown. By analyzing BDNF-expressing neurons in paraventricular hypothalamus (PVH), we have uncovered neural circuits that control energy balance. The Bdnf gene in the PVH was mostly expressed in previously undefined neurons, and its deletion caused hyperphagia, reduced locomotor activity, impaired thermogenesis, and severe obesity. Hyperphagia and reduced locomotor activity were associated with Bdnf deletion in anterior PVH, whereas BDNF neurons in medial and posterior PVH drive thermogenesis by projecting to spinal cord and forming polysynaptic connections to brown adipose tissues. Furthermore, BDNF expression in the PVH was increased in response to cold exposure, and its ablation caused atrophy of sympathetic preganglionic neurons. Thus, BDNF neurons in anterior PVH control energy intake and locomotor activity, whereas those in medial and posterior PVH promote thermogenesis by releasing BDNF into spinal cord to boost sympathetic outflow.

Peripheral and spinal circuits involved in mechanical allodynia.

Sensory information from nociceptors and touch receptors, such as Aβ-fiber mechanoreceptors and C-fiber low-threshold mechanoreceptors (C-LTMRs), is normally relayed and processed by separate neural circuits in the spinal cord.19 After nerve injury or inflammation, however, touch-related information is also relayed to nociceptive circuits in the superficial dorsal horn, which results in touch-evoked pain.16 Here, we depict the spinal circuits that facilitate this modality crosstalk and form the cellular basis of mechanical allodynia. A role of Aβ fibers in allodynia was originally proposed by human studies showing that compression block of Aβ fibers abolishes touch-evoked pain.3,7 Subsequent studies confirmed that after nerve injury, neurokinin-1 receptor (NK1R) expressing projection neurons in lamina I receive excitatory input from Aβ fibers via a preexisting polysynaptic connection that includes somatostatin (SOM) expressing interneurons, which also receive input from nociceptors.2,6,17 This connection is normally inhibited by dynorphin-/GAD67-expressing GABAergic interneurons in lamina II,4,6 the activity of which is controlled by polysynaptic input from Aβ fibers and by mono- and poly-synaptic inputs from Aδ- and C-fibers nociceptors. Another circuit links Aβ-fibers with lamina I NK1R− projection neurons through PKCγ+/SOM+ interneurons, central cells and SOM+ interneurons in outer lamina II. Information flow through this connection is controlled by feed-forward inhibition mediated by glycinergic and dynorphin-expressing interneurons.6,13 The Aβ-fiber subtypes that provide input to these circuits are unknown. Studies in humans14 and mice with impaired glutamate release from central synapses of C-LTMRs18 suggested that these afferents, which normally signal pleasant touch,10 might also signal mechanical allodynia. This hypothesis was, however, challenged by others who could not find differences in allodynia in mice lacking C-LTMRs11 and who were unable to induce tactile allodynia in patients lacking Aβ fibers.9 A possible explanation for this controversy is that C-LTMRs co-release the protein TAFA4, which counterbalances excitatory actions of glutamate.5 Thus, C-LTMR–specific loss of glutamatergic neurotransmission only,18 would result in a different phenotype than does the complete loss of C-LTMRs.11 C-fiber low-threshold mechanoreceptors project to lamina IIi and provide polysynaptic input to lamina I spinoparabrachial neurons,1 most of which express NK1R. They do not project to PKCγ+ neurons in the same lamina, which receive input from Aβ fibers.15 Putative C-LTMRs, as identified by means of conduction velocity and fiber diameter, also provide excitatory drive onto GABAergic islet cells in lamina II, which regulate information flow from C-fiber nociceptors via central and vertical cells to NK1R+ projection neurons.12 Existence of this C-LTMR–driven pain-inhibiting connection is consistent with recent findings suggesting an analgesic effect of C-LTMR signaling,5 which is reduced during tactile allodynia.9 Whether there is a direct crosstalk between C-LTMR– and Aβ-fiber–processing circuits or whether these 2 pathways only converge at the level of lamina I projection neurons is still unclear. However, given the evidence for a role of both fiber types in mechanical allodynia and considering the columnar organization of the spinal projections of Aβ fibers and C-LTMRs from the same skin area,8 a functional connection between these circuits seems highly likely.

