Excitation vs. Inhibition - Version 86

Cross references:    Excitation    Inhibition  
Sympathetic Nervous System     
Parasympathetic Nervous System       
Initiation of Locomotion in Lampreys      Inhibition of Locomotion in Lampreys    

Primary afferents evoke excitatory amino acid receptor-mediated EPSPs that are modulated by presynaptic GABAB receptors in lamprey.      
    1.  "
The primary afferent neurons (dorsal cells) are of two types in lamprey, which are fast (touch) and slowly adapting (pressure), respectively. Intracellular stimulation of such sensory neurons evokes mono- and polysynaptic excitatory postsynaptic potentials (EPSPs) in spinobulbar neurons (giant interneurons) and in unidentified interneurons. Paired intracellular recordings between identified sensory cells and spinobulbar neurons made it possible to study the synaptic transmission in detail. It is shown that both touch and pressure primary afferents utilize excitatory amino acid (EAA) transmission and, furthermore, that these effects are subject to a presynaptic GABAB receptor modulation. 
    2. The monosynaptic mixed electrical and chemical EPSPs in giant interneurons had a mean peak amplitude of 3.2 +/- 1.3 (SD) mV, a time to peak of 4.7 +/- 1.2 ms, and a duration at one-half peak amplitude of 9.4 +/- 3.2 ms. Corresponding results were obtained with dorsal root or dorsal column stimulation. Seventy percent of the fast-adapting dorsal cells of the "touch" type evoked monosynaptic mixed EPSPs in giant interneurons, whereas only 3% of the slowly adapting "pressure" dorsal cells did.  
    3. The chemical part of the monosynaptic EPSPs evoked in giant interneurons was, in all cases tested, blocked by application of EAA antagonists, like the nonselective antagonist kynurenic acid (KYAC; 2 mM). The selective kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5 microM) had a similar effect, whereas the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphono-5-valeric acid (AP-5; 200-400 microM) did not change the EPSP, even in the absence of magnesium ions. 
     4. The monosynaptic excitatory synaptic transmission was modulated by application of the selective GABAB receptor agonist L-baclofen (5-10 mM local droplet application or 100-1,000 microM bath applied) or by gamma-aminobutyric acid (GABA; 100-1,000 microM), also when GABAA receptor-evoked effects were blocked by bicuculline (10 microM). L-baclofen or GABA in combination with bicuculline did not evoke any effects in the postsynaptic neuron on membrane potential, input resistance, or spike threshold. Therefore the effects of the GABAB receptor activation most likely occurs at the presynaptic afferent level.  
    5.  In conclusion, the monosynaptic excitation from skin mechanoreceptors evoked in spinobulbar neurons is mediated by EAA receptors of the kainate/AMPA type. GABAB receptor activation causes a depression of this EPSP, most likely because of a presynaptic action. GABA interneurons are known to form close appositions on sensory axons in the lamprey."  
My comments:   
"It is shown that both touch and pressure primary afferents utilize excitatory amino acid (EAA) transmission and, furthermore, that these effects are subject to a presynaptic GABAB receptor modulation."   
    Although not stated explicitly, this makes it sound as though the "
presynaptic GABAB receptors" which modulate the "EEA transmission" are on the same neurons that transmit the EEAs.  
2.  I think that the term "presynaptic" probably always indicates neuromodulators rather than neurotransmitters. 
Please see:    Neuromodulators vs Neurotransmitters  .
3.  The distinction between "fast (touch) and slowly adapting (pressure)" primary afferent neurons echos the distinction between fast twitch and slow twitch muscles.  Is this just a coincidence, or do fast primary afferent neurons contact fast twitch muscles and slow primary afferent neurons contact slow twitch muscles?  
Lamprey Fast-Slow Twitch
    1180 Related citations
     Since this is the first reference I've found that discusses both excitation and inhibition, I may skim the 1180 Related citations.   But first, the next step might be to list references for the two types. 

Finding new references: 

1972    1168<1180 
Gamma-aminobutyric acid antagonism and presynaptic inhibition in the frog spinal cord.  
     The convulsant alkaloid bicuculline blocked presynaptic inhibition, dorsal root potentials, primary afferent depolarization, and depolarizing effects of gamma-aminobutyric acid on dorsal root terminals of the amphibian spinal cord, but did not block effects of other putative amino acid transmitters. These actions of bicuculline suggest that gamma-aminobutyric acid may be the transmitter involved in spinal presynaptic inhibition."  

1974    1163<1180 
On the transmitter function of 5-hydroxytryptamine at excitatory and inhibitory monosynaptic junctions.

1975  1148<1180 
Localization and electrical characteristics of a giant synapse in the spinal cord of the lamprey.