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Yawning: its cycle, its role
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Fetal yawning assessed by 3D and 4D sonography
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Pharmacol Biochem Behav.
1998;60(2):585-92.
Sexual behavior in male rats after radiofrequency or dopamine-depleting lesions in nucleus accumbens
 
Liu YC, Sachs BD, Salamone JD.
Department of Psychology, and Biobehavioral Sciences, University of Connecticut, USA.

Chat-logomini

Abstract
 
Considerable neurochemical evidence links dopamine (DA) in nucleus accumbens (NAcc) to male sexual behavior. The present experiments were conducted to extend this information to the male's sexual response to remote stimuli from estrous female (noncontact erection; NCE). Male rats were tested for copulation and NCE after either 6-hydroxydopamine (6-OHDA) or radiofrequency (RF) lesions in NAcc). Males with an average 78% depletion of DA in NAcc had a lower incidence of NCE, longer latency to display NCE, and fewer erections. DA-depleted males also had less locomotor activity after injections of d-amphetamine, and reductions in apomorphine-induced yawning, but a normal incidence of penile erection. Males with RF lesions of the NAcc had longer NCE latencies. All males copulated to ejaculation after either 6-OHDA or RF lesions with little or no deficit, although the 6-OHDA-treated males had longer intromission latencies. The NCE deficit supports the hypothesized role of NAcc DA in arousal processes in responding to remote cues from estrous females. The minimal effect of lesions on copulation suggests that the presence of additional proximal stimulation during copulation may overcome the deficits induced by DA depletions or lesions in NAcc.
 

 
NEUROCHEMICAL and lesion studies have yielded conflicting evidence about the role of dopamine (DA) in nucleus accumbens (NAcc) in male sexual behavior. Microdialysis studies have demonstrated that extracellular DA is increased when male rats are exposed to inaccessible estrous females (6,29), and remains elevated during copulatory behavior and until ejaculation (6,32,44). DA-related voltammetric signals increase during sexual behavior (19,21,31) and with the presentation of novel odors associated with estrous females (45). Yet despite the evidence indicating that accumbens DA release is associated with aspects of male sexual behavior, the precise role of DA in sexual behavior remains unclear. Recent voltammetric evidence indicates that DA-related signals increase in response to chasing, mounting, and intromission (22). Yet these markers of extracellular DA actually decrease with ejaculation (22), suggesting that accumbens DA release does not mediate the hedonic aspects of ejaculation. In addi-
 
tion, evidence from studies employing cell body or DAdepleting lesions of the NAcc indicates that damage to this brain area has little or no effect upon male sexual behavior. Depletions of accumbens DA by local injections of the neurotoxic agent, 6-hydroxydopamine (6-OHDA), delayed the initiation of copulation in male rats (9,33), but did not change the number of mounts or intromissions, nor did they alter the intromission ratio, a common index of erectile function in copula (9). Neither electrolytic lesions (2) nor blockade of DA receptors in accumbens by local injections of haloperidol (30) caused more than minor impairment of copulatory behavior. Injections of apomorphine into the ventral tegmental area to decrease DA neuron activity resulted in motor slowing, but did not alter the selection of estrous females by male rats in an X-maze procedure (15). Thus, despite the neurochemical changes seen in NAcc that are associated with male sexual behavior, pharmacological or neurotoxic disruption of accumbens DA, as well as lesions of the intrinsic cell bodies of this structure, leave fundamental aspects of sexual behavior and motivation intact.
 
Of course, it remains possible that aspects of sexual behavior that are not usually assessed in standard tests of copulation would be affected by accumbens DA depletions. For example, responding on a higher order schedule for sexual reinforcement was affected by accumbens DA depletions (9). This is consistent with other data indicating that accumbens DA is involved in responsiveness to conditioned stimuli or remote cues (3,9,33,39,40,42). Recently, procedures have been developed for the assessment of noncontact erections (NCEs) (38). NCEs, which occur in response to remote cues from estrous females, are thought to be related to the autonomic arousal that is characteristic of sexual responsiveness (34,35). NCEs are analogous to human psychogenic erections, usually defined as erections initiated by brain centers in response to visual, auditory, chemosensory, or imaginative stimuli (23,34,35). Previous research has indicated that brain structures involved in NCEs can be dissociated from those that mediate copulatory behavior. In rats, excitotoxic or radiofrequency lesions of the medial preoptic area substantially impaired copulatory behavior, but did not alter NCEs (18). Conversely, radiofrequency lesions of the medial amygdala (17) or bed nucleus of the stria terminalis (18) severely impaired NCEs, but they caused only moderate impairment of copulation. Thus, it is possible that manipulations of the NAcc could alter NCEs, despite the fact that these same conditions would have little effect upon copulatory behavior.
 
The present experiments were conducted to explore the role of the NAcc in mediating NCE in male rats. We examined the effects of 6-OHDA depletion of DA in the NAcc on NCE and copulatory behavior. In a separate experiment, we also studied the behavioral effects of radiofrequency lesions in the NAcc. Males with 6-OHDA lesions were also tested for apomorphine-induced penile erection and yawning, and for d-amphetamine-induced locomotor activity.
 