Increases in Functional Connectivity between Prefrontal Cortex and Striatum during Category Learning

Functional connectivity between the prefrontal cortex (PFC) and striatum (STR) is thought critical for cognition and has been linked to conditions like autism and schizophrenia. We recorded from multiple electrodes in PFC and STR while monkeys acquired new categories. Category learning was accompanied by an increase in beta band synchronization of LFPsbetween, but not within, the PFC and STR. Afterlearning, different pairs of PFC-STR electrodes showed stronger synchrony for one or the other category, suggesting category-specific functional circuits. This category-specific synchrony was also seen between PFC spikes and STR LFPs, but not the reverse, reflecting the direct monosynaptic connections from the PFC to STR. However, causal connectivity analyses suggested that the polysynaptic connections from STR to the PFC exerted a stronger overall influence. This supports models positing that the basal ganglia "train" the PFC. Category learning may depend on the formation of functional circuits between the PFC and STR.

Resting state functional MRI of the brain

The article presents an introduction to studies of the brain using functional magnetic resonance imaging during rest (rsfMRI). These studies are based on the fact that the resting brain exhibits a certain level of constant background activity. These spontaneous rsfMRT activities are characterized by fluctuations of the blood oxygenation level-dependent (BOLD) signal (typically in the low frequency part of the power spectrum < 0.1 Hz), which correlate with the local neuronal activity and can be seen as a result of neuronal coupling of monosynaptic and polysynaptic connections. The first network, described in detail is the so-called default mode network (DMN). This includes the medial prefrontal cortex (MPC), the posterior cingulate cortex (PCC), the precuneus (PrC), parts of the medial temporal lobe and the lateral inferior parietal lobe. In addition, a number of other resting state networks (RSNs), such as a motor, somatosensory, visual, auditory and cognitive system has been described, which partly process long-term connections from the cerebral cortex to the diencephalon, brain stem and cerebellum.

Identification of CNS neurons with polysynaptic connections to both the sympathetic and parasympathetic innervation of the submandibular gland.

Coordinated modulation of sympathetic and parasympathetic nervous activity is required for physiological regulation of tissue function. Anatomically, whilst the peripheral sympathetic and parasympathetic pathways are separate, the distribution of premotor neurons in higher brain regions often overlaps. This co-distribution would enable coordinated regulation and might suggest individual premotor neurons could project to both sympathetic and parasympathetic outflows. To investigate this one submandibular gland was sympathectomized. One of two isogenic strains of the pseudorabies virus, expressing different fluorophores, was injected into the cut sympathetic nerve and the other into the submandibular gland. Independent labeling of the peripheral sympathetic and parasympathetic pathways was observed. Dual-labeled neurons were observed in many CNS regions known to be involved in regulating salivary function. We propose these observations highlight a common pattern of organization of the CNS, providing the anatomical framework for the fine control of organ function required for homeostatic regulation and the coordination of organ responses to enable complex behaviors.

Neuronal transport defects of the MAP6 KO mouse – a model of schizophrenia – and alleviation by Epothilone D treatment, as observed using MEMRI

The MAP6 (microtubule-associated protein 6) KO mouse is a microtubule-deficient model of schizophrenia that exhibits severe behavioral disorders that are associated with synaptic plasticity anomalies. These defects are alleviated not only by neuroleptics, which are the gold standard molecules for the treatment of schizophrenia, but also by Epothilone D (Epo D), which is a microtubule-stabilizing molecule. To compare the neuronal transport between MAP6 KO and wild-type mice and to measure the effect of Epo D treatment on neuronal transport in KO mice, MnCl2 was injected in the primary somatosensory cortex. Then, using manganese-enhanced magnetic resonance imaging (MEMRI), we followed the propagation of Mn2+ through axonal tracts and brain regions that are connected to the somatosensory cortex. In MAP6 KO mice, the measure of the MRI relative signal intensity over 24h revealed that the Mn2+ transport rate was affected with a stronger effect on long-range and polysynaptic connections than in short-range and monosynaptic tracts. The chronic treatment of MAP6 KO mice with Epo D strongly increased Mn2+ propagation within both mono- and polysynaptic connections. Our results clearly indicate an in vivo deficit in neuronal Mn2+ transport in KO MAP6 mice, which might be due to both axonal transport defects and synaptic transmission impairments. Epo D treatment alleviated the axonal transport defects, and this improvement most likely contributes to the positive effect of Epo D on behavioral defects in KO MAP6 mice.

