Mykhaylo Moldavan

Nociceptin/orphanin FQ (N/OFQ) inhibits excitatory and inhibitory synaptic signaling in the suprachiasmatic nucleus (SCN)

Environmental synchronization of the endogenous mammalian circadian rhythm involves glutamatergic and GABAergic neurotransmission within the hypothalamic suprachiasmatic nucleus (SCN). The neuropeptide nociceptin/orphanin FQ (N/OFQ) inhibits light-induced phase shifts, evokes K(+)-currents and reduces the intracellular Ca(2+) concentration in SCN neurons. Since these effects are consistent with a modulatory role for N/OFQ on synaptic transmission in the SCN, we examined the effects of N/OFQ on evoked and spontaneous excitatory and inhibitory currents in the SCN. N/OFQ produced a consistent concentration-dependent inhibition of glutamate-mediated excitatory postsynaptic currents (EPSC) evoked by optic nerve stimulation. N/OFQ did not alter the amplitude of currents induced by application of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-d-aspartate (NMDA) nor the amplitude of miniature EPSC (mEPSC) consistent with a lack of N/OFQ effect on postsynaptic AMPA or NMDA receptors. N/OFQ significantly reduced the mEPSC frequency. The inhibitory actions of N/OFQ were blocked by omega-conotoxin GVIA, an N-type Ca(2+)channel antagonist and partially blocked by omega-agatoxin TK, a P/Q type Ca(2+) channel blocker. These data indicate that N/OFQ reduces evoked EPSC, in part, by inhibiting the activity of N- and P/Q-type Ca(2+) channels. In addition, N/OFQ produced a consistent reduction in baseline Ca(2+) levels in presynaptic retinohypothalamic tract terminals. N/OFQ also inhibited evoked GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSC) in a concentration dependent manner. However, N/OFQ had no effect on currents activated by muscimol application or on the amplitude of miniature IPSC (mIPSC) and significantly reduced the mIPSC frequency consistent with an inhibition of GABA release downstream from Ca(2+) entry. Finally, N/OFQ inhibited the paired-pulse depression observed in SCN GABAergic synapses consistent with a presynaptic mechanism of action. Together these results suggest a widespread modulatory role for N/OFQ on the synaptic transmission in the SCN.

GABAB receptor‐mediated frequency‐dependent and circadian changes in synaptic plasticity modulate retinal input to the suprachiasmatic nucleus

•  Light entrains the circadian clock by activating intrinsically photosensitive retinal ganglion cells projecting axons through the retinohypothalamic tract (RHT) to the hypothalamic suprachiasmatic nucleus (SCN). •  Initial release probability and synaptic plasticity changes in RHT‐SCN synapses depended on the strength of GABAB receptor (GABABR)‐mediated presynaptic inhibition. •  The RHT axon terminals are under the tonic inhibitory control of GABAB receptors. CGP55845 (3 μm) application increased the evoked excitatory postsynaptic current amplitude 30% throughout the light–dark cycle. •  During the light and dark phases the RHT inputs to 55% and 33% of recorded neurons, respectively, were under GABAB inhibitory control indicating that the tonic GABA inhibition contributes to the circadian variation of transmitter release. •  GABABR‐mediated presynaptic inhibition depended on the sensitivity of RHT terminals to GABABR agonists and diurnal changes of the extracellular GABA concentration around RHT axon terminals in the SCN, and decreased with increasing frequency of RHT stimulation.

Retinohypothalamic Tract Synapses in the Rat Suprachiasmatic Nucleus Demonstrate Short-Term Synaptic Plasticity

The master circadian pacemaker located in the suprachiasmatic nucleus (SCN) is entrained by light intensity-dependent signals transmitted via the retinohypothalamic tract (RHT). Short-term plasticity at glutamatergic RHT-SCN synapses was studied using stimulus frequencies that simulated the firing of light sensitive retinal ganglion cells. The evoked excitatory postsynaptic current (eEPSC) was recorded from SCN neurons located in hypothalamic brain slices. The eEPSC amplitude was stable during 0.08 Hz stimulation and exhibited frequency-dependent short-term synaptic depression (SD) during 0.5 to 100 Hz stimulus trains in 95 of 99 (96%) recorded neurons. During SD the steady-state eEPSC amplitude decreased, whereas the cumulative charge transfer increased in a frequency-dependent manner and saturated at 20 Hz. SD was similar during subjective day and night and decreased with increasing temperature. Paired-pulse stimulation (PPS) and voltage-dependent Ca(2+) channel (VDCC) blockers were used to characterize a presynaptic release mechanism. Facilitation was present in 30% and depression in 70% of studied neurons during PPS. Synaptic transmission was reduced by blocking both N- and P/Q-type presynaptic VDCCs, but only the N-type channel blocker significantly relieved SD. Aniracetam inhibited AMPA receptor desensitization but did not alter SD. Thus we concluded that SD is the principal form of short-term plasticity at RHT synapses, which presynaptically and frequency-dependently attenuates light-induced glutamatergic RHT synaptic transmission protecting SCN neurons against excessive excitation.

GABAA Receptor-Mediated Neurotransmission in the Suprachiasmatic Nucleus

GABAergic neurotransmission is a fundamental component of the suprachiasmatic nucleus (SCN) neural network, and virtually all SCN neurons communicate using GABA as a neurotransmitter. GABAergic neurotransmission plays a critical role in light-induced phase shifts, synchronization of the dorsal and ventral SCN, and, although controversial, synchronization of the circadian phase of individual SCN neurons. The circadian clock regulates the strength of GABAA receptor-mediated neurotransmission although the signaling mechanisms mediating this regulation are not known. GABA released from axon terminals acts on synaptic GABAA receptors producing postsynaptic currents that have a rapid onset and offset and desensitize in the continued presence of GABA. In the SCN, the postsynaptic GABAA receptor-mediated currents may be excitatory or inhibitory depending on the time of day. Once released GABA is removed from the synaptic cleft by specific sodium–chloride-dependent transporters (GAT). Some GABA can diffuse out of the synaptic cleft and act on extrasynaptic GABAA receptors. These extrasynaptic GABAA receptors have high affinity for GABA and show little or no desensitization. They mediate a “tonic” GABAA current that could modulate the input–output characteristics of individual SCN neurons. While significant scientific questions remain about the roles of GABAergic neurotransmission in the circadian timing signals, recent findings have yielded important advances in our understanding of GABAergic neurotransmission in the SCN.