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PubMed2002

Localisation of cannabinoid CB(1) receptor immunoreactivity in the guinea pig and rat myenteric plexus.

Activation of cannabinoid CB(1) receptors inhibits gastrointestinal motility, propulsion, and transit, whereas selective antagonism of these receptors has the opposite effects, suggesting the presence of endocannabinoid tone. Supporting evidence for presynaptic CB(1) receptors on myenteric neurons has been found in vitro. In this study, selective CB(1) receptor antibodies and neuronal markers were used to identify and characterise myenteric neurons expressing cannabinoid receptors. Whole mounts of rat and guinea pig myenteric preparations were dually labelled with antibodies against the CB(1) receptor and choline acetyltransferase, neurofilament proteins, calbindin, calretinin, synapsin I, microtubule-associated protein-2, calcitonin gene-related peptide, or substance P. The pattern of CB(1) receptor labelling and the neurochemical classification of CB(1) receptor-positive cells were markedly influenced by the species and fixation procedure. Virtually all choline acetyltransferase-immunoreactive myenteric neurons expressed CB(1) receptors in ganglia from both species. Subpopulations of neurons identified with calbindin, calretinin, and microtubule-associated protein-2 did not express CB(1) receptors. A few calcitonin gene-related peptide- and substance P-positive somata coexpressed CB(1) receptor immunoreactivity but showed little colocalisation on individual fibres. There was a close association between CB(1) receptor immunoreactivity and fibres labelled for synaptic protein, suggesting a role in the modulation of transmitter release. Functional responses to cannabinoids in the presence of hexamethonium suggest further that CB(1) receptors occur on excitatory motoneurons. In conclusion, CB(1) receptors are expressed on a variety of cholinergic sensory, interneuronal, and motor neurons in myenteric ganglia.

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PubMed2000

Synapse formation during neuron differentiation: an in situ study of the myenteric plexus during murine embryonic life.

Ultrastructural steps characterizing synapse formation in vivo and appearance in neuroblasts of properties suggestive of synaptic function acquisition have scarcely been studied. Synapse formation and proteosynthetic apparatus organization were thus studied under transmission electron microscope in mouse myenteric neurons from embryonic day 12.5 (E12.5) until birth. Expression of Ret and p75(NTR), markers of neural crest cells, as well as that of neuron-specific enolase (NSE), synaptophysin (SY), and synaptosomal-associated protein (SNAP), markers of synaptic function acquisition, were immunohistochemically evaluated. At E12.5 many cells were Ret- and p75(NTR)-immunoreactive (IR), whereas a few were NSE-IR and had neuronal ultrastructural characteristics. Two types of contacts between poorly or nondifferentiated cells and axons of presumed extrinsic (synapse-like contacts) or local (immature synapses) origin were identified, along with SY-IR elements. By E16. 5, many cells had developed a proteosynthetic apparatus, synapse-like contacts were no longer present, and immature synapses were gradually differentiating. Concurrently, there was an increase in NSE-IR cells, some of which were also SNAP-IR, and in SY-IR varicosities. At E18.5, ultrastructurally mature neurons and synapses had increased in number as had NSE-IR and SNAP-IR cells and SY-IR varicosities. These data indicate that 1) one type of contact (synapse-like) is present at E12.5 between very immature cells and presumed vagal fibers, with a possible transient role for the onset of the differentiative process of these cells; and 2) another type of contact (typical synapses) lasts until E18.5, with a similar but long-lasting role that progressively shifts to the classical function (neurotransmission) as the synapse matures and the embryo reaches the day of birth.

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