Date Published: July 31, 2015
Publisher: Public Library of Science
Author(s): Ayat Kaeidi, Hossein Azizi, Mohammad Javan, S. Mohammad Ahmadi Soleimani, Yaghoub Fathollahi, Saeed Semnanian, James Edgar McCutcheon.
Studies have shown that following opiate withdrawal, the spontaneous discharge rate of locus coeruleus (LC) neurons remarkably increases. Combination of intrinsic mechanisms with extrinsic excitatory modulations mediates the withdrawal-induced hyperactivity of LC neurons. The nucleus paragigantocellularis (PGi) provides the main excitatory inputs to LC and plays a pivotal role in opiate withdrawal. In the present study the direct facilitatory role of PGi on opiate withdrawal-induced hyperactivity of LC neurons was investigated using a newly developed brain slice, containing both LC and PGi. HRP retrograde neuronal tracing was used to verify the existence of both LC and PGi neurons in the developed slice. The spontaneous discharge rate (SDR), resting membrane potential (RMP) and spontaneous excitatory post-synaptic currents (sEPSCs) were recorded in LC neurons using whole cell patch clamp recording. Results showed that the net SDR and the net RMP of LC neurons in slices containing both LC and PGi neurons are significantly higher than slices lacking intact (uncut) PGi inputs. Also, the frequency of sEPSCs in those LC neurons receiving PGi inputs significantly increased compared to the slices containing no intact PGi inputs. Altogether, our results propose that increase in PGi-mediated excitatory transmission might facilitate the opiate withdrawal-induced hyperactivity of LC neurons.
Locus coeruleus (LC) nucleus, which is bilaterally located on the floor of the fourth ventricle, is the largest cluster of noradrenergic neurons in brain stem [1,2]. This region expresses a high density of opioid receptors and because of its relatively homogenous structure, serves as a good model for studying opiate actions . Different in vivo and in vitro investigations have shown that acute morphine administration decreases LC neuronal activity, whereas these neurons undergo significant tolerance to opiate effects during chronic opiate exposure . This finding has been previously confirmed by the return of LC neuronal firing rate toward the pretreatment levels . In addition, following opiate withdrawal, the spontaneous firing rate of LC neurons increases dramatically above the normal level [5,6].
Identification of the neuronal network mediating autonomic, neurochemical and behavioral responses are essential for understanding opioid dependence and withdrawal. Among these diversified networks strong projections from PGi to LC have been verified by various anterograde and retrograde neuronal tracers and these routes have been confirmed via antidromic electrophysiologic stimulations .
In conclusion, our results strengthen the idea that intact PGi inputs, preserved in the OTH slices, facilitate the cellular expression of morphine withdrawal in the LC neurons. This is further supported by the electrophysiological differences observed among slice preparations. However, such facilitative effect of intact PGi afferents on cellular expression of morphine withdrawal in LC neurons has not been directly investigated in vitro. Also, these findings show that the hyperactivity of LC neurons during opiate withdrawal might be due to an increment in their resting membrane potential. This in turn could be mediated by an increase in the excitatory tone of PGi neurons to LC.