Behavior Modulates Effective Connectivity between Cortex and Striatum

It has been notoriously difficult to understand interactions in the basal ganglia because of multiple recurrent loops. Another complication is that activity there is strongly dependent on behavior, suggesting that directional interactions, or effective connections, can dynamically change. A simplifying approach would be to examine just the direct, monosynaptic projections from cortex to striatum and contrast this with the polysynaptic feedback connections from striatum to cortex. Previous work by others on effective connectivity in this pathway indicated that activity in cortex could be used to predict activity in striatum, but that striatal activity could not predict cortical activity. However, this work was conducted in anesthetized or seizing animals, making it impossible to know how free behavior might influence effective connectivity. To address this issue, we applied Granger causality to local field potential signals from cortex and striatum in freely behaving rats. Consistent with previous results, we found that effective connectivity was largely unidirectional, from cortex to striatum, during anesthetized and resting states. Interestingly, we found that effective connectivity became bidirectional during free behaviors. These results are the first to our knowledge to show that striatal influence on cortex can be as strong as cortical influence on striatum. In addition, these findings highlight how behavioral states can affect basal ganglia interactions. Finally, we suggest that this approach may be useful for studies of Parkinson's or Huntington's diseases, in which effective connectivity may change during movement.

Differing synaptic strengths between homologous mechanosensory neurons

Leeches have four mechanosensory pressure neurons (P cells) in each midbody ganglion. Within a ganglion, P cells show complex electrical and chemical connections that vary between species. In Hirudo verbana, stimulating one P cell causes a weak depolarization followed by a strong hyperpolarization in the other P cells; however, stimulating a P cell in Erpobdella obscura produces strong depolarizations in the other P cells. In this study, we examined interactions between P cells in the American medicinal leech Macrobdella decora. Not only is Macrobdella more closely related to Hirudo than to Erpobdella, but Hirudo and Macrobdella also have very similar behavioral responses to mechanical stimulation. Despite the phylogenetic relationship and behavioral similarities between the two species, we found that intracellular stimulation of one P cell in Macrobdella causes a depolarization in the other P cells, rather than the hyperpolarization seen in Hirudo. Experiments performed in a high Mg(2+), 0 Ca(2+) saline solution and a high Mg(2+), high Ca(2+) saline solution suggest that the P cells in Macrobdella have a monosynaptic excitatory connection, a polysynaptic inhibitory connection, and a weak electrical coupling, similar to the connections between P cells in Hirudo. The difference in net response of P cells between these two species seems to be based on differences in the strengths of the chemical connections. These results demonstrate that even when behavioral patterns are conserved in closely related species, the underlying neural circuitry is not necessarily tightly constrained.

Central and peripheral monosynaptic, polysynaptic and collaterals connectivity in the rat

Central and peripheral monosynaptic, polysynaptic and collaterals connectivity in the rat

