Mammary duct luminal epithelium controls adipocyte thermogenic programme

Sympathetic activation during cold exposure increases adipocyte thermogenesis via the expression of mitochondrial protein uncoupling protein 1 (UCP1)1. The propensity of adipocytes to express UCP1 is under a critical influence of the adipose microenvironment and varies between sexes and among various fat depots2-7. Here we report that mammary gland ductal epithelial cells in the adipose niche regulate cold-induced adipocyte UCP1 expression in female mouse subcutaneous white adipose tissue (scWAT). Single-cell RNA sequencing shows that glandular luminal epithelium subtypes express transcripts that encode secretory factors controlling adipocyte UCP1 expression under cold conditions. We term these luminal epithelium secretory factors 'mammokines'. Using 3D visualization of whole-tissue immunofluorescence, we reveal sympathetic nerve-ductal contact points. We show that mammary ducts activated by sympathetic nerves limit adipocyte UCP1 expression via the mammokine lipocalin 2. In vivo and ex vivo ablation of mammary duct epithelium enhance the cold-induced adipocyte thermogenic gene programme in scWAT. Since the mammary duct network extends throughout most of the scWAT in female mice, females show markedly less scWAT UCP1 expression, fat oxidation, energy expenditure and subcutaneous fat mass loss compared with male mice, implicating sex-specific roles of mammokines in adipose thermogenesis. These results reveal a role of sympathetic nerve-activated glandular epithelium in adipocyte UCP1 expression and suggest that mammary duct luminal epithelium has an important role in controlling glandular adiposity.

Innervation of the canine mammary gland: an immunohistochemical study

The distribution of peptidergic nerves in canine mammary tissues was studied by immunohistochemical techniques. In addition, the general and the noradrenergic innervations were demonstrated using protein gene product 9.5 and tyrosine hydroxylase immunoreactivities as markers, respectively. Tissue specimens from the caudal mammary glands were obtained from adult, non-lactating, female dogs. The overall innervation of the mammary gland tissue was sparse and primarily associated with the arterial vasculature. Nerve fibres positive for protein gene product 9.5 were rarely found in the secretory parenchyma. The nipple was not richly innervated, although it displayed a greater amount of nerve fibres than the mammary parenchyma. Nerve fibres supplying nonvascular structures of the nipple expressed immunoreactivity for the sensory neuropeptides calcitonin gene-related peptide, substance P and neuropeptide K, but not for vasoactive intestinal peptide, peptide histidine isoleucine and C-flanking peptide of neuropeptide Y. Somatostatin immunoreactivity was not detected in mammary gland tissue. Our results indicate that the innervation of the canine mammary gland is mainly affiliated with the vasculature and comprises peptidergic nerves which may be involved in the regulation of local blood flow. The presence of sensory neuropeptides in nerves supplying the mammary nipple suggest that these peptides may play a role in the afferent pathway of the milk ejection reflex.

Distribution of Neurons Supplying the Porcine Mammary Gland

The distribution of neurons projecting to the mammary gland was investigated by using the retrograde tracing method in juvenile pigs (n = 12). Fluorescent retrograde tracer Fast Blue (FB) was injected into the nipple (n = 3) or parenchyma (n = 3) of the second, right thoracic mamma or into the nipple (n = 3) and parenchyma (n = 3) of the last, right abdominal mamma. FB-positive (FB+) mammary gland-projecting neurons were found in some right dorsal root ganglia (DRG) and sympathetic chain ganglia (SChG) only. After injection of the tracer into the second, right thoracic mamma, FB+ neurons were observed in Th9-Th12 DRG but most of them were located in Th11 and Th12 ganglia. As concerns SChG, FB+ neurons were found in Th1-Th4, Th7-Th14 and L1-L4 ganglia. The vast majority of them were located in Th10 and Th11 SChG, which appeared to be the main sources of efferent innervation of this mamma. Neurons projecting to the last right abdominal mamma were found in L1-L3 DRG and L1-L4 SChG but most of them were located in L1-L2 ganglia and L1-L2 ganglia, respectively. This study for the first time has disclosed the localization of neurons supplying the mammary gland in larger domestic animal species, the pig, by using the retrograde tracing method.

Perinatal exposure to androgen suppresses sexual dimorphism in nerve trunk diameter, axon number, and fiber size spectrum: a quantitative ultrastructural study of the adult rat mammary nerve

The adult mammary nerves (MNs) from female, male, and testosterone-androgenized female rats were studied by light and electron microscopy. The female MNs trunk has twice the diameter of that of the male. Morphometry showed a significantly more myelinated (307 +/- 6) and unmyelinated axons (1654 +/- 10) in the female MN than the male MN (278 +/- 6 and 1373 +/- 28, respectively). Perinatal exposure of the female to testosterone significantly reduced the number of both axon types in the MN in adulthood (244 +/- 6 myelinated and 1300 +/- 32, unmyelinated). Another sexual dimorphism is a distinct group of large (>7.0 microm in diameter) myelinated axons known to conduct sensory information (i.e., touch and vibration). Because the male and the perinatally-androgenized female MNs lack these fibers, it is concluded that gonadal sex hormones may promote the differentiation of specific sets of axons committed to transmission of sensory cues relevant to reproduction.

Estrogen increases calcitonin gene-related peptide-immunoreactive sensory innervation of rat mammary gland

Estrogen plays important roles in preparing mammary tissue for lactation. However, estrogen also influences innervation in some tissues. We examined the effect of estrogen on peripheral innervation of mammary tissues of ovariectomized adult virgin female rats. Seven days after ovariectomy, 17beta-estradiol or placebo pellets were implanted subcutaneously, and tissues were harvested 1 week later. Estrogen treatment decreased mammary gland mass and adipocyte content, while ductal content increased and vascular composition was unaffected. Estrogen increased total areas occupied by nerves in mammary gland sections immunostained for the pan-neuronal marker protein gene product 9.5, and this increase persisted after normalizing for treatment-induced differences in gland mass. Although a significant increase in tyrosine hydroxylase-immunoreactive sympathetic nerve area was observed, no difference was detected following correction for differences in gland size, implying a conserved number of sympathetic nerves in the face of reduced gland volume. Calcitonin gene-related peptide-immunoreactive sensory nerve sectional area was also increased, and corrected nerve area remained 88% greater, indicating nerve proliferation during estrogen treatment. Total, sensory, and sympathetic innervation of the nipple and adjacent dermal tissue were unaffected by estrogen. We conclude that chronic estrogen elevation induces selective proliferation of rat mammary gland calcitonin gene-related peptide-containing nerves, which are associated primarily with blood vessels and are probably nociceptors. Because they are likely to subserve a vasodilatory function, increased innervation may promote increased blood flow necessary for milk formation during suckling. Moreover, these findings may help explain abundant anecdotal reports of increased breast sensitivity in humans under high estrogen conditions.

