The Horsehead Nebula, thanks to its proximity (1'' = 0.002 pc) and simple geometry is a perfect benchmark case to study the interplay between structure and chemistry in PDRs (Photon Dominated Regions). Our past studies of many tracers enabled us to obtain a clear picture of the density and temperature structure of the region. The presence of a steep density gradient (from less than 10^3 cm-3 to 10^5 cm-3 in less than 10'' = 0.02 pc) allows to probe different environments, from far-UV photon-dominated regions to shielded cold gas, in less than 50''. With the increased sensitivity available, extragalactic observations of gas tracers other than CO such as HCN, HNC, HCO^+ become more common. These observations are used in the extragalactic context to probe the density of the interstellar medium, to identify the heating mechanisms : FUV photons --- related to starburst phenomena, whose interaction with the ISM can be understood using PDR models ) or X rays --- related to AGN, using XDR models ---, and to probe star formation using specific molecular lines as tracers of embedded star forming regions. Nevertheless these observations are of low linear resolution and the details of the physics and chemistry of the interstellar medium are smeared by averaging over different environments. We will study how the understanding of these tracers in well known Galactic environments, such as the Horsehead PDR, can help the interpretation of extragalactic observations. We present high resolution multiline observations of HCN, HNC, CN and HCO^+ by the IRAM PdBI and 30m telescope instruments. We study the emission line ratios of these different molecules with the objective of comparing these high resolution observations of a well characterized Galactic region to extragalactic observations of the same tracers. Among the questions raised are : What is the importance of hyperfine anomalies in understanding the HCN and HNC observations? Can HCN be used as a tracer of star formation at subpc/pc scales? Is the HNC/HCN ratio a tracer of PDR? Are the HCO^+/HCN and HCO^+/HNC ratios good probes of density?< Réduire