Excitability of dopamine neurons: modulation and physiological consequences.

This aim of this chapter is to review literature on the excitability and function of dopamine neurons that originate in the midbrain and project to cortico-limbic and motor structures (A9 and A10 dopamine pathways). Electrophysiological studies on rodent or non-human primates have shown that these dopamine neurons are silent or spontaneously active. The spontaneously active neurons show slow regular firing, slow irregular firing or fast bursting activity. In the first section, we will review how neuronal firing is modulated by intrinsic factors, such as impulse-regulating somatodendritic dopamine autoreceptors, a balance between inward voltage-gated sodium and calcium currents and outward potassium currents. We will then review the major excitatory and inhibitory pathways that play important roles in modulating dopamine cell excitability. In the second section, we will discuss how, in addition to being modulated by intrinsic and synaptic factors, excitability of dopamine neurons can also be modulated by life experiences. Dopamine neurons change their firing rate throughout the developmental period, their activity can be modified by stressful life events, and the firing mode can change as a consequence of acute or repeated exposure to psychoactive drugs. Finally, these cells change their firing pattern in response to behaviorally relevant stimuli and learning experiences. We will conclude by discussing how changes in the physiology of the dopamine neurons could participate in the development or exacerbation of psychiatric conditions such as drug addiction.

Estrogen regulates functional inhibition of hippocampal CA1 pyramidal cells in the adult female rat.

Previous studies have focused considerable attention on the effects of estrogen on excitatory synaptic input to hippocampal CA1 pyramidal cells. Estrogen increases the density of dendritic spines and synapses on CA1 pyramidal cells and increases the sensitivity of these cells to excitatory synaptic input. Little is known, however, about the effects of estrogen on inhibitory synaptic input to CA1 pyramidal cells. We have used immunohistochemistry for glutamic acid decarboxylase and whole-cell voltage-clamp recording of IPSCs and EPSCs at multiple time points after estrogen treatment to (1) investigate estrogen regulation of synaptic inhibition in CA1 and (2) evaluate how estrogen affects the interaction between inhibitory and excitatory input to CA1 pyramidal cells. We find that estrogen transiently suppresses GABA(A)-mediated inhibition of CA1 pyramidal cells at a time point before changes in excitatory input to these cells occur. This finding is consistent with the suggestion that transient disinhibition of CA1 pyramidal cells is involved in estrogen-induced dendritic spine formation. We have also found that at a later time after estrogen, inhibition of CA1 pyramidal cells recovers in parallel with enhancement of NMDA-mediated excitatory input. The concurrent enhancement of GABA(A) and NMDA-mediated input to CA1 pyramidal cells restores a balance of excitatory and inhibitory input to these cells and increases the potential dynamic range of CA1 pyramidal cell responses to synaptic input.

The role of dopamine in the nucleus accumbens and striatum during sexual behavior in the female rat.

Dopamine in dialysate from the nucleus accumbens (NAcc) increases during sexual and feeding behavior and after administration of drugs of abuse, even those that do not directly activate dopaminergic systems (e.g., morphine or nicotine). These findings and others have led to hypotheses that propose that dopamine is rewarding, predicts that reinforcement will occur, or attributes incentive salience. Examining increases in dopamine in NAcc or striatum during sexual behavior in female rats provides a unique situation to study these relations. This is because, for the female rat, sexual behavior is associated with an increase in NAcc dopamine and conditioned place preference only under certain testing conditions. This experiment was conducted to determine what factors are important for the increase in dopamine in dialysate from NAcc and striatum during sexual behavior in female rats. The factors considered were the number of contacts by the male, the timing of contacts by the male, or the ability of the female to control contacts by the male. The results indicate that increased NAcc dopamine is dependent on the timing of copulatory stimuli, independent of whether the female rat is actively engaged in regulating this timing. For the striatum, the timing of copulatory behavior influences the magnitude of the increase in dopamine in dialysate, but other factors are also involved. We conclude that increased extracellular dopamine in the NAcc and striatum conveys qualitative or interpretive information about the rewarding value of stimuli. Sexual behavior in the female rat is proposed as a model to determine the role of dopamine in motivated behavior.

Estrogen modulates sexually dimorphic contextual fear conditioning and hippocampal long-term potentiation (LTP) in rats(1).