[The most important local anesthesia in cattle: a review]

Surgical interventions in cattle are frequently performed under local analgesia. Local analgesia may be carried out in the standing animal without or with slight sedation or with the animal in recumbency after deep sedation. Injection of local analgesics is less time consuming than induction and maintenance of general analgesia and is, therefore, frequently used in private veterinary practice. Precise anatomical knowledge of the nerve supply to the area to be operated is a prerequisite for the successful introduction of a local analgesia. The goal of the present review is to summarize nerve supply and indications for surgery in the area of the head, male genital tract, teat and the claws of the hind limb of cattle.

Anatomic organization of the ascending branch of the milk-ejection reflex in sheep: primary afferent neurons

The present study examines the anatomic characteristics of the primary afferent neurons that innervate the nipples and pseudonipples of ewes and the nipples of lambs. For this purpose, horseradish-peroxidase coupled to wheat germ agglutinin (WGA-HRP) was injected intradermally into the whole extent, the tip, or the base of the nipples and pseudonipples, as well as into a region of the posterior surface of the udder. After survival periods of 72-96 hours, dorsal root ganglia (DRG), segments of the spinal cord and medulla oblongata were sectioned and reacted histochemically with tetramethylbenzidine to reveal the transganglionically transported tracer. Injections of WGA-HRP in the nipples and pseudonipples of the ewe resulted in labeled cells in the second to fifth ipsilateral lumbar spinal ganglia (L(2)-L(5)) and third and fourth (L(3) and L(4)) lumbar spinal ganglia, respectively. Labeled cells after WGA-HRP injections in the nipples of the lamb were found in the ipsilateral L(3)-L(5) spinal ganglia. Central projections of the DRG-labeled cells were found in the medial part of laminae I-III of the ipsilateral L(3) and L(4) spinal segments (ewe and lamb) and in the ipsilateral dorsal column nuclei (ewe). Central projections of the DRG-labeled cells after injections in the pseudonipples of the ewe were located in the medial part of laminae I-III of the ipsilateral L(3) spinal segment. The results of this study demonstrate that, whereas the innervation of the nipples of the ewe originates from four successive lumbar spinal ganglia (L(2)-L(5)), the innervation of the nipples of the lamb and the pseudonipples of the ewe originates from three (L(3)-L(5)) and two (L(3) and L(4)) successive ganglia, respectively.

Vasoactive intestinal polypeptide (VIP)--immunoreactive nerve fibres in the mammary gland of the pig

Immunohistochemical studies have been performed to investigate the coexistence of VIP with dopamine-beta-hydroxylase (D(beta)H), vesicular acetylcholine transporter (VAChT), somatostatin (SOM) or neuropeptyd Y (NPY) within nerve fibres supplying the immature mammary gland in the pig. Generally, a moderate number of the VIP-immunoreactive (VIP-IR) nerve fibres were located in the nipple and parenchyma of the gland. VIP-IR fibres surrounded smooth muscle cells (SMC), blood vessels (BV) and lactiferous ducts (LD). Double-labelling immunohistochemistry revealed that some of VIP-IR nerve fibres also contained immunoreactivity to D(beta)H. VIP/D(beta)H-IR nerves were associated with BV and SMC and single fibres were observed around the LD in both nipple and parenchyma of the gland. VIP/VAChT-IR nerve fibres were not observed. The majority of VIP-IR fibres associated with SMC were also SOM-IR. Less numerous VIP/SOM-IR fibres supplied the BV and were located around the LD of the gland. A small number of VIP-IR nerves also displayed immunoreactivity to NPY. VIP/NPY-IR nerve fibres supplied the BV of the gland.

Suckling-induced activation of neural c-fos expression at lower thoracic rat spinal cord segments

Suckling stimulation is essential for neuroendocrine and sympathetic reflex activation during lactation. In the present study, the induction of c-fos gene expression was used to identify neuronal populations in the spinal cord activated by acute 5 min suckling or by electrical stimulation of the central stump of the first abdominal mammary nerve in lactating rats previously separated from their litters for 6 or 18 h. In addition, to investigate whether spinal sympathetic preganglionic neurons are activated by suckling, dual immunostaining (Fos and choline acetyltransferase) was performed. Fos was expressed at low levels in continuously suckled and 6 h nonsuckled mothers, but no expression was found after 18 h of nonsuckling. On the other hand, in 6 h nonsuckled rats, significant increments in Fos expression occurred in several regions after acute suckling and after electrical stimulation. Also, the pattern of Fos expression in each spinal laminae was different for the two stimuli, i.e. more intense effects of suckling in deep laminae V-X and more intense effects in laminae I-IV with electrical stimulation. Double-labeling after suckling was found only in sympathetic preganglionic neurons from the intermedio-medial cell column, whereas after electrical stimulation, double label was observed only in neurons from the intermedio-lateral cell column. On the other hand, no effect upon Fos protein expression was observed after suckling and only a minor effect after electrical stimulation of mammary nerve in 18 h nonsuckled rats. These results are consistent with previous findings on the sympathetic reflex regulation of the mammary gland, as well as on the importance of the nonsuckling interval for optimal functioning of lactation.

