Immunohistochemical evidence for degeneration of cholinergic neurons in the forebrain of the rat following injection of AF64A-picrylsulfonate into the dorsal hippocampus

Regulator of G protein signaling (RGS16) inhibits hepatic fatty acid oxidation in a carbohydrate response element-binding protein (ChREBP)-dependent manner

G protein-coupled receptor (GPCR) pathways control glucose and fatty acid metabolism and the onset of obesity and diabetes. Regulators of G protein signaling (RGS) are GTPase-activating proteins (GAPs) for G(i) and G(q) α-subunits that control the intensity and duration of GPCR signaling. Herein we determined the role of Rgs16 in GPCR regulation of liver metabolism. Rgs16 is expressed during the last few hours of the daily fast in periportal hepatocytes, the oxygen-rich zone of the liver where lipolysis and gluconeogenesis predominate. Rgs16 knock-out mice had elevated expression of fatty acid oxidation genes in liver, higher rates of fatty acid oxidation in liver extracts, and higher plasma β-ketone levels compared with wild type mice. By contrast, transgenic mice that overexpressed RGS16 protein specifically in liver exhibited reciprocal phenotypes as well as low blood glucose levels compared with wild type littermates and fatty liver after overnight fasting. The transcription factor carbohydrate response element-binding protein (ChREBP), which induces fatty acid synthesis genes in response to high carbohydrate feeding, was unexpectedly required during fasting for maximal Rgs16 transcription in liver and in cultured primary hepatocytes during gluconeogenesis. Thus, RGS16 provides a signaling mechanism for glucose production to inhibit GPCR-stimulated fatty acid oxidation in hepatocytes.

Electrophysiological and morphological heterogeneity of slow firing neurons in medial septal/diagonal band complex as revealed by cluster analysis

Slow firing septal neurons modulate hippocampal and neocortical functions. Electrophysiologically, it is unclear whether slow firing neurons belong to a homogeneous neuronal population. To address this issue, whole-cell patch recordings and neuronal reconstructions were performed on rat brain slices containing the medial septum/diagonal band complex (MS/DB). Slow firing neurons were identified by their low firing rate at threshold (<5 Hz) and lack of time-dependent inward rectification (Ih). Unsupervised cluster analysis was used to investigate whether slow firing neurons could be further classified into different subtypes. The parameters used for the cluster analysis included latency for first spike, slow after-hyperpolarizing potential, maximal frequency and action potential (AP) decay slope. Neurons were grouped into three major subtypes. The majority of neurons (55%) were grouped as cluster I. Cluster II (17% of neurons) exhibited longer latency for generation of the first action potential (246.5+/-20.1 ms). Cluster III (28% of neurons) exhibited higher maximal firing frequency (25.3+/-1.7 Hz) when compared with cluster I (12.3+/-0.9 Hz) and cluster II (11.8+/-1.1 Hz) neurons. Additionally, cluster III neurons exhibited faster action potentials at suprathreshold. Interestingly, cluster II neurons were frequently located in the medial septum whereas neurons in cluster I and III appeared scattered throughout all MS/DB regions. Sholl's analysis revealed a more complex dendritic arborization in cluster III neurons. Cluster I and II neurons exhibited characteristics of "true" slow firing neurons whereas cluster III neurons exhibited higher frequency firing patterns. Several neurons were labeled with a cholinergic marker, Cy3-conjugated 192 IgG (p75NTR), and cholinergic neurons were found to be distributed among the three clusters. Our findings indicate that slow firing medial septal neurons are heterogeneous and that soma location is an important determinant of their electrophysiological properties. Thus, slow firing neurons from different septal regions have distinct functional properties, most likely related to their diverse connectivity.

