Sensory experience selectively regulates transmitter synthesis enzymes in interglomerular circuits

Abstract

Sensory experience influences brain organization and function. A particularly striking example is in the olfactory bulb where reduction of odorant sensory signals profoundly down-regulates dopamine in glomerular neurons. There are two large populations of glomerular inhibitory interneurons: (1) GABAergic periglomerular (PG) cells, whose processes are limited to a single glomerulus, regulate intraglomerular processing and (2) DAergic-GABAergic short axon (SA) cells, whose processes contact multiple glomeruli, regulate interglomerular processing. The inhibitory neurotransmitter GABA is synthesized from L-glutamic acid by the enzyme glutamic acid decarboxylase (GAD) of which there are two major isoforms: GAD65 and GAD67. GAD65 is expressed in uniglomerular PG cells. GAD67 is expressed by SA cells, which also co-express the rate-limiting enzyme for dopamine synthesis, tyrosine hydroxylase (TH). Deafferentation or sensory deprivation decreases TH expression but it is not known if sensory input alters GAD isoforms. Here we report that either deafferentation or reduction of sensory input by nares occlusion significantly reduced GAD67 protein and the number of SA cells expressing GAD67. However, neither manipulation altered GAD65 protein or the number of GAD65 PG cells. These findings show that sensory experience strongly impacts transmitter regulation in the circuit that controls neural processing across glomeruli but not in the circuit that regulates intraglomerular processing.

Introduction

Neural activity regulates expression of neurotransmitter synthesis enzymes in many brain regions. The inhibitory neurotransmitter GABA is synthesized from L-glutamic acid by the enzyme glutamic acid decarboxylase (GAD), for which there are two major isoforms: 65 and 67 kDa. These isoforms are differentially expressed by various neuron subpopulations and differentially distributed within cells (Erlander et al., 1991). The two GAD isoforms differ in activity-dependent expression in different brain structures (Rimvall and Martin, 1994, Jinno and Kosaka, 2009). Differential regulation of these two GAD isoforms by experience may lead to plasticity of inhibitory neural circuits. We investigated this possibility in glomerular inhibitory circuits of the olfactory bulb.

Olfactory sensory neurons (OSNs) in the nasal epithelium project axons that terminate on the dendrites of second-order interneurons and output neurons (mitral/tufted cells (M/T)) in glomeruli of the main olfactory bulb. Three principle types of juxtaglomerular (JG) interneurons – external tufted (ET) cells, periglomerular (PG) cells and short axon (SA) cells (Golgi, 1875, Price and Powell, 1970, Cajal, 1911, Pinching and Powell, 1971, Aungst et al., 2003, Shao et al., 2009, Kiyokage et al., 2010) – form several distinct glomerular circuits that regulate the glomerular input–output function (Shao et al., 2009, Kiyokage et al., 2010). These interneurons differ in morphotype: ET cells and PG cells processes are predominantly confined to a single parent glomerulus (uniglomerular) while SA cells connect with multiple glomeruli. They also differ in chemotype: PG cells are GABAergic; SA cells use both GABA and dopamine (DA) as transmitters (Kosaka and Kosaka, 2008, Shao et al., 2009, Kiyokage et al., 2010). SA cells express GAD67, whereas PG cells predominantly express GAD65 (Kiyokage et al., 2010). These three JG neurons form two basic glomerular inhibitory circuits: (1) GAD65 GABAergic PG cells mediate intraglomerular inhibition and (2) GAD67 GABAergic-DAergic SA cells mediate interglomerular inhibition.

Afferent sensory input is necessary for maintenance of tyrosine hydroxylase (TH), the rate-limiting enzyme for DA biosynthesis in SA neurons (Kawano and Margolis, 1982, Margolis et al., 2006, Baker et al., 1983, Kosaka et al., 1987, Baker et al., 1988, Baker et al., 1993, Brunjes, 1994, Cho et al., 1996, Philpot et al., 1998, Saino-Saito et al., 2004). Blocking afferent activity by means of deafferentation or sensory deprivation reduces TH expression (Baker et al., 1983, Baker et al., 1993, Brunjes, 1994, Cho et al., 1996, Philpot et al., 1998). DAergic SA cells predominantly contain GAD67 (Kiyokage et al., 2010) and PG cells predominantly express GAD65 (Parrish-Aungst et al., 2007). Since afferent activity regulates TH we asked if sensory activity alters the expression of GAD67 in SA cells and/or GAD65 expression in PG cells, i.e., are interglomerular and intraglomerular inhibitory circuits differentially impacted by sensory experience?