Mechanisms of chronic pain

Chronic pain is classified as nociceptive or neuropathic, depending on whether the integrity of the somatosensory nervous system is compromised by the underlying disease. Nociceptive pain results from the activation of receptors (nociceptors) sensitive to noxious stimuli. Prolonged or intense exposure to these stimuli, for example, chemical mediators released during inflammation, enhances the responsiveness of nociceptive nerve fibers [1]. This process, termed peripheral sensitization, involves a shift in the activation threshold of nociceptors and upregulation of voltage-gated sodium channels. Peripheral sensitization leads to increased action potential firing and transmitter release in the dorsal horn of the spinal cord, where somatosensory information is processed. Dorsal horn neurons react to the rising input with heightened excitability, a process termed central sensitization. Enhanced depolarization leads to the recruitment of N-methyl-D-aspartate (NMDA)-type glutamate receptors. NMDA and neuropeptide receptor activation produces a sharp increase in intracellular calcium, triggering signaling pathways and gene expression changes that promote a long-term shift in the activity of nociceptive circuits [2]. In some aspects, central sensitization even resembles long-term potentiation of excitatory transmission in the hippocampus. Central sensitization generates an exaggerated response to painful stimuli (hyperalgesia) and contributes to pain elicited by normally nonpainful stimuli (allodynia). Clinical findings suggest that pain hypersensitivity produces structural changes in the brain over time. These changes are, however, reversible upon pain relief. The pathophysiology of neuropathic pain is fundamentally different. Peripheral nerve lesion evokes stimulus-independent (ectopic) activity in nerve fibers. Innate immune cells react at the lesion site, in the dorsal root ganglion, where the cell bodies of peripheral somatosensory neurons reside, and in the dorsal horn of the spinal cord [3]. Active microglia of the dorsal horn releases chemical mediators that modulate the activity of neurons in the vicinity. One of these mediators, brain-derived neurotrophic factor, reduces the inhibitory effect of γ-aminobutyric acid (GABA) and glycine. Disinhibition opens polysynaptic connections in the dorsal horn, further enhancing the abnormal input from the lesioned nerve. Similarly to nociceptive pain, central sensitization occurs. Worsened by a relative deficit in transmitter uptake, increased glutamatergic transmission causes excitotoxic cell death, reducing the number of inhibitory interneurons. Their loss and a shift in descending modulatory pathways from the brainstem produce a profound imbalance between inhibition and excitation. The complexity of chronic pain mechanisms poses a major therapeutic challenge. Without biomarkers, it will remain difficult to develop targeted strategies for chronic pain reduction or prevention in the individual patient.

Frequency response of vestibular reflexes in neck, back, and lower limb muscles

Vestibular pathways form short-latency disynaptic connections with neck motoneurons, whereas they form longer-latency disynaptic and polysynaptic connections with lower limb motoneurons. We quantified frequency responses of vestibular reflexes in neck, back, and lower limb muscles to explain between-muscle differences. Two hypotheses were evaluated: 1) that muscle-specific motor-unit properties influence the bandwidth of vestibular reflexes; and 2) that frequency responses of vestibular reflexes differ between neck, back, and lower limb muscles because of neural filtering. Subjects were exposed to electrical vestibular stimuli over bandwidths of 0-25 and 0-75 Hz while recording activity in sternocleidomastoid, splenius capitis, erector spinae, soleus, and medial gastrocnemius muscles. Coherence between stimulus and muscle activity revealed markedly larger vestibular reflex bandwidths in neck muscles (0-70 Hz) than back (0-15 Hz) or lower limb muscles (0-20 Hz). In addition, vestibular reflexes in back and lower limb muscles undergo low-pass filtering compared with neck-muscle responses, which span a broader dynamic range. These results suggest that the wider bandwidth of head-neck biomechanics requires a vestibular influence on neck-muscle activation across a larger dynamic range than lower limb muscles. A computational model of vestibular afferents and a motoneuron pool indicates that motor-unit properties are not primary contributors to the bandwidth filtering of vestibular reflexes in different muscles. Instead, our experimental findings suggest that pathway-dependent neural filtering, not captured in our model, contributes to these muscle-specific responses. Furthermore, gain-phase discontinuities in the neck-muscle vestibular reflexes provide evidence of destructive interaction between different reflex components, likely via indirect vestibular-motor pathways.

Studying large-scale brain networks: electrical stimulation and neural-event-triggered fMRI

The brain is example of an adaptive, complex system. It is characterized by ultra-high structural complexity and massive connectivity, both of which change and evolve in response to experience. Information related to sensors and effectors is processed in both a parallel and a hierarchical fashion. The connectivity between different hierarchical levels is bidirectional, and its effectiveness is continuously controlled by specific associational and neuromodulatory centers. In the study of such systems one major problem is the adequate definition for an elementary operational unit (often called an agent), because any such module can be a complex system in its own right and may be recursively decomposed into other sets of units. A second difficulty arises from the synergistic organization of complex systems and of the brain in particular. Synergy here refers to the fact that the behavior of an integral, aggregate, whole system cannot be trivially reduced to, or predicted from, the components themselves. Localizing and comprehending the neural mechanisms underlying our cognitive capacities demands the combination of multimodal methodologies, i.e. it demands concurrent study of components and networks; one way of doing this, is to combine invasive methods which afford us direct access to the brain's electrical activity at the microcircuit level with global imaging technologies such as magnetic resonance imaging (MRI). In my talk, I'll discuss two such methodologies: Direct Electrical Stimulation and fMRI (DES-fMRI) and Neural-Event-Triggered fMRI (NET-fMRI). DES-fMRI can be used in hopes of gaining insight into the functional or effective connectivity underlying DES-induced behaviors. Yet, our first findings suggest that DES has an important limitation: It clearly demarcates all monosynaptic targets of a stimulated site, but it largely fails to reveal polysynaptic cortico-cortical connectivity. NET-fMRI, on the other hand, appears to offer great potential for mapping whole-brain activity that is associated with individual local events. In the second part of my talk, I'll describe the characteristic states of widespread cortical and subcortical networks that are associated with the occurrence of hippocampal sharp waves and ripples; the brief aperiodic episodes associated with memory consolidation.

