VGluT3-expressing CCK-positive basket cells construct invaginating synapses enriched with endocannabinoid signaling proteins in particular cortical and cortex-like amygdaloid regions of mouse brains

J Neurosci. 2015 Mar 11;35(10):4215-28. doi: 10.1523/JNEUROSCI.4681-14.2015.

Abstract

Invaginating synapses in the basal amygdala are a unique type of GABAergic synapses equipped with molecular-anatomical organization specialized for 2-arachidonoylglycerol (2-AG)-mediated endocannabinoid signaling. Cholecystokinin (CCK)-positive basket cell terminals protrude into pyramidal cell somata and form invaginating synapses, where apposing presynaptic and postsynaptic elements are highly loaded with cannabinoid receptor CB₁ or 2-AG synthetic enzyme diacylglycerol lipase-α (DGLα), respectively. The present study scrutinized their neurochemical and neuroanatomical phenotypes in adult mouse telencephalon. In the basal amygdala, vesicular glutamate transporter-3 (VGluT3) was transcribed in one-fourth of CB₁-expressing GABAergic interneurons. The majority of VGluT3-positive CB₁-expressing basket cell terminals apposed DGLα clusters, whereas the majority of VGluT3-negative ones did not. Importantly, VGluT3-positive basket cell terminals selectively constructed invaginating synapses. GABAA receptors accumulated on the postsynaptic membrane of invaginating synapses, whereas metabotropic glutamate receptor-5 (mGluR₅) was widely distributed on the somatodendritic surface of pyramidal cells. Moreover, CCK₂ receptor (CCK₂R) was highly transcribed in pyramidal cells. In cortical regions, pyramidal cells equipped with such VGluT3/CB₁/DGLα-accumulated invaginating synapses were found at variable frequencies depending on the subregions. Therefore, in addition to extreme proximity of CB₁- and DGLα-loaded presynaptic and postsynaptic elements, tripartite transmitter phenotype of GABA/glutamate/CCK is the common neurochemical feature of invaginating synapses, suggesting that glutamate, CCK, or both can promote 2-AG synthesis through activating Gαq/₁₁ protein-coupled mGluR₅ and CCK₂R. These molecular configurations led us to hypothesize that invaginating synapses might be evolved to provide some specific mechanisms of induction, regulation, and cooperativity for 2-AG-mediated retrograde signaling in particular cortical and cortex-like amygdaloid regions.

Keywords: amygdala; cannabinoid CB1 receptor; cerebral cortex; cholecystokinin-positive basket cell; invaginating synapse; sn-1-specific diacylglycerol lipase.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Transport Systems, Acidic / genetics
  • Amino Acid Transport Systems, Acidic / metabolism*
  • Amino Acid Transport Systems, Acidic / ultrastructure
  • Amygdala / cytology*
  • Animals
  • Cerebral Cortex / cytology*
  • Cholecystokinin / genetics
  • Cholecystokinin / metabolism*
  • Endocannabinoids / metabolism*
  • Glutamate Decarboxylase / genetics
  • Glutamate Decarboxylase / metabolism
  • Lipoprotein Lipase / metabolism
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Microscopy, Immunoelectron
  • Neurons / metabolism*
  • Neurons / ultrastructure
  • RNA, Messenger / metabolism
  • Receptor, Cannabinoid, CB1 / genetics
  • Receptor, Cannabinoid, CB1 / metabolism
  • Receptor, Cholecystokinin B / genetics
  • Receptor, Cholecystokinin B / metabolism
  • Signal Transduction / genetics
  • Signal Transduction / physiology*
  • Synapses / metabolism*
  • Synapses / ultrastructure

Substances

  • Amino Acid Transport Systems, Acidic
  • Endocannabinoids
  • RNA, Messenger
  • Receptor, Cannabinoid, CB1
  • Receptor, Cholecystokinin B
  • vesicular glutamate transporter 3, mouse
  • Cholecystokinin
  • Lipoprotein Lipase
  • Glutamate Decarboxylase
  • glutamate decarboxylase 1