Neonatal decortication and adult female sexual behavior

Life without a brain: Neuroradiological and behavioral evidence of neuroplasticity necessary to sustain brain function in the face of severe hydrocephalus

A two-year old rat, R222, survived a life-time of extreme hydrocephaly affecting the size and organization of its brain. Much of the cortex was severely thinned and replaced by cerebrospinal fluid, yet R222 had normal motor function, could hear, see, smell, and respond to tactile stimulation. The hippocampus was malformed and compressed into the lower hindbrain together with the hypothalamus midbrain and pons, yet R222 showed normal spatial memory as compared to age-matched controls. BOLD MRI was used to study the reorganization of R222’s brain function showing global activation to visual, olfactory and tactile stimulation, particularly in the brainstem/cerebellum. The results are discussed in the context of neuroadaptation in the face of severe hydrocephaly and subsequent tissue loss, with an emphasis on what is the “bare minimum” for survival.

Physiological and Anatomical Outputs of Rat Genital Cortex

Rat somatosensory genital cortex contains a large sexually monomorphic representation of the penis in males and the clitoris in females. Genital cortex microstimulation-evoked movements of legs, trunk and genitals, which showed sex-specific differences related to mating behaviors and included thrusting in males and lordosis-like movements in females. Erections/tumescence of penis or clitoris could not be evoked, however. Anterograde tracer injections into penis/clitoris cortex revealed eleven corticocortical and 10 subcortical projection targets, which were qualitatively similar in both sexes. Corticocortical genital-cortex-projections innervated about 3% of the cortical surface and most were analog to other somatosensory projections targeting motor cortex, secondary somatosensory cortex, parietal cortex and perirhinal cortex. Corticocortical projections that differed from other parts of somatosensory cortex targeted male scrotum cortex, female vulva cortex, the somatosensory–ear–auditory-cortex-region and the caudal parietal area. Aligning cytoarchitectonic borders with motor topography, sensory genital responses and corticocortical projections identified a candidate region for genital motor cortex. Most subcortical genital-cortex-projections were analog to other thalamic, tectal or pontine projections of somatosensory cortex. Genital-cortex-specific subcortical projections targeted amygdala and nucleus submedius and accumbens. Microstimulation-effects and projections support a sexual function of genital cortex and suggest that genital cortex is a major hub of sexual sensorimotor processing in rodents.

Development of rat female genital cortex and control of female puberty by sexual touch

Rat somatosensory cortex contains a large sexually monomorphic genital representation. Genital cortex undergoes an unusual 2-fold expansion during puberty. Here, we investigate genital cortex development and female rat sexual maturation. Ovariectomies and estradiol injections suggested sex hormones cause the pubertal genital cortex expansion but not its maintenance at adult size. Genital cortex expanded by thalamic afferents invading surrounding dysgranular cortex. Genital touch was a dominant factor driving female sexual maturation. Raising female rats in contact with adult males promoted genital cortex expansion, whereas contact to adult females or nontactile (audio-visual-olfactory) male cues did not. Genital touch imposed by human experimenters powerfully advanced female genital cortex development and sexual maturation. Long-term blocking of genital cortex by tetrodotoxin in pubescent females housed with males prevented genital cortex expansion and decelerated vaginal opening. Sex hormones, sexual experience, and neural activity shape genital cortex, which contributes to the puberty promoting effects of sexual touch.

We recently identified the somatosensory representation of rat genitals; remarkably, this cortical region—genital cortex—is sexually monomorphic, despite the marked sexual dimorphism of external genitals in rats. Most intriguing was the observation that genital cortex doubles in size during puberty. In order to shed light on this unusual expansion, we studied genital cortex development and sexual maturation in the female rat. We first showed that sex hormones are likely to cause the pubertal expansion of genital cortex. Next, we examined whether sexual experience affects the development of female genital cortex. Raising females together with adult males advanced genital cortex expansion, but cohousing with adult females or exposure to nontactile male cues was not sufficient to drive genital cortex growth. Surprisingly, artificial genital touch led to an early onset of female puberty and growth of genital cortex. In line with this finding, we find that if genital cortex activity is blocked, the advancing effects of adult males on puberty and genital cortex growth are inhibited. Together, our results point to an important role of genital cortex in the puberty-promoting effects of sexual touch.