The present study examined the role of ovarian steroids in contextual fear conditioning and hippocampal synaptic plasticity in female rats. In experiment 1, adult female rats were ovariectomized and submitted to contextual fear conditioning, a procedure in which rats received unsignaled footshock in a novel observation chamber; freezing behavior served as the measure of conditional fear. Ovariectomized female rats froze at levels comparable to male rats, both of which froze significantly more than sham-operated female rats. In experiment 2, estrogen replacement in ovariectomized female rats reduced fear conditioning to a level comparable to that of sham-operated females in experiment 1. In experiment 3, the influence of estrogen on the induction of long-term potentiation (LTP) at perforant path-dentate granule cell synapses in ovariectomized female rats was examined. Estrogen decreased both population spike LTP and EPSP-spike potentiation at perforant path synapses. Taken together, these experiments indicate that ovarian steroids regulate both sexually dimorphic behavior and hippocampal plasticity in a fear-conditioning paradigm.

Estradiol induces a phasic Fos response in the hippocampal CA1 and CA3 regions of adult female rats.

Previous studies have shown that estradiol induces structural and functional changes in hippocampal CA1 pyramidal cells of the adult female rat. Estradiol increases the density of dendritic spines and axospinous synapses on CA1 pyramidal cells, and increases these cells' sensitivity to NMDA receptor-mediated synaptic input. Curiously, while estradiol effects are observed in CA1 pyramidal cells, the majority of the evidence indicates that these cells lack genomic estradiol receptors. In contrast, genomic estradiol receptors are expressed in at least some hippocampal interneurons in CA1. The goal of the present study was to determine which hippocampal neuronal populations are activated by estradiol, as determined by induction of c-Fos immunoreactivity, as well as the time-course of this activation. We quantified c-Fos expression in each of the major subdivisions of the hippocampus in adult female rats at various time points during the same estradiol treatment regimen known to regulate dendritic spines and synapses on CA1 pyramidal cells. Our results show a phasic estradiol-induced c-Fos response in the pyramidal cell layers of both CA1 and CA3. c-Fos was induced within 2 h of treatment, decreased at 6 and 12 h, and subsequently increased again at 24 h after treatment with estradiol. Double labeling for c-Fos and GAD 65 or GAD 67 suggests that c-Fos is induced primarily in principal cells, though a small proportion of GABAergic cells is also labeled. These estradiol-induced changes in c-Fos expression may reflect phasic neuronal activation and coupling to gene expression, which could be involved in estradiol's effects on excitatory synaptic connectivity in the hippocampus.

Rapid effects of estrogen or progesterone on the amphetamine-induced increase in striatal dopamine are enhanced by estrogen priming: a microdialysis study.

There are estrous cycle-dependent differences in amphetamine-stimulated behaviors and striatal dopamine (DA) release; intact female rats exhibit a greater behavioral response to amphetamine on estrus than on other days of the cycle. Following ovariectomy amphetamine-induced behavior is attenuated, as is the striatal DA response to amphetamine in vitro. Repeated estrogen treatment in ovariectomized rats reinstates both of these responses to a level comparable to estrous females. In addition, 30 min after a single treatment with a physiological dose of estrogen there is enhanced amphetamine-induced behavior and increased amphetamine-induced striatal DA detected during microdialysis. This experiment was conducted to determine whether the acute effect of estradiol and the effect of repeated exposure to estrogen are functionally related. We report here that prior treatment with estrogen (three daily treatments of 5 microg estradiol benzoate) results in a significant enhancement of the effect of acute estrogen (5 microg estradiol benzoate) or progesterone (500 microg) on amphetamine-induced striatal DA release and stereotyped behaviors. Both the peak response and the duration of the response are greater in estrogen-primed animals treated with estrogen or progesterone 30 min prior to amphetamine, than in all other groups. Either prior treatment with estrogen (last dose 24 h before) or a single acute injection of estrogen result in an enhanced peak response to amphetamine, with no effect on the duration of amphetamine-induced striatal DA release. Treatment with progesterone in animals not primed with estrogen was not different from treatment with oil vehicle. These results demonstrate that there are both acute and long-term effects of estrogen on the striatum that underlie the dynamic changes in stimulated DA release and amphetamine-induced behaviors during the reproductive cycle.