We study the electrophysiology of noradrenergic sympathetic neurons and dopamine neurons in the substantia nigra. These neurons are implicated in a wide range of human pathologies ranging from hypertension and post-traumatic stress disorder to Parkinson's disease, drug addiction, learning disorders and schizophrenia. However, the questions we ask are much more basic. Rather than focus upon disease models, we combine electrophysiology with computational approaches to identify the cellular and molecular origins of synaptic integration and neuromodulation in the context of circuit function.
As an example, our recent work on integration in sympathetic neurons shows how they can function as synaptic amplifiers whose gain may be regulated by excitatory muscarinic modulation. This is potentially important for a couple of reasons. First, these cells are embedded in negative feedback loops whose ability to control cardiovascular function depends critically on gain. Our work therefore identifies a synaptic mechanism of amplification that may serve to enhance homeostatic blood pressure control. Second, the type of muscarinic excitation found in sympathetic neurons also operates in the cerebral cortex, where it influences memory and is a drug target in Alzheimer's disease.
Other recent work from our lab has focused on the pacemaker properties of dopamine neurons using brain slice preparations. This project is a collaboration with my colleague Ed Levitan in the Pharmacology Department, something very typical of our CNUP community. The thing that ties this project to the work on sympathetic neurons is a technique called dynamic clamp. With it one can implement virtual ion channels in cells using computer feedback. In our most recent project on dopamine neurons using the dynamic clamp we have studied the role of L-type calcium channels in pacemaking. Others in the field have argued that calcium entering through the L channels is essential. We have shown that after blocking the L channels with a drug that has been suggested as a potential treatment in Parkinson's disease, one can restore pacemaking. This provides some of the best available evidence that L channels support pacing and it also proves that calcium entry is secondary, since virtual L type currents are not carried by calcium.
Students and Postdocs in the Horn lab have opportunities to learn electrophysiology from the bottom up and to combine it with computational modeling, neuroanatomy and molecular biology.
Kullmann, P.H.M. and J.P. Horn Homeostatic regulation of M-current modulates synaptic integration in secretomotor, but not vasomotor, sympathetic neurons in the bullfrog. Journal of Physiology, 588: 923-938, 2010.
Rimmer, K. and J.P. Horn Weak and straddling secondary nicotinic synapses can drive firing in rat sympathetic neurons and thereby contribute to ganglionic amplification. Frontiers in Neurology/Autonomic Neuroscience, 1:130. doi: 10.3389/fneur.2010.00130, 2010.
Putzier, I., Kullmann, P.H.M., Horn, J.P. and E.S. Levitan CaV1.3 channel voltage dependence, not Ca2+ selectivity, drives pacemaker activity and amplifies bursts in nigral dopamine neurons. Journal of Neuroscience 29: 15414-15419, 2009. PubMed
Putzier, I., Kullmann, P.H.M., Horn, J.P. and E.S. Levitan. Dopamine neuron responses depend exponentially on pacemaker interval. Journal of Neurophysiology, 101: 926 _ 933, 2009. PubMed
Li, C. and J.P. Horn. Differential inhibition of Ca2+ channels by α2-adrenoceptors in three functional subclasses of rat sympathetic neurons. Journal of Neurophysiology 100: 3055 _ 3063, 2008. PubMed
Horn, J.P. and P.H.M. Kullmann. Dynamic clamp analysis of synaptic integration in sympathetic ganglia. Neurophysiology 39: 486-492, 2007. PubMed
Li, C. and J.P. Horn. Physiological classification of sympathetic neurons in the rat superior cervical ganglion. Journal of Neurophysiology 95: 187-195, 2006. PubMed