6-hydroxydopamine lesions enhance progesterone-facilitated lordosis of rats and hamsters, independent of effects on motor behavior

The Ventral Tegmental Area (VTA) is an important brain area for progesterone (P(4))'s effects to facilitate female sexual behavior of rodents. We investigated the importance of dopaminergic neurons in the VTA, and two dopaminergic projection sites, the Nucleus Accumbens (NAc), and Caudate Nucleus of the Striatum (CN), in modulating P(4)-facilitated sex and motor behavior. Ovariectomized (ovx) rats and hamsters, administered estradiol benzoate (10 microg) and P(4) (0, 50, 100, 200, or 500 microg), were tested for motor behavior in a chamber that automatically records horizontal beam breaks, and for sexual behavior in response to a sexually-experienced male. Animals were tested once a week until each P(4) dosage was received; animals then had bilateral 6-hydroxydopamine (6-OHDA) or sham lesions to the VTA, NAc, or CN and were re-tested at each P(4) dosage on subsequent weeks. Fixed brains were stained with cresyl violet and processed for dopamine transporter (DAT) immunoreactivity. The number of cresyl violet stained cells was significantly lower in all 6-OHDA infusion sites compared to non-6-OHDA infusion sites of rats and hamsters. Also, in rats, the number of DAT-immunoreactive neurons was lower in all 6-OHDA infusion sites compared to non-6-OHDA infusion sites. In rats, 6-OHDA but not sham, lesions to the VTA, NAc, or CN produced P(4)-dependent increases in lordosis quotients and resulted in modest increases in motor behavior. In hamsters, 6-OHDA, but not sham, lesions to the VTA, NAc, or CN produced P(4)-dependent increases in total lordosis durations and produced modest decreases in motor behavior. This suggests that the dopaminergic output neurons of midbrain VTA may play an important role in modulation of P(4)-facilitated sexual lordosis among rodents.

Representations of motivational drives in mesial cortex, medial thalamus, hypothalamus and midbrain

We propose that neural representations of motivational drives, including sexual desire, hunger, thirst, fear, power-dominance, the motivational aspect of pain, the need for sleep, and nurturance, are represented in four areas in the brain. These are located in the medial hypothalamic/preoptic area, the periaqueductal gray matter (PAG) in the midbrain/pons, the midline and intralaminar thalamic nuclei, and in the anterior part of the mesial cortex, including the medial prefrontal and anterior cingulate areas. We attempt to determine the locations of each of these representations within the hypothalamus/preoptic area, periaqueductal gray and cortex, based on the available literature on activation of brain structures by stimuli that evoke these forms of motivation, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on hodological data. We discuss the hierarchical organization of the representations for a given drive, outputs from these representations to premotor structures in the medulla, caudate-putamen, and cortex, and their contributions to involuntary, learned-sequential (operant) and voluntary behaviors.

Transneuronal tracing from sympathectomized lumbar epaxial muscle in female rats

Pseudorabies virus (PRV) has been used as a transneuronal tracer to study central neural networks, including the central control of the lordosis-producing, lumbar epaxial muscles. Within muscles, however, the sympathetic innervation of blood vessels poses a confounding source of tracer labeling in the CNS. The present study destroyed sympathetic nerves before injection of PRV, thereby allowing for a more selective uptake by somatic motoneurons. Specifically, a focal sympathectomy was created by the injection of dopamine-beta-hydroxylase immunotoxin (DHIT). When PRV was injected into control rats, both somatic motoneurons within the ventral horn of the spinal cord and sympathetic preganglionic neurons within the intermediolateral column (IML) of the spinal cord became labeled. Additionally, labeled neurons were observed in many brain regions, including those previously implicated in the control of the lordosis reflex (e.g., the medullary reticular formation; MRF) and those previously implicated in the control of vasomotor tone (e.g., the rostral ventrolateral medulla; RVLM). When injected into DHIT-pretreated animals, PRV labeling in ventral horn neurons persisted in many animals; however, labeling in IML was eliminated in almost every case. In these animals, PRV labeling was absent in brain areas traditionally associated with vasomotor tone, such as RVLM, whereas labeling persisted in brain areas previously implicated in the control of the lordosis response, such as MRF. The results support the connectivity of spinal and medullary structures with the somatic control of the lordosis-producing muscles and provide a more detailed description of these portions of the putative lordosis-relevant neurocircuitry.

