The effect of direct administration of drugs into the licking generator in rats

Spontaneous licking of thirsty rats was investigated under the effect of GYKI (an AMPA/kainate receptor antagonist) and of 2-amino-5-phosphonopentanoic acid (AP5, an NMDA receptor antagonist) applied intracranially into the central rhythm generator of licking. Adult Long Evans male rats were stereotaxically implanted with guiding cannulae aimed at the oral part of nucleus reticularis gigantocellularis (NRG). After a recovery (1 week at least), animals after 24 h water deprivation were trained to drink in an experimental box. Then 1 microl of GYKI (1 mM solution) or 1 microl of AP5 (20 mM solution) was administrated by microinjection through the guiding cannula directly into the target structure. Lick-lick interval (LLI) was recorded by an electrical lick sensor and analysed with a laboratory computer Pentium. Localisation of the administration was checked by a routine histological method. GYKI administration significantly prolonged the LLI i.e. slowed down licking frequency. The effect was immediate and began to dwindle in the period between 10 min to 2 h. Licking frequency under the influence of AP5 was faster (shorter LLI). This effect culminated after 30 min and almost disappeared after 2 h. Both our findings are in a good accordance with those of Grillner's group that NMDA receptors are important for slow swimming movements while non-NMDA receptors are responsible for fast ones.