The output of the hippocampus is inhibited during social behavior in the male rat.

During sexual behavior in the male rat, peptidergic cells in the medial amygdaloid nucleus become active and release a vasopressin-like peptide. The present experiments were designed to examine hippocampal changes as a result of this peptide's action during sexual behaviors. Chronic field-potential recordings from the hippocampus of male rats were acquired in a wide variety of social and nonsocial circumstances. Hippocampal responses that resemble the known action of the vasopressin-like peptide were seen only with social stimuli such as sexual stimuli and stimuli that led to aggressive behavior between males. The results show that the occasions of peptide action in the hippocampus correlate with the occasions of peptide release as determined by recording from the peptidergic cell bodies. The results are interpreted to indicate that the amygdala projection to the hippocampus has a special role to play in social behavior.

Regulation of lysophospholipase activity of the 85-kDa phospholipase A2 and activation in mouse peritoneal macrophages.

The regulation of the lysophospholipase activity of the 85-kDa cytosolic phospholipase A2 (PLA2) was studied in vitro and in stimulated macrophages. Bovine serum albumin was found to inhibit lysophospholipase activity of the recombinant 85-kDa PLA2 when assayed at a relatively low substrate concentration. Inhibition could be reversed if the substrate concentration was increased or if Ca2+ was present in the assay. Incubation of recombinant enzyme with macrophage membranes and lipid extracts from macrophage membranes resulted in the release of arachidonic acid, as well as, stearic acid, which is enriched at the sn-1 position of macrophage phospholipids. This suggests that with a bilayer substrate the PLA2 can sequentially deacylate the sn-2 then sn-1 acyl groups. This was verified by demonstrating that the phospholipids, phosphatidylcholine and phosphatidylinositol, were hydrolyzed to glycerophosphocholine and glycerophosphoinositol by incubation with recombinant 85-kDa PLA2. The 85-kDa enzyme was identified as the main lysophospholipase activity in mouse peritoneal macrophage cytosols. Addition of Ca2+ to the assay enhanced activity, but this effect decreased as the substrate concentration was increased. Incubation of macrophages with zymosan increased the lysophospholipase activity of the 85-kDa PLA2 in cytosols. Phosphorylation of recombinant PLA2 with mitogen-activated protein kinase resulted in an increase in lysophospholipase, as well as, PLA2 activity. In macrophages stimulated with zymosan release of stearic acid (18:0) and palmitic acid (16:0) was observed in addition to arachidonic acid (20:4). These results are consistent with a role of the 85-kDa PLA2 in regulating lysophospholipid levels in macrophages during zymosan stimulation.

Activity of peptidergic neurons in the amygdala during sexual behavior in the male rat.

The medial amygdaloid nucleus (AME) occupies a central position in the circuitry that organizes sexual behavior in the male rat. It receives a projection from olfactory structures that are activated by pheromonal cues indicating receptivity in the female and projects in turn to limbic and hypothalamic structures that are thought to organize aspects of coitus. Electrical stimulation of the AME elicits a behavioral state that is indistinguishable by several measures from the post-ejaculatory interval. We used chronic single-unit recording techniques to determine the behavioral conditions in which the AME is normally active. We found that the cells indeed fired selectively during the presence of a receptive female, but that the discharge considerably anticipated copulation in time. We propose that sexual behavior in the male rat is a reaction chain of fixed action patterns, each one acting as a releaser for the next. The AME mediates an early event in the reaction chain, namely recognition of the receptive female, but electrical activation of the AME causes the reaction chain to proceed to its culminating behavior, the post-ejaculatory interval.

A peptidergic circuit for reproductive behavior.

A projection from the medial amygdaloid nucleus to the hippocampus and septum probably uses vasopressin as a transmitter. The nucleus synthesizes vasopressin and activation of the nucleus has a hippocampal effect that is completely blocked by a vasopressin antagonist. The afferent and efferent projections of this peptidergic nucleus suggest a possible role for the system in sexual behavior. Stimulation of the nucleus inhibits the output of the hippocampus in both genders and reorganizes behavior for a period of 15-20 min. In males, the effect of peptidergic activation is to produce a behavior that resembles the post-ejaculatory interval in coitus. This state is characterized by an EEG that resembles slow-wave sleep and by ultrasonic vocalizations at a characteristic frequency of 22 kHz. Castration in either gender causes depletion of the peptide from the target fields and eliminates the peptidergic signal in the hippocampus after about 15 weeks. The effects of castration in males can be reversed by testosterone replacement. The fluctuation of estrogen levels in rat plasma during the estrus cycle happens too quickly to impact the peptidergic system, and thus there is no significant change in the strength of the peptidergic signal among the proestrus, estrus, metestrus and diestrus stages. This fact permits study of the physiology of the system without concern for stage of estrus but does not permit conclusions regarding its function in females.