Projects a. Non-classical mechanisms of presynaptic inhibition. Primary afferent neurotransmission is the fundamental first step in the central processing of sensory stimuli. Somatosensory transmission is controlled by a unique ionotropic form of presynaptic inhibition that powerfully limits body sensations. Using multi-pronged approaches that include pioneering electrophysiological studies in the in vitro nerves-attached rodent spinal cord, we are challenging the deeply ingrained doctrine that a 3-synapse circuit with GABAergic interneurons acting on GABAA receptors is responsible. We identified the presence of more direct pathways (Shreckengost et al 2010). We also observed an underlying complex pharmacology that supports promiscuous blending of cys-loop receptors and questions the proprietary role of GABA and GABAA receptors in this process. Cholinergic transmission in particular may contribute significantly, including via direct release from primary afferents. β-alanine and taurine transmission are also implicated (Hochman et al 2010, NYAS). b. Modulatory actions of serotonin, noradrenaline and dopamine. These neuromodulatory transmitters arise from descending systems to control spinal sensory integration. We provided the first comparative analysis of their actions on synaptic and cellular properties of spinal sensory encoding neurons (Garraway & Hochman, 2001). These modulators acted in a comparable and predictable manner; to profoundly depress sensory input yet increase cell excitability. With Jorge Quevedo, in a just submitted manuscript, we looked more specifically at modulation of afferents arising from muscle and cutaneous nerves, and again found broadly uniform actions – depression of afferent input. My lab then looked at actions arising from visceral afferents, again observing broad depression. Yet, in all above studies, subtly different responses for each monoamine were seen. Thus, the different monoamines, recruited by different brain systems during different behavioral drives, all include the general feature of reducing body sensations, particularly pain. 2. Neural encoding of limb movement In contrast to their broadly depressant actions on sensory systems, the actions of the monoamines on motor systems are generally facilitatory, but also differentiable. For example, noradrenaline promotes self-reinforcing positive feedback in spinal motor circuits while serotonin promoted negative feedback (Machacek & Hochman 2006). We published an “Innovative Methodology” paper (Hayes et al 2009) on a newly-developed isolated in vitro rat spinal cord with intact hindlimbs freely stepping on a custom-built treadmill. It combines the neural accessibility of in vitro preparations with modulatory influence of sensory feedback from physiological hindlimb movement. A currently submitted manuscript provides the first-ever intracellular neuronal recordings during mechanically-unrestrained locomotion. Using additional pioneering techniques in another submitted manuscript, we investigated how hindlimb mechanics influence sensory input during locomotion. We found that stance-phase force on the opposite limb strongly and linearly encoded the magnitude and timing of afferent presynaptic inhibition in the swinging limb, thus binding interlimb sensorimotor states by adjusting sensory inflow to the swing limb based on forces generated by the stance limb. These studies suggest that stroke or spinal cord injury rehabilitative approaches that involve loading the unaffected limb may provide a novel means of reducing spasticity and hyperreflexia in the affected limb. 3. An unrecognized family of neuromodulators control locomotion Based on their low concentrations in mammalian brain, octopamine, β-phenylethylamine, tyramine, and tryptamine are classified as "trace" amines (TAs) and viewed as metabolic by-products. TAs are related to the classical monoamine transmitters and are synthesized from the same precursor aromatic amino acids. The recent discovery of a family of G protein-coupled receptors preferentially activated by TAs rekindled interest in TAs, but without a known circuitry, their function remains elusive and understudied. We now have anatomical evidence of an endogenous TA system in spinal cord, and show that the TAs recruit locomotor circuits and modulate ongoing locomotion. Our newest evidence suggest the TAs represent a parallel biochemical modulatory system that alters circuit function via a membrane transporter shuttling system that operates independent of synaptic actions. The long-term goal is to understand the physiological relevance of the TAs as an intrinsic modulatory system for subsequent therapeutic manipulation of spinal circuit function. 