Over the past decade, the possibility of intranasal drug administration to directly enhance brain levels of the administered ligand has received a great deal of clinical attention.
Practical, non-invasive, rapid and simple, OXT nasal spray has been used in numerous clinical trials where empathic feelings, social cognition, anxiety, and eye gaze behavior have been investigated mainly in single-dose studies with healthy subjects (MacDonald et al., 2011; Striepens et al., 2011; Zak et al., 2005).
Up till now, only very few preclinical studies have successfully reported behavioral changes after intranasal OXT delivery (Bales et al., 2013; Chang et al., 2012; Parker et al., 2005). In particular, chapter 6 of this thesis presents a series of studies conducted on male WTG rats where intranasal OXT treatment significantly induced anti-aggressive and pro-social changes, similarly to icv manipulation.
In general, the use of nasal spray as a tool for central delivery of drugs was promoted enormously after the revelation of a rise in the human CSF AVP level within 10 min after the intranasal AVP application (Born et al., 2002). Similarly, two studies, one in humans and one in rodents, have monitored the concentration of OXT in CSF and in plasma after intranasal OXT application. Without distinguishing between exogenous and endogenous source, a peak in OXT content in the extracellular fluid within the LS and the dorsal hippocampus has been found in rats and mice between 30 and 60 min after intranasal application of OXT (Neumann et al., 2013). In line, Striepens and colleagues have shown for the first time in humans increased CSF OXT concentration (+64%) 75 min after intranasal application of OXT at 24 IU dose (Striepens et al., 2013).
These studies, although minimal in number, seem to justify intranasal OXT application as methodology to manipulate the central OXTergic system, and consequently the behavior. However, the study of Striepens suggests that in the clinical studies where nasal OXT is used at the dose of 24 IU, the time interval between the intranasal application and any behavioral or neuroimaging experiments should be at least of 75 min, in contrast with the typical 45-50 min (Domes et al., 2010; Guastella et al., 2008; Macdonald and Feifel, 2013). Moreover, based on the different time-course and magnitude of the nonapeptides changes in the CSF of humans (Born et al., 2002; Striepens et al., 2013), it has been hypothesized that higher intranasal dose may result in a greater concentration gradient, especially detectable when sampling by lumbar spine puncture.
To possibly explain the entry of drugs into the central nervous system after intranasal application, several hypotheses have been formulated (Figure 8) (Dhuria et al., 2010;
Thorne et al., 1995; Thorne et al., 2004).
(1) OXT may enter the central nervous system via adsorptive or receptor-mediated or non-specific fluid phase endocytosis into olfactory sensory neurons. These neurons are bipolar with one end located in the nasal olfactory epithelium on the roof of the nasal cavity, and the other end extending through the holes in the cribriform plate of the ethmoid bone and
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Figure 8. Proposed mechanisms for oxytocin transport into the central nervous system following intranasal application (Thorne et al., 2004).
ending in the olfactory bulb (Watelet and Van Cauwenberge, 1999). Although it has been suggested that there is no morphological barrier between the nasal submucosa interstitial space and the olfactory perineuronal space (Erlich et al., 1986; Jackson et al., 1979), this intraneuronal route has been reported to be slow (hours to reach the olfactory bulb and days to reach other brain region), not explaining the behavioral effects researchers have reported within minutes after the nasal puff (Chen et al., 1998; Frey et al., 1997).
(2) Alternatively, OXT might be transported through extraneuronal pathway, probably relying on bulk flow transport, through perineural channels. After reaching the lamina propria a. OXT may enter channels created by olfactory ensheathing cells surrounding the olfactory
nerves, and directly access the brain parenchymal tissue and/or CSF. This pathway would allow OXT to reach the CSF within minutes (Chen et al., 1998; Frey et al., 1997).
b. The trigeminal neuronal pathway may also be involved in rapidly delivering OXT to the brain or spinal cord, following the trigeminal nerve that innervates the respiratory and olfactory epithelium of the nasal cavity and enters the subarachnoid space through the cribriform plate near the olfactory bulb, and/or through the anterior lacerated foramen near the pons (Dhuria et al., 2010; Thorne et al., 1995).
Next to these hypothesis, researchers are also discussing the possibility that:
(3) increased central level of OXT after intranasal application could actually occur indirectly through “feed forward” stimulation of the endogenous OXTergic system. The signal might come via
a. afferent feedback from the periphery, due to elevated circulating OXT (Churchland and Winkielman, 2012), or
b. neuronal inputs from OXTR expressing cells in the inferior part of the brain (olfactory areas) (Ostrowski, 1998; Tribollet et al., 1992) activated by synthetic OXT.
In fact, in both humans (Born et al., 2002; Striepens et al., 2013) and rodents (Neumann et al., 2013), increased OXT concentration after nasal delivery has been consistently found also in the plasma with an earlier onset of the peak (10-15 min), greater magnitude (+225%) and faster back-to-baseline recovery (within 75 min) as compared to CSF changes (Striepens et al., 2013). This fast increase of OXT concentration in the blood may be due to its rapid absorption by the heavily vascularized nasal mucosa, draining then into the bloodstream through both fenestrated epithelium and facial veins (Macdonald and Feifel, 2013). The enzymatic degradation of OXT in the blood is then probably responsible for its fast decrease (Veening et al., 2010). In general, the lack of temporal correspondence between central and peripheral OXT concentrations invites caution in using the plasma OXT measurements as accurate reflection of central OXT levels (Kagerbauer et al., 2013), and does not exclude that the later rise in CSF OXT content may be due to indirect activation of the endogenous central OXTergic system. Interestingly, recent studies in male rats have reported increased hypothalamic OXTergic neuronal activity (Carson et al., 2010) and pro-social effects (Ramos et al., 2013) after intraperitoneal OXT injection.
The hypothesis of indirect activation of the endogenous OXTergic system after intranasal OXT application has been recently challenged in rodent studies, including the one descripted in chapter 6 of this thesis. Contrary to Ludwig and colleagues who failed to show any change in behavior or neuronal activity after intranasal AVP application (Ludwig et al., 2013), increased neuronal activation in both OXTergic and other neurons was found in the PVN and SON of male WTG rats after intranasal application of OXT. This finding suggests that intranasal OXT may activate the endogenous hypothalamic OXTergic system and it leads to the hypothesis that intranasal OXT may also alter the neuronal activity of effector brain regions potentially responsible for the anti-aggressive and pro-affiliative effects observed in these animals after intranasal OXT application (chapter 6).
However, activation of OXTergic neurons does not necessarily imply evidence of following endogenous OXT central release, neither does it exclude the entry of exogenous OXT.
In conclusion, more studies are needed to describe mechanistic nose-to-brain communication and to optimize the experimental conditions, considering how much influence these can have on the drug deposition within the nasal cavity and presumably also on the diffusion to the brain (Dhuria et al., 2010). Up till now, no studies have actually discriminated between the exogenous and endogenous source of the rise in CSF OXT level
after nasal application, neither have they temporally and spatially described the distribution
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of synthetic OXT into the brain. Moreover, in order to advance the current understanding of the pharmacokinetics, pharmacodynamics, methodologies and general safety related to intranasal OXT application, especially for clinical use, research should direct attention to the impact of dose-depend effects, multi-dose strategy, chronic treatment, long-term effects, as well as contextual cues and individual differences moderating the effects.