The influence of the amygdala on the basal septum and preoptic area of the rat

Psychological and neurobiological mechanisms underlying the decline of maternal behavior

The maternal behavior decline is important for the normal development of the young and the wellbeing of the mother. This paper reviews limited research on the factors and mechanisms involved in the rat maternal behavior decline and proposes a multi-level model. Framed in the parent-offspring conflict theory (an ultimate cause) and the approach-withdrawal model (a proximate cause), the maternal behavior decline is viewed as an active and effortful process, reflecting the dynamic interplay between the mother and her offspring. It is instigated by the waning of maternal motivation, coupled with the increased maternal aversion by the mother in responding to the changing sensory and motoric patterns of pup stimuli. In the decline phase, the neural circuit that mediates the inhibitory ("withdrawal") responses starts to increase activity and gain control of behavioral outputs, while the excitatory ("approach") maternal neural circuit is being inhibited or reorganized. Various hormones and certain monoamines may play a critical role in tipping the balance between the excitatory and inhibitory neural circuits to synchronize the mother-infant interaction.

Plasticity of paternity: Effects of fatherhood on synaptic, intrinsic and morphological characteristics of neurons in the medial preoptic area of male California mice

Parental care by fathers enhances offspring survival and development in numerous species. In the biparental California mouse, Peromyscus californicus, behavioral plasticity is seen during the transition into fatherhood: adult virgin males often exhibit aggressive or indifferent responses to pups, whereas fathers engage in extensive paternal care. In this species and other biparental mammals, the onset of paternal behavior is associated with increased neural responsiveness to pups in specific brain regions, including the medial preoptic area of the hypothalamus (MPOA), a region strongly implicated in both maternal and paternal behavior. To assess possible changes in neural circuit properties underlying this increased excitability, we evaluated synaptic, intrinsic, and morphological properties of MPOA neurons in adult male California mice that were either virgins or first-time fathers. We used standard whole-cell recordings in a novel in vitro slice preparation. Excitatory and inhibitory post-synaptic currents from MPOA neurons were recorded in response to local electrical stimulation, and input/output curves were constructed for each. Responses to trains of stimuli were also examined. We quantified intrinsic excitability by measuring voltage changes in response to square-pulse injections of both depolarizing and hyperpolarizing current. Biocytin was injected into neurons during recording, and their morphology was analyzed. Most parameters did not differ significantly between virgins and fathers. However, we document a decrease in synaptic inhibition in fathers. These findings suggest that the onset of paternal behavior in California mouse fathers may be associated with limited electrophysiological plasticity within the MPOA.

Storing maternal memories: hypothesizing an interaction of experience and estrogen on sensory cortical plasticity to learn infant cues

Much of the literature on maternal behavior has focused on the role of infant experience and hormones in a canonical subcortical circuit for maternal motivation and maternal memory. Although early studies demonstrated that the cerebral cortex also plays a significant role in maternal behaviors, little has been done to explore what that role may be. Recent work though has provided evidence that the cortex, particularly sensory cortices, contains correlates of sensory memories of infant cues, consistent with classical studies of experience-dependent sensory cortical plasticity in non-maternal paradigms. By reviewing the literature from both the maternal behavior and sensory cortical plasticity fields, focusing on the auditory modality, we hypothesize that maternal hormones (predominantly estrogen) may act to prime auditory cortical neurons for a longer-lasting neural trace of infant vocal cues, thereby facilitating recognition and discrimination. This couldthen more efficiently activate the subcortical circuit to elicit and sustain maternal behavior.

Effects of prolonged social isolation on responses of neurons in the bed nucleus of the stria terminalis, preoptic area, and hypothalamic paraventricular nucleus to stimulation of the medial amygdala

The studies presented demonstrate changes in hypothalamo-pituitary-adrenocortical secretion, and in electrical activity and synaptic responses of neurons in the bed nucleus of the stria terminalis, preoptic area, and hypothalamic paraventricular nucleus of rats exposed to early, long-term social isolation. Rats isolated from all social contact from an early preweaning time showed reduced basal plasma corticosterone concentrations, compared with littermate controls raised under social conditions. Isolated animals also exhibited a selective decrease in the spontaneous electrical activity of neurons within the hypothalamic paraventricular nucleus and lateral preoptic area, but not in adjacent structures. Moreover, isolation also altered the response of neurons in certain nuclei to electrical stimulation of the medial amygdala. Thus, a reduction in excitatory responses, and an increase in inhibition and nonresponsiveness, of preoptic area and paraventricular nucleus neurons was recorded, compared with control rats. Neurons in the bed nucleus of the stria terminalis were less affected, but showed an increase in the duration of excitatory responses following medial amygdala stimulation. These results, obtained from urethane-anesthetized rats, together with the reduced basal plasma corticosterone concentrations, suggest a reduction in limbic-hypothalamo-pituitary-adrenocortical (LHPA) activity following maternal deprivation and prolonged social isolation. This may result from altered limbic activity, specifically in the amygdala and its pathways to the paraventricular nucleus (PVN). Such alterations may include the stria terminalis, in so much as increased efficacy of inhibitory components and reduced efficacy of excitatory components was observed. The neural mechanisms underlying these alterations could involve an altered synaptology of the regions examined and/or a disruption of glucocorticoid feedback events.

