Norepinephrine stimulation of adenylate cyclase potentiates protein kinase C action: electrophysiological studies in the dentate gyrus

Long-term potentiation in the dentate gyrus in freely moving rats is reinforced by intraventricular application of norepinephrine, but not oxotremorine

Growing evidence suggests that processes of synaptic plasticity, such as long-term potentiation (LTP) occurring in one synaptic population, can be modulated by consolidating afferents from other brain structures. We have previously shown that an early-LTP lasting less than 4 h (E-LTP) in the dentate gyrus can be prolonged by stimulating the basolateral amygdala, the septum or the locus coeruleus within a specific time window. Pharmacological experiments have suggested that noradregeneric (NE) and/or cholinergic systems might be involved in these effects. We have therefore investigated whether the direct intraventricular application of agonists for NE- or muscarinic receptors is able to modulate synaptic plasticity. E-LTP was induced at the dentate gyrus of freely moving rats using a mild tetanization protocol that induces only an E-LTP. NE or oxotremorine (OXO) were applied icv 10 min after the tetanus. Results show that low doses of NE (1.5 and 5 nM) effectively prolong LTP. A higher dose (50 nM) was not effective. None of the OXO doses employed (5, 25, and 50 nM) showed similar effects. These results stress the importance of transmitter-specific modulatory influences on the time course of synaptic plasticity, in particular NE whose application mimics the reinforcing effect of directly stimulating limbic structures on LTP.

Reduced membrane-associated protein kinase C(beta) immunoreactivity in the hippocampus of reserpinized rat brain

Protein kinase C(beta) immunoreactivity was determined in the membrane and cytosol fractions of the hippocampus, frontal cortex and cerebellum after repetitive, intraperitoneal administration of reserpine. A marked decrease in the immunoreactivity was observed in the membrane of the hippocampus, while no significant change was found in any other subfractions of the frontal cortex or cerebellum. These findings suggest a novel mechanism regulating the distribution of protein kinase C. The topographical selectivity in the present study may indicate the importance of the alteration of protein kinase C in the pathophysiological mechanism in Alzheimer's disease.

Actions of norepinephrine in the rat hippocampus

Acting at postsynaptic alpha 1- and beta 1-receptors, norepinephrine (NE) exerts a complex action in rat hippocampus. It is currently believed that beta 1-receptor activation enhances excitability of recorded neurons, whereas alpha 1 activation suppresses reactivity to afferent stimulation. These reported effects of alpha-agonists are not consistent with alpha 1 effects found elsewhere in the brain. We have conducted experiments in the anesthetized rat and found that an amphetamine-induced increase in the dentate gyrus population spike can be blocked by a beta-antagonist but also by an alpha 1-antagonist. We have conducted experiments in the brain slide preparation and found that an alpha-agonist, phenylephrine (PHE), selectively enhances responses to N-methyl-D-aspartate (NMDA) but not to quisqualate. We propose that the product of activation of both alpha- and beta-receptor types will enhance reactivity of hippocampal cells to afferent stimulation.

Intrahippocampal injection of pertussis toxin blocks adenosine suppression of synaptic responses

Adenosine exerts prominent inhibitory effects on synaptic transmission via a presynaptic action. Using the hippocampal slice preparation, we have found in electrophysiological experiments that this action of adenosine is blocked by intrahippocampal injections of pertussis toxin. In biochemical studies, we have confirmed that this treatment affects the GTP-binding proteins, Gi and Go, in this preparation. These results indicate that both pre- and postsynaptic actions of adenosine involve pertussis toxin-sensitive GTP-binding proteins.

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

We have used the hippocampal slice preparation to investigate the regulation of protein tyrosine phosphorylation in brain. After pharmacological treatment of intact slices, proteins were separated by electrophoresis, and levels of protein tyrosine phosphorylation were assessed by immunoblotting with specific anti-phosphotyrosine antibodies. Phorbol esters, activators of the serine- and threonine-phosphorylating enzyme protein kinase C, selectively increase tyrosine phosphorylation of a soluble protein with an apparent molecular mass of approximately 40 kilodaltons. Muscarinic agonists such as carbachol and oxotremorine M that strongly activate the inositol phospholipid system also increase tyrosine phosphorylation of this protein. Neurotransmitter activation of the inositol phospholipid system and protein kinase C appears to trigger a cascade leading to increased tyrosine phosphorylation.