Reproducible increases in intramammary pressure after spinal cord stimulation in lactating rats

Oxytocin release and milk removal in ruminants

Before milking, less than 20% of the milk yielded by dairy cows is stored within the cistern, where it is immediately available for removal. Most of the milk is available for the milking machine only after milk ejection, which occurs in response to tactile teat stimulation and oxytocin release. For complete milk removal, milk ejection is necessary throughout the entire milking process. The continuation of stimulatory effect of the milking machine until the end of milking is, therefore, essential. Premilking teat stimulation causes induction of alveolar milk ejection before the start of milking. Thus, bimodal milk flow curves (i.e., interruption of milk flow after removal of the cisternal milk) are avoided. Continual ejection of milk is dependent on the presence of elevated oxytocin concentrations during the entire milking. Any interruption of the milk ejection process can disturb milk removal. Disruption of milk removal can be caused by peripheral inhibition of oxytocin effects on the mammary gland or by inhibition of oxytocin release by the central nervous system. Peripheral inhibition is induced by elevated concentrations of catecholamines through stimulation of alpha-adrenergic receptors in the mammary gland, likely via changes in ductal resistance. Inhibition of oxytocin release by the central nervous system has been observed in primiparous cows immediately after parturition, during peak estrus, and during milking in unfamiliar surroundings; concentrations of beta-endorphin and cortisol are elevated in this situation. However, the role of endogenous opioid peptides in the inhibition of oxytocin release in cows remains unclear. In conclusion, during machine-milking, the physiological requirements of the cows need to be considered, and, most importantly, stressors must be minimized.

Oxytocin release evoked by electrical stimulation of the medial forebrain in the rat: analysis of stimulus parameters and supraoptic neuronal activity

The role of the medial forebrain area (vertical limb of the diagonal band, medial septum and medial nucleus accumbens) in the control of oxytocin secretion in lactating rats was investigated. Electrical stimulation of the medial forebrain evoked a reproducible rise in intramammary pressure, equivalent to that caused by i.v. injection of 1 mU oxytocin. No pressor effect accompanied this response. Radioimmunoassay of plasma samples showed that stimulation caused a significant rise in the concentration of circulating oxytocin. The effects of changing the parameters of stimulation to the medial forebrain were compared with those evoked by stimulation of the neural stalk. The optimal frequency for stimulation of the forebrain was found to be four-fold lower (10-20 Hz) than that for stimulation of the neural stalk (50 Hz). During continuous prolonged stimulation of the forebrain (20 Hz; 2 min) only a single transient response was obtained, whereas a protracted response was obtained as a result of prolonged stimulation of the stalk. Recordings were made from antidromically identified neurosecretory cells in the supraoptic nucleus. Electrophysiological responses to electrical stimulation of the medial forebrain were characterized by two main features. (1) Single-pulse stimulation produced only a small excitation (one or two action potentials), while high-frequency trains produced a profound facilitation of this response, with each pulse evoking short-duration 'bursting' behaviour in the supraoptic neurons. (2) During long trains of stimulation this frequency-dependent facilitation declined and could only be renewed after a period of rest.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of early- and late-recruited oxytocin bursting cells at the beginning of suckling in rats

