Research ReportIndividual and additive effects of neuromodulators on the slow components of afterhyperpolarization currents in layer V pyramidal cells of the rat medial prefrontal cortex
Introduction
Most ascending neuromodulatory systems, including those of serotonin (5-HT) (O'Hearn and Molliver, 1984), acetylcholine (ACh) (Mesulam et al., 1983), noradrenaline (NA) (Jones and Moore, 1977) and dopamine (DA) (Lindvall and Björklund, 1974), innervate the medial prefrontal cortex (mPFC) of mammalian brain and exhibit overlapping terminal fields (Lewis et al., 1992). Virtually all the original nuclei of these corticopetal systems in the brainstem or the basal forebrain receive reciprocal inputs, either directly or indirectly, from the PFC (for a review, see Briand et al., 2007). Except for 5-HT, they show the two distinct patterns of activity, tonic and phasic ones, which are evident in the pattern of either firing activity or neurotransmitter release. Tonic activity is state-specific and regulates arousal (Pribram and McGuinness, 1975), reflecting alterations in the activity over several minutes. On the contrary, phasic components are evoked by defined cognitive activity lasting for several seconds (Briand et al., 2007).
These systems affect the excitability of neurons in cortical local circuits, which mainly consist of pyramidal and nonpyramidal neurons. Each agonist exerts a depolarizing action on a specific subpopulation of GABAergic interneurons depending on the cell type, and makes them discharge a train of action potentials (Kawaguchi, 1997, Férézou et al., 2002, Gorelova et al., 2002). Although the effects on the membrane potential of pyramidal cells appear less prominent than those on nonpyramidal cells, two remarkable effects on the intrinsic membrane properties of these cells have been established. One is the potent inhibition of slow afterhyperpolarization potentials (sAHP), and the other is the replacement of sAHP by slow afterdepolarization (sADP) (Schwindt et al., 1988b, Haj-Dahmane and Andrade, 1998). sAHP arises from activation of calcium-activated potassium currents triggered by calcium influx during a train of action potentials, lasts for seconds (for a review, see Vogalis et al., 2003), causes spike-frequency adaptation, and is thus a major determinant of cellular excitability (Madison and Nicoll, 1982). Medium AHP (mAHP), which decays faster than sAHP (Stocker et al., 1999), has also been shown to follow a burst of action potentials, to be caused by calcium-activated potassium currents, and to affect early interspike interval (Faber and Sah, 2002). These two afterpolarizations could be separable on pharmacological grounds: mAHP is sensitive to the blockade by apamin, a bee toxin, whereas sAHP is not (Faber and Sah, 2002). Unlike sAHP and mAHP, sADP emerges solely in the presence of agonists (Schwindt et al., 1988b). The sADP temporally overlaps with the sAHP and only appear when the inward currents mediating the sADP exceed the outward current underlying sAHP (IsAHP) (Schwindt et al., 1988b).
Strong inhibition of sAHP has been shown for a range of neuromodulatory agonists including NA (Madison and Nicoll, 1982), the muscarinic agonist carbachol (CCh) (Schwindt et al., 1988b), and 5-HT (Pedarzani and Storm, 1993). This inhibition results in raised firing rate due to reduced spike-frequency adaptation (Madison and Nicoll, 1982, Pedarzani and Storm, 1993). The effects of DA on the sAHP of the pyramidal cells, however, are rarely addressed in the PFC despite its importance in PFC-related cognition (Sawaguchi and Goldman-Rakic, 1991). Moreover, given the enormous complexity of the effects of DA on current-evoked spike firing (Gorelova and Yang, 2000, Gulledge and Jaffe, 1998, Gulledge and Jaffe, 2001, Yang and Seamans, 1996), it is difficult to deduce the effects of DA on the sAHP.
Several lines of evidence indicate a concomitant increase in the release of multiple neuromodulator systems in the rat PFC that is induced by behavioral task or exposure to stress. Visual attention tasks enhance temporally overlapping efflux of NA and ACh in the mPFC (Dalley et al., 2001). Stress due to conditioned fear increases both 5-HT (Yoshioka et al., 1995) and ACh (Acquas et al., 1996) release in the PFC. Thus it is possible that interaction could take place between these systems at a cellular level, because neuromodulators released concomitantly in the overlapping target regions might exert their actions partly via the convergence of signaling pathways on the sAHP, that is, the usual inhibition of sAHP and the less frequent generation of sADP. Evidence describing such interaction between these systems at a cellular level, however, has remained scarce.
The purpose of the present study is two-fold. First, the difference in the individual effects of four neuromodulator agonists, 5-HT, CCh, NA and DA, on the mAHP and sAHP of layer V pyramidal neurons of the rat mPFC was examined. In particular, the link between their modulation of the IsAHP and the change in spike-frequency was tested, because very little is known about such a link for DA, whereas it has been established for the other three agonists (Madison and Nicoll, 1982, Schwindt et al., 1988b, Pedarzani and Storm, 1993). Second, the question of whether co-application of two distinct agonists, that have been shown to increase the spike-frequency, also show measurable additive effects on the currents underlying slow afterpotentials was examined. The inhibition of IsAHP and the generation of IsADP were considered collectively as an index to assess repetitive firing properties.
Section snippets
Results
Recordings were made from visually identified large pyramidal neurons (n=95) in layer V of the rat medial prefrontal cortex (mPFC). The primary focus of this study is to examine both the individual and additive effects of distinct neuromodulators on the IsAHP and the repetitive firing properties of layer V pyramidal cells. The number of cells tested for the actions of individual agonists and their combinations (all at 30 μM) was as follows: serotonin (5-HT), n = 13; noradrenaline (NA), n = 6;
Discussion
The major findings of the present study in the layer V pyramidal neurons of the rat mPFC are summarized as follows. (1) Inhibitory effects on the Ca2+-activated IsAHP were comparable among the four neuromodulator agonists, 5-HT, CCh, NA and DA, and the influences of these agents on ImAHP were much smaller than those on IsAHP. However, their effects on repetitive action potential firing properties were different because 5-HT, CCh and NA all increased the frequency of action potential firing by
Experimental procedures
Animal-use procedures were in accordance with the Tottori University Guide for the Care and Use of Laboratory Animals and were approved by the Tottori University Animal Care and Use Committee. All the experiments conformed to international guidelines on the ethical use of animals.
Acknowledgments
The authors thank Dr. M.Brown for critical reading the manuscript. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan.
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