Neuroscience Letters, 141 (1992) 218 222 co, 1992 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/92/$ 05.00

NSL 08766

Elicitation of penile erection following activation of the hippocampal formation in the rat K u a n g - K u o Chen a, Julie Y.H. Chan c, Luke S. Chang a, Ming-Tsun Chen a and Samuel H.H. Chan b "Division of Urology, Department of Surgery, blnstitute of Pharmacology, National Yang-Ming Medical College and 'Department of Medical Research, Veterans General HospitaLTaipei, Taipei ( Taiwan, ROC) (Received 18 February t992; Revised version received 14 April 1992; Accepted 16 April 1992

Key words." Penile erection: Intracavernous pressure; Ammon's horn; Dentate gyrus; Hippocampal formation; Rat We explore the possible involvement of the hippocampal formation in penile erection, using male, adult Sprague-Dawley rats that were anesthetized with pentobarbital sodium. The intracavernous pressure (ICP) was used as the experimental index for penile erection. Electrical activation of the hippocampal formation resulted in two patterns, viz, multiple and single episodes of elevation in ICP, along with visible penile erection and ejaculation. The former pattern exhibited an increase in ICP that was more sustained, with higher peak amplitude and longer latency. Furthermore, they originated respectively from the granule cells of the dentate gyrus and pyramidal cells of the CA1 and CA3 fields of the Ammon's horn. Chemical stimulation of the hippocampus with glutamate also elicited significant increase in ICP. These results thus provided direct evidence to establish that the hippocampal formation may be involved in central neural regulation of the erectile process.

Penile erection is a sophisticated physiologic response which depends upon the integration of vascular, endocrine, neurologic and psychologic mechanisms. In the peripheral nervous system, it is generally assumed that penile erection is mediated by opposing sympathetic and parasympathetic outflows to penile vascular smooth muscles [2, 8]. Apart from acetylcholine and norepinephrine, the participation of neuropeptides (e.g. vasoactive intestinal peptide, neuropeptide Y, somatostatin) and nitric oxide has been reported [8]. Our knowledge on the central nervous control of penile erection is scanty at best. In his classical studies on squirrel monkeys, MacLean and associates [3, 6] reported a remarkable correlation between penile erection elicited by electrical stimulation of septum, rostral diencephalon and frontal cortex and recruitment of highvoltage potentials and afterdischarges in the hippocampus. Although these authors concluded that the hippocampus only plays an adjunctive role in genital function, their electrophysiologic results have long since been assumed to imply a direct involvement of the hippocampus in penile erection [8]. Our laboratory recently established an animal model Correspondence." S.H.H. Chan, Institute of Pharmacology, National Yang-Ming Medical College, Taipei 11221, Taiwan, Republic of China. Fax: (886) (2) 8264372.

[1], which uses the intracavernous pressure (ICP) in anesthetized rats as the index for a more objective, accurate and quantitative assessment of penile erection. The ability to record intracavernousty alongside hemodynamic parameters immediately differentiates the direct, penile action from indirect, circulatory effect. Thus, we have in our hands an animal model that, with the entire neuraxis intact, is appropriate for the neurophysiologic and neuropharmacologic study of the central integrative mechanisms in the erectile process. This is the impetus for the present study, in which we intend to directly establish the hitherto assumed role of the hippocampus in penile erection. As demonstrated, electrical or chemical activation of the hippocampal formation indeed elicited an increase in ICE along with visible penile erection and ejaculation in many instances. Male, adult Sprague-Dawley rats (200-300 g) anesthetized with pentobarbital sodium (50 mg/kg, i.p., with 10 mg/kg/h i.v. supplements) were used. They were placed on a heating pad to maintain the body temperature at 37°C. An endotracheal tube was placed for a patent airway. The femoral artery was cannulated for the measurement of systemic arterial pressure through a pressure transducer (Gould 23ID). Heart rate was detected by a biotachometer triggered by the arterial pressure pulses. The femoral vein was also cannulated for the administration of drugs.


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Fig. I. Two response patterns, multiple episodes (A) and single episode (B) of increase in intracavernous pressure (1CP), elicited by electrical stimulation {bar) of the hippocarnpal formation. Note lack of simultaneous change of systemic arterial pressure (SAP) and heart rate (HR) immediately before or during the period of elevation of ICP in both instances.