Transneuronal labelling of nerve cells in the CNS of female rat from the mammary gland by viral tracing technique

Using the viral transneuronal tracing technique, the cell groups in the CNS transneuronally connected with the female mammary gland were detected. Lactating and non-lactating female rats were infected with pseudorabies virus injected into the mammary gland. The other group of animals was subjected to virus injection into the skin of the back. Four days after virus injection, infected neurons detected by immunocytochemistry, were present in the dorsal root ganglia ipsilateral to inoculation and in the intermediolateral cell column of the spinal cord. In addition, a few labelled cells could be detected in the dorsal horn and in the central autonomic nucleus (lamina X) of the spinal cord. At this survival time several brain stem nuclei including the A5 noradrenergic cell group, the caudal raphe nuclei (raphe obscurus, raphe pallidus, raphe magnus), the A1/C1 noradrenergic and adrenergic cell group, the nucleus of the solitary tract, the area postrema, the gigantocellular reticular nucleus, and the locus coeruleus contained virus-infected neurons. In some animals, additional cell groups, among others the periaqueductal gray and the red nucleus displayed labelling. In the diencephalon, a significant number of virus-infected neurons could be detected in the hypothalamic paraventricular nucleus. In most cases, virus-labelled neurons were present also in the lateral hypothalamus, in the retrochiasmatic area, and in the anterior hypothalamus. In the telencephalon, in some animals a few virus-infected neurons could be found in the preoptic area, in the bed nucleus of the stria terminalis, in the central amygdala, and in the somatosensory cortex. At the longer (5 days) survival time each cell group mentioned displayed immunopositive neurons, and the number of infected cells increased. The pattern of labelling was similar in animals subjected to virus inoculation into the mammary gland and into the skin. The distribution and density of labelling was similar in lactating and non-lactating rats. The present findings provide the first morphological data on the localization of CNS structures connected with the preganglionic neurons of the sympathetic motor system innervating the mammary gland. It may be assumed that the structures found virus-infected belong to the neuronal circuitry involved in the control of the sympathetic motor innervation of the mammary gland.

Adrenergic and cholinergic innervation of the mammary gland in the pig

Adrenergic and acetylcholinesterase-positive (AChE-positive) innervation of the mammary gland in the sexually immature and mature pigs was studied using histochemical methods. Upon examining the adrenergic and cholinergic innervation, the adrenergic innervation was found to be much more developed. The majority of both sub-populations of the nerve fibres studied was localized in the subcutaneous tissue of the mammary gland. Adrenergic and AChE-positive nerve fibres also supplied structures of the nipple (subcutaneous tissue, blood vessels, smooth muscles fibres) and glandular tissue (blood vessels, lactiferous ducts). The glandular tissue contained the smallest number of adrenergic and AChE-positive nerve fibres. No distinct differences were observed in the adrenergic and AChE-positive innervation of the porcine mammary gland between the juvenile and non-pregnant adult animals.

Mammary gland sympathetic innervation is a major component in type 1 deiodinase regulation

Recent observations have shown that in lactating rats previously deprived of suckling, either suckling stimulus or ip injection of norepinephrine was capable of increasing mammary deiodinase type 1 (M-D1) mRNA content and enzyme activity. In the present work, we show that intact efferent sympathetic mammary innervation is required to restore both mammary D1 mRNA content and enzyme activity, whereas suckling-induced secretion of catecholamines from the adrenal glands does not seem to participate in M-D1 enzyme regulation. The data also indicate that the sympathetic reflex activation in response to suckling involves two complementary autonomic components: (1) activation, presumably through mammary segmental arrangement affecting neighboring mammary glands; and (2) an individual reflex regulatory mechanism capable of maintaining M-D1 activity within each mammary gland. In addition to these findings, we show that the suckling-induced sympathetic activation of M-D1 activity could be blocked by prior activation of ductal mechanoreceptors. This set of regulatory and counterregulatory mechanisms seems to ensure the optimal control of mammary energetic expenditure according to litter size.

Pituitary adenylate cyclase activating polypeptide (PACAP) in the rat mammary gland

Pituitary adenylate cyclase activating polypeptide (PACAP), a member of the vasoactive intestinal polypeptide (VIP) family of peptides, is present in the brain and in neuronal elements of a number of peripheral organs. Since no information on PACAP in the mammary gland exists, we have investigated, by radioimmunoassay and immunohistochemistry, the occurrence and distribution of PACAP immunoreactivity in the mammary gland of lactating and non-lactating rats. A specific monoclonal mouse anti-PACAP antibody'has been used to show that the peptide is located in nerve fibres associated with bundles of circular and longitudinal smooth muscle surrounding the lactiferous duct of the nipple. PACAP-immunoreactive nerve fibres and nerve bundles are present in the subepidermal connective tissue of the nipple and in the mammary parenchyma, some of the fibres being in close contact with blood vessels. Occasionally, a few delicate varicose fibres are associated with secretory alveoli and lactiferous ducts. The majority of PACAP-positive nerve fibres are, however, located in the glabrous skin of the nipple and the hairy skin adjacent to the nipple forming a subepithelial plexus from which delicate varicose nerve fibres enter the overlying epithelium. Double immunostaining for PACAP and a marker for sensory neurons, calcitonin gene-related peptide, has disclosed that the two peptides are almost completely co-localized. A minor population of the PACAP-immunoreactive nerve fibres shows co-existence with VIP. Although no obvious changes at the immunohistochemical level could be observed during pregnancy or lactation, elevated concentrations of immunoreactive PACAP-38 in mammary extracts have been found during lactation. Our data suggest that PACAP is involved in the nervous control of mammary gland function, probably in the transmission of suckling stimuli.

Central effects of catecholamines upon mammary contractility in rats are neurally mediated

We injected, i.e., intracerebroventricularly (ICV) or systematically, small amounts of adrenaline (ADR), noradrenaline (NA), isoproterenol (ISOP) and dopamine (DA) in urethane-anesthetized lactating rats, and determined the effects on isometrically recorded intramammary pressure (IMP) responses to exogenous oxytocin (OXY). While centrally administered ADR, NA and DA provoked increased IMP responses to OXY, the beta-adrenergic agonist ISOP induced the opposite effect. These effects were reversible, dose related and also occurred in hypophysectomized rats. However, when injected systematically, all adrenergic agonists but DA depressed IMP responses to OXY. Further experiments showed that central effects of catecholamines were exerted by regulating ductal tone, through the direct innervation of the mammary glands. Thus, whereas complete blockage of these effects occurred after selective denervation of the mammary glands, increased ductal tone resulted from ICV administration of ISOP. Finally, evidence was also obtained that antagonistic alpha- and beta-adrenergic mechanisms may interact with each other to regulate milk ejection, and with afferent signals from the mammary glands. Thus, beta-adrenergic inhibition upon IMP was counteracted by either NA administration or by activation of ductal mechanoreceptors. Together, these results suggest that regulation of milk ejection may involve neurally mediated influences on mammary contractility. Such actions would interact closely with afferents from the mammary gland influencing ductal tone.

