Endogenous cannabinoids (endocannabinoids) are endogenous compounds that resemble the active ingredient of marijuana and activate the cannabinoid receptor in the brain. They mediate retrograde signaling from principal cells to both inhibitory ["depolarization-induced suppression of inhibition" (DSI)] and excitatory ("depolarization-induced suppression of excitation") afferent fibers. Transient endocannabinoid release is triggered by voltage-dependent Ca(2+) influx and is upregulated by group I metabotropic glutamate receptor activation. Here we show that muscarinic acetylcholine receptor (mAChR) activation also enhances transient endocannabinoid release (DSI) and induces persistent release. Inhibitory synapses in the rat hippocampal CA1 region of acute slices were studied using whole-cell patch-clamp techniques. We found that low concentrations (0.2-0.5 microm) of carbachol (CCh) enhanced DSI without affecting basal evoked IPSCs (eIPSCs) by activating mAChRs on postsynaptic cells. Higher concentrations of CCh (> or =1 microm) enhanced DSI and also persistently depressed basal eIPSCs, mainly by releasing endocannabinoids. Persistent CCh-induced endocannabinoid release did not require an increase in [Ca2+]i but was dependent on G-proteins. Although they were independent at the receptor level, muscarinic and glutamatergic mechanisms of endocannabinoid release shared intracellular machinery. Replication of the effects of CCh by blocking acetylcholinesterase with eserine suggests that mAChR-mediated endocannabinoid release is physiologically relevant. This study reveals a new role of the muscarinic cholinergic system in mammalian brain.

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Previous histological and immunocytochemical studies suggest that reorganization of the dentate granule cell axons, the mossy fibers, can occur in epileptic human hippocampus (Sutula et al., 1989; Houser et al., 1990; Babb et al., 1991) and in animal models of epilepsy (Tauck and Nadler, 1985; Sutula et al., 1988; Cronin et al., 1992). However, neuroanatomical analyses of the trajectory and morphology of reorganized axons are not yet available. The present study was conducted to investigate single dentate granule cell axonal systems in human epileptic hippocampus. Individual mossy fibers were directly visualized by injecting a tracer (biocytin or Lucifer yellow) intracellularly in hippocampal slices prepared from temporal lobes that were surgically removed from patients for treatment of intractable epilepsy. Two major arborization patterns were identified: (1) the parent axons extended to and coursed through the hilus toward CA3, leaving collaterals along their paths in the hilus (N = 19 neurons); (2) in addition to the aforementioned axonal system, collateral(s) branched from the parent axon near the soma and projected to the granule cell layer and molecular layer, forming an aberrant axonal pathway (N = 9 neurons). These aberrant collaterals bore large boutons similar to those of the hilar axons and formed extensive plexuses in the granule cell layer and/or in the molecular layer. The summed length of collaterals in the granular/molecular layers was 1110.8 microns on average, which was one-fourth of the total summed length of the mossy fibers (3698.5 microns on average). The size of the somata in neurons that had aberrant collaterals was significantly larger than that of neurons without such collaterals (p < 0.025). In four cases, filopodium-like fine processes were present near the axon hillock and proximal parts of the parent axon, suggesting that the aberrant collateral formation might be an ongoing process in these tissues. The lack of control slices from normal living human hippocampus makes it difficult to assess to what extent the present findings are epilepsy associated. However, the presence of aberrant mossy fiber collaterals in the hippocampi used in the present study has been confirmed by Timm's staining and/or dynorphin immunohistochemistry in comparison with nonepileptic autopsy material, indicating its relation to epilepsy (Babb et al., 1991, 1992). At present, there seems to be a consensus that the projection of mossy fiber collaterals to the supragranular layer is a rare occurrence in normal rats (Lorento de Nó, 1934; Claiborne et al., 1986; Seress et al., 1991; present study), normal monkeys (Seress et al., 1991), and normal humans (Houser et al., 1990).(ABSTRACT TRUNCATED AT 400 WORDS)

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