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Concomitant in Vivo Voltammetric and Electrophysiological Analysis Indicate that Nociceptin/ Orphanin FQ Affects Dopamine and then Serotonin Activities in Brain Substancia Nigra

Nociceptin/orphanin-FQ (NOCI) together with its receptor NOP are widely expressed in cortical and subcortical motor areas and it is known that NOCI acts as an anxiolytic attenuating the behavioral inhibition of animals acutely exposed to stressful/anxiogenic conditions.

Influence of NOCI upon the dopaminergic system has been observed in the ventral tegmental area and in the nucleus accumbens as well as an inhibitory action of NOCI is described upon serotoninergic mechanisms at cells and terminal levels. In particular, it is known that serotoninergic fibers from the raphe system project to the substancia nigra (SN) and that this modulation is behaviourally relevant.

In the present work, the effect of exogenous NOCI injected into the SN upon DA and 5-HT levels have been analyzed by means of differential pulse voltammetry and nafion-carbon fiber microelectrodes. Electrophysiological monitoring of multicell activity was concomitantly performed with the same microsensor.

It appeared that both levels of these biogenic amines were specularly altered, with possibly a driving influence of the DA activity upon the serotoninergic function(s).


Introduction
Nociceptin/orphanin-FQ (NOCI) is an opioid-like neuropeptide that activates a G-protein coupled receptor: the NOP receptor [1] NOCI and its receptor are widely expressed in cortical and subcortical motor areas [2].
In 1997 Jenck and Coll. [3] demonstrated that NOCI acts as an anxiolytic, attenuating the behavioral inhibition of animals acutely exposed to stressful/ anxiogenic conditions although the anxiolytic mechanism of NOCI is at present not completely clarified.
It has been reported that treatment with NOCI reduces the firing activity of dopamine (DA) cells in the ventral tegmental area (VTA) [4]and inhibits DA release in the nucleus accumbens [5]. This is resulting in altered regulation of motor control [1,6].
Concerning serotonin (5-HT), it is known that serotonergic fibers from the raphe system project to the substancia nigra (SN) and that this modulation is behaviourally relevant [7]. In particular, an inhibitory action of NOCI is described upon serotoninergic mechanisms exerted at two different levels:     Electrophysiology is then performed in the time gap between each DPV scan.
In such a way, these two types of in vivo recordings could be performed concurrently in situ, allowing direct comparison of cell firing and monoamine release in the same brain region and in real time. In addition, the influence of pharmacological treatments upon both electrochemical and electrophysiological signals is therefore studied concurrently in real time and in the same animal.
Note here the dose-dependent effect of local injection (arrow) of NOCI 1nm and 10nm, while NOCI 0.1nm is having no effect on the signal as well as aCSF (control).
Data are presented as % of basal pre-treatment levels ± sem, (n=4 each treatment).
Stats are shown in the figure. rapid decrease of (multi)-cell firing in the SN (figure 3).

Discussion
The present original in vivo data demonstrates that NOCI locally injected in the SN directly affects cell firing as well as DA and 5-HT levels in this brain region. It is known that serotoninergic fibers from the raphe system project to the Substancia Nigra (SN) [16] and in particular that serotonin released from terminals in SN, derived from cell bodies in the raphe dorsalis nucleus, decreases the activity of the nigro-striatal dopaminergic system [17]. It has been also observed that this modulation is behaviourally Additionally, the present data showing large decrease of 5-HT levels in SN following local treatment with NOCI and a parallel dose-dependent, rapid decrease of (multi)-cell firing in the SN are in accord with results showing reduction of electrical activity of 5-HT neurons in RDN [18] as well as 5-HT release i.e. in cerebral cortex [19] after NOCI injection. Also, the inhibitory effect of NOCI on the nigral release of 5-HT can be related to the altered motor activity monitored i.e. using the fixed-speed rotarod test [6].
When considering the present data in a whole, it appears that there is a rapid effect of NOCI upon both  The interaction between these two aminergic systems has been already reported in such diverse functions as temperature regulation [20], sleep [21], sexual behavior [22] and extrapyramidal function [23] although many results are conflicting [24].
In particular, it has been already reported that electrical stimulation of SN resulted in a rapid increase of the catecholaminergic DPVoltammetric signal followed by a slower decrease of the serotoninergic peak, both recorded simultaneously in the rat striatum [25].
Additionally, the implication of NOCI within the regulation of feeding, body weight homeostasis, stress, the stress-related psychiatric disorders of depression and anxiety, and in drug and alcohol dependence, pathological situations involving both DA and 5-HT, has been described (for a review see [34]