Calcitonin gene-related peptide (CGRP) in the nipple of the rat mammary gland

The distribution of nerve fibres immunoreactive to calcitonin gene-related peptide (CGRP) was investigated by immunohistochemistry in nipples and mammary glands from lactating and non-lactating rats and compared to the immunoreactivity of other neuropeptides including substance P (SP), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and somatostatin (SOM). The study revealed an extensive innervation of the mammary nipples, in which CGRP-immunoreactive (IR) nerve fibres were abundantly present in the epidermis, dermal connective tissue and intralobular connective tissue of the mammary gland parenchyma. Several of the dermal CGRP-IR fibres seemed to follow blood vessels, or formed "ringlet-like" structures. The latter were mostly observed in the dermal connective tissue of the nipple from the lactating rat and may have a mechanoreceptive function, e.g. for the suckling stimuli. The location of SP-IR appeared to be comparable to CGRP-IR, but in fewer fibres. Dense NPY-IR networks of nerve fibres were closely associated with the fascicles of smooth musculature in the core of the nipple base. In contrast, VIP-IR fibres were only sparsely present, and SOM-IR was not detected in the mammary nipples. The immunoreactive content of CGRP and SP was determined by radioimmunoassays. The total amount of immunoreactive CGRP was significantly higher in the nipples from the pregnant and the lactating rats when compared to SP. The maximum concentration of CGRP (65.9 +/- 4.0 pmol/g) measured in the nipples of the pregnant (day 10) rats exceeded almost ninefold the maximum concentration of SP (7.7 +/- 2.0 pmol/g).(ABSTRACT TRUNCATED AT 250 WORDS)

Mammary somatosensory pathways are not required for suckling-mediated inhibition of luteinizing hormone secretion and delay of ovulation in cows

Four experiments were conducted to evaluate endocrine, lactational, and reproductive features of an experimental animal model employing complete neural disconnection of the udder in beef cows and then to utilize the validated model to study the role of mammary somatosensory pathways in suckling-mediated anovulation. For experiment 1, crossbred beef cows (n = 16) were randomly assigned to suckled/sham-operated control, weaned (calf removed)/sham-operated control, and suckled/mammary-denervated groups between Days 14 and 18 postcalving. Ten additional cows were randomly divided into weaned or suckled unoperated control groups (experiment 2). Complete mammary anesthesia was attained in all denervated cows (experiment 1), but sensory perception was not affected in sham-operated controls. Prolactin release patterns were markedly depressed by denervation; however, oxytocin release, milk production, and calf growth rates were not affected. Although acute sham surgery attenuated weaning-induced increases in LH pulse and ovulation frequency (experiment 1), normal responses to weaning were observed in unoperated controls (experiment 2) as well as in sham-operated cows 1 yr later (experiment 3). Finally, denervated-suckled cows (n = 22) that had been denervated before conception (experiment 4) exhibited LH secretion patterns and mean postpartum intervals to luteal activity similar to those of intact-suckled cows (n = 16). In contrast, the intact-weaned group (n = 16; calves weaned at birth) responded within 2 wk postcalving with an increased frequency of LH pulses, and intervals to onset of luteal activity were shortened compared to those in the other groups. Suckling-mediated anovulation is not dependent upon mammary somatosensory cues.

Prolactin and propranolol prevent the suckling-induced inhibition of lactation in rabbits

The normal once-a-day frequency of suckling in the rabbit was increased on day 31 (late lactation) by the addition of two extra sucklings (8 and 16 hr after) the daily suckling. In confirmation of previous data, two additional sucklings significantly decreased milk yield acutely on day 31 in comparison with the average 4-day milk yield before and after day 31. The decrease in milk secretion after the two additional sucklings was prevented by a single injection of 3 mg prolactin (given 24 hr before the two extra sucklings) and/or by injections of the beta-adrenergic-blocking drug, propranolol (100 micrograms/kg b. wt. given 30 min before each additional suckling). Since prolactin secretion is decreased in these species and the mammary gland is less responsive to the hormone during late lactation, the present results suggest that in addition to these factors, suckling-induced activation of sympathetic influences may contribute to the decline in milk production at this stage of lactation. Taken together, these results suggest that suckling may regulate lactation in the rabbit through antagonistic mechanisms at different stages of lactation.

Plasma levels of vasoactive intestinal polypeptide and oxytocin in response to suckling, electrical stimulation of the mammary nerve and oxytocin infusion in rats

The aim of the present study was to measure plasma levels of vasoactive intestinal polypeptide (VIP) and oxytocin following suckling, electrical stimulation of the mammary nerve and oxytocin infusion in lactating rats. Trunk blood was collected by decapitation after 5 and 20 min of suckling in conscious lactating rats. Repeated blood samples were drawn from the carotid artery in anesthetized rats, in connection with suckling, oxytocin infusion (0.22 nmol/l/kg/h) and electrical stimulation of the mammary nerve (5 V, 5 Hz, 2 ms). VIP and oxytocin levels were measured by radioimmunoassay. In conscious rats, VIP levels rose significantly from 18 +/- 5 to 102 +/- 30 pM after 5 min of suckling and to 123 +/- 25 pM after 20 min of suckling when milk ejection occurred. Oxytocin levels rose significantly from 90 +/- 24 to 269 +/- 45 pM during milk ejection. Suckling, oxytocin infusion and mammary nerve stimulation in anesthetized rats raised VIP levels significantly from 13 +/- 2, 18 +/- 5 and 10 +/- 2 to 43 +/- 8, 45 +/- 16 and 53 +/- 22 pM, respectively, whereas oxytocin levels rose from 111 +/- 34 to 294 +/- 66 pM after 20 min of suckling and to a peak value of 500 +/- 70 pM after oxytocin infusion. This study shows that VIP is elevated in plasma in lactating rats when the pups are suckling. The results showing that VIP levels rise following mammary nerve stimulation and oxytocin infusions indicate that both neurogenic and hormonal mechanisms can contribute to the regulation of VIP levels in plasma.

