Research reportRegional brainstem expression of Fos associated with sexual behavior in male rats
Introduction
In rats, the distribution of the protein product of various immediate early genes (IEG) has been used to map regions in the nervous system associated with both appetitive and consumatory aspects of sexual behavior. The display of several IEGs, such as Fos, Jun, and EGR-1, has been observed in areas of the brain and spinal cord after male rats have copulated to ejaculation [1]. The most widely studied IEG product, Fos, has been found in the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BNST), medial amygdala (MeA), nucleus accumbens (NAcc), periaqueductal gray (PAG), and central tegemental field (CTF) following mating [1], [9], [33]. The MPOA is an area of well-known importance in copulation; it is sensitive to gonadal steroids and is intimately connected to other regions in the brain regulating aspects of sexual behavior [11], [12], [13], [14], [38]. In the spinal cord, copulation-induced Fos expression is observed in galanin-expressing lumbar spinothalamic (LSt) cells [39], as well as cells of lamina X [10]. Moreover, motoneurons of the spinal nucleus of the bulbocavernosus (SNB), which innervate muscles regulating penile reflexes [26], express the IEG products Jun [10] and Fos [28] following consumatory sexual behavior.
Given that the brainstem is reciprocally connected with the hypothalamus, as well as areas in the spinal cord mediating penile reflexes, several sites in the pons and medulla may also regulate the display of copulation. Regions within the caudal ventral medulla (CVM), which are known to regulate penile reflexes [23], innervate the perineal motoneurons of the SNB and the dorsolateral nucleus (DLN), in addition to other lumbosacral cell groups [25]. One group in particular, the lateral paragigantocellular reticular nucleus (LPGi), which receives projections from the MPOA and the PAG [6], [29], is the primary site which provides supraspinal inhibition of perineal motoneurons [23]. In addition, the LPGi may be functionally and anatomically similar to the neighbouring cells of the alpha and ventral divisions of the gigantocellular reticular nucleus (termed the nGi in the current study), and thus, the nGi has additionally been implicated in the control of penile reflexes [16]. Destruction of the medullary raphe pallidus (RPa), which projects to pudendal motoneuron groups [15], produces results similar to those of destruction of the LPGi [45], suggesting this area may also regulate penile reflexes.
Simultaneous injections of tracers into the PAG and lumbosacral spinal cord produced double-labelling of fibres in close apposition to the cells of Barrington's nucleus (Bar), located in the pons [6]. Another region within the pons, the locus coeruleus (LC), also receives input from the LPGi [40]. Electrical and chemical stimulation of the MPOA augmented Fos production in both Bar and the LC, leading the authors to suggest that these regions potentially participate in the regulation of reproductive behaviors [32].
The LPGi–nGi is also intimately connected with the cochlear nuclei complex (CNC) [2], [22], suggesting that auditory processing may modulate CVM activity. Although the role of the CNC has not been studied, it is interesting that the CNC reportedly receives somatosensory input from the cord [44].
Lastly, retrograde tract tracing has identified several regions of the medulla as probable sources of supraspinal projections to the pudendal motoneurons of the SNB and nearby lumbar nuclei. These regions include the parvocellular reticular nucleus (PCRt), spinal and medial vestibular nuclei (SpVe and MVe, respectively), and raphe obscuris (ROb), with additional heavy labelling of the lateral vestibular nucleus (LVe) and the pontine reticular nucleus (PnRt) [27]. Given the diffuse nature of SNB innervation by the reticular regions, its potential contribution to the regulation of copulation has not been assessed as destruction of these important regions might be fatal [27]. Accordingly, observing Fos immunoreactivity in these regions after mating may clarify their potential involvement in this behavior. Thus, mapping the distribution of activated cells in postcopulatory males may elucidate the role of the brainstem in regulating masculine sexual behavior, and refine the circuit diagram for this important behavior.
Section snippets
Materials and methods
Twenty-one adult male Long–Evans rats served as experimental animals while 12 adult female Sprague–Dawley rats were used as stimulus females. Animals were placed on a 12/12 h L/D cycle three to a cage or singly housed, with food and water available ad libitum. Animals were treated according to the Canadian Council on Animal Care guidelines and all protocols were approved by the University Animal Care Committee at Simon Fraser University.
Females were bilaterally ovariectomized (OVX) under
Behavioral observations
On measures of M, I, ML, and IL, the performance of the copulators was comparable to published norms [10]. Cops displayed an average of 8 (±2.36 S.E.M.) mounts and 10 (±1.1 S.E.M.) intromissions before achieving a single ejaculation. The average latencies to mount, intromit, and ejaculate were 70 s (±21.74 S.E.M.), 91 s (±32.85 S.E.M.), and 617 s (±123.94 S.E.M.), respectively. The average interintromission interval was 57.59 s (±10.28 S.E.M.), with an average hit rate of 62% successful
Discussion
Consistent with previous reports establishing that supraspinal ventral medullary neurons are important for the normal display of sexual behavior, the lateral paragigantocellular reticular nucleus and the nucleus gigantocellularis (LPGi–nGi) contained more Fos immunoreactivity in Cops than in the NCs and the SIs (Fig. 6, Fig. 7 for orientation). In addition, the NCs and SIs did not differ in the amount of Fos staining. This pattern of results suggests that the augmentation of Fos above baseline
Acknowledgements
This work was supported by a grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada to NVW. We wish to thank the animal care staff at SFU for their hard work, and the helpful comments of two anonymous reviewers.
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