The rat was mounted on a stereotaxic headholder (Kopf) in a prone position. The lower part of the body was slightly rotated to adequately expose the lower abdomen. The penis was degloved, and a 26-gauge needle connected to a Gould 23ID pressure transducer was meticulously and accurately inserted into one of the corpora cavernosa to measure the ICP. Stereotaxically positioned (1.8 2.6 mm posterior to the bregma, 3.0 4.5 mm from the cortical surface and 0.5 1.5 mm lateral to the midline) bipolar concentric electrode (Rhodes SN-100, tip diameters: 100 and 250 #m) and 28-gauge stainless steel needle were used respectively for electrical and chemical activation of the hippocampal formation. The former was delivered via a 30-s train of 30 120 #A, 40 160 Hz, 1-ms rectangular pulses. The latter was achieved by microinjection of glutamate (0.5 nmol, 50 hi) with a 1-#1 Hamilton syringe. The resultant effects on ICP, systemic arterial pressure and heart rate were continuously monitored on a polygraph (Gould ES2800). In addition, the status of the penis was carefully observed for visible erection and/or ejaculation. The brain was removed after each experiment and fixed in 30% sucrose in 10% formaldehyde-saline. Frozen 25-/am sections stained with either Cresyl violet or Neu-

tral red were used for histologic verifications of the sites of electrical or chemical stimulation. The latter was aided by the addition of 1% Evans blue in the microinjection medium. As we observed previously [1], the resting ICP in all rats (tt-27) studied was 5.2 +_ 0.6 mmHg (mean + S.E.M.). The elevation in ICP upon electrical activation of the hippocampal formation exhibited at least 4 characteristics. First, there were two basic response patterns: multiple episodes (Fig. IA) and single episode (Fig. 1B) of rise in ICE The elevation of ICP in the former was more sustained, and displayed higher peak amplitude and longer latency (Table 1). Close histologic examination revealed that these two response patterns originated respectively from activation of the granule cells at the dentate gyrus and pyramidal cells primarily at the CA1 and CA3 fields of the Ammon's horn (Fig. 2). However, they were not related to the intensity and frequency of the electrical stimulation. In two rats, we actually encountered an immediate rise in ICP (26 and 68 mmHg) simply following the insertion of the electrode through the CA3 field. Second, visible penile erection (straightening of penis with disappearance of the angle between the glans and


shaft) was observed during multiple and single episodes of elevation in ICP (Table I), at a threshold pressure of approximately 40 mmHg. Some of them were accompanied by ejaculation (Table I). Third, a majority (11/16) of the erectile effect was not associated with remarkable changes in mean systemic arterial pressure (99.4 + 8.0 vs. 98.4 + 7.6 mmHg, mean + S.E.M., n = l l ) and heart rate (433.6 + 9.8 vs. 434.3 + 10.4 bpm, mean + S.E.M., n-- 11 ) before and during the elevation of ICP (Fig. 1). Furthermore, transient increase in arterial pressure with phenylephrine (5 pg/kg, i.v.) did not elicit significant alteration in ICE Fourth, activation of sites rostral to the dentate gyrus and Ammon's horn, or adjacent to the immediate vicinity of the granule and pyramidal cell layers (Fig. 2), was relatively ineffective. The latter, however, was not attributed to end-organ failure, since the penis still responded positively to intracavernous injection of papaverine (0.4 mg) with an increase in ICP [1]. We also carried out chemical activation (n=3) with glutamate to confirm that our observed elevation in ICP by electrical stimulation of the hippocampal formation was indeed exerted on neuronal perikarya. As exemplified by Fig. 3, microinjection of the excitatory amino acid (0.5 nmol, 50 nl) into the pyramidal cell layers of the CA3 field induced a multiple episodal rise in ICP, with a latency of 72 s and a peak effect of 66 mmHg. This elevated ICP persisted for approximately 60 min before returning to the basal level. The present study thus provided evidence to establish that the hippocampal formation is directly involved in the initiation of penile erection in anesthetized rats. We further demonstrated that, specifically at the cellular level, the granule cells of the dentate gyrus and pyrami-


Resting ICP (mmHg) Duration (s) Peak increase in ICP (mmHg) Latency (s) Visible erection Ejaculation following visible erection

Multiple episode

Single episode

5.4 _+0.8 (3-10) 2175.0 _+ 719.0 (30(~6000) 74.2 _+ 10.0 (26-100) 87.3 _+ 18.7 (26-160) 7/8

5.0 +_0.8 (3 10) 171.3 _+ 86.9 ( 2 0 750) 28.0 _+8.4 (10-77) 15.3 _+6.4 (3-52) 2/8








Fig. 2. Diagrammatic representations of different rostral-caudal levels (ram) of the hippocampal formation relative to the bregma illustrating the sites upon which electrical activation elicited multiple (e) and single ( : ) episodes of increase in intracavernous pressure. Although active sites were bilaterally represented, they are indicated on the left side of the diagram for clarity. Sites that were ineffective were indicated by (2?). Abbreviations: CAI, CA1 field of the Ammon's horn; CA2, CA2 field of the Ammon's horn; CA3, CA3 field of the Ammon's horn; cc, corpus callosum; cg, cingulum; DG, dentate gyrus; fi, fimbria hippocampus: LV, lateral ventricle; SFO, subfornical organ; 3V, third ventricle.

