Learning-induced changes in D2 receptors of rat brain are sexually dimorphic

Sex Differences in Dopamine Receptors and Relevance to Neuropsychiatric Disorders

Dopamine is an important neurotransmitter that plays a key role in neuropsychiatric illness. Sex differences in dopaminergic signaling have been acknowledged for decades and have been linked to sex-specific heterogeneity in both dopamine-related behaviours as well as in various neuropsychiatric disorders. However, the overall number of studies that have evaluated sex differences in dopamine signaling, both in health and in these disorders, is low. This review will bring together what is known regarding sex differences in innate dopamine receptor expression and function, as well as highlight the known sex-specific roles of dopamine in addiction, depression, anxiety, schizophrenia, and attention deficit hyperactivity disorder. Due to differences in prognosis, diagnosis, and symptomatology between male and female subjects in disorders that involve dopamine signaling, or in responses that utilize pharmacological interventions that target dopamine receptors, understanding the fundamental sex differences in dopamine receptors is of vital importance for the personalization of therapeutic treatment strategies.

Effect of early isolation on signal transfer in the entorhinal cortex–dentate–hippocampal system

Deprivation of socio-sensory interactions during early life impairs brain function in adulthood. In previous investigations we showed that early isolation severely affects neuron development in several structures of the hippocampal region, including the entorhinal cortex. In the present study we investigated the effects of early isolation on signal processing along the entorhinal cortex-dentate-CA3-CA1 system, a major memory circuit of the hippocampal region. Male and female guinea-pigs were assigned at 6-7 days of age to either a social or an isolated environment. At 90-100 days of age the animals were anesthetized and field potentials were recorded from the entorhinal cortex-dentate-CA3-CA1 circuit, driven by dorsal psalterium commissural volleys. Analysis of the input-output function in the different structures showed that in isolated males there was a small reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in layer II of the entorhinal cortex. No changes occurred in isolated females. In isolated males and females there was a reduction in the input-output function of the population excitatory postsynaptic potential and population spike evoked in the dentate gyrus, CA3 and CA1, but this effect was larger in males. In isolated males, but not in females, the population spike/population excitatory postsynaptic potential ratio was reduced in all investigated structures, indicating that in males the size of the discharged neuron population was reduced more than due to the decreased input. Results show that isolation reduces the synaptic function in the whole entorhinal cortex-dentate gyrus-CA3-CA1 system. While the entorhinal cortex was moderately impaired, the dentate-hippocampal system was more severely affected. The impairment in the signal transfer along the entorhinal cortex-dentate gyrus-CA3-CA1 system was heavier in males, confirming the larger susceptibility of this sex to early experience. This work provides evidence that malfunctioning of a major hippocampal network may underlie the learning deficits induced by impoverished surroundings during early life.

The role of D1 and D2 receptors in the cocaine conditioned place preference of male and female rats

The rewarding effects of cocaine have been shown to be sexually dimorphic; female rats develop cocaine conditioned place preference at lower doses and with fewer cocaine pairings than male rats. The present study was conducted to determine whether D1 and D2 receptors contribute to sex differences in cocaine conditioned place preference using a 4-day paradigm. Fifteen minutes prior to receiving saline or cocaine (5mg/kg for females and 20mg/kg for males), rats were pretreated with either SCH 23390, a D1 receptor antagonist, (0.10, 0.25, or 0.50mg/kg), eticlopride, a D2 receptor antagonist, (0.05, 0.10, or 0.25mg/kg), or vehicle (saline). Antagonism of D1 receptors by SCH 23390 fully blocked cocaine conditioned place preference in male rats, while only the two lower doses of SCH 23390 blocked cocaine conditioned place preference in female rats. Conversely, antagonism of D2 receptors using eticlopride had no effect on cocaine conditioned place preference in male or female rats. Due to the known role of D1 receptors in cocaine conditioned place preference, sex differences in D1 receptor sensitivity may explain the differences observed in cocaine reward between male and female rats.

Sex differences in brain and behavior: emphasis on nicotine, nitric oxide and place learning

Although males and females are unmistakably different, the recognition of sex as a key variable in science and medicine is considered a revolution in some circles. Sex differences transcend reproductive functions, are evident in the structural and functional organization of the brain, and are reflected in group differences in cognitive abilities and behavior. Males and females have different neural organizational patterns for information processing and different strategies in problem solving. Research on sex differences not only provides descriptive data, but also allows us to elucidate mechanisms that underlie our behavior. In this review, sex differences in the central actions of nicotine (an addictive substance) and nitric oxide, and performance on active avoidance and place learning tasks are discussed as examples, and biobehavioral approaches relating to these topics are presented.

