Recent data indicate that there is a cardiotopic organization of negative chronotropic
and negative dromotropic neurons in the nucleus ambiguus (NA). Negative dromotropic
neurons are found in the rostral ventrolateral NA (rNA-VL), negative chronotropic
neurons are found in the caudal ventrolateral NA (cNA-VL), and both types of neurons
are found in an intermediate level of the ventrolateral NA (iNA-VL). Substance P (SP)
immunoreactive nerve terminals synapse upon negative chronotropic vagal motoneurons
in the iNA-VL, and SP microinjections in the NA cause bradycardia. In the present
report we have attempted to: (1) define the type of tachykinin receptor which mediates
the negative chronotropic effect of SP microinjections into the iNA-VL; (2) define
the physiological effect of microinjections of a selective SP agonist into the rNA-VL
on atrioventricular (AV) conduction: and (3) find ultrastructural evidence for synaptic
interactions of SP-immunoreactive nerve terminals with negative dromotropic vagal
motoneurons in the rNA-VL. Microinjections of the excitatory amino acid glutamate
(Glu) into the iNA-VL to activate all local vagal preganglionic neurons caused both
bradycardia and a decrease in the rate of AV conduction. Injections of the selective
neurokinin-1 (NK-1) receptor agonist drug GR-73632 also caused bradycardia, however
the rapid onset of agonist induced desensitization prevented an evaluation of potential
effects on AV conduction in the iNA-VL. These data suggest that the SP-induced bradycardia
which can be elicited from the NA is mediated, at least in part, by NK-1 receptors.
Microinjections of Glu into the rNA-VL caused a decrease in AV conduction without
an effect on cardiac rate. On the other hand, GR-73632 microinjections into rNA-VL
did not affect AV conduction. Following injections of the beta subunit of cholera
toxin conjugated to horseradish peroxidase (CTB-HRP) into the left atrial fat pad
ganglion which selectively mediates changes in AV conduction, retrogradely labeled
neurons were histochemically visualized in the rNA-VL. These tissues were subsequently
processed for the simultaneous immunocytochemical visualization of SP, and examined
by electron microscopy. Histochemically labeled neurons were large, multipolar, with
abundant cytoplasm containing large masses of rough endoplasmic reticulum, and exhibited
distinctive dendritic and somatic spines. Unlabeled nerve terminals were noted to
form either asymmetric or symmetric synapses with dendrites, dendritic spines, and
perikarya of histochemically labeled neurons. SP-immunoreactive nerve terminals were
also detected in the rNA-VL. SP terminals typically contained numerous small pleomorphic
vesicles, multiple large dense core vesicles, and several mitochondria, and they synapsed
upon unlabeled dendritic profiles. A total of 154 SP-immunoreactive nerve terminals
were observed on photomicrographs of tissues which also contained histochemically
labeled profiles. None made an identifiable synapse with a retrogradely labeled profile
on the sections examined. In summary, both physiological and ultrastructural data
indicate that SP terminals in the iNA-VL do modify the output of negative chronotropic
vagal motoneurons. This effect is mediated by NK-1 receptors. On the other hand both
physiological and ultrastructural data indicate that SP terminals in the rNA-VL do
not modify the output of negative dromotropic vagal motoneurons. Therefore different
mechanisms (neurotransmitters or receptors) mediate the central vagal control of cardiac
rate and AV conduction.