Depression of neuronal activity in the hypothalamic ventromedial nucleus of the rat following microinjection of morphine in the amygdala or the periaqueductal gray

The effects of morphine administered intravenously and intracerebrally in the cortical amygdala and periaqueductal gray were examined on spontaneous neuronal activity in the hypothalamic ventromedial nucleus in both intact and castrated adult male rats. Spontaneous extracellular single unit activity in the ventromedial nucleus was significantly faster in chronically castrated (19-23 days) rats compared with intact control rats (6.0 +/- 1.1 and 2.5 +/- 0.3 spikes/s, respectively). Neurons in the ventromedial nucleus exhibited a significant attenuation of neuronal activity following the intravenous administration of morphine; the maximal per cent decrease (to 25% of the control unit activity) was the same in both castrated and intact rats, but cell firing in castrated rats was affected more at lower doses of morphine. When microinjected bilaterally in the cortical amygdala or periaqueductal gray (5 micrograms/site), morphine produced the same maximal depression of unit activity in the ventromedial nucleus as when administered intravenously. The time required to significantly attenuate unit activity in the ventromedial nucleus was the same (3 min) whether morphine was administered intravenously or bilaterally in the periaqueductal gray. When microinjected bilaterally in the cortical amygdala, the depressant effect of morphine on cell firing in the ventromedial nucleus was significant first at 10 min. These data correlate well with the time courses for depression of serum levels of luteinizing hormone produced by morphine microinjected in the same doses in the same extrahypothalamic sites in the rat. In view of the relatively high levels of specific opiate receptor binding in both the periaqueductal gray and amygdala, this report further supports a role for modulation by extrahypothalamic areas of opioid-induced changes in the functional activity of the hypothalamic-pituitary axis, perhaps via the ventromedial nucleus.

Electrophysiological analysis of inhibitory synaptic mechanisms in the preoptic area of the rat

1. Extracellular recordings were made from single neurones in the medial preoptic-anterior hypothalamus of rats anaesthetized with urethane or pentobarbitone 2. Stimulation of the arcuate--ventromedial area evoked inhibition in 76% of neurones; inhibition occurred as the initial response in 26% of neurones, followed orthodromic excitation in 26% and antidromic excitation in 24%. Stimulation of the periaqueductal grey evoked inhibition as the initial response in 63% of neurones. 3. Stimulation of both the arcuate--ventromedial area and the periaqueductal grey generated synaptically evoked high-frequency discharges in a few neurones. 4. Short ionophoretic pulses of beta-alanine, gamma-aminobutyric acid (GABA), glycine and taurine inhibited the discharge of most preoptic-anterior hypothalamic neurones. Bicuculline methochloride and picrotoxin antagonized both GABA responses and synaptic inhibition. Strychnine antagonized beta-alanine, glycine and taurine responses without altering synaptic inhibition. 5. Reduction of hypothalamic serotonin (5-HT) levels to about 40% of control values by micro-injection of 5,7-dihydroxytryptamine into the ventral tegmental area, or parenteral injection of parachlorophenylalanine, did not alter the profile of responses evoked by either arcuate-ventromedial or periaqueductal grey stimulation. Micro-injection of tetanus toxin (100-200 MLD) into the preoptic-anterior hypothalamus significantly reduced the frequency of occurrence of inhibition evoked from both sites. 6. Pentobarbitone administered acutely or as the sole anaesthetic increased both the duration and frequency of occurrence of inhibitory responses evoked from the arcuate-ventromedial nuclei. 7. It is suggested that a GABA-linked inhibitory synaptic mechanism, activated by arcuate-ventromedial and periaqueductal grey stimulation, operates in the preoptic-anterior hypothalamus. Such convergent inhibition may be a result of activity in local interneurone circuitry.

Slow wave field potentials in the rat brain during the estrous cycle

Twenty-one rats were implanted with chronic electrodes in the amygdala, preoptic area, and arcuate nucleus-median eminence region in an effort to learn more about electrical activity in the limbic system and the relationship to the estrous cycle. Slow wave field potentials from these three areas were recorded for ten minutes of each half-hour between 10:00 a.m. and 3:30 p.m. each day of the four-day estrous cycle. These recordings were analyzed for amount of correlation and direction of conduction between pairs of electrodes and for conduction times between pairs of sites. Slow wave field potential conduction from the amygdala to the preoptic area during the "critical period" on the day of proestrus lasts longer and takes longer en route than at other times of the cycle. This is statistically significant at p < 0.002 and implies a change in the conducting pathway during this time.