Electronmicroscopic and electrophysiological studies of the teat branch of the XIII thoracic nerve: relationship with lactation in the rat

Electrical stimulation of the XIII thoracic nerve (the 'mammary nerve') causes milk ejection and the release of prolactin and other hormones. We have analysed the route of the suckling stimulus at the level of different subgroups of fibres of the teat branch of the XIII thoracic nerve (TBTN), which innervates the nipple and surrounding skin, and assessed the micromorphology of the TBTN in relation to lactation. There were 844 +/- 63 and 868 +/- 141 (S.E.M.) nerve fibres in the TBTN (85% non-myelinated) in virgin and lactating rats respectively. Non-myelinated fibres were enlarged in lactating rats; the modal value being 0.3-0.4 micron 2 for virgin and 0.4-0.5 micron 2 for lactating rats (P greater than 0.001; Kolmogorov-Smirnov test). The modal value for myelinated fibres was 3-6 micron 2 in both groups. The compound action potential of the TBTN in response to electrical stimulation showed two early volleys produced by the A alpha- and A delta-subgroups of myelinated fibres (conduction velocity rate of 60 and 14 m/s respectively), and a late third volley originated in non-myelinated fibres ('C') group; conduction velocity rate 1.4 m/s). Before milk ejection the suckling pups caused 'double bursts' of fibre activity in the A delta fibres of the TBTN. Each 'double burst' consisted of low amplitude action potentials and comprised two multiple discharges (33-37 ms each) separated by a silent period of around 35 ms. The 'double bursts' occurred at a frequency of 3-4/s, were triggered by the stimulation of the nipple and were related to fast cheek movements visible only by watching the pups closely. In contrast, the A alpha fibres of the TBTN showed brief bursts of high amplitude potentials before milk ejection. These were triggered by the stimulation of cutaneous receptors during gross slow sucking motions of the pup (jaw movements). Immediately before the triggering of milk ejection the mother was always asleep and a low nerve activity was recorded in the TBTN at this time. When reflex milk ejection occurred, the mother woke and a brisk increase in nerve activity was detected; this decreased when milk ejection was accomplished. In conscious rats the double-burst type of discharges in A delta fibres was not observed, possibly because this activity cannot be detected by the recording methods currently employed in conscious animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurophysiologic maps of the cutaneous innervation of the external genitalia of the ewe

The area of skin supplied by the afferent fibers in a peripheral nerve is called the cutaneous area (CA) of that nerve. The CA responsive to movement of wool or hair in the genital regions were mapped in 17 ewes, with the identifications of the peripheral nerves and of the spinal nerves contributing to the pudendal plexus being checked at necropsy. Differences were found in the origins and extent of CA of the cutaneous branches from the sacral plexus. The CA of the caudal rectal nerves and of a nerve that passed caudally between the caudal vertebrae and the ventral sacrococcygeus muscle lay lateral to the anus and in the adjacent skin of the tail. The CA of the proximal cutaneous branch and of the distal cutaneous branch from the pudendal nerve (or plexus) overlapped craniocaudally (by approx one-half) the CA of the distal cutaneous branch extending ventrally and ending just caudal to the ipsilateral mammary gland. The deep perineal nerve innervated the skin immediately lateral to the anus and vulva. The dorsal nerve of the clitoris innervated hairs on the ipsilateral half of the vulva. Other fibers in the pudendal nerve were presumed to pass into the mammary branch of the nerve. They innervated the skin ventral to the vulva, the ipsilateral mammary gland, and (in some ewes) areas of the skin cranial to the mammary gland. The CA of the genitofemoral nerve included the ipsilateral teat and the inguinal fossa.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitatory effect of oxytocin on oxytocin cell background activity in the rat is suckling-dependent

The injection of oxytocin into the third ventricle during suckling in lactating rats not only enhances neurosecretory bursts but also the background activity of oxytocin cells. However, removing the young rats 10 min after the oxytocin injection is immediately followed by a decrease in background activity, while injecting oxytocin alone (i.e. without suckling) or suckling alone (i.e. without oxytocin injection) has no effect. These results, which show that the facilitatory effect of oxytocin on oxytocin cell background activity is suckling-dependent, suggest that oxytocin could act either on the afferent pathways for milk ejection or on oxytocin cells themselves, but only if they received inputs from mammary glands. Various hypotheses on the site and mode of action of oxytocin are discussed.

Afferent projections from the mammary glands to the spinal cord in the lactating rat--I. A neuroanatomical study using the transganglionic transport of horseradish peroxidase-wheatgerm agglutinin

Horseradish peroxidase-wheatgerm agglutinin was injected subcutaneously into one or more nipples of lactating rats to determine the spinal organization of sensory afferents emanating from the mammary glands. After survival periods of 45-96 h, dorsal root ganglia and segments of the spinal cord and/or medulla oblongata were sectioned and reacted histochemically with tetramethylbenzidine to reveal the transganglionically transported tracer. For each nipple injected, the peroxidase reaction product was found in somata, ranging in diameter from 15 to 60 microns, and fibres in 5-11 contiguous dorsal root ganglia. The number of labelled profiles was highest in the 2-4 central-most ganglia of the series and generally decreased progressively rostrally and caudally. After separate injections into each of the six ipsilateral nipples, labelling occurred in all ipsilateral dorsal root ganglia between the 5th cervical and 6th lumbar spinal segments. Substantial overlap of the spinal projections from adjacent mammary glands was seen, a given dorsal root ganglion innervating 2-3 different glands. Label in the spinal cord was restricted to the medial portion of the superficial dorsal horn. It occurred in what appeared to be terminal fields and fibres essentially in the substantia gelatinosa, but was also seen to extend into the marginal zone and sometimes into deeper regions of the dorsal horn. Label was found in both the gracile and cuneate nuclei of the medulla oblongata, though only occasionally and then only very sparsely. The substantial spread and segmental overlap of labelled mammary afferents, and the fact that most labelled afferents terminated in the dorsal horn, suggest that this spinal region may be an important site for the integration of sensory input from the mammary glands that may play a role in the sensory induction of reflex milk ejection.

