Elsevier

Neuropharmacology

Volume 54, Issue 1, January 2008, Pages 117-128
Neuropharmacology

Cholecystokinin inhibits endocannabinoid-sensitive hippocampal IPSPs and stimulates others

https://doi.org/10.1016/j.neuropharm.2007.06.023Get rights and content

Abstract

Cholecystokinin (CCK) is the most abundant neuropeptide in the central nervous system. In the hippocampal CA1 region, CCK is co-localized with GABA in a subset of interneurons that synapse on pyramidal cell somata and apical dendrites. CCK-containing interneurons also uniquely express a high level of the cannabinoid receptor, CB1, and mediate the retrograde signaling process called DSI. Reported effects of CCK on inhibitory post-synaptic potentials (IPSPs) in hippocampus are inconsistent, and include both increases and decreases in activity. Hippocampal interneurons are very heterogeneous, and these results could be reconciled if CCK affected different interneurons in different ways. To test this prediction, we used sharp microelectrode recordings from pyramidal cells with ionotropic glutamate receptors blocked, and investigated the effects of CCK on pharmacologically distinct groups of IPSPs during long-term recordings. We find that CCK, acting via the CCK2 receptor, increases some IPSPs and decreases others, and most significantly, that the affected IPSPs can be classified into two groups by their pharmacological properties. IPSPs that are increased by carbachol (CCh-sIPSPs), are depressed by CCK, ω-conotoxin GVIA, and endocannabinoids. IPSPs that are enhanced by CCK (CCK-sIPSPs) are blocked by ω-agatoxin IVA, and are unaffected by carbachol or endocannabinoids. Interestingly, a CCK2 antagonist enhances CCh-sIPSPs, suggesting normally they may be partially suppressed by endogenous CCK. In summary, our data are compatible with the hypothesis that CCK has opposite actions on sIPSPs that originate from functionally distinct interneurons.

Introduction

Cholecystokinin (CCK) is the most abundant neuropeptide in the central nervous system (CNS) (Beinfeld et al., 1981), and is highly expressed in a subset of GABAergic interneurons of the hippocampus (Dockray, 1976, Innis et al., 1979). CCK is released mainly as CCK8-S, and also, at low concentrations, as CCK4 or CCK8-U (Rehfeld, 1985). CCK-releasing interneurons in the hippocampus all contain GABA (Somogyi et al., 1984) and most express cannabinoid receptors (Katona et al., 1999, Freund, 2003). The axons of many CCK-positive neurons terminate on hippocampal pyramidal cell somata in stratum (s.) pyramidale and their proximal dendrites of s. radiatum (Freund and Buzsaki, 1996). Central (CCK2) receptors are widely distributed throughout the CNS (Zarbin et al., 1983) and modulate stress, anxiety and exploratory behaviors (Singh et al., 1991, Matto et al., 1997).

Physiological actions of CCK in the hippocampus have been attributed to CCK2 (Bohme et al., 1988, Carlberg et al., 1992); however CCK2 has not yet been localized to specific neuronal sub-types in hippocampus or elsewhere (cf. Mercer et al., 2000). Reports of CCK's physiological actions are inconsistent, with both excitation (Dodd and Kelly, 1979, Boden and Hill, 1988, Bohme et al., 1988, Shinohara and Kawasaki, 1997), and inhibition (MacVicar et al., 1987, Perez de la Mora et al., 1993) of pyramidal cells having been demonstrated. CCK may inhibit pyramidal cells indirectly (Perez de la Mora et al., 1993) by increasing GABA release from interneurons (Miller and Lupica, 1994, Miller et al., 1997, Ferraro et al., 1999, Deng and Lei, 2006). Some discrepancies have been ascribed to dosage and application method, or to different effects of CCK on interneurons and pyramidal cells (Miller et al., 1997). The close association of CCK only with certain interneurons suggests that some of the reported discrepancies in CCK effects might reflect its actions on distinct classes of interneurons (Freund and Buzsaki, 1996).

The primary aim of the present study was to test the hypothesis that CCK affects different interneurons in different ways, by using pharmacological tools to identify classes of interneuron outputs. Focusing on the rat hippocampal CA1 region, we show that CCK2 activation mediates the effects of CCK agonists, and directly stimulates persistent spontaneous (sIPSP) activity in control conditions. However, other sIPSPs are initiated in the presence of carbachol (CCh) and CCK2 activation inhibits the CCh-sIPSPs. This does not represent opposing effects of CCK and CCh on the same interneurons however, because the IPSPs in these two different conditions are sharply distinguished by their sensitivity to endocannabinoids, calcium channel antagonists, muscarinic agonists, and GABAB agonists. We also report the first evidence that endogenously released CCK suppresses CCh-sIPSPs. Our data are consistent with the hypothesis that the disparate actions of CCK on inhibition reflect opposite effects on distinct interneuron classes. Indeed, the pharmacological profiles of these two classes of sIPSPs correspond well with the properties of PV- and CCK-expressing interneurons as described in the literature (see Table 1). We suggest that CCK could thereby link the actions of different interneurons, a hypothesis that may have implications for understanding some of the oscillatory electrical activity in hippocampus (Buzsaki, 2002, Baraban and Tallent, 2004, Freund, 2003, Whittington and Traub, 2003).

Section snippets

Materials and methods

Male Sprague–Dawley rats, 5–7 weeks old (Charles River Laboratories) were deeply anaesthetized with halothane and decapitated in accordance with the guidelines set forth by the Institutional Animal Care and Use Committee of the University of Maryland, School of Medicine. The brain was rapidly removed from the skull and both hippocampi dissected free. Transverse hippocampal sections (400 μm thick) were cut on a Vibratome (Series 1000, Technical Products International). Slices were kept in a

CCK analogs initiate sIPSP activity in control conditions by activating CCK2

We used high resistance microelectrodes for pyramidal cell recording under current clamp conditions because the experiments, involving multiple pharmacological tests on single cells, demanded long-term (>2 hour), stable recording conditions. The advantages of these electrodes - less disruption of cell internal environment and long lasting maintenance of normal cell properties - outweighed the drawback of somewhat noisier recordings. With D-AP5 and NBQX present, the average pyramidal cell

Discussion

Despite the close association of CCK with GABAergic interneurons, prior reports of its actions on synaptic inhibition have not yielded a coherent picture. Our findings can reconcile some of the previous contradictory results, and in addition, suggest that CCK may mediate interactions between different classes of interneurons. The main observation was that CCK has opposite effects on two kinds of pyramidal cell sIPSPs, suppressing the endocannabinoid-sensitive, CCh-sIPSPs, but inducing the

Acknowledgements

We thank Scott Thompson, Celine Dinocourt, David Edwards, and Carlos Lafourcade for their comments on a draft of this manuscript. This work was supported by NIH grants NS30219 and DA140625 to B.E.A. M.A.K. was supported in part by the Cellular and Integrative Neurosciences Training Grant (NS07275) to the University of Maryland.

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