DISCUSSION
 
In these experiments, cell body or DA-depleting lesions of the NAcc significantly impaired NCE behavior. After accumbens DA depletions, the NAccd males had significantly longer latencies to the first erection in both NCE tests compared to vehicle-control males (Fig. 2). The incidence of NAccd males displaying NCE (Fig. 1) and the number of erections (Table 2) tended to be smaller than vehicle controls in both tests, but were significant only in test 2. Males with RF lesions of NAcc had the same trend as 6-OHDA-treated males for the incidence of displaying NCE (Fig. 1), NCE latencies (Fig. 4), and the number of erections (Table 2) in both tests, but group differences were reliable only for NCE latency in test 2. The significantly longer latency to first NCE (three of four tests across experiments) and the modest reduction in number of NCEs (one of four tests across experiments) in males with NAcc lesions or DA depletions suggests that NAcc is a part of the neural circuitry that regulates the display of NCEs. These results support the view that the NAcc participates in sexual arousal processes (9,14,30,44).
 
Previous work has shown that olfactory cues are essential for evoking NCE (8,37), and lesions of medial amygdala eliminate this sexual response (17). The NAcc receives olfactory input, particularly through projections from the medial amygdala to the shell part of the accumbens (4,12,46). It has been suggested that the NAcc is a "limbic-motor interface" for translating sensory and emotional processes into motor output (25), and that dopaminergic modulation of the activity of neurons in accumbens could serve a "gating" function to influence output (40,41). In addition, DA in NAcc has been implicated in the modulation of behavioral responsiveness to conditioned or remote incentive stimuli (9,33,39,40,42). Thus, in view of the fact that NCE tests measure behavioral responses to remote cues from inaccessible estrous females (38), the results obtained from the present studies are consistent with previous hypotheses of NAcc function. However, the present results are unique in that the response being disrupted by interference with NAcc is not a skeletal motor response such as locomotion or lever pressing, but apparently includes an autonomically mediated response, i.e., penile erection. This inference should be tested by direct measures of erection, rather than the indirect ones used in these studies, because of the possibility that disruption of NAcc function interfered only with the motor component of NCE.
 
In the 6-OHDA experiment, DA in NAcc was depleted by 78% in 6-OHDA-treated males relative to vehicle-treated rats. These depletions were substantial enough to affect NCE and reduce amphetamine-induced locomotion, although 6-OHDA treatment had little effect on several aspects of copulatory be-
 
havior. Despite the fact that the intromission latencies of NAccd males were longer than vehicle-injected males, experimental and control males were similar on all other measures of copulatory behavior in the 6-OHDA experiment, as well as in the RF lesion study. In particular, it should be emphasized that the lesion and control groups in both experiments had similar intromission ratios, suggesting that NAcc lesions did not impair erectile function during copulation. The relative preservation of copulatory function after NAcc lesions or DA depletions is generally consistent with previous studies, reviewed above, using various techniques to impair NAcc function (2,30,33). In addition, the lack of effect on nose pokes after cell body or DA-depleting lesions suggests that any deficit in NCE was not due to reduced attention to the female. Thus, despite the fact that cell body or DA-depleting lesions of NAcc disrupted NCE, several other aspects of sexual behavior were left intact after these manipulations. Clearly, it would be an oversimplification to state that accumbens DA depletions or lesions impaired "motivation," "reward," "preparatory behavior," or even "erections," without further qualification; the deficits in erectile function produced by lesions or DA depletions of accumbens were highly context specific.
 
There are many reports that extracellular DA levels in the NAcc increase during copulation until ejaculation (6,10,21,29, 32,44). However, depleting accumbens DA had relatively little effect on copulatory behavior [(33) and present article]. Previous reports have indicated that the response-suppressing effects of interference with DA function can be overridden by increasing the sensory input (40). For example, changing environmental stimulation reversed the decline in locomotion in-
 
duced by treating rats with haloperidol (20). In the present experiments, the proximal stimuli from the female during the copulation tests may have overcome the lesion-induced deficit that was manifest in the NCE tests, in which only remote cues from the female were available to the male.
 
Systemic administration of low doses of the DA agonist, apomorphine, can induce penile erection and yawning in rats (11,24,26). This effect can be blocked by pretreatment with the DA antagonist, haloperidol (11,43). Several previous studies indicated that the behavioral effects of systemic apomorphine treatment are mediated by multiple sites within the brain. For example, electrolytic lesion of the paraventricular hypothalamus (PVH) abolished apomorphine-induced erections and yawning (1). Depletions of striatal DA by injections of 6-OHDA into caudate putamen also abolished apomorphine-induced erections and yawning (7). Microinjection of apomorphine (1 g) into PVH induced both penile erection and yawning, but similar injections into NAcc were ineffective (24). Our data suggest that NAcc DA does play a role in apomorphine-induced yawning, but little or no role in apomorphine-induced penile erection. It is possible that accumbens DA depletions reduced the effect of apomorphine on yawning because the DA depletions resulted in DA receptor supersensitivity, which eliminated the yawning effect of apomorphine that normally occurs at a relatively low dose. In addition, the lack of effect of DA depletions on apomorphine-induced penile erections suggests that erections induced by remote cues from females and those induced by apomorphine are regulated by different brain mechanisms.
 
In summary, male rats with depletions of accumbens DA had deficits in NCE, which supports the hypothesized role of NAcc DA in sexual arousal processes involved in responding to remote cues from estrous females. After incurring cell body or DA-depleting lesions of NAcc, these males had little or no deficit in nose pokes or copulatory behavior. We therefore infer that the deficit in NCE was not due to a reduction of attention to the females or to a severe deficit in sexual motivation.