1. Paired or single recordings of paraventricular and/or supraoptic oxytocin cells at the beginning of suckling in urethane-anaesthetized rats enabled us to study cell recruitment and compare the characteristics of the early- and late-recruited cells. This was done under different experimental conditions, i.e. when the reflex was triggered in less than 1 h suckling (control), and when its triggering was facilitated either by the intraventricular (i.c.v.) injection of oxytocin, of apomorphine (a dopamine agonist) or by the intravenous (i.v.) injection of propranolol (a beta-adrenoceptor antagonist) into suckled rats with no milk ejection. 2. Under control conditions, the amplitude (total number of spikes) of the successive bursts of the early-recruited cells progressively increased, generally reaching maximum by the 6th burst. This increase was more rapid and greater after oxytocin than under control conditions or after apomorphine injection, and was delayed and reduced after propranolol. The burst frequency was higher after oxytocin and apomorphine injections than under control conditions and very low after propranolol. 3. Late-recruited cells were observed under all experimental conditions, except after oxytocin injection, since all cells displayed bursts right away. Moreover, when injected during the recruitment period of a control reflex, oxytocin greatly speeded up the recruitment of the late-recruited cells. These cells generally displayed smaller amplitude bursts than the early-recruited cells. Moreover, the increase in burst amplitude was less marked for the late- than for the early-recruited cells and often was not sustained. 4. Neither the likelihood of recruitment of an oxytocin cell nor its burst amplitude could be correlated with background activity level and there was no clear relationship between the recruitment period or the bursting characteristics on one hand and the background activity on the other. 5. In conclusion, the differences between the early- and late-recruited cells in recruitment time and in burst amplitude reflected differences in cell excitability which may depend mainly on the presence of oxytocin in the magnocellular nuclei.

Prolonged electrical stimulation of the nipples evokes intermittent milk ejection in the anaesthetised lactating rat

Intermittent and continuous electrical stimulation of the nipples elicited milk-ejection responses in the lactating rat. The responses occurred intermittently, with similar amplitudes and periodicity as those seen in suckled rats. The responses were always associated with synchronization of the electroencephalogram (EEG), but some rats with synchronized EEG activity did not milk eject during stimulation. Since continuous stimulation also resulted in intermittent milk ejection it seems unlikely that the periodicity of the reflex in suckled rats depends upon changes in the intensity of sensory stimulation. The techniques of nipple stimulation may be a useful method with which to study neural pathways and other phenomena such as gating involved in oxytocin release and milk ejection. The success of the technique depends on a variety of factors such as parameters of the stimulation and state of anaesthesia.

Afferent projections from the mammary glands to the spinal cord in the lactating rat--II. Electrophysiological responses of spinal neurons during stimulation of the nipples, including suckling

In lactating rats, the milk ejection reflex is evoked and maintained by stimulation of the nipples by the suckling young. In order to understand the processing of the suckling stimulus within the spinal cord, urethane-anaesthetized lactating rats were prepared for electrophysiological recording from the thoraco-lumbar spinal cord during stimulation of the nipples. Single shocks to inguinal or abdominal nipples evoked a cord dorsum potential, consisting of an early (2.6 ms) afferent volley followed by a negative wave (100-200 microV; latency 5-7 ms, duration 5-10 ms). Evoked potentials were also recorded at various depths within the spinal cord, with a maximum amplitude (200-400 microV) at a depth of 400-800 microns, 400-800 microns lateral to the mid-line. At a given recording site, the response was maximal for one particular nipple but submaximal potentials could be evoked from adjacent nipples. Simultaneous stimulation of adjacent nipples caused summation of the response. Unit recordings were made from 35 spinal neurons. Upon electrical stimulation of the nipples, the cells responded with an early train of spikes (latency 5-15 ms), and in 6 cells, a later response (140-180 ms), with a higher stimulation threshold, was also observed. All cells examined showed convergence and summation from different nipples. Twenty out of 27 cells were also activated by stretching of the nipples, which evoked a rapidly adapting response; rhythmical stretching produced a more sustained increase in activity. The cells also responded to other natural stimuli such as touch and pressure or stroking the hair around the nipple and had large receptive fields. Six cells were tested with the suckling stimulus. There was a brisk increase in firing as the pup grasped the nipple and then intermittent (every 18-30 s) episodes of enhanced activity, which directly correlated with the suckling movements. These episodes continued for the duration of the suckling test and were enhanced when a second pup was placed on an adjacent nipple. Finally, from a few experiments when a stimulating electrode was placed within the contralateral antero-lateral funiculus at the level of C2-C3 for antidromic identification, it was seen that some of the cells activated from the nipples projected to higher levels. The short latency responses to nipple stimulation, including suckling, suggest that the suckling stimulus reaches the spinal cord ungated.(ABSTRACT TRUNCATED AT 400 WORDS)

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.