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The first mammalian neuropeptide to be characterized was substance P (sP) , and it is now recognized that sP is a member of a structurally related family of peptides, the tachykinins. Extensive studies have demonstrated. that sP and some related tachykinin peptides play key roles as neurotransmitters and neuromodulators. The synthesis of different members of the tachykinin family is in part due to the modifications' of three sP-encoding preprotachykinin (PPT) mRNA's that are derived from a single sP gene. At least four tachykinin peptides can be synthesized as a result of these differential modifications including sP, neurokinin A, neuropeptide and neuropeptide K (NPK). Whereas the behavioral significance of sP has been extensively studied, there has been very little examination of the behavioral significance of NPK. This is especially true of the examination of male reproductive behavior. Dornan and Malsbury (1) reported that bilateral injections of sP into the media~ preoptic area (MPOA) facilitated male rat sexual behavior. High concentrations of PPT mRNA's are found within the MPOA, and as previously mentioned, NPK is derived from PPT mRNA's. At present, however, it is not known whether NPK plays a role in the neural regulation of male copulatory behavior. In the following experiment, we examined the role of NPK within the MPOA in the regulation of male rat copulatory behavior.Sexually experienced adult male Long-Evans rats were used. Chronic cannulae implants were stereotaxically placed 2mm above the MPOA, and following 7-12 days of recovery, the animals were behaviorally tested to obtain a baseline measure of copulatory behavior. One week following baseline testing, the animals were randomly placed into 4 groups and were bilaterally injected with .5ml solutions of either saline, 100ng NPK, 1000ng NPK or 2000ng NPK and the effects of these injections on copulatory behavior were determined. Following testing, brains were histologically analyzed to confirm the placements of the cannulae. Analysis revealed that bilateral injections of 1000ng NPK into the MPOA produced an abolishment of copulation in a significant number of animals, whereas injections of 100 and 2000ng NPK produced no significant effect on the copulatory behavior. It was noted thatinjections of2000ng NPK producedan abolishment ofcopulation in50 % of the animals, but due to the small number of animals in the group (n = 4), the result was not significant. Although the complete role of the tachykinins in the regulation of copulatory behavior has yet to be elucidated, it seems that sP and NPK, two tachykinins synthesized from the sP gene, exert opposite effects in the regulation of male copulatory behavior.


Biology | Psychology