Polysynaptic connections between Barrington's nucleus and sacral preganglionic neurons

The pontine micturition center, or Barrington's nucleus, is an essential component in the micturition reflex. The purpose of the present study is to examine the connections between Barrington's nucleus and sacral preganglionic neurons with an electrophysiological method using an intracellular recording technique. When the bladder pressure was near 0mmH2O, electrical stimulation of Barrington's nucleus either evoked no postsynaptic potential or evoked very small excitatory postsynaptic potentials (EPSPs) with ambiguous onset, in sacral preganglionic neurons. However, when the bladder showed micturition contraction, electrical stimulation of the nucleus evoked clear EPSPs in sacral preganglionic neurons. Latencies of EPSPs ranged from 21.9 to 47.5ms. The onset of the EPSPs was not fixed in any of the preganglionic neurons, indicating that they were evoked polysynaptically. The results thus demonstrated that the descending pathway from Barrington's nucleus to the sacral preganglionic neurons is polysynaptic. The results also showed that the descending pathway is strongly facilitated during the voiding phase, whereas this facilitatory effect is very weak when the bladder pressure is low. The study supports the idea that continuous firings of Barrington's nucleus are needed to activate the sacral preganglionic neurons that innervate the bladder muscle.

Network interneurons underlying ciliary locomotion in Hermissenda.

In the nudibranch mollusk Hermissenda, ciliary locomotion contributes to the generation of two tactic behaviors. Light elicits a positive phototaxis, and graviceptive stimulation evokes a negative gravitaxis. Two classes of light-responsive premotor interneurons in the network contributing to ciliary locomotion have been recently identified in the cerebropleural ganglia. Aggregates of type I interneurons receive monosynaptic excitatory (I(e)) or inhibitory (I(i)) input from identified photoreceptors. Type II interneurons receive polysynaptic excitatory (II(e)) or inhibitory (II(i)) input from photoreceptors. The ciliary network also includes type III inhibitory (III(i)) interneurons, which form monosynaptic inhibitory connections with ciliary efferent neurons (CENs). Illumination of the eyes evokes a complex inhibitory postsynaptic potential, a decrease of I(i) spike activity, a complex excitatory postsynaptic potential, and an increase of I(e) spike activity. Here, we characterized the contribution of identified I, II, and III(i) interneurons to the neural network supporting visually guided locomotion. In dark-adapted preparations, light elicited an increase in the tonic spike activity of II(e) interneurons and a decrease in the tonic spike activity of II(i) interneurons. Fluorescent dye-labeled type II interneurons exhibited diverse projections within the circumesophageal nervous system. However, a subclass of type II interneurons, II(e(cp)) and II(i(cp)) interneurons, were shown to terminate within the ipsilateral cerebropleural ganglia and indirectly modulate the activity of CENs. Type II interneurons form monosynaptic or polysynaptic connections with previously identified components of the ciliary network. The identification of a monosynaptic connection between I(e) and III(i) interneurons shown here suggest that they provide a major role in the light-dependent modulation of CEN spike activity underlying ciliary locomotion.

