Orienting and defensive behaviors elicited by superior colliculus stimulation in rats: effects of 5-HT depletion, uptake inhibition, and direct midbrain or frontal cortex application

• Published in: Behavioural brain research 2003-09, Vol.144 (1), p.95-103
• Main Authors: Dringenberg, Hans C., Dennis, Kristopher E.B., Tomaszek, Summer, Martin, Jody
• Format: Article
• Language: English
• Subjects: 5-HT; Animals; Behavior, Animal - drug effects; Behavioral inhibition; Defensive behavior; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Electric Stimulation; Fenclonine - administration & dosage; Fluoxetine; Fluoxetine - administration & dosage; Frontal Lobe - drug effects; Frontal Lobe - physiology; Head Movements; Male; Mesencephalon - drug effects; Mesencephalon - physiology; Microdialysis - methods; Neural Inhibition; Optic tectum; Orienting; Piperazines - administration & dosage; Pyridines - administration & dosage; Rats; Rats, Long-Evans; Reverse microdialysis; Running - physiology; Serotonin; Serotonin - deficiency; Serotonin - metabolism; Serotonin Antagonists - administration & dosage; Serotonin Uptake Inhibitors - pharmacology; Stimulation, Chemical; Superior Colliculi - anatomy & histology; Superior Colliculi - drug effects; Superior Colliculi - metabolism; Superior colliculus; Time Factors
• ISSN: 0166-4328; 1872-7549
• DOI: 10.1016/S0166-4328(03)00065-2

Electrical or chemical stimulation of the superior colliculus (SC) in rats produces orienting and defensive responses. Defensive behaviors are modulated by serotonin (5-hydroxytryptamine, 5-HT), and serotonergic fibers provide a dense innervation of the SC. Here, we examined the role of 5-HT in modulating the behavioral responses of rats elicited by electrical SC stimulation. Low-intensity (107±12 μA) stimulation of the SC elicited orienting head movements, while higher intensities (204±20 μA) produced running and jumping responses. Treatment with the 5-HT depletor p-chlorophenylalanine (300 mg/kg/day×3, i.p.) lowered current thresholds to elicit orienting and running by 40 and 21%, respectively. Conversely, concurrent administration of the 5-HT uptake inhibitor fluoxetine (10 mg/kg, i.p.) and the 5-HT 1A receptor antagonist WAY 100635 (0.5 mg/kg, s.c.) increased threshold currents to produce head and running movements by 41 and 18%, respectively. We investigated the anatomical substrate of this inhibitory effect of 5-HT with intracerebral 5-HT application by means of reverse microdialysis. Application of 5-HT (1–50 mM) into the midbrain immediately adjacent to the SC stimulation electrode resulted in a pronounced (approximately four-fold for 50 mM 5-HT) dose- and time-dependent increase in stimulation thresholds to elicit head movements. Application of 5-HT into the frontal cortex (up to 100 mM) had no significant effect on SC-evoked behavioral responses. These results show that 5-HT exerts an inhibitory influence over orienting and defensive behaviors initiated in the mammalian SC. It appears that this inhibitory effect is mediated, to a large extent, by a direct action of 5-HT at the level of the midbrain.