"Nice guys finish last": influence of mate choice on reproductive success in Long-Evans rats.

The present study was designed to determine if male physiology and male reproductive behavior predict reproductive success in Long-Evans rats. Mating behavior was observed in sexually naïve, naturally cycling female rats during behavioral estrous that were given the opportunity to mate with two males simultaneously. DNA analysis of offspring born following these mating encounters was used to identify the paternity of each pup. In order to assess the effect of mate choice during these mating encounters on reproductive success, one male rat in each pair was categorized as the preferred mate if the female spent more time (>50%) with him during the mating test of the present study. Furthermore, each male in the pairs was categorized as "attractive" or "non-attractive" by computing the number of females that preferred each male across many mating tests. Similar to results reported in Lovell et al. (2007), during 76% of these mating tests the same male rat in each pair was preferred by different female rats. Overall attractiveness of individual male rats predicted reproductive success in the present study. Interestingly, "attractive" males sired significantly FEWER pups than "non-attractive" males. Neither behavioral (e.g., latency to first sexual stimulation, number of sexual stimulations) nor physiological measures (e.g., body weight, urinary testosterone levels) of male rats predicted their reproductive success. In conclusion, the present results indicate that certain features of some males are more attractive to females, but attractive males are at a reproductive disadvantage (as measured by the number of pups sired). Although basal urinary testosterone levels did not differ between males that sired the majority of pups in a litter and males that sired few or none of the pups in a litter, aggression and/or other physiological measures of fertility (e.g., penile reflexes) may differ between males that are attractive to females and those that have a reproductive advantage.

The effects of intra-amygdaloid infusions of a D2 dopamine receptor antagonist on Pavlovian fear conditioning.

The present study examined the effects of bilateral intra-amygdaloid infusions of the D2 receptor antagonist, eticlopride, on the acquisition and expression of Pavlovian fear conditioning as measured by freezing to acoustic and background contextual stimuli in the rat. Infusions of eticlopride before acquisition or before both acquisition and retention testing significantly attenuated conditioned freezing to tone presentations during the retention test 24 hr later. No effects, however, were observed on freezing that emerged during acquisition. Furthermore, these effects were not attributable to state-dependent learning effects or alterations in baseline activity or shock reactivity. In conclusion, these results suggest that amygdaloid dopamine transmission at D2 receptors contributes to the formation and/or consolidation of fear memories.

The effects of neurotoxic hippocampal lesions on two effects of context after fear extinction.

Three conditioned suppression experiments with rats examined the role of the hippocampus in 2 effects of context after extinction. Reinstatement is the context-specific recovery of fear to an extinguished conditioned stimulus (CS) that occurs following independent presentations of the unconditioned stimulus (US), after extinction. Renewal is the recovery of fear when the CS is presented in the context in which it was conditioned, after extinction in a different context. Results indicated that neurotoxic lesions of the hippocampus, performed before conditioning, abolished reinstatement, which depends on context-US associations, but not renewal, which does not. This dissociation is not the result of differences in the recentness of context learning that ordinarily governs the 2 effects. The results suggest that the hippocampus is necessary for some, but not all, types of contextual learning.

Amygdaloid D1 dopamine receptor involvement in Pavlovian fear conditioning.

The amygdala has long been implicated in conditioned fear. The mesencephalic dopaminergic system provides a rich innervation to the amygdala [J.H. Fallon, P. Ciofi, Distribution of monoamines within the amygdala, in: J.P. Aggleton (Ed.), The Amygdala: Neurobiological Aspects of Emotion, Memory and Mental Dysfunction, Wiley, New York, 1992, pp. 97-114; L.J. Freedman, M.D. Cassell, Distribution of dopaminergic fibers in the central division of the extended amygdala of the rat. Brain Research 633 (1994) 243-252; E. Asan, The catecholaminergic innervation of the rat amygdala. Advances in Anatomy Embryology and Cell Biology 142 (1996) 1-107]. Specific activation of the mesoamygdaloid dopaminergic system has been reported to occur in response to conditioned fear-arousing stimuli [M.L. Coco, C.M. Kuhn, T.D. Ely, C.D. Kilts, Selective activation of mesoamygdaloid dopamine neurons by conditioned stress: attenuation by diazepam. Brain Research 590 (1992) 39-47] suggesting that dopamine release in the amygdala may contribute to the acquisition and/or expression of conditioned fear. Using a 2x2 factorial design, Experiment 1A investigated the effects of bilateral intra-amygdaloid infusions of the selective D1 receptor antagonist, SCH 23390 (2.0 microgram 0.5 microliter-1 side-1), on the acquisition and expression of Pavlovian conditioned fear measured by freezing to acoustic and background contextual stimuli. Infusions of SCH 23390 prior to acquisition training, prior to retention testing or prior to both significantly attenuated conditioned freezing during retention testing. Experiment 1B investigated the dose-dependent effects of pre-training infusions of SCH 23390 (0.5, 1.0 and 2.0 microgram) on conditioned fear. Pre-training infusions of SCH 23390 dose-dependently attenuated conditioned freezing during retention testing. Experiment 2A investigated the effects of bilateral infusions of the selective D1 receptor agonist, SKF 82958 (2.0 microgram 0.5 microliter-1 side-1) on the acquisition and expression of conditioned fear. Infusions of SKF 82958 prior to training facilitated conditioned freezing during retention testing. Experiment 2B investigated the dose-dependent effects of pre-training infusions of SKF 82958 (1.0, 2.0 and 4.0 microgram) on conditioned fear. Pre-training infusions of SKF 82958 dose-dependently facilitated conditioned freezing during retention testing. In conclusion, these results suggest that dopamine transmission within the amygdala contributes to the acquisition and expression of Pavlovian fear conditioning.

Intrahippocampal infusions of a metabotropic glutamate receptor antagonist block the memory of context-specific but not tone-specific conditioned fear.

The role of metabotropic glutamate receptors (mGluRs) in the acquisition of learning and memory using fear conditioning as a behavioral model was examined. The mGluR antagonist (R, S)-alpha-methyl-4-carboxyphenylglycine (MCPG) was infused into the hippocampus 30 min before fear conditioning, and freezing was measured during both acquisition and retention tests. The results show that pretraining antagonism of MCPG-sensitive mGluRs in the hippocampus impaired context-specific memory for an aversive event during testing. The memory for tone-specific fear, however, remained intact despite pretraining infusion of MCPG. Treating rats with MCPG did not affect context- or tone-specific fear during acquisition. Results suggest that mGluR activation may play an important role in hippocampally mediated memory consolidation.