John P. Walrond, PhD
Research Interests -- Structure and Function of Central and Peripheral Nicotinic Cholinergic Synapses
Studies of synaptic transmission at the neuromuscular synapse have made important contributions to understanding synaptic structure and function. However, this synapse is highly specialized to ensure that each action potential in the motor neuron releases sufficient neurotransmitter to elicit an action potential in the postsynaptic muscle fiber. Such a one-to-one relationship is unusual for synapses in the central nervous system where action potential generation in the postsynaptic neuron results from the integration of many small potentials produced by numerous synaptic inputs. Regulating the output of neurons in the central nervous system depends largely on setting the "gain" of the pre- and postsynaptic cells either by altering the amount of neurotransmitter released presynaptically or by affecting the amplitude of the postsynaptic potential. Nicotinic acetylcholine receptors in the brain appear to be involved in this process. The brain contains numerous nicotinic acetylcholine receptor subtypes which are categorized according to the a and b subunits that they contain. The alpha subunits in the central nervous system include those numbered a2-9, although a8 has not been found in mammals. There are two beta subunits.
Neurodegenerative diseases, including Alzheimer's disease and Parkinsonism, are associated with the loss of cholinergic neurons in the central nervous system. In addition, the relationships between nicotinic acetylcholine receptor function and addictive behaviors, particularly tobacco consumption, have important implications for public health policies. Despite the importance of these receptors in normal and diseased nervous systems, little is known about their distribution and function at the cellular level.
Electrophysiological, pharmacological, immunohistochemical, and ultrastructural techniques are used in my laboratory to study nictonic acetylcholine receptors in the central and peripheral nervous systems. Electrophysiological and pharmacological studies in my lab have shown that alkaloids isolated from Delphinium spp. including methyllycaconitine (MLA), nudicauline, 14-deacetylnudicauline (14-DN), barbinine and deltaline specifically block nicotinic acetylcholine receptors in the brain and at neuromuscular junctions. These alkaloids are most specific for blockade of a7-containing receptors, but they are also effective at lower concentrations at a4-containing receptors in the CNS and at a1-containing receptors at the neuromuscular junction.
There are no cholinergic neuronal cell bodies in the hippocampal formation, and nuclei in the septum appear to provide most of the cholinergic input to this region. To study hippocampal nicotinic synapses, we are co-culturing acutely dissociated septal and hippocampal neurons. At least half of the septal neurons in culture are cholinergic and appear capable of forming synapses. Currently we are using immunocytochemistry at the light and electron microscopic levels to investigate the distribution of cholinergic synapses and nicotinic acetylcholine receptor subtypes on individual neurons. Cultured neurons are especially tractable for these studies because the cell types are identifiable and diffusional barriers for pharmacological and immunocytochemical reagents are minimal. These preparations are also well suited for electrophysiological studies of nicotinic synaptic function since the identity and proximity of pre- and postsynaptic neurons can be controlled. Through these studies we aim to provide insights into how nicotinic acetylcholine receptors affect neuronal output in the central nervous system.
Walrond JP, Reese TS. 1985. Structure of nerve terminals on twitch and tonic muscle fibers in Anolis lizard intercostal muscles. J Neurosci 5:1118-1131.
Walrond JP, Govind CK, Huestis SE. 1993. Two structural adaptations for regulating transmitter release at lobster neuromuscular synapses. J Neurosci 13:4831-4845.
Garcia KD, Mynlieff M, Sanders DB, Walrond JP, Beam KG. 1996. Lambert-Eaton sera reduces low-voltage and high-voltage activated Ca2+ currents in murine dorsal root ganglion neurons. Proc Natl Acad Sci USA 93:9264-9269.
Dobelis P, Madl JE, Pfister JA, Manners GD, Walrond JP. 1999. Effects of delphinium alkaloids on neuromuscular transmission. J Pharmacol Exp Ther 291:538-546.