4. Deciphering neural circuit dysfunction in Restless Legs Syndrome (RLS) RLS is a neural disorder involving abnormal sensations that are reduced during motor activity, worsen at rest, and with a diurnal nighttime prevalence. Primary treatment is directed at increasing CNS dopaminergic D2-like [= D2D3D4] receptor activity. We studied the actions of dopamine and D2-like receptors on reflex function. We demonstrated that dopamine and D2-like agonists depress the monosynaptic reflex, and that at low but physiologic levels of dopamine, this modulation is mediated by D3 receptors (Clemens and Hochman 2004). This dopamine-induced depression was absent in mice lacking the functional D3 receptor (D3KO) suggesting that impairments in dopamine signaling via spinal D3 receptors may account for the heightened spinal reflex amplitudes found in RLS patients. A subsequent “Medical Hypothesis” article (Clemens et al. Neurology, 67:125-130, 2006) forwarded the hypothesis that RLS reflects dysfunction in spinal dopaminergic signaling. Predicted changes in peripheral-spinal loop circuits would account for the RLS phenotype and sites of therapeutic action. Our additional electrophysiological, pharmacological, anatomical and molecular studies support this increasingly accepted model of RLS that still remains the only neural circuit model proposed (Clemens et al 2005; Zhu et al 2007, 2008). RLS as a peripheral chronic pain syndrome. We now hypothesize that RLS is a chronic muscle pain syndrome associated with vascular dysfunction caused by impaired circulation, and perhaps associated with venous insufficiency. To examine peripheral neural signaling changes, we developed an adult mouse in vitro skeletal muscle/nerve-attached preparation and are studying muscle afferent signaling changes following induced states of muscle dysfunction (intermittent hypoxia, inflammation, and venous return reductions). 5. Plasticity of spinal cord function after injury: Autonomic dysreflexia The injured spinal cord often becomes hyper-responsive resulting in autonomic dysfunction and devastating chronic pain syndromes. Spinal preganglionic sympathetic neurons (SPNs) are the sympathetic output neurons of the CNS and are located in thoracic and upper lumbar spinal segments. Loss of brain controls to SPNs after spinal injury leads to an unregulated circuitry strongly influenced by afferent input from pain systems, which can produce autonomic dysreflexia. We hypothesize that excitatory nociceptive input converts and traps SPNs into an up-state in excitability (plateau potentials) and so generating continuous SPN firing to produce potentially life-threatening hypertensive crisis. Using an adult spinal slice preparation and recording SPN firing properties changes after cord injury, we predict the up-state is maintained by constitutively-active monoamine receptors, and will focus on the actions of inverse agonists as prospective therapeutics. preganglionic sympathetic neurons represent the final common output of the CNS sympathetic nervous system. These neurons are located in thoracic and upper lumbar spinal segments. Loss of descending controls to this system after spinal injury leads to an unregulated circuitry that is strongly influenced by input from pain systems. The result is excess sympathetic activation which can produce autonomic dysreflexia. Amanda Zimmerman is recording from sympathetic preganglionic neurons to study the actions of neuromodulatory transmitters that regulate their function. 6. Development of an animal model of "meditation" Slowed, deep breathing may be a fundamental aspect of the relaxation response in several forms of meditation and yoga. We hypothesize that deep breathing is the essential autonomic unit of stress reduction and the major untested mechanism triggering the therapeutic benefit of the relaxation response. Physiologically, the view forwarded is that slow-deep breathing leads to a fundamental state change in the composition and signaling properties of stretch-sensitive sensory nerve signaling to brain from the lungs. This unique signaling signature oscillates at the same rhythmic frequency as the slow cortical oscillations observed in the cerebral cortex during restful deep sleep (0.1 hertz), and projects to similar brain circuits to imprint a parasympathetic ‘wellness’ state. We are developing behavioral (rodent model of deep breathing using operant conditioning) and electrophysiological approaches (identification of brain circuits activated by deep breathing) to test our hypothesis. Techniques |