Effects of neonatal decortication on the social play of juvenile rats

The effects of radical neonatal decortication on the social play of juvenile rats, as well as the effects of neonatal ablation of frontal or parietal cortex, were examined in this series of experiments. When total decorticates were tested in like-lesioned pairs, the frequency of pinning was reduced by about 50% and their average pin durations were shorter. Nevertheless, the play of decorticates appeared essentially normal in general appearance, and did not differ from controls in a measure of overall play vigor using an electronic activity platform. Further, there were no differences in pin frequencies when controls and decorticates were paired together in cross-lesion testing. Separate tests of play solicitation behaviors did not detect any differences between controls and decorticates suggesting that play motivation was essentially intact after decortication. No deficits in pinning resulted from frontal ablations; however, pin durations were shorter in like-lesion testing. In cross-lesion testing, there was an increase in dorsal contacts and a trend toward shortening of pin durations. Parietal aspirations resulted in a 65% reduction in pin frequency, without substantially altering dorsal contacts. Anesthetization of the anterior surface of the animal's back with xylocaine reduced pinning in controls but eliminated pinning in parietals. Although the results generally indicate little participation of the neocortex in the instigation of rough-and-tumble play, the reliable numerical changes that were observed may be explained by apparent motor changes as well as reduced somatosensory sensitivity.

The mating movements of male decorticate rats: evidence for subcortically generated movements by the male but regulation of approaches by the female

The study shows that although many features of copulation in decorticate male rats are normal, copulatory success is importantly dependent upon the control of approaches exerted by the normal female rat. Copulation by neonatally decorticated adult rats and normal adult rats was studied in cohabitation and videotaped tests. Seven of 10 decorticate rats and 6 of 6 normal rats sired pups in the cohabitation test. When initially paired with ovariectomized and primed female rats, in the videotaped tests, all normal rats, but only one decorticated rat, copulated. All decorticate rats made movements indicative of sexual interest including: treading on the female's back, passing over the female, and sniffing the female's genitals. After activating stimulation, 5 of 6 remaining decorticated males copulated. After one successful mount the remaining copulatory patterns proceeded relatively normally. Numbers of mounts, intromissions, ejaculations, postejaculatory songs, and the intromission and ejaculatory patterns were like those of control rats, although the decorticate rats had fewer mount bouts and showed abnormalities in the execution of movements. Precopulatory movements were notated, using the Eshkol-Wachmann system, and compared with copulatory movements. Non-copulatory and copulatory approaches were similar, except that clasping appeared to be the key movement involved in the transition of an approach movement into a copulatory movement. The analysis also showed that the females' movements of hopping, turning, and kicking were important for regulating the males' approaches, and were instrumental in the success achieved by the decorticated males. The study shows that although the cortex, insofar as it facilitates the appearance of certain movements and contributes to their efficiency, is involved in male sexual activity, in its absence well organized sexual activity is possible, although this is dependent, in part, upon the behaviour of the female.

Decortication abolishes place but not cue learning in rats

The experiments examined whether decorticate rats are able to acquire a place learning strategy, as compared with a cue learning strategy, to successfully navigate from one place to another and whether the hippocampus, in the absence of the neocortex, contributes to successful performance. Decorticate rats, with or without hippocampectomy, were unable to locate an "invisible" platform submerged at a fixed place in a tank of cool water (made opaque by milk), rather they scrabbled at the edges of the tank and failed even to initiate search strategies. They were able to learn to swim directly to the platform if it was visible. Their ability to find the hidden platform was not enhanced by presurgical experience or two-stage ablations with training before and after ablations. When pretrained on the cue task and tested on the place task, they learned to inhibit scrabbling at the tank edges and "search" in a haphazard fashion for the hidden platform, but they never learned to swim directly toward it. When decorticate rats, trained on the cue task, received superior colliculus or basal ganglia removal in a second operation, cue learning was abolished. Hippocampal removal after decortication left performance on the cue task unaffected. The results demonstrate: (1) the integrity of the neocortex is essential for place learning; (2) the brainstem, including superior colliculus and basal ganglia, is sufficient for cue learning; and (3) in the absence of the neocortex the hippocampus plays no role in guiding either type of navigation. It is concluded that sensorimotor subsystems of the forebrain play a special role as detector-response systems for guiding behaviour in response to constellations of distal stimuli, whereas subcortical structures are sufficient for navigation to a single stimulus.

A balanced brain asymmetry modulates T cell-mediated events

Partial ablation of the left fronto-parietal cerebral cortex decreases the number of spleen T cells, impairs IgG-alpha SRBC and T mitogen-induced responses, and delays the response to alloantigens. In contrast, these events are increased following a symmetric lesion of the right neocortex. The findings extend previous results showing that the neocortex modulates NK activity and the efficacy of T cell-specific serum factors. B cells and macrophages are not affected. In these assays, mice subjected to ablation of one lateral cerebral neocortex serve as controls for symmetrically lesioned mice, in addition to no surgery or sham-operated controls. The findings suggest that brain lateralization for cognitive processes should be extended to T cell immune recognition. The phenomenon is present at a population level.