[Localization of choline acetyltransferase in the alveolar portion of the mammary gland of the white mouse]

The light optic investigation has been performed on the 10th--15th days of lactation. The enzymatic activity has been determined by Burt method (1971). The product of histochemical reaction is revealed in the secretory cell and in the basal parts of the alveoli, that corresponds to location of myoepithelial cells. Localization of the reaction product in the secretory cells of the neighbouring alveoli is heterogeneous. The enzyme of acetylcholine synthesis is revealed along the course of single neural fibers included into the composition of the ileo-hypogastric nerve of the mammary gland. The results obtained, together with the data of certain physiological investigations make it possible to substantiate participation of the cholinergic mechanism in the alveolar cells activity and consider participation of the cholinergic process in intercellular interactions, that ensure functional conjugation of myoepithelial and secretory cells.

Comparison of plasma profiles of oxytocin and prolactin following suckling in the rat

The purpose of these studies was to determine the effect of suckling on the plasma oxytocin (OT) concentration profile in conscious primiparous rats during midlactation. Comparisons were made with plasma prolactin (PRL) levels obtained in the same rats. OT levels in the majority of rats exhibited a single peak during the first 5-30 min, then fell rapidly during the course of a 45-min period of suckling. The plasma OT levels were sustained over a longer period in mothers suckling 8 rather than 6 pups; the amplitudes of the OT response were similar, however. By contrast, plasma PRL profiles indicated that a steady secretion of the hormone occurred throughout the suckling period, with suckling of 8 pups resulting in significantly higher plasma levels than suckling of 6 pups. A considerably greater increase in the peak plasma OT concentration resulted when hungry foster litters of 6 pups were suckled after the mothers' own 6 pups had been suckled. Plasma PRL levels during the two sucklings, though, were similar. The rapid onset of the OT response to suckling was seen more clearly in urethane-anesthetized rats following mammary nerve stimulation. Plasma OT levels rose to a peak within 5 s after the onset, then fell to prestimulus levels by the end of the 65-second stimulation period. These results suggest that different regulating mechanisms are involved in the secretory responses of OT and PRL to suckling and that different thresholds of activation are likely to exist for the two hormones.

Food intake, aggression, and fear behavior in the mother rat: Control by neural systems concerned with milk ejection and maternal behavior

Mother rats eat more, are more aggressive, and show less fear behavior (freezing) than during other stages of the reproductive cycle. Electrolytic lesions in the peripeduncular area of the lateral midbrain made nursing mother rats eat less and interact peacefully with male intruders. This midbrain area forms part of the ascending milk-ejection pathway, so it seems plausible that the suckling stimulus maintains hyperphagia and aggression in mother rats. Because no alteration in fear behavior was observed in mothers with lesions, it was predicted that the reduction in freezing was related primarily to maternal responsiveness to pup cues other than suckling. In line with this hypothesis, it was found that the experimental induction of maternal behavior in ovariectomized, hormone-treated females was associated with a significant decrease in fear behavior, with no concomitant changes in food intake or aggression.

Release of catecholamines follows suckling or electrical stimulation of mammary nerve in lactating rats

The hypothesis that catecholamines may be released by mammary gland stimulation during lactation was tested by measuring, with an HPLC electrochemical method, plasma epinephrine (E) and norepinephrine (NE) concentrations during suckling in conscious rats and during electrical stimulation (pulses: 1 msec duration, 10/sec at 5-30 V) of the central end of a cut abdominal mammary nerve in urethane-anesthetized rats. Plasma E and NE concentrations were significantly elevated in two different strains of rats (Wistar and Holtzman) within 5 min of suckling. The concentration of E and NE did not change in control unsuckled rats during the same time period. As a complementary indication of sympathetic activation, it was observed that piloerection occurred during suckling. Plasma E levels (but not NE levels) increased significantly within 30 sec of a 2-min period of nerve stimulation in lactating rats on either day 7 or day 21 of lactation, as well as in nonlactating rats. The effect was significantly greater in nonlactating rats. The levels of E and NE were not altered after sham stimulation, whereas adrenalectomy abolished the rise in plasma E after mammary nerve stimulation. Blockade of the rise in plasma E also occurred after rapid injection of 100 microliters milk intraductally into each of two thoracic mammary glands, 15 sec before the onset of mammary nerve stimulation. These results show that E and NE can be released in response to suckling, and that activation of ductal mechanoreceptors may inhibit such release. These mechanisms may operate to regulate the rate of milk removal during suckling in the rat.

Neural and anatomic characteristics of peripheral afferent fibers in the milk ejection reflex

Conduction velocities were measured and certain morphologic characteristics were examined of the abdominal mammary nerve in two- to ten-day postpartum rats. This nerve enters the spinal cord at the spinal segmental level T-12. Overall conduction velocity was (Mean +/- S.D.) 18.9 +/- 2.25 m/sec with a major peak at 9.7 +/- 0.72 m/sec. The distribution of conduction velocities in the nerve was similar to that of a typical spinal nerve. Nerve fiber diameters measured between about 1 and 25 microns with peaks at 4.9, 10.5, and 18.9 microns. Injection into the peripheral nerve of fluorescent dye, Lucifer yellow CH (LY), or wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) after ventral root rhizotomy permitted study of the distribution of primary afferents in the spinal cord. The terminal field of these fibers centered around the dorsal cap of Clarke's column and the lateral spinal nucleus, bilaterally. The distribution of WGA-HRP was more restricted than that of LY. A large number of LY-staining fibers were also found ipsilaterally in the medial portion of the intermediomedial column. A smaller amount of LY-staining was present contralaterally in the area of the spinothalamic tract. It is concluded that afferent impulses resulting from mammary stimulation in the milk ejection reflex are probably carried in a mixed spinal nerve whose primary afferent field lies mainly in ipsilateral spinal structures, although there is some evidence for crossing fibers. The data suggest that considerable opportunity exists for interaction with major sensory afferent fiber systems as well as with autonomic fibers. Hence, the spinal path of afferent information relevant for the milk ejection reflex may well be diffuse and it may involve several sensory modalities.

Is oxytocin really necessary for efficient milk removal in dairy cows?

