Responses of single units in the medulla to genital stimulation in estrous and anestrous cats

Contributions of the medullary raphe and ventromedial reticular region to pain modulation and other homeostatic functions

The raphe magnus is part of an interrelated region of medullary raphe and ventromedial reticular nuclei that project to all areas of the spinal gray. Activation of raphe and reticular neurons evokes modulatory effects in sensory, autonomic, and motor spinal processes. Two physiological types of nonserotonergic cells are observed in the medullary raphe and are thought to modulate spinal pain processing in opposing directions. Recent evidence suggests that these cells may modulate stimulus-evoked arousal or alerting rather than pain-evoked withdrawals. Nonserotonergic cells are also likely to modulate spinal autonomic and motor circuits involved in thermoregulation and sexual function. Medullary serotonergic cells have state-dependent discharge and are likely to contribute to the modulation of pain processing, thermoregulation, and sexual function in the spinal cord. The medullary raphe and ventromedial reticular region may set sensory, autonomic, and motor spinal circuits into configurations that are appropriate to the current behavioral state.

The activity of ON and OFF cells at the rostroventromedial medulla is modulated by vagino-cervical stimulation

In anesthetized rats it was tested whether or not the activity of the ON and OFF cells within the rostral ventromedial medulla (RVM) is modulated by the mechanical stimulation of the uterine cervix (VS). ON cells were identified by an abrupt increase in their firing rate before the tail flick in response to a noxious heat. OFF cells were identified by a sudden decrease in their firing rate before the tail flick. All (27 out of 27) identified ON cells decreased their firing rate immediately after VS was applied. The effect of VS on the activity of the cells persisted for the entire stimulation period. On the other hand, all (19 out of 19) identified OFF cells increased their firing rate immediately after VS. The effect of VS on the activity of these cells also persisted for the entire stimulation period. The activity of the neutral cells showed no change, neither during the application of noxious heat, nor during VS. These results suggest that the analgesic-like effect produced by VS can be mediated by the activity of the antinociceptive circuit at the RVM. Alternatively, it can be suggested that the afferent inflow from the genital tract can induce the activity of the antinociceptive circuit at RVM, either by projections to the periaqueductal gray matter or by direct projections to RVM.

Estradiol and progesterone influence the response of ventromedial hypothalamic neurons to tactile stimuli associated with female reproduction

Stimulation of the vagina and cervix, provided by the male during copulation or manually with a probe, causes many behavioral and endocrine changes associated with female reproduction in rats. Previously, we found that vaginal-cervical stimulation (VCS), by mating or manual probing, increases the expression of Fos-immunoreactivity (Fos-IR) in discrete populations of neurons in the preoptic area, mediobasal hypothalamus and midbrain, suggesting that these neurons respond to VCS. The purpose of the present study was to determine if hormonal priming would increase the number of Fos-IR cells following VCS. Contrary to our hypothesis, in Experiment 1 priming animals with a behaviorally effective dose of 17 beta-estradiol benzoate followed 48 h later by progesterone caused a trend towards a decrease in the number of VCS-induced Fos-IR cells in the ventromedial hypothalamus. In Experiment 2, which was done to confirm this decrease in VCS-induced Fos-IR neurons by hormones, this effect was found to be statistically significant. Furthermore, this hormone-induced decrease in VCS-responsive cells was localized to the ventromedial nucleus of the hypothalamus, an area rich in estrogen and progestin receptors. No effects of hormone treatment on VCS-induced Fos-IR were observed in any other brain regions analyzed. These findings suggest that steroid hormones may elicit some of their effects on female reproductive behavior and physiology by altering the responsiveness of ventromedial nucleus neurons to vaginal-cervical stimulation.

Forebrain influences on brainstem and spinal mechanisms of copulatory behavior: a current perspective on Frank Beach's contribution

In a 1967 Physiological Reviews paper, Frank Beach put forth four propositions regarding forebrain and hormonal control of brainstem-spinal mechanisms of copulatory behavior. Simply stated, he proposed that: 1) the forebrain exerted an inhibitory control over species-typical copulatory reflexes through descending effects on brainstem-spinal mechanisms and 2) gonadal hormones influence these reflexes largely by actions on forebrain control processes rather than by direct effects on the brainstem or spinal cord. This theoretical scheme was of great heuristic significance during the subsequent two decades of research, which has largely supported and delineated in greater mechanistic detail the processes Beach hypothesized to exist. This subsequent research has also shown the central nervous system actions of gonadal hormones to be more widespread and complex than Beach proposed. Some of these recent research findings are presented, with emphasis on neurophysiological studies which have identified hormone-induced functional changes in forebrain and brainstem neurons. It is proposed that these functional changes may represent a mechanism for the behavior-controlling actions of hormones that were hypothesized by Beach.

The identification of a brainstem site controlling spinal sexual reflexes in male rats

It has long been assumed that the brainstem exerts a tonic descending inhibitory influence on spinal sexual reflexes, but the source of this inhibition is unknown. Sexual responses (penile erection and ejaculatory movements) were elicited in anesthetized male rats using urethral stimuli. Using brainstem transections and electrolytic and neurotoxic lesions, we have identified a group of neurons in the paragigantocellular reticular nucleus in the ventral medulla which mediates this descending inhibition. Correlative neuroanatomical studies indicate that the inhibition is mediated via a direct projection to pelvic efferent neurons and interneurons. These results promise new approaches to the treatment of human sexual disorders.

