Cholinergic phosphatidylinositol modulation of inhibitory, G protein-linked neurotransmitter actions: electrophysiological studies in rat hippocampus.

Abstract

In electrophysiological studies using the rat hippocampal slice preparation, cholinergic agonists and phorbol 12,13-diacetate, a stimulator ofprotein kinase C, block the inhibitory actions of baclofen, a -aminobutyric acid B receptor agonist, and adenosine. Relative potencies of cholinergic agonists in stimulating the phosphatidylinositol system, as measured biochemically, parallel their activity in blocking adenosine assessed electrophysiologically. Electrical stimulation ofcholinergic afferents also reverses adenosine's inhibitory action. These fridings indicate that stimulation of protein kinase C by the phosphatidylinositol system mediates cholinergic blockade of adenosine and baclofen. As these inhibitory agonists act by way of receptors linked to GTP-binding proteins, protein kinase C's inactivation of the GTP-binding protein involved may account for this cholinergic action.Several lines of evidence indicate a prominent role for the phosphatidylinositol (PtdIns) cycle in mediating transmitter actions in the brain. Protein kinase C (PKC) and inositol trisphosphate receptors localized by autoradiographic techniques occur in very high concentrations in synaptic areas of the brain (1,2). Phorbol esters that stimulate PKC (3) exert selective actions on ionic conductances in brain neurons (4-7). Direct injections of inositol trisphosphate into dorsal raphe neurons mimic transmitter actions thought to be linked to the PtdIns system (7).Recent studies suggest that PKC can block inhibitory transmitter actions associated with GTP-binding regulatory proteins (G proteins). Thus, in platelets, phorbol esters antagonize the ability of epinephrine to inhibit adenylate cyclase (8). In oocytes, phorbol esters block the increased potassium conductance elicited by adenosine (9). In brain slices, phorbol esters diminish the ability of adenosine to inhibit adenylate cyclase (10). In previous electrophysiologic studies of hippocampal CA1 pyramidal neurons, we found that the synaptically activated late hyperpolarization (11) (18,19). These interactions might reflect the ability of PKC to phosphorylate and thus inactivate the G proteins involved in mediating inhibitory transmitter actions (20). In the present study, we provide direct evidence that muscarinic cholinergic synaptic transmission, acting by way of the PtdIns cycle and PKC, blocks inhibitory effects of neurotransmitters mediated by receptor-regulated G proteins. METHODSElectrophysiological Recordings from Hippocampal Slices. Slices were obtained from adult male Sprague-Dawley rats by using techniques that have been described in detail elsewhere (21,22). For some field potential recordings, slices were obtained from aged Fisher 344 rats (30 months, National Institutes of Health). One 400-,um-thick slice was held submerged in the recording chamber at 30'C. Temperature was regulated by a heating-cooling module (Cambion, Cambridge, MA) and was monitored within 1 mm of a slice by a hypodermic thermistor probe. Other slices were maintained in an incubation chamber at...

Protein kinase C activators block specific calcium and potassium current components in isolated hippocampal neurons

“…A G protein serves as a substrate for PKC phosphorylation in platelets; the G protein regulating Z,, in brain may be a substrate for PKC, or, conversely, PKC may be regulated by a G protein. Interestingly, phorbol esters, and muscarinic agonists that cause PI turnover, block the ability of adenosine to activate a K conductance in hippocampal neurons (Worley et al, 1987) and oocytes (Dascal et al, 1985). An adenosine-induced increase in K conductance is also mediated by a G protein (Trussel and Jackson, 1986).…”

Barium, Glibenclamide and CGS21680 Prevent Adenosine A₁ Receptor Changes of ES Coupling and Spike Threshold

“…Previous investigators have also demonstrated a lack of relationship between cAMP levels and the electrophysiological effects of adenosine [14][15][16][17] , except in forskolin-treated hippocampal slices [18] .…”

Muscarinic agonists and phorbol esters increase tyrosine phosphorylation of a 40-kilodalton protein in hippocampal slices.

“…USA 86 (1989) late inositol phospholipid turnover share a common action. They block the inhibitory effects of several receptors coupled to potassium channels via pertussis-toxin-sensitive GTPbinding proteins (10,31). For example, potassium-channel activation elicited by adenosine or serotonin is blocked by these agents.…”

“…Muscarinic stimulation of the inositol phospholipid system in the hippocampus displays a characteristic pharmacology (10,17,18,21). Several muscarinic agonists such as carbachol, oxotremorine M, and oxotremorine 2 possess high efficacy, while others such as oxotremorine, arecoline, and pilocarpine behave as weak partial agonists.…”