dal cells of the CA1 and CA3 fields of the Ammon's horn are respectively responsible for the eticitation of multiple and single episodes of increase in ICP. Our study was greatly facilitated by the experimental index used for penile erection [1]. Its superb detection sensitivity of 2 mmHg allowed us to identify the minimal peak amplitude of ICP upon hippocampal activation to be 10 mmHg (Table I). Although this is already close to a 100% increase over the basal value, it is still: much below 40 mmHg, which was the threshold pressure for visible erection, the basis for behavioral evaluations. In addition, our continuous monitor of fluctuations in ICR along with systemic arterial pressure and heart rate, enabled us to define the latency, duration and temporal patterns of alterations in ICP following hippocampal stimulation, and immediately dissociated these results from indirect, circulatory influences. The identification of multiple and single episodes of rise in ICP following perikaryal activation respectively of the granule and pyramidal cells is intriguing. Contempo-


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Fig. 3. Time course changes of iniracavernous pressure (ICP), systemic arterial pressure (SAP) and heart rate (HR) following microinjection of glutamate 10.5 nmol, 50 nl) into the CA3 field of the hippocampal R)rmation. Note again the lack of simultaneous hemodynamic changes betk~re and during the period of increased ICE

rary studies on the hippocampus stipulate that the circuitous flow of information between the granule cells of the dentate gyrus, mossy fibers, the pyramidal cells of the CA3 and CA I fields and Schaffer collaterals [5], is responsible for long-term potentiation, the electrophysiologic basis for memory [4, 7]. Speculatively, this same circuit(s) may also account for our findings. The stream of neuronal signals, initiated upon activation of the granule cells, may impinge asynchronously (via mossy fibers and Schaffer collaterals) on the pyramidal cells, resulting in the multiple episodes of rise in ICE On the other hand, synchronous discharge elicited upon direct activation of the pyramidal cells, the efferent neurons of the hippocampus, may only give rise to a single episode of elevation in ICE Our observed elicitation of multiple (cf. Fig. 3), instead of single episodes of elevation in ICE upon microinjection of glutamate into the CA3 area may lend credence to the above notion. It is plausible that the progressive excitation of the pyramidal cells by diffusion of the excitatory amino acid mimics an asynchronous activation of these neurons. MacLean and Ploog [6] also reported that during afterdischarges in the hippocampus following stimulation of highly effective brain loci for penile erec-

tion, erections may become throbbing in character and reach maximal degree. Nonetheless, this speculation must await further verification, as is the possible involvement of humeral factors that may be released upon hippocampal activation. At the same time. the relative position of the hippocampal formation in the central neural circuit responsible for penile erection remains to be elucidated. Supported in part by Research Grant NSC-81-0412B010-10 to S.H.H.C. by the National Science Council, Taiwan. Republic of China. I Chen, K.K., Chan, J.Y.H., ('hang, L.S., ('hen, M.T. and Chan, S.H.H.. Intracavernous pressure as an experimental index in a rat model for the evaluation of penile erection. J. Urol.. 147 11992) 1124-1128. 2 de Groat, W.C. and Steers, W.D., Neuroan'ltomv and neurophysielegy of penile erection. In E.A. Tanagho, T.F. Lue and R.D. M e ( l u r e (Eds.), Contemporary Management of Impotence and hifertility, Williams & Wilkins. Baltimore, 1988, pp. 3 27. 3 Dua, S. and MacLean, P.D., Localization for penile erection in medial frontal lobe, Am. J. Physiol., 2{)7 (1964) 1425 1434. 4 Gustal~son, B. and WigstrOm, H., Physiological mechanisms underlying long-term potentiation, Trends Neurosci.. I1 (1988) 156 162.

222 5 Kandel, E.R., Spencer, W.A. and Brinley Jr., F.J., Electrophysiology of hippocampal neurons. I. Sequential invasion and synaptic organization, J. Neurophysiol., 24 (1961) 225-242. 6 MacLean, P.D. and Ploog, D.W., Cerebral representation of penile erection, J. Neurophysiol., 25 (1962) 29-55.

7 Nicoll, R.A., Kauer, J.A. and Malenka, R.C., The current excitement in long-term potentiation, Neuron, 1 (1988) 97 -103. 8 Steers, W.D., Neural control of penile erection, Sere Urol., 8 (1990) 66 79.

Elicitation of penile erection following activation of the hippocampal formation in the rat.

We explore the possible involvement of the hippocampal formation in penile erection, using male, adult Sprague-Dawley rats that were anesthetized with...
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