Effects of AF64A on gene expression of choline acetyltransferase (ChAT) in the septo-hippocampal pathway and striatum in vivo

AF64A (ethylcholine mustard aziridinium ion) was stereotaxically administered bilaterally (1 nmol/side) into rat lateral cerebral ventricles. Choline acetyltransferase (ChAT) activity and ChAT mRNA levels were measured at predetermined time points in the septo-hippocampal pathway and striatum, both well identified as rich in cholinergic neurons. AF64A caused a rapid but transient increase in ChAT mRNA (167%, P < 0.05) and ChAT activity (164%, P < 0.01) in the septum. By day 7 post treatment, there was a significant decrease in ChAT mRNA (42.5% of control, P < 0.05) in the septum although the ChAT activity still stayed high. This decreased ChAT mRNA level in the septum lasted for at least four weeks, and was paralleled by a long-lasting decrease in ChAT activity in the hippocampus. In the striatum, on the other hand, there were no observed changes in either ChAT activity or ChAT mRNA. These data suggest that the long term effect of AF64A on the septo-hippocampal cholinergic pathway may, at least in part, be due to an action of AF64A on gene expression in the cholinergic neuron. The difference in the response to AF64A between the septo-hippocampal and striatal cholinergic systems might be due to their difference in neuron types.

Morphological assessment of ethyl choline mustard aziridinium-induced neurotoxicity in rat brain reaggregate cultures

Foetal rat brain reaggregate cultures have been employed to investigate the morphological changes associated with the neurotoxic action of ethylcholine mustard aziridinium (ECMA). In a companion study we provided evidence for apparent selective cholinergic neurotoxicity. Exposure of 9-day-old cultures to 12.5 microM ECMA for 3 days produced dilatation of selected axon preterminals and terminals in the outer core tissue layer. Axoplasm in these dilated terminals was electron lucent and contained a flocculent, plasma-like material with remnants of the smooth endoplasmic reticulum. Their synaptic vesicle content was much reduced or, absent. Microglial cells were engaged in phagocytosis of these effete structures and a few necrotic neurons were enveloped by glial processes. Exposure to 50 microM ECMA produced widespread necrosis with some surviving neurons, surrounded by the still-persisting capsular layer. Treatment with 100 microM ECMA generated a greater extent of tissue necrosis, with only a few surviving neurons and glial cells being contained within the necrotic tissue mass. Reaggregates frequently disintegrated following capsule loss. Our results indicate that the initial morphological manifestation of ECMA-induced toxicity is dilatation of axon terminals, that are probably of cholinergic origin and are targeted due to their possession of the high-affinity choline transport system which is unique to these neurons.

Involvement of the septohippocampal cholinergic system in representational memory

To develop an animal model for testing muscarinic agonists, we examined the effects of cholinergic lesions with the ethylcholine aziridinium ion (AF64A) on two types of memory tasks. The tasks provided a distinction between representational and dispositional memory that could be measured in a single paradigm. Young, male Long-Evans rats were trained in a modified T-maze to learn both a discrimination task and a paired-run alternation task. Once animals learned the tasks, they were administered either saline or AF64A (5 nmol into each hippocampus) via stereotaxic technique. One week following surgery, saline-treated animals exhibited comparable performances (P greater than 0.2) on both the discrimination task (90.0 +/- 2.6% correct) and the alternation task (79.5 +/- 5.7%). In contrast, animals treated with AF64A showed a significant impairment of performance (P less than 0.005) on the alternation task (56.1 +/- 1.7%) as compared to the discrimination task (81.6 +/- 5.0%). Performance of the alternation task was significantly lower for AF64A-treated animals than for controls (P less than 0.02). AF64A-treated animals subsequently injected with pilocarpine (1.0 mg/kg, i.p.) showed moderate improvements in performance on the alternation task, while performance on the discrimination task remained unaffected. Immunocytochemical studies of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH) immunoreactivity indicated a loss of ChAT-positive cells in the septal region in AF64A-injected animals while TH-positive cells in the ventral tegmental area were unaffected by the treatment. The data suggest that AF64A can be used to produce selective lesions of the septohippocampal cholinergic system, which plays a greater role in representational memory than in dispositional memory.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrophy of cholinergic neurons within the rat nucleus basalis magnocellularis following intracortical AF64A infusion

The rat nucleus basalis magnocellularis (nBM) was morphometrically analyzed following multiple intracortical AF64A infusions. At 3 weeks post-infusion, brains were histochemically double-stained for acetyl-cholinesterase and Nissl substance following diisopropylfluorophosphate pretreatment. Intracortical AF64A induced significant atrophy, but not degeneration, of nucleus basalis cholinergic cell bodies. These results suggest that retrograde cellular atrophy is associated with inhibition of presynaptic high-affinity choline transport on cortical terminals of nBM cholinergic neurons.