Responses of cat preoptic neurons to stimulation of the medial frontal cortex and the medial basal hypothalamus

Responses of single preoptic neurons to electrical stimulation of the medial frontal cortex, the mediobasal hypothalamus (MBH) and the medial forebrain bundle (MFB) were recorded in anaesthetised cats. Single pulse stimulation of the medial frontal cortex orthodromically drove 96 otherwise quiescent preoptic neurons, which were found more frequently in the dorsal preoptic region, inhibited 53% of the spontaneously active preoptic neurons and excited 16%. Testing of cortically influenced preoptic neurons with MBH or MFB stimulation resulted in antidromic invasion of 6% (MBH) and 9% (MFB). Convergence of orthodromic inputs from medial frontal cortex and MBH was detected in 78% of spontaneously active preoptic neurons, and three-way convergence including input from MFB was noted in 17% of neurons tested with all stimulators. Some cortex-response neurons were found to also respond to vaginal or anal probing, paw squeezing and haemorrhage. The role of this input to the preoptic region from medial frontal cortex remains to be elucidated, but may include neuroendocrine, behavioural and homeostatic functions.

The effects of lesions of medullary midline and lateral reticular areas on inhibition in the dorsal horn produced by periaqueductal grey stimulation in the cat

In barbiturate-anaesthetized cats, the excitation of lumbar dorsal horn neurones by impulses in unmyelinated primary afferent fibres was inhibited by electrical stimulation in the periaqueductal grey matter. This inhibition was slightly reduced by extensive electrocoagulation of the medullary midline and para-medial areas including the raphé, but significantly reduced by small bilateral lesions in the region of the caudal lateral reticular nuclei. When the lateral lesions were made subsequent to midline coagulation, the inhibition from periaqueductal grey stimulation was abolished. An important component of spinal inhibition from periaqueductal grey stimulation appears to relay in lateral reticular areas of the medulla.

The estrogen-sensitive neural system in the brain of female cats

Estrogens participate in the regulation of gonadotropin secretion and in the control of sexual and aggressive behavior. To elucidate the neuroanatomical substrate for these actions in the female cat, the topographical distribution of estrogen-concentrating cells in the brain was studied by autoradiography. One hour after the intravenous injection of 1 mCi (3 microgram) of 3H-estradiol in three adult ovariectomized cats, radioactivity was concentrated in the nuclei of neurons in several brain areas. Within the telencephalon, high concentrations of moderately to heavily labeled cells were found in the lateral and posterior septal nuclei (n.), bed n. of the stria terminalis (medial division), medial and central amygdaloid n., and amygdalo-hippocampal area. There were also many less strongly labeled cells in the ventral dentate gyrus. Within the diencephalon, many heavily labeled cells were distributed in the medial and periventricular preoptic n., anterior hypothalamic area, and infundibular, ventromedial, and periventricular hypothalamic n. In the brain stem, some labeled cells were located throughout the periaqueductal gray, and in the laminar spinal trigeminal n., n. of the solitary tract, n. incertus, marginal n. of the superior cerebellar peduncle, rostral linear n., and interfascicular n. Although minor differences were noted, the topographical distribution of estradiol-concentrating cells in the female cat brain agreed closely with that reported in other vertebrate species.

Antinociceptive effects of vaginal stimulation in rats: neurophysiological and behavioral studies

The present studies extend previous findings that probing the vaginal cervix of rats blocks withdrawal reflexes and induces immobilization44. In the present studies, we report that this effect is apparently not due to an action on the final motor pathway, for limb or facial movement induced by electrical stimulation of the pyramidal tract was not suppressed by the probing. In contrast, the sensory response of neurons in the ventrobasal complex of the thalamus to noxious pinch stimulation was markedly attenuated by probing the vaginal cervix. However, the response of these neurons to gentle tactile stimulation was not attenuated, indicating a selective antinociceptive effect of the probing. The antinociceptive effect was not necessarily related to changes in arousal. These findings were supported by behavioral studies in which probing the vaginal cervix blocked vocalization in response to tail shock, and elevated the current threshold for eliciting vocalization in response to tail shock. Furthermore, during the probing, the rats were found to be capable of vocalizing in response to presumably non-noxious (lifting) stimulation, even though their vocalization response to noxious tail shock was suppresed. These studies suggest that probing the vaginal cervix rats exerts an analgesic action.

Response properties and anatomical organization of pontine and medullary units responsive to vaginal stimulation in the cat

The response properties and anatomical organization in the brain stem of 234 pontine and 60 medullary genitally sensitive (GS) neurons were studied by single-unit recording in acutely prepared estrous and anestrous cats which were decerebrated under sodium pentothal or anesthetized with urethane. Pontine unit responses to vaginal probing were very pronounced and stimulus-bound, and involved a variety of response patterns. Most of the pontine GS neurons also responded to nociceptive stimuli and to a lesser degree, to innocuous somatic, extragenital visceral, and auditory stimuli. Among pontine GS cells recorded in estrous cats, responsiveness to innocuous somatosensory stimuli was more prevalent than among units in anestrous preparations (71% vs. 40%), and on the average, the units in estrous cats responded to stimulation of more regions of the body than did comparable units in anestrous animals. In recordings from the medulla in the decerebrate preparations, exclusive responsiveness to vaginal stimulation was observed in 22% of the GS neurons, contrasting with earlier results from urethane-anesthetized animals in which a greater degree of specificity was found. A close anatomical proximity existed between some medullary GS neurons and cells of nucleus ambiguus, including antidromically identified recurrent laryngeal nerve motor neurons, and between pontine cells and the trigeminal motor nuclei. Antidromic responses of GS neurons to stimulation of the brain stem revealed a projection from medullary GS neurons to the lateral tegmental field of the pons. Relatively few pontine GS neurons projected to the ipsilateral medulla. A strong synaptic influence upon medullary GS neurons, possibly or midbrain origin, descends from the lateral tegmental field of the pons.