Correlation of morphology, electrophysiology and chemistry of neurons in the myenteric plexus of the guinea-pig distal colon

Intracellular recordings were made from myenteric neurons of the guinea-pig distal colon to determine their electrical behaviour in response to intrac ellular current injection and stimulation of synaptic inputs. The recording microelectrode contained the intracellular marker biocytin, which was inje cted into impaled neurons so that electrophysiology, shape and immunohistoc hemistry could be correlated. Myenteric neurons in the distal colon were di vided into four morphological groups based on their shapes and projections. One group (29 of the 78 that were characterized electrophysiologically, mo rphologically and immunohistochemic ally) was the multiaxonal Dogiel type I I neurons, the majority (25/29) of which were calbindin immunoreactive, Eac h of these neurons had an inflection on the falling phase of the action pot ential that, in 24/29 neurons, was followed by a late afterhyperpolarizing potential (AHP). Slow excitatory postsynaptic potentials were recorded in 2 0 of 29 Dogiel type II neurons in response to high. frequency internodal st rand stimulation and two neurons responded with slow inhibitory postsynapti c potentials. Low amplitude fast excitatory postsynaptic potentials occurre d in 3 of 29 Dogiel type II neurons. Neurons of the other three groups were all uniaxonal: neurons with Dogiel type I morphology, filamentous ascendin g interneurons and small filamentous neurons with local projections to the longitndinal or circular muscle or to the tertiary plexus. Dogiel type I ne urons were often immunoreactive for nitric oxide synthase or calretinin, as were some small filamentous neurons, while all filamentous ascending inter neurons tested were calretinin immunoreactive. All uniaxonal neurons exhibi ted prominent fast excitatory postsynaptic potentials and did not have a la te AHP following a single action potential, that is, all uniaxonal neurons displayed S type electrophysiological characteristics. However, in 6/19 Dog iel type I neurons and 2/8 filamentous ascending interneurons, a prolonged hyperpolarizing potential ensued when more than one action potential was ev oked. Slow depolarizing postsynaptic potentials were observed in 20/29 Dogi el type I neurons, 6/8 filamentous ascending interneurons and 8/12 small fi lamentous neurons. Six of 29 Dogiel type I neurons displayed slow inhibitor y postsynaptic potentials, as did 2/8 filamentous ascending interneurons an d 4/12 small filamentous neurons, These results indicate that myenteric neu rons in the distal colon of the guinea-pig are electrophysiologically simil ar to myenteric neurons in the ileum, duodenum and proximal colon. Also, th e correlation of AH electrophysiological characteristics with Dogiel type I I morphology and S electrophysiological characteristics with uniaxonal morp hology is preserved in this region. However, filamentous ascending interneu rons have not been encountered in other regions of the gastrointestinal tra ct and there are differences between the synaptic properties of neurons in this region compared to other regions studied, including the presence of sl ow depolarizing postsynaptic potentials that appear to involve conductance increases and frequent slow inhibitory postsynaptic potentials. (C) 1999 Pu blished by Elsevier Science B.V. All rights reserved.