GABAergic Neurons as Putative Neurochemical Substrate Mediating Aversive Effects of Nicotine

Nicotine, the main addictive component of tobacco smoke, has both rewarding and aversive properties. Recent studies have suggested that GABAergic neurons, one of the main neurochemical components of the reward-addiction circuitry, may also play a role in the aversive responses to nicotine. In the present study of transgenic mice expressing Green Fluorescent Protein (GFP) in Glutamate Decarboxylase 67 (GAD67) neurons, we hypothesized that a subpopulation of GABAergic neurons in the Ventral Tegmental Area (VTA) are the targets of aversive doses of nicotine in the CNS. We tested this hypothesis using c-Fos immunohistochemical techniques to identify GAD67-GFP positive cells within the VTA, that are activated by a single intraperitoneal (i.p.) injection of a low (40 ug/kg) or a high (2 mg/kg) dose of nicotine. We also assessed the anatomical location of GAD67-GFP positive cells with respect to tyrosine hydroxylase (TH) Immunoreactive (IR) dopaminergic cells in VTA. Consistent with our previous studies low- and high-dose nicotine both induced c-Fos activation of various intensities at multiple sites in VTA. Double labeling of c-Fos activated cells with GAD67-GFP positive cells identified a subpopulation of GABAergic neurons in Substantia Nigra Compact part Medial tier (SNCM) that were activated by high- but not by low-dose nicotine. Of 217 GABAergic cells counted at this site, 48.9% exhibited nicotine induced c-fos immunoreactivity. GAD67-GFP positive cells in other regions of VTA were not activated by the nicotine doses tested. Double labeling of GAD67-GFP positive cells with TH IR cells showed that the GABAergic neurons that were activated by high-dose nicotine were located in close proximity to the dopaminergic neurons of substantia nigra compact part and VTA. Dose-dependent activation of GAD67-GFP positive neurons in SNCM, by a nicotine dose known to produce aversive responses, implies that GABAergic neurons at these sites may be an important component of the nicotine aversive circuitry.

Glutamic acid decarboxylase 67 expression by a distinct population of mouse vestibular supporting cells

The function of the enzyme glutamate decarboxylase (GAD) is to convert glutamate in γ-aminobutyric acid (GABA). Glutamate decarboxylase exists as two major isoforms, termed GAD65 and GAD67, that are usually expressed in GABA-containing neurons in the central nervous system. GAD65 has been proposed to be associated with GABA exocytosis whereas GAD67 with GABA metabolism. In the present immunofluorescence study, we have investigated the presence of the two GAD isoforms in the semicircular canal cristae of wild type and GAD67-GFP knock-in mice. While no evidence for GAD65 expression was found, GAD67 was detected in a distinct population of peripherally-located supporting cells, but not in hair cells or in centrally-located supporting cells. GABA, on the other hand, was found in all supporting cells. The present result indicate that only a discrete population of supporting cells use GAD67 to synthesize GABA. This is the first report of a marker that allows to distinguish two populations of supporting cells in the vestibular epithelium. On the other hand, the lack of GABA and GAD enzymes in hair cells excludes its involvement in afferent transmission.

Diversity among principal and GABAergic neurons of the anterior olfactory nucleus

Understanding the cellular components of neural circuits is an essential step in discerning regional function. The anterior olfactory nucleus (AON) is reciprocally connected to both the ipsi- and contralateral olfactory bulb (OB) and piriform cortex (PC), and, as a result, can broadly influence the central processing of odor information. While both the AON and PC are simple cortical structures, the regions differ in many ways including their general organization, internal wiring and synaptic connections with other brain areas. The present work used targeted whole-cell patch clamping to investigate the morphological and electrophysiological properties of the AON's two main neuronal populations: excitatory projection neurons and inhibitory interneurons. Retrograde fluorescent tracers placed into either the OB or PC identified projection neurons. Two classes were observed with different physiological signatures and locations (superficial and deep pyramidal neurons), suggesting the AON contains independent efferent channels. Transgenic mice in which GABA-containing cells expressed green fluorescent protein were used to assess inhibitory neurons. These cells were further identified as containing one or more of seven molecular markers including three calcium-binding proteins (calbindin, calretinin, parvalbumin) or four neuropeptides (somatostatin, vasoactive intestinal peptide, neuropeptide Y, cholecystokinin). The proportion of GABAergic cells containing these markers varied across subregions reinforcing notions that the AON has local functional subunits. At least five classes of inhibitory cells were observed: fast-spiking multipolar, regular-spiking multipolar, superficial neurogliaform, deep neurogliaform, and horizontal neurons. While some of these cell types are similar to those reported in the PC and other cortical regions, the AON also has unique populations. These studies provide the first examination of the cellular components of this simple cortical system.