This review examines the potential importance of mechanisms other than the release of oxytocin for efficient milk removal. First, evidence is presented that oxytocin release is not always essential for efficient milk removal. Second, potential roles for the release of oxytocin during suckling or milking not directly related to milk removal are discussed. Third, alternative mechanisms that potentiate or induce milk ejection are introduced. Finally, the role of the autonomic nervous system in the milk removal process is examined.

The milk ejection pathway in brain studied with the 2-deoxyglucose method

The neural pathways involved in the milk ejection reflex have been studied with the aid of the 2-deoxyglucose (2DG) method. All the experiments were carried out on Wistar female rats, 9-11 days post-partum, which had been separated from their pups (except for one) overnight. The effect of suckling on the relative metabolic activity (RMA) of the brain was studied in conscious rats and in rats anaesthetized with urethane. Control animals were similarly treated but were not suckled. In addition, the effect of mammary nerve stimulation on RMA was studied in animals anaesthetized with urethane; sham-operated animals served as controls for this group. Suckling (minimum of 10 pups) in conscious animals had no apparent effect on the RMA of any of the brain areas measured. However, in anaesthetized rats, suckling produced a significant increase in the RMA of the paraventricular and supraoptic nuclei (PVN and SON), but had no effect on the RMA of any other brain area or the pituitary gland. Stimulation of the mammary nerve, with a stimulus that causes milk ejection and an increase in prolactin release, produced a significant increase in the RMA of the PVN, SON, the pars distalis and pars nervosa and the spinothalamic tract, and a significant decrease in the ventromedial and mediodorsal nuclei of the thalamus, the zona incerta, the red nucleus and the ventral nucleus of the lateral lemniscus. These results show that suckling significantly increases the metabolic activity of afferent terminals in the PVN and SON. Activation of the cell bodies of the PVN and SON, as assessed by increased RMA of the pituitary gland, could be evoked by the more intense stimulus of mammary nerve stimulation. The ascending pathway from the mammary nerve involves the spinothalamic tract but could not be traced beyond the midbrain. The lack of effect of suckling in conscious animals may have been due to the inhibitory influence of stress mediated by forebrain structures.

In situ voltammetric microelectrodes: application to the measurement of median eminence catecholamine release during simulated suckling

Catechol-sensitive microelectrodes (10-30 microns) were developed and then used to study the dynamic regulatory role of the prolactin inhibiting factor, dopamine, under conditions of simulated suckling. Current flow resulting from the electrochemical oxidation of catecholamines at the microelectrode surface was linearly related to the concentration of catecholamines present in solution over the range of 5-100 microM. Endogenous catecholamine levels in the rat median eminence were readily detectable and the electrochemical signal corresponding to dopamine release responded in an appropriate manner to various pharmacologic manipulations. We then implanted carbon microelectrodes into the medial median eminence region among capillaries of the primary portal plexus of urethane anesthetized lactating rats. Catecholamine release into the extracellular fluid was electrochemically measured once each minute before, during and after electrical stimulation (15 Hz, 5-30 V, 15 min) of a surgically isolated mammary nerve trunk. This simulated suckling paradigm reliably evoked prolactin secretory episodes qualitatively similar to those observed during suckling of the nipples by the young. During the period of nerve stimulation, a transient (3-5 min) 65% decline in electrochemically detectable catecholamine release was observed. Following cessation of nerve stimulation an oscillatory pattern of catecholamine release was observed with an overall trend toward an increased level of release. This latter observation corresponds with previous reports of increased hypothalamic dopamine turnover during or following suckling and with the increased levels of dopamine measured in hypophysial portal blood following mammary nerve stimulation. The transient nature of the decline of catecholamine release during the nerve stimulation period may explain why a similar observation has not been forth-coming from experiments utilizing the stalk blood collection technique (unless the standard collection periods are considerably shortened). These observations lead us to reject the hypothesis of a mirror image relationship between stalk blood dopamine and peripheral prolactin levels. Instead, we suggest that a transient decline in dopamine secretion coincident with the onset of suckling acts to prepare the pituitary lactotrophs to respond to a prolactin releasing factor which then facilitates prolactin secretion.

The decrease in hypothalamic dopamine secretion induced by suckling: comparison of voltammetric and radioisotopic methods of measurement

Previous in situ voltammetric microelectrode measurements of median eminence dopamine release during mammary nerve stimulation of anesthetized lactating rats revealed a transient (1-3 min) 70% decline of dopamine concentrations. This dopamine was believed to be destined for secretion into the hypophysial portal circulation, but direct experimental support for this supposition was lacking. Thus, in the present study, [3H]dopamine release into brief sequential samples of hypophysial portal blood was compared with dopamine release in the median eminence measured by voltammetry. Lactating female rats were urethane anesthetized, and the median eminence pituitary region was exposed. [3H]Tyrosine was injected into a jugular cannula (100 microCi) followed by continuous infusion (5 microCi/min). In a preliminary experiment, this regimen produced a steady state level of [3H]dopamine in the portal blood within 45 min. In subsequent experiments, portal blood was collected as sequential 3-min samples, and electrochemical sampling from a microelectrode placed in the median eminence occurred at 1-min intervals. Electrochemical current resulting from the oxidation of dopamine in the medial median eminence was unvarying throughout the 75-min experiment in control rats (n = 4) and during the 30-min control period preceding mammary nerve stimulation in the other group (n = 4). These results were parallel by [3H] dopamine levels in portal blood during the same periods of time. All animals showed simultaneous decreases in oxidation current and [3H]dopamine levels within 1-4 min after initiation of mammary nerve stimulation (respectively, 35 +/- 7% and 62.5 +/- 5.9%, mean +/- SEM). Significant increases in oxidation current, taking the form of brief 2- to 6-min pulses began within an average of 18.5 min after initiation of stimulation. Similar increases in [3H]dopamine levels in portal blood were also observed. These and earlier results demonstrate that mammary nerve stimulation (and by extension, suckling) induces a momentary, but profound, decrease in hypothalamic dopamine secretion which precedes or accompanies the rise in PRL secretion evoked by the same stimulus.

