Cannabinoid Signaling in the Auditory Brain Stem of the Chick (Gallus Domesticus)

Nucleus magnocellularis (NM) is a cochlear nucleus in the avian auditory brain stem which solely receives excitatory input from the auditory nerve fibers of cranial nerve VIII (cnVIII) and bilaterally innervates nucleus laminaris. The primary function of NM neurons is to code the temporal characteristics of acoustic stimuli and pass on this information to nucleus laminaris for use in coincidence detection and ultimately sound localization. Robust and dynamic stimuli could easily overwhelm many other synapses, but NM not only faithfully encodes important features of sounds, but can also enhance the information. Synaptic depression is one consequence of the large, rapid currents produced in NM in response to acoustic stimuli. The progressive decline in postsynaptic responses could lead to a failure in temporal coding. Therefore, depression must therefore be managed to allow coding not just at the onset, but throughout the duration of an ongoing stimulus. Many synaptic adaptations can be found at the cnVIII-NM synapse which act to dynamically adjust neuronal signaling in order to maintain consistent coding. GABAB receptors, in particular, present an interesting situation where activity-dependent inhibition can lead to an enhancement of neural signaling. The increase in synaptic reliability is presumably mediated through a conservation of neurotransmitter. The cannabinoid (CB) system represents another type of signaling that can mediate negative feedback, reducing neurotransmitter release. Furthermore the cannabinoid receptor one (CB1) is present in many sensory systems and is found throughout the brain of the chick. Relevant to auditory processing, the ganglion cells which form cnVIII produce CB1 mRNA. Immunohistochemical labeling of CB1 revealed that the calyceal terminals, not the cell bodies, of NM contain the functional receptor. Activation of these receptors with WIN 55,212-2 (WIN), a CB agonist, reduces excitatory postsynaptic currents, most likely through lowering of vesicle release probability. The high safety factor of neurotransmission at the calyx synapse means that a reduction in peak amplitude does not necessarily inhibit action potentials, but does appear to reduce the degree of observed synaptic depression. Endogenous CB production has been shown to occur in an activity-dependent manner, through either activation of metabotropic glutamate receptors or postsynaptic depolarization/intracellular Ca2+ rises. High frequency stimulation was able to induce an enhancement effect in current clamp which was blocked by pretreatment with a CB antagonist. The stimulation protocol was subsequently used under voltage clamp, but did not appear to initiate endogenous cannabinoid production as measured by paired-pulses. This finding suggests that metabotropic glutamate receptor activation is not sufficient to elicit cannabinoid production or paired-pulses are not a good measure in NM. If cannabinoids are produced endogenously at this synapse then the Ca2+-sensitive production pathway could be necessary as it require postsynaptic depolarization which does not occur under voltage clamp. A second way to measure CB effects is to measure the amplitude and frequency of spontaneous postsynaptic events. Picrotoxin, a GABAA antagonist, was used to isolate effects on glutamatergic signaling for study. In most cases picrotoxin had no effect; however, the drug was able to cause a near complete cessation of spontaneous postsynaptic currents in NM. A CB antagonist AM251 had an effect in the other direction and was able to preferentially increase the frequency, but not the amplitude of spontaneous events. These findings demonstrate that the vast majority of spontaneous depolarizing events are from random GABA release and there is a basal level of CB production even in an unstimulated slice. Another possibility is that the deafferentation-induced rise in intracellular Ca2+ caused this CB release. Unfortunately at this time we did not record from NM neurons with both picrotoxin and AM251 present in the bath. This limits our ability to interpret the data as it is not clear if the additional spontaneous events are from increased GABA or glutamate release. We did not detect any CB1 mRNA labeling of the superior olive which is responsible for the GABAergic input to NM. Also, WIN had an effect on depression even with picrotoxin present. This indirect evidence points to glutamate release as the underlying cause for the increased events. This study was by no means an exhaustive examination of CB signaling in the chick auditory brain stem. Rather, we made the first steps toward a new line of research as we now clearly know that CB1 is present and able to modulate signaling at the cnVIII-NM synapse. At this time we do not fully know how, to what degree, and when CB signaling is engaged. The functional purpose of CB would appear to enhance the coding of temporal events, however, we do not know at what scale. CB1 activation could function in a synapse-specific manner, complementary to GABAB activation, reducing synaptic depression at high rates of stimulation. Another, simpler role could be to maximize the signal to noise ratio by keeping the frequency of spontaneous glutamate release low. Further experiments will be needed to clarify these intriguing results.