ROLE AND PLACE OF TIOCETAM IN STRATEGY OF NEUROPROTECTION IN SEVERE CCT AND ISCHEMIC STROKE

The question about the possible ways of pharmacological correction of the second damage of cerebral tissue in brain ischemia, trauma and hemorrhage is a topical one. Trophic dysregulation, biochemical and functional differentiation of neurons are the universal component of nervous tissue damage pathogenesis. Reparative therapy is directed on the improvement of plasticity of healthy tissue which surrounds the area of ischemic-damaged brain, on activation of polysynaptic connections formation, on the increase of density of receptors and traditionally includes nootropic preparations. The comatose state of patient is a contraindication to application of these agents, because they provide a stimulating action. The application of Tiocetam (combination of piracetam with thyotriazolinum) is perspective in solution of this problem. In intensive therapy of acute cerebral insuffi ciency caused by the severe craniocerebral trauma or brain stroke a problem of determination of terms of beginning of treatment by Tiocetam and criteria of choice of adequate dose of this preparation is a topical one. The method of determination of Tiocetam adequate dose is developed on the basis of comparison of methods of estimation of impairment of consciousness on Glasgow Coma Scale and integral quantitative analysis of level of disorganization of EEG-patterns with research of brain responsiveness to Tiocetam introduction.

Regulation of polysynaptic subthalamonigral transmission by D2, D3 and D4 dopamine receptors in rat brain slices

Dopamine depletion in experimental models of Parkinson's disease promotes burst firing of neurons in the subthalamic nucleus (STN) and substantia nigra zona reticulata (SNR). A synaptically generated form of burst firing has been shown to arise from complex excitatory postsynaptic currents (EPSCs) that are evoked in SNR neurons by STN stimulation. The present experiments were designed to characterize actions of dopamine on complex EPSCs in slices of rat brain. Using patch pipettes to record whole-cell currents under voltage clamp, dopamine (30 μm) caused a reversible 64% reduction in complex EPSC charge. This effect was partially mimicked by D(2), D(3) and D(4) receptor agonists, and the action of dopamine could be nearly completely blocked by the combined effects of the D(2/3) antagonist sulpiride and the D(4) antagonist L-745,870. Local application of dopamine to the STN caused a larger inhibition of the complex EPSC (55% reduction) than did dopamine application to the SNR (15% reduction). Simple, monophasic EPSCs, which were evoked in SNR neurons by stimulating the SNR close to the recording pipette, were inhibited to a smaller extent compared to complex EPSCs. Bursts of action potentials evoked in SNR neurons by STN stimulation were inhibited by dopamine to a greater extent than was spontaneous firing. These results show that dopamine D(2)-like receptors inhibit complex EPSCs and burst discharges in the SNR by acting within the STN to suppress transmission in the subthalamonigral pathway. Dopamine receptor-mediated inhibition of polysynaptic connections in the STN might be beneficial in the treatment of Parkinson's disease.

Sympathetic Response to Insulin Is Mediated by Melanocortin 3/4 Receptors in the Hypothalamic Paraventricular Nucleus

Hyperinsulinemia increases sympathetic nerve activity and contributes to cardiovascular dysfunction in obesity and diabetes. Neurons of the hypothalamic paraventricular nucleus (PVN) regulate sympathetic nerve activity through mono- and poly-synaptic connections to preganglionic neurons in the spinal cord. The purpose of the present study was to determine whether PVN neurons mediate the sympathetic response to insulin. Hyperinsulinemic-euglycemic clamps were performed in α-chloralose-anesthetized, male Sprague-Dawley rats (280-420 g) by an infusion of insulin (3.75 mU/kg per min) and 50% dextrose (0.75-2.0 mL/h) for 120 minutes. At 90 minutes, insulin significantly increased lumbar sympathetic nerve activity without any change in renal sympathetic nerve activity, heart rate, or blood glucose levels. Inhibition of the PVN with bilateral injection of the GABA(A) receptor agonist muscimol completely reversed the sympathoexcitatory response. However, direct injection of insulin into the PVN did not alter lumbar sympathetic nerve activity, and thereby suggests that insulin activates neurons upstream of the PVN. Interestingly, the sympathetic response to insulin was eliminated by PVN injection of the melanocortin 3/4 receptor antagonist SHU9119, but was unaffected by the angiotensin II type 1 receptor antagonist losartan. A final set of experiments suggests activation of PVN neurons during hyperinsulinemia increases glutamatergic drive to the rostral ventrolateral medulla. Collectively, these findings indicate that insulin activates a melanocortin-dependent pathway to the PVN that increases glutamatergic drive to the rostral ventrolateral medulla and alters cardiovascular function.