Evidence for the involvement of hypothalamic dopamine and thyrotrophin-releasing hormone in suckling-induced release of prolactin

The changes in adenohypophysial and hypothalamic content and in hypothalamic release of dopamine and thyrotrophin-releasing hormone (TRH) into the hypophysial portal system during the suckling-induced release of prolactin were investigated. An increase in peripheral plasma levels of prolactin was induced by mammary nerve stimulation in urethane-anaesthetized and by suckling in unanaesthetized lactating rats. In the unanaesthetized rat, suckling caused a decrease of dopamine levels in hypothalamus and adenohypophysis and a short-lasting small increase in hypothalamic TRH. Mammary nerve stimulation induced a transient decrease in dopamine levels and an increase in TRH levels in hypophysial stalk blood. To assess the significance of the observed changes in dopamine and TRH levels for prolactin release, these changes in dopamine and TRH were mimicked in lactating rats anaesthetized with urethane and pretreated with α-methyl-p-tyrosine (AMpT, a competitive inhibitor of catecholamine synthesis). Reducing hypothalamic dopamine secretion by treatment with AMpT increased peripheral plasma levels of prolactin from 15 to 477 ng/ml; an infusion with dopamine, resulting in plasma levels similar to those measured in hypophysial stalk plasma, reduced plasma levels of prolactin to 127 ng/ml. Neither a 50% reduction in dopamine infusion rate for 15 min nor administration of 100 ng TRH caused an appreciable change in plasma prolactin levels. However, when dopamine infusion was reduced by 50% for 15 min just before TRH was injected, then an increase in plasma levels of prolactin from 172 to 492 ng/ml was observed. Thus, the effectiveness of TRH in releasing prolactin in the lactating rat was enhanced when a transient decrease of dopamine levels occurred before treatment with TRH. It is concluded that the changes observed in dopamine and TRH levels in hypophysial stalk blood are involved in the suckling-induced prolactin release in an important manner.

Responses of single receptor units of the rat mammary gland parenchyma to tactile stimulation

Slowly adapting mechanoreceptor units with regular and irregular discharges were found in the mammary gland parenchyma. The receptive fields of both types of receptor units had a diameter of 1-1.5 mm. In some cases the receptor units had spontaneous activity at rest (0.5-3 spikes/sec). During stimulation by sawtooth mechanical stimuli with different rates of rise the momentary frequency of the receptor units was a linear function of amplitude of the mechanical stimulus. The threshold of afferent action potentials of the receptor units increased with an increase in the rate of rise of the mechanical stimulus.

Effect of electrical stimulation of mammary nerve upon pituitary and plasma prolactin concentrations in anesthetized lactating rats

A 50-70% depletion in the anterior pituitary concentration of PRL occurred within 5 min of electrical stimulation of a single abdominal mammary nerve of urethane-anesthetized lactating rats previously nonsuckled for 6-8 h. The stimulus parameters were: 1-msec pulses, 10-20/sec at 5-30 V applied 5 sec on and 10 sec off. The anterior pituitary concentration of PRL remained low for another 15-30 min after depletion, then repleted to prestimulus levels by the 90th min. The same pattern of depletion-repletion occurred when mammary nerve stimulation was applied for 5 min as when it was applied for 180 min. In other experiments, plasma PRL concentration rose swiftly and attained a maximal level, 4- to 5-fold in Wistar rats and 8- to 10-fold in Holtzman rats, above basal concentrations within 10-15 min of mammary nerve stimulation. The maximal level was sustained throughout the time the nerve was stimulated and for 45 min after the stimulation was stopped, i.e. the sustained secretion of PRL into the plasma occurred at the same time that depleted PRL was repleting. Ligation of both adrenals or iv injection of the beta-adrenergic blocker, propranolol, before stimulation of the nerve had no effect upon the plasma PRL profile in response to mammary nerve stimulation. These data indicate that mammary nerve stimulation mimics that of suckling upon depletion, repletion, and the release of PRL into the circulation and add further support to the hypothesis that these phases are independent processes.

[Effect of inhibitors of the active transport of ions on the spontaneous and evoked electrical activity of parenchymal mammary gland receptors]

In the potassium-free saline spontaneous activity of the mammary gland parenchyma receptor increased within first 30--40 min., then decreased and, finally, ceased. Sensitivity of the receptor to the mechanical stimuli decreased gradually. After washing in normal saline the evoked and spontaneous activities recovered. Ouabain (5 . 10(-5)--10(-4) g/ml) exerted an effect similar to that of the potassium--free solution except that the impulse activity could not be restored by the washing in physiological saline.

[Properties of mechanoreceptor units of mammary gland parenchyma]

Two types of mechanosensitive units were found in the parenchyma of the rat mammary gland (SA and RA). Most of SA units had a spontaneous activity 1-3/sec. and responded to an abrupt displacement with a decline in the activity for 5-10 sec. RA had no such a response. The relation between average firing rate and displacement velocity at a constant displacement amplitude was linear for both SA and RA. RA however had the velocity threshold higher than SA. The number and frequency of SA afferent impulses decreased at a repeated stimulation. Electron microscopy revealed that free nerve endings correspond to SA and RA.

[Slowly-adapting mechanoreceptor units of the guinea pig mammary nipple]

Slowly--adapting mechanoreceptor units of areola and nipple (MAI and MAII) at rest exhibited spontaneous 1--2/sec firing in 6% only. At constant mechanical displacement of the skin surface. MAI had an irregular firing while MAII--regular one. Mean frequency of MAI and MAII APs linearly depended on the amplitude of mechanical stimulus. trepeated stimulation of MAI and MAII reduced number of APs for each successive stimulus. At sinusoidal mechanical stimulation, the smallest AP threshold for MAI and MAII was about 10--15 Hz.

Nature of adrenoceptor sites in bovine teat muscles

The motility of smooth muscles excised from the wall of bovine teats was studied "in vitro". These muscle preparations often show spontaneous rhythmic contractions. Administration of isoprenaline results in relaxation and decreased spontaneous motility, these effects being blocked by propranolol. Noradrenaline elicits contraction and stimulates rhythmical activity, these effects being inhibited by dibenamine. The effect of adrenaline is variable as it induces contraction or inhibition or biphasic responses. Dibenamine blocked contractions, whereas propranolol inhibited relaxations. It appears that alpha and beta adrenoceptors are present in teat muscles. Stimulation of the former elicits activation, whereas stimulation of the latter results in inhibition. These "in vitro" results are largely in agreement with "in vivo" responses described previously.