Nitric oxide synthetase inhibition hinders facilitation of active avoidance learning by nicotine in rats

Nicotine produces dose-dependent enhancement of performance in an active avoidance test, and also increases the levels of NO2- and NO3-, which are stable metabolites of nitric oxide (NO), in various brain regions of rats. On the basis of these two observations, we hypothesized that the beneficial effect of nicotine on learning could result from increased NO in relevant brain regions. We therefore tested active avoidance performance in rats given L-Nomega-nitroarginine (L-NA) to inhibit NO synthetase (NOS) prior to nicotine administration. Male Sprague-Dawley rats received L-NA (30 or 50 mg/kg), nicotine (0.4 mg/kg), saline or combinations of these treatments before learning trials. Rats were also tested on the inclined plane, to assess the possible effects due to impairment of motor function by drug treatments on active avoidance learning. L-NA treatment impaired the acquisition of active avoidance learning, and this defect was partially overcome by the co-administration of nicotine. Nicotine facilitated learning and significantly increased the number of correct responses. The threshold for the effect of NOS inhibition on performance exceeded 30 mg/kg L-NA, whereas 50 mg/kg impaired learning and also eliminated the nicotine-induced enhancement of learning. On the fifth day of learning trials, no facilitation of learning by nicotine was observed in rats receiving either dose of L-NA. Our results suggest that NO is involved in the facilitation of active avoidance learning by nicotine.

The effects of aging on cerebellar beta-adrenergic receptor activation and motor learning in female F344 rats

Female Fischer 344 (F344) rats were tested for age-related differences in both cerebellar beta-adrenergic receptor activation and motor learning. Three and 20 month old rats were tested on a motor learning paradigm that required the rats to adjust their gait to navigate a runway of unevenly spaced pegs. Rates of acquisition were compared between age groups. After behavioral testing, cerebellar beta-adrenergic receptor function was assessed using extracellular recordings of Purkinje cells during iontophoresis of GABA and isoproterenol (ISO). Aged female rats showed deficits in motor learning and in beta-adrenergic receptor activation. In young rats, ISO modulated GABAergic inhibition of Purkinje cell firing in more cells compared with aged rats. Thus, aging produces deficits in both motor learning and in beta-adrenergic receptor activation in aged female F344 rats.

Sex differentiation of rat hippocampal GABAergic neurons

In order to analyse mechanisms of sex differentiation of the hippocampus at the cellular level, the differentiation of hippocampal GABAergic neurons was studied in vitro. Serum-supplemented and serum-free dissociated cell cultures were raised from the hippocampus of embryonic day 17 male and female rat embryos for up to 14 days in vitro. This time period roughly corresponds to the critical phase for sex differentiation of the rat brain as determined in vivo. Serum-free cultures were treated with testosterone and/or 17 beta-oestradiol for the entire culture period. Control cultures from male donors contained twice as many GABA-immunoreactive neurons as those from female donors, while there was no sex difference in overall counts of neurons stained for microtubule-associated protein 5. Measurements of high-affinity uptake of [3H]GABA essentially confirmed this sex difference. The development of the sex difference could not be influenced by long-term treatment with androgen or oestrogen. It is concluded that sex differentiation of a specific subpopulation of hippocampal neurons may take place independently of the environment provided by gonadal steroids and in the absence of extrinsic connections with the hypothalamus or other relays of the limbic circuit.

Prenatal alcohol exposure and the effects of environmental enrichment on hippocampal dendritic spine density

The effects of environmental enrichment on synaptic spine density in hippocampal area CAI were examined in rats exposed prenatally to alcohol. Pregnant dams were given ethanol via intragastric intubation (6 g/kg/day) from gestational days 8 through 19, or given isocaloric sucrose. An untreated control group was also used. After weaning, offspring from the three groups were then reared for 10 weeks in either isolated (caged alone, not handled) or enriched (group housed with "toys," handled) conditions. Animals were then sacrificed, the brains Golgi impregnated, and CAI pyramidal cell apical and basilar spine densities quantified. Among isolated animals there were no significant differences between control and alcohol-exposed groups. In general, environmental enrichment increased apical or basilar spine densities in untreated and sucrose controls. However, in prenatal alcohol-exposed animals, environmental enrichment did not increase spine densities. Because the environmental enrichment acted postnatally, these findings suggest that the effects of prenatal alcohol exposure included decreased neural plasticity enduring into early adulthood. Such a reduction in neuroanatomical plasticity in hippocampus may be associated with cognitive impairments found following prenatal alcohol exposure.