Psychopharmacology (1990) 100:308-315 Psychopharmacology @] Springer-Verlag 1990 Reinforcing effects of peripherally administered substance P and its C-terminal sequence pGlu6-SP6-11 in the rat M.-S. Oitzl*, R.U. Hasentihrl, and J.P. Huston Institute of Physiological Psychology, University of Dfisseldorf, Universitfitsstrasse 1, D-4000 Dfisseldorf, Federal Republic of Germany Abstract. Reinforcing effects of intraperitoneally (IP) ad- ministered substance P (SPI-11), its amino-terminal frag- ment SP1-7 (SPN) and an analog of the carboxy terminus (pGlu6-SP6-11 : SPC) were studied in rats. Two conditioned place preference paradigms were used. After three pairings of the drug with a certain environment the effect of the treatment was evaluated in the drug-free state during a test trial. The reinforcing effects of SP (37 nmol) and the equi- molar dose of SPC were expressed by a significant increase in the amount of time the animals spent in the treatment environment. Other doses of SP (3.7 and 185 nmol) and SPC (7.4 and 185 nmol) and none of the doses of SPN (37, 185, 370 nmol) influenced the place preference behavior of the rats. The reinforcing effects of SP parallel the known facilitating effects of peripherally administered SP on mem- ory. The amino acids that encode the reinforcing effects of SP may lie within the C-terminal sequence of the SP molecule. Key words: Substance P ......Reinforcement - SP fragments Structure-activity - Conditioned place preference Rat The neuropeptide substance P (SP; a peptide of the tachy- kinin family) is found in various parts of the mammalian nervous system, where it appears to function as a neu- rotransmitter and/or neuromodulator (Ljungdahl et al. 1978; Nicoll et al. 1980; Costa 1988). Data are accumulat- ing that SP plays a physiological role in learning: intracrani- al or peripheral injections of this peptide can facilitate or impair learning (Huston and Stfiubli 1981 ; Schlesinger et al. 1983a, b). Furthermore, SP was found to serve as a rein- forcer for learning a T-maze when injected into the hypo- thalamus or medial septal nucleus (St/iubli and Huston 1985). Reinforcing effects of SP were also demonstrated with a place preference task when SP was injected into the lateral hypothalamus or into the region of the nucleus basalis magnocellularis (Holzhfiuer-Oitzl et al. 1987, 1988). Injections of SP into these regions of the brain had been reported to facilitate performance in learning tasks (Huston and St/iubli 1981; Kafetzopoulos etal. 1986; Nagel and Huston 1988). Systemically applied SP was also found to * Present address: Rudolf Magnus Institute for Medical Pharmaco- logy, University of Utrecht, Vondellaan 6, NL-3521 GD Utrecht, The Netherlands Offprint requests to: J.P. Huston enhance memory in mice and rats (Wetzel and Matthies 1982; Schlesinger et al. 1983 a, b, 1986; Tomaz and Huston 1986; Pelleymounter et al. 1988). Given the parallelism be- tween the memory promoting and reinforcing action of SP obtained in studies using the intracranial route of drug ad- ministration (Huston and Oitzl 1989), we hypothesized that peripherally administered SP should also have reinforcing effects (experiment 1 and 2). Pharmacological studies suggest that SP is enzymati- cally cleaved into various amino- and carboxy-terminal fragments, which can have biological activity on their own (Blumberg and Teichberg 1982; Iversen et al. 1985). The N- and C-terminal fragments of SP can exert different ef- fects on behavior (Hall arid Stewart 1983, 1984; Pelley- mounter et al. 1986). Structure-activity studies have been performed also with other peptides, such as ACTH and vasopressin, demonstrating different effects of various frag- ments of the peptides (De Wied and Jotles 1982). In order to determine the active sequence of the SP molecule that encodes the reinforcing action of peripherally injected SP, we tested its amino terminal fragment SP1-7 (SPN) and the analog of the carboxy-terminal fragment SP6-11 (pGlu6-SP6-11" SPC; experiment 2). The reinforcing effects of SP were assessed with two versions of the conditioned place preference paradigm: with a conventional 3-compartment procedure (experiment 1) and the recently developed "corral method" in an open field (experiment 2). The corral method (Hasen6hrl et al. 1989) has some advantages over the three-compartment task: one is that rats do not show an initial preference for a certain part of the apparatus. This method was also used for testing the fragments of the SP molecule. In both tasks reinforcing effects of the treatment are indicated by a significant increase in the amount of time spent in the environment (i.e., compartment or corral) which had been paired previously with the presumed reinforcer (i.e., either SP or fragments of the SP molecule). Additionally, it was decided to analyze the behavioral pattern of animals show- ing place preference. Thus, the aim of the present experi- ments was to examine possible reinforcing effects of periph- erally administered SP and two of its fragments. Experiment 1 : Three-compartment place preference Methods Subjects. Male Wistar rats (260-300 g; n = 36) were housed in groups of four to six animals per cage. Rats were kept