In Affective Neuroscience: The Foundations of Human and Animal Emotions (1998), Jaak Panksepp proposes that nurturance in mammals probably "arose from neurochemical processes that controlled mating and egg laying in reptiles." The precursor of mammalian oxytocin, the neurochemical vasotocin, explains Panksepp, "controls sexual urges in reptiles" and "helps deliver reptilian young into the world." Regarding reptiles, Panksepp writes: "Although a number of species—for instance, crocodiles—do exhibit some parental care, it is meager by mammalian standards." He provides a delightful story to illustrate hands-off reptilian parenting:
When a sea turtle, after thousands of miles of migration, lands on its ancestral beach and begins to dig its nest, an ancient birthing system comes into action. The hormone vasotocin is secreted from the posterior pituitary to facilitate the delivery of the young. Vasotocin levels in the mother turtle's blood begin to increase as she lands on the beach, rise further as she digs a pit large enough to receive scores of eggs, and reach even higher levels as she deposits one egg after the other. With her labors finished, she covers the eggs, while circulating vasotocin diminishes to insignificant levels… . Her maternal responsibilities fulfilled, she departs on another long sea journey. Weeks later, the newly hatched turtles enter the world and scurry independently to the sea without the watchful, caring eyes of mother to guide or protect them.
As mammals evolved, vasotocin evolved into oxytocin and argine-vasopressin (AVP), the neurochemicals so very important to mating behavior. As we discuss in The MATING System, the Brain, and Gender Determination, these two mammalian neurochemicals differ from vasotocin, the reptilian neurochemical, by only one amino acid. Panksepp points out that oxytocin, the same neurochemical which prompts receptivity in female mammals—including the lordosis reflex in the rat—also prepares "the mother's brain for nurturance." He writes: "The initial clue that there is an intrinsic bodily signal to promote maternal behavior was the fact that transfusion of blood from a female rat that had just given birth could instigate maternal behaviors in a virgin female."
Maternal care, hormones, and brain anatomy:
In rats, Panksepp reports that a "heightened maternal desire corresponds to the peak" of hormonal changes, "reaching an apex several hours before birth." Rat mothers begin to build nests for their offspring during this time. Panksepp explains that "estrogen, which has remained at modest levels throughout pregnancy, rapidly increases as parturition nears. Progesterone, which has been high throughout pregnancy, begins to plummet. And, of course, there is a precipitous rise in prolactin, which induces the mother's acinar glandular tissues to manufacture milk." Panksepp writes: "Prolactin may be the critical ingredient in sustaining the natural behavior sequence, not only because brain injections of prolactin promote nurturance, but females who are nonmaternal because they have been surgically deprived of their pituitary glands do gradually become maternal when replacement injections of prolactin are provided."
Regarding oxytocin, Panksepp notes that "during the last few days of pregnancy and the first few days of lactation, there are remarkable increases in oxytocin receptors in several brain areas, as well as increases in the number of hypothalamic neurons that begin to manufacture this neuropeptide." He writes: "During lactation, oxytocin cells begin to communicate with each other directly via the development of gap junctions between adjacent oxytocinergic neurons, allowing them to synchronize their neural messages precisely." Regarding specific structures in the brain where this activity takes place, Panksepp points out that "the greatest oxytocin receptor proliferation is observed in the bed nucleus of the stria terminalis (BNST); when that area is damaged, maternal behavior is severely impaired." We discuss the stria terminalis in other sections of CorticalBrain.com but it is reiterated here the excellent description found in MedlinePlus Dictionary. The stria terminalis is "a bundle of nerve fibers that passes from the amygdala along the demarcation between the thalamus and caudate nucleus mostly to the anterior part of the hypothalamus with a few fibers crossing the anterior commissure to the amygdala on the opposite side."
Panksepp notes that "lesions of the BNST and the nearby POA [preoptic area within the hypothalamus] can eliminate essentially all aspects of maternal behavior." The preoptic area lies in the same general subcortical area as the BNST; this area is outlined with a red circle in the MRI image above left (links to source). Oxytocin prompts signaling in neurons that exit "from the POA laterally and descend in the medial forebrain bundle, with key terminals being in the VTA [ventral tegmental area]." The VTA is located in the midbrain, very near to the BNST, as illustrated in the MRI image above left. The illustration to the right shows the position of the preoptic area within the hypothalamus. This image is from Endocrinology, S.S. Nussey and S.A. Whitehead, obtained from the NCBI bookshelf (links to source).
As we discuss in Dopamine action, synthesis, and pathways, neurons in the VTA within the midbrain synthesize dopamine, which is essential to motivated behavior. Panksepp writes: "It has been established that the oxytocinergic synapses that terminate on dopamine cells on the VTA do, in fact, promote maternal behavior." He explains that oxytocin injections into the VTA "can induce maternal behavior… ."
Panksepp points out that "well-established maternal behavior no longer requires brain oxytocin arousal; oxytocin blockade impairs maternal behavior only if administered to mothers during the birth of their first litter of pups. In animals that have been allowed to exhibit maternal behavior for several days, oxytocin antagonists have no outward effect on maternal competence." In other words, even when drugs block oxytocin, such action does not block previous learning. In addition to neurons in the pre-optic nucleus of the hypothalamus, neurons in the paraventricular nucleus (PVN) of the hypothalamus also produce oxytocin. Panksepp writes: "PVN lesions administered prior to parturition weaken subsequent maternal behavior, but those administered after several days of normal maternal functioning do not." His postulates that "a great deal of learning is probably controlled in the higher reaches of CARE circuits such as the anterior cingulate cortex and bed nucleus of the stria terminalis [BNST]."
Unfortunately, the circumstances within which human infants are born are not always supportive of necessary learning curves and attachment. Panksepp illustrates:
For instance, not too long ago in certain arctic aboriginal groups, such as the Netsilik Eskimo of northern Canada, long-term social concerns often overrode short-term emotional ones. Female babies who had little hope of finding an appropriate mate, because no male babies of comparable age had been born in the tribe, would be left to die in the snow, with little outward distress or remorse exhibited by the parents.
In addition to adverse cultural pressures in various human cultures, it is my opinion that another anxiety-provoking circumstance—uncertain paternity—can also affect a woman's attachment to her offspring. The fear that the questionable paternity issue will be discovered can only lead to a great deal of anxiety—the focus of which, of course, is the baby. this kind of situation happens much more often than we humans are willing to admit.
Access to sex, male oxytocin, and reduced aggression:
Above, we discuss the neurochemical details of a mother's attachment to offspring. But what about neurochemicals and paternal attachment? Panksepp is clear on this: "Oxytocin administration reduces all forms of aggression that have been studied." In rodents, Panksepp points out that "free access to sexual gratification can lead to an enormous threefold elevation in oxytocin levels in some parts of the male brain. Apparently, sex promotes the synthesis of nurturant and antiaggressive neurochemistries." Male rat behavior bears this out. Panksepp reports that male rats will often "kill the young in a territory they have successfully invaded." However, once they have mated with a female, most often any rats born three weeks later are spared. This makes evolutionary sense since, as Panksepp explains, for rats, "it typically takes three weeks from the time of successful fertilization to the time of birth." In other words, the increased oxytocin in male rats that have had sex makes the male rats more nurturing and less aggressive. But how do researchers know it is the oxytocin that promotes nurturing behaviors? In the laboratory, when a male rat is placed into a new territory where there are young rats, he would be expected to kill the rat babies. But when oxytocin is administered to the male rate in this situation, Panksepp notes that the tendency to commit infanticide "dramatically diminishes."
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