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.