Projects Non-classical control of body sensations Primary afferent neurotransmission is the fundamental first step in the central processing of sensory stimuli. It is controlled by pre- and post-synaptic inhibitory mechanisms. Presynaptic inhibition (PSI) is “more powerful than postsynaptic inhibition in depressing the central excitatory actions of almost all primary afferent fibers”[Eccles] and is the reason why you can't tickle yourself. For decades, it has been assumed though never directly demonstrated that this primary afferent depolarization (PAD) of touch and movement-encoding sensory afferents is mediated by a minimally trisynaptic pathway, and that GABAergic interneurons are essential. We are undertaking studies that challenge this doctrine. We are testing the hypotheses that much of this afferent stimulation-evoked PADis instead generated by more direct synaptic pathways, that may be independent of classical GABAA receptors and independent of GABA. This project involves pioneering electrophysiological studies in the in vitro nerves-attached rodent spinal cord blending essential pharmacological, molecular and biophysical characterizations of the transmitter(s) released & ionotropic receptor subunits expressed in primary afferents. Restless Legs Syndrome (RLS) and spinal cord dopamine RLS is a CNS disorder involving abnormal sensations
that are reduced during motor activity, worsen at rest, and have a marked circadian pattern. Primary treatment
is directed at increasing CNS dopaminergic activity, particularly activation of D2-like receptors.
Significant alterations in brain dopamine function are not unequivocally observed in RLS – suggesting
that the obvious brain dopamine ‘hotspots’ are not involved. However, an anatomically discrete dopaminergic
system in hypothalamus (A11) has projections to two spinal cord functional systems (sympathetic NS output and
deep afferent input) whose altered activity would result in actions wholly consistent with the production of RLS,
but yet remains virtually unstudied... until now! In collaboration with
Dr. Stefan Clemens and
Dr. David Rye, we have undertake behavioral,
electrophysiological, pharmacological, anatomical (immunolabeling & in situ hybridization) and molecular
studies (expression profiling) to examine hypothalamospinal dopamine function
(Clemens et al 2004)
(Clemens et al 2005)
(Clemens et al 2006)
(Zhu et al 2007)
(Zhu et al 2008). Neuromodulatry control of spinal sympathetic autonomic function: Spinal 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. Plasticity of inhibitory circuit function after cord injury: (i)In collaboration with Dr. Jorge Quevedo from CINVESTAV, Jacob Shreckengost is examing plasticity in presynaptic inhibition of primary afferents. (ii) Using GAD67-EGFP transgenic mice, Dr. Kim Dougherty studied the properties of visually-identified GABAergic neurons in lamina I of the spinal cord (Dougherty et al 2005) (Dougherty et al 2008) (Dougherty et al 2009). This region is involved in spinal pain processing and modification in the lamina I inhibitory apparatus may contribute significantly to the high incidence of chronic pain syndromes following cord injury. Gene expression changes after spinal cord injury: In collaboration with Deb Baro, we have used DNA microarrays in conjunction with laser-capture microdissection (LCM) to provide a comprehensive dissection of gene expression changes in motoneurons and sympathetic preganglionic neurons following chronic spinal injury (Cui et al 2006). Our goal is to identify expression changes in important cell populations to direct novel approaches for spinal cord repair. Regulation of spinal locomotor activity Remarkably, the neonatal rat spinal cord can be surgically isolated, even with hindlimbs attached, and preserved for many hours in an oxygenated saline bath. Serotonin activates the spinal locomotor circuitry, enabling an in vitro study of spinal cord locomotor mechanisms. One goal is to identify pharmacotherapeutic strategies that complement known restrictions in the regenerative repair process to enable a limited reconnection from brain command pathways to activate the locomotor central pattern generator (CPG). These studies are currently undertaken by Heather Hayes, JoAnna Todd and Lisa Gozal. In collaboration with Steve Deweerth, we are testing the interfacing of multi-electrode arrays with spinal cord systems that initiate locomotor activity and regulate its frequency (Meacham et al 2007). Unusual modulatory mechanisms controlling spinal cord function Based on their low concentrations in mammalian brain, octopamine, beta -phenylethylamine (PEA), tyramine, and tryptamine are classified as "trace" amines (TAs) . TAs are related to the classical monoamine transmitters and are synthesized from the same precursor aromatic amino acids (AAs). Their CNS distribution is heterogeneous, and they have very high turnover rates, equivalent to that of DA and NA which may be a more significant index of their importance than their endogenous concentrations. Interest in TAs faded in the 80's viewing them as metabolic by-products. The seminal discovery in 2001 of a new family of G-protein coupled receptors preferentially activated by TAs rekindled interest in TAs. However, without a known circuitry, the function of TAs in CNS remains elusive and understudied. Lisa Gozal has demonstrated that the TAs; (i) recruit locomotor circuits in the isolated spinal cord, (ii) can generate complex locomotor patterns, and (iii) modulate 5-HT's locomotor actions. The long-term goal is to understand the physiological relevance of the TAs as an intrinsic spinal modulatory system. TA-based actions may become valuable therapeutically to control spinal